An enantiomerically pure bilirubin. Absolute configuration of (αR,α′R)-dimethylmesobilirubin-XIIIα

Article abstract of DOI:10.1016/S0957-4166(01)00455-4

Enantiomerically pure (+)-(αR,α′R)-dimethylmesobilirubin-XIIIα 1 and its (αS,α′S) enantiomer ent-1 were synthesized in ten steps from simple precursors. Resolution was achieved at an early stage in the synthesis, with a racemic monopyrrole precursor rac-6 being converted to its amides 8 with (1S)-camphor-2,10-sultam. Resolution of 8 to 99% d.e. was accomplished in three crystallizations, and the absolute configuration of the acid 6 was deduced by X-ray crystallography of the more crystalline, diastereomerically pure amide 7. Circular dichroism spectroscopy of 1 showed intense bisignate Cotton effects: Δε₄₃₅^max = +344, Δε₃₉₁^max = -193 (CHCl₃), as expected for a molecular exciton, and consistent with a P-helical intramolecularly hydrogen-bonded ridge-tile conformation. The Cotton effect magnitudes of 1 match almost exactly those found for (-)-(βS,β′S)-dimethylmesobilirubin-XIIIα 11 and (+)-(αR,β′R)-dimethylmesobilirubin-XIIIα. However, the Cotton effect of the pseudo-meso diastereomer (αR,β′S)-dimethylmesobilirubin-XIIIα 12 is not zero. Its large positive exciton couplet and ¹H NMR NOE analysis confirm that an α-CH₃ exerts a greater steric demand than a β-CH₃ - by a factor of ～3.

Full text of DOI:10.1016/S0957-4166(01)00455-4

TETRAHEDRON:
ASYMMETRY
Pergamon
Tetrahedron: Asymmetry 12 (2001) 2551–2564
An enantiomerically pure bilirubin. Absolute configuration of
(aR,a%R)-dimethylmesobilirubin-XIIIa
Stefan E. Boiadjiev and David A. Lightner*
Department of Chemistry, University of Nevada, Reno, NV 89557, USA
Received 30 August 2001; accepted 27 September 2001
Abstract—Enantiomerically pure (+)-(aR,a%R)-dimethylmesobilirubin-XIIIa 1 and its (aS,a%S) enantiomer ent-1 were synthesized
in ten steps from simple precursors. Resolution was achieved at an early stage in the synthesis, with a racemic monopyrrole
precursor rac-6 being converted to its amides 8 with (1S)-camphor-2,10-sultam. Resolution of 8 to 99% d.e. was accomplished in
three crystallizations, and the absolute configuration of the acid 6 was deduced by X-ray crystallography of the more crystalline,
diastereomerically pure amide 7. Circular dichroism spectroscopy of 1 showed intense bisignate Cotton effects: Dmm43a5x=+344,
Dmm39a1x=−193 (CHCl3), as expected for a molecular exciton, and consistent with a P-helical intramolecularly hydrogen-bonded
ridge-tile conformation. The Cotton effect magnitudes of 1 match almost exactly those found for (−)-(bS,b%S)-dimethylmesobiliru-
bin-XIIIa 11 and (+)-(aR,b%R)-dimethylmesobilirubin-XIIIa. However, the Cotton effect of the pseudo-meso diastereomer
1
(aR,b%S)-dimethylmesobilirubin-XIIIa 12 is not zero. Its large positive exciton couplet and H NMR NOE analysis confirm that
an a-CH3 exerts a greater steric demand than a b-CH3—by a factor of ꢀ3. © 2001 Elsevier Science Ltd. All rights reserved.
1. Introduction
chiral compound, such as quinine9 or serum albumin,10
and observed by circular dichroism spectroscopy (CD).
Bilirubin (Fig. 1), the lipophilic yellow pigment of
jaundice, belongs to the class of pigments called ‘linear
tetrapyrroles’1 but its solution and biological
properties2 do not correlate well with either the linear
structure (Fig. 1A) or a porphyrin-like structure (Fig.
1B), which are predicted to be rather polar. In biliru-
bin, and its mesobilirubin analogs, where vinyls are
replaced by ethyl while retaining propionic acids at
C(8) and C(12) (Fig. 1C), the most stable conformation
is bent in the middle, in the shape of a ridge-tile.3–6 In
the ridge-tile conformation, which is flexible, not rigid,
each propionic acid group is engaged in intramolecular
hydrogen bonding to the opposing dipyrrinone lactam
and pyrrole (Fig. 1D), thus affording considerable con-
formational stabilization and rendering the pigments
surprisingly lipophilic.7
It was found earlier that selective stabilization of one
enantiomer can also be achieved through intramolecu-
lar non-bonded steric interactions, e.g. when stereo-
genic centers are created by a single methyl substitution
at either the a or the b carbons of both propionic acid
chains.11–13 Such forced ‘intramolecular resolution’ was
first achieved in a,a%-dimethylmesobilirubin-XIIIa 111
and its b,b%-dimethyl analog.12 The absolute configura-
tion at the b,b% stereogenic centers of the latter was
established unequivocally from an X-ray crystal struc-
ture of the resolved diastereomeric salt (with brucine) of
a monopyrrole b-methylpropionic acid precursor to the
central rings of the mesobilirubin. The absolute
configuration of the former, resolved by partitioning
into chloroform from its complex with human serum
albumin in pH 9.0 solution, was deduced from the
NMR 1H{1H}-NOE spectrum of its bis-amide from
(−)-a-phenethylamine.11 As such it does not have the
same clarity as that from the crystallographic study of
the b,b%-dimethylrubin. In addition the e.e. of the
‘resolved’ rubin was not apodictic, and that led recently
to difficulties in quantitative determinations of func-
tional group steric size using circular dichroism on the
bilirubin platform.14
The ridge-tile conformations of Fig. 1D have near-C2-
symmetry. They are dissymmetric and enantiomeric. In
an isotropic medium, bilirubin and its synthetic analog
mesobilirubin-XIIIa8 thus consist of a 50:50 mixture of
equilibrating conformational enantiomers. Displace-
ment of the equilibrium toward one or the other of the
enantiomers has been achieved by complexation with a
In the following, we present an improved synthesis and
unequivocal determination of the absolute configura-
* Corresponding author. E-mail: lightner@scs.unr.edu
0957-4166/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0957-4166(01)00455-4

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S. E. Boiadjie6, D. A. Lightner / Tetrahedron: Asymmetry 12 (2001) 2551–2564
Figure 1. Bilirubin in its unstable linear (A) and porphyrin-like (B) conformations, and in its most stable ridge-tile conformations
(D). (C) Mesobilirubin-XIIIa, a close analog of bilirubin that also prefers ridge-tile conformations. In (D), the enantiomeric,
intramolecularly hydrogen-bonded ridge-tile conformers interconvert rapidly. The double-headed arrows represent the dipyrrinone
long wavelength electric transition moment vectors (dipoles). The relative helicities (M, minus, or P, plus) of the vectors are shown
(inset) for each enantiomer. Dashed lines represent hydrogen bonds.
tion and e.e. of a,a%-dimethylmesobilirubin-XIIIa. In
addition, we provide revised circular dichroism (CD)
data for the 100% e.e. (+)-(aR,a%R) enantiomer 1 and
(aR,b%S) regio isomer 12. With CD data from the latter
and previously published CD data, we are able to
construct a consistent quantitative picture of alkyl (Me,
Et, i-Pr, t-Bu), aryl (Bn, Ph), SMe and OMe steric size.
(from reaction with oxalyl chloride) then coupling with
the sodium salt of 9 (formed from 9 and NaH). The
resulting mixture of diastereomeric amides 8 [mp 158–
184°C, [h]2D0=−46.6 (c 1.2, CHCl3)], was separated by
crystallization from ethyl acetate–hexane. Three crystal-
lizations achieved a sharp melting point (191–193°C);
constant rotation: [h]2D0=−92.2 (c 1.2, CHCl3); and
]99% d.e. The progress of the resolution and% d.e.
1
could be followed by H NMR, observing the methyl
region (Fig. 2) or the b-CH2 region. The absolute
configuration of the acid component of this highly
resolved amide 7 was determined by X-ray crystallogra-
phy (below).
2. Results and discussion
2.1. Synthesis
The target rubin 1 was prepared as outlined in Scheme
1 from small components using a design strategy previ-
ously discussed.15 The key intermediate, racemic car-
boxylic acid rac-6, was obtained in high yield by
selective partial saponification of diester 10, and then
converted to its amides with (1S)-camphor-2,10-sultam
9. Amide formation was achieved by reaction of 9 and
rac-6 in the presence of dicyclohexylcarbodiimide and
DMAP, or by conversion of rac-6 to its acid chloride
Hydrolysis of sultam amides poses a problem. Harsh
conditions that might lead to racemization at the propi-
onic acid a-carbon should be avoided. Hydrolysis of
the carbomethoxy group should also be avoided as the
resulting pyrrole a-carboxylic acids are typically sensi-
tive toward decarboxylation. Following Oppolzer,16
selective cleavage of the amide group of 7 was achieved
in good yield by reaction with LiOH–H2O2 in THF–

S. E. Boiadjie6, D. A. Lightner / Tetrahedron: Asymmetry 12 (2001) 2551–2564
2553
Scheme 1. Reagents and conditions: (a) NaOH; (b) HCl; (c) NaBH4; (d) p-chloranil/HCO2H/CH2Cl2; (e) HNO3; (f) MeOH, D; (g)
LiOH/H2O2/THF/H2O; (h) recrystallization from EtOAc–hexane; (i) (COCl)2, NaH; (j) DCC, DMAP; (k) HONꢀC(CO2Me)2, Zn,
HOAc; (l) KF/EtOH, D.
final step. However, we could detect none of the latter,
either chromatographically or by NMR.
H2O to afford monoacid 6 and recoverable sultam 9.
Complete saponification of 6, followed by careful
acidification in cold conc. HNO3–50% aq. NaNO3
afforded the diacid of 6 in quantitative yield, and the
diacid was coupled with bromomethylene–pyrrolinone
5 in refluxing CH3OH to give dipyrrinone (−)-4 in 70%
yield. The latter was oxidatively self-coupled in the
presence of p-chloranil to afford verdin 3 in excellent
yield, and 3 was reduced to rubin dimethyl ester 2. In a
separate experiment, the verdin 3 was first saponified
then reduced with NaBH4 to give only (+)-1 in overall
high yield. If partial racemization had occurred during
the conversion of 7 to 1, a mixture of 1 and its
(R,S)-diastereomer11 would have been present at the
2.2. Molecular geometry and absolute configuration
from X-ray crystallography
Crystals of (−)-7 were grown in ethyl acetate by diffu-
sion of hexane vapor. Molecules of 7 pack in an
orthorhombic lattice with the sultam SO2 group lying
close to one face of the pyrrole ring, and near the
C(5)-CH3. This conformation (Fig. 3) is also predicted
by molecular mechanics calculations (Sybyl)17 to be the
most stable of the three staggered rotamers that follow
from rotation about the amide CaꢁCb bond. In the

2554
S. E. Boiadjie6, D. A. Lightner / Tetrahedron: Asymmetry 12 (2001) 2551–2564
(aR) absolute configuration to acid 6, with [h]2D0=−51.2
(c 1.2, CHCl3), as shown in Scheme 1. In other ways,
the molecular geometry of crystalline
unexceptional.
7 appears
2.3. Local stereochemistry and pigment conformational
enantiomerism
Intramolecular hydrogen bonding of the type shown in
Fig. 1D is known to be the most important factor in
stabilizing the ridge-tile conformation,3–6,18,19 which is
not rigid but lepidopterous.6 Ridge-tiles with small
dihedral angles (q) are higher energy, as are ridge-tiles
with larger dihedral angles.5,6 The dynamic process of
opening the dihedral angle and slipping over the barrier
for the interconversion of enantiomeric ridge-tiles (Fig.
1D) is calculated to be of high energy (ꢀ20 kcal/mol)5
and experimentally determined as rapid (3–95 s−1 at
50–95°C).18 By taking advantage of intramolecular
non-bonded steric interactions in the ridge-tile, one can
displace the conformational enantiomeric equilibrium
(Fig. 1D) toward either enantiomer simply by replacing
one of the pro-stereogenic a (or b) hydrogens with a
methyl group.11–14 An (aR)-methyl thrusts into the C(7)
or C(13) ring methyl of the dipyrrinone to which the
acid is hydrogen bonded when the pigment is in the
M-helical conformation—but not in the P (as may be
seen for the (aR)-CH3 in the propionic acid at C(8) of
Fig. 4). An (aS)-methyl would destabilize the P-helical
conformation but not the M. Similarly, a (bS)-methyl
would destabilize the P-helical conformation but not
the M (as may be seen for the (b%S)-CH3 in the propi-
onic acid at C(12) of Fig. 4). Consequently, one can tilt
the equilibrium toward the M or P helicity enantiomer
especially in non-polar solvents that promote hydrogen
bonding, simply by the choice of local stereochemistry
at the a (or b) carbon of the propionic acid chain. But
one methyl group is sufficient.20 Two methyls acting in
concert are quite effective in displacing the M?P
conformational equilibrium and produce very large cir-
cular dichroism (CD) Cotton effects from the pigment’s
Figure 2. Partial 1H NMR spectra in CDCl3 of (lower) the
diastereomeric camphor sultam amides 8, showing the C(8)
and C(9) camphor methyls and a-methyl region for a 50:50
diastereomer ratio, and (upper) a 99:1 diastereomer ratio in
isolated 7.
crystal, the propionic amide chain is oriented so that
the pyrrole ring and amide carbonyl are syn-clinal
rather than anti, and from the crystal structure one can
know the relative configuration of the pyrrole and
sultam components. Since the absolute configuration of
the (−)-camphor-2,10-sultam is known (1S,2R,4R), one
can readily deduce the absolute configuration of the
pyrrole component of (−)-7 as (aR) and thus assign the
Figure 3. Crystal structure drawing and crystallographic atom numbering of amide 7. The sultam unit lies below the pyrrole, and
the stereochemistry at C(122) is evidently (R), given that the (−)-camphor-2,10-sultam has known (1S,2R,4R) absolute
configuration.

S. E. Boiadjie6, D. A. Lightner / Tetrahedron: Asymmetry 12 (2001) 2551–2564
2555
Figure 4. Ball and stick conformational representations for the M- and P-chirality, intramolecularly hydrogen-bonded, intercon-
verting ridge-tile conformations of (aR,b%S)-dimethylmesobilirubin-XIIIa (12). One of the methyl substituents on the propionic
acid side chains attached to pyrrole ring carbons C(8) and C(12) is in a sterically-crowded position, whether in the M or P
conformer. When the M-chirality conformer inverts into the P-chirality, steric crowding of the (aR)-methyl by the C(7)-methyl
is relieved and replaced by steric crowding of the (b%S)-methyl by the C(10) methylene group.
long wavelength absorption in UV–vis near 430 nm, as
predicted from exciton coupling theory,21,22 opening
the door to sensitive quantitative investigations.
(Table 1). Confirming our suspicions, the new data
show larger Dm values than those seen earlier.11 Signifi-
cantly, the absolute values of the Cotton effect magni-
tudes are essentially identical to those of
(bS,b%S)-dimethylmesobilirubin-XIIIa 11.12 For exam-
ple, the exciton CD curves of 1 and 11 are essentially
mirror images (Fig. 5). Interestingly, when a and b
methyls act in opposition (anti-chiral14) to control the
stereochemistry of the pigment, as in the pseudo-meso
isomer, (aR,b%S)-dimethylmesobilirubin-XIIIa 12 (Fig.
4), the (aR)-methyl clearly exerts a greater steric
demand than the (b%S)-methyl and thus controls the
signs of the exciton Cotton effects (Table 1). Thus, as
seen in Fig. 5, positive exciton chirality CD of 12
indicates a P-helicity conformation, which we estimate
dominates the M by a factor of ꢀ3 (KM?P=2.97, or
75% P, and DG=−0.638 kcal/mol at 22°C). In the
other non-polar solvents (hexane, benzene, CH2Cl2,
ClCH2CH2Cl) the equilibrium population of P is
nearly the same. In the very polar, aprotic solvent
N-methylformamide, the P-helicity conformation
dominates (KM?P=1.64, or 62% P); and even in the
polar solvent ethanol, the P-helicity conformation still
dominates (KM?P=1.54, or 61% P). It is not entirely
clear why the a-methyl should exert a greater effective
steric size than a b-methyl, but the observation opened
the door to assessing group steric size by CD spec-
troscopy.
Previous
investigations
with
dimethylbilirubins:
(aR,a%R), (aS,a%S);11 (bS,b%S), (bR,b%R);12 (aR,b%S)
and (aS,b%S)13 confirmed this allosteric model and per-
mitted a study13 by CD spectroscopy to determine the
relative importance of the steric demand of a and b
methyls where they acted not in concert but in opposi-
tion, as in (aR,b%S) of Fig. 4. It was learned that an a
methyl exerted a greater steric demand than a b
methyl, which led to an assessment of group steric
size.14 However, certain quantitative aspects of the CD
studies remained puzzling, particularly when the a,a%-
methyls acted cooperatively (as in R,R, or S,S) to
unbalance the conformational enantiomerism of Fig.
1D. The amplitude of the exciton CD Cotton effect
was considerably lower than that when b,b%-methyls
acted in unison.13,14 We suspected that the resolved
a,a%-dimethyl rubin was of <100% e.e., but we could
not ascertain this with certainty or even know its
e.e.11,13
In the current work, with precursor 6 of ]99% e.e.
and evidence against any partial racemization of the
stereogenic center during the course of its conversion
to 1, we redetermined the CD Cotton effect data for 1

2556
S. E. Boiadjie6, D. A. Lightner / Tetrahedron: Asymmetry 12 (2001) 2551–2564
2.4. Reassessment of functional group steric size
of an exciton CD couplet.23 A-values from various
(aR)-substituted, (b%S)-methylmesobilirubins in CH2Cl2
solvent are compiled in Table 2. The positive CD
A-values observed for each example except OCH3
confirm that the (aR)-substituent dominates the (b%S)-
methyl, i.e. it is at least as large in steric size as a
methyl group (as discussed in the preceding section).
The negative CD A-value of the (aR)-OCH3, (b%S)-CH3
rubin indicates that the (b%S)-CH3 exerts a greater steric
demand than an (aR)-OCH3, consistent with the a-
Previous studies indicated that one might obtain both
qualitative and quantitative information on the relative
steric size of functional groups (such as SCH3 and
OCH3,22 or Et, Bn, i-Pr, t-Bu and Ph14) from the CD
spectra of analogs of 12, wherein the a-substituent is
drawn from those cited. The CD A-value, where A=
Dmm1 ax(u1)−Dmm2 ax(u2) for u1>u2, of the exciton couplet,
has been suggested as a useful measure of the intensity
Table 1. CD and UV–vis spectral data from (aR,a%R), (aR,b%S) and (bS,b%S)-dimethylmesobilirubins-XIIIa at 22°Ca
Pigment
Solvent
Hexane
mb
Ac
CD
Dm2(u2)
UV
Dm1(u1)
u at Dm=0 mmax
u (nm)
aR,a%R
1.9
+615
+422 (430)
−193 (390)
403
64,700
438
417
435
431
411
440
440
435
439
438
433
437
430
431
436
419
431
435
433
430
435
434
430
431
423
427
431
425
426
429
424
425
429
422
423
430
426
427
430
432
425
430
430
426
434
430
432
56,800
61,600
60,400
53,600
63,100
59,900
60,800
61,200
57,000
55,400
59,900
55,800
55,800
60,900
52,400
57,900
60,600
57,500
54,800
59,700
56,400
57,100
59,500
55,400
57,100
59,600
55,600
57,600
60,400
56,800
60,800
58,400
54,500
56,700
59,200
54,600
55,200
61,900
60,400
56,700
66,500
62,000
66,000
59,000
54,200
57,700
aR,b%S
bS,b%S
+340
−617
+229 (428)
−423 (430)
−111 (386)
+194 (388)
401
402
aR,a%R
aR,b%S
bS,b%S
aR,a%R
aR,b%S
bS,b%S
aR,a%R
aR,b%S
bS,b%S
aR,a%R
aR,b%S
bS,b%S
aR,a%R
aR,b%S
bS,b%S
aR,a%R
aR,b%S
bS,b%S
aR,a%R
aR,b%S
bS,b%S
aR,a%R
aR,b%S
bS,b%S
aR,a%R
aR,b%S
bS,b%S
aR,a%R
aR,b%S
bS,b%S
aR,a%R
aR,b%S
bS,b%S
aR,a%R
aR,b%S
bS,b%S
aR,a%R
aR,b%S
bS,b%S
aR,a%R
aR,b%S
bS,b%S
CCl4
2.2
2.3
4.7
7.3
8.9
+586
+319
−572
+565
+277
−553
+537
+265
−523
+544
+115
−526
+526
+273
−499
+535
+276
−525
+515
+172
−504
+427
+94
+396 (435)
+216 (433)
−393 (434)
+370 (434)
+179 (432)
−362 (434)
+344 (435)
+165 (430)
−337 (434)
+351 (432)
+70 (433)
−338 (433)
+334 (432)
+174 (431)
−319 (433)
+339 (433)
+177 (432)
−332 (433)
+328 (430)
+107 (426)
−322 (430)
+257 (431)
+57 (424)
−284 (434)
+238 (429)
+23 (422)
−285 (431)
+310 (428)
+99 (428)
−315 (429)
+300 (432)
+79 (428)
−292 (432)
−23 (429)
−26 (425)
+23 (425)
+382 (427)
+80 (427)
−359 (427)
+286 (434)
−81 (427)
−287 (433)
−190 (391)
−103 (391)
+179 (392)
−195 (391)
−98 (388)
+191 (390)
−193 (391)
−100 (386)
+186 (389)
−193 (389)
−45 (394)
+188 (390)
−192 (389)
−99 (388)
+180 (392)
−196 (390)
−99 (389)
+193 (389)
−187 (387)
−65 (384)
+182 (387)
−170 (388)
−37 (382)
+168 (389)
−154 (384)
−16 (380)
+177 (386)
−180 (385)
−58 (384)
+181 (384)
−192 (385)
−54 (382)
+187 (386)
+29 (382)
+31 (382)
−6 (369)
408
406
406
407
406
406
407
405
407
406
412
406
407
405
407
407
406
407
404
402
404
406
400
405
404
398
405
403
403
403
406
402
406
403
401
385
401
400
400
408
403
407
Benzene
CHCl3
THF
CH2Cl2
ClCH2CH2Cl
(CH3)2CO
CH3CH2OH
CH3OH
10.4
20.7
24.3
32.6
36.2
−452
+392
+39
−462
+490
+157
−496
+492
+133
−479
−52
CH3CN
CF3CH2OH
(CH3)2SO
CH3NHCHO
CH3CO2H
46.5
−57
+29
182.4
+625
+131
−559
+458
−136
−469
−243 (384)
−51 (385)
+200 (383)
−172 (390)
+55 (386)
+182 (388)
a Conc. ꢀ1.5×10−5 M.
b Dielectric constant taken from: Gordon, A. J.; Ford, R. A. The Chemist’s Companion; Wiley: New York, 1972; pp. 4–8.
c A=Dmm1 ax(u1)−Dm2max(u2).

S. E. Boiadjie6, D. A. Lightner / Tetrahedron: Asymmetry 12 (2001) 2551–2564
2557
can be compared with the order obtained from the
conformational A-values derived from cyclohexane sys-
tems and their axial ? equatorial conformational equi-
libria.24 In this comparison, we note that the same
relative order is followed more or less, with the notable
exception of the SCH3 group, which is found to have a
larger steric size than OCH3 (or even CH3) by CD. We
note too that isopropyl is found to have a larger steric
size than phenyl by CD. It is not entirely clear why the
relative steric sizes from the two different templates
(bilirubin and cyclohexane) fail to agree consistently on
the steric size of the group. However, it should be
recognized that the gauche butane type interactions
between the axial substituent on a cyclohexane ring and
cyclohexane ring carbons 3 and 5 are oriented in a
rather different buttressing arrangement than is the
a-substituent of the bilirubin template. In the latter, the
(aR) substituent is compressed into a C(7) or C(13)
methyl group, in a face-to-face orientation, which is
different from the gauche butane steric interaction
encountered between an axial substituent and the C(3)
and C(5) ring methylenes of chair cyclohexane. Such
distinctions would doubtless come into play in a major
way when the substituent has long bonds (as in SCH3)
and jammed into the C(7) or C(13) CH3 of the bilirubin
template.
Figure 5. Circular dichroism curves of 1.5×10−5 M solutions
of dimethylmesobilirubins-XIIIa: 1 (aR,a%R) (solid line), 12
(aR,b%S) (dashed line) and 11 (bS,b%S) (dotted line) at 22°C in
chloroform.
methoxy group having a smaller steric size than an
a-methyl.
Our failure to extract consistent quantitative values
for steric size, such as DG for the M?P equilibrium of
Fig. 4 is probably related to deformations of the ridge-
tile motif. That is, although the steric size of sub-
stituents directs the equilibrium, large groups probably
Attempts to interpret the CD A-values in terms of DG
for the M?P equilibrium (Fig. 1D) were not satisfac-
tory; so, we simply used the CD A-values in order to
show the relative order of steric size (Table 2), which
Table 2. Functional group steric size from exciton chirality CD A-values, as compared with conformational A-values
Functional group (R)
CD A-values in CH2Cl2
(ab%-anti-chiral)a (ab%-syn-chiral)b
Conformational A-valuesc
Relative steric sized from
A-values
CD A-values
CH3
+273 (R,S)
+351 (R,S)
+374 (R,S)
+362 (S,S)
+453 (S,S)
+441 (S,S)
+424 (R,S)
−350 (R,S)
+490e (R,R)
−447 (S,S)
−489 (S,S)
−526 (R,S)
−498 (R,S)
−546 (R,S)
−494 (S,S)
−463 (S,S)
1.74
1.76
1.79
2.87
2.21
4.9
1.00
1.01
1.03
1.65
1.27
2.8
1.00
1.29
1.37
1.33
1.66
1.62
1.55
1.28
CH2C6H5
CH2CH3
C6H5
CH(CH3)2
C(CH3)3
SCH3
1.00
0.75
0.57
0.43
OCH3
a From Table 1 and Ref. 22. The CD data for aR-SCH3: Dm4m3a1x=+274, Dm405=0, Dm3m8a6x=−150; for aS-SCH3: Dmm43a2x=−313, Dm406=0,
Dm3m8a7x=+181; for aR-OCH3: Dm4m3a2x=−221, Dm406=0, Dmm38a7x=+129; for aS-OCH3: Dm4m3a1x=−290, Dm405=0, Dm3m8a7x=+173.
b From Ref. 14.
c From Ref. 21 (Table 2.2) by NMR: CH3, CH2CH3, CH(CH3)2 (CFCl3–CDCl3, 300 K), CH2C6H5 (CD2Cl2, 202 K), C(CH3)3 (CD2Cl2, 153 K),
C6H5 (CD2Cl2, 173 K), OCH3 (CFCl3, 180 K), SCH3 [(CD3)2CO/CHClꢀCCl2, ꢀ180 K].
d Based on CD A-values, uncorrected for minor percents of M-helical isomer in the aR,b%S isomers, where the aR substituent is CH3 (22%),
CH2CH3 (9%), C6H5 (8%) and C6H5CH2 (6%)—as detected by 1H NMR in CDCl3. In this solvent, no M-helical isomers could be detected when
the substituents were CH(CH3)2 and C(CH3)3.
e New value; in Ref. 14 value is −445 for the (S,S) antipode.

2558
S. E. Boiadjie6, D. A. Lightner / Tetrahedron: Asymmetry 12 (2001) 2551–2564
Table 3. Influence of diethyl amine on the CD spectra and CD A-values of (a-substituted, b%S-methyl)-mesobilirubins-XIIIa
Configuration at a center
A
CD
Dm2(u2)
UV
Dm1(u1)
u at Dm=0
+167 (435)
−609 (438)
+228 (435)
+548 (439)
+125 (435)
+586 (438)
+343 (436)
+620 (437)
+73 (428)
+591 (439)
+99 (428)
+646 (438)
+360 (437)
+598 (436)
+568 (441)
+564 (437)
mmax
u (nm)
Me
Bn
Et
Ra
Rb,c
Ra
Sb
Ra
Sb
Sa
+277
−110 (390)
+377 (394)
−145 (390)
−348 (394)
−86 (388)
−369 (394)
−209 (392)
−386 (393)
−48 (388)
−368 (393)
−65 (387)
−407 (394)
−231 (392)
−374 (392)
−358 (396)
−352 (393)
407
409
408
410
407
409
408
408
402
409
402
409
409
407
412
408
62,000
59,100sh
62,700
62,800
60,900
57,700sh
54,900
55,200
60,200
57,400
59,100
59,200
66,200
60,700
61,200
61,200
67,800
64,800sh
58,400
59,400
60,200
58,300sh
61,400
62,500
63,200
60,200sh
60,600
60,700
57,700
59,900
60,200
60,000
442
419–427
444
410
435
409–423
442
407
439
411–424
438
403
439
414
440
406
437
408–422
442
407
435
410–424
442
407
438
413–424
439
405
443
410
−986
+373
+896
+211
+955
+552
+1006
+121
+959
+164
+1053
+591
+972
+926
+916
Ph
Rb
Sa
i-Pr
Rb
t-Bu
SMe
OMe
Sa
Rb
Ra
Sb
Ra
Sb
440
406
a anti-chiral configuration.
b syn-chiral configuration.
c (b%R)-configuration.
also distort the ridge-tile somewhat. And alterations of
the ridge-tile template geometry change the relative
orientation of the dipyrrinone components (and hence
the relative orientation of their relevant electric dipole
transition moments), leading to corresponding changes
in CD magnitudes. This phenomenon has been dis-
cussed earlier.5,25
bling or even tripling. Curiously, and for reasons
unclear, the negative CD A-value seen in CH2Cl2 for
the methoxy group becomes positive in diethylamine
and greatly intensified. The data illustrate the sensitivity
of the equilibrium to conformational changes. In the
case of diethylamine solvent, presumably a change in
the ridge-tile interplanar angle is needed in order to
+
accommodate the formation of Et2NH2 salts of the
+
propionic acids as well as to incorporate the Et2NH2
2.5. Chirality inversion
cation into the hydrogen-bonding matrix—as has been
observed in the crystal structure of bis-isopropylammo-
nium bilirubinate.4
For the series of pigments where the a-substituent and
(b%S)-methyl act in opposition sterically to direct the
M?P equilibrium (Fig. 4), the intense exciton chirality
CD Cotton effects observed for the various functional
groups of Table 2 remain strong and positive with a
change of solvent from CH2Cl2 to Et2NH (Table 3). In
some cases, the CD A-value is greatly increased, dou-
Most unusual, but consistent with earlier reports,25 for
the examples of Table 2 where the a-functional group
and (b%S)-methyl work in concert sterically to control
the M?P equilibrium (thus leading to CD A-values

S. E. Boiadjie6, D. A. Lightner / Tetrahedron: Asymmetry 12 (2001) 2551–2564
2559
>445 in CH2Cl2 solvent), in Et2NH solvent the exciton
chirality is found to be inverted from negative to posi-
tive (Table 3). Without exception, the CD A-values are
intensified to the range +900 to +1000. Although it is
unclear why the a-iPr rubin exhibits a slightly smaller
A-value, the consistently positive sign indicates an
inversion of molecular chirality, from the dominant M
in CH2Cl2 to the dominant P in Et2NH.25 Just why the
rubin should adopt the conformation in which both the
a and b% groups are in the (apparently) more sterically
compressed sites remains unclear. Here again, one is
dealing with ammonium salts, and the conformational
preference may in some way be associated with the
inclusion of the cation in the ridge-tile structure.
2.6. Hydrogen-bonded dimers
1H NMR studies of monopyrrole mono-acid 6 in
CDCl3 showed expected chemical shifts. Assignment of
the C(5) methyl singlet signal as more shielded than the
C(3) methyl singlet was based on NOE experiments
from the NH. It was interesting to note, however, that
although the C(3) methyl signal at 2.264 ppm remained
invariant when comparing 6 to rac-6 (Fig. 6), the C(5)
methyl signal of the former was slightly more
deshielded (2.135 ppm) than that of the latter (2.110
ppm) (all l referred to strictly 5×10−3 M solutions). The
NH signal was also slightly more deshielded in 6 (9.858
ppm) than in rac-6 (9.821), but the a-methyl of 6
(1.164/1.178 ppm) was more shielded than in rac-6
(1.180/1.194 ppm). These small differences in chemical
shift might have escaped attention had not the 34% e.e.
sample exhibited two signals for the C(5) methyl (2.102/
2.117, Fig. 6A), with the C(3) methyl signal remaining
at 2.264 ppm. Neither of the two C(5) signals lies at the
same chemical shift as that seen in either 6 or rac-6, but
the C(5) methyl of rac-6 appears at the average of the
two signals seen in 34% e.e. 6.
It might be assumed that 6 forms dimers in CDCl3
because in a dilution study in CDCl3 (Fig. 6B), the C(5)
methyl resonances of 34% e.e. 6 at 5×10−2 M shift
upfield and coalesce at 5×10−5 M. Vapor pressure
osmometry measurements of rac-6 in CHCl3 at 45°C
gave an apparent molecular weight of 311 (versus that
calculated for the monomer: 239), thus confirming some
extent of dimer formation. In contrast, the dimethyl
ester of 34% e.e. 6 shows only one signal for the C(5)
methyl and no changes in chemical shift upon the
1000-fold dilution of Fig. 6B. For 6, only one type of
dimer is possible: 6–6 (or R,R). For rac-6 there are two
types: R,R (and S,S) and R,S, as shown in Fig. 7.
Molecular mechanics (Sybyl) calculations indicate that
the heterochiral (R,S) dimer is more stable than the
homochiral (R,R or S,S) by ꢀ2.5 kcal/mol. Thus, in
rac-6 one might expect a heterodimer mainly; whereas
in 6, only a homodimer is possible. The difference
might account for the different chemical shifts, e.g. the
1
C(5) methyl, seen in the H NMR spectra of 6 and
rac-6 (Fig. 6A). In 34% e.e. 6, one can expect to see
signals for both the heterodimer (more deshielded) and
homodimer (more shielded) in the ratio ꢀ2:1, as
observed.
1
Figure 6. (A) Partial H NMR spectra of the C(3) and C(5)
methyl groups region in 5×10−3 M CDCl3 solutions of pyrrole
acid 6 with different enantiomeric excess. (B) Partial 1H
NMR spectra of pyrrole acid with 34% e.e. in CDCl3 solu-
tions showing the range of C(3)-CH3 and C(5)-CH3 signals at
decreasing concentration.
3. Concluding comments
Methyl
4-(2%R-carboxypropyl)-3,5-dimethyl-1H-pyr-
role-2-carboxylate rac-6 was synthesized and resolved
to 100% e.e. as its amide 7 with (1S)-camphor-

2560
S. E. Boiadjie6, D. A. Lightner / Tetrahedron: Asymmetry 12 (2001) 2551–2564
Figure 7. Line drawings (upper) and ball and stick representations of the hydrogen-bonded dimers of monopyrrole acid 6: (A)
homodimer, R,R, and (B) heterodimer R,S. The heterodimer is computed to be ꢀ2.5 kcal/mol lower energy.
2,10-sultam. The absolute configuration of the acid
component of the crystalline, resolved sultam amide
was determined by X-ray crystallography, the absolute
configuration of the camphor sultam being known.
Both the racemic acid rac-6 and resolved (aR) acid 6
form hydrogen-bonded dimers in CDCl3, with the het-
erodimer (R,S) being favored over the homodimer. The
resolved acid 6 was smoothly converted in three steps
to (aR,a%R)-dimethylmesobilirubin-XIIIa 1, which
exhibited a large positive exciton chirality CD spectrum
in most non-polar solvents, but a large negative exciton
chirality CD in diethylamine.
enced to the residual CHCl3 1H signal at 7.26 ppm, and
CDCl3 13C signal at 77.00 ppm. A J-modulated spin–
echo experiment (Attached Proton Test), and HMBC
and HMQC experiments were used to assign 13C NMR
spectra. Optical rotations were measured on a Perkin–
Elmer model 141 polarimeter. Melting points were
taken on a Mel-Temp capillary apparatus and are
uncorrected. Gas chromatography–mass spectrometry
analyses were carried out on a Hewlett–Packard 5890A
capillary gas chromatograph (30 m DB-1 column)
equipped with a Hewlett–Packard 5970 mass selective
detector. Radial chromatography was carried out on
Merck Silica Gel PF254 with gypsum preparative layer
grade, using a Chromatotron (Harrison Research, Inc.,
Palo Alto, CA). Vapor pressure osmometry measure-
4. Experimental
ments were taken on
a Gonotec Osmomat 070
All UV–vis spectra were recorded on a Perkin–Elmer
Lambda-12 or Cary 219 spectrometer, and all circular
dichroism (CD) spectra were recorded on a JASCO
J-600 instrument. NMR spectra were obtained on
Varian Unity Plus spectrometer operating at 1H fre-
quency of 500 MHz in CDCl3 solvent (unless otherwise
noted). Chemical shifts are reported in l (ppm) refer-
osmometer. Combustion analyses were carried out by
Desert Analytics, Tucson, AZ.
Spectral data were obtained in spectral grade solvents
(Aldrich or Fisher). Commercial reagents and HPLC
grade solvents were dried and purified following stan-
dard procedures.26

S. E. Boiadjie6, D. A. Lightner / Tetrahedron: Asymmetry 12 (2001) 2551–2564
2561
4.1. Methyl 4-acetyl-2-methyl-5-oxohexanoate
4.3. Methyl 4-(2%-carboxypropyl)-3,5-dimethyl-1H-
pyrrole-2-carboxylate rac-6
To a mixture of 214 mL (2 mol) of purified methyl
methacrylate, 410 mL (4 mol) of pentane-2,4-dione
and 300 mL of 95% ethanol, 58 g (1 mol) of finely
ground anhydrous potassium fluoride was added dur-
ing 45 min, and the contents were mechanically stirred
and heated at gentle reflux for 70 h. After cooling for
1 h in an ice bath, the precipitated inorganics were
separated by filtration and washed with ethyl acet-
ate:hexane (1:1 by vol). The solvents from the filtrate
were evaporated under vacuum. The residue was
filtered again, and the filtrate was doubly distilled
under vacuum to afford 143.6 g (36%) of b-diketone,
bp 103–110°C/1.5 mmHg (lit.11 bp 118–121°C/3
mmHg). The material was used directly in the follow-
ing step.
To a solution of 50.66 g (200 mmol) of dimethyl ester
10 in 500 mL of methanol was added a solution of
24.00 g (600 mmol) of NaOH in 100 mL of water, and
the mixture was stirred for 24 h at rt. The methanol
solvent was partially (ꢀ400 mL) evaporated under
vacuum; then 400 mL of 0.2% aqueous NaOH was
added to the residue to yield a homogeneous solution,
which was cooled in an ice bath and very slowly
acidified with conc. aq. HCl. The precipitated solid
was collected by filtration, washed with water (3×100
mL) and dried under vacuum to afford 46.88 g (98%)
1
of monoacid rac-6. It had mp 162–164°C; H NMR: l
1.18 (3H, d, J=7.0 Hz), 2.10 (3H, s), 2.26 (3H, s),
3
2
2.48 (1H, ABX, J=7.4 Hz, J=14.3 Hz), 2.61 (1H,
m), 2.79 (1H, ABX, 3J=7.5 Hz, 2J=14.3 Hz), 3.83
(3H, s), 9.29 (1H, br.s), 11.24 (1H, very br.s) ppm; 13C
NMR: l 10.82, 11.33, 16.62, 28.33, 40.80, 51.13,
116.69, 118.82, 127.72, 131.44, 162.86, 181.65 ppm.
4.2. Methyl 3,5-dimethyl-4-(2%-methoxycarbonylpropyl)-
1H-pyrrole-2-carboxylate (10)
To a solution of 20 g (0.5 mol) of NaOH in 230 mL
of glacial acetic acid cooled with ice bath, was added
80 mL (0.7 mol) of dimethyl malonate followed by a
cooled solution of 103.5 g (1.5 mol) of sodium nitrite
in 160 mL of water at 0°C over 3 h. After stirring for
16 h at rt, the mixture was saturated with 150 g NaCl,
and the product was extracted with ether (4×200 mL).
The solvent was removed under vacuum to afford a
solution of dimethyl oximinomalonate in acetic acid,
and this was used immediately in the pyrrole synthesis
below.
4.4. N-[3-(2,4-Dimethyl-5-methoxycarbonyl-1H-pyrrol-
3-yl)-2-methylpropanoyl]-(1%S)-camphor-2%,10%-sultam 8
4.4.1. Method A.27 To a suspension of 2.39 g (10
mmol) of acid rac-6 in 110 mL of anhydrous CH2Cl2
under N2 was added 2.9 mL (33 mmol) of oxalyl
chloride, and the mixture was heated at reflux for 3 h.
The solvents were completely evaporated under vac-
uum, and traces of residual (COCl)2 were removed by
co-evaporation with dry toluene (25 mL). Simulta-
neously, in a separate flask equipped with side arm,
(1S)-camphor-2,10-sultam anion was prepared in 100
mL of dry toluene by stirring 928 (2.16 g, 10 mmol)
with 600 mg (20 mmol) NaH (80% suspension in oil)
for 2 h under N2. This suspension was transferred
through the connected side arm during 10 min to a
solution of the acid chloride in 25 mL of dry toluene
at 0°C, and the mixture was stirred at rt for 16 h. The
reaction was quenched by adding cold water, and the
product was extracted into CHCl3 (4×50 mL). The
extracts were washed with 1% aqueous NaOH (2×50
mL), water (3×100 mL), dried (anhydr. MgSO4) and
filtered. The filtrate was evaporated under vacuum,
and the residue was purified by radial chromatogra-
phy. Recrystallization from EtOAc–hexane afforded
3.10 g (71%) of pyrrolepropanoyl sultam 8.
In a 2 L three neck round bottom flask equipped for
mechanical stirring were placed 100 g (0.5 mol) of
b-diketone from above, 475 mL of glacial acetic acid,
102.5 g (1.25 mol) of anhydrous sodium acetate, and
98 g (1.5 gA) of zinc; then the mixture was warmed to
75°C. A solution of the oxime (prepared above) in 20
mL of AcOH was slowly added to the vigorously
stirred reaction mixture over 2 h at such a rate as to
maintain the reaction temperature at 80–85°C. The
mixture was then heated at reflux for 3.5 h, after
which it was poured (while hot) into 6 L of water/ice
and kept overnight at 0°C. The precipitated solid was
separated by filtration and washed with water (3×500
mL). The crude product was dissolved in CHCl3 (800
mL) and filtered from unreacted zinc. The filtrate was
washed with water (3×200 mL), dried (anhydr.
MgSO4), filtered, and the solvent was evaporated
under vacuum. The residue was recrystallized from
methanol–water (3:1 by vol) to afford 70.51 g (56%) of
4.4.2. Method B.29 To a solution of 21.53 g (100
mmol) (1S)-camphor-2,10-sultam28 9 in 450 mL of
anhydrous CH2Cl2 were added 22.70 g (110 mmol) of
DCC, 1.22 g (10 mmol) of DMAP and 28.72 g (120
mmol) of acid rac-6. The mixture was stirred for 16 h
at rt. The separated dicyclohexylurea was removed by
filtration, and washed with cold CH2Cl2. The filtrate
was washed sequentially with 4% aqueous NaOH (2×
50 mL), 3% HCl (100 mL), and water (until neutral),
then dried over anhydr. MgSO4. After filtration, the
solvent was removed under vacuum, and the residue
was purified by radial chromatography. Recrystalliza-
tion from EtOAc–hexane afforded 31.34 g (72%) of 8.
1
pure pyrrole 10. It had mp 97–98°C; H NMR: l 1.11
(3H, d, J=7.0 Hz), 2.19 (3H, s), 2.25 (3H, s), 2.43
(1H, ABX, 3J=8.3 Hz, 2J=14.3 Hz), 2.57 (1H, m),
3
2
2.77 (1H, ABX, J=6.8 Hz, J=14.3 Hz), 3.64 (3H,
s), 3.82 (3H, s), 8.60 (1H, br.s) ppm; 13C NMR: l
10.64, 11.43, 16.52, 28.11, 40.48, 50.79, 51.49, 116.73,
118.92, 127.32, 130.73, 162.22, 176.81 ppm. MS: m/z
+
’
(%) 253 [M ] (7), 222 (6), 166 (72), 134 (100). Anal.
calcd for C13H19NO4: C, 61.64; H, 7.56; N, 5.53.
Found: C, 61.79; H, 7.78; N, 5.58.
1
It had mp 158–184°C, [h]2D0=−46.6 (c 1.2, CHCl3); H

2562
S. E. Boiadjie6, D. A. Lightner / Tetrahedron: Asymmetry 12 (2001) 2551–2564
NMR: l 0.68, 0.96 (2×1.5H, 2×s), 0.87, 1.15 (2×1.5H,
2×s), 1.13, 1.20 (2×1.5H, 2×d, J=7.0, 6.5 Hz), 1.33
(2H, m), 1.62 (0.5H, m), 1.71 (0.5H, m), 1.88 (2.5H, m),
2.06 (1.5H, m), 2.20, 2.22 (2×1.5H, 2×s), 2.25, 2.29
(2×1.5H, 2×s), 2.48 (1H, m), 2.79 (1H, m), 3.22, 3.32
ered 9 after recrystallization from EtOH–H2O. The
material is sufficiently pure for recycling. The alkaline
aqueous solution was acidified with conc. HCl to pH
<3, and the pyrrole product was extracted into CH2Cl2–
CHCl3 (1:1 by vol; 4×75 mL). The combined extracts
were washed with water (2×20 mL), and the solvents
were evaporated to afford a residue that was purified by
radial chromatography. Recrystallization afforded 2.78
g (77%) of pyrrole monoacid 6 with mp 139–141°C and
[h]2D0=−51.2 (c 1.2, CHCl3).
2
(2×0.5H, 2×m), 3.36, 3.44 (2×1H, 2×AB, J=13.8, 13.7
3
3
Hz), 3.78, 3.88 (2×0.5H, 2×dd, J=4.6, 7.6 Hz, J=5.8,
6.8 Hz), 3.802, 3.804 (2×1.5H, 2×s), 8.51, 8.56 (2×0.5H,
2×br.s) ppm; 13C NMR: l 10.58, 10.71, 11.52, 11.66,
16.92, 17.78, 19.68, 19.76, 19.79, 20.77, 26.29, 26.37,
26.38, 29.73, 32.74, 32.78, 38.16, 38.35, 40.52, 40.92,
44.59, 44.67, 47.41, 47.66, 47.92, 48.24, 50.76, 50.79, br.
53.02, 64.82, 64.97, 116.56, 116.63, 118.43, 118.68,
127.88, 128.01, int. 131.01, 162.12, 162.21, 176.03,
176.24 ppm. Anal. calcd for C22H32N2O5S: C, 60.52; H,
7.39; N, 6.42. Found: C, 60.74; H, 7.27; N, 6.40.
4.7. 5-Bromomethylene-4-ethyl-3-methyl-2-oxo-1H-
pyrrole 5
This synthetic intermediate was prepared in four steps
from ethyl acetoacetate and pentane-2,4-dione as
described previously.31 It had mp 138–140°C (lit.31a mp
4.5. (−)-N-[3-(2,4-Dimethyl-5-methoxycarbonyl-1H-
pyrrol-3-yl)-(2R)-methylpropanoyl]-(1%S)-camphor-2%,10%-
sultam 7
1
138–139°C); H NMR: l 1.13 (3H, t, J=7.7 Hz), 1.85
(3H, s), 2.40 (2H, q, J=7.7 Hz), 5.90 (1H, s), 7.44 (1H,
br.s) ppm; 13C NMR: l 8.27, 14.12, 17.75, 86.64,
129.45, 141.39, 145.06, 171.58 ppm. MS: m/z (%) 217,
215 [M ] (30), 136 (100), 121 (10), 108 (85).
To a warm solution of 21.83 g (50 mmol) of the 1:1
diastereomeric mixture 8 in 60 mL of ethyl acetate was
slowly added 35 mL of hexane, then seed crystals of the
pure (2R)-7 diastereomer were added. The solution was
maintained warm for ꢀ1 h until the initial fast crystal-
lization subsided, the mixture was slowly cooled and
kept at rt for 24 h. The crystals were separated by
filtration to afford 12.74 g of a fraction enriched to 75%
d.e. of (2R)-7. This fraction was dissolved at reflux in
75 mL of EtOAc, and while cooling it very slowly,
seeds of pure (2R)-7 were added followed by hexane
added in small portions (40 mL over 6 h). After 24 h,
the crystalline fraction (8.56 g) was separated and sub-
jected to another recrystallization while maintaining the
same ratios of solvents. The third recrystallization gave
6.15 g (56%) of the (2R)-7 diastereomer, with 99% d.e.
+
’
4.8. (−)-3-Ethyl-8-[(2%R)-(methoxycarbonyl)propyl]-
2,7,9-trimethyl-10H-dipyrrin-1-one 4
A mixture of 2.39 g (10 mmol) of (2%R)-monoacid 6,
2.00 g (50 mmol) of NaOH, 20 mL of ethanol, 5 mL of
water, and 2.00 g of NaNO3 was heated at reflux for 4
h. After cooling, the ethanol was evaporated completely
under vacuum. To the residue was added 30 mL of
0.5% aq. NaOH, and the mixture was cooled to −15°C.
Slow acidification with a mixture of conc. HNO3–50%
aq. NaNO3 (1:5 by vol) precipitated the product, which
was collected by filtration, washed with cold water (2×5
mL) and dried overnight under vacuum (P2O5) to
afford 2.26 g (quantitative yield) of diacid. The diacid
was used without further characterization in the follow-
ing step.
1
determined by H NMR. It had mp 191–193°C; [h]2D0=
1
−92.2 (c 1.2, CHCl3); H NMR: l 0.96 (3H, s), 1.13
(3H, d, J=7.0 Hz), 1.14 (3H, s), 1.36 (2H, m), 1.89 (3H,
m), 2.05 (2H, m), 2.22 (3H, s), 2.28 (3H, s), 2.47 (1H,
3
2
3
ABX, J=9.0 Hz, J=14.3 Hz), 2.79 (1H, ABX, J=
To a solution of the diacid above in 70 mL of anhyd-
rous methanol was added 2.16 g (10 mmol) of brom-
omethylene oxopyrrole 5; then the mixture was heated
under vigorous reflux for 10 h. The volume of MeOH
was reduced by one-half by distillation, and the residue
was chilled overnight at −15°C. The crude product was
collected by filtration, washed with cold MeOH (2×10
mL) and dried under vacuum. Purification by radial
chromatography and recrystallization from CH2Cl2–
MeOH yielded 2.31 g (70%) of dipyrrinone 4. It had mp
213–214°C (lit.11 mp for the racemate 218–219°C),
2
5.5 Hz, J=14.3 Hz), 3.22 (1H, ABXY3, m), 3.41, 3.47
(1H, AB, 2J=13.8 Hz), 3.80 (3H, s), 3.87 (1H, dd,
3J=6.0, 6.5 Hz), 8.64 (1H, br.s) ppm; 13C NMR: l
10.69, 11.60, 17.74, 19.75, 20.74, 26.34, 26.35, 32.70,
38.32, 40.88, 44.56, 47.63, 48.21, 50.72, 52.98, 64.93,
116.52, 118.63, 127.95, 131.09, 162.14, 175.99 ppm.
4.6. (−)-Methyl 4-(2%R-carboxypropyl)-3,5-dimethyl-1H-
pyrrole-2-carboxylate 6
1
To a solution of 6.55 g (15 mmol) of sultam 7 in 90 mL
of purified THF cooled to 0°C was added a solution of
2.52 g (60 mmol) of lithium hydroxide monohydrate in
22 mL of H2O. This was followed after 5 min by adding
13.5 mL (120 mmol) of 30% H2O2;16,30 then the mixture
was slowly warmed and stirred at rt for 16 h. The THF
solvent was evaporated under vacuum, the residue was
diluted with 100 mL of 1% aq. NaOH, and the cam-
phor auxiliary was extracted with hexane (3×50 mL)
and CH2Cl2 (2×50 mL) to give 2.30 g (71%) of recov-
[h]2D0=−55.8 (c 0.3, CHCl3); H NMR: l 1.13 (3H, d,
J=7.0 Hz), 1.18 (3H, t, J=7.5 Hz), 1.95 (3H, s), 2.12
(3H, s), 2.39 (3H, s), 2.46 (1H, ABX, 3J=8.7 Hz,
2J=14.2 Hz), 2.55 (2H, q, J=7.5 Hz), 2.59 (1H,
3
2
ABXY3, m), 2.82 (1H, ABX, J=6.3 Hz, J=14.2 Hz),
3.67 (3H, s), 6.13 (1H, s), 10.28 (1H, br.s), 11.16 (1H,
br.s) ppm; 13C NMR: l 8.48, 9.71, 11.73, 15.01, 16.56,
17.91, 28.39, 40.66, 51.55, 101.14, 118.04, 122.31,
122.41, 125.00, 126.99, 132.24, 148.30, 174.06, 177.05
ppm.

S. E. Boiadjie6, D. A. Lightner / Tetrahedron: Asymmetry 12 (2001) 2551–2564
2563
4.9. (+)-3,17-Diethyl-8,12-di[(2%R)-(methoxycarbonyl)-
propyl]-2,7,13,18-tetramethyl-(21H,24H)-bilin-1,19-
dione [(aR,a%R)-dimethylmesobiliverdin-XIIIa dimethyl
ester] 3
deoxygenated with N2) was added 0.2 M aq. NaOH (90
mL), and the mixture was stirred at 50°C for 4 h under
N2. After cooling it was diluted with 30 mL of 0.2 M
NaOH and extracted with 75 mL of CHCl3 which was
discarded. The alkaline green–blue solution was acid-
ified with 10% HCl to pH ꢀ3, and the product was
extracted with CH2Cl2 (4×100 mL). The extracts were
washed with water (2×100 mL), dried (anhydr.
Na2SO4), and filtered. The solvent was evaporated
under vacuum to give a quantitative yield of verdin
diacid, which was used without delay and characteriza-
tion in the following reduction step.
A mixture of 661 mg (2 mmol) of dipyrrinone 4, 1.24 g
(5 mmol) of p-chloranil, 440 mL of CH2Cl2 and 22 mL
of formic acid was heated at reflux for 24 h; then the
volume was reduced by distillation to one-half, and
reflux was continued for 6 h. The mixture was chilled
overnight at −20°C. The separated solid was filtered
and washed with cold CH2Cl2. The cold filtrate was
carefully neutralized with satd aq. NaHCO3, then
washed with 3% aq. NaOH (2×100 mL), followed by
water until neutral. It was dried (anhydr. Na2SO4),
filtered, and the solvent was evaporated under vacuum.
The residue was purified by radial chromatography and
recrystallization from CHCl3–hexane to afford 543 mg
To a solution of the blue diacid above in 70 mL of dry
and deoxygenated MeOH was added 946 mg (25 mmol)
of sodium borohydride in small portions over 30 min
while purging the mixture with N2. After 15 min more
stirring the yellow solution was diluted with 150 mL of
H2O. Then AcOH (2 mL) was added followed by
enough 5% HCl to bring the pH ꢀ3. The product was
extracted into CHCl3 (4×75 mL), washed with H2O
(3×75 mL), dried (anhydr. Na2SO4), and filtered. The
solvent was evaporated under vacuum, and the residue
was purified by radial chromatography. Recrystalliza-
tion from CH2Cl2–MeOH afforded 142 mg (92%) of
rubin diacid 1. It had mp 313–315°C (dec.) (lit.11 mp for
the racemate 290°C (dec.)) and [h]2D0=+5530 (c 5.0×
10−3, CHCl3) (lit.11 [h]D20=+4500 (c 5.4×10−4 M,
(84%) of verdin 3. It had mp 217–218°C (lit.11 mp for
20
the racemate+meso diastereomers 200–205°C), [h]
=
436
+2190 (c 4.3×10−3, CHCl3); H NMR: l 1.22 (6H, t,
J=7.6 Hz), 1.23 (6H, d, J=6.7 Hz), 1.83 (6H, s), 2.08
(6H, s), 2.52 (4H, q, J=7.6 Hz), 2.66 (4H, ABX, m),
3.01 (2H, ABXY3, m), 3.65 (6H, s), 5.93 (2H, s), 6.68
(1H, s), 8.11 (2H, br.s), 9.12 (1H, very br.s) ppm; 13C
NMR: l 8.30, 9.64, 14.42, 17.09, 17.83, 28.61, 41.15,
51.75, 96.17, 114.54, 128.38, 128.45, 136.62, 139.85,
141.47, 146.69, 149.88, 172.42, 176.38 ppm.
1
1
CH2Cl2)); H NMR: l 1.12 (6H, t, J=7.5 Hz), 1.45
(6H, d, J=7.3 Hz), 1.86 (6H, s), 2.15 (6H, s), 2.42 (2H,
4.10. (+)-3,17-Diethyl-8,12-di[(2%R)-(methoxycarbonyl)-
propyl]-2,7,13,18-tetramethyl-(10H,21H,23H,24H)-bilin-
1,19-dione [(aR,a%R)-dimethylmesobilirubin-XIIIa
dimethyl ester] 2
3
2
ABX, J=2.8 Hz, J=14.5 Hz), 2.48 (4H, q, J=7.5
3
2
Hz), 2.90 (2H, ABX, J=12.0 Hz, J=14.5 Hz), 3.04
(2H, ABXY3, m), 4.04 (2H, s), 6.05 (2H, s), 9.09 (2H,
br.s), 10.54 (2H, br.s), 13.67 (2H, br.s) ppm; 13C NMR:
l 7.92, 10.25, 14.86, 17.82, 19.65, 22.14, 28.03, 39.14,
100.61, 119.23, 123.20, 123.90, 124.13, 128.24, 133.14,
148.40, 174.89, 182.34 ppm.
To a blue solution of 129 mg (0.2 mmol) of verdin 3 in
3 mL of CHCl3 and 35 mL of CH3OH (both dry and
deoxygenated with N2) was added NaBH4 (454 mg, 12
mmol) in small portions over 15 min under N2. After 10
min, the yellow mixture was diluted with 200 mL of
water. Five drops of AcOH were added followed by 5%
HCl to acidify the mixture to pH ꢀ3. The mixture was
extracted with CHCl3 (4×50 mL), washed with H2O
until neutral, dried (anhydr. Na2SO4), and filtered. The
solvent was evaporated under vacuum, and the residue
was purified by radial chromatography. Recrystalliza-
tion (CH2Cl2–MeOH) afforded 107 mg (83%) of rubin
dimethyl ester 2. It had mp 244–246°C (dec.) (lit.32 mp
230°C (dec.)); [h]2D0=+240 (c 5.0×10−3, CHCl3); 1H
NMR: l 1.00 (6H, t, J=7.7 Hz), 1.13 (6H, d, J=6.5
Hz), 1.44 (6H, s), 2.09 (6H, s), 2.32 (4H, q, J=7.7 Hz),
2.62 (4H, m), 2.99 (2H, m), 3.66 (6H, s), 4.11 (2H, s),
5.91 (2H, s), 10.29 (2H, br.s), 10.69 (2H, br.s) ppm; 13C
NMR: l 7.71, 9.91, 14.72, 16.48, 17.76, 22.77, 28.72,
41.00, 51.52, 100.19, 117.74, 123.05, 123.44, 123.94,
128.85, 131.41, 146.99, 174.28, 177.01 ppm.
Acknowledgements
We thank the National Institutes of Health (HD-17779)
for support of this work. Dr. S. E. Boiadjiev is on leave
from the Institute of Organic Chemistry, Bulgarian
Academy of Science, Sofia, Bulgaria. We thank Dr. T.
W. Bell for use of the vapor pressure osmometry
apparatus and Dr. V. J. Catalano for assistance with
the X-ray crystallography.
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