Atropisomerism in linear tetrapyrroles

Article abstract of DOI:10.1016/S0040-4020(02)00827-X

Novel bilirubin and biliverdin congeners with propionic acids replaced by o-carboxyphenyl exhibit diastereomerism due to axial chirality about the carbon-carbon single bond linking the o-carboxyphenyl group to a pyrrole ring. Evidence for atropisomerism was found even in the monopyrrole precursor, ethyl 3,5-dimethyl-4-(o-carboxyphenyl)pyrrole-2-carboxylate. Like bilirubin, o-carboxyphenyl rubin 1a adopts an intramolecularly hydrogen-bonded ridge-tile conformation in nonpolar solvents. In solutions containing optically active amines or human serum albumin 1a exhibits intense bisignate exciton coupling-type induced circular dichroism for its long wavelength absorption near 400nm.

Full text of DOI:10.1016/S0040-4020(02)00827-X

Tetrahedron 58 (2002) 7411–7421
Atropisomerism in linear tetrapyrroles
*
Stefan E. Boiadjiev and David A. Lightner
Department of Chemistry, University of Nevada, Reno, NV 89557-0020, USA
Received 20 May 2002; revised 3 July 2002; accepted 11 July 2002
Abstract—Novel bilirubin and biliverdin congeners with propionic acids replaced by o-carboxyphenyl exhibit diastereomerism due to axial
chirality about the carbon–carbon single bond linking the o-carboxyphenyl group to a pyrrole ring. Evidence for atropisomerism was found
even in the monopyrrole precursor, ethyl 3,5-dimethyl-4-(o-carboxyphenyl)pyrrole-2-carboxylate. Like bilirubin, o-carboxyphenyl rubin 1a
adopts an intramolecularly hydrogen-bonded ridge-tile conformation in nonpolar solvents. In solutions containing optically active amines or
human serum albumin 1a exhibits intense bisignate exciton coupling-type induced circular dichroism for its long wavelength absorption near
400 nm. q 2002 Elsevier Science Ltd. All rights reserved.
1. Introduction
m and p-carbo(alko)xyphenyl isomers to the Suzuki
coupling method and pursue a more classical synthesis of
4 (Scheme 1). For the latter, we required 3-(o-carboxy-
phenyl)pentane-2,4-dione (5a), which could be produced in
high yield by reaction of the sodium o-bromobenzoate with
the sodium salt of pentane-2,4-dione.5,6 The resulting acid
(5a) was converted to its methyl ester (5b) by reaction with
diazomethane, but Fischer–Knorr condensation of it with
diethyl oximinomalonate using zinc in acetic acid led to
only a 26% isolated yield of pyrrole methyl ester 4b—along
with substantial isocoumarin side-product, 4-acetyl-3-
methyl-1H-2-benzopyran-1-one.6 This mixture was sepa-
rated only with considerably difficulty. The problems
encountered in converting 5b to 4b were largely overcome
by treating the acid (5a) under the same Fischer–Knorr
pyrrole-forming condensation conditions to afford a 65%
isolated yield of pyrrole acid 4a.
Atropisomerism has become synonymous with biphenyl or
biaryl stereochemistry, where sufficiently high barriers to
rotation about the interconnecting sp2–sp2 C–C bond may
lead to isolatable stereoisomers.1 Studies of atropisomerism
about a pyrrole to phenyl sp2–sp2 C–C bond are few,2 and
in the following we describe one such example (the
monopyrrole 4) and show how it can be detected in a
dipyrrole (3c) and in various new linear tetrapyrrole
derivatives. Among the latter, we describe the syntheses
and stereochemistry of novel bilirubin (1) and biliverdin
(2) analogs whose two propionic acids are replaced by
o-carboxyphenyls (Fig. 1).
2. Results and discussion
2.1. Synthesis
Saponification of 4a or 4b to its diacid and condensation
with 5-bromomethylene-4-ethyl-3-methyl-2-oxo-1H-pyr-
role7 afforded yellow dipyrrinone 3a in 60% yield after
treatment with CH2N2. Attempted oxidative coupling of
dipyrrinone acid 3b gave no verdin, apparently due to
interference (by proton transfer) from the free CO2H group;
however, oxidative coupling of dipyrrinone ester 3a gave a
mixture of verdins (2) in 84% yield. Standard verdin
reduction with NaBH4 gave rubin ester 1b, which was
saponified to rubin diacid 1a in 91% isolated yield.
Pyrroles of the type illustrated by 4 (Fig. 1), whether with an
o-carboethoxyphenyl, o-carbomethoxyphenyl or o-carboxy-
phenyl group on the pyrrole nucleus are unknown, as are
variants with these same o-substituents located m or p on the
benzene ring. In principle, they might be prepared3 by a
classical Fischer–Knorr synthesis from the appropriate
3-phenylpentane-2,4-dione, a reaction not yet reported, and
a dione apparently unknown. An alternative synthesis might
follow one developed by Chang and Bag4 using a Suzuki
coupling of a b-bromopyrrole with phenylboronic acid.
Although m and p-carboxy (or carboalkoxy) phenylboronic
acids have been described in the literature, the o-isomer had
not. We therefore decided to reserve the pyrrole syntheses of
2.2. Characterization
The structures of monopyrroles 4, dipyrrole 3 and
tetrapyrroles 1 and 2 follow logically from the method of
synthesis (see Scheme 1) and the compounds were
characterized by spectroscopy, especially 13C NMR,
which showed the expected characteristic carbon reso-
nances for the pyrrole units and the o-carboxy (or
Keywords: bilirubin; conformation; hydrogen bonding.
*
Corresponding author. Tel.: þ1-775-784-4980; fax: þ1-775-784-6804;
e-mail: lightner@scs.unr.edu
0040–4020/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4020(02)00827-X

7412
S. E. Boiadjiev, D. A. Lightner / Tetrahedron 58 (2002) 7411–7421
Figure 1. Atropisomeric mesobilirubins (1), mesobiliverdins (2) and
monopyrroles (4), with restricted rotation about the pyrrole–phenyl bond.
The rubins (1) and verdins (2) are shown for simplicity in the conventional
linear representations.
Scheme 1. Reagents and conditions: (a) NaOH/EtOH–H2O, D, then HCl;
(b) NaBH4/CH3OH; (c) p-chloranil/HCOOH–CH2Cl2; (d) NaOH/EtOH–
H2O, D, then HNO3; (e) 3-methyl-4-ethyl-5-bromomethylene-2-pyrroli-
none, CH3OH, D; (f) CH2N2; (g) CH3CH2OH, DCC, DMAP; (h) diethyl
oximinomalonate, Zn, HOAc, NaOAc; (i) Cu2Br2/EtOH, D, then HCl.
carboalkoxy) phenyl group. In 1–4, the four benzene ring
hydrogens constitute an ABCD system of chemical shifts
and splittings. In 4, for example, the most deshielded
hydrogen (HD) is ortho to the CO2R group, as predicted by
theory and confirmed by its NOE to the –OCH3 hydrogens
of 4b. The most shielded hydrogen (HA) is ortho to the
pyrrole ring (meta0 to the CO2R), as shown by an NOE
between it and the pyrrole methyls at C(3) and C(5).
The apparent first order coupling constants for HD and HA
are in accord with predictions of one vicinal coupling
(3J,7–8 Hz) and one W-coupling (4J,1 Hz) with the inner
aromatic hydrogens. The assignments of the two remaining
(inner) aromatic hydrogens, HB and HC, were made as meta
and para to the CO2R group, respectively, on the basis of an
NOE seen between HA and HC, and between HB and HD.
The apparent first-order coupling constants of HB and HC
are consistent with predictions: two vicinal couplings
(3J,7–8 Hz) and one W-coupling (4J,1 Hz).
The UV–Vis spectra of verdin 2 differed little from that of
mesobiliverdin-XIIIa dimethyl ester, while those of rubin
1a differed little from the UV–Vis spectra of mesobilirubin-
XIIIa, indications that the o-carboxyphenyl groups do not
perturb the tetrapyrrole chromophores and suggestive of an
orthogonal relationship between the pyrrole ring and
attached phenyl ring.
Rubin 1a was more soluble in CHCl3 than mesobilirubin-
XIIIa and sufficiently soluble in CHCl3 for molecular

S. E. Boiadjiev, D. A. Lightner / Tetrahedron 58 (2002) 7411–7421
7413
Figure 2. (A) Partial 1H NMR spectrum of monopyrrole 4b in CDCl3 at 258C showing an ABX3 spin system for the CH2 protons of the ethyl ester. The inverted
spectrum is of the spin-simulated ABX3 system, showing only the AB part and generated using nA¼2154.4 Hz, nB¼2161.1 Hz, nX¼679.8 Hz, JAB¼10.9 Hz,
and JAX¼JBX¼7.1 Hz. The chemical shifts nA and nB, and the coupling constant JAB were determined by spin-decoupling of X3 and were used as input
simulation parameters. (B) Stereo-diagrams illustrating diastereotopic CHAHB hydrogens of the ethyl ester atropisomeric 4b enantiomers.
weight determination by vapor pressure osmometry at 458C
for concentrations in the range 1022–1023 M. As with most
rubins capable of intramolecular hydrogen bonding, 1a was
monomeric in CHCl3: measured MW 655^30 g/mol (FW
685 g/mol). Surprisingly, its dimethyl ester (1b) was also
monomeric in CHCl3: measured MW 733^20 g/mol (FW
713 g/mol). This finding is unusual, as most rubin dimethyl
esters are dimeric in CHCl3.8 Apparently, the phenyls
interfere with the conformation required for dimer
formation.
somewhat more polar than its mesobilirubin-XIIIa analog—
possibly because the large p-electron rich phenyl groups of
the o-benzoic acids are adsorbed to the silica more strongly
than the simple CH2–CH2– units of the propionic acid
chains.
2.3. Stereochemistry
First in monopyrrole 4b, and then in its progeny, we
detected an unexpected event in the 1H NMR that provided
the first indication of axial chirality, that 4b is a mixture
of enantiomers. Although all proton and carbon (APT)
resonances had the expected, typical chemical shifts and
splitting patterns in CDCl3, the methylene group of the ethyl
ester did not. In the 1H NMR spectrum it showed at least 14
lines, suggesting an ABX3 spin system (Fig. 2). Selective
decoupling of X3 at 1.36 ppm, revealed an AB pattern: two
different, tightly coupled resonances at 4.310 and
On silica gel TLC chromatographic analysis, however,
rubin ester 1b (Rf 0.10, 1% CH3OH in CH2Cl2) is more
polar than rubin acid 1a (Rf 0.54), as is typically found for
rubins capable of intramolecular hydrogen bonding, e.g.
mesobilirubin-XIIIa (Rf 0.77, 1% CH3OH in CH2Cl2) and
its dimethyl ester (Rf 0.21). Somewhat contradictorily, these
chromatographic data suggest that while 1a is most
probably intramolecularly hydrogen-bonded, it is also
2
4.323 ppm, and J¼10.9 Hz. We take this as evidence for

7414
S. E. Boiadjiev, D. A. Lightner / Tetrahedron 58 (2002) 7411–7421
Figure 3. Partial 1H NMR spectra of verdin 2 in CDCl3 at 258C showing: (A) doubled sets of signals in the aromatic region, 2:3 or 3:2 integral ratio, and (B)
doubled OCH3 ester signals, 3:2 (right) and (left) doubled C(10) methine resonances. The resonance at 5.98 is due to the C(5)/C(15) methines.
diastereotopicity and thus the presence of an axis of chirality
in 4b. Although NMR evidence for atropisomerism could be
detected in 4b, attempts to resolve the acid (4a) by classical
methods, including crystallization of salts with alkaloids
and separation of its l-menthyl ester derivatives, were
unsuccessful.
diastereotopicity of the sort seen in 4b. However, in ethyl
ester 3c, again the ethyl ester CH2 hydrogens exhibited
diastereotopicity. The chemical shift difference of the HA
and HB resonances in 3c is similar to that of 4b.
Consistent with the greater stability of the syn-Z configu-
ration of their component dipyrrinones, NMR NOE and
crystallographic studies show that verdins and rubins adopt
the favored syn-Z configuration.9 The general shape of
verdins is thus porphyrin-like, with a lock-washer type
conformation, while that of rubins is bent into a ridge-tile
shape.9 The local stereochemistry about the pyrrole-to-
phenyl bond is manifested differently in the tetrapyrroles
due to an accumulation of two elements of chirality. Thus,
verdin 2 can be expected to exist in at least two limiting
diastereoisomeric forms, one with the o-CO2CH3 groups syn
(relative to the average horizontal plane of the lock-washer
Extensive NMR and X-ray crystallographic studies firmly
established that dipyrrinones with NHs and b-alkyl groups
adopt the syn-Z configuration.9 This is particularly true in
CDCl3 solvent, where dipyrrinones form tightly inter-
molecularly hydrogen-bonded dimers,9 –12 but also in
(CD3)2SO, where the solvent participates in hydrogen
bonding to the NHs.9 Where E-configuration isomers of
dipyrrinones of this type have been prepared, they are
unstable and revert to the more stable Z.9,13–15 NOE studies
of 3 are consistent with the syn-Z configuration. Thus NOEs
are seen between the pyrrole and lactam NHs, and between
the C(5) methine and the C(7) methyl and C(3)–CH2. NOEs
were not detected between the C(5)–H and the pyrrole NH
(as would come from the anti-Z configuration), nor between
the C(5)–H and the lactam NH (as would come from the
E-configuration). For dipyrrinones 3a and 3b, derived from
monopyrroles 4b and 4a, we found no evidence for
1
shaped verdin) and one with them anti (Fig. 1). In the H
NMR spectrum of 2 one sees a doubling of the aromatic
hydrogen signals (Fig. 3A) with the two sets of signals
appearing in a 40:60 ratio. Doubled resonances also appear
for the C(7)/C(13) methyls, the ester OCH3 and the
C(10) methine (Fig. 3B). The C(5)/C(15) methines do
not, however, show a doubling (Fig. 3B, left). The

S. E. Boiadjiev, D. A. Lightner / Tetrahedron 58 (2002) 7411–7421
7415
Figure 4. Partial 1H NMR spectra of the C(10) CH2 region of 1a in (A) CDCl3, (B) (CD3)2SO, (C) CD3OD at 258C. Only a singlet is found in (A). In (B), and to
a very small extent in (C), a singlet is superimposed on a pair of doublets (65 and 12%, respectively). The singlet corresponds to C(10) CH2 of the anti
conformation, the AB pattern corresponds to the syn.
considerable shielding of the C(10) methine from its normal
chemical shift of 6.9–7.0 ppm is probably due to p-facial
shielding by the nearby phenyl rings.
1b and 2, e.g. by exhibiting only a singlet for the C(10) CH2,
an indication that 1a adopted exclusively the anti confor-
mation. Consistent with this assignment, the C(7) and C(13)
methyls (1.81 ppm) and C(10) CH2 (3.62 ppm) are more
shielded in 1a than in mesobilirubin-XIIIa (2.14 and
4.06 ppm, respectively). The anti diastereomer (Fig. 1) of
1a can adopt a ridge-tile conformation with both CO2H
groups hydrogen-bonded intramolecularly to dipyrrinones
(Fig. 5),16 and the dipyrrinone NH chemical shifts
(9.46 ppm, pyrrole; 10.47 ppm, lactam) provide supporting
evidence. (In mesobilirubin-XIIIa, known to adopt an
intramolecularly hydrogen-bonded ridge-tile conformation
in CHCl3, the pyrrole NH resonates at 9.16 ppm, and the
lactam at 10.59 ppm.) More direct evidence for hydrogen
bonding and the syn-Z configuration of the dipyrrinones in
1a-anti was found by NOE experiments. A ROESY
spectrum indicated strong NOEs between the pyrrole and
lactam NHs, and between the C(5)/C(15) methine and the
C(3)/C(17)–CH2CH3 and the C(7)/C(13) methyls—all
consistent with a syn-Z conformation in the dipyrrinones.
A steady state NOE difference spectrum in CDCl3
confirmed the syn-Z stereochemistry and also showed an
NOE from the lactam NH to the o-carboxylic acid proton.
The latter confirms that intramolecular hydrogen bonding is
likely. A pulsed field gradient spin-echo NOE experiment
showed that when the C(10) CH2 protons were irradiated,
NOEs were found to the phenyl ring o0-hydrogens as well as
to the pyrrole NHs, thus strongly suggesting a rigid
conformation around the pyrrole–phenyl bond in nonpolar
solvents.
Signal doubling of the type seen by 1H NMR of verdin 2 is
also seen in rubin ester 1b and can be detected in diacid 1a
(Fig. 4). The C2-symmetric rubins (1a-anti, Fig. 1, and
1b-anti) have their C(10) CH2 hydrogens in identical
electronic environments and these hydrogens appear as a
singlet in CDCl3. In the CS-symmetric rubins (1a-syn, Fig.
1, and 1b-syn) the C(10) CH2 hydrogens lie in different
electronic environments and their resonances appear at
different chemical shifts as a four-line pattern, characteristic
of an AB spin system. The integral ratio of the C(10) CH2
resonances of 1a thus serves as a sensitive probe of
conformational (atropisomerism) preference.
The 1H and 13C NMR of 1a in CDCl3 differed from that of
Apparently, the atropisomeric conformational homogeneity
found for 1a in CDCl3 is forced by intramolecular hydrogen
bonding (Fig. 5) because when this constraint is lifted,
as found in polar solvents, the syn isomer may be detected
1
by H NMR (again by the C(10) CH2 signal). In the most
dramatic case, in (CD3)2SO, for example, we find a 35:65
ratio of anti/syn (Fig. 4). The same signal doubling and
intensity ratio are observed in the 13C NMR spectrum of 1a
in (CD3)2SO. In CD3OD, however, the anti still predomi-
nates, 88:12, over the syn. The C(10) CH2 resonances from
1a in (CD3)2SO did not change noticeably upon brief
warming from 50 to 1108C, nor did they in CDCl2CDCl2
from 20 to 1408C for a prolonged time. A survey of the
influence of solvents on the COOH, NH and C(10) CH2
Figure 5. Ball and Stick (Ref. 16) conformational representation of the
most stable conformer of 1a, with two dipyrrinones shaped like a ridge-tile
and hydrogen bonded intramolecularly to the opposing o-carboxyphenyl
groups.

7416
S. E. Boiadjiev, D. A. Lightner / Tetrahedron 58 (2002) 7411–7421
Table 1. Solvent influence on the carboxylic acid, pyrrole, lactam and C(10)–CH2 1H NMR chemical shifts and anti/syn diastereomer ratio of 1a at 258C
Solvent
1a
Acid
Lactam NH
Pyrrole NH
C(10)–CH2
syn
Ratio
anti
anti
syn
C6D6
CDCl3
THF-d8
CD2Cl2
CDCl2CDCl2
(CD3)2CO
CD3OD
2.3
4.7
7.3
8.9
10.4
20.7
32.6
36.2
36.7
46.5
6.2
14.70
13.60
13.75
13.74
13.45
13.79
–
10.80
10.47
10.21
10.46
10.19
10.40
–
10.35
9.57
9.46
9.51
9.37
9.30
9.48
–
9.30
3.39
3.62
3.58
3.61
3.54
3.63
3.72
3.54
3.83
3.73
3.93
3.80
100
100
100
100
100
100
88
100
42
35
72
0
0
0
0
0
0
12
0
58
65
28
24
3.75, 3.83
CD3CN
–
(CD3)2NCDO
(CD3)2SO
CD3COOD
Pyridine-d5
13.10
12.32
–
sh 9.67, 9.64
9.77, 9.71
–
10.38, 10.28
10.24, 10.08
–
3.80, 3.88
3.58, 3.78
3.88, 4.04
4.18, 4.31
12.3
12.47
sh 10.91, 10.59
10.90, 10.52
76
Concentration 2£1023 M.
a
Solvent dielectric constant for protiosolvents from Gordon, A. J.; Ford, R. A. The Chemist’s Companion; Wiley: New York, 1972; pp 4–13.
Table 2. Solvent dependence of the NH 1H NMR chemical shifts and diastereomer ratio of 1b
Solvent
Pyrrole NH
Lactam NH
Ratioa anti/syn
C6D6
CDCl3
CD3CN
(CD3)2SO
11.04, 11.44
11.19, 11.68
37:63
79:21
48:52
46:54
8.92, 10.85 (9.84, 10.97)b
10.79, 10.88
10.13, 10.20
9.82, 10.50 (10.21, 10.58)b
10.53
9.72, 9.79
Solutions 2£1023 M at 258C.
a
Measured by the integral intensity of singlet anti-1b and AB syn-1b signals of the C(10)–CH2 (the italicized chemical shifts dominate).
Concentration 8£1023 M.
b
chemical shifts and the anti/syn diastereomer ratio is
summarized in Table 1. Solvents that interfere with
intramolecular hydrogen bonding appear to have the
greatest influence in raising the relative amount of syn
diastereomer, where at least one carboxyl group cannot
engage in intramolecular hydrogen bonding and is freely
solvated.
In the rubin dimethyl ester (1b), where intramolecular
hydrogen bonding is not expected to play a dominating
role, the anti/syn diastereomer ratios are also solvent
dependent. Here, however, substantial amounts of syn
diastereomer are present in aprotic solvents (Table 2), and
the ratios do follow a logical progression. Curiously,
whereas the NH chemical shifts of 1b-anti are noticeably
concentration-dependent, with the pyrrole NH being
deshielded by ,0.9 ppm and the lactam NH deshielded
by ,0.4 ppm following a 3–4 fold concentration increase,
in 1b-syn they are not affected much. Although the data
might suggest an aggregation phenomenon, vapor phase
osmometry studies of 1b (and 1a) clearly indicate that
these compounds are monomeric in CHCl3 in the concen-
tration range 1022–1023 M at 458C. While it is not unusual
to find that bilirubins are monomeric in CHCl3, when they
are capable of intramolecular hydrogen bonding it is
unusual to find bilirubin esters that are not dimeric in
CHCl3.8
2.4. Rotation barrier
Variable high temperature 1H NMR experiments of verdin 2
in CDCl2CDCl2 solvent showed coalescence at 413 K
(Fig. 6) of the diastereotopic methyl ester CH3 groups.
Diastereotopic because 2 is a mixture of interconverting M
Figure 6. Variable temperature partial 1H NMR spectrum of 2 in
CDCl2CDCl2 solvent.

S. E. Boiadjiev, D. A. Lightner / Tetrahedron 58 (2002) 7411–7421
7417
max
431
Figure 7. UV–Vis spectra of 1.60£1025 M rubin 1a. In CHCl3: 1
sh
419
max
59,300, 1 57,400. In CH3OH: 1
421
sh
62,300, 1 57,800. In (CH3)2SO:
401
Figure 9. Induced CD of rubin 1a in CHCl3 in presence of cinchona
alkaloids.
max
1
max
399
59,700, 1
60,600.
416
of nearly the same values as in (CH3)2SO. The batho-
chromic spectral shifts observed for the solvent changes in
the UV–Vis spectra of 1a (Fig. 7) are probably associated
with a change toward more complete intramolecular
hydrogen bonding in CHCl3 (Fig. 5).
and P helical conformers (with a low interconversion barrier
,10 kcal/mol), each capable of atropisomerism about the
pyrrole to phenyl bond axis of chirality. Analysis by
variable temperature 1H NMR (Fig. 6) indicates a rate
constant (Kc) at coalescence temperature of ,40 s21 from
pffiffi
the equation: Kc ¼ pDn 2. Using the Eyring equation
(DG ‡¼RTc (23.762ln(Kc/Tc)), where Kc¼40 s21 and
Tc¼413 K, we estimate an interconversion barrier of
DG ‡¼21.4 kcal/mol for rotation about the pyrrole to phenyl
bond—or too low to isolate the atropisomers.
In the presence of chiral amines (cinchona alkaloids),
intense bisignate long wavelength CD Cotton effects are
observed for 1a (but not 1b) (Fig. 9). As with bilirubin, the
signed order of the Cotton effects in CHCl3 correlates with a
negative exciton chirality (and negative torsion angle
between relevant electric dipole transition moments) when
quinine or cinchonidine are present, and with a positive
exciton chirality when cinchonine or quinidine are present.
The induced CD magnitudes are similar to those observed
for bilirubin.
2.5. Circular dichroism
The two dipyrrinone chromophores in bilirubins interact
as an exciton system, as observed by UV–Vis spectro-
scopy and especially circular dichroism (CD) spectroscopy.
Rubin 1a and its dimethyl ester (1b) offer no exception.
Thus, in (CH3)2SO, their UV–Vis spectra are dimpled, with
In alkaline buffer, and in the presence of human serum
albumin (HSA), bisignate CD Cotton effects are also
observed. The exciton chirality of 1a in the presence of
HSA is the same as that observed for bilirubin, and the
magnitude of the CD is also similar (Fig. 10).
max
399
sh
max
max
1
1
60,600, 1
59,900 for 1b (Fig. 8). In CHCl3 and in CH3OH, the
59,700 for 1a (Fig. 7) and 1
62,500,
397
416
418
spectra of 1a are also dimpled (Fig. 7), with lmax and 1 max
Figure 8. UV–Vis spectra of 1.89£1025 M rubin dimethyl ester 1b. In
max
415
max
62,300, 1
399
max
61,800. In CH3OH: 1
411
CHCl3: 1
61,900, 1s3h99 61,400. In
Figure 10. Induced CD of rubin 1a from complexation with HSA in
aqueous alkaline buffers.
(CH3)2SO: 1s4h18 59,900, 1
62,500.
max
397

7418
S. E. Boiadjiev, D. A. Lightner / Tetrahedron 58 (2002) 7411–7421
3. Conclusions
Fisher. Argon and nitrogen were from Air Products.
Solvents and reagents were used directly as provided by
the vendor, except for the Fisher HPLC-grade solvents,
which were further purified by standard procedures as
described in detail in Ref. 18. The spectral data were
obtained in spectral grade solvents, used as provided by
Aldrich or by Fisher.
Synthetic rubin 1a is found to adopt preferentially a
conformation shaped like a ridge-tile or half-opened book
where the o-benzoic acids are engaged in intramolecular
hydrogen bonding to the pigment’s dipyrrinone com-
ponents. As in natural bilirubin, the yellow pigment of
jaundice, such intramolecular hydrogen bonding stabilizes
the ridge-tile conformation. Unlike bilirubin the o-benzoic
acid components present the opportunity for atropisomer-
ism, for which a barrier of ,21 kcal/mol has been estimated
by DNMR measurements.
4.1.1. Ethyl 4-(o-carboxyphenyl)-3,5-dimethyl-1H-pyr-
role-2-carboxylate (4a). To a preheated (,808C) mixture
of 9.81 g (150 mgA) of zinc, 10.3 g (125 mmol) of
anhydrous sodium acetate and 40 mL of acetic acid were
added simultaneously in small portions 11.01 g (50 mmol)
of 3-(o-carboxyphenyl)-pentane-2,4-dione (5a)5 and a
solution of 18.92 g (100 mmol) of diethyl oximinomalo-
nate19 in 12 mL of acetic acid during 45 min while
maintaining the temperature at 90–958C. After the additions
were complete, the mixture was heated at vigorous reflux for
4 h and then poured into 400 mL of ice and water. After
stirring for 30 min, the aqueous layer was decanted, and the
semisolid product was dissolved in a minimum volume of
458C ethanol (,65 mL) and then precipitated by slow
addition of ice-cold water over 1 h. After cooling for an
additional hour in an ice bath, the crude product was
collected by filtration, washed with water and dried.
Purification of the product by radial chromatography
(1.5–5.0% v/v CH3OH in CH2Cl2) and recrystallization of
the isolated solid from ethyl acetate–hexane afforded 9.33 g
(65%) of pyrrole 4a. It had mp 199–2008C; 1H NMR: d 1.35
(3H, t, J¼7.1 Hz), 2.03 (3H, s), 2.14 (3H, s), 4.32 (1H,
ABX3, 3J¼7.1 Hz, 2J¼11.0 Hz), 4.34 (1H, ABX3,
3J¼7.1 Hz, 2J¼11.0 Hz), 7.22 (1H, dd, 3J¼7.6 Hz,
4. Experimental
4.1. General procedures
Nuclear magnetic resonance spectra were obtained on a
Varian Unity Plus spectrometer at 11.75 T magnetic field
1
strength operating at H frequency of 500 MHz and 13C
frequency of 125 MHz. CDCl3 solvent was used throughout
(unless otherwise specified), and chemical shifts are
reported in d ppm, referenced to the residual CHCl3 1H
signal at 7.26 ppm and CDCl3 13C signal at 77.00 ppm.
J-modulated spin-echo (Attached Proton Test) and gHMBC
experiments were used to obtain the 13C NMR assignments.
The apparent multiplicities and values of spin–spin
4
coupling constants (3J, J)17 of all aromatic protons were
confirmed by single-frequency homonuclear decoupling
experiments and by the Varian simulation routine. All
UV–Vis spectra were recorded on a Perkin–Elmer Lambda
12 spectrophotometer and the circular dichroism spectra
were recorded on a JASCO J-600 dichrograph. Gas
chromatography–mass spectrometry analyses were carried
out on a Hewlett–Packard 5890A gas chromatograph (30 m
DB-1 column) equipped with a Hewlett–Packard 5970
mass selective detector. HPLC analyses were carried out on
a Perkin–Elmer series 410 high-pressure liquid chromato-
graph with a Perkin–Elmer LC-95 UV–Vis spectrophoto-
metric detector (set at 420 nm for rubinoid compounds)
equipped with a Beckman Altex ultrasphere IP 5 mm C-18
ODS column (25£0.46 cm) kept at 348C. The flow rate was
1 mL per minute, and the mobile phase was 0.1 M di-n-
octylamine acetate buffer in 5% H2O in methanol (v/v) with
pH 7.7 at 228C. Radial chromatography was carried out on
Merck silica gel PF254 with CaSO4 binder preparative layer
grade, using a Chromatotron (Harrison Research, Inc., Palo
Alto, CA) with 1, 2, or 4 mm thick rotors and analytical
thin-layer chromatography was carried out on J. T. Baker
silica gel IB-F plates (125 mm layer). Melting points were
determined on a Mel-Temp capillary apparatus and are
uncorrected. The combustion analyses were carried out by
Desert Analytics, Tucson, AZ.
3
4
4J¼1.0 Hz), 7.42 (1H, ddd, J¼ 7.6, 7.5 Hz, J¼1.0 Hz),
3
4
7.55 (1H, ddd, J¼7.6, 7.5 Hz, J¼1.2 Hz), 8.04 (1H, dd,
3J¼7.6 Hz, J¼1.2 Hz), 9.50 (1H, brs), 10.34 (1H, very
4
brs) ppm; 13C NMR: d 11.40, 11.73, 14.52, 60.11, 117.25,
123.12, 127.06, 127.33, 130.75, 130.92, 131.05, 131.98,
132.86, 135.77, 162.58, 171.22 ppm. MS m/z (rel. abund.):
287 (Mþz, 100%), 270 (41%), 242 (55%), 214 (61%) amu.
Anal. calcd for C16H17NO4 (287.3): C, 66.88; H, 5.96; N,
4.88. Found: C, 66.79; H, 5.78; N, 4.79.
4.1.2. Ethyl 4-(o-methoxycarbonylphenyl)-3,5-dimethyl-
1H-pyrrole-2-carboxylate (4b). Methyl ester 4b was
obtained by the same procedure as for 4a by using 3-(o-
methoxycarbonylphenyl)pentane-2,4-dione (2)5—except
that after separating the crude product, it was dissolved in
150 mL of CH2Cl2 and treated with 100 mL of 1 M aq
NaOH with vigorous stirring for 6 h. The organic layer was
washed with water, dried (anh. MgSO4), and filtered. The
solvent was evaporated under vacuum. Radial chroma-
tography of the residue (hexane–ethyl acetate, gradient
5.5:1 to 2.5:1 v/v) followed by recrystallization of the
isolated solid from ethanol–water afforded 3.95 g (26%) of
pyrrole 4b. It had mp 135–1368C; 1H NMR: d 1.36 (3H, t,
J¼7.1 Hz), 2.09 (3H, s), 2.10 (3H, s), 3.72 (3H, s), 4.30 (1H,
ABX3, 3J¼7.1 Hz, 2J¼10.9 Hz), 4.32 (1H, ABX3, 3J¼
7.1 Hz, 2J¼10.9 Hz), 7.21 (1H, dd, 3J¼7.6 Hz, 4J¼1.2 Hz),
4-Dimethylaminopyridine (DMAP) and dicyclohexylcarbo-
diimide (DCC) were from Aldrich. Di-n-octylamine was
from Fluka. Methanol, chloroform, ethyl acetate and
dichloromethane were HPLC grade from Fisher. Sodium
borohydride, p-chloranil, sodium acetate, sodium hydrox-
ide, anhydrous sodium sulfate, anhydrous magnesium
sulfate, sodium nitrate, sodium bicarbonate, nitric acid,
hydrochloric acid, 96% formic acid and ethanol were from
3
4
7.39 (1H, ddd, J¼7.6, 7.4 Hz, J¼1.2 Hz), 7.52 (1H, ddd,
4
3
4
3J¼7.4, 7.6 Hz, J¼1.3 Hz), 7.90 (1H, dd, J¼7.6 Hz, J¼
1.3 Hz), 8.69 (1H, brs) ppm; 13C NMR: d 11.15, 11.84,
14.58, 52.02, 59.70, 117.13, 123.74, 126.91, 127.05,
129.69, 129.91, 131.31, 132.06, 132.67, 135.58, 161.81,

S. E. Boiadjiev, D. A. Lightner / Tetrahedron 58 (2002) 7411–7421
7419
168.25 ppm. MS m/z (rel. abund.): 301 (Mþz, 100%), 270
(8%), 256 (10%), 223 (83%) amu. Anal. calcd for
C17H19NO4 (301.3): C, 67.76; H, 6.36; N, 4.65. Found: C,
67.65; H, 6.38; N, 4.74.
8.08, 9.96, 11.64, 14.85, 17.16, 97.62, 121.86, 122.16,
122.40, 122.84, 126.35, 127.68, 129.04, 129.54, 130.43,
132.07, 133.91, 134.90, 147.23 ppm.
To a mixture of 350 mg (1.00 mmol) of crude acid 3b from
above, 4 mL of anh CH2Cl2, 248 mg (1.20 mmol) of DCC,
and 12.2 mg (0.1 mmol) of DMAP was added 175 mL
(3.00 mmol) of anhydrous ethanol, and the mixture was
stirred for 16 h. The solid by-product was separated by
filtration. The filtrate was diluted with 100 mL of CH2Cl2
and washed consecutively with 5% aq NaHCO3 (100 mL),
5% aq HCl (50 mL) and water (2£50 mL). After drying (anh
Na2SO4), filtration and evaporation, the residue was purified
by radial chromatography (2–3% CH3OH in CH2Cl2 v/v).
Recrystallization of the pure solid fractions from CH2Cl2–
CH3OH afforded 309 mg (82%) of dipyrrinone ethyl ester
3c. It had mp 246–2478C; 1H NMR: d 1.13 (3H, t,
J¼7.0 Hz), 1.19 (3H, t, J¼7.7 Hz), 1.95 (3H, s), 2.00 (3H,
s), 2.30 (3H, s), 2.56 (2H, q, J¼7.7 Hz), 4.15 (1H, ABX3,
3J¼7.0 Hz), 4.16 (1H, ABX3, 3J¼7.0 Hz), 6.19 (1H, s), 7.26
4.1.3. 3-Ethyl-8-(o-methoxycarbonylphenyl)-2,7,9-tri-
methyl-1,10-dihydro-11H-dipyrrin-1-one (3a). A mixture
of 4.31 g (15.0 mmol) of monoester 4a (or 15.0 mmol of
diester 4b), 6.00 g (150 mmol) of sodium hydroxide, 90 mL
of ethanol, and 25 mL of water was heated at reflux for 4 h.
After cooling, the ethanol solvent was evaporated under
vacuum, and the residue was diluted with 25 mL of 50% aq
NaNO3. The mixture was cooled to 2208C and slowly
acidified with a solution of conc HNO3 in 50% aq NaNO3
(1:5 v/v). The product was separated by filtration, washed
with cold water and dried overnight under vacuum. This
crude diacid (obtained in a quantitative yield) was used
immediately in the following step without further
characterization.
3
4
3
A mixture of the diacid from above, 3.46 g (16.0 mmol) of
5-bromomethylene-4-ethyl-3-methyl-2-oxo-1H-pyrrole,7
100 mL of anhydrous methanol and 1 drop of conc H2SO4
was heated at reflux for 10 h. Then approximately 35 mL of
CH3OH were removed by distillation, and the remaining
solution was chilled overnight at 2208C. The product was
separated by filtration, suspended in 100 mL of CH3OH and
45 mL of CHCl3 and treated for 10 min with ethereal
diazomethane (generated from 100 mmol of N-nitroso-N-
methylurea). Excess CH2N2 was destroyed with acetic acid.
Then the solvents were evaporated under vacuum, and the
residue was purified by radial chromatography (2–3%
CH3OH in CH2Cl2 v/v). Recrystallization of the isolated
solid from CH2Cl2–CH3OH afforded 3.29 g (60%) of
dipyrrinone 3a. It had mp 289–2918C (decomp); 1H
NMR: d 1.19 (3H, t, J¼7.6 Hz), 1.95 (3H, s), 2.01 (3H, s),
2.31 (3H, s), 2.56 (2H, q, J¼7.6 Hz), 3.73 (3H, s), 6.20 (1H,
(1H, dd, J¼7.5 Hz, J¼1.0 Hz), 7.38 (1H, ddd, J¼7.5,
4
3
4
7.6 Hz, J¼1.0 Hz), 7.51 (1H, ddd, J¼7.5, 7.6 Hz, J¼
3
4
1.3 Hz), 7.88 (1H, dd, J¼7.5 Hz, J¼1.3 Hz), 10.58 (1H,
brs), 11.40 (1H, brs) ppm; 13C NMR: d 8.51, 10.10, 12.03,
13.92, 15.04, 17.97, 60.74, 101.41, 122.43, 122.50, 123.19,
125.01, 126.65, 127.34, 129.64, 131.00, 131.80, 132.44,
132.82, 136.10, 148.36, 168.50, 174.14 ppm. Anal. calcd for
C23H26N2O3 (378.5): C, 72.99; H, 6.93; N, 7.40. Found: C,
72.64; H, 6.77; N, 7.39.
4.1.5. 3,17-Diethyl-8,12-bis-(o-methoxycarbonylphenyl)-
2,7,13,18-tetramethyl-(21H,24H)-bilin-1,19-dione (2). A
mixture of 729 mg (2.00 mmol) of dipyrrinone 3a, 1.23 g
(5.00 mmol) of p-chloranil, 440 mL of CH2Cl2, and 22 mL
of 96% formic acid was heated at reflux for 24 h. The
volume of the mixture was reduced to one-half by
distillation, and reflux was continued for 6 h. Then the
mixture was chilled overnight at 2208C. A solid separated
and was removed by filtration and discarded. The blue
filtrate was carefully neutralized with 5% aq NaHCO3 until
effervescence ceased then washed with 4% aq NaOH
(2£100 mL), H2O (4£100 mL), and dried (anh Na2SO4).
After filtration and evaporation of the solvent under
vacuum, the crude product was purified by radial chroma-
tography (gradient CH2Cl2–CH3CO2H–CH3OH¼100:3:3
to 100:3:7 v/v/v). The combined pure fractions were washed
with 1% aq NaHCO3 and H2O, then dried (anh Na2SO4).
After filtration, the solution was evaporated and the residual
was recrystallized from CHCl3–hexane to afford 594 mg
(84%) of syn and anti mesobiliverdins (2). The solid had mp
3
4
s), 7.26 (1H, dd, J¼7.6 Hz, J¼0.9 Hz), 7.38 (1H, ddd,
3J¼7.6, 7.4 Hz, 4J¼0.9 Hz), 7.52 (1H, ddd, 3J¼7.6, 7.4 Hz,
3
4
4J¼1.1 Hz), 7.89 (1H, dd, J¼7.6 Hz, J¼1.1 Hz), 10.57
(1H, brs), 11.38 (1H, brs) ppm; 13C NMR: d 8.53, 10.11,
12.06, 15.03, 17.96, 52.07, 101.47, 122.46, 122.52, 122.90,
124.87, 126.56, 127.32, 129.80, 131.16, 131.83, 132.14,
132.59, 136.30, 148.37, 168.67, 174.16 ppm. Anal. calcd for
C22H24N2O3 (364.4): C, 72.50; H, 6.64; N, 7.69. Found: C,
72.16; H, 6.41; N, 7.63.
4.1.4. 3-Ethyl-8-(o-ethoxycarbonylphenyl)-2,7,9-tri-
methyl-1,10-dihydro-11H-dipyrrin-1-one (3c). A mixture
of 729 mg (2.00 mmol) of methyl ester 3a, 30 mL of 10% aq
NaOH, and 50 mL of ethanol was heated at vigorous reflux
for 4.5 h. The ethanol solvent was removed by distillation,
and the residue was cooled in an ice bath. Enough 10% aq
HCl was added slowly to bring pH to ,3. After stirring for
15 min, the crude dipyrrinone acid was separated by
filtration, washed with water (3£50 mL) and dried under
vacuum to afford 670 mg (96%) of 3b, which did not melt
but began to decompose at temperatures $1908C). It had 1H
NMR ((CD3)2SO): d 1.09 (3H, t, J¼7.6 Hz), 1.79 (3H, s),
1.90 (3H, s), 2.06 (3H, s), 2.51 (2H, q, J¼7.6 Hz), 5.96 (1H,
1
284–2888C; H NMR: d 1.25 (6H, t, J¼7.7 Hz), 1.86 (6H,
s), 1.92 (2.4H, s), 1.95 (3.6H, s), 2.54 (4H, q, J¼7.7 Hz),
3.68 (2.4H, s), 3.71 (3.6H, s), 5.98 (2H, s), 5.99 (0.6H, s),
6.01 (0.4H, s), 7.15 (0.8H, dd, 3J¼7.8 Hz, 4J¼1.0 Hz), 7.17
3
4
3
(1.2H, dd, J¼7.8 Hz, J¼1.0 Hz), 7.36 (2H, ddd, J¼7.5,
7.6 Hz, 4J¼1.0 Hz), 7.47 (0.8H, ddd, 3J¼7.5, 7.8 Hz,
4J¼1.3 Hz), 7.50 (1.2H, ddd, J¼7.6, 7.8 Hz, J¼1.3 Hz),
7.86 (1.2H, dd, 3J¼7.6 Hz, 4J¼1.3 Hz), 7.89 (0.8H, dd,
3J¼7.6 Hz, 4J¼1.3 Hz), 8.29 (2H, brs), 8.75 (1H, very
brs) ppm; 13C NMR: d 8.36, 9.94, 9.95, 14.42, 17.85, 52.18,
52.22, 96.29, 117.30, 117.50, 127.59, 127.61, 128.14,
128.39, 128.53, 128.55, 130.16, 130.36, 131.19, 131.42,
131.48, 131.57, 131.94, 132.12, 133.94, 134.19, 139.83,
3
4
3
3
s), 7.18 (1H, d, J¼7.6 Hz), 7.36 (1H, t, J¼7.6 Hz), 7.50
3
3
(1H, t, J¼7.6 Hz), 7.73 (1H, d, J¼7.6 Hz), 9.84 (1H, s),
10.53 (1H, s), 12.57 (1H, brs) ppm; 13C NMR ((CD3)2SO): d

7420
S. E. Boiadjiev, D. A. Lightner / Tetrahedron 58 (2002) 7411–7421
139.86, 140.21, 141.78, 141.86, 146.68, 149.40, 149.54,
167.64, 168.01, 172.57, 172.59 ppm. Anal. calcd for
C43H42N4O6 (710.8): C, 72.66; H, 5.96; N, 7.88. Found:
C, 72.60; H, 5.99; N, 7.92.
was added and the mixture was heated at reflux for 90 min
under Ar. After cooling, the mixture was diluted with
150 mL of H2O and 150 mL of CHCl3 then poured into
150 mL of 1% aq HCl. The product was extracted with
CHCl3 (4£50 mL), washed with H2O (4£100 mL) and dried
(anh Na2SO4). After filtration, the solvent was evaporated
under vacuum, and the residue was purified by radial
chromatography (gradient 1–2% CH3OH in CH2Cl2 v/v)
and recrystallization to afford 248 mg (91%) of bright
yellow syn and anti mesobilirubins (1a). The solid had mp
333–3388C (decomp .3058C); 1H NMR (in CDCl3
solvent—exclusively anti): d 1.14 (6H, t, J¼7.6 Hz), 1.81
(6H, s), 1.91 (6H, s), 2.49 (4H, q, J¼7.6 Hz), 3.62 (2H, s),
4.1.6. 3,17-Diethyl-8,12-bis-(o-methoxycarbonylphenyl)-
2,7,13,18-tetramethyl-(10H,21H,23H,24H)-bilin-1,19-
dione (1b). To a blue solution of 355 mg (0.500 mmol) of
mesobiliverdins 2 in 8 mL of CHCl3 and 75 mL of
anhydrous CH3OH kept at 108C was slowly added sodium
borohydride (1.13 g, 30.0 mmol) during 20 min. while
purging the mixture with N2. After stirring for 15 min, the
yellow mixture was diluted with 300 mL of ice-cold water,
slowly acidified by addition of 8 mL of acetic acid followed
by 6 mL of 10% aq HCl. The product was extracted with
CHCl3 (4£100 mL). The combined extracts were washed
with water until neutral, and then dried (anh Na2SO4). After
filtration, the solvent was evaporated under vacuum, and the
residue was purified by radial chromatography (gradient
1–3% CH3OH in CH2Cl2 v/v) and recrystallization from
CH2Cl2–CH3OH to afford 327 mg (92%) of mesobilirubin
dimethyl esters (1b). The solid had mp 268–2718C (decomp
.2508C); 1H NMR (in CDCl3 solvent ratio anti/syn¼
79:21): d 1.05 (1.2H, t, J¼7.6 Hz), 1.13 (4.8H, t, J¼7.6 Hz),
1.61 (1.2H, s), 1.80 (4.8H, s), 1.85 (4.8H, s), 1.89 (1.2H, s),
2.38 (0.8H, q, J¼7.6 Hz), 2.46 (3.2H, q, J¼7.6 Hz), 3.66
(1.2H, s), 3.95 (4.8H, s), 3.73 (1.6H, s), 3.74, 3.95 (0.4H,
AB, 2J¼15.8 Hz), 5.97 (1.6H, s), 5.99 (0.4H, s), 6.98–7.11
(2H, 2£m), 7.33–7.41 (4H, m), 7.72–7.74 (0.4H, m), 7.80
3
4
6.08 (2H, s), 7.18 (2H, dd, J¼7.5 Hz, J¼1.0 Hz), 7.47
3
4
3
(2H, ddd, J¼7.5, 7.8 Hz, J¼1.0 Hz), 7.56 (2H, ddd, J¼
7.5, 7.5 Hz, 4J¼1.5 Hz), 8.29 (2H, dd, 3J¼7.8 Hz, 4J¼
1.5 Hz), 9.46 (2H, s), 10.47 (2H, s), 13.60 (2H, brs) ppm; 1H
NMR (in (CD3)2SO solvent ratio anti/syn¼35:65): d 1.09
(2.1H, t, J¼7.6 Hz), 1.10 (3.9H, t, J¼7.6 Hz), 1.73 (3.9H, s),
1.77 (2.1H, s), 1.78 (2.1H, s), 1.79 (3.9H, s), 2.51 (2£2H,
2
2£q, J¼7.6 Hz), 3.57, 3.78 (1.3H, AB, J¼16.8 Hz), 3.74
(0.7H, s), 5.91 (0.7H, s), 5.93 (1.3H, s), 6.63 (1.3H, dd,
4
3
4
3J¼7.0 Hz, J¼1.0 Hz), 6.82 (0.7H, dd, J¼7.0 Hz, J¼
1.0 Hz), 7.23–7.31 (2£2H, 2£m), 7.66 (1.3H, dd, 3J¼
7.5 Hz, 4J¼1.4 Hz), 7.70 (0.7H, dd, 3J¼7.5 Hz, 4J¼1.4 Hz),
9.71 (0.7H, s), 9.77 (1.3H, s), 10.08 (0.7H, s), 10.24 (1.3H,
s), 12.32 (2H, s) ppm; 13C NMR (in CDCl3 solvent): d 8.03,
9.86, 14.84, 17.86, 24.20, 100.89, 123.56, 123.88, 124.17,
124.68, 127.75, 128.78, 129.91, 131.52, 131.53, 132.73,
133.73, 138.51, 148.54, 171.92, 174.92 ppm; 13C NMR (in
(CD3)2SO solvent the shifts of dominant syn are italicized):
d 8.08, 9.86, 9.89, 14.87, 17.17, 17.18, 23.91, 24.16, 97.77,
97.86, 122.02, 122.05, 122.10, 122.15, 122.60, 122.70,
123.06, 123.11, 126.18, 126.24, 128.24, 128.41, 128.82,
129.04, 130.06, 130.20, 130.43, 130.46, 132.02, 132.08,
132.89, 132.96, 134.80, 134.89, 147.09, 147.21, 168.94,
169.07, 171.96, 171.97 ppm. Anal. calcd for C41H40N4O6
(684.8): C, 71.91; H, 5.89; N, 8.18. Found: C, 72.06; H,
5.85; N, 8.14.
3
4
(1.6H, dd, J¼7.7 Hz, J¼1.1 Hz), 8.92 (1.6H, brs), 9.82
1
(1.6H, brs), 10.50 (0.4H, brs), 10.85 (0.4H, brs) ppm; H
NMR (in (CD3)2SO solvent ratio anti/syn¼46:54): d 1.09
(3H, t, J¼7.6 Hz), 1.10 (3H, t, J¼7.6 Hz), 1.71 (3H, s), 1.72
(3H, s), 1.78 (3H, s), 1.79 (3H, s), 2.51 (2£2H, 2£q, J¼
7.6 Hz), 3.51 (3H, s), 3.54 (3H, s), 3.56, 3.75 (1H, AB,
2J¼16.9 Hz), 3.67 (1H, s), 5.91 (1H, s), 5.92 (1H, s), 6.72
3
4
3
(1H, dd, J¼7.5 Hz, J¼1.1 Hz), 6.78 (1H, dd, J¼7.6 Hz,
4J¼1.1 Hz), 7.26–7.33 (3H, m), 7.37 (1H, ddd, J¼7.5,
3
4
3
4
7.5 Hz, J¼1.5 Hz), 7.67 (1H, dd, J¼7.5 Hz, J¼1.5 Hz),
3
4
7.69 (1H, dd, J¼7.7 Hz, J¼1.5 Hz), 9.72 (1.1H, s), 9.79
(0.9H, s), 10.13 (0.9H, s), 10.20 (1.1H, s) ppm; 13C NMR (in
CDCl3 solvent the shifts of dominant anti isomer are
italicized): d 7.97, 8.13, 9.86, 10.14, 14.73, 14.78, 17.80,
17.83, 23.42, 23.72, 51.84, 52.78, 99.55, 100.63, 122.77,
122.95, 123.00, 123.40, 123.45, 123.75, 123.87, 126.28,
126.65, 128.74, 129.00, 129.35, 129.66, 130.90,
131.07, 131.08, 131.11, 131.70, 132.44, 132.60, 132.94,
135.60, 136.00, 147.13, 147.43, 168.21, 169.57, 173.75,
174.31 ppm; 13C NMR (in (CD3)2SO solvent): d 8.07, 8.08,
9.68, 9.70, 14.85, 14.86, 17.15, 17.16, 23.76, 23.99, 51.68,
51.72, 97.69, 121.77, 121.85, 121.97, 122.13, 122.15,
122.27, 123.13, 123.17, 126.24, 126.38, 128.30, 128.45,
128.97, 129.14, 129.88, 130.33, 130.73, 131.00, 131.21,
131.35, 132.17, 135.15, 135.33, 147.12, 147.22, 167.36,
167.45, 171.96, 171.98 ppm. Anal. calcd for C43H44N4O6
(712.8): C, 72.45; H, 6.22; N, 7.86. Found: C, 72.15; H,
6.33; N, 7.84.
Acknowledgements
We thank the US National Institutes of Health (HD 17779)
for support of this work. Stefan E. Boiadjiev is on leave
from the Institute of Organic Chemistry, Bulgarian
Academy of Sciences, Sofia.
References
1. Eliel, E.; Wilen, S. H. Stereochemistry of Carbon Compounds.
Wiley: New York, 1994; Chapter 14.
´
´
2. (a) Alazard, J. P.; Boye, O.; Gillet, B.; Guenard, D.; Beloeil,
J. C.; Thal, C. Bull. Soc. Chim. Fr. 1993, 779–787.
(b) Furusho, Y.; Aida, T.; Inoue, S. J. Chem. Soc., Chem.
Commun. 1994, 653–655. (c) (N-Aryl pyrroles) Fogassy, K.;
4.1.7. 8,12-bis-(o-Carboxyphenyl)-3,17-diethyl-2,7,13,
18-tetramethyl-(10H,21H,23H,24H)-bilin-1,19-dione
(1a). A solution of 285 mg (0.400 mmol) of mesobilirubin
dimethyl esters (1b) and 30 mL of ethanol was saturated
with Ar for 20 min. Then 6 mL of 1 M aq NaOH (6 mmol)
´ ´
Harmat, V.; Bocskei, Z.; Tarkanyi, G.; Toke, L.; Faigl, F.
¨
´
Tetrahedron: Asymmetry 2000, 11, 4771–4780. (d) Klvana,
¨
ˇ
R.; Pohl, R.; Pawlas, J.; Cejka, J.; Dvorakova, H.; Hrabal, R.;
ˇ´
´
¨
Bohm, S.; Kratochvil, B.; Kuthan, J. Collect. Czech. Chem.
Commun. 2000, 65, 651–666.

S. E. Boiadjiev, D. A. Lightner / Tetrahedron 58 (2002) 7411–7421
7421
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