Hemirubin: An Intramolecularly Hydrogen-Bonded Analogue for One-Half Bilirubin

Article abstract of DOI:10.1021/jo972227r

A model for one-half bilirubin, the neurotoxic yellow-orange pigment of jaundice, 9-[2-(2-carboxyethyl)benzyl]-2,3,7,8-tetramethyl-l,10-dihydrodipyrrin (1, hemirubin) was synthesized following SnCl₄-catalyzed Friedel-Crafts acylation at C(9) of

Full text of DOI:10.1021/jo972227r

J . Org. Chem. 1998, 63, 2665-2675
2665
Hem ir u bin : An In tr a m olecu la r ly Hyd r ogen -Bon d ed An a logu e for
On e-Ha lf Bilir u bin
Qingqi Chen and David A. Lightner*
Department of Chemistry, University of Nevada, Reno, Nevada 89557-0020
Received December 9, 1997
A model for one-half bilirubin, the neurotoxic yellow-orange pigment of jaundice, 9-[2-(2-
carboxyethyl)benzyl]-2,3,7,8-tetramethyl-1,10-dihydrodipyrrin (1, hemirubin) was synthesized fol-
lowing SnCl4-catalyzed Friedel-Crafts acylation at C(9) of 2,3,7,8-1-oxo-1,10-dihydrodipyrrin (7)
with methyl o-(chlorocarbonyl)hydrocinnamate. Unlike earlier bilirubin model compounds, hemiru-
bin is predicted and found to engage in intramolecular hydrogen bonding. Like bilirubin, the
propionic acid carboxyl group of hemirubin is linked to the opposing dipyrrinone by intramolecular
hydrogen bonds, and thus, 1 shares in some of the solution properties of its parent bilirubin, e.g.,
an acid that is less polar than its methyl ester.
In tr od u ction
Typical dipyrrinones are bright yellow compounds with
an intense UV-visible absorption near 400 nm (ꢀ ∼30,000
L mol-1 cm-1) due to a long axis-polarized π f π*
excitation in the 14π electron conjugated chromophore
(Figure 1).1 They are known from 15N NMR1, X-ray
crystallography,1,2 and molecular mechanics calcula-
tions1,3 to prefer the lactam tautomer and the Z-config-
uration CdC at C(4) and to show substantial double-bond
and single-bond character in the C(4)-C(5) and C(5)-
C(6) bonds, respectively. The dipyrrinone chromophore
is found in nature in tetrapyrrole bile pigments, most
notably in (4Z,15Z)-bilirubin-IXR (Figure 1), the end
product of heme metabolism in mammals and colorful
herald of hepatobiliary disease.4 Bilirubin, the yellow-
orange pigment of jaundice,5 has two dipyrrinones, each
with a single propionic acid. These dipyrrinones are
conjoined at and capable of independent rotations about
a -CH2- group, and it is through such rotations that
the propionic carboxyl group on each dipyrrinone is
brought into sufficiently close proximity to engage the
other dipyrrinone’s lactam and pyrrole moieties in a
matrix of intramolecular hydrogen bonds (Figure 1).
Collectively, these six hydrogen bonds act as a potent
stabilizer of bilirubin conformation forcing it to adopt a
ridge-tile shape6 and controlling its solution, spectro-
scopic, and metabolic properties.
Whether in bilirubin or as independent units, dipyr-
F igu r e 1. (Top) dipyrrinone chromophore. The double-headed
arrow approximates the long axis polarization of the intense
∼400 nm electronic transition. (Middle) bilirubin in the
unstable linear representation is composed of two dipyrrinone
chromophores. (Bottom) most stable bilirubin conformation
shaped like a ridge-tile with hydrogen bonding between
carboxylic acid groups and opposing dipyrrinones shown by
dashed lines.
rinones are known to be avid participants in hydrogen
bonding.1 They adopt essentially planar conformations
(1) For leading references, see: Falk, H. The Chemistry of Linear
Oligopyrroles and Bile Pigments; Springer-Verlag: New York, 1989.
(2) (a) Cullen, D. L.; Black, P. S.; Meyer, E. F., J r.; Lightner, D. A.;
Quistad, G. B.; Pak, C.-S. Tetrahedron 1977, 33, 477-483. (b) Cullen,
D. L.; Pe`pe, G.; Meyer, E. F., J r.; Falk, H.; Grubmayr, K. J . Chem.
Soc., Perkin Trans 2 1979, 999-1004. (c) Gossauer, A.; Blacha, M.;
Sheldrick, W. S. J . Chem. Soc., Chem. Commun. 1976, 764-765. (d)
Sheldrick, W. S.; Borkenstein, A.; Blacha-Puller, M.; Gossauer, A. Acta
Crystallogr. 1977, B33, 3625-3635. (e) Sheldrick, W. S. Israel J . Chem.
1983, 23, 155-166.
(3) Falk, H.; Mu¨ller, N. Tetrahedron 1983, 39, 1875-1885.
(4) Ostrow, J . D., Ed. Bile Pigments and J aundice; Marcel-Dekker:
New York, 1986; and references therein.
(5) McDonagh, A. F.; Lightner, D. A. Pediatrics 1985, 75, 443-455.
(6) Person, R. V.; Peterson, B. R.; Lightner, D. A. J . Am. Chem. Soc.
1994, 116, 42-59.
(torsion angle C(4)-C(5)-C(6)-N ∼0°) in the crystal,
where they are present as intermolecularly hydrogen-
bonded planar dimers (Figure 2).1,2 Such dimers persist
in solutions of nonpolar solvents. In CHCl3, for example,
dipyrrinones are strongly associated with dimerization
constants of 1700 M (37 °C) for kryptopyrromethenone7
and 25 000 M (22 °C) for methyl xanthobilirubinate,8 as
measured by vapor-pressure osmometry and 1H NMR
S0022-3263(97)02227-5 CCC: $15.00 © 1998 American Chemical Society
Published on Web 03/19/1998

2666 J . Org. Chem., Vol. 63, No. 8, 1998
Chen and Lightner
F igu r e 3. Folded, intramolecularly hydrogen-bonded confor-
mation of hemirubin (1, X ) H2) and its 10-oxo analogue (2, X
) O). Hydrogen bonds are represented by dashed lines. The
shape is similar to one half of bilirubin (Figure 1).
four intermolecular hydrogen bonds in favor of gaining
the added stabilization due to a total of six hydrogen
bonds in the stacked dimer (Figure 2). This preference
of dipyrrinones for hydrogen bonding to available car-
boxyl groups is not surprising since the planar dipyr-
rinones of bilirubin are tightly hydrogen bonded, albeit
intramolecularly, to neighboring carboxylic acid groups
(Figure 1).
In bilirubin and its analogues the component dipyr-
rinones participate in a unique type of hydrogen bonding
with carboxylic acid groups (Figure 1), as confirmed in
the solid by X-ray crystallography2c,13 and in solution by
F igu r e 2. Dipyrrinone dimers. (Top) most stable dimer of
methyl xanthobilirubinate with four intermolecular hydrogen
1
bonds. Consistent with this dimeric representation, H NMR
NOEs measured in CDCl3 are found between the methyls at
C(2) and C(9) (ref 8). (Bottom) schematic representation for
formation of the most stable dimer of xanthobilirubic acid. The
dipyrrinones are stacked, thereby allowing the formation of
six intermolecular hydrogen bonds, as in bilirubin (Figure 1)
and alleviating a nonbonded steric repulsion between the
methyls at C(9).
1
13C{1H} heteronuclear Overhauser effects14 and H NMR
spectroscopy10-12 and supported by molecular orbital and
molecular dynamics computations.6,15 Until recently,16,17
bilirubin represented the only well-established example
of carboxylic acid to amide hydrogen bonding. In the
following, we show how a single dipyrrinone with only
one propionic acid may be designed and constructed to
engender intramolecular hydrogen bonding of the type
characteristic of bilirubin. We call the new pigment
hemirubin (1, Figure 3).
spectroscopy, respectively. The dimers are held together
by a matrix of four intermolecular hydrogen bonds, with
a calculated stabilization enthalpy of 20-30 kcal/mol.9
While a coplanar dipyrrinone-to-dipyrrinone motif (Fig-
ure 2) is probably the most common type of hydrogen
bonding in dipyrrinone dimers1 (and is found even in
bilirubin dimethyl ester1,10a,11), when carboxylic acid
groups are present another type of dimer can become
more stable. Thus, unlike simple dipyrrinones and
dipyrrinone esters, dipyrrinone acids may form π-facial
stacked dimers where each carboxylic acid group makes
three hydrogen bonds with a dipyrrinone, as was found
recently in xanthobilirubic acid and its homologues.12
These dipyrrinone acids eschew the planar dimer and its
Resu lts a n d Discu ssion
Syn th esis. Falk et al.18 had previously reported on
the condensation of 2,3,7,8-tetramethyldipyrrinone (7)
with o-formylbenzoic acid to afford the corresponding
benzaldipyrrinone (12) (Scheme 1), and we had doubly
(13) (a) Bonnett, R.; Davies, J . E.; Hursthouse, M. B.; Sheldrick, G.
M. Proc. R. Soc. London, Ser. B 1978, 202, 249-268. (b) LeBas, G.;
Allegret, A.; Mauguen, Y.; DeRango, C.; Bailly, M. Acta Crystallogr.,
Sect. B 1980, B36, 3007-3011. (c) Becker, W.; Sheldrick, W. S. Acta
Crystallogr., Sect. B 1978, B34, 1298-1304. (d) Mugnoli, A.; Manitto,
P.; Monti, D. Acta Crystallogr., Sect. C 1983, 38, 1287-1291.
(14) (a) Nogales, D.; Lightner, D. A. J . Biol. Chem. 1995, 270, 73-
77. (b) Do¨rner, T.; Knipp, B.; Lightner, D. A. Tetrahedron 1997, 53,
2697-2716.
(7) Falk, H.; Grubmayr, K.; Ho¨llbacher, G.; Hofer, O.; Leodolter, A.;
Neufingerl, E.; Ribo´, J . M. Monatsh. Chem. 1977, 108, 1113-1130.
(8) Nogales, D. F.; Ma, J .-S.; Lightner, D. A. Tetrahedron 1993, 49,
2361-2372.
(9) Molecular Mechanics calculations and molecular modeling were
carried out on an Evans and Sutherland ESV-10 workstation using
version 6.0 of SYBYL (Tripos Assoc., St. Louis, MO) as described in
ref 6. The ball-and-stick drawings were created from the atomic
coordinates of the molecular dynamics structures using Mu¨ller and
Falk’s “Ball and Stick” program for the Macintosh.
(10) (a) Kaplan, D.; Navon, G. Israel J . Chem. 1983, 23, 177-186.
(b) Kaplan, D.; Navon, G. Biochem. J . 1982, 201, 605-613. (c) Navon,
G.; Frank, S.; Kaplan, D. J . Chem. Soc., Perkin Trans 2 1984, 1145-
1149.
(11) Trull, F. R.; Ma, J . S.; Landen, G. L.; Lightner, D. A. Israel J .
Chem. 1983, 23, 211-218.
(12) (a) Boiadjiev, S. E.; Anstine, D. T.; Lightner, D. A. J . Am. Chem.
Soc. 1995, 117, 8727-8736. (b) Boiadjiev, S. E.; Anstine, D. T.;
Maverick, E.; Lightner, D. A. Tetrahedron: Asymmetry 1995, 6, 2253-
2270.
(15) (a) Falk, H. Molecular Structure of Bile Pigments. In Bilirubin;
Heirwegh, K. P. M., Brown, S. B., Eds.; CRC Press: Boca Raton, FL,
1982; Vols. 1 and 2, pp 7-29 and references therein. (b) Shelver, W.
H.; Rosenberg, H.; Shelver, W. H. Intl. J . Quantum Chem. 1992, 44,
141-163. (c) Shelver, W. L.; Rosenberg, H.; Shelver, W. H. J . Mol.
Struct. 1994, 312, 1-9.
(16) Wash, P. L.; Maverick, E.; Chiefari, J .; Lightner, D. A. J . Am.
Chem. Soc. 1997, 119, 3802-3806.
(17) (a) Garcia-Tellado, F.; Gaswami, S.; Chang, S.-K.; Geib, S. J .;
Hamilton, A. D. J . Am. Chem. Soc. 1990, 112, 7393-7394. (b)
Hamann, B. C.; Branda, N. R.; Rebek, J ., J r. Tetrahedron Lett. 1993,
34, 6837-6840. (c) Kelly, T. R.; Kim, M. H. J . Am. Chem. Soc. 1994,
116, 7072-7080.
(18) Eichinger, D.; Falk, H. Monatsh. Chem. 1987, 118, 91-103,
261-271.

Hydrogen-Bonded Analog for One-Half Bilirubin
J . Org. Chem., Vol. 63, No. 8, 1998 2667
Ta ble 1. Attem p ted Con d en sa tion s of Dip yr r in on e 7 w ith Ben za ld eh yd e 13 To Give th e Ben za ld ip yr r in on e
rctn
catalyst
solvent
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
none
T (°C)
time
results (% yield)
15 (100)
HBr-HOAc
HBr-HOAc
HBr-HOAc
HBr-HOAc
TFA
25
0
-20
-60
rt
2 min
1 min
3 h
12 h
3 h
15 (100)
15 (60) and unreacted 13
15 (80) and unreacted 13
15 (100)
TFA
TFA
TsOH
CH2Cl2
MeOH
CH2Cl2
-20
-20
rt
3 h
1 h
5 h
15 (50) and recovered 13
15 (30) and recovered 13
15 (80)
TsOH
TsOH
TsOH
AcOH
AcOH
none
none
Bu4NOH
MeOH
CH2Cl2-MeOH
none
CH2Cl2
CH2Cl2-MeOH
CH2Cl2
rt
rt
rt
rt
rt
rt
reflux
reflux
12 h
10 h
10 h
2 h
30 h
5 h
15 (60)
15 (60)
15 (75)
15 (63) and recovered 13
15 (80)
15 (30) and recovered 13
NR recovered 7
NR
MeOH
MeOH-THF
2-10 h
2 h
Sch em e 1
Sch em e 2
Sch em e 3
condensed the same dipyrrinone with the aromatic dial-
dehydes, terephalaldehyde or isophthalaldehyde, to af-
ford the corresponding tetrapyrrole dibenzalbisdipyrrino-
nes.19 Thus, with the expectation benzlpyrrolinones can
be prepared readily from aromatic aldehydes and R-free
dipyrrinones, we envisioned the preparation of 1 to follow
from a straightforward coupling of dipyrrinone 7 with
o-formylhydrocinnamic acid or ester (13) (Scheme 1).
â-Alkylated dipyrrinones are relatively easy to synthe-
size, especially the 2,3,7,8-tetramethyldipyrrinone 7,20,21
and o-formylhydrocinnamic acid (13) can be prepared in
high yield from â-tetralone (Scheme 2). Unexpectedly,
however, reaction of 7 with 13 under reaction conditions
that gave 12 did not stop at the monoadduct stage and
form a stable benzaldipyrrinone (Scheme 3). Apparently,
the initially formed monoadduct did not undergo smooth
elimination to yield a stable benzaldipyrrinone. Either
through protonation of the erstwhile benzalpyrrolinone
or by elimination of water from a protonated aldol-like
intermediate, the resulting carbocation attacked a second
molecule of 7 to afford mainly verdin 15 and only a very
small amount of the corresponding rubin, which was
unstable and rapidly converted to 15 (Scheme 3). Ex-
tensive modification of the acid catalysis reaction condi-
tions (Table 1) consistently afforded 15, and we could
isolate none of the desired monocondensation product.
It would appear that attachment of the electron-with-
drawing COOH group directly to the phenyl ring of the
aldehyde (as in o-formylbenzoic acid) is responsible for
the acid-catalyzed reaction of Scheme 1 having stopped
at the single condensation stage, because reaction of 7
(19) Nogales, D.; Anstine, D. T.; Lightner, D. A. Tetrahedron 1994,
50, 8579-8596.
(20) Montforts, F.-P.; Schwartz, U. M. Liebigs Ann. Chem. 1985,
1228-1253.
(21) Xie, M.; Lightner, D. A. Tetrahedron 1993, 49, 2185-2200.

2668 J . Org. Chem., Vol. 63, No. 8, 1998
Chen and Lightner
Sch em e 4
Sch em e 7
Sch em e 5
Sch em e 8. Syn th etic Sch em ea
Sch em e 6
with o-tolualdehyde also gave only the verdin (16)
(Scheme 3). Base catalysis gave no reaction. Surmising
that the desired reaction required a deactivated benzal-
dehyde, we synthesized 2-methyl-5-nitrobenzaldehyde
and found that it underwent smooth conversion in TFA
to monocondensation product in 30% yield without
formation of verdin. Unfortunately, for reasons that are
not entirely clear, the more relevant nitrobenzaldehyde
14 (synthesized according to Scheme 2) gave only the
verdin (17) in reaction with 7. Similarly, reaction of
o-formylcinnamic acid (prepared from o-bromobenzalde-
hyde according to Scheme 4) with 7 gave none of the
target monocondensation product. In the midst of these
difficulties, a different condensation reaction appeared
promising: reaction of 7 with R-bromo-o-xylene was
found to give a monocondensation product (Scheme 5).
However, the reaction failed when o-(bromomethyl)hy-
drocinnamic acid methyl ester was reacted with 7 under
the same conditions.
In view of the failure of the appropriate aldehyde or
benzyl bromide condensations to give the target mono-
condensation products, an alternative route was sought
wherein further reaction of the initial product would be
unlikely: Friedel-Crafts acylation of dipyrrinone 7 with
acid chloride 8 (Scheme 6). Friedel-Crafts-type acyla-
tions at an R- or â-free position of pyrroles and at â-free
(8H) dipyrrinones have been reported,22 but such reac-
tions at an R-free (9H) dipyrrinone are less well-known.
We found that the readily available o-methylbenzoyl
chloride reacted with dipyrrinone 7 in good yield to afford
the adduct (6, Scheme 7). Reduction of 6 to 3 failed with
KBH4 in methanol at reflux but proceeded smoothly in
refluxing isopropyl alcohol. These positive results
a
Key: aCH3OH/H2SO, 77-85%; bKBH4/(CH3)2CHOH refl., 63-
70%; cNaOH/CH3OH, 75%; dSnCl4/CH2Cl2, 70-95%; eSOCl2, 66%;
fNi(Al)/NaOH, 86%; g30% CH3CO3H, 68%.
prompted us to proceed with the synthesis of 1 as
outlined in the Scheme 8.
The required acid chloride 8 was prepared from
â-naphthol. Oxidation with 30% peroxyacetic acid af-
forded o-carboxylcinnamic acid (11), which was reduced
with Raney nickel alloy in aqueous NaOH to give 10. The
latter was selectively esterified in methanol-H2SO4 to
afford 9, from which acid chloride 8 was obtained
following reaction with thionyl chloride. Ester-acid
chloride 8 underwent a Friedel-Crafts acylation with
dipyrrinone 7 (prepared as reported previously)21 in
(22) Thyrann, T.; Lightner, D. A. Tetrahedron 1996, 52, 447-460.

Hydrogen-Bonded Analog for One-Half Bilirubin
J . Org. Chem., Vol. 63, No. 8, 1998 2669
Ta ble 2. 13C NMR Ch em ica l Sh iftsa of Dip yr r in on e 7, Hem ir u bin 1 (X ) H2, R ) H), Its Meth yl Ester 4 (X ) H2, R ) CH3),
o-Xylyld ip yr r in on e 3, a n d Rela ted 9-Su bstitu ted Oxod ip yr r in on es 2 (X ) O, R ) H), 5 (X ) O, R ) CH3), a n d 6
dipyrrinone chemical shift in (CD3)2SO and (CDCl3)
carbon
1
7
1
4
3
2
5
6c
172.3
(174.3)
118.5
141.5
131.6
98.14
(101.1)
124.5
124.1
122.0
120.1
171.8
(174.4)
116.1
141.4
128.9
97.74
(101.0)
123.5
122.7
122.3
128.4
28.45
(25.58)
137.5
138.4
126.0
126.0
126.0
130.6
27.23
(26.01)
34.04
(34.81)
173.7
(178.9)
8.20
172.1
(173.6)
122.6
141.7
128.9
98.04
(100.5)
125.5
125.4
123.0
128.9
29.00
(27.65)
137.8
138.5
126.6
126.5
126.1
129.3
27.54
(29.63)
34.12
(34.43)
172.1
(173.6)
8.53
173.2
(173.2)
125.9
141.8
134.3
95.65
(96.07)
131.2
126.2
126.1
135.0
29.43
(30.52)
135.4
135.8
130.0
128.0
127.9
130.0
19.50
(19.48)
173.0
(173.3)
123.7
142.1
136.5
95.71
(98.74)
130.9
126.5
127.3
130.3
186.0
(177.6)
141.0
138.2
128.0
131.2
129.8
130.8
27.98
(25.89)
35.31
(34.21)
173.7
(175.5)
8.59
172.6
(173.2)
123.7
142.1
136.5
95.68
(96.21)
127.4
126.6
127.3
128.0
185.9
(186.3)
141.0
137.9
129.9
130.8
129.9
131.0
27.93
(28.01)
35.03
(35.19)
173.0
(173.5)
8.55
173.0
[172.8]
123.6
142.2
135.0
95.78
[94.84]
127.8
126.1
127.0
134.7
186.2
[185.9]
141.26
136.4
129.7
130.8
127.3
130.8
18.95
[18.43]
(1)b
2
3
4
5
(5)b
6
7
8
9
10
(10)b
1′
2′
3′
4′
5′
6′
R
(R)b
â
(â)b
CO2R
(CO2R)b
2-CH3
3-CH3
7-CH3
8-CH3
8.57
9.36
9.83
8.13
10.33
10.26
9.42
8.57
9.78
9.35
9.46
9.33
8.73
9.78
9.64
9.06
9.78
9.36
10.78
9.77
9.34
10.77
10.26
10.49
a
b
δ, ppm downfield from (CH3)4Si for 10-2 M solutions in (CD3)2SO. Values in parentheses are from 10-3 M solutions in CDCl3. c Values
in square brackets are from 10-3 M solutions in 10% (CD3)2SO-90% CCDl3 by volume.
dichloromethane using SnCl4 catalyst to give the benzoyl-
dipyrrinone 5 in high yield. Saponification of 5 afforded
the corresponding acid (2). Reduction of 5 to hemirubin
1 was achieved using KBH4 in refluxing isopropyl alcohol.
Thus, both hemirubin 1 and its 10-oxo analogue (2) could
be obtained in a straightforward way. For comparison
of spectroscopic properties, their methyl esters (4) and
(5) were also prepared.
Molecu la r Str u ctu r e. The constitutional structures
of 1-6 (Schemes 7 and 8), which follow from the structure
of the common starting dipyrrinone (7)21 and the syn-
thetic methods, were confirmed by their 13C NMR spectra
run in (CD3)2SO. In (CD3)2SO solvent, 1-6 are expected
to be monomeric and show chemical shifts (Table 2)
characteristic of the dipyrrinone fragment and the o-
substituted phenyl fragmentsconsistent with the postu-
lated structures. In CDCl3 solvent, where intramolecular
hydrogen bonding is more likely to occur in 1 and 2, and
intermolecularly hydrogen-bonded dipyrrinone-to-dipyr-
rinone dimers are very probable in 3-7, shifts were noted
at particularly sensitive carbons (10, R, and CO2R). In
1, which is capable of exhibiting intramolecular hydrogen
bonding, C(10) is strongly shielded in the spectrum run
in CDCl3, relative to that in (CD3)2SO, whereas in its
ester (4) the shielding is only half as large. In the oxo-
analogue (2) of 1, the C(10) resonance found in CDCl3 is
also strongly shielded relative to its ester (5) and its
o-toluoyldipyrrinone analogue (6). Similarly, the R-car-
bon resonance of 1 measured in CDCl3 is more shielded
than in (CD3)2SO, whereas in the methyl ester (4) it is
more deshielded. The same observation is made at the
R-carbon resonances of the oxo analogues 2 and 5.
The geometric structural assignments, particularly the
Z-configuration of the C(4) exocyclic double bond of the
dipyrrinone moiety in 1-7, is confirmed by the observa-
tion of strong homonuclear nuclear Overhauser effects
(NOEs) in CDCl3 between the lactam and pyrrole NHs
and moderate NOEs between the C(5)-H and the C(3)
and C(7) methyls. Other NOEs of significance were
detected between the C(10) hydrogens of 1, 3, and 4 and
(i) their benzene ring C(6) hydrogens, (ii) their pyrrole
NHs, and (iii) their C(8) methyls. NOEs were found
between the â-CH2 of 1-4 and the benzene ring C(3′)
hydrogens. Significantly, as in bilirubin and meso-
bilirubin,10,14b weak NOEs were found between the car-
boxylic acid hydrogen and the lactam hydrogen in 1 and
2. These data indicate a proximal spatial relationship
between those groups that is consistent with the in-
tramolecular hydrogen bonding motif shown in the
structural representation of Figure 3.
Consistent with these observations, T1 relaxation
measurements of the CO2H carbons in 1 and 2 were 3.31
and 3.21 s, respectively, in CDCl3. In bilirubin, which is
known to engage in intramolecular hydrogen bonding,

2670 J . Org. Chem., Vol. 63, No. 8, 1998
Chen and Lightner
Ta ble 3. Com p a r ison of NH 1H NMR Ch em ica l Sh iftsa of Dip yr r in on es in CDCl3 a n d (CD3)2SO Solven ts
a
b
δ, downfield from Me4Si. Run as 10-2 M (CD3)2SO and 10-3 M CDCl3 solutions at 23 °C.
T1 for its CO2H carbons is slightly longer (3.9 s), but
∼10.5 ppm). In tetrapyrroles, such as bilirubin, where
the dipyrrinones are intramolecularly hydrogen bonded
to the opposing propionic acids but are not stacked
(Figure 1), the lactam and pyrrole chemical shifts lie near
δ ∼10.6 and 9.2, respectively, in CDCl3. The N-H
chemical shifts of hemirubin 1, notably the shielded
pyrrole NH near 9 ppm (Table 3), are similar to those of
bilirubin, whereas those of hemirubin ester (3) and the
o-xylyldipyrrinone 4 are more like those found in dipyr-
rinone 7. The data point to intramolecular hydrogen
bonding between dipyrrinone and lactam in hemirubin
and intermolecular dipyrrinone-to-dipyrrinone hydrogen
bonding in 3 and 4. In (CD3)2SO solvent, the N-H
chemical shifts of 1, 3, 4, and 7 are all very similar,
indicative of hydrogen bonding to the solvent.
methyl xanthobilirubinate, whose ester group does not
engage in hydrogen bonding, has a much longer T1. The
data again are consistent with intramolecularly hydrogen-
bonded carboxylic acid groups in 1 and 2.
1H NMR a n d Hyd r ogen Bon d in g. Dipyrrinones are
avid hydrogen bonders.1,8 Diagnostic of this behavior and
1
typical of hydrogen bonding, the intrinsic N-H H NMR
chemical shifts of the monomer (δ ∼8 ppm)8 become
strongly deshielded in nonpolar solvents such as CDCl3
to approximately 10 and 11 ppm11 for the pyrrole and
lactam hydrogens, respectively. These more deshielded
chemical shifts, found in a wide variety of dipyrrinones
with hydrocarbon, ester, and amide substituents, are
characteristic of the traditional dipyrrinone to dipyr-
rinone planar dimer (Figure 2, top)1,8,11 However, when
the dipyrrinones engage in hydrogen bonding to CO2H
groups to give a stacked dimer (Figure 2, bottom), the
NH chemical shifts are relatively more shielded (espe-
cially pyrrole NH ∼8.9 ppm, but also the lactam NH
Similarly, for 10-oxohemirubin 2, the N-H chemical
shifts in CDCl3 are indicative of intramolecular hydrogen
bonding. In the ester (5), the pyrrole and lactam NH
chemical shifts lie at 9.36 and 9.57 ppm, respectively,
whereas in 2, the corresponding chemical shifts lie at 8.15

Hydrogen-Bonded Analog for One-Half Bilirubin
J . Org. Chem., Vol. 63, No. 8, 1998 2671
F igu r e 4. Interconverting, enantiomeric ridge-tile conforma-
tions of hemirubin (1, X ) H2) and 10-oxohemirubin (2, X )
O). The M and P helicities are correlated with the negative
and positive torsion angles [C(1′)-C(10)-C(9)-N], respec-
tively. Hydrogen bonds are shown by dashed lines.
and 10.71 ppmsvalues consistent with intramolecular
hydrogen bonding. Although the C(10) carbonyl causes
some differences in NH chemical shifts relative to those
of 1, the strong shielding of the pyrrole NH is typical of
a dipyrrinone hydrogen bonded to a carboxylic acid in
an orientation where the pyrrole N-H is above or below
the aromatic ring π-system. Such an arrangement is
very much like that in ridge-tile bilirubin (Figure 1), and
so it seems probable that hemirubin and its 10-oxo
analogue fold into the conformation shown in Figure 3.
Like the bilirubin ridge-tile conformation, 1 and 2 may
adopt either of two interconverting enantiomeric ridge-
tile conformations (Figure 4). For bilirubin, the inter-
conversion barrier has been determined experimentally
to be ∼20 kcal/mol. The lowest energy interconversion
pathway has been computed to be ∼20 kcal/mol over a
transition state involving the breaking of at least three
of the six intramolecular hydrogen bonds (while three
hydrogen bonds are retained).6 Interconversion of enan-
tiomeric 1 or 2 also requires breaking three hydrogen
bonds, but unlike the interconversion of bilirubin con-
formational enantiomers there are no residual 3 hydro-
gen bonds for use in stabilizing the transition state. The
interconversion barrier is thus probably lower in the
hemirubins, as supported by the observation that the 1H
NMR signals of the propionic -CRH2-CâH2- segment of
1 and 2 indicate a simple A2B2 splitting pattern. In
contrast, bilirubin exhibits an ABMX splitting pattern
characteristic of restricted segmental motion in the
propionic acid chains on the NMR time scale.
F igu r e 5. Ball and stick representations for the minimum
energy conformations of hemirubin (1) and its 10-oxo analogue
(2). The energy minimization and structure graphical repre-
sentations were carried out according to refs 6 and 9. In 1,
the interplanar angle between the phenyl ring and average
plane of the dipyrrinone is 97°, in 2 it is 103°. The C(4)-C(5)-
C(6)-N torsion angles of 1 and 2 are 18° and 29°, respectively;
the N-C(9)-C(10)-C(1′) torsion angles of 1 and 2 are 64° and
24°, respectively. The COOH‚‚‚OdC (lactam) hydrogen bond
distances are 1.553 and 1.535 Å in 1 and 2, respectively; the
HO-CdO‚‚‚HN (lactam) hydrogen bond distances are 1.528
and 1.549 Å in 1 and 2, respectively, and the HO-CdO‚‚‚HN
(pyrrole) hydrogen bond distances are 1.584 and 1.885 Å in 1
and 2, respectively.
distorted from that geometry, presumably because its
C(10) carbon is sp2 hybridized rather than sp3.
Molecu la r Dyn a m ics Ca lcu la tion s. In support of
the conclusions reached (above) by NMR spectroscopic
analysis, molecular dynamics calculations9 of hemirubin
(1) and its 10-oxo analogue (2) show that these com-
pounds prefer intramolecularly hydrogen-bonded confor-
mations (Figure 5), which are computed to lie some 13
kcal/mol lower in energy than the non-hydrogen-bonded
forms. The intramolecularly hydrogen-bonded conforma-
tions shown in Figure 5 have torsion angles similar to
those found in bilirubin and mesobilirubin.6,13 The
dipyrrinone moiety in 1 and 2 is twisted somewhat, with
C(4)-C(5)-C(6)-N torsion angles of ∼18° and ∼29°,
respectively. The C(1′)-C(10)-C(9)-N torsion angles
are 64° and 24° in 1 and 2, respectivelysas compared to
∼60° in mesobilirubin.6 In addition, the interplanar
dihedral angles are 97° and 103° in 1 and 2, respectively,
as compared to ∼96° in mesobilirubin.6 In sum, the
molecular dynamics calculations suggest that 1 adopts
a molecular geometry very similar to that found in
bilirubin and mesobilirubin, whereas 2 is somewhat
Op tica l Sp ectr a . The UV-vis spectral data for 1-7
in solvents with a wide range of polarity are shown in
Table 4. The long-wavelength absorption of hemirubin
(1) has nearly the same λmax and ꢀmax as its methyl ester
(4) and is rather similar to its o-xylyl (3) analogue in
methanol and in dimethyl sulfoxide, solvents likely to
interfere with hydrogen bonding. In nonpolar solvents,
however, the spectra of 1 and 4 (or 3) are noticeably
different, with 1 showing a strong bathochromic shift of
λ
max, whereas λmax of 3 and 4 are relatively unchanged.
While the spectral shifts do not unambiguously confirm
an intramolecularly hydrogen bonded structure for 1,
they lend support to the conclusions based on NMR
spectral analysis and are consistent with the ability of 1
to adopt a unique conformational structure in nonpolar
solvents. The UV-vis spectral data for 2 are less solvent
dependent. The major long wavelength absorption near
400 nm has an inflection or shoulder near 420-430 nm
in the various solvents of Table 4.

2672 J . Org. Chem., Vol. 63, No. 8, 1998
Chen and Lightner
Ta ble 4. Solven t Dep en d en ce of UV-vis Da ta for Hem ir u bin (1) a n d Rela ted Dip yr r in on es (2-7)a
λmax (ꢀmax)b for
solvent
C6H6
ꢀc
1
4
3
2
5
6
7
2.3
421 (29 500)
408 (30 500)
406 (24 300)
398 (22 800)
420 (19 000)
401 (24 280)
420 (21 000)
403 (31 300)
424 (26 800)
398 (28 000)
419 (24 300)
405 (33 700)
427 (31 800)
398 (30 100)
419 (25 400)
406 (33 100)
429 (27 400)
402 (35 800)
425 (30 400)
396 (33 000)
417 (26 500)
405 (37 600)
428 (35 300)
407 (34 400)
429 (32 500)
407 (32 800)
429 (28 400)
402 (33 800)
424 (29 300)
396 (31 300)
417 (29 300)
405 (36 000)
427 (34 200)
392 (23 500)
CHCl3
4.7
32.6
36.2
49
420 (29 800)
413 (34 300)
409 (30 500)
410 (34 200)
408 (32 400)
411 (34 500)
400 (33 800)
410 (35 300)
407 (24 900)
412 (28 300)
400 (23 600)
411 (28 300)
394 (25 300)
396 (27 200)
395 (26 700)
395 (26 300)
CH3OH
CH3CN
(CD3)2SO
a
b
Data obtained at 22 °C on (2-4) × 10-5 M solutions. λmax in nm, ꢀmax in M-1 cm-1
.
c Dielectric constants: Gordon, A. J .; Ford, R. A.
The Chemist’s Companion; Wiley: New York, 1972; pp 4-8.
mined as reported previously.12,14,16 NMR spectra were ob-
tained at 300 and 500 MHz and reported in δ (ppm). HMQC,
HMBC, NOE, and T1 experiments were carried out on a 500
MHz spectrometer. Melting points are uncorrected. Combus-
tion analyses were carried out by Desert Analytics, Tucson,
AZ. Flash column chromatography was carried out using
Woelm silica gel F, thin-layer chromatography grade. Radial
chromatography was carried out on Merck silica gel PF254 with
gypsum (preparative-layer grade). HPLC analyses were car-
ried out on a Beckman-Altex ultrasphere-IP 5 µm C(18) ODS
column (25 × 0.46 cm) and ODS precolumn (4.5 × 0.46 cm).
The flow rate was 1.0 mL/min, and the elution solvent was
0.1 M di-n-octylamine acetate in 5% aqueous methanol (pH
7.7, 31 °C). All solvents were reagent grade obtained from
Aldrich, Acros Organics, or Fisher. Deuterated chloroform and
dimethyl sulfoxide were from Cambridge Isotope Laboratories.
Ethyl acetate, 2,4-pentanedione, diisopropropylamine, n-
butyllithium in hexane, tin tetrachloride, and potassium
borohydride were from Aldrich; peracetic acid, 2-toluoyl chlo-
ride, sodium hydroxide, thionyl chloride, titanium tetrachlo-
ride, and 2-propanol were from Fisher-Acros; hydrochloric acid
and glacial acetic acid, were from EM Science, Inc.; nickel-
aluminum alloy was from W. R. Grace & Co.; 2-bromotoluene
was from J . T. Baker Co.
Solu t ion a n d Ch r om a t ogr a p h ic P r op er t ies.
Hemirubin (1) is less soluble in chloroform than bilirubin.
It is more polar and is soluble in dilute aqueous bicar-
bonate, where bilirubin is insoluble. Consistent with
these properties, 1 has a shorter retention time (∼9.2
min) on reversed-phase HPLC than mesobilirubin-XIIIR
(∼16.5 min). It also has a longer retention time than its
methyl ester 4 (∼5.2 min), which is close to that of the
o-xylyldipyrrinone 3 (∼6.0 min), suggesting that the ester
is more polar than the acid. This unusual HPLC
behavior (ester more polar than the parent acid) parallels
that observed for bilirubin and mesobilirubin-XIIIR and
their dimethyl esters and is yet a further indication of
intramolecular hydrogen bonding in the acids. Such
distinctions in HPLC behavior are not evident for the 10-
oxohemirubin (2) and its methyl ester (5), however, which
have identical retention times (∼4.3 min) that are very
similar to that of the (o-methylbenzoyl)dipyrrinone 6
(∼4.6 min) and comparable to 3 and 4 but not 1.
On silica gel TLC, 1 travels more rapidly than 3 or 4,
again consistent with reduced polarity due to intramo-
lecular hydrogen bonding. This behavior parallels that
of bilirubin relative to its dimethyl ester. Curiously, the
10-oxohemirubin (2) behaves on TLC as if it were more
polar than its ester (5), which would appear to suggest
less effective intramolecular hydrogen bonding than in
1. Taken collectively, the chromatographic behavior of
1 is consistent with a structure where the polar dipyr-
rinone and carboxylic acid groups are tied together by
intramolecular hydrogen bonding (Figure 3), whereas for
2 the data suggest less effective hydrogen bonding of its
CO2H group.
3-(2-Ca r boxyp h en yl)p r op en oic a cid (11) and 3-(2-ca r -
boxyp h en yl)p r op a n oic a cid (10)23 were prepared in 68%
and 86% yields, respectively, according to literature proce-
dures.23
Meth yl 3-(2-Ca r boxyp h en yl)p r op a n oa te (9). 3-(2-Car-
boxyphenyl)propanoic acid (15.4 g, 697 mmol) and methanol
(500 mL) were placed in a 1000 mL round-bottom flask. To
this mixture was added concentrated sulfuric acid (8 mL)
dropwise over a period of 10 min with magnetic stirring. The
solution was allowed to stir another 30 min at room temper-
ature; the solvent was then removed (rotovap) at 30-35 °C to
1/10 volume. Water (200 mL) was added, followed by dropwise
addition of sodium hydroxide (100 mL, 2 M). With the
appropriate rate of addition, the temperature of the reaction
mixture rose slowly to 30-35 °C but should not exceed 40 °C.
A solid white material began to separate from the solution;
when the NaOH addition was complete, the mixture was
stirred for 1 h. After being cooled to 0 °C on an ice bath, the
solid was collected by suction filtration and washed with water
(3 × 100 mL). Air drying gave 14.0 g (85%) of crude product
as white crystalline solid, which is sufficiently pure for use in
the next step: mp 80-82 °C (lit.24 mp 83-84 °C); 1H NMR
((CD3)2SO, 300 MHz) δ 2.56 (t, J ) 7.32 Hz, 2H, CH2CH2COO),
3.13 (t, J ) 7.32 Hz, 2H, CH2CH2COO), 3.53 (s, 3H, OCH3),
7.25-7.35 (m, 2H, 2Ar-H), 7.40-7.45 (m, 2H, 2Ar-H), 12.91
Con clu sion s
Hemirubin (1), a model for one-half bilirubin, has been
synthesized and, like bilirubin, is found to adopt a folded,
intramolecularly hydrogen-bonded conformation as its
energy-minimum structure. As such, it is less polar than
its methyl ester and much less polar than bilirubin
analogues, e.g., xanthobilirubic acid, that are incapable
of intramolecular hydrogen bonding. Whether this ana-
logue behaves similarly to bilirubin in hepatic metabo-
lism, requiring glucuronidation of its hydrogen-bonded
carboxylic acid in order to be excreted into bile, remains
to be investigated.
(brs, 1H, COOH) ppm; 13C NMR ((CD3)2SO):
δ 29.27
(CH2CH2COO), 35.35 (CH2CH2COO), 51.45 (OCH3), 94.14,
126.6, 130.6, 131.0, 132.0, 141.7, 168.8 (COOH), 172.9 (COOMe)
ppm.
Exp er im en ta l Section
(23) Page, G. A.; Tarbell, G. S. Organic Syntheses; Wiley: New York,
1963; Collect. Vol. IV, pp 136-140.
(24) Arth, C.; Clemens, M.; Muse, W. Liebigs Ann. Chem. 1994, 259-
263.
Gen er a l Meth od s. All UV-vis and infrared (IR) spectra
and nuclear magnetic resonance (NMR) spectra were deter-

Hydrogen-Bonded Analog for One-Half Bilirubin
J . Org. Chem., Vol. 63, No. 8, 1998 2673
9-(2-Meth ylben zoyl)-2,3,7,8-tetr am eth yldipyr r in on e (6).
2-Meth ylben zoyl Ch lor id e. 2-Methylbenzoic acid (20 g,
0.147 mol) and thionyl chloride (61 g, 37 mL, 0.51 mol, 3.5
mol equiv) were placed in a 100 mL round-bottom flask. The
mixture was allowed to warm to 50 °C (heat by mantle) under
protection of a drying tube equipped with anhyd calcium
chloride. After all solid material was dissolved, the solution
was stirred for 10 min at the same temperature. Excess
thionyl chloride was distilled under water aspiration, and the
residue was distilled at high vacuum. The desired product
was collected at 54-56 °C /1 mmHg to give 20 g (90% yield):
bp 54-56°/1 mmHg; 1H NMR (CDCl3, 300 MHz) δ 2.56 (s, 3H,
CH3), 7.27-7.37 (m, 2H, 2Ar-H), 7.49-7.54 (m, 1H, Ar-H), 8.20
(d, J ) 7.81 Hz, 1H, Ar-H) ppm; 13C NMR (CDCl3) δ 21.67
(CH3), 126.2, 131.7, 132.3, 133.6, 133.9, 141.1, 167.2 (COOCl).
The compound was used directly in the next step.
mg, 3.7 mmol, 24 mol equiv) was then added in small portions
over 20 min. The mixture was allowed to reflux for another 2
h after the addition was complete. The reaction was then
cooled to room temperature and poured into ice. The mixture
was extracted with dichloromethane (3 × 50 mL), and the
extract was washed with aqueous hydrochloric acid (2 × 50
mL, 10%) and then water (50 mL). The organic extract was
dried over anhyd sodium sulfate and evaporated (rotovap), to
give a residue that was crystallized from methanol to yield
the desired product (33.5 mg, 70%): mp 278-280 °C; IR (film)
ν 3336, 2919, 2860, 1657, 1637, 1458, 1387, 1273, 1182, 938,
732, 691 cm-1; UV-vis (Table 4); 1H NMR (CDCl3, 300 MHz)
δ 1.80 (s, 3H, CH3), 1.95 (s, 3H, CH3), 2.19 (s, 3H, CH3), 2.24
(s, 3H, CH3), 2.29 (s, 3H, CH3), 4.09 (s, 2H, CH2), 6.80 (s, 1H,
-CHd), 6.94-7.57 (m, 4H, phenyl-H), 10.01 (brs, 1H, NH),
10.90 (brs, 1H, NH) ppm; 1H NMR ((CD3)2SO, 300 MHz) δ 1.72
(s, 3H, CH3), 1.75 (s, 3H, CH3), 2.01 (s, 3H, CH3), 2.03 (s, 3H,
CH3), 2.27 (s, 3H, CH3), 3.82 (s, 2H, CH2), 5.94 (s, 1H, -CHd),
6.70-6.72 (m, 1H, phenyl-H), 7.03-7.13 (m, 2H, 2 phenyl-H),
7.14 (m, 1H, phenyl-H), 9.73 (brs, 1H, NH), 10.29 (brs, 1H,
NH) ppm; 13C NMR (CDCl3) δ 7.26 (CH3), 8.76 (CH3), 9.88
(CH3), 9.98 (CH3), 19.48 (CH3), 30.52 (CH2), 96.07, 116.6, 120.9,
123.1, 124.1, 125.6, 125.9, 126.4, 128.4, 128.7, 130.0, 135.5,
136.2, 143.8, 173.2 (CdO) ppm; 13C NMR ((CD3)2SO) δ 8.13
(CH3), 9.42 (CH3), 10.26 (CH3), 10.33 (CH3), 19.50 (CH3), 29.43
(CH2), 95.65, 125.9, 126.1, 126.2, 127.9, 128, 123.0, 130.0,
131.2, 134.3, 135.0, 135.4, 135.8, 141.8, 173.2 (CdO) ppm;
MS(FAB) m/z 320 (M+).
Con d en sa tion . 2-Methylbenzoyl chloride (50 mg, 0.32
mmol, 6.9 mol equiv) and dry dichloromethane (50 mL) were
added to a 100 mL three-neck flask fitted with a magnetic
stirrer and nitrogen inlet. The flask was cooled with an ice-
salt bath, and nitrogen was bubbled through the solution. After
20 min, tin tetrachloride (three drops) was introduced, followed
by addition 2,3,7,8-tetramethyldipyrrinone (7) (10 mg, 0.046
mol). The mixture was stirred for another 20 min after the
addition was complete at 0 °C. Before it was poured into ice,
the mixture was allowed to stir for 1 h at room temperature.
The aqueous mixture was extracted with dichloromethane (3
× 50 mL), and the combined extract was washed with
hydrochloric acid (2 × 30 mL, 10%) followed by water (2 × 30
mL) and saturated aqueous sodium bicarbonate (50 mL). The
organic extracts were dried over anhyd sodium sulfate, the
solvent was removed, and the residue was purified by flash
chromatography using dichloromethane-methanol (97/3 by
vol) as eluant to yield the desired compound. After recrystal-
lization from methanol, the desired product was obtained (10.8
mg 70% yield): mp 302-304 °C; IR (film): ν 3450, 3343, 2843,
1660, 1617, 1438, 1406, 1385, 1273, 1169 cm-1; UV-vis (Table
4); 1H NMR (CDCl3, 300 MHz) δ 1.61 (s, 3H, CH3), 1.95 (s,
3H, CH3), 2.02 (s, 3H, CH3), 2.11 (s, 3H, CH3), 2.31 (s, 3H,
CH3), 5.95 (s, 1H, -CHd), 7.25-7.34 (m, 4H, 4 Ar-H), 8.65
(br s, 1H, NH), 9.40 (br s, 1H, NH) ppm; 1H NMR (90% CDCl3
and 10% (CD3)2SO, 300 MHz) δ 1.28 (s, 3H, CH3), 1.44 (s, 3H,
CH3), 1.52 (s, 3H, CH3), 1.70 (s, 3H, CH3), 2.66 (s, 3H, CH3),
5.31 (s, 1H, -CHd), 6.64-6.75 (m, 4H, 4 Ar-H), 10.04 (br s,
Anal. Calcd for C21H24N2O (320.4): C, 78.75; H, 7.50; N,
8.75.
Anal. Calcd for C21H24N2O‚0.5H2O (329.4): C, 76.49; H,
7.58; N, 8.49. Found: C, 76.03; H, 7.30; N 8.03.
9-[2-[2-(Met h oxyca r b on yl)et h yl]b en zoyl]-2,3,7,8-t et -
r a m eth yld ip yr r in on e (5). The acid chloride of 10 was first
prepared and then condensed with dipyrrinone 7.
Meth yl 3-(2-Ca r boxyp h en yl)p r op a n oyl Ch lor id e (9).
Methyl 3-(2-carboxyphenyl)propanoate (5.0 g, 24 mmol) and
thionyl chloride (10 g, 6.3 mL, 84 mmol, 3.5 mol equiv) were
placed in a 100 mL round-bottom flask. The mixture was
equipped with a CaCl2 drying tube and allowed to warm to 50
°C using a heating mantle. After all of the solid was dissolved,
the solution was stirred for 10 min at the same temperature.
Excess thionyl chloride was removed by distillation under
vacuum (water aspirator). The residue was distilled at high
vacuum. The desired product (3.4 g, 66% yield) was collected
at 120 °C /1 mmHg and used directly in the next step: bp 120
1
1H, NH), 10.44 (br s, 1H, NH) ppm; H NMR ((CD3)2SO, 300
MHz): δ 1.49 (s, 3H, CH3), 1.77 (s, 3H, CH3), 1.96 (s, 3H, CH3),
2.05 (s, 3H, CH3), 2.18 (s, 3H, CH3), 5.93 (s, 1H, -CHd), 7.18-
7.36 (m, 4H, 4 Ar-H), 10.50 (br s, 1H, NH), 11.13 (br s, 1H,
NH) ppm; 13C NMR (90% CDCl3 and 10% (CD3)2SO) δ 8.01
(CH3), 8.89 (CH3), 9.46 (CH3), 10.45 (CH3), 18.43 (CH3), 94.84,
123.2, 124.3, 125.1, 126.3, 126.7, 127.4, 127.5, 128.7, 130.0,
130.2, 134.3, 135.4, 141.1, 172.8 (CdO, lactam), 185.9 (CdO)
ppm; 13C NMR ((CD3)2SO) δ 8.57 (CH3), 9.35 (CH3), 9.78 (CH3),
10.49 (CH3), 18.95 (CH3), 95.78, 123.6, 126.1, 127.0, 127.3,
127.8, 129.7, 130.8, 130.9, 134.7, 135.0, 136.4, 141.3, 142.2,
173.0 (CdO, lactam), 186.2 (CdO) ppm; MS(FAB) m/z ) 334
(M+).
1
°C/1 mmHg; H NMR (CDCl3, 300 MHz) δ 2.60 (t, J ) 7.82
Hz, 2H, CH2CH2COO), 3.18 (t, J ) 7.82 Hz, 2H, CH2CH2COO),
3.61 (s, 3H, OCH3), 7.31-7.38 (m, 2H, 2Ar-H), 7.50-7.55 (m,
1H, Ar-H), 8.17 (d, J ) 8.30 Hz, 1H, Ar-H) ppm; 13C NMR
(CDCl3) δ 29.45 (CH2CH2COO), 34.61 (CH2CH2COO), 51.38
(OCH3), 126.9, 126.9, 131.2, 133.7, 134.2, 143.0, 167.3 (COCl),
172.6 (COOMe) ppm.
2-[2-(Methoxycarbonyl)ethyl]benzoyl chloride (596 mg, 2.78
mmol, 2.0 mol equiv) and dry dichloromethane (50 mL) were
added to a 100 mL three-neck flask fitted with a magnetic
stirrer and nitrogen inlet. The flask was cooled with an ice-
salt bath, and nitrogen was bubbled throughout the solution.
