Synthesis of diastereoisomeric pairs of novel analogues of d4T having an isochroman glycon moiety; their enzymatic kinetic resolution, their enantiopure synthesis, molecular modeling and NMR structural study

Article abstract of DOI:10.1016/j.tet.2005.07.084

An efficient route, starting from 2-bromobenzaldehyde, is described to synthesize racemic diastereoisomeric thymine derivatives of isochroman, which are aromatic analogues of Stavudine, an approved anti-HIV drug. The relative configurations were determined by NOE proton NMR experiments in connection with molecular modeling. Following the separation of the latter diastereoisomers, kinetic resolution was achieved via a transesterification reaction catalyzed by lipases. Using this method, moderate ee's were obtained (0.74-0.98). Thus, an alternative strategy starting from d-mannitol was proposed to provide pure enantiomers. The attribution of absolute configurations was made by chemical filiation on the basis of the configurations obtained from d-mannitol. The structural attributions were confirmed by studying the behavior of proton NMR shifts of the corresponding isochroman Mosher's esters.

Full text of DOI:10.1016/j.tet.2005.07.084

Tetrahedron 61 (2005) 10583–10595
Synthesis of diastereoisomeric pairs of novel analogues of d4T
having an isochroman glycon moiety; their enzymatic kinetic
resolution, their enantiopure synthesis, molecular modeling and
NMR structural study
c
d
d
e
Christophe Len,a,b, Serge Pilard, Emmanuelle Lipka, Claude Vaccher, Marie-Pierre Dubois,
*
Yana Shrinska,e Vinh Trane and Claude Rabillere
a
` ´ ´ ´
Synthese et Reactivite des Substances Naturelles, UMR 6514 CNRS, FR 2703, Universite de Poitiers, 40, avenue du Recteur Pineau,
F-86022 Poitiers cedex, France
b
´
Laboratoire des Glucides, FRE 2779 CNRS, Universite de Picardie, 33, rue Saint Leu, F-80039 Amiens, France
c
´
Plate-Forme Analytique, Universite de Picardie, 33, rue Saint Leu, F-80039 Amiens, France
d
´
Laboratoire de Chimie Analytique, EA 1043, Universite de Lille 2, BP 83-3, rue du Pr. Laguesse, F-59006 Lille, France
e
´ ´
Biotechnologie, Biocatalyse et Bioregulation, UMR 6204 CNRS, Universite de Nantes, 2, rue de la Houssiniere,
`
F-44322 Nantes cedex 03, France
Received 23 May 2005; revised 19 July 2005; accepted 26 July 2005
Available online 19 September 2005
Abstract—An efficient route, starting from 2-bromobenzaldehyde, is described to synthesize racemic diastereoisomeric thymine derivatives
of isochroman, which are aromatic analogues of Stavudine, an approved anti-HIV drug. The relative configurations were determined by NOE
proton NMR experiments in connection with molecular modeling. Following the separation of the latter diastereoisomers, kinetic resolution
was achieved via a transesterification reaction catalyzed by lipases. Using this method, moderate ee’s were obtained (0.74–0.98). Thus, an
alternative strategy starting from D-mannitol was proposed to provide pure enantiomers. The attribution of absolute configurations was made
by chemical filiation on the basis of the configurations obtained from D-mannitol. The structural attributions were confirmed by studying the
behavior of proton NMR shifts of the corresponding isochroman Mosher’s esters.
q 2005 Elsevier Ltd. All rights reserved.
1. Introduction
Natural nucleosides are of great biological importance in
metabolic pathways.1 For many years, the typical structure
of nucleosides was described by scientists as two molecular
fragments: D-ribose or D-deoxyribose as the sugar moiety
connected by a b-glycosyl linkage to different heterocyclic
bases such as thymine, uracil, cytosine, adenine and
guanine. This dogma disappeared when different groups
reported the isolation of natural nucleosides having
D-arabinose or 20,30-didehydro-20,30-dideoxy-D-glucose
instead of the D-ribose part (Fig. 1). In 1950, Bergmann
et al. reported the isolation of spongouridine (1) and
spongothymidine (2) from marine Caribbean sponges
Cryptotheca crypta, which had D-arabinose as the sugar
Keywords: d4T Analogue synthesis; Lipase resolution; Enantiopure
synthesis; NMR configuration attribution; Molecular modeling.
*
Corresponding author. Tel.: C33 549366389; fax: C33 549453501;
e-mail: christophe.len@univ-poitiers.fr
Figure 1. Natural compounds having a nucleoside moiety.
0040–4020/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2005.07.084

10584
C. Len et al. / Tetrahedron 61 (2005) 10583–10595
2. Results and discussion
2.1. Synthesis of racemic unsaturated nucleosides and
determination of their relative configuration
A retrosynthetic analysis suggested that the starting material
for the synthesis of nucleoside analogue 8 was the
2-bromobenzaldehyde (9). This compound had the advan-
tage of being stable, inexpensive and easily available.
Figure 2. Stavudine 5 and abacavir 6.
moiety.2 In 1958, Y. Yonehara et al. reported the discovery
of a metabolite of Streptomyces griseochromogenes,
Blasticidin S (3),3 which controls rice blast Pyricularia
oryzae.4 In 1978, K. Suetomi et al. reported the isolation of
antifungal mildiomycin (4) from a culture of Streptoverti-
cillium rimofaciens.5
The aldehyde 9 was converted into the protected compound
10 by acetonide formation in the presence of propan-1,3-
diol and catalytic amounts of PTSA in 84% yield (Scheme
1). The aromatic ring of 10 was metalated and allylated to
the olefin 11 in 87% yield. Based on the Sharpless
mnemonic device,18–20 asymmetric dihydroxylation (AD)
of the terminal olefin 11 using AD-mix a was expected to
afford the S configuration that was required for the synthesis
of the nucleoside analogue 8. Thus, the alkene 11 was
reacted with AD-mix a in a mixture of tert-butyl alcohol
and water using classical Sharpless dihydroxylation meth-
odology. Unfortunately, this oxidation furnished the diol as
a mixture of the expected Sharpless diol having the S
configuration and the unexpected Sharpless diol having the
R configuration (eeZ0.2). On the contrary, the catalytic AD
system displayed very high enantioselectivity in the
preparation of the diol 19 (eeZ0.98) starting from the
styrene derivative 18 (Scheme 2).16 The presence of a
methylene group in 11 between the vinyl and the phenyl
groups probably decreased the selectivity (eeZ0.98 for 19
vs 0.2 for 12S) due to the loss of rigidity.
These discoveries led to a large number of nucleoside
analogues that were tested for the treatment of viral
diseases.6 Among the US FDA approved compounds
used in the treatment of acquired immunodeficiency
syndrome (AIDS), the 20,30-didehydro-30-deoxythymi-
dine d4T (5)7–9 and the carbocyclic 2-amino-6-cyclo-
propylaminopurine analogue abacavir (6)10,11 showed
potent anti-human immunodeficiency virus (HIV)
activity (Fig. 2).
However, side effects and drug-resistant variants remained a
problem with these antiviral agents.12–14 Moreover, the
introduction of the 20,30-double bond in compound 5
resulted in an increased lipophilicity compared to the
corresponding natural and saturated 20,30-dideoxynucleo-
side series but decreased the chemical stability in acidic
medium. In the course of the search for new antiviral agents
with a higher therapeutic index, the obvious emphasis was
on the design of drugs with potent activity, high stability,
low cytotoxicity, minimal side effects. In our previous
studies, we reported the synthesis of pyrimidine nucleoside
analogues of d4T based on the 1,3-dihydrobenzo[c]furan
core 7 (Fig. 3).15,16 This class of nucleoside with a modified
glycon part was attractive because: (i) it retained the
phosphorylation site; (ii) the presence of the benzene ring as
electron-withdrawing group stabilized the glycosidic bond
compared to the olefinic analogue: 20,30-didehydro-20,30-
dideoxynucleoside; (iii) the introduction of the aromatic
residue increased the lipophilicity compared to d4T.17 In an
attempt to expand the variety of nucleoside antiviral drugs, a
novel range of unsaturated nucleoside analogues of d4T 8
were synthesized to explore their potential as antiviral
drugs.
Considering this low enantiofacial discrimination, the allyl
derivative 11 afforded the diol rac-12 as a racemic mixture,
using OsO4 in the presence of N-methylmorpholine-N-oxide
(NMO) as co-oxidant, in 75% yield. The subsequent
removal of the acetal group using HCl 10% in methanol
for 2 h resulted in spontaneous cyclization to afford the
isochroman derivatives rac-13 as the major stereoisomers.
It was notable that the cyclization between the primary
hydroxyl and formyl groups giving 1,3,4,5-tetrahydro-
benzo[c]oxepine derivative was not observed during this
rearrangement. The alcohol rac-13 was converted into the
corresponding acetate rac-14, which was used as a glycosyl
donor in a Vorbruggen condensation reaction.21 Thus,
treatment of the acetylated derivative rac-14 with silylated
thymine in the presence of SnCl4 afforded a mixture of
diastereoisomeric nucleoside analogues rac-15 and rac-16
that were readily separated by chromatography. The
condensation reaction gave the two isomers rac-15 and
rac-16 due to the lack of a participating effect by
neighboring groups. Classical removal of the acetyl group
of rac-15 and rac-16 by treatment with saturated
methanolic ammonia produced the desired free nucleosides
rac-8 and rac-17, respectively, in quantitative yield. The
nucleoside analogues rac-8 and rac-17 obtained by the
method shown in Scheme 1 were of course a pair of
enantiomers. Unfortunately, the heterocyclic compounds
rac-8 and rac-17 gave poor quality crystals thus, precluding
the determination of their configurations by X-ray crystallo-
graphy. Thus, the absolute configurations of the two
0
0
asymmetric carbons C1 and C3 included in the isochroman
core were determined by NMR experiments and
Figure 3. Isobenzofuran and isochroman derivatives 7 and 8.

C. Len et al. / Tetrahedron 61 (2005) 10583–10595
10585
Scheme 1. Reagents and conditions: (i) propan-1,2-diol, PTSA, toluene reflux; (ii) BuLi, THF then CH2CHCH2Br; (iii) OsO4, H2O, NMO, pyridine, tert-
BuOH; (iv) HCl, MeOH; (v) Ac2O, pyridine; (vi) silylated thymine, C2H4Cl2, SnCl4; (vii) NH3, MeOH.
0
than the H4 a proton while in the former the reverse situation
was observed. This difference was very probably due to
different anisotropic contributions on these proton chemical
independent chemical correlation. The relative configur-
ations for the nucleoside analogues rac-8 and rac-17 were
assigned as 10R, 30S (10S,30R) and 10S,30S (10R,30R),
respectively, on the basis of proton NMR NOE experiments.
0
0
0
0
shifts exerted by C1 –O2 and C9 –O9 bonds, aromatic and
0
Thus, in the racemic mixture rac-8, irradiation of H1 proton
0
gave enhanced signals for H3 proton. The same was true for
0
0
thymine rings. The contribution of the C1 –O2 bond could
not explain this effect since molecular modeling (Fig. 4 and
0
H1 proton when H3 proton was irradiated. Conversely, no
0
0
Table 1) indicated that the positions of H4 a and H4 b protons
0
NOE effect was observed for the same protons of rac-17
0
while the irradiation of H3 proton showed an interaction
0
toward the oxygen atom O2 of the pyran cycle remained the
same whichever diastereoisomer was considered.
with H6 proton and vice-versa. The study of the
conformational analysis made by means of molecular
modeling confirmed these attributions. Thus, for the lowest
energy conformer found for rac-8, (Fig. 4 and Table 1), the
The effect of the aromatic ring was also unable to explain
the chemical shift differences since, for each diastereo-
0
0
isomer, the same orientations of H4 a and H4 b protons toward
the plane of the cycle were observed on the molecular
0
0
proximity of H3 and H1 explained the NOE interaction
observed. Similarly, the lowest energy conformer of rac-17
0
(Fig. 4), which presented a closer disposition for H3 and H6
protons than that of rac-8, accounted for the proton dipolar
interaction observed for these protons in the former
compound.
The comparison of the chemical shifts and coupling
0
constants of H4 protons for the two diastereoisomers was
also very interesting. For rac-8, the following NMR
0
0
parameters were measured for H4 a and H4 b protons
(subscripts a and b refer, respectively, to the lower cis and
to the higher trans 3J3 ,4 values): dH-4 aZ2.86 ppm, J3 ,4 a
Z
0
0
0
0
0
0
0
0
3.2 Hz and dH-4 bZ2.70 ppm, J3 ,4 bZ10.7 Hz, while for
rac-17: dH-4 aZ2.72 ppm, J3 ,4 aZ2.8 Hz and dH-4 b
2.86 ppm, J3 ,4 bZ11.5 Hz. Thus, for the latter, the H4 b
proton (high coupling constant) resonated at a lower field
0
0
0
0
Z
0
0
0
Figure 4. Lowest energy conformers obtained by molecular modeling for
rac-8 and rac-17.
Scheme 2. Reagents and conditions: (i) AD-mix a, tert-BuOH, H2O.

10586
C. Len et al. / Tetrahedron 61 (2005) 10583–10595
˚
Table 1. Angles (deg) and interatomic distances (A) for the lowest energy conformer of rac-8 and rac-17
rac-8
10S,30R
(10R,30S)
rac-17
10S,30S
(10R,30R)
rac-8
10S,30R
(10R,30S)
rac-17
10S,30S
(10R,30R)
Angles
(deg)
Distances
˚
(A)
a
b
92
74
dH6–OH
0
dH6–H4 a
0
dH6–H4 b
4.6
5.1
3.8
4.9
3.1
2.7
6.3
5.1
5.1
4.2
4.9
2.7
3.5
3.4
5.4
5.9
a
K57
C59
C56
C173
C37
K81
66
94
12
25
C66
K177
K53
K170
K37
C81
26
15
30
19
a
b0
a
0
ga
gb
ma
mb
dH6–H3
dC(O)–H4 a
a
a
a
0
a
0
a
dC(O)–H4 b
0
dCC–H4 a
0
dCC–H4 b
a
a
a
a
0
3C(O)–H4 a
0
3C(O)–H4 b
0
fCC–H4 a
0
fCC–H4 b
a
a
a
0
0
The subscripts ‘a’ and ‘b’ for 4 protons refer to their coupling constant with H3 ; H4 a has the lower cis J value while H4 b has the higher trans J value
0
0
.
0 0
shielding with a more intense effect for H4 b since 3C(O)–H4 b
0
is lower than 3C(O)–H4 a. These remarks, made on the basis of
the most stable conformers obtained from a molecular
modeling study, which are in agreement with measured
model. Similarly, the influence exerted by the thymine ring
looked non-determinant because, in each of the lowest
energy conformers of rac-8 and rac-17, the H6 proton
0
pointed out in the direction of H4 protons (the values for
0
0
0
chemical shifts of H4 protons, gave confirmation of the
diastereoisomer structural attribution.
fCC–H4 a and fCC–H4 b angles were ,respectively, 12 and 258
for rac-8, 30 and 198 for rac-17). Conversely, the different
0
0
anisotropic contributions of the C9 –O9 bond in the
0
chemical shifts of H4 protons for each diastereoisomer
seemed able to explain the effect. Thus, molecular modeling
indicated that, for diastereoisomer rac-8, the values for
2.2. Lipase-catalyzedkineticresolutionofrac-8 andrac-17
0
0
angles 3C(O)–H4 a and 3C(O)–H4 b were, respectively, 66 and
948. According to the Mc Connell and Pople relationship22
giving the anisotropic contribution of an axial symmetry
The synthesis of enantiomerically pure nucleoside ana-
logues was undertaken. For this purpose, several methods
have been described in the literature, for example, enantio-
selective reaction using Jacobsen epoxidation,23 enzymatic
resolution24 or formation of diastereoisomeