(m.n)-Homorubins: Syntheses and structures

Article abstract of DOI:10.1007/s00706-014-1288-4

Five new homorubin analogs of bilirubin with their two dipyrrinone components conjoined to (CH₂)₂, (CH₂)₃, and (CH₂)₄ units were synthesized with propionic acid chains shortened to acetic and elongated to butyric, and examined by spectroscopy and molecular mechanics computations for an ability to form conformation-determining hydrogen bonds. With m designating the number of conjoining CH₂ units and n indicating the number of CH₂ units of the alkanoic acid chains of ( m.n)-homorubins, (2.1)-, (3.2)-, (4.2)-, and (4.3)-homorubins were prepared and compared with previously synthesized (2.2) and (2.3), which adopt intramolecularly hydrogen-bonded conformations in CHCl₃.

Full text of DOI:10.1007/s00706-014-1288-4

Monatsh Chem (2014) 145:1777–1801
DOI 10.1007/s00706-014-1288-4
ORIGINAL PAPER
(m.n)-Homorubins: syntheses and structures
•
William P. Pfeiffer David A. Lightner
Received: 24 April 2014 / Accepted: 22 July 2014 / Published online: 10 September 2014
Ó Springer-Verlag Wien 2014
Abstract Five new homorubin analogs of bilirubin with
their two dipyrrinone components conjoined to (CH2)2,
(CH2)3, and (CH2)4 units were synthesized with propionic
acid chains shortened to acetic and elongated to butyric,
and examined by spectroscopy and molecular mechanics
computations for an ability to form conformation-deter-
mining hydrogen bonds. With m designating the number of
conjoining CH2 units and n indicating the number of CH2
units of the alkanoic acid chains of (m.n)-homorubins,
(2.1)-, (3.2)-, (4.2)-, and (4.3)-homorubins were prepared
and compared with previously synthesized (2.2) and (2.3),
which adopt intramolecularly hydrogen-bonded confor-
mations in CHCl3.
of bilirubin with vinyl groups reduced to ethyls, e.g.,
mesobilirubin-XIIIa (Fig. 1c, where m = 1, n = 2), also
adopt an intramolecularly hydrogen-bonded structure [9,
10] and were found to exhibit similar solution and meta-
bolic properties [11, 12]. To learn whether intramolecular
hydrogen bonding might persist in a centrally homologated
mesobilirubin, yet remain stable, we linked the two dip-
yrrinones to two CH2 connector groups. We communicated
[13] our synthesis of (2.2)-homorubin (Fig. 1c, where
m = n = 2) and compared its properties to those of mes-
obilirubin-XIIIa. The work indicated the presence of
bilirubin-like intramolecular hydrogen bonding and a he-
patobiliary metabolism similar to that of bilirubin and
mesobilirubin-XIIIa. More recently we reported on a (2.3)-
homorubin, with butyric acids replacing propionic, and this
too proved to be intramolecularly hydrogen bonded,
although its shape was modified to accommodate the
longer butyric acid [14].
Keywords Bilirubinoids ꢀ Synthesis ꢀ
Molecular structure ꢀ Conformational analysis
Introduction
Prompted by our first communication on homorubin
[13], we undertook the syntheses and analyses of a series of
(m.n)-homorubins (Fig. 1c), where m varied between 1 and
4 and n varied between 1 and 3. At first, we conducted a
series of conformational analysis studies by molecular
dynamics using Sybyl to learn whether the carboxylic acids
could engage in intramolecular hydrogen bonding to the
dipyrrinones and to estimate the stabilization energies and
number of effective hydrogen bonds (Table 1). We found
the predictions of Table 1 for (2.2)- and (2.3)-homorubins
to correlate nicely with their solution and spectroscopic
properties and thus extended the investigations to other
(m.n)-homorubins, notably (2.1), (3.2), (4.1), (4.2), and
(4.3), whose syntheses and analyses are presented in the
following. We were especially interested in pairs of the
same molecular formulas, (2.2) and (3.1); (3.3) and (4.2),
and the triplet (2.3), (3.2), and (4.1). Unfortunately, we
Bilirubin (Fig. 1a) is the yellow pigment of human bile and
jaundice [1], produced copiously as the end product of
porphyrin metabolism [2]. Its most stable conformation in
the crystal [1, 3–5] and in solution [1, 6–9] is one where the
two dipyrrinone chromophores are rotated about C(10) so
as to bring each dipyrrinone into hydrogen bonding with
one of its two propionic acid groups (Fig. 1b). The analogs
W. P. Pfeiffer
Apex International, Eden Prairie, MN 55344, USA
D. A. Lightner (&)
Department of Chemistry, University of Nevada, Reno, NV
89557-0216, USA
e-mail: lightner@unr.edu
123

1778
W. P. Pfeiffer, D. A. Lightner
(A)
(B)
(C)
CO2H
HO2C
H
H
H
H
8
10CH2
12
8
O
(CH2)n
10
C
H
N
O
N
N
O
N
H H
H H
. .
O
.
N
N
.
.
C
.
.
O
.
.
. .
H
O
O
O
.
O
C
H
C
10(CH2)m
O
. . . .
H H
H
H
N
N
O
H
H
N
N
H H
N
N
(CH2)n
O
O
12
Bilirubin
Bilirubin
(m.n)-Homorubins: m > 1
Mesobilirubin-XIIIα: m = 1, n = 2
Fig. 1 a Bilirubin-IXa in a porphyrin-like representation and b in its
intramolecularly hydrogen-bonded planar projection. c The planar
projection of intramolecularly hydrogen-bonded homorubins, where
the integers m and n indicate the number of CH2 groups in the linker
at position 10 and in the alkanoic acids, respectively. Hydrogen bonds
are represented by dotted lines
in the synthesis of the first homorubin, where m = 2, with
propionic acid chains at the usual locations, C(8) and C(12)
[13]—and recently for the (2.3)-homorubin with butyric
acid chains [14].
Table 1 Potential energies (EP) of (m-n)-homorubins as computed by
molecular dynamics for: hydrogen bonded (w/H-bonds), non-hydro-
gen bonded in the same conformation without (w/o H-bonds), and the
difference in energy (EP(w) - EP(w/o))
m n EP/kJ mol-1 Number of EP/kJ mol-1
DEP/kJ mol-1
w/o H-bonds EP(w) - EP(w/o)
w/H-bonds
H-bonds
(4.n)-Homorubins
2
2
2
3
3
3
4
4
4
1 -57
6
6
6
6
6
6
4
4
6
29
12
-5
12
14
35
64
27
1
-86
-83
-58
-96
-93
-103
-75
-82
-78
2 -71
3 -63
1 -84
2 -79
3 -68
1 -11
2 -55
3 -77
It also turned out to work well in the syntheses of the (4.1)-,
(4.2)-, and (4.3)-homorubins, 4-1, 4-2, and 4-3, which were
all synthesized (Scheme 1) following saponification of
their dimethyl esters (4-1e, 4-2e, and 4-3e) prepared by
condensing the well-known 5-bromomethylene 2-pyrroli-
none (12) [17, 18] with a 1,4-dipyrrylbutane core (5-n).
Under the reaction conditions, 5-n underwent decarboxyl-
ation of both pyrrole a-CO2H groups to give reactive a,a-
dipyrroles, which coupled with 12 in standard fashion [17–
19]. It was thought that the most practical way to introduce
the 1,4-disubstituted butane unit of 5-n would be as a
diacetylene unit (as in 6-ne) formed by Pd(0)-catalyzed
coupling of the two mono-acetylenes. The target mono-
acetylenes would thus be 7-n for which we envisioned
introduction of the acetylene by a Sonogashira reaction
[20–22] of trimethylsilyl acetylene with an a-iodopyrrole
(9-n). The a-iodopyrroles 10-n were prepared in four steps
from simple, acyclic starting materials: diethyl oximino-
malonate and the known acyl-oxo-esters 12-n [23] to afford
mono-pyrrole diesters 11-n, prepared in connection with
earlier synthetic work [23].
See Fig. 1b for intramolecularly hydrogen-bonded structure
Calculated and determined using Sybyl version 6.0
were unable to prepare the (3.1)- and (3.3)-homorubins and
thus report only the (3.2) of the m = 3 set. The (2.2)- and
(2.3)-homorubins are known [13, 14]; the (1.2) is meso-
bilirubin-XIIIa, also known [15].
Results and discussion
Homorubin synthesis aspects
The syntheses of (m.n)-homorubins, where m is an even
integer proved to be conducted best by the ‘‘1 ? 2 ? 1’’
route [16], where a dipyrrole core was expanded to the
desired tetrapyrrole by the addition of two equivalents of a
reactive mono-pyrrole. This approach worked successfully
The a-CH3 of 11-n was converted to a-I [24] first by
perchlorination with SO2Cl2, followed by hydrolysis of the
newly formed a-CCl3 to convert it to a-CO2H (10-n). The
last was converted to a-I upon treatment [24] with KI/I2
ethanol (10-1) or methanol (10-2, 10-3). The resultant a-I
123

(m.n)-Homorubins: syntheses and structures
1779
Scheme 1
CO2H
HO2C
CO2CH3
(CH2)n
CH3O2C
(CH2)n
(CH2)n
(CH2)n
NaOH
THF-CH3OH
(CH2)4
(CH2)4
O
O
O
O
N
H
N
H
N
H
N
H
N
H
N
H
N
H
N
H
Δ
4-1e: n = 1 (42%)
4-2e: n = 2 (45%)
4-3e: n = 3 (37%)
4-1: n = 1 (43%)
4-2: n = 2 (62%)
4-3: n = 3 (62%)
12:
CH3OH, Δ
O
CHBr
N
H
CO2H
(CH2)n
HO2C
(CH2)n
CO2Et
(CH2)n
EtO2C
(CH2)n
NaOH
(CH2)4
(CH2)
HO2C
CO2H
4
EtO2C
CO2Et
THF CH3OH
N
H
N
H
N
N
H
H
5-1: n = 1 (76%)
5-2: n = 2 (99%)
5-3: n = 3 (74%)
5-1e: n = 1 (97%)
5-2e: n = 2 (99%)
5-3e: n = 3 (99%)
H2 Pd(C) or PtO2
CO2Et
EtO2C
(CH2)n
(CH2)nCO2Et
CH
(CH2)nCO2Et
C-TMS
(CH2)n
K2CO3
[(C6H5)2P]4Pd(0)
Cu(I)I
C
C
EtO2C
EtO2C
N
H
N
C
C C C
EtO2C
CO2Et
N
H
N
H
H
6-1e: n = 1 (91%)
6-2e: n = 2 (70%)
6-3e: n = 3 (74%)
7-1: n = 1 (66%)
7-2: n = 2 (79%)
7-3: n = 3 (70%)
8-1: n = 1 (90%)
8-2: n = 2 (76%)
8-3: n = 3 (84%)
(CH2)nCO2Et
(CH2)nCO2Et
(CH2)nCO2Et
(CH2)nCO2Et
(EtO2C)2C
SO2Cl2
I2/KI
NOH
EtO2C
EtO2C
CH3
CO2H
EtO2C
I
Zn/HOAc
N
H
N
H
N
H
O
O
12-1: n = 1
12-2: n = 2
12-3: n = 3
11-1: n = 1 (60%)
11-2: n = 2 (42%)
11-3: n = 3 (60%)
10-1: n = 1 (63%)
10-2: n = 2 (67%)
10-3: n = 3 (52%)
9-1: n = 1 (78%)
9-2: n = 2 (63%)
9-3: n = 3 (87%)
pyrrole 9-n formed in good yield and served as the entry to
diacetylenic dipyrrole 6-ne. An acetylene group was
substituted smoothly and in high yield for a-I using tri-
methylsilyl (TMS) acetylene in the presence of
bis(triphenylphosphine)palladium(II) chloride plus cop-
per(I) iodide [20–22, 25]. After initial attempts to desilylate
8-n using 1 M aq. NaOH gave very poor yields, the TMS
actylene group of 8-n was deprotected by adding anhyd.
K2CO3 in anhydrous ethanol to a solution of 8-n in dry
tetrahydrofuran (THF). Thus, 7-n was formed in good
yield, and it was self-coupled to 6-ne using tetrakis(tri-
phenylphosphine)palladium(0) and Cu(I) catalyst in the
presence of chloroacetone and triethylamine [26].
Hydrogenation of diacetylenes 6-1e and 6-2e over 5 %
Pd(C) catalyst in ethyl acetate at 1 bar gave nearly quan-
titative yields of the target 1,4-dipyrrylbutane esters 5-1e
and 5-2e. Attempts to hydrogenate 6-3e under the same
conditions, or at 2.8 bar were unsuccessful; however,
success was achieved using PtO2 catalyst at 1 bar over
24 h. Saponification of 5-ne in aqueous ethanolic NaOH,
followed by removal of the ethanol and acidification with
20 % HNO3 produced white precipitates of 5-n, which
were collected and dried. Acids 4 tended to be less stable
than their (m = 2) analogs, with their colors changing to
pink or brown when exposed to air for more than a few
hours. They were converted to tetrapyrrole esters 4-ne by
123

1780
W. P. Pfeiffer, D. A. Lightner
suspending them in CH3OH, bringing the suspension to
reflux, and adding two equivalents of 5-bromomethyl-2-
pyrrolinone (12). After 3 h at reflux, the product 4-ne
started to precipitate. Surprisingly and unexpectedly, all of
the homorubin esters 4-ne are very insoluble in many
organic solvents. For example, heat and ultrasonication are
required to dissolve them completely in dimethyl sulfoxide
(DMSO) for characterization by nuclear magnetic reso-
nance (NMR) spectroscopy. After sitting a few days in
DMSO, they begin to precipitate. The best purification
method for the esters 4-ne was to dissolve them in hot
dimethyl formamide (DMF) (*110 °C) and allow them to
precipitate upon cooling. The precipitates were filtered,
washed with ethanol to remove any traces of DMF, then
washed again with CH2Cl2 to remove any additional
impurities that may have been in the precipitate. These
solubility properties of 4-ne are very peculiar compared to
the shorter bridged dimethyl analogs where (m = 1) and
(m = 2). One might assume that by interspersing more
CH2 units (m = 4) between the dipyrrinone units in 4-ne,
the compounds would be more soluble in organic com-
pounds, but that is not the case. Strangely, during the
saponification of 4-ne to 4-n, although esters 4-ne could
only be suspended (and not dissolved) in hot THF-CH3OH,
upon addition of aq. NaOH they go immediately into
solution. After cooling, acidification led to precipitated 4-n
in respectable yields.
reflux; (3) molten KOAc–NaOAc; or (4) by heating to
230 °C.
Therefore, we resorted to a ‘‘2 ? 2’’ route (Scheme 3)
described by Chen et al. [27] to prepare a conjugated bis-
dipyrrinone propene. As modified to neoxanthobilirubinic
acid methyl ester [28], reaction of 14 with dimethylami-
noacrolein yielded the vinylogous Vilsmeier product 15 in
good yield. Condensation of 14 and 15 in acetic acid–HBr
afforded the desired product 13. Reduction of 13 first with
NaBH4 to reduce the azafulvene unit, followed by catalytic
hydrogenation of the residual C=C afforded the m = 3,
n = 2 homorubin dimethyl ester (3-2e), from which the
(3.2)-homorubin (3-2) was obtained after saponification.
Neoxanthobilirubinic acid methyl ester (14) was syn-
thesized from 17 [29] and 18 [30], which were prepared
from small molecules, as outlined in Scheme 3. Nitroe-
thane and propionaldehyde were carried through five steps,
involving the Barton–Zard pyrrole reaction to the known
4-ethyl-3-methyl-3-pyrrolin-2-one (17), as described pre-
viously [29]. Similarly, as described earlier, succinic
anhydride and acetylacetone were carried through a series
of four steps to produce pyrrole diester 19 [18]. The pyrrole
a-CH3 was oxidized to pyrrole a-aldehyde 18 [30] by ceric
ammonium nitrate [31, 32]. Condensation of 17 with 18 in
4 M KOH–CH3OH yielded dipyrrinone diacid 16 [33],
which was smoothly a-decarboxylated and esterified in
CH3OH–H2SO4 to afford dipyrrinone 14 [33] in 59 % yield
from 17 and 18. The synthesis of tetrapyrrole 13 required
the dipyrrinone coupling partner 15, which was prepared
from 14 in good yield by reaction with dimethylaminoac-
rolein. Dipyrrinones 14 and 15 were smoothly condensed
to form the conjugated 1,3-bis-dipyrrinone propene 13 in
86 % yield, and 13 was reduced to 3-2e as described above.
Saponification of 3-2e yielded the desired 3-2.
(3.n)-Homorubins
The synthesis of a m = 3 homorubin, e.g., a three-carbon
homolog of mesobilirubin-XIIIa, was initially intended to
follow a ‘‘1 ? 2 ? 1’’ route similar to that used for the
m = 2 and m = 4 homorubins. However, attempts to
form a 1,3-bis-pyrrylpropane central unit failed. These
included initially attractive routes toward the synthesis of
the (3.1)-homorubin analog of 4-1e. However, as outlined
in Scheme 2 (1), though the conversion of iodopyrrole 9-1
(Scheme 1) to its deiodinated a-H analog proceeded
smoothly, as did a vinylogous Vilsmeier reaction to pro-
duce the b-(a-pyrrolyl)-acrolein, the last failed to
condense with the a-H pyrrole. In Scheme 2 (2) the b-(a-
pyrrolyl) acrolein formed in (1) could be reduced to the
corresponding allylic alcohol, but as such or as its tosylate
failed to condense with the a-H pyrrole precursor. Similar
failures were encountered with the C=C reduced analogs.
In Scheme 2 (3), we found an unacceptably low yield
(*10 %) of the final nitro-dipyrrolylpropene product. In
Scheme 2 (4), although the pyrrole a-aldehyde could be
condensed smoothly with malonic acid, neither the
resultant unsaturated diacid nor its hydrogenation product
could be smoothly decarboxylated in: (1) ethanol–conc.
HCl (1:1 by vol.) at reflux; (2) aniline or piperidine at
(2.1)-Homorubins
Reverting to the ‘‘1 ? 2 ? 1’’ approach to linear tetra-
pyrrole synthesis, we employed a route similar to that used
for preparing (2.2)-homorubin [13, 14] and (2.3)-homoru-
bin [14]. Here and in the (2.2)- and (2.3)-homorubin, the
required center dipyrrole sections differed only by the
length of the carboxylic acid chain: acetic vs. the propionic
and butyric of earlier work [13, 14]. Thus, as shown in
Scheme 4, the a-CH3 of the known mono-pyrrole diester
11-1 [23] was converted to the a-CHO of 22 in 66 % yield
by oxidation using ceric ammonium nitrate [31, 32], and 22
was coupled to afford dipyrrylethene 21 in 46 % yield by
the McMurry coupling procedure [13, 14, 34] using Ti°.
The central C=C of 21 in ethyl acetate was smoothly
hydrogenated (H2/Pd(C)) to give a 99 % yield of dipyrry-
lethane tetra-ester 20e, which was saponified in ethanolic
NaOH to its tetra-acid 20 in 99 % yield. The last, when
123