After 20 min, 2,3,7,8-tetramethyldipyrrinone (7) (300 mg, 1.39
mmol mol) was added, followed by tin tetrachloride (3.61 g,
1.62 mL, 13.9 mmol, 10 mol equiv). The mixture was stirred
for another 20 min at 0 °C after the addition was complete.
Before it was poured into an ice-HCl solution (100 g of ice +
50 mL of concentrated hydrochloric acid), the mixture was
allowed to stir for 2 h at room temperature. The aqueous
mixture was extracted with dichloromethane (3 × 50 mL). The
combined extract was washed with hydrochloric acid (2 × 30
mL, 10%), water (2 × 30 mL), and saturated aqueous sodium
bicarbonate (50 mL). The organic extract was dried over
anhyd sodium sulfate, the solvent was removed, and the
residue was purified by flash chromatographic column using
dichloromethane-methanol (97/3, by vol) as eluant to yield
the desired compound, which was further purified by recrys-
tallization from methanol. Pure title product was obtained
(545 mg, 95%): mp 212-214 °C; IR (film) ν 3336, 2835, 2830,
Anal. Calcd for C21H22N2O2 (334.4): C, 75.45; H, 6.59; N,
8.38. Found: C, 75.20; H, 6.33; N, 8.06.
9-(2-Meth ylben zyl)-2,3,7,8-tetr a m eth yld ip yr r in on e (3).
Meth od A: Con d en sa tion of Dip yr r in on e w ith 2-(Br o-
m om eth yl)tolu en e (5). To a mixture of 2,3,7,8-tetrameth-
yldipyrrinone (10 mg, 46 mmol) and 2-(bromo)methyltoluene
(100 mg, 540 mmol, 11.7 mol equiv) in methanol (10 mL) was
added p-toluenesulfonic acid monohydrate (10 mg, 58 mmol,
1.26 mol equiv). The solution was stirred under nitrogen
protection for 12 h at 40 °C. After being cooled to room
temperature, the mixture was kept for 5 h at -20 °C, and the
resulting precipitate was collected by suction titration. The
solid was crystallized from dichloromethane-hexane to yield
10.9 mg of 5 (73% yield).
Meth od B: Red u ction of 6. 9-(2-Methylbenzoyl)-2,3,7,8-
tetramethyldipyrrinone (6) (50 mg, 0.15 mmol) and 2-propanol
(50 mL) were placed in a 100 mL flask. The mixture was
heated to reflux under nitrogen; potassium borohydride (200

2674 J . Org. Chem., Vol. 63, No. 8, 1998
Chen and Lightner
1736, 1672, 1617, 1560, 1508, 1490, 1432, 1405, 1166, 934, 757
cm-1; UV-Vis (Table 4); 1H NMR (CDCl3, 300 MHz) δ 1.69 (s,
3H, CH3), 1.94 (s, 3H, CH3), 2.04 (s, 3H, CH3), 2.10 (s, 3H,
CH3), 2.63 (t, J ) 7.32 Hz, 2H, CH2CH2COO), 3.00 (t, J ) 7.32
Hz, 2H, CH2CH2COO), 3.62 (s, 3H, OCH3), 5.95 (s, 1H, -CHd),
7.26-7.38 (m, 4H, 4 Ar-H), 9.36 (br s, 1H, NH), 9.57 (br s, 1H,
(CDCl3, 500 MHz) δ 1.84 (s, 3H, CH3), 2.05 (s, 3H, CH3), 2.13
(s, 3H, CH3), 2.14 (s, 3H, CH3), 2.98 (t, J ) 5.5 Hz, 2H,
CH2CH2COO), 3.07 (t, J ) 5.50 Hz, 2H, CH2CH2COO), 4.08
(s, 2H, -CH2-), 6.07 (s, 1H, -CHd), 6.98 (d, J ) 7.5 Hz, 1H,
Ar-H), 7.05 (t, J ) 7.5 Hz, 1H, Ar-H), 7.13 (t, J ) 8.0 Hz, 1H,
Ar-H), 7.20 (d, J ) 7.5 Hz, 1H, Ar-H), 8.98 (s, 1H, NH), 10.53
(s, 1H, NH), 12.61 (br s, 1H, COOH) ppm; 1H NMR ((CD3)2SO,
500 MHz) δ 1.76 (s, 3H, CH3), 1.79 (s, 3H, CH3), 2.03 (s, 3H,
CH3), 2.05 (s, 3H, CH3), 2.47 (t, J ) 7.50 Hz, 2H, CH2CH2COO),
2.90 (t, J ) 7.50 Hz, 2H, CH2CH2COO), 3.92 (s, 2H, CH2) 5.95
(s, 1H, -CHd), 6.78 (d, J ) 7.50 Hz, 1H, Ar-H), 7.06-7.14
(m, 2H, 2 Ar-H), 7.16 (d, J ) 8.00 Hz, 1H, Ar-H), 9.72 (s, 1H,
NH), 10.28 (s, 1H, NH), 12.21 (s, 1H, COOH) ppm; 13C NMR
(CDCl3) δ 7.91 (CH3), 8.86 (CH3), 9.41 (CH3), 9.47 (CH3), 24.58
(CH2CH2COO), 27.01 (CH2), 34.80 (CH2CH2COO), 101.0, 126.1,
126.2, 128.6, 128.6, 129.3, 129.2, 130.8, 135.4, 138.0, 138.4,
148.7, 153.1, 153.7, 174.5 (CdO, lactam), 178.9 (COOH) ppm;
13C NMR ((CD3)2SO) δ 8.20 (CH3), 8.73 (CH3), 9.33 (CH3), 9.46
(CH3), 27.24 (CH2CH2COO), 28.45 (CH2), 34.04 (CH2CH2COO),
97.73, 116.1, 122.3, 122.7, 123.5, 126.0, 126.0, 126.0, 128.4,
128.9, 130.6, 137.5, 138.4, 141.4, 171.8 (CdO, lactam), 173.7
(COOH) ppm; MS(FAB) m/z 378 (M+).
1
NH) ppm; H NMR ((CD3)2SO, 300 MHz) δ 1.47 (s, 3H, CH3),
1.75 (s, 3H, CH3), 1.95 (s, 3H, CH3), 2.04 (s, 3H, CH3), 2.53 (t,
J ) 7.32 Hz, 2H, CH2CH2COO), 2.78 (t, J ) 7.32 Hz, 2H,
CH2CH2COO), 3.49 (s, 3H, OCH3), 5.91 (s, 1H, -CHd), 7.19-
7.38 (m, 4H, 4 Ar-H), 10.46 (br s, 1H, NH), 11.13 (br s, 1H,
NH) ppm; 13C NMR (CDCl3) δ 8.34 (CH3), 9.09 (CH3), 9.59
(CH3), 10.56 (CH3), 28.01 (CH2CH2COO), 35.19 (CH2CH2COO),
51.40 (OCH3), 96.21, 126.1, 127.3, 128.6, 128.8, 129.6, 129.7,
130.5, 130.7, 130.7, 137.0, 138.0, 140.1, 141.5, 173.2 (CdO,
lactam), 173.5 (COOMe), 186.3 (CdO) ppm; 13C NMR ((CD3)2SO)
(Table 2); MS(FAB) m/z 406 (M+).
Anal. Calcd for C24H26N2O4 (406.5): C, 70.93; H, 6.40; N,
6.89. Found: C, 70.50; H, 6.39; N, 6.89.
9-[2-(2-Car boxyeth yl)ben zoyl]-2,3,7,8-tetr am eth yldipyr -
r in on e (2). 9-[2-(2-Methoxycarbonyl)benzoyl]-2,3,7,8-tetram-
ethyldipyrrinone (5) (50 mg, 0.123 mmol) was dissolved in a
solution of methanol (10 mL) and aqueous sodium hydroxide
(10%, 3 mL). The mixture was stirred and warmed to reflux
under nitrogen atmosphere protection for 2 h. After being
cooled to room temperature, the mixture was brought to pH 7
by adding acetic acid dropwise. The solution was extracted
with dichloromethane (3 × 50 mL), and the combined extracts
were washed with water (2 × 50 mL). The organic extract
was dried over anhyd sodium sulfate, the solvent was removed
(rotovap), and the residue was crystallized from methanol to
yield the desired product (36.2 mg) in 75% yield: mp 200-
202 °C; IR (film) ν 3376, 2840, 1690, 1520, 1490, 1430, 1405,
1350, 1268, 1166, 933, 836, 757, 631 cm-1; UV-vis (Table 4);
1H NMR (CDCl3, 500 MHz) δ 1.85 (s, 3H, CH3), 2.08 (s, 3H,
CH3), 2.08 (s, 3H, CH3), 2.43 (s, 3H, CH3), 2.83 (t, J ) 6.50
Hz, 2H, CH2CH2COO), 3.08 (t, J ) 6.50 Hz, 2H, CH2CH2COO),
6.05 (s, 1H, -CHd), 7.15-7.17 (m, 2H, 2 Ar-H), 7.40 (m, 2H,
2 Ar-H), 8.15 (s, 1H, NH), 10.69 (s, 1H, NH), 12.37 (br s, 1H,
COOH) ppm; 1H NMR ((CD3)2SO, 500 MHz) δ 1.51 (s, 3H,
CH3), 1.79 (s, 3H, CH3), 1.98 (s, 3H, CH3), 2.07 (s, 3H, CH3),
2.46 (t, J ) 7.50 Hz, 2H, CH2CH2COO), 2.77 (t, J ) 7.50 Hz,
2H, CH2CH2COO), 5.95 (s, 1H, -CHd), 7.23 (d, J ) 7.50 Hz,
1H, Ar-H), 7.30 (t, J ) 7.50, 7.0 Hz, 1H, Ar-H), 7.38 (d, J )
7.50 Hz, 1H, Ar-H), 7.41 (t, J ) 7.50, 7.0 Hz, 1H, Ar-H), 10.50
Anal. Calcd for C23H26N2O3 (378.5): C, 73.01; H, 6.88; N,
7.41. Found: C, 73.16; H, 6.92, N, 7.40.
9-[2-[2-(Meth oxyca r bon yl)eth yl]ben zyl]-2,3,7,8-tetr a m -
eth yld ip yr r in on e (4). 9-[2-(2-Carboxyethyl)benzyl]-2,3,7,8-
tetramethyldipyrrinone (1) (15 mg, 39.68 mmol) was dissolved
in dry methanol (50 mL). To this mixture was added concen-
trated sulfuric acid (1 mL) dropwise over 5 min. The solution
was stirred at room temperature for 1 h before dichlo-
romethane (100 mL) and water (100 mL) were added. The
organic layer was separated, and the aqueous phase was
extracted with dichloromethane (2 × 50 mL). The combined
organic extract was successively washed with water (2 × 70
mL), saturated aqueous sodium bicarbonate (70 mL), and brine
(70 mL). The organic extract was dried over anhyd sodium
sulfate, the solvent was removed (rotovap), and the residue
was purified by radial chromatography using 2% methanol in
dichloromethane as eluant. The main yellow fraction was
collected and recrystallized from methanol and then from
dichloromethane-hexane to afford the expected product (12 mg)
in 77% yield: mp 236-238 °C dec; IR (KBr, film) ν 3344, 2923,
2854, 1741, 1735, 1662, 1636, 1362, 1276, 1176, 934, 756, 691
cm-1; UV-vis (Table 4); 1H NMR (CDCl3, 300 MHz) δ 1.65 (s,
3H, CH3), 1.91 (s, 3H, CH3), 2.07 (s, 3H, CH3), 2.10 (s, 3H,
CH3), 2.54 (t, J ) 7.32 Hz, 2H, CH2CH2COO), 3.04 (t, J ) 7.32
Hz, 2H, CH2CH2COO), 3.66 (s, 3H, OCH3), 4.16 (s, 2H, CH2),
6.11 (s, 1H, -CHd), 7.00-7.14 (m, 4H, 4 Ar-H), 10.15 (s, 1H,
NH), 10.79 (s, 1H, NH) ppm; 1H NMR ((CD3)2SO, 300 MHz) δ
1.74 (s, 6H, 2 CH3), 2.01 (s, 3H, CH3), 2.03 (s, 3H, CH3), 2.49
(t, J ) 7.32 Hz, 2H, CH2CH2COO), 2.90 (t, J ) 7.32 Hz, 2H,
CH2CH2COO), 3.55 (s, 3H, OCH3), 3.90 (s, 2H, -CH2-), 5.93
(s, 1H, -CHd), 6.77 (d, J ) 6.35 Hz, 1H, Ar-H), 7.08-7.12
(s, 1H, NH), 11.17 (s, 1H, NH), 12.10 (s, 1H, COOH) ppm; 13
C
NMR (CDCl3) δ 8.02 (CH3), 8.70 (CH3), 9.49 (CH3), 10.33 (CH3),
25.89 (CH2CH2COO), 34.21 (CH2CH2COO), 98.74, 125.3, 125.3,
126.9, 127.8, 128.2, 129.0, 129.0, 129.2, 129.8, 129.9, 137.6,
139.4, 141.4, 173.3 (CdO, lactam), 175.5 (COOH), 177.6 (CdO)
ppm; 13C NMR ((CD3)2SO) δ 8.59 (CH3), 9.36 (CH3), 9.78 (CH3),
10.78 (CH3), 27.98 (CH2CH2COO), 35.31 (CH2CH2COO), 95.71,
123.7, 126.5, 127.3, 128.0, 129.8, 130.3, 130.8, 130.9, 131.2,
136.5, 138.2, 141.0, 142.1, 173.0 (CdO, lactam), 173.7 (COOH),
186.0 (CdO) ppm; MS(FAB) m/z 392 (M+).
(m, 3H, 3 Ar-H), 9.71 (s, 1H, NH), 10.26 (s, 1H, NH) ppm; 13
C
NMR (CDCl3) δ 7.99 (CH3), 8.02 (CH3), 9.41 (CH3), 9.61 (CH3),
27.65 (CH2CH2COO), 29.63 (CH2), 34.43 (CH2CH2COO), 51.43
(OCH3), 100.5, 116.9, 122.5, 123.6, 124.5, 126.3, 126.4, 128.5,
128.9, 129.1, 132.3, 137.1, 138.0, 141.9, 173.6 (CdO, lactam,
and COO) ppm; 13C NMR ((CD3)2SO) δ 8.53 (CH3), 9.06 (CH3),
9.64 (CH3), 9.78 (CH3), 27.54 (CH2CH2COO), 29.00 (CH2), 34.11
(CH2CH2COO), 51.51 (OCH3), 98.04, 122.6, 123.0, 125.4, 125.5,
126.1, 126.5, 126.6, 128.9, 129.0, 129.3, 137.9, 138.5, 141.7,
172.1 (CdO, lactam, and COO) ppm; MS(FAB) m/z 392 [M+].
Anal. Calcd for C24H28N2O3 (392.5): C, 73.47; H, 7.14; N,
7.14. Found: C, 73.35; H, 7.19; N, 7.14.
Anal. Calcd for C23H24N2O4 (392.5): C, 70.39; H 6.16; N,
7.14. Anal. Calcd for C23H24N2O4‚CH3OH (423): C, 68.09; H,
6.38; N, 6.66. Found: C, 68.50; H, 6.09; N, 6.66.
9-[2-(2-Ca r boxyeth yl)ben zyl]-2,3,7,8-tetr a m eth yld ip yr -
r in on e (1, Hem ir u bin ). 9-[2-[2-(Methoxycarbonyl)ethyl]ben-
zoyl]-2,3,7,8-tetramethyldipyrrinone (5) (115 mg, 0.28 mmol)
and 2-propanol (50 mL) were placed in a 100 mL flask. The
mixture was heated to reflux under nitrogen; then potassium
borohydride (200 mg, 3.70 mmol, 13.27 mol equiv) was added
in small portions over 20 min. The mixture was allowed to
heat at reflux for another 2 h after the addition was complete.
The reaction was cooled to room temperature and poured onto
ice. The mixture was extracted with dichloromethane (3 ×
50 mL), and the extract was washed with aqueous hydrochloric
acid (2 × 50 mL, 10%) and water (50 mL). The organic layer
was dried over anhyd sodium sulfate, and the solvent was
removed (rotovap). The residue was crystallized from metha-
nol to yield the desired product (67 mg) in 63% yield: mp 220°
dec; IR (film) ν 3425, 3340, 2916, 2863, 1163, 1443, 1355, 1273,
1179, 940, 756, 731, 695 cm-1; UV-vis (Table 4); 1H NMR
Ack n ow led gm en t. We thank the National Insti-
tutes of Health (HD-17779) for support of this research
and the National Science Foundation (CHE-9214294
and DIR-9102893) for funds to purchase the 500 MHz
NMR spectrometer and mass spectrometer used in this
study. We are indebted to Mr. Michael Huggins for
determining the energy-minimum structures of 1 and
2 and for creating their ball and stick displays.

Hydrogen-Bonded Analog for One-Half Bilirubin
J . Org. Chem., Vol. 63, No. 8, 1998 2675
Su p p or tin g In for m a tion Ava ila ble: Spectra for com-
pounds 1-7 in (CD3)2SO solvent are reported herein (14
pages). This material is contained in libraries on microfiche,
immediately follows this article in the microfilm version of the
journal, and can be ordered from the ACS; see any current
masthead for ordering information.
J O972227R