(m.n)-Homorubins: syntheses and structures
1781
Scheme 2
EtO2C
EtO2C
EtO2C
EtO2C
(1)
EtO2C
I
N
(CH3)2NCH
CHCHO
HCl/KI
XH
H
POCl3
HBr-AcOH
EtO2C
EtO2C
EtO2C
I
N
H
N
H
N
H
O
9-1
EtO2C
EtO2C
EtO2C
EtO2C
(2)
N
NaBH4
XH
p-TSA or
H
OH
EtO2C
EtO2C
N
H
N
H
montmorrilonite K-10
O
Tos-Cl
EtO2C
EtO2C
N
EtO2C
XH
p-TSA or
OTos montmorrilonite K-10
EtO2C
N
H
EtO2C
(3)
EtO2C
EtO2C
EtO2C
CO2Et
H
, piperidine
EtO2C
N
H
1) MeNO2/MeNH2
HCl
O
H
EtO2C
2) NaBH4
EtO2C
EtO2C
CO2Et
N
H
NO2
N
H
N
H
N
H
NO2
O
O
(4)
EtO2C
EtO2C
EtO2C
CH2(COOH)2/aniline
H2/Pd/C
COOH
COOH
COOH
COOH
H
EtO2C
EtO2C
EtO2C
N
H
N
H
N
H
CO2
CO2
dissolved in refluxing CH3OH in the presence of 5-bro-
moethylene-3-pyrrolin-2-one (12), underwent
(2.2)- and (2.3)-homorubins, we investigated how homol-
ogating the central CH2 unit of mesobilirubin-XIIIa affects
(1) the ability of the pigment to engage in intramolecular
hydrogen bonding and (2) the most stable shape of the
(2.2)-homorubin molecule [13], which was found to adopt
a layered conformation with intramolecular hydrogen
bonding. More recently, we showed that even with
homologating the propionic acid chains to butyric, this
pigment also strongly favors intramolecular hydrogen
bonding and also with more of a layered shape [14]. These
studies led to an investigation of other homorubins, for
which Sybyl molecular dynamics calculations predicted
that while six intramolecular hydrogen bonds persisted in
the (2.1)-, (2.2)-, (2.3)-, (3.2)-, and (4.3)-homorubins, four
hydrogen bonds were engaged in the (4.1)- and (4.2)-ho-
morubins (Table 1). The energy-minimum molecular
geometry, defined by torsion angles (A–F) and dihedral
decarboxylation of the a-CO2H groups and coupling to 1 to
afford tetrapyrrole dimethyl ester 2-1e in 51 % yield,
which was saponified to the desired m = 2, n = 1 ho-
morubin (2-1) in 52 % yield.
Conformational analysis
Bilirubin-IXa (Fig. 1a, b) and mesobilirubin-XIIIa
(Fig. 1c) preferentially adopt an intramolecularly hydro-
gen-bonded conformation. This conformation, shaped like
a half-open book (Fig. 2a), named ‘‘ridge-tile’’ [3], which
minimizes destabilizing non-bonded steric interactions, is
more stable than all others, and as such it plays a domi-
nating role in the pigment’s physico-chemical properties
and metabolism [1, 11–15, 29]. In our earlier studies with
123

1782
W. P. Pfeiffer, D. A. Lightner
Scheme 3
CO2R
CO2R
CO2CH3
CO2CH3
1) NaBH4
(CH2)3
CH CH CH
2) H2/PtO2
O
O
O
N
O
N
H
N
H
N
H
N
H
N
N
H
N
H
H
3-2e: R = CH3 (40%)
3-2 : R = H (74%)
13 (86%)
NaOH
HBr/HOAc
CO2CH3
CO2H
CO2CH3
H
CH3OH/H2SO4
+
OHC CH CH
Δ
O
O
CO2H
N
O
N
N
N
N
H
N
H
H
H
H
H
16
14 (59%)
15 (70%)
(CH3)2NCH CH CHO
POCl3
KOH/CH3OH
Δ
CO2Et
CO2Et
CO2Et
+
O
CO2Et
OHC
N
H
N
N
H
H
19
18
17
Scheme 4
CO2R
CH2
RO2C
RO2C
CO2R
CH2
12:
O
CH2
CH2 CH2
CH2
CHBr
N
H
CH2 CH2
CO2R
O
O
RO2C
N
H
N
H
CH3OH/HBr
N
H
N
H
N
H
N
H
2-1e (m = 2, n =1): R = CH3
2-1 (m = 2, n =1): R = H
20 : R = H
20e: R = Et
EtO2C
CO2Et
CO2Et
CAN
CO2Et
CH2
CH CH
CH2
Ti°
EtO2C
CH3
N
CHO
CO2Et
EtO2C
EtO2C
N
H
N
H
N
H
H
11-1
22
21
angles (h) in and about the dipyrrinone units (Fig. 2b),
indicates a quasi-ridge-tile shape for the (2.1)- and (4.1)-
homorubins (Fig. 2b) and a different geometry for the
(2.2)-, (2.3)-, (3.2)-, (4.2)-, and (4.3)-homorubins, where
the dipyrrinones tend to lie in parallel planes (Fig. 2c). The
shapes of the energy-minimized molecules are suggested
by their ball and stick representations in Fig. 3. It should be
noted that while all of the conformations are stabilized by
intramolecular hydrogen bonding, two do not have a full
complement of six hydrogen bonds. The carboxyl OH
123

(m.n)-Homorubins: syntheses and structures
1783
Fig. 2 a Intramolecularly hydrogen-bonded bilirubin in its energet-
ically favored ridge-tile shape, shown as interconverting enantiomeric
conformations. b, c The [m.n]-homorubins: bilirubin analogs centrally
homologated (m) at C(10) with carboxylic acid chains (n) lengthened
or shortened: The interplanar angle (h) of bilirubin is *100°; that of
the homorubins depends on torsion angles A–F and hence the value of
m and n. The (2.1)- and (4.1)-homorubins look more like (B). The
(2.2)-, (2.3)-, (3.2)-, (4.2)-, and (4.3)-homorubins look more like
c. Hydrogen bonds are represented by dotted lines
distance to the lactam C=O is predicted to be too long to
permit a hydrogen bond in the (4.2)-homorubin, as is the
pyrrole N–H to carboxy C=O hydrogen bond of the (4.1)-
homorubin (Table 2).
acid chains (2-1 and 4-1) to those of the previously studied
mesobilirubin-XIIIa with propionic acids replaced by
acetic acids [15]. In like manner, Table 4 compares 13C
NMR chemical shifts for the pigments with propionic acid
chains, (2.2)-, (3.2)-, and (4.2)-homorubins with those of
mesobilirubin-XIIIa [15], again finding good correlation.
Finally, Table 5 shows a good correlation for the 13C NMR
chemical shifts of the rubins with butyric acid chains,
n = 3 mesobilirubin-XIIIa, and the (2.3)- and (4.3)-ho-
morubins. The data clearly show strong similarities across
the range of homorubin structures possessing the same
alkanoic acid chain length.
Structural aspects and 13C NMR
The assigned constitutional structures of the homorubins
follow from the method of synthesis and are confirmed by
their 13C NMR spectral data, as obtained in (CD3)2SO and
shown in Tables 3, 4, 5. Table 3 shows good correlation
for the 13C NMR chemical shifts of homorubins with acetic
123

1784
W. P. Pfeiffer, D. A. Lightner
Fig. 3 Ball and stick representations of intramolecularly hydrogen-bonded minimum energy quasi-ridge-tile conformations (a, b) of (2.1)- and
(4.1)-homorubins, and conformations (c–g) of (2.2)-, (2.3)-, (3.2)-, (4.2)-, and (4.3)-homorubins, respectively
1
Hydrogen bonding aspects and H NMR
1
in the crystal [3–5, 10]. In CDCl3, the H NMR chemical
shifts of lactam and pyrrole NHs typically lie between 7
and 8 ppm in the absence of hydrogen bonding and
undergo considerable deshielding when hydrogen bonded
[35, 36]. Previous studies of bilirubinoid pigments showed
a deshielding to *10.5 ppm (lactam) and *9 ppm
Much concerning hydrogen bonding can be learned from
an examination of bilirubinoid pigment N–H chemical
shifts in CDCl3, a solvent in which bilirubin and meso-
bilirubin retain the intramolecular hydrogen bonding found
123

(m.n)-Homorubins: syntheses and structures
1785
˚
Table 2 Hydrogen bond distances/A of (m.n)-homorubins
(m.n)
OHꢀꢀꢀO=C
NHꢀꢀꢀO=C
NHꢀꢀꢀO=C
NHꢀꢀꢀO=C
NHꢀꢀꢀO=C
OHꢀꢀꢀO=C
Lactam
Lactam
Pyrrole
Pyrrole
Lactam
Lactam
(1.2)a
(2.1)
(2.2)
(2.3)
(3.2)
(4.1)
(4.2)
(4.3)
1.53
1.61
1.56
1.49
1.60
1.59
1.52
1.62
1.84
1.69
2.33
1.53
1.70
1.56
1.56
1.59
2.25
1.56
1.57
1.59
2.25
1.59
1.52
1.62
1.84
1.77
1.86
1.54
1.65
1.53
1.61
1.56
1.49
1.66
1.58
b
1.55
b
1.49
b
1.48
b
1.57
1.77
1.59
2.18
1.54
1.48
Computed by Syble vers.6.0 for the energy-minimized conformations
a
Mesobilirubin-XIIIa
b
Outside H-bond distance
(pyrrole) when the dipyrrinone and carboxylic acid groups
were intramolecularly hydrogen bonded in CDCl3 [37].
Whereas when intermolecular hydrogen bonding prevails
between dipyrrinones, the chemical shifts are found near
11 ppm (lactam) and 10 ppm (pyrrole) in CDCl3 [36, 37].
An unusual behavior was observed in the 1H NMR
spectrum of the (3.2)-homorubin (3-2) that had not been
observed previously for other bilirubin analogs. When the
1H NMR was determined in CDCl3 at 25 °C, two addi-
tional and unexpected resonances were seen in the N–H
chemical shift region of the spectrum (Fig. 4). Their
chemical shifts (10.92 and 10.17 ppm, Table 6) corre-
spond to lactam and pyrrole N–Hs, respectively, and are
characteristic of intermolecular hydrogen-bonded dip-
yrrinones [36, 39]. Upon heating, these signals disappear,
indicating that the secondary structure which gave rise to
them is not as dynamically stable as the structure that
provides the other resonances. Upon cooling, the reverse
occurred and the resonances’ ‘‘new resonances’’ reap-
peared. Therefore, one may venture to assume that some
sort of limited intermolecular interaction occurs in non-
polar solvents.
1
In contrast, in (CD3)2SO solvent, the H NMR NH chem-
ical shifts generally lie 9.5–9.9 ppm (lactam) and
10.1–10.3 ppm (pyrrole). Kaplan and Navon [38] showed
that in (CD3)2SO solvent molecules are bound up in the
intramolecular hydrogen bonding matrix. Thus, an exami-
1
nation of the NH H NMR chemical shifts in CDCl3 is an
excellent way of investigating intramolecular hydrogen
bonding (in a non-polar solvent) [6]. The important indi-
cator in CDCl3 is a pyrrole N–H chemical shift near 9 ppm,
more shielded than expected because the N–H lies over or
near the face of a second pyrrole ring and experiences
diamagnetic anisotropic shielding by the pyrrole’s p-sys-
tem. This implies a conformation for the pigment, and in
bilirubin and mesobilirubin-XIIIa, such a conformation is
the same as that seen in the crystal [3–5, 10] or that shown
in Fig. 1a—and well established in CDCl3 by sophisticated
NMR spectroscopic analysis [6–8].
Solubility and chromatographic considerations
Investigation of the different solubility, chromatographic,
and spectral properties gives insight into whether the ho-
morubin analogs display intramolecular hydrogen bonding,
as in mesobilirubin. The first two methods include solu-
bility behavior and thin layer chromatography.
Mesobilirubin is soluble in chloroform, indicating that it is
somewhat non-polar, consistent with intramolecular
hydrogen bonding. (2.2)-Homorubin (2-2) seems to have
solubility properties similar to those of mesobilirubin,
though slightly less soluble in non-polar solvents, consis-
tent with intramolecular hydrogen bonding. The (2.3)- and
(3.2)-homorubins (2-3 and 3-2) are also somewhat soluble
in CHCl3 to the extent that a reasonable 1H NMR spectrum
could then be obtained for each species. In contrast, the
(2.1)-, (4.1)-, (4.2)-, and (4.3)-homorubins are less soluble
in non-polar solvents, thus indicating increased polarity
1
The H NMR chemical shifts of the lactam and pyrrole
N–Hs are given in Table 6 along with data from mesobili-
rubin-XIIIa. In CDCl3, the deshielded lactam N–H chemical
shifts, ranging from 9.6 to 10.6 ppm, are characteristic of
hydrogen bonding and consistent with intramolecular
hydrogen bonding, as one might deduce similarly, from the
pyrrole N–H chemical shifts in CDCl3. A rather large range
of pyrrole N–H chemical shifts, from 8.01 (3-2) to 9.04 (2-3)
ppm, is seen. The more shielded values, e.g., those\9 ppm,
might be attributed to either more effective diamagnetic
shielding and/or less effective hydrogen bonding of the
pyrrole N–H in 2-2, 3-2, 1-2, and 1-3. The first rationale is
supported by the energy-minimized intramolecularly
hydrogen-bonded structures of Fig. 3.
123

1786
W. P. Pfeiffer, D. A. Lightner
1
Table 3 13C and H NMR chemical shifts and assignments of mesobilirubin-XIIIa and the (2.1)- and (4.1)-homorubins (2-1) and (4-1) with
acetic acid chains
Atom numbera
MBR-XIIIa
(2.1)-Homorubin (2-1)
(4.1)-Homorubin (4-1)
m = 1, n = 1b
Carbon
Proton
Carbon
Proton
Carbon
Proton
1, 19
172.6
131.2
8.4
–
171.8
133.7
7.8
–
171.94
135.0
8.1
–
2, 18
21, 181
–
–
–
1.75
–
1.78
–
1.97
–
3, 17
147.4
17.7
15.1
123.4
97.9
122.7
128.6
9.7
146.7
17.0
14.5
122.5
97.5
122.1
128.0
9.1
147.3
17.2
14.8
122.8
97.7
121.9
127.5
9.4
31, 171
32, 172
4, 16
2.44
1.06
–
2.51
1.08
–
2.46
1.05
–
5, 15
5.88
–
5.96
–
5.91
–
6, 14
7, 13
71, 131
–
–
–
1.95
–
1.99
–
1.97
–
8, 12
81, 121
9, 11
114.7
29.8
123.6
23.8
–
113.8
29.5
122.9
26.1
–
113.6
29.9
122.9
25.4
29.8
173.1
–
3.11
–
3.21
–
3.22
–
10(10a)
10b
3.95
–
2.79
–
2.5
1.53
11.98
9.81
10.24
COOH
21, 24
22, 23
174.5
–
11.87
9.74
10.28
172.8
–
12.06
9.80
10.24
–
–
–
Chemical shifts recorded in ppm downfield from (CH3)4Si. Spectra from (CD3)2SO at 23 °C
a
See Fig. 2 for atom numbering. Superscripts refer to the carbons in the b-substituent chains, e.g., 21 is the first carbon attached to the ring on
carbon C(2)
b
Data from Ref. [15]
with the possibility of less effective intramolecular
hydrogen bonding.
homorubin dimethyl esters are similar, they display sig-
nificant differences in their solubility properties. It might
be expected that, with their longer saturated hydrocarbon
chain between the dipyrrinone units, the (4.n)-tris-ho-
morubins should be more soluble in organic solvents than
the corresponding (2.n)- and (3.n)-homorubins. However, it
turns out that the tris-homologs are not very soluble in
many organic solvents, which raised difficulties in their
purification. The only purification method found was to
dissolve them in hot dimethyl formamide, cool to room
temperature, collect the precipitate, and wash it with 95 %
ethanol. Unfortunately, due to their poor solubility prop-
erties in CDCl3, NMR data could not be obtained in this
solvent. To obtain NMR data, the tris-homorubin esters (4-
1, 4-2, and 4-3) were dissolved by sonication in warm
(CD3)2SO. Yet after standing in the NMR tube for more
than a day, they would precipitate.
Thin-layer chromatography (TLC) and high-perfor-
mance liquid chromatographic (HPLC) behavior provided
further insight into pigment polarity and thus intramole-
cular hydrogen bonding. As intramolecular hydrogen
bonding weakens or is absent, the pigment’s polarity
increases. In TLC, increased polarity is seen by slower
movement (smaller Rf values) on silica gel; in reversed
phase HPLC, it is seen in shorter retention times. The
chromatographic behavior of the homorubins of this work
relative to that of mesobilirubin-XIIIa is displayed in
Table 7. From the HPLC retention times, it would appear
that the (3.2)-, (4.1)-, and (4.2)-homorubins, especially the
last, would appear to be more polar than the others, which
are more like mesobilirubin in chromatographic behav-
ior—and probably engaged in more effective
intramolecular hydrogen bonding. In regard to polarity
(and the effectiveness of intramolecular hydrogen bond-
ing), the TLC behavior is in qualitative agreement.
The dimethyl esters present a somewhat different solu-
bility picture. Although the structures of the various
The solubility properties of the (2.1)-homorubin dime-
thyl ester (2–1e) permitted purification by washing with
ethyl acetate or acetone. Its poor solubility properties
mirror those of the mesobilirubin analog with acetic acid
chains replacing propionic [15]. Apparently, poor solubility
123

(m.n)-Homorubins: syntheses and structures
1787
Table 4 13C and 1H NMR chemical shifts and assignments of mesobilirubin-XIIIa (n = 2) and homorubins (2-2), (3-2), and (4-2) with
propionic acid chains
Atom numbera
MBR-XIIIa
(2.2)-Homorubin
(2-2)
(3.2)-Homorubin
(3-2)
(4.2)-Homorubin
(4-2)
m = 1, n = 2b
Carbon
Proton
Carbon
Proton
Carbon
Proton
Carbon
Proton
1, 19
172.4
130.8
8.5
–
172.0
133.0
8.1
–
171.9
133.8
8.1
–
171.7
133.8
7.8
–
2, 18
21, 181
–
–
–
–
1.75
–
1.78
–
2.22
–
1.98
–
3, 17
147.7
17.6
15.3
123.0
98.2
122.4
128.3
9.6
147.2
17.2
14.8
122.3
97.8
122.1
127.6
9.3
147.2
17.2
14.9
122.3
97.7
122.0
127.4
9.3
147.0
17.0
14.5
121.9
97.4
121.8
127.3
9.0
31, 171
32, 172
4, 16
2.40
1.06
–
2.43
1.08
–
2.54
1.04
–
2.50
1.02
–
5, 15
5.92
–
5.94
–
5.89
–
5.89
–
6, 14
7, 13
71, 131
–
–
–
–
1.97
–
2.00
–
1.74
–
1.74
–
8, 12
119.7
19.7
34.8
123.40
24.0
–
119.0
19.3
34.9
122.8
26.4
–
118.7
19.5
35.3
122.7
25.6
31.0
174.2
–
118.5
19.4
35.4
122.5
25.3
29.9
173.7
–
81, 121
82, 122
9, 11
2.49
2.40
–
2.53
2.45
–
2.50
2.25
–
2.53
2.45
–
10(10a)
10b
3.93
–
2.82
–
2.47
1.75
12.06
9.80
10.17
2.22
1.53
12.02
9.75
10.15
COOH
21, 24
22, 23
174.5
–
11.87
9.74
10.28
174.2
–
11.95
9.79
10.17
–
–
–
–
Chemical shifts recorded in ppm downfield from (CH3)4Si. Spectra run in (CD3)2SO
a
Superscripts refer to the carbons in the b-substituent chains, e.g., 21 is the first carbon attached to the ring on carbon C(2)
b
Data from Ref. [15]
properties are associated with the short acetic acid ester
the molecule [1, 9]. Although this may sound confusing,
mesobilirubin does not show any significant shift in
absorbance when changing from a non-polar to a polar
solvent. Since mesobilirubin is known from NMR studies
to adopt the intramolecularly hydrogen-bonded conforma-
tion in CDCl3 solvent, one might expect to see a significant
change in the UV–Vis absorbance when changed from a
non-polar to a polar solvent. However, the absorbance does
not vary significantly when changing solvents; thus, it may
be argued that mesobilirubin prefers the folded conforma-
tion (but not necessarily hydrogen bonded) regardless of
the solvent used. Therefore, the UV–Vis absorption prop-
erties of the homorubin analogs were measured in a variety
of solvents (Tables 8, 9, 10) to observe any solvent
dependence of the conformation.
side chain. Similar properties were observed in the acetic
ester ethene (21) and butadiyne (5-1e) diester precursors. In
contrast, (2.2)-, (2.3)-, and (3.2)-homorubin dimethyl esters
(2-2e, 2-3e, and 3-2e) were quite soluble in many organic
solvents such as ethyl acetate, CH2Cl2, and (CH3)2SO.
Diester 3-2e seemed to be just as soluble in these organic
solvents as the (2.2)- and (2.3)-homorubin dimethyl esters.
Because the solubility properties are so much worse for the
(4.1) analogs than the (2.n) analogs, one might expect that
the (3.2)-homorubin 3-2e would exhibit solubility proper-
ties between those of the (2.n) and (4.n) homologs;
however that was not observed.
UV–Vis spectral aspects
Table 8 includes data for homorubins with acetic acid
chains (n = 1) and dipyrrinones linked by CH2 units
varying from one to four (m = 1, 2, 4). All examples
exhibit very similar emax values, but kmax remains fairly
constant for the m = 1, n = 1 rubin suggesting little sol-
vent influence on its conformation. Unlike that of 2-1,
Further evidence of intramolecular hydrogen bonding in
the alkanoic acid homologs comes from solvent-dependent
UV–Vis spectra. A noticeable change in the UV–Vis
spectrum when switching from a non-polar to a polar sol-
vent unusually indicates a change in the conformation of
123

1788
W. P. Pfeiffer, D. A. Lightner
Table 5 13C and 1H NMR chemical shifts and assignments of mesobilirubin-XIIIa (n = 3) and (2.3)- and (4.3)-homorubins (2-3) and (4-3) with
butyric acid side chains
Atom numbera
MBR-XIIIa
(2.3)-Homorubin
(2-3)
(4.3)-Homorubin
(4-3)
m = 1, n = 3b
Carbon
Proton
Carbon
Proton
Carbon
Proton
1, 19
172.5
130.7
8.5
–
171.9
132.9
9.0
–
171.9
134.0
8.1
–
2, 18
21, 181
–
–
–
1.75
–
1.78
–
1.74
–
3, 17
147.8
17.6
15.3
123.0
98.2
122.4
128.3
9.6
147.0
17.0
14.5
122.1
97.9
122.2
127.5
7.8
147.2
17.2
14.8
122.3
97.7
121.9
127.2
9.3
31, 171
32, 172
4, 16
2.12
1.06
–
2.44
1.08
–
2.46
1.04
–
5, 15
5.93
–
5.96
–
5.89
–
6, 14
7, 13
71, 131
–
–
–
1.98
–
1.97
–
1.98
–
8, 12
120.7
24.1
23.4
33.8
123.3
26.1
–
119.8
25.7
22.9
33.1
122.5
26.3
–
119.2
25.4
23.0
33.2
122.6
26.1
30.1
174.4
–
81, 121
82, 122
83, 123
9, 11
2.46
1.30
1.99
–
2.21
1.4
2.08
–
2.14
1.53
2.47
–
10(10a)
10b
3.89
–
2.79
–
2.26
1.53
11.96
9.81
10.14
COOH
21, 24
22, 23
174.9
–
11.50
9.79
10.28
174.2
–
12.03
9.87
10.15
–
–
–
Chemical shifts recorded in ppm downfield from (CH3)4Si. Spectra run in (CD3)2SO
a
See Fig. 2 for atom numbering. Superscripts refer to the carbons in the b-substituent chains, e.g., 21 is the first carbon attached to the ring on
carbon C(2)
b
Data from Ref. [15]
Circular dichroism aspects
where kmax varies more with solvent polarity, the data for
4-1 suggest that the pigments adopt conformations where
the dipyrrinones act independently, viz., not part of or only
in a weak exciton system. In Table 9, comparison is made
for homorubins with propionic acid chains (n = 2) and
CH2 linkers varying between m = 1 to m = 4. As the well-
studied exciton of mesobilirubin-XIIIa (MBR-XIIIa,
n = 1, n = 2) [1, 41], there is little variation in kmax for
2-2 and 3-2 with these three apparently adopting confor-
mations where the chromophores may interact. In contrast,
4-2 behaves like 4-1 with two non-interacting or only
weakly interactive dipyrrinone chromophores. In Table 10,
where the pigments have butyric acid chains (n = 3) and
vary by the number of CH2 linkers (m = 1, 2, 4), again
where m = 1, the pigment MBR-XIIIa behaves like a
molecular exciton (as in its analogs of Tables 8, 9),
whereas, 2-3 and 4-3 seem more likely to adopt confor-
mations where the dipyrrinone chromophores might
interact only in non-polar chloroform and benzene.
When a molecule absorbs UV–Visible left and right cir-
cularly polarized light unequally, it exhibits circular
dichroism (CD). From CD measurements one can often
deduce the handedness or stereochemistry of the molecule.
In certain cases, achiral molecules or even racemic mix-
tures can be complexed with chiral molecules, and an
induced circular dichroism results [1, 40]. An example of
the latter can be seen with bilirubin. Since bilirubin exists
as two interconverting conformation enantiomers (Fig. 2a),
it is a racemic mixture in isotropic media and does not
show CD. However, chirality can be induced when it binds
to a chiral molecule such as quinine [41] or the protein
human serum albumin (HSA) [42, 43], giving rise to an
induced CD (ICD) and a bisignate curve. The observed
bisignate CD comes from exciton coupling of two electric
dipole transitions [1, 41]: one from each of the pigment’s
twin dipyrrinone chromophores. In the case of
123

(m.n)-Homorubins: syntheses and structures
1789
selection or to a disposition of the dipyrrinone electric
transition dipole moments for the (2.2) and (2.3)—as was
seen in earlier studies of the influence of CHCl3 and vol-
atile anesthetics [42, 43].
Table 6 Comparison of mesobilirubin-XIIIa and the homorubins’
lactam and pyrrole N–H chemical shifts in CDCl3 and (CD3)2SO
Analog
CDCl3
(CD3)2SO
Lactam
Pyrrole
Lactam Pyrrole
Mesobilirubin-XIIIaa 10.57
9.15
9.02
9.72
9.78
10.27
10.21
Concluding comments
(2.1)-Homorubin
(2-1)
9.61
(2.2)-Homorubin
(2-2)a
10.18
10.36
8.59
9.04
9.79
9.87
10.16
10.15
10.17
10.24
10.15
10.14
Like the previously reported (2.2)- and (2.3)-homorubins
[13, 14], the homorubins of the current work were all
predicted to be able to engage in conformation-determining
intramolecular hydrogen bonding by molecular mechanics
computations. These predictions appear to be borne out
experimentally from solubility and chromatographic data
along with 1H NMR N–H chemical shifts for the ho-
morubins of this work, although the higher homologs (4.1),
(4.2), and (4.3) and the lower homolog (2.1) are less sol-
uble in CHCl3 than the (2.2), (2.3), and (3.2). The
decreased solubility of the lower and higher homologs
suggests an increased polarity that would correlate with
less effective intramolecular hydrogen bonding.
(2.3)-Homorubin
(2-3)b
(3.2)-Homorubin
(3-2)
9.67 (10.92)c 8.01 (10.17)c 9.80
(4.1)-Homorubin
(4-1)
Insoluble
9.79
Insoluble
8.22
9.81
9.79
9.81
(4.2)-Homorubin
(4-2)
(4.3)-Homorubin
(4-3)c,d
10.36
8.54
Run at 2 9 10-4 M concentration of pigment at 25 °C
a
Data from Ref. [14]
b
Data from Refs. [13] and [14]
c
These resonances disappear at 45 °C
Due to poor solubility, the concentration was not determined
d
Experimental
All nuclear magnetic (NMR) spectra were obtained on a
Varian unity plus at 11.75 T magnetic strength operating at
500 MHz (1H), 125 MHz (13C) and a Varian GE at 7.06 T
magnetic strength operating at 300 MHz (1H) and 75 MHz
(13C), respectively, in CDCl3 unless otherwise indicated.
Chemical shifts were reported in ppm referenced to the
residual CHCl3 proton signal at 7.26 ppm and 13C at
77.23 ppm unless otherwise noted. A combination of het-
eronuclear multiple bond correlation (HMBC) spectra,
heteronuclear single bond correlation (HSQC) spectra,
two-dimensional correlation spectroscopy (COSY), and
1H{1H} nuclear Overhauser effect (NOE) data were used to
assign 1H and 13C NMR spectra. Melting points were taken
on a Thomas–Hoover capillary apparatus. Analytical
samples were dried under vacuum in a drying pistol (Ab-
derhalden) at refluxing ethanol or toluene temperature
using P2O5 as desiccant. Combustion analyses were per-
formed by Desert Analytics, Tucson, AZ and gave results
within 0.4 % of theoretical values. For a few compounds,
FAB-HRMS mass determinations of the molecular ion
were obtained from the Nebraska Center for Mass Spec-
trometry, Lincoln, Nebraska. UV–Vis spectra were
recorded on a Perkin-Elmer Lambda-12 spectrometer.
Circular dichroism spectra were measured on a Jasco J-600
spectrometer. For final purification, radial chromatography
was carried out on Merck silica gel PF254 with gypsum
binder, preparative layer grade, using a Chromatotron
(Harrison Research, Palo Alto, CA, USA). Analytical thin-
mesobilirubin, from the long wavelength UV–Vis excita-
tion near 420 nm [37, 40]. For the homorubins, ICD
experiments were conducted using HSA and quinine as the
chiral complexing agents, with the data shown in Tables 11
and 12.
The presence of bisignate curves indicates that the ho-
morubins have fixed conformations when bound to HSA.
The results show that the m = 2 homorubins have similar
binding characteristics to HSA as mesobilirubin, as all
exhibit positive exciton chirality. The remaining analogs
also have the ability to bind HSA, but the manner in which
they bind cannot be determined. The ICD data for all of the
homorubins of this work are far weaker in intensity than
that observed for mesobilirubin-XIIIa [37] or for bilirubin
itself [41]—the unique exception being that of (2.2)-ho-
morubin (2-2). Whether the weak ICDs reflect poor
enantioselection or poor binding (unlikely in the case of
HSA) cannot be determined at present.
With quinine as a chiral complexing agent, the data of
Table 12 indicate the previously noted Cotton effect sign
reversals for mesobilirubin-XIIIa relative to those seen
(Table 11) in aqueous buffered albumin, the same (albeit
weak) exciton chirality for the (4.n)-homorubins. Curi-
ously, a reversed and strong exciton chirality is seen for the
(2.1)-homorubin—whereas the (2.2)- and (2.3)-homorubins
exhibit barely detectable CDs. It is unclear whether the
weak CDs seen are due to a lack of pigment enantiomer
123

1790
W. P. Pfeiffer, D. A. Lightner
Table 8 Comparison of UV–Vis long wavelength absorption spectral
data of mesobilirubin and homorubins with acetic acid chains
Solvent
MBR-XIIIa
m = 1,
n = 1a
(2.1)-Homorubin
2-1
(4.1)-Homorubin
4-1
kmax emax
kmax
emax
kmax
emax
Chloroform 423 57,100 416
44,300
49,800
48,800
–
410
413
409
–
47,500
51,300
49,200
–
DMSO
424 49,400 421
Methanol
422 50,200 415
396b 45,600
–
Acetonitrile –
–
–
–
422
411
401
44,600
48,300
46,500
405
408
413
45,400
45,100
45,300
Acetone
Benzene
–
–
emax in dm3 mol-1 cm-1 and kmax in nm
At room temperature for 1 9 10-5 M solutions
a
Data from Ref. [15]
b
Shoulder or inflection determined by first and second derivatives
Perkin-Elmer Series four high-performance liquid chro-
matograph with an LC-95 UV–Vis spectrophotometric
detector (set at 420 or 640 nm) equipped with a Beckman-
Altex ultrasphere-IP 5 lm C-18 ODS column
(25 9 0.46 cm). The flow rate was 1.0 cm3/min, and the
elution solvent was 0.1 M di-n-octyl amine acetate in 5 %
aq. methanol (pH 7.7, 37 °C). All reagents and solvents
used in the syntheses were obtained from Fisher-Acros,
Aldrich, and Alfa Aesar. Deuterated chloroform, dichlo-
romethane, dimethylsulfoxide, and methanol were from
Cambridge Isotope Laboratories. Molecular dynamics
computations were performed on an SGI Octane worksta-
tion using versions 6.0 and 7.1 of SYBYL (Tripos Assoc.,
St. Louis, MO, USA) force field with Gasteiger–Hu¨ckel
charges. Ball and stick drawings were created from atomic
coordinates of the molecular dynamics structure using
Mu¨ller and Falk’s ‘‘Ball and Stick’’ program (Cherwell
Scientific, Oxford, UK) for the Macintosh (http://www.orc.
uni-linz.ac.at/mueller/ball_and_stick.shtml). All solvents
were reagent grade, from Fisher-Acros.
Fig. 4 Variable temperature 1H NMR of the N–H region
(11.5–7.5 ppm) of homorubin 3-2 in CDCl3. Temperatures (°C) are
noted above the scans
Table 7 HPLC retention times and TLC Rf values of homorubins
compared to mesobilirubin-XIIIa
Pigment
Retention time/min
Rf
Mesobilirubin
14.85
19.54
12.83
15.37
9.67
0.89
0.58
0.63
0.92
0.36
0.32
0.39
0.45
2-1
2-2
2-3
3-2
4-1
4-2
4-3
Some synthetic precursors were prepared as in previous
work: 3-pyrrolin-2-one (17) [29], ethyl 3-[2-(ethoxycar-
bonyl)ethyl]-4,5-dimethyl-1H-pyrrole-2-carboxylate (19)
[18], ethyl 5-carboxyethyl-2,4-dimethyl-1H-pyrrole-3-ace-
tate (11-1) [23], and the corresponding 3-propanoate (11-2)
and 3-butanoate (11-3) [23].
9.96
8.47
10.81
Flow rate 1 cm3/min on an ion pairing ODS C-18 reversed-phase
analytical column (25 cm 9 4.6 mm) eluting with 0.1 M di-n-octyl-
amine acetate in CH3OH, 5 % by vol. H2O, pH 7.7
2,20-(1,2-Ethanediyl)bis[5-[(3-ethyl-1,5-dihydro-4-
methyl-5-oxo-2H-pyrrol-2-ylidene)methyl]-4-methyl-(Z)-
1H-pyrrole-3-acetic acid] [(2.1)-homorubin]
(2-1, C32H38N4O6)
CH2Cl2–CH3OH (95:5 by vol.) eluent on silica gel
layer chromatography was carried out on J.T. Baker silica
gel IB-F plates (125 l layers). Flash chromatography was
carried out using Woelm silica gel F, thin-layer chroma-
tography grade. HPLC analyses were carried out on a
Homorubin dimethyl ester 2-1e (100 mg, 0.166 mmol) was
suspended in 5 cm3 N2-purged CH3OH and 20 cm3 THF
123

(m.n)-Homorubins: syntheses and structures
1791
Table 9 Comparison of UV–Vis long wavelength absorption spectral data of mesobilirubin and homorubins with propionic acid chains
Solvent
MBR-XIIIa
(2.2)-Homorubin
2-2
(3.2)-Homorubin
3-2
(4.2)-Homorubin
4-2
m = 1, n = 2a
kmax
emax
kmax
emax
kmax
emax
kmax
emax
49,900
Chloroform
DMSO
431
418b
55,400
53,400
59,100
50,000
52,100
45,600
56,400
48,900
55,000
51,500
55,900
51,600
424
57,000
65,700
65,100
57,100
50,100
61,200
437
69,100
61,700
67,700
41,400
72,700
65,200
420
428
425
428
418
407
423
424
429
430
430
440
412
414
410
410
418
55,000
55,700
47,800
52,400
52,600
401b
427
403b
Methanol
Acetonitrile
Acetone
420
396b
424
422b
Benzene
453
418b
emax in dm3 mol-1 cm-1 and kmax in nm
At room temperature for 1 9 10-5 M solutions
a
Data from Ref. [37]
b
Shoulder or inflection determined by first and second derivatives
and heated to reflux. Nitrogen-purged NaOH (2 cm3, 1 M)
was then added and reflux was continued for 4 h. The
solution was cooled to 5 °C and acidified with 10 % nitric
acid until a precipitate formed. The precipitate was filtered
and dried (in vacuo) then redissolved in a minimal amount
of CH2Cl2:CH3OH (75:25 v/v) and passed through a short
column of silica gel using CH2Cl2–CH3OH (95:5 v/v) as
eluent. All of the first yellow band was collected giving 2-
1. Yield 50 mg (0.087 mmol, 52 %); m.p. 270 °C (dec); IR
(1H, s), 10.36 (1H, bs), 10.95 (1H, bs) ppm; 13C NMR:
d = 7.1, 9.5, 14.7, 17.7, 26.4, 30.5, 51.8, 99.5, 112.4,
122.9, 123.5, 123.6, 128.1, 136.5, 146.7, 172.7, 173.6 ppm;
UV–Vis (acetonitrile): kmax (e) = 382 (53,200), 403 (sh,
44,300) nm (mol-1 dm3 cm-1); UV–Vis (acetone): kmax
(e) = 387
(54,800),
413
(sh,
44,300)
nm
(mol-1 dm3 cm-1); UV–Vis (benzene): kmax (e) = 392
(55,700), 413 (sh, 49,400) nm (mol-1 dm3 cm-1); UV–Vis
(CH2Cl2): kmax (e) = 383 (50,000), 407 (sh, 42,100) nm
(mol-1 dm3 cm-1); UV–Vis (CHCl3): kmax (e) = 395
(51,200), 408 (sh, 48,900) nm (mol-1 dm3 cm-1); UV–
Vis ((CH3)2SO): kmax (e) = 415 (57,100), 400 (sh, 56,700)
nm (mol-1 dm3 cm-1); UV–Vis (CH3OH): kmax (e) = 402
(55,000) nm (mol-1 dm3 cm-1).
-1
;
ꢀ
(KBr): m = 3,369, 2,970, 1,664, 1,637, 1,274, 1,171 cm
NMR data in Table 3; UV–Vis data in Table 8.
2,20-(1,4-Ethanediyl)bis[5-[(3-ethyl-1,5-dihydro-4-methyl-
5-oxo-2H-pyrrol-2-ylidene)methyl]-4-methyl-(Z)-1H-pyr-
role-3-acetic acid] dimethyl ester [(2.1)-homorubin
dimethyl ester] (2-1e, C34H42N4O6)
2,20-(1,2-Ethanediyl)bis(5-carboxy-4-methyl-1H-pyrrole-3-
acetic acid) (20, C18H20N2O8)
To a 25 cm3 round-bottom flask equipped with a magnetic
stirrer and a heating mantle was added 160 mg bis-
pyrrylethane tetra-acid 20 (0.408 mmol), 200 mg 5-bro-
momethylene-3-pyrrolin-2-one (12, 0.98 mmol), 15 cm3
CH3OH, and 8 drops of H2O. The flask was equipped with
a condenser and heated under reflux for 8 h. The flask was
then cooled and placed in the freezer overnight. The
resulting precipitate was collected (vacuum filtration) and
washed with cold ethyl acetate to give dimethyl ester 2-1e
as a yellow-orange solid. Yield 126 mg (0.21 mmol,
To a 100 cm3 round-bottom flask equipped with a magnetic
stirrer, heating mantle, and a condenser was added 495 mg
bis-pyrrylethane 20e (0.82 mmol), 20 cm3 ethanol (95 %),
and 20 cm3 NaOH (2 M). The mixture was heated at reflux
for 3 h before it was cooled to room temperature and the
ethanol was removed (rotovap). Water (10 cm3) was
added, and the solution was cooled to 0 °C using an ice
bath. Nitric acid (20 %) was added until a white precipitate
formed; then a few more drops were added, and the
mixture was stirred at 0 °C for 15 min more. The
precipitate was collected by filtration, washed with cold
H2O, and dried (vacuum desicator) overnight to afford the
desired 20 as a white powder. Yield: 380 mg (0.82 mmol,
ꢀ
51 %); m.p. 245 °C (dec); IR (KBr): m = 3,365, 1,736,
1
1,655, 1,637, 1,234, 1,163 cm-1; H NMR: d = 1.04 (3H,
t, J = 7.2 Hz), 1.22 (3H, s), 2.13 (3H, s), 2.33 (2H, q,
J = 7.2 Hz), 3.14 (2H, s), 3.50 (2H, s), 3.70 (3H, s), 5.85
123

1792
W. P. Pfeiffer, D. A. Lightner
99 %); m.p.: 137–138 °C; IR (KBr): mꢀ = 3,321, 2,985,
Table 10 Comparison of UV–Vis long wavelength absorption
spectral data of mesobilirubin and homorubins with butyric acid
chains
1
1,735, 1,672, 1,437, 1,301, 1,251, 1,020 cm-1; H NMR:
d = 1.26 (6H, t, J = 7.2 Hz), 1.28 (6H, t, J = 7.2 Hz),
2.23 (6H, s), 2.78 (4H, s), 3.40 (4H, s), 4.17 (4H, q,
J = 7.2 Hz), 4.20 (4H, q, J = 7.2 Hz), 9.09 (2N–H, bs)
ppm; 13C NMR: d = 10.1, 14.1, 14.3, 25.8, 29.9, 59.3,
60.9, 113.8, 117.7, 127.0, 132.9, 161.0, 172.7 ppm.
Solvent
MBR-XIIIa
(2.3)-Homorubin (4.3)-Homorubin
4-3
m = 1, n = 3a 2-3
kmax emax
kmax
emax
kmax
emax
Acetonitrile
Acetone
407
411
50,300
49,500
51,800
51,700
56,500
412
412
420
422
413
46,800
53,700
52,900
49,400
53,400
2,20-(1,2-Ethenediyl)-bis[5-(ethoxycarbonyl)-4-methyl-1H-
pyrrole-3-acetic acid] diethyl ester (21, C26H34N2O8)
To a 500 cm3 round-bottom flask equipped with a magnetic
stirrer containing 100 cm3 dry THF under N2 was added
8.96 g TiCl4 (5.3 cm3, 47.9 mmol). Upon addition of the
TiCl4 a bright yellow precipitate formed. The mixture was
cooled in an ice/salt bath to -5 °C. Zinc dust (6.2 g) was
added to the mixture over 5 min. The mixture was allowed
to warm to room temperature and then heated under reflux
for 1 h. The mixture went from yellow to green to dark
violet to black. A solution of 2.51 g formylpyrrole 22
(9.6 mmol) in 50 cm3 dry THF was added to the black
mixture. The flask was equipped with a condenser and a
heating mantle and heated under reflux for 2 h. The mixture
was then cooled to 5 °C and 200 cm3 NH4OH (50 %) was
slowly added to the mixture. The mixture was poured into a
separatory funnel containing 200 cm3 CH2Cl2 and shaken.
The organic layer was filtered using aspirator vacuum, and
more CH2Cl2 was added to the separatory funnel, shaken,
and again filtered. This process was repeated until no more
yellow solution was extracted. The organic layer was
washed with H2O (2 9 100 cm3) and brine
(1 9 100 cm3), dried over anhyd. Na2SO4, and the CH2Cl2
was removed (rotovap) to afford a thick yellow paste.
Ethanol (95 %) was added to thin the paste, and the mixture
was filtered and dried (vacuum dessicator) resulting in the
yellow 21. Yield: 1.10 g (2.19 mmol, 46 %); m.p.: 206–
Benzene
420
Chloroform 438
59,700 421
DMSO
427
398b 53,000
58,200 405
Methanol
428
398b 54,500
52,000 417
55,300
416
55,500
emax in dm3 mol-1 cm-1 and kmax in nm
At room temperature for 1 9 10-5 M solutions
a
Data from Ref. [15]
b
Shoulder or inflection determined by first and second derivatives
ꢀ
99 %); m.p.: 150–152 °C (dec); IR (KBr): m = 3,450,
1,712, 1,635, 1,460, 1,355, 1,274 cm-1 1H NMR
;
((CD3)2SO): d = 2.12 (6H, s), 2.65 (4H, s), 11.05 (2N–
H, bs), 12.01 (4COOH, bs) ppm; 13C NMR ((CD3)2SO):
d = 10.2, 25.9, 29.3, 114.3, 117.0, 125.7, 134.2, 162.2,
172.9 ppm.
2,20-(1,2-Ethanediyl)bis[5-(ethoxycarbonyl)-4-methyl-1H-
pyrrole-3-acetic acid] diethyl ester (20e, C26H36N2O8)
To a 100 cm3 round-bottom flask equipped with a magnetic
stirrer was added 100 mg bis-pyrrylethene 21 (0.20 mmol),
100 cm3 ethyl acetate, and 50 mg 5 % Pd(C). The flask
was equipped with a hydrogen balloon and stirred until the
blue fluorescence was no longer visible. More of 21 was
added, and the balloon was refilled. This process was
repeated as many times as needed (5 times for this
procedure). The mixture was then filtered through Celite,
and the ethyl acetate was removed (rotovap). The white
solid was recrystallized from ethyl acetate-hexane to give
the desired diethyl ester 20e. Yield: 495 mg (0.99 mmol,
ꢀ
207 °C; IR (KBr): m = 3,324, 2,984, 1,694, 1,448, 1,262,
1,161, 958, 772, 665, 631 cm-1; 1H NMR: d = 1.27 (6H, t,
J = 7.2 Hz), 1.36 (6H, t, J = 7.2 Hz), 2.25 (6H, s), 3.52
(4H, s), 4.11 (4H, q, J = 7.2 Hz), 4.30 (4H, q, J = 7.2 Hz),
6.74 (2H, s), 9.17 (2N-H, bs) ppm; 13C NMR ((CD3)2SO):
d = 10.2, 13.9, 14.3, 29.1, 59.2, 59.9, 115.0, 117.5, 119.2,
Table 11 Induced circular
Analog
Circular dichroismb
UV–Visb
dichroism and UV–Vis spectra
for mesobilirubin-XIIIa and
homorubin analogs in phosphate
Demax (k1)
k at De = 0
Demax (k2)
emax (k1)
Mesobilirubin-XIIIaa
?50 (440)
?9.5 (443)
?94 (433)
?5.5 (405)
-19 (437)
-6 (432)
407
417
414
405
417
415
417
418
-44 (390)
-11.8 (399)
-80 (395)
-5.7 (392)
?18 (397)
?8.4 (394)
?8.7 (394)
?8.6 (395)
44,800 (434)
47,700 (420)
52,000 (411)
47,400 (434)
57,100 (428)
44,900 (420)
53,950 (421)
52,000 (420)
buffer at pH 7.4 with
pigment:HSA ratio 2:1
(2.1)-Homorubin m = 2, n = 1 (2-1)
(2.2)-Homorubin m = 2, n = 2 (2-2)
(2.3)-Homorubin m = 2, n = 3 (2-3)
For 1 9 10-5 M pigment
(3.2)-Homorubin m = 3, n = 2 (3-2)
solutions in a 1 cm cell at 25 °C
Data from Ref. [37]
(4.1)-Homorubin m = 4, n = 1 (4-1)
a
(4.2)-Homorubin m = 4, n = 2 (4-2)
-6 (436)
b
De and emax in
(4.3)-Homorubin m = 4, n = 3 (4-3)
-6.2 (435)
dm mol-1 cm-1 and k in nm
123

(m.n)-Homorubins: syntheses and structures
1793
Table 12 Induced circular
Analog
Circular dichroismb
UV–Visb
dichroism and UV–Vis spectra
for mesobilirubin-XIIIa and
homorubins in CHCl3 with a
1:300 pigment:quinine molar
Demax (k1)
k at De = 0
Demax (k2)
emax (k1)
Mesobilirubin-XIIIaa
-49 (428)
?121 (434)
*0
402
409
–
?27 (383)
-80 (395)
*0
54,100 (429)
47,700 (421)
44,700 (420)
63,300 (426)
49,300 (426)
43,900 (417)
52,900 (428)
52,100 (423)
ratio
(2.1)-Homorubin (2-1)
(2.2)-Homorubin (2-2)
(2.3)-Homorubin (2-3)
(3.2)-Homorubin (3-2)
(4.1)-Homorubin (4-1)
(4.2)-Homorubin (4-2)
(4.3)-Homorubin (4-3)
*0
–
*0
For 1 9 10-5 M pigment
?20 (443)
?6.9 (443)
?4 (429)
?6.4 (430)
419
415
410
407
-13 (400)
?8.4 (394)
3.6 (389)
-3 (390)
solutions in a 1 cm cell at 25 °C
a
Data from Ref. [37]
De and emax in dm mol-1
b
cm-1 and k in nm
126.7, 132.6, 160.8, 170.8 ppm; UV–Vis (CH2Cl2): kmax
(e) = 380 (39,800) nm (mol-1 dm3 cm-1); UV–Vis
(CH3OH): kmax (e) = 378 (47,400), 399 (35,600), 361 (sh,
36,600), 399 (36,000) nm (mol-1 dm3 cm-1); UV–Vis
((CH3)2SO): kmax (e) = 382 (48,900), 405 (43,200), 367
(sh, 32,700) nm (mol-1 dm3 cm-1).
(0.037 mmol), and the mixture was brought to reflux under
N2. A solution of 3 cm3 CH3OH and 1 cm3 NaOH (2 M)
was added and the mixture was heated at reflux for 4 h
under an N2 atmosphere. After the solution was cooled to
room temperature, 5 cm3 H2O was added and the organic
solvents were removed (rotovap). The solution was then
cooled to 5 °C, acidified with nitric acid (2 M), and the
solid was separated by centrifugation and washed with cold
water. The yellow solid was dried under high vacuum to
give the desired 3-2. Yield: 18 mg (0.027 mmol, 74 %); IR
5-(Ethoxycarbonyl)-2-formyl-3-methyl-1H-pyrrole-4-
acetic acid ethyl ester (22, C13H17NO5)
To a 1 dm3 round-bottom flask equipped with a magnetic
stirrer was added pyrrole 5.0 g 11-1 (20 mmol) and
300 cm3 acetic acid. After the pyrrole had fully dissolved,
250 cm3 H2O and 100 cm3 THF were added and the
solution was cooled to 0 °C. Ceric ammonium nitrate
(CAN, 43.5 g, 80 mmol) was added in portions to the
solution and stirred for 1.5 h. The solution was poured into
a 2 dm3 separatory funnel containing 300 cm3 CH2Cl2
after which 1 dm3 H2O was added. The organic layer was
washed several times with water to remove as much acetic
acid as possible, then washed with 10 % aq. NaHCO3,
followed by brine. The organic layer was dried over anhyd.
Na2SO4 and removed (rotovap). The remaining oil was
passed through a short column of silica gel using CH2Cl2 as
the eluent, collected, and evaporated (rotovap). The residue
was recrystallized (CH2Cl2–hexane) to give light yellow
crystals of 22. Yield: 3.5 g (13 mmol, 66 %); m.p.:
ꢀ
(KBr): m = 3,356, 2,967, 2,933, 1,667, 1,634, 1,436, 1,361,
1,174, 980, 677 cm-1; H NMR data in Table 4; UV–Vis
data in Table 9.
2,20-(1,3-Propanediyl)bis[5-[(3-ethyl-1,5-dihydro-4-
methyl-5-oxo-2H-pyrrol-2-ylidene)methyl]-4-methyl-(Z)-
1H-pyrrole-3-propionic acid] dimethyl ester [(3.2)-
homorubin dimethyl ester] (3-2e, C37H48N4O6)
To a 50 cm3 round-bottom flask was added 180 mg 13
(0.279 mmol) and 10 cm3 THF. After all the 13 had
dissolved, 10 cm3 CH3OH was added followed by the
addition of 70 mg NaBH4. The solution first turned green,
then yellow, indicating that the reduction of the azafulvene
moiety was complete. To the yellow solution was added
0.5 cm3 acetic acid to destroy any unreacted NaBH4; then,
10 cm3 H2O was added, and a yellow precipitate formed.
The organic solvents were removed (rotovap), and the solid
residue was taken up in 30 cm3 CH2Cl2. After washing the
residue with H2O (3 9 30 cm3) and brine (1 9 30 cm3)
and then drying it over anhyd. Na2SO4, the solvent was
removed (rotovap). The resulting yellow solid was dis-
solved in CH2Cl2 and passed through a short column of
silica gel. The isolated, chromatographed yellow solid was
determined to be the 1,3-bis propene dipyrrinone on the
basis of the observation of four N–H resonances, a
multiplet near 7.2 ppm and a doublet at 6.45 ppm, which
ꢀ
76–77 °C; IR (KBr): m = 3,268, 2,980, 1,736, 1,692,
1,560, 1,484, 1,262, 1,098, 785 cm-1 1H NMR:
;
d = 1.25 (3H, t, J = 7.1 Hz), 1.37 (3H, t, J = 7.1 Hz),
2.31 (3H, s), 3.74 (2H, s), 4.15 (2H, q, J = 7.1 Hz), 4.35
(2H, q, J = 7.1 Hz), 9.61 (1H, bs), 9.78 (N–H, s) ppm; 13
C
NMR: d = 9.8, 14.1, 14.3, 29.6, 61.0, 61.3, 124.9, 125.6,
127.4, 130.3, 160.8, 170.3, 179.5 ppm.
2,20-(1,3-Propanediyl)bis[5-[(3-ethyl-1,5-dihydro-4-
methyl-5-oxo-2H-pyrrol-2-ylidene)methyl]-4-methyl-(Z)-
1H-pyrrole-3-propionic acid] [(3.2)-homorubin]
(3-2, C35H44N4O6)
To a 50 cm3 round-bottom flask was added 10 cm3 THF
(N2-purged) followed by 25 mg dimethyl ester 3-2e
1
are assigned to the two vinyl proton resonances in the H
NMR spectrum. All of the aliphatic 13C resonances were
doubled. A portion of the solid was taken directly to the
next step.
123

1794
W. P. Pfeiffer, D. A. Lightner
To a 50 cm3 round-bottomed flask was added 10 cm3
redissolved in CH2Cl2, and passed through a short column
of silica gel using CH2Cl2:CH3OH (98:2 v/v) as eluent
while collecting all of the blue compound. Hexane
(10 cm3) was added to the blue solution, and the solvent
volume was decreased by evaporation until a precipitate
formed in the flask. The mixture was placed in the freezer
for 2 h, and the precipitate formed was collected by
filtration to afford the desired blue 13. Yield: 230 mg
(0.36 mmol, 86 %); m.p.: 208–209 °C; IR (KBr):
THF and 10 cm3 CH3OH followed by 10 mg platinum
oxide. The flask was then placed on a hydrogenation
apparatus and charged with hydrogen. After 30 min, 50 mg
of the reduced product from above (0.079 mmol) was
dissolved in 5 cm3 THF and added to the pre-hydrogenated
platinum oxide and again placed in the hydrogenation
apparatus. The mixture was stirred for 24 h, after which the
catalyst was removed by filtration and the solvent was
removed to give a yellow residue. The residue was dissolved
in CH2Cl2 and eluted through a short column of silica gel
using CH2Cl2:CH3OH (98:2 v/v), collecting the first yellow
band. Hexane (5 cm3) was added to the eluted yellow
solution, and the solvent was evaporated (rotovap) until a
precipitate formed. The flask was placed in the freezer for
2 h and then filtered to obtain the desired yellow dimethyl
ester 3-2e. Yield: 20 mg (0.031 mmol, 40 %); m.p.: 206–
ꢀ
m = 3,424, 2,969, 1,734, 1,699, 1,654, 1,560, 1,280,
1,167, 1,090, 858, 669 cm-1 1H NMR: d = 0.97 (6H,
;
bs), 1.29 (3H, bs), 1.48 (3H, bs), 2.01 (6H, s), 2.25 (4H,
bd), 2.50 (4H, bt, J = 7.3 Hz), 2.87 (4H, bt, J = 7.3 Hz),
3.70 (6H, s), 5.59 (2H, bs), 6.79 (2H, d, J = 13.7 Hz), 8.89
(1H, t, J = 13.7 Hz), 10.36 (N–H, bs), 10.84 (N–H, bs),
12.07 (N–H, bs) ppm; UV–Vis (acetonitrile): kmax
(e) = 399 (59,100) nm (mol-1 dm3 cm-1); UV–Vis (ace-
tone): kmax (e) = 403 (62,700) nm (mol-1 dm3 cm-1);
UV–Vis (benzene): kmax (e) = 397 (53,300), 413 (sh,
57,000) nm (mol-1 dm3 cm-1); UV–Vis (CH2Cl2): kmax
(e) = 398 (54,000) nm (mol-1 dm3 cm-1); UV–Vis
(CHCl3): kmax (e) = 402 (57,000) nm (mol-1 dm3 cm-1);
UV–Vis ((CH3)2SO): kmax (e) = 412 (58,200) nm
(mol-1 dm3 cm-1); UV–Vis (CH3OH): kmax (e) = 421
(62,000), 401 (sh, 54,800) nm (mol-1 dm3 cm-1).
ꢀ
207 °C; IR (KBr): m = 3,366, 1,255, 1,715, 1,640, 1,636,
1
1,169, 669 cm-1; H NMR: d = 1.04 (6H, t, J = 7.3 Hz),
1.34 (6H, s), 2.10 (6H, s), 2.33 (6H, m), 2.79 (8H, m), 3.73
(6H, s), 5.83 (2H, s), 10.35 (N–H, bs), 11.02 (N–H, bs) ppm;
13C NMR: d = 7.7, 9.5, 14.8, 17.3, 19.9, 25.3, 30.2, 35.6,
51.6, 99.8, 118.7, 122.9, 123.0, 123.4, 127.4, 135.1, 146.8,
173.6, 173.7 ppm; UV–Vis (acetonitrile): kmax (e) = 377
(83,300) nm (mol-1 dm3 cm-1); UV–Vis (acetone): kmax
(e) = 378 (83,700) nm (mol-1 dm3 cm-1); UV–Vis (ben-
zene): kmax (e) = 382 (85,800), 431 (sh, 18,300) nm
(mol-1 dm3 cm-1); UV–Vis (CH2Cl2): kmax (e) = 379
(86,800), 430 (sh, 17,500) nm (mol-1 dm3 cm-1); UV–Vis
(CHCl3): kmax (e) = 380 (88,600), 430 (sh, 18,700) nm
(mol-1 dm3 cm-1); UV–Vis ((CH3)2SO): kmax (e) = 400
(65,900), 419 (sh, 64,500) nm (mol-1 dm3 cm-1); UV–Vis
(CH3OH): kmax (e) = 413 (65,500) nm (mol-1 dm3 cm-1).
5-[(3-Ethyl-1,5-dihydro-4-methyl-5-oxo-2H-pyrrol-2-ylide-
ne)methyl]-2-(3-oxo-1(Z)-propenyl)-4-methyl-(Z)-1H-pyr-
role-3-propionic acid methyl ester (15, C20H24N2O4)
To a 100 cm3 round-bottomed flask equipped with a
magnetic stirrer was added 2 cm3 CH2Cl2. After cooling to
0 °C, 120 mg dimethylamino-acrolein (1.21 mmol) was
added followed by the slow addition of 185 mg phospho-
rous oxychloride (1.21 mmol) in 2 cm3 CH2Cl2. The
solution was allowed to warm to room temperature, stirred
for 30 min and then cooled to 0 °C. A solution of 300 mg
dipyrrinone 14 (1.01 mmol) in 5 cm3 CH2Cl2 was added
slowly, immediately giving a red solution. The solution
was stirred for 30 min before a solution of sodium acetate
(1 g in 50 cm3 H2O) was added; then, stirring was
continued for 8 h. The organic layer was washed with
H2O (2 9 50 cm3), dried over anhyd. Na2SO4, and
removed (rotovap), to yield a red-yellow solid. The solid
was flash chromatagraphed using CH2Cl2:CH3OH (98:2 v/
v) as eluent. A yellow band was collected, and the solvent
removed (rotovap). Recrystallization of the residue from
CH2Cl2–hexane gave the desired 15. Yield: 81 mg
(0.24 mmol, 70 % yield); m.p.: 233-234 °C; IR (KBr):
5-[(3-Ethyl-1,5-dihydro-4-methyl-5-oxo-2H-pyrrol-2-ylide-
ne)methyl]-2-[3-[5-[(3-ethyl-1,5-dihydro-4-methyl-5-oxo-
2H-pyrrol-2-ylidene)methyl]-3-[2-(methoxycar-
bonyl)ethyl]-4-methyl-2H-pyrrol-2-ylidene]-1-propenyl]-
4-methyl-1H-pyrrole-3-propionic acid methyl ester
(13, C37H44N4O6)
To a 100 cm3 round-bottom flask equipped with a magnetic
stirrer was added 10 cm3 CH3OH, 20 cm3 CH2Cl2, 150 mg
propenal dipyrrinone 15 (0.42 mmol), and 127 mg dipyrri-
none 14 (0.42 mmol). Hydrobromic acid in acetic acid
(30 %, 2 cm3) was slowly added, and the solution was
stirred for 30 min. Upon addition of the acid the solution
turned red and then dark green. Ammonium hydroxide
(*2 cm3) was added to neutralize the solution as it
changed color from green to blue. The solution was then
poured into a separatory funnel containing 40 cm3 CH2Cl2
and the mixture was washed with H2O (2 9 50 cm3) and
brine (1 9 50 cm3), then dried over anhyd. Na2SO4. The
solvent was removed (rotovap) and the residue was
ꢀ
m = 3,350, 2,933, 1,733, 1,669, 1,648, 1,615, 1,272, 1,167,
1
1,119 cm-1; H NMR: d = 1.18 (3H, t, J = 7.3 Hz), 1.91
(3H, s), 2.16 (3H, s, 3H), 2.52 (4H, m), 2.94 (2H, t,
J = 7.3 Hz), 3.69 (3H, s), 6.09 (1H, s), 6.95 (1H, dd,
J = 7.8 Hz, 15.6 Hz), 7.42 (1H, d, J = 15.6 Hz), 9.60
123

(m.n)-Homorubins: syntheses and structures
1795
(1H, d, 7.8 Hz), 10.40 (N–H, bs), 11.21 (N–H, bs) ppm;
13C NMR: d = 8.7, 9.3, 14.8, 17.9, 19.8, 34.9, 51.8, 98.7,
123.7, 125.7, 126.1, 129.6, 129.8, 130.1, 123.1, 138.5,
148.7, 173.0, 175.1, 193.4 ppm; UV–Vis (acetonitrile):
kmax (e) = 433 (26,100) nm (mol-1 dm3 cm-1); UV–Vis
(acetone): kmax (e) = 435 (34,300) nm (mol-1 dm3 cm-1);
UV–Vis (benzene): kmax (e) = 470 (21,800) nm
(mol-1 dm3 cm-1); UV–Vis (CH2Cl2): kmax (e) = 440
(21,100) nm (mol-1 dm3 cm-1); UV–Vis ((CH3)2SO):
kmax (e) = 451 (32,400) nm (mol-1 dm3 cm-1); UV–Vis
(CH3OH): kmax (e) = 445 (33,800) nm (mol-1 dm3 cm-1).
2-(Ethoxycarbonyl)-5-formyl-4-methyl-1H-pyrrole-3-pro-
pionic acid ethyl ester (18, C14H15NO5)
To a 1,000 cm3 round-bottom flask equipped with a
magnetic stirrer was added 5.0 g pyrrole 19 [18]
(18.7 mmol), 100 cm3 THF, 300 cm3 acetic acid, and
250 cm3 H2O. The mixture was stirred until all of the
pyrrole had dissolved then cooled to 5 °C. Ceric ammonium
nitrate (CAN, 41.0 g, 74.8 mmol) was added, and the
mixture was stirred for 1 h. Water (500 cm3) was then
added, and the solution was extracted with CH2Cl2
(2 9 175 cm3), washed with H2O (2 9 150 cm3), 5 % aq.
NaHCO3 (1 9 200 cm3), H2O (1 9 150 cm3), and brine
(1 9 150 cm3). The organic layer was dried (anhyd.
Na2SO4), filtered, and evaporated (rotovap). The oily residue
was crystallized from CH2Cl2–hexane to give the desired
a-aldehyde 18. Yield: 4.6 g (16.4 mmol, 91 %); m.p.:
5-[(3-Ethyl-1,5-dihydro-4-methyl-5-oxo-2H-pyrrol-2-ylide-
ne)methyl]-4-methyl-(Z)-1H-pyrrole-3-propionic acid
methyl ester (14, C17H21N2O3)
To a 100 cm3 round-bottom flask equipped with a
magnetic stirrer and heating mantle was added 1.15 g
formylpyrrole 18 (7.2 mmol), 1.80 g 3-pyrrolin-2-one 17
[29] (6.5 mmol), and 40 cm3 CH3OH. Aqueous KOH
(100 cm3, 4 M) was added, and the flask was fitted with
a condenser. The solution was heated at reflux under N2
in the dark. After 16 h, the solution was cooled to room
temperature and washed with CH2Cl2. The aqueous layer
was then cooled in an ice bath and acidified with 20 %
HCl to form a yellow-green precipitate of 16. The
precipitate was centrifuged and washed with water in the
centrifuge tube. The precipitate was filtered (suction
filtration) using a minimal amount of water. Suction was
continued for *3 h. The green solid (16) was suspended
in 100 cm3 CH3OH and brought to reflux. Sulfuric acid
(4 cm3, 2 M) was added and reflux was continued until
most all of the solid was in solution (*2 h). The mixture
was cooled to room temperature and neutralized with
5 % aq. NaHCO3. The CH3OH was then removed
(rotovap), and the residue was dissolved in 100 cm3
CH2Cl2. The organic layer was washed with H2O
(2 9 100 cm3) and brine (1 9 100 cm3), dried over
anhyd. Na2SO4, and the solvent was removed (rotovap).
The residue was redissolved in CH2Cl2, passed through a
short column of silica gel, and the yellow fraction
collected while eluting by CH2Cl2:CH3OH (98:2 v/v).
After removal of the solvent, the residue was recrystal-
lized from CH3OH giving the desired 14. Yield: 1.16 g
(3.84 mmol, 59 %); m.p.: 174-175 °C (Ref. [34], 175–
ꢀ
83–84 °C (Ref. [30], 84–86 °C); IR (KBr): m = 3,142,
2,982, 1,731, 1,704, 1,557, 1,274, 1,251, 1,129, 1,024, 780,
669 cm-1; H NMR: d = 1.24 (3H, t, J = 7.3 Hz), 1.38
(3H, t, J = 7.3 Hz), 2.33 (1H, s), 2.53 (2H, t, J = 7.3 Hz),
1
3.03 (2H, t, J = 7.3 Hz), 4.11 (2H, q, J = 7.3 Hz), 4.35
(2H, q, J = 7.3 Hz), 9.57 (N–H, bs), 9.77 (1H, s) ppm; 13
C
NMR: d = 8.3, 14.2, 14.2, 19.8, 35.5, 60.3, 61.1, 124.2,
125.8, 129.7, 131.4, 160.3, 172.8, 179.1 ppm.
2,20-(1,4-Butanediyl)bis[5-[(3-ethyl-1,5-dihydro-4-
methyl-5-oxo-2H-pyrrol-2-ylidene)methyl]-4-methyl-(Z)-
1H-pyrrole-3-acetic acid] [(4.1)-homorubin]
(4-1, C34H42N2O6)
Homorubin dimethyl ester 4-1e (50 mg, 0.079 mmol) was
suspended in 20 cm3 N2-purged CH3OH and 5 cm3 THF
and heated to reflux. Nitrogen-purged aq. NaOH (2 cm3,
1 M) was added, and reflux was continued for 4 h under a
blanket of N2 before it was cooled to 5 °C and acidified
with nitric acid (10 %) to form a precipitate. The precip-
itate was collected by filtration and dried in vacuo. Then it
was redissolved in a minimal amount of dimethyl form-
amide and heated to *90 °C. Upon cooling to room
temperature, the precipitate that had formed was collected
by filtration, washed with ethanol, and dried to afford the
desired 4-1. Yield: 20 mg (0.033 mmol, 43 %); m.p.:
ꢀ
240 °C (dec); IR (KBr): m = 3,356, 2,968, 1,667, 1,635,
1,456, 1,270, 1,171, 980, 668 cm-1; NMR data in Table 3;
UV–Vis data in Table 8.
ꢀ
2,20-(1,4-Butanediyl)bis[5-[(3-ethyl-1,5-dihydro-4-methyl-
5-oxo-2H-pyrrol-2-ylidene)methyl]-4-methyl-(Z)-1H-pyr-
role-3-acetic acid] dimethyl ester [(4.1)-homorubin
dimethyl ester] (4-1e, C36H46N4O6)
To a 25 cm3 round-bottom flask equipped with a magnetic
stirrer and heating mantle was added 117 mg bis-pyrrylbu-
176 °C); IR (KBr): m = 3,759, 2,916, 1,716, 1,661,
1,636, 1,173 cm-1
;
1H NMR: d = 1.18 (3H, t,
J = 7.3 Hz), 1.95 (3H, s), 2.15 (3H, s), 2.55 (2H, q,
J = 7.3 Hz), 2.58 (2H, d, 7.3 Hz), 2.79 (2H, d,
J = 7.3 Hz), 3.69 (3H, s), 6.15 (1H, s), 6.85 (1H, s),
10.53 (N–H, bs), 11.08 (N–H, bs) ppm; 13C NMR:
d = 8.1, 9.4, 14.9, 18.0, 20.7, 34.8, 51.6, 101.1, 120.7,
122.6, 123.3, 123.4, 124.4, 128.5, 148.5, 173.7,
174.3 ppm.
tane
tetra-acid
5-1
5-bromomethylene-3-pyrrolin-2-one (12, 0.558 mmol), and
7 cm3 CH3OH. The mixture was heated at reflux under a
(0.279 mmol),
120 mg
123

1796
W. P. Pfeiffer, D. A. Lightner
blanket of N2 (after *30 min it becomes a solution). After
3 h, a precipitate began to form. Heat was removed after
12 h, and the mixture was placed in the freezer overnight.
The precipitate was collected by filtration, then suspended in
5 cm3 dimethyl formamide and heated until all the solid had
dissolved (*110 °C). As the solution cooled to room
temperature a precipitate formed. The precipitate was
isolated by filtration and washed with ethanol (95 %) to
yield dimethyl ester 4-1e as a yellow-green solid. Yield:
71 mg (0.117 mmol, 42 %); m.p.: 250 °C (dec); IR (KBr):
recrystallized from CH2Cl2–hexane to afford white crystals
of 5-1e. Yield: 195 mg (0.37 mmol, 97 %); m.p.: 140-
ꢀ
142 °C; IR (KBr): m = 3,305, 2,981, 2,938, 1,732, 1,668,
1
1,450, 1,431, 1,281, 1,084, 922 cm-1; H NMR: d = 1.22
(6H, t, J = 7.3 Hz), 1.33 (6H, t, J = 7.3 Hz), 1.61 (4H, t,
J = 7.2 Hz), 2.27 (6H, s), 2.57 (4H, t, J = 7.2 Hz), 3.36
(4H, s), 4.10 (4H, q, J = 7.3 Hz), 4.28 (4H, q,
J = 7.3 Hz), 8.96 (2N–H, bs) ppm; 13C NMR: d = 10.6,
14.2, 14.5, 25.6, 28.8, 30.2, 59.7, 60.7, 114.4, 117.6, 127.4,
134.7, 167. 7, 171.8 ppm.
ꢀ
m = 3,336, 2,937, 1,733, 1,667, 1,634, 1,457, 1,436, 1,269,
2,20-(1,4-Butadiynediyl)bis[(5-ethoxycarbonyl)-4-methyl-
1H-pyrrole-3-acetic acid] diethyl ester
1,111, 979, 678 cm-1; 1H NMR ((CD3)2SO): d = 1.02 (6H,
t, J = 7.3 Hz), 1.51 (3H, brs), 1.78 (6H, s), 2.00 (6H, s),
2.45–2.51 (8H, brm), 3.35 (4H, s), 3.51 (6H, s), 5.94 (2H, s),
9.83 (2N–H, brs), 10.31 (2N–H, brs) ppm; 13C NMR
((CD3)2SO): d = 8.1, 9.3, 14.8, 17.4, 25.4, 29.6, 29.7, 51.4,
97.5, 112.9, 122.0, 122.7, 122.9, 127.7, 134.9, 147.3, 171.9,
172.0 ppm; UV–Vis (acetonitrile): kmax (e) = 399 (59,100)
nm (mol-1 dm3 cm-1); UV–Vis (acetone): kmax (e) = 403
(62,200) nm (mol-1 dm3 cm-1); UV–Vis (benzene):
kmax (e) = 397 (53,300), 413 (sh, 57,000) nm
(mol-1 dm3 cm-1); UV–Vis (CH2Cl2): kmax (e) = 398
(54,000) nm (mol-1 dm3 cm-1); UV–Vis (CHCl3):
kmax (e) = 402 (56,800) nm (mol-1 dm3 cm-1); UV–Vis
(6-1e, C28H32N2O8)
A mixture of 500 mg ethynyl pyrrole 7-1 (1.90 mmol) and
214 mg chloroacetone in 25 cm3 dry benzene was added to
a solution of 47 mg tetrakis(triphenylphosphine)palla-
dium(0), 24 mg copper(I) iodide, and 380 mg
triethylamine in 25 cm3 dry benzene. After stirring the
reaction mixture at room temperature for 16 h the solvent
was removed (rotovap) and the residue was taken into
CH2Cl2. The solution was passed through a short column of
silica gel, eluting first with CH2Cl2 then CH2Cl2:CH3OH
(97.25:0.75 v/v). All of the yellow solution was collected,
and the solvent removed (rotovap). The yellow residue was
recrystallized from CH2Cl2–hexane to give light yellow
crystals of the desired 6-1e. Yield: 449 mg (0.86 mmol,
91 %); m.p.: 212-213 °C; IR (KBr): mꢀ = 3,254, 2,981,
2,931, 2,142, 1,733, 1,633, 1,468, 1,268, 1,177, 1,032,
((CH3)2SO):
kmax
(e) = 412
(58,200)
nm
(mol-1 dm3 cm-1); UV–Vis (CH3OH): kmax (e) = 421
(62,000) nm (mol-1 dm3 cm-1).
2,20-(1,4-Butanediyl)bis(5-carboxy-4-methyl-1H-pyrrole-3-
acetic acid) (5-1, C20H24N2O8)
1
750 cm-1; H NMR: d = 1.27 (6H, t, J = 7.3 Hz), 1.36
Bis-pyrrylbutane 5-1e (210 mg, 0.395 mmol) was dis-
solved in 10 cm3 warm CH3OH. Aqueous NaOH (10 cm3,
2 M) was added, and the solution was heated under reflux
for 6 h. The CH3OH was then removed (rotovap), and the
aqueous layer was cooled to 5 °C and then acidified with
nitric acid (20 %) until a white precipitate formed out of
the solution. The solid was collected by filtration and dried
in vacuo to give the desired 5-1. Yield: 120 mg
(0.29 mmol, 72 %). The tetra-acid was characterized as
follows and used directly in the next step. M.p.: 128 °C
(dec); 1H NMR ((CD3)2SO): d = 1.43 (4H, brs), 2.10 (6H,
s), 2.44 (4H, brs), 3.20 (4H, s), 10.92 (2N–H, brs), 11.87
(2COOH, brs) ppm; 13C NMR ((CD3)2SO): d = 10.3, 24.8,
29.0, 29.6, 113.9, 116.7, 125.9, 135.2, 162.2, 172.8 ppm.
(6H, t, J = 7.3 Hz), 2.26 (6H, s), 3.55 (4H, s), 4.17 (4H, q,
J = 7.3 Hz), 4.37 (4H, q, J = 7.3 Hz), 9.22 (2N–H, bs)
ppm; 13C NMR: d = 10.0, 13.8, 14.0, 30.8, 59.6, 60.2,
74.8, 78.2, 113.7, 120.9, 125.1, 125.7, 159.8, 170.0 ppm;
UV–Vis (CH2Cl2): kmax (e) = 329 (40,800) nm
(mol-1 dm3 cm-1); UV–Vis (CH3OH): kmax (e) = 328
(44,700) nm (mol-1 dm3 cm-1); UV–Vis ((CH3)2SO):
kmax (e) = 331 (38,600) nm (mol-1 dm3 cm-1).
5-(Ethoxycarbonyl)-2-ethynyl-4-methyl-1H-pyrrole-3-
acetic acid ethyl ester (7-1, C14H17NO4)
The TMS-acetylene pyrrole 8-1 (2.0 g, 5.96 mmol) was
dissolved in 30 cm3 dry THF. Anhydrous ethanol (50 cm3)
and 1.30 g K2CO3 were then added, and the mixture was
stirred for 4 h at room temperature. The mixture was
poured into 40 cm3 H2O and the ethanol and THF were
removed (rotovap). The aqueous mixture was extracted
with CH2Cl2 (3 9 25 cm3). The combined organic layers
2,20-(1,4-Butanediyl)bis[(5-ethoxycarbonyl)-4-methyl-1H-
pyrrole-3-acetic acid] diethyl ester (5-1e, C28H40N2O8)
Bis-pyrryl butadiyne 6-1e (200 mg, 0.38 mmol) was par-
tially dissolved in 50 cm3 ethyl acetate, followed by the
addition of 50 mg palladium on carbon (5 %). The flask
was equipped with a hydrogen balloon, and the mixture
was stirred until it became a dark black mixture (*1 h).
The mixture was then filtered through Celite, and the
solvent was removed to give a white solid. The latter was
were
washed
with H2O
(2 9 75 cm3), brine
(1 9 100 cm3), and dried over anhyd. Na2SO4. The solvent
was removed (rotovap), and the crude residue was taken
into a small amount of CH2Cl2 and filtered through a short
column of silica gel, from which the entire first yellow
band to elute was collected. After removing the solvent, the
123

(m.n)-Homorubins: syntheses and structures
1797
yellow-brown solid was recrystallized from hexane to
afford the desired ethyl ester 7-1. Yield: 1.04 g
(3.97 mmol, 66 %); m.p.: 110–111 °C; IR (KBr):
mꢀ = 3,265, 2,993, 2,091, 1,731, 1,683, 1,458, 1,270,
2-Carboxy-5-(ethoxycarbonyl)-4-methyl-1H-pyrrole-3-
acetic acid ethyl ester (10-1, C13H17NO6)
Sulfuryl chloride (7.93 g, 4.75 cm3, 59.4 mmol) was added
dropwise to a stirred solution of 5 g pyrrole 11-1
(19.8 mmol) dissolved in 100 cm3 anhyd. ether at 5 °C.
Stirring was continued for 3 days at room temperature;
then, the ether was removed. The remaining oil was taken
into 50 cm3 dioxane and sodium acetate (5 g in 50 cm3
H2O) was added, and the solution was stirred overnight.
The solution was extracted with ether (3 9 50 cm3), after
which the ether was extracted with sat. aq. Na2CO3. The
aq. layer was then cooled in an ice bath and acidified with
nitric acid (20 %) as a precipitate formed. The precipitate
was collected by filtration, washed with water, and dried in
vacuo to afford ethyl ester 10-1. Yield: 3.5 g (12.4 mmol,
1,171, 1,043, 1,022 cm-1 1H NMR: d = 1.24 (3H, t,
;
J = 7.3 Hz), 1.34 (3H, t, J = 7.3 Hz), 2.25 (3H, s), 3.34
(1H, s), 3.52 (2H, s), 4.14 (2H, q, J = 7.3 Hz), 4.32 (2H, q,
J = 7.3 Hz), 9.29 (N–H, bs) ppm; 13C NMR: d = 10.4,
14.5, 14.4, 30.7, 60.3, 60.7, 74.3, 82.7, 114.8, 120.2, 123.5,
126.5, 160.8, 170.8 ppm.
5-(Ethoxycarbonyl)-4-methyl-2-[(trimethylsilyl)ethynyl)]-
1H-pyrrole-3-acetic acid ethyl ester (8-1, C17H25NO4Si)
To a solution of 3 g iodo-pyrrole 9-1 (8.22 mmol) in 85 cm3
diethylamine were added under N2 0.90 g (trimethyl-
silyl)acetylene
(9.20 mmol),
101 mg
ꢀ
63 %); m.p.: 187–188 °C; IR (KBr): m = 3,291, 2,983,
bis(triphenylphoephine)pallandium(II) chloride, and 54 mg
copper(I) iodide. The mixture was stirred at 50 °C for 1.5 h.
The solvent was removed (rotovap) under water aspirator
vacuum, and the residue was taken into CH2Cl2 and passed
through a short column of silica gel using CH2Cl2 as the
eluent. All of the first yellow band to elute was collected, the
solvent was removed (rotovap), and the residue was
recrystallized from hexane to afford the desired ethyl ester
8-1. Yield: 2.47 g (7.40 mmol, 90 %) m.p.: 85–86 °C; IR
1,737, 1,714, 1,472, 1,213, 1,201, 1,020, 928 cm-1; H
1
NMR ((CD3)2SO): d = 1.05 (3H, t, J = 7.3 Hz), 1.29 (3H,
t, J = 7.3 Hz), 2.15 (3H, s), 3.76 (2H, s), 4.03 (2H, q,
J = 7.3 Hz), 4.22 (2H, q, J = 7.3 Hz), 11.75 (N–H, bs),
12.95 (COOH, bs) ppm; 13C NMR ((CD3)2SO): d = 9.9,
14.2, 14.2, 29.9, 59.9, 60.1, 121.5, 122.7, 123.5, 126.6,
160.5, 161.8, 170.8 ppm.
2,20-(1,4-Butanediyl)bis[5-[(3-ethyl-1,5-dihydro-4-
ꢀ
(KBr): m = 3,276, 2,664, 2,149, 1,745, 1,671, 1,475, 1,275,
methyl-5-oxo-2H-pyrrol-2-ylidene)methyl]-4-methyl-(Z)-
1H-pyrrole-3-propanoic acid] [(4.2)-homorubin]
1
1,160, 1,027, 845 cm-1; H NMR: d = 231 (9H, s), 1.25
(3H, t, J = 7.3 Hz), 1.35 (3H, t, J = 7.3 Hz), 2.25 (3H, s),
3.51 (2H, s), 4.14 (2H, q, J = 7.3 Hz), 4.30 (2H, q,
J = 7.3 Hz), 8.98 (N–H, bs) ppm; 13C NMR: d = -0.34,
10.3, 14.0, 14.3, 30.7, 60.1, 60.5, 94.7, 100.6, 115.7, 119.7,
123.2, 126.5, 160.5, 170.7 ppm.
(4-2, C36H46N4O6)
Homorubin dimethyl ester 4-2e (30 mg, 0.046 mmol) was
suspended in 3 cm3 N2-purged CH3OH and 6 cm3 THF
and heated to reflux. Nitrogen-purged aq. NaOH (2 cm3,
1 M) was added, and reflux was continued for 4 h under N2
before the solution was cooled to room temperature. Then
3 cm3 H2O was added, and the organic solvents were
removed (rotovap). The mixture was cooled to 5 °C and
acidified with nitric acid (10 %) until a precipitate formed.
The precipitate was collected by filtration and dried in
vacuo; then, it was redissolved in a minimal amount of hot
dimethyl formamide (*90 °C) and allowed to cool to
room temperature during which time a precipitate formed.
The solid was collected by filtration, washed with ethanol
(95 %), and dried in vacuo to give the desired 4-2. Yield:
18 mg (0.028 mmol, 62 %); m.p.: 290 °C (dec); IR (KBr):
5-(Ethoxycarbonyl)-2-iodo-4-methyl-1H-pyrrole-3-acetic
acid ethyl ester (9-1, C12H16INO4)
A solution of 2.10 g NaHCO3 (25.0 mmol) in 4 cm3 H2O
was added at 65 °C to a stirred solution of 2.34 g
carboxypyrrole 10-1 (8.2 mmol) in 40 cm3 ethanol. Then
a solution of 2.10 g I2 (8.20 mmol) and 2.74 g KI
(16.6 mmol) in 60 cm3 ethanol and 15 cm3 H2O was
added dropwise during 1 h while the temperature was
maintained at 60 °C. After stirring at 60 °C for 1 h, the
solution was stirred at room temperature for an additional
2 h. To the light pink solution was added 60 cm3 H2O
containing 1 g Na2S2O2, thus giving a white precipitate.
The mixture was placed in the freezer for 1 h; then, the
solid was collected by filtration and dried to afford ethyl
ester 9-1. Yield: 2.30 g (6.30 mmol, 78 %); m.p.:
ꢀ
m = 3,419, 2,934, 1,652, 1,635, 1,270, 1,173, 980,
679 cm-1; NMR data in Table 4; UV–Vis data in Table 9.
2,20-(1,4-Butanediyl)bis[5-[(3-ethyl-1,5-dihydro-4-methyl-
5-oxo-2H-pyrrol-2-ylidene)methyl]-4-methyl-(Z)-1H-pyr-
role-3-propanoic acid] dimethyl ester [(4.2)-homorubin
dimethyl ester] (4-2e, C38H50N4O6)
To a 50 cm3 round-bottom flask equipped with a magnetic
stirrer and a heating mantle were added 230 mg bis-
pyrrylbutane tetra-acid 5-2 (0.51 mmol), 250 mg 5-bro-
momethylene-3-pyrrolin-2-one (12, 1.16 mmol), 15 cm3
ꢀ
117–118 °C; IR (KBr): m = 3,275, 2,983, 1,737, 1,681,
1
1,245, 1,211, 1,132, 1,063 cm-1; H NMR: d = 1.26 (3H,
t, J = 7.3 Hz), 1.36 (3H, t, J = 7.3 Hz), 2.31 (3H, s), 3.40
(2H, s), 4.15 (2H, q, J = 7.3 Hz), 4.33 (2H, q, J = 7.3 H),
9.18 (N–H, bs) ppm; 13C NMR: d = 11.0, 14.2, 14.5, 32.8,
60.3, 60.8, 74.2, 123.9, 129.3, 127.4, 160.6, 170.7 ppm.
123

1798
W. P. Pfeiffer, D. A. Lightner
CH3OH, and 5 drops H2O. The mixture was heated at
reflux under a blanket of N2. After 5 h, a precipitate began
to form out of the solution, and after 8 h heating was
stopped, and the mixture was placed in the freezer
overnight. The precipitate was collected by filtration and
dissolved in hot dimethyl formamide (*110 °C) and then
allowed to cool to room temperature, during which time a
precipitate formed. The precipitate was collected by
filtration, washed with ethanol (95 %) and dried in vacuo
to afford a yellow-green solid dimethyl ester 4-2e. Yield:
150 mg (0.228 mmol, 45 %); m.p.: 280 °C (dec); IR
fluorescence was no longer observed (*4 h). The mixture
was filtered through Celite and the solvent was removed
(rotovap) to give a quantitative yield of the desired 5-2e.
Yield: 402 mg (0.724 mmol); m.p.: 116–117 °C; IR (KBr):
ꢀ
m = 3,302, 2,932, 1,733, 1,662, 1,274, 1,169, 1,095,
1,028 cm-1 1H NMR: d = 1.23 (6H, t, J = 7.3 Hz),
;
1.33 (6H, t, J = 7.2 Hz), 1.60 (4H, brs), 2.26 (6H, s), 2.39
(4H, t, J = 7.3 Hz), 2.54 (4H, brs), 2.69 (4H, t,
J = 7.3 Hz), 4.10 (4H, q, J = 7.3 Hz), 4.28 (4H, q,
J = 7.3 Hz), 8.71 (2N–H, bs) ppm; 13C NMR: d = 10.5,
14.2, 14.6, 19.6, 25.7, 29.2, 35.4, 59.6, 60.3, 117.5, 20.0,
126.9, 133.6, 161.6, 173.0 ppm.
ꢀ
(KBr): m = 3,356, 2,934, 1,735, 1,668, 1,635, 1,270,
1,173, 980, 669 cm-1 1H NMR ((CD3)2SO): d = 1.04
;
2,20-(1,4-Butadiynediyl)bis[5-(ethoxycarbonyl)-4-methyl-
1H-pyrrole-3-propanoic acid] diethyl ester
(6H, t, J = 7.3 Hz), 1.51 (4H, brm), 1.74 (6H, s), 1.97 (6H,
s), 2.31 (4H, t, J = 7.2 Hz), 2.45-2.51 (8H, brm), 3.52 (6H,
s), 5.88 (2H, s, 2H), 9.79 (2N–H, brs), 10.15 (2N–H, brs)
ppm; 13C NMR ((CD3)2SO): d = 8.0, 9.1, 14.7, 17.1, 19.4,
25.4, 30.0, 35.0, 51.1, 97.5, 118.2, 122.0, 122.1, 122.8,
127.6, 133.9, 147.2, 171.9, 172.71 ppm; UV–Vis (aceto-
nitrile): kmax (e) = 399 (57,400) nm (mol-1 dm3 cm-1);
UV–Vis (acetone): kmax (e) = 405 (59,000) nm
(mol-1 dm3 cm-1); UV–Vis (benzene): kmax (e) = 401
(55,600), 415 (sh, 55,100) nm (mol-1 dm3 cm-1); UV–Vis
(CH2Cl2): kmax (e) = 397 (54,000) nm (mol-1 dm3 cm-1);
UV–Vis (CHCl3): kmax (e) = 402 (54,000) nm
(mol-1 dm3 cm-1); UV–Vis ((CH3)2SO): kmax (e) = 413
(58,200) nm (mol-1 dm3 cm-1); UV–Vis (CH3OH): kmax
(e) = 412 (58,300) nm (mol-1 dm3 cm-1).
(6-2e, C30H36N2O8)
A mixture of the 690 mg 5-(ethoxycarbonyl)-2-ethynyl-4-
methyl-1H-pyrrole-3-propanoic acid ethyl ester (7-2,
2.5 mmol) and 280 mg chloroacetone in 30 cm3 dry
benzene was added to a solution of 62 mg tetrakis(tri-
phenylphosphine)palladium(0), 31 mg copper(I) iodide,
and 500 mg triethylamine in 30 cm3 dry benzene. After
stirring the mixture at room temperature for 16 h, the
solvent was removed (rotovap). The residue was taken into
CH2Cl2, passed through a short column of silica gel first
using CH2Cl2 as eluent and then CH2Cl2:CH3OH (99:1 v/
v). The entire yellow band was collected, and the solvent
was removed to give a yellow residue. This was recrys-
tallized from hexane to give the desired 6-2e as light
yellow crystals. Yield: 479 mg (1.75 mmol, 70 %); m.p.:
165–166 °C; IR (KBr): mꢀ = 3,256, 2,977, 2,143, 1,736,
2,20-(1,4-Butanediyl)bis(5-carboxy-4-methyl-1H-pyrrole-3-
propanoic acid) (5-2, C22H28N2O8)
Bis-pyrrylbutane tetra-ester 5-2e (315 mg, 0.540 mmol)
was dissolved in 10 cm3 warm ethanol. Aqueous NaOH
(20 cm3, 2 M) was added, and the solution was heated
under reflux for 6 h. The ethanol was removed (rotovap),
the aqueous layer was cooled to 5 °C and then acidified
with nitric acid (20 %) until a white precipitate formed out
of the solution. The solid was collected by filtration and
dried in vacuo to afford the desired 5-2. Yield: 240 mg
(0.535 mmol, 99 %). It was characterized as follows and
1,671, 1,465, 1,447, 1,264, 1,176, 1,161, 1,094, 776 cm-1
;
1H NMR: d = 1.26 (6H, t, J = 7.3 Hz), 1.36 (6H, t,
J = 7.3 Hz), 2.29 (6H, s), 2.55 (4H, t, J = 7.3 Hz), 2.86
(4H, t, J = 7.3 Hz), 4.14 (4H, q, J = 7.3 Hz), 4.33 (4H, q,
J = 7.3 Hz), 9.06 (2N–H, bs) ppm; 13C NMR: d = 10.0,
14.0, 14.3, 20.3, 34.5, 60.2, 60.2, 74.5, 79.2, 114.6, 121.0,
125.9, 131.6, 160.3, 172.4 ppm; UV–Vis (CH2Cl2): kmax
(e) = 333 (42,500) nm (mol-1 dm3 cm-1); UV–Vis
(CH3OH): kmax (e) = 329 (51,400) nm (mol-1 dm3 cm-1);
UV–Vis ((CH3)2SO): kmax (e) = 330 (39,600) nm
(mol-1 dm3 cm-1).
1
carried directly to the next step. M.p.: 172 °C (dec); H
NMR ((CD3)2SO): d = 1.60 (2H, brs), 2.13 (3H, s, 3H),
2.25 (2H, brs), 2.4–2.5 (4H, brs), 10.79 (N–H, brs), 12.01
(COOH, 2H) ppm; 13C NMR ((CD3)2SO): d = 10.1, 19.2,
24.8, 29.4, 35.2, 116.7, 118.8, 125.1, 134.2, 162.1,
173.7 ppm.
5-(Ethoxycarbonyl)-2-ethynyl-4-methyl-1H-pyrrole-3-pro-
panoic acid ethyl ester (7-2, C15H19NO4)
Ethyl ester 8-2 (1.11 g, 3.18 mmol) was dissolved in
15 cm3 dry THF. Anhydrous ethanol (40 cm3) and 800 mg
K2CO3 were added, and the mixture was stirred for 4 h at
room temperature. The mixture was then poured into
60 cm3 water, and the ethanol and THF were removed
(rotovap). The resulting aqueous solution was extracted
with CH2Cl2 (3 9 50 cm3), and the organic layer was
washed with H2O (2 9 75 cm3) and brine (1 9 100 cm3)
and dried over anhyd. Na2SO4. The solvent was removed
2,20-(1,4-Butanediyl)bis[5-(ethoxycarbonyl)-4-methyl-1H-
pyrrole-3-propanoic acid] diethyl ester
(5-2e, C30H44N2O8)
Diethyl ester 6-2e (400 mg, 0.72 mmol) was dissolved in
200 cm3 ethyl acetate, followed by the addition of 50 mg
palladium on carbon (5 %). The flask was fitted with a
hydrogen balloon and the mixture was stirred until
123

(m.n)-Homorubins: syntheses and structures
1799
(rotovap) and the crude residue was taken into a small
amount of CH2Cl2 and filtered through a short column of
silica gel, of which all of the first yellow band was
collected. After removing the solvent (rotovap) the yellow-
brown solid was recrystallized from hexane, giving the
desired ethyl ester 7-2. Yield: 690 mg (2.50 mmol, 79 %);
m.p.: 104–105 °C; IR (KBr): mꢀ = 3,272, 2,994, 1,717,
2-Carboxy-5-(ethoxycarbonyl)-4-methyl-1H-pyrrole-3-
propionic acid ethyl ester (10-2, C14H19NO6)
As for the synthesis of 10-1, 7.5 g sulfuryl chloride
(4.5 cm3, 57 mmol) was added dropwise with stirring to a
solution of 5 g pyrrole 11-2 (18.7 mmol) dissolved in
100 cm3 anhyd. ether, and cooled to 5 °C. The reaction and
workup proceeded as for 10-1 to yield ethyl ester 10-2.
Yield: 3.7 g (12.5 mmol, 67 %); m.p.: 145–146 °C (Ref.
1,671, 1,273, 1,172, 1,089, 1,033 cm-1
;
1H NMR:
ꢀ
d = 1.23 (3H, t, J = 7.3 Hz), 1.34 (3H, t, J = 7.3 Hz),
2.27 (3H, s), 2.52 (3H, t, J = 7.3 Hz), 2.82 (2H, t,
J = 7.3 Hz), 3.35 (1H, s), 4.12 (2H, q, J = 7.3 Hz), 4.31
(2H, q, J = 7.3 Hz), 9.16 (N–H, bs) ppm; 13C NMR:
d = 10.2, 14.1, 14.4, 20.2, 34.6, 60.2, 60.3, 74.7, 82.6,
119.9, 121.8, 125.8, 129.3, 161.8, 172.8 ppm.
[31] 146 °C); IR (KBr): m = 3,293, 2,978, 2,618, 1,732,
1
1,698, 1,671, 1,472, 1,276, 1,178, 1,016 cm-1; H NMR
((CD3)2SO): d = 1.23 (3H, t, J = 7.3 Hz), 1.38 (3H, t,
J = 7.3 Hz), 2.30 (3H, s), 2.55 (2H, t, J = 7.3 Hz), 3.07
(2H, t, J = 7.3 Hz), 4.12 (2H, q, J = 7.3 Hz), 4.35 (2H, q,
J = 7.3 Hz), 11.53 (N–H, bs), 12.86 (COOH, bs) ppm;
13C NMR ((CD3)2SO): d = 9.7, 14.0, 14.2, 19.9, 34.6,
60.3, 60.7, 120.5, 122.9, 126.9, 131.4, 160.7, 164.9,
172.9 ppm.
5-(Ethoxycarbonyl)-4-methyl-2-[(trimethylsilyl)ethynyl]-
1H-pyrrole-3-propanoic acid ethyl ester [32]
(8-2, C18H27NO4Si)
2,20-(1,4-Butanediyl)bis[5-[(3-ethyl-1,5-dihydro-4-
methyl-5-oxo-2H-pyrrol-2-ylidene)methyl]-4-methyl-(Z)-
1H-pyrrole-3-butanoic acid] [(4.3)-homorubin]
(4-3, C38H50N4O6)
As for the synthesis of 8-1, using an ethynylation procedure
developed earlier [32], to a solution of 2 g iodopyrrole 9-2
(5.30 mmol) in 60 cm3 diethylamine were added under N2
0.78 g (trimethylsilyl)acetylene (7.90 mmol), 65 mg
bis(triphenylphosphine)palladium(II) chloride, and 65 mg
copper(I) iodide. The reaction and workup proceeded as for
8-1 to give the desired ethyl ester 8-2. Yield: 1.40 g
Homorubin dimethyl ester 4-3e (30 mg, 0.046 mmol) was
suspended in 5 cm3 N2-purged CH3OH and 15 cm3 THF
and heated to reflux. Nitrogen-purged aq. NaOH (2 cm3,
1 M) was then added, and reflux was continued for 4 h.
The mixture was cooled to room temperature, 5 cm3 H2O
was added and then the solvent was removed. Workup
continued as for 4-1 to give the desired 4-3. Yield: 15 mg
(0.037 mmol, 62 %); m.p.: 225 °C (dec); IR (KBr):
mꢀ = 3,356, 2,934, 1,667, 1,634, 1,257, 1,362, 1,246,
1,173, 979, 680 cm-1; NMR data in Table 5; UV–Vis
data in Table 10.
ꢀ
(4.0 mmol, 76 %); m.p.: 83–84 °C; IR (KBr): m = 3,277,
2,982, 2,149, 1,731, 1,674, 1,456, 1,273, 1,162, 1,095,
1,031, 841 cm-1; 1H NMR: d = 0.219 (9H, s), 1.22 (3H, t,
J = 7.3 Hz), 1.33 (3H, t, J = 7.3 Hz), 2.25 (3H, s), 2.52
(3H, t, J = 7.3 Hz), 2.80 (2H, t, J = 7.3 Hz), 4.12 (2H, q,
J = 7.3 Hz), 4.29 (2H, q, J = 7.3 Hz), 9.15 (N–H, bs)
ppm; 13C NMR: d = -0.3, 10.2, 14.2, 14.4, 20.3, 34.5,
60.1, 60.2, 94.4, 100.5, 114.9, 119.6, 125.8, 129.3, 160.8,
172.8 ppm.
2,20-(1,4-Butanediyl)bis[5-[(3-ethyl-1,5-dihydro-4-methyl-
5-oxo-2H-pyrrol-2-ylidene)methyl]-4-methyl-(Z)-1H-pyr-
role-3-butanoic acid] dimethyl ester [(4.3)-homorubin
dimethyl ester] (4-3e, C40H54N4O6)
5-(Ethoxycarbonyl)-2-iodo-4-methyl-1H-pyrrole-3-propa-
noic acid ethyl ester (9-2, C13H18INO4)
To a 50 cm3 round-bottom flask equipped with a magnetic
stirrer and a heating mantle were added 300 mg bis-
pyrrylbutane tetra-acid 5-3 (0.63 mmol), 377 mg 5-bro-
momethylene-3-pyrrolinone (1, 1.74 mmol), 18 cm3
CH3OH, and 7 drops of H2O. The reaction and workup
proceeded as for 4-1e to yield crude 4-3e as yellow-green
solid. Yield: 165 mg (0.23 mmol, 37 %); m.p.: 260 °C
(dec); IR (KBr): = 3,356, 2,935, 1,734, 1,664, 1,625,
As for 9-1, a solution of 2.7 g NaHCO3 (24.6 mmol) in
6 cm3 H2O was added at 65 °C to a stirred solution of 3.0 g
carboxypyrrole 10-2 (10.2 mmol) in 50 cm3 CH3OH. A
solution of 2.6 g I2 (10.2 mmol) and 3.4 g KI (20.4 mmol)
in 60 cm3 CH3OH and 15 cm3 H2O was added dropwise
during 1 h while the temperature was maintained at 60 °C.
The reaction and workup proceeded as for 8-1 to afford
ethyl ester 9-2. Yield: 2.40 g (6.33 mmol, 63 %); m.p.:
1,508, 1,246, 1,173, 778, 678 cm-1
;
1H NMR:
ꢀ
104–105 °C (Ref. [31], 106–107 °C); IR (KBr): m = 3,266,
2,982, 1,718, 1,654, 1,417, 1,235, 1,161, 1,039,
((CD3)2SO): d = 1.04 (6H, t, J = 7.3 Hz), 1.55 (8H,
brm), 1.74 (6H, s), 1.97 (6H, s), 2.24 (8H, m), 2.4-2.5 (8H,
m) ppm; 13C NMR((CD3)2SO): d = 8.1, 9.3, 14.9, 17.2,
23.0, 25.4, 26.0, 30.1, 32.87, 51.2, 97.9, 119.2, 122.0,
122.4, 122.6, 127.2, 134.1, 147.3, 172.0, 173.3 ppm; UV–
Vis (acetonitrile): kmax (e) = 399 (54,900) nm
(mol-1 dm3 cm-1); UV–Vis (acetone): kmax (e) = 408
1,015 cm-1 1H NMR: d = 1.26 (3H, t, J = 7.3 Hz),
;
1.35 (3H, t, J = 7.3 Hz), 2.31 (3H, s), 2.42 (2H, t,
J = 7.3 Hz), 2.70 (2H, t, J = 7.3 Hz), 4.13 (2H, q,
J = 7.3 Hz), 4.32 (2H, q, J = 7.3 Hz), 9.19 (N–H, bs)
ppm; 13C NMR: d = 10.6, 14.1, 14.3, 22.0, 34.5, 60.1,
60.3, 72.5, 124.0, 126.5, 128.6, 160.4, 172.5 ppm.
123

1800
W. P. Pfeiffer, D. A. Lightner
(61,900) nm (mol-1 dm3 cm-1); UV–Vis (benzene): kmax
2,136, 1,738, 1,668, 1,471, 1,435, 1,258, 1,177, 1,095,
1,022, 867, 776 cm-1 1H NMR: d = 1.27 (6H, t,
;
(e) = 402
(60,400),
417
(sh,
60,000)
nm
(mol-1 dm3 cm-1); UV–Vis (CH2Cl2): kmax (e) = 398
(57,000) nm (mol-1 dm3 cm-1); UV–Vis (CHCl3): kmax
(e) = 405 (59,200) nm (mol-1 dm3 cm-1); UV–Vis
J = 7.3 Hz), 1.38 (6H, t, J = 7.3 Hz), 1.85 (4H, m), 2.29
(6H, s), 2.33 (4H, t, J = 7.2 Hz), 2.52 (4H, t, J = 7.2 Hz),
4.14 (4H, q, J = 7.3 Hz), 4.36 (4H, q, J = 7.3 Hz), 9.23
(2N–H, bs) ppm; 13C NMR: d = 10.3, 14.3, 14.4, 24.2,
25.4, 33.6, 60.2, 60.4, 74.8, 78.5, 113.6, 121.2, 126.1,
132.7, 160.6, 173.3 ppm; UV–Vis (CH2Cl2): kmax
(e) = 333 (44,500) nm (mol-1 dm3 cm-1); UV–Vis
(CH3OH): kmax (e) = 330 (48,300) nm (mol-1 dm3 cm-1);
UV–Vis ((CH3)2SO): kmax (e) = 357 (36,500) nm
(mol-1 dm3 cm-1).
((CD3)2SO):
kmax
(e) = 414
(60,000)
nm
(mol-1 dm3 cm-1); UV–Vis (CH3OH): kmax (e) = 415
(63,200) nm (mol-1 dm3 cm-1).
2,20-(1,4-Butanediyl)bis(5-carboxy-4-methyl-1H-pyrrole-3-
butanoic acid) (5-3, C24H32N2O8)
Bis-pyrryl butane 5-3e (300 mg, 0.51 mmol) was dissolved
in 10 cm3 warm CH3OH. Aqueous NaOH (20 cm3, 2 M)
was added, and the solution was heated at reflux for 6 h.
The reaction and workup proceeded as for 5-1, resulting in
the desired 5-3. Yield: 200 mg (0.379 mmol, 74 %). It was
characterized through the following methods and carried
5-(Ethoxycarbonyl)-2-ethynyl-4-methyl-1H-pyrrole-3-
butanoic acid ethyl ester (7-3, C16H21NO4)
Ethyl ester 8-3 (1.70 g, 4.63 mmol) was dissolved in
22 cm3 dry THF. Anhydrous ethanol (45 cm3) and 960 mg
potassium carbonate were then added, and the mixture was
stirred for 4 h at room temperature. The reaction and
workup proceeded as for 7-2 to afford the desired ethyl
ester 7-3. Yield: 960 mg (3.29 mmol, 70 %); m.p.: 102–
1
directly to the next step. M.p.: 140 °C (dec); H NMR
((CD3)2SO): d = 1.43 (4H, brs), 1.51 (4H, dd,
J = 7.3 Hz), 2.11 (6H, s), 2.14 (4H, t, J = 7.3 Hz), 2.24
(4H, t, J = 7.3 Hz), 2.43 (4H, brs), 10.82 (2N–H, brs),
11.85 (4H, COOH) ppm; 13C NMR ((CD3)2SO): d = 10.4,
22.9, 24.9, 26.1, 29.6, 33.2, 116.7, 119.7, 125.3, 134.4,
162.4, 174.5 ppm.
ꢀ
103 °C; IR (KBr): m = 3,270, 2,938, 2,128, 1,718, 1,670,
1
1,272, 1,186, 1,154, 1,026 cm-1; H NMR: d = 1.24 (3H,
t, J = 7.3 Hz), 1.35 (3H, t, J = 7.3 Hz), 1.84 (2H, m), 2.27
(3H, s), 2.30 (2H, t, J = 7.3 Hz), 2.54 (2H, t, J = 7.3 Hz),
3.30 (1H, s), 4.11 (2H, q, J = 7.3 Hz), 4.31 (2H, q,
J = 7.3 Hz), 9.01 (N–H, bs) ppm; 13C NMR: d = 10.4,
14.3, 14.5, 24.0, 25.4, 33.7, 60.2, 60.3, 75.0, 82.5, 113.9,
120.1, 126.0, 130.4, 160.8, 173.5 ppm.
2,20-(1,4-Butanediyl)bis[5-(ethoxycarbonyl)-4-methyl-1H-
pyrrole-3-butanoic acid] diethyl ester
(5-3e, C32H48N2O8)
Bis-pyrrylbutadiyne 6-3e (300 mg, 0.510 mmol) was dis-
solved in 200 cm3 ethyl acetate followed by the addition of
30 mg platinum oxide. The flask was fitted with a hydrogen
balloon and stirred until no fluorescence was observed
(*24 h). The reaction was worked up as for 5-2e to give a
quantitative yield of the desired 5-3e. Yield: 302 mg
5-(Ethoxycarbonyl)-4-methyl-2-[(trimethylsilyl)ethynyl]-
1H-pyrrole-3-butanoic acid ethyl ester
(8-3, C19H29NO4Si)
As in the preparation of 8-2, to a solution of 2.0 g ethyl
ester 9-3 (5.09 mmol) in 60 cm3 diethylamine were added
under N2 0.775 g (trimethylsilyl)acetylene (7.9 mmol),
65 mg bis(triphenylphosphine)palladium(II) chloride, and
36 mg copper(I) iodide. The reaction and workup pro-
ceeded as for 8-2 to provide the desired ethyl ester 8-3.
Yield: 1.55 g (4.28 mmol, 84 %); m.p.: 71–72 °C; IR
ꢀ
(0.510 mmol); m.p.: 122–123 °C; IR (KBr): m = 3,293,
1
2,938, 1,734, 1,654, 1,264, 1,160, 1,095 cm-1; H NMR:
d = 1.25 (6H, t, J = 7.3 Hz), 1.34 (6H, t, J = 7.3 Hz),
1.60 (4H, brs), 1.73 (4H, m), 2.26 (6H, s), 2.28 (4H, t,
J = 7.3 Hz), 2.39 (4H, t, J = 7.3 Hz), 4.12 (4H, q,
J = 7.3 Hz), 4.24 (4H, q, J = 7.3 Hz), 8.56 (2N–H, bs)
ppm; 13C NMR: d = 10.5, 14.3, 14.6, 23.4, 25.8, 33.8,
59.6, 60.2, 117.4, 121.0, 127.1, 133.5, 161.6, 173.5 ppm.
ꢀ
(KBr):m = 3,278, 2,965, 2,148, 1,734, 1,675, 1,250, 1,176,
1
1,097, 1,026, 843 cm-1; H NMR: d = 0.25 (9H, s), 1.24
(3H, t, J = 7.3 Hz), 1.35 (3H, t, J = 7.3 Hz), 1.85 (2H,
m), 2.25 (3H, s), 2.30 (3H, t, J = 7.3 Hz), 2.53 (2H, t,
J = 7.3 Hz), 4.11 (2H, q, J = 7.3 Hz), 4.30 (2H, q,
J = 7.3 Hz), 8.81 (N–H, bs) ppm; 13C NMR: d = -0.1,
10.3, 14.3, 14.5, 24.2, 25.3, 33.9, 60.2, 60.2, 95.8, 100.3,
115.0, 119.8, 126.1, 130.3, 160.8, 173.5 ppm.
2,20-(1,4-Butadiynediyl)bis[5-(ethoxycarbonyl)-4-methyl-
1H-pyrrole-3-butanoic acid] diethyl ester
(6-3e, C32H40N2O8)
A mixture of 950 mg acetylenic pyrrole 7-3 (3.26 mmol)
and 365 mg chloroacetone in 35 cm3 dry benzene was
added to a solution of 80 mg tetras(triphenylphosphine)pal-
ladium(0), 40 mg copper(I) iodide, and 650 mg
triethylamine in 35 cm3 dry benzene. The reaction and
workup proceeded as for 6-2e to give the desired 6-3e as
light yellow crystals. Yield: 700 mg (1.2 mmol, 74 %);
5-(Ethoxycarbonyl)-2-iodo-4-methyl-1H-pyrrole-3-buta-
noic acid ethyl ester (9-3, C14H20INO4)
A solution of 3.38 g NaHCO3 (40.25 mmol) in 8 cm3 H2O
was added at 65 °C to a stirred solution of 5.00 g 10-3
(16.1 mmol) in 80 cm3 CH3OH. A solution of 4.09 g I2
ꢀ
m.p.: 124–125 °C; IR (KBr): m = 3,270, 2,982, 2,938,
123

(m.n)-Homorubins: syntheses and structures
1801
(16.1 mmol) and KI in 80 cm3 CH3OH and 18 cm3 H2O
was added dropwise during 1 h while the temperature was
maintained at 60 °C. After stirring at 60 °C overnight, the
solution was allowed to cool to room temperature. Then
300 cm3 H2O was added to form a precipitate. The
precipitate was collected by filtration, washed with water,
dried under high vacuum, and recrystallized from CH2Cl2–
light petroleum ether to afford ethyl ester 9-3. Yield: 5.50 g
(14.1 mmol, 87 %); m.p.: 100–102 °C; IR (KBr):
125. In: Scriver CR, Sly WS, Childs B, Beaudet AL, Valle D,
Kinzler KW, Vogelstein B (eds) The metabolic and molecu-
lar bases of inherited disease, 8th edn. McGraw-Hill, Inc, New
York
3. Bonnett R, Davies JE, Hursthouse MB, Sheldrick GM (1978)
Proc R Soc Lond B202:249
4. LeBas G, Allegret A, Mauguen Y, DeRango C, Bailly M (1980)
Acta Cryst B36:3007
5. Mugnoli A, Manitto P, Monti D (1983) Acta Cryst C39:1287
6. Kaplan D, Navon G (1983) Isr J Chem 23:177
7. Nogales D, Lightner DA (1995) J Biol Chem 270:73
¨
8. Dorner T, Knipp B, Lightner DA (1997) Tetrahedron 5:2697
ꢀ
m = 3,254, 2,936, 1,718, 1,684, 1,419, 1,255, 1,185,
9. Person RV, Peterson BR, Lightner DA (1994) J Am Chem Soc
116:42
1,154 cm-1 1H NMR: d = 1.25 (3H, t, J = 7.3 Hz),
;
1.35 (3H, t, J = 7.3 Hz), 1.78 (2H, brm), 2.30 (3H, s), 2.34
(3H, t, J = 7.3 Hz), 2.41 (2H, t, J = 7.3 Hz), 4.12 (2H, q,
J = 7.3 Hz), 4.32 (2H, q, J = 7.3 Hz), 9.05 (N–H, bs)
ppm; 13C NMR: d = 10.8, 14.3, 14.5, 25.3, 25.9, 33.7,
60.2, 60.2, 72.2, 124.2, 126.7, 129.7, 160.6, 173.3 ppm.
10. Becker W, Sheldrick WS (1978) Acta Cryst B34:1298
11. Boiadjiev SE, Lightner DA (1999) Tetrahedron Asymmetry
10:607
12. McDonagh AF, Lightner DA (1991) The importance of molecular
structure in bilirubin metabolism and excretion, Chap 5. In: Bock
KW, Gerok W, Matern S (eds) Hepatic metabolism and dispo-
sition of endo and xenobiotics (Falk Symposium No. 57). Kluwer,
Dordrecht, p 47
13. Pfeiffer WP, Lightner DA (1994) Tetrahedron Lett 35:9673
14. Pfeiffer WP, Dey SK, Falk H, Lightner DA (2014) Monatsh
Chem 145:963
15. Shrout DP, Puzicha G, Lightner DA (1992) Synthesis, 328
16. Boiadjiev SE, Lightner DA (1994) Synlett, 777
17. Shrout DP, Lightner DA (1990) Synthesis, 1062
18. Trull FR, Franklin RW, Lightner DA (1987) J Heterocycl Chem
24:1573
19. Boiadjiev SE, Lightner DA (2001) Tetrahedron Asymmetry
12:2551
20. Sonogoshira K, Tohda Y, Hagihara N (1975) Tetrahedron Lett,
4467
21. Tu B, Lightner DA (2003) J Org Chem 68:8950
22. Tu B, Ghosh B, Lightner DA (2004) Monatsh Chem 135:519
23. Puzicha G, Shrout DP, Lightner DA (1990) J Heterocycl Chem
27:2117
24. Jackson AH, Jenkins RM, Joros MD, Matlin SA (1983) Tetra-
hedron 39:1849
2-Carboxy-5-(ethoxycarbonyl)-4-methyl-1H-pyrrole-3-
butanoic acid ethyl ester (10-3, C15H21NO6)
Sulfuryl chloride (29 g, 159 mmol) was added dropwise to
a stirred solution of 15 g pyrrole 11-3 (53 mmol) dissolved
in 280 cm3 anhyd. ether and cooled to 5 °C. Stirring was
continued for 3 days at room temperature, and the ether
was removed (rotovap). The remaining oil was taken into
210 cm3 dioxane, sodium acetate (15 g in 150 cm3 H2O)
was added, and the solution was stirred overnight. The
solution was then extracted with ether (3 9 200 cm3), after
which the ether was washed with sat. aq. Na2CO3. The
aqueous layer was then cooled in an ice bath and acidified
with nitric acid (20 %) to form a precipitate. The precip-
itate was collected by filtration, washed with H2O, and
dried in vacuo resulting in 10-3. Yield: 8.7 g (28 mmol,
ꢀ
52 %); m.p.: 140–141 °C; IR (KBr): m = 3,303, 2,985,
2,608, 1,736, 1,715, 1,704, 1,472, 1,272, 1,093, 1,013, 913,
25. Dieck HA, Heck FR (1975) J Organomet Chem 93:259
26. Rossi R, Carpita A, Bigelli C (1985) Tetrahedon Lett, 523
27. Chen Q-Q, Falk H (1995) Monatsh Chem 126:1233
28. Fischer H, Adler E (1931) Hoppe-Seyler’s Z Physiol Chem
200:209
668 cm-1
;
1H NMR ((CD3)2SO): d = 1.14 (3H, t,
J = 7.3 Hz), 1.28 (3H, t, J = 7.3 Hz), 1.68 (2H, m), 2.17
(3H, s), 2.22 (2H, t, J = 7.3 Hz), 2.68 (2H, t, J = 7.3 Hz),
3.99 (2H, q, J = 7.3 Hz), 4.21 (2H, q, J = 7.3 Hz), 11.63
(N–H, bs), 12.80 (COOH, bs) ppm; 13C NMR ((CD3)2SO):
d = 10.0, 14.4, 14.5, 23.5, 25.7, 33.5, 59.9, 60.1, 121.8,
127.9, 126.0, 129.8, 160.8, 162.0, 172.9 ppm.
29. Chen Q, Huggins MT, Lightner DA, Norona W, McDonagh AF
(1999) J Am Chem Soc 121:9253
30. Kar A, Lightner DA (1998) Tetrahedron 54:5151
31. Thyrann T, Lightner DA (1995) Tetrahedron Lett 36:4345
32. Paine JB, Dolphin D (1976) Can J Chem 54:411
33. Ghosh B, Lightner DA (2003) J Heterocycl Chem 40:1113
34. McMurry JE (1989) Chem Rev 89:1513
35. Trull FR, Ma JS, Landen GL, Lightner DA (1983) Israel J Chem
23:211
36. Nogales DF, Ma J-S, Lightner DA (1993) Tetrahedron 49:2361
37. Brower JO, Lightner DA, McDonagh AF (2000) Tetrahedron
56:7869
Acknowledgments We thank the late Prof. A. F. McDonagh (Univ.
California, San Francisco) for the metabolism studies and the
National Institutes of Health for partial support.
38. Kaplan D, Navon G (1982) Biochem J 201:605
39. Huggins MT, Lightner DA (2001) Monatsh Chem 132:203
40. Lightner DA, An JY, Pu Y-M (1987) Tetrahedron 43:4287
References
´
41. Lightner DA, Gawronski JK, Wijekoon WMD (1987) J Am
Chem Soc 109:6354
42. Lightner DA, Reisinger M, Landen GL (1986) J Biol Chem
261:6034
43. Pu Y-M, McDonagh AF, Lightner DA (1993) J Am Chem Soc
115:377
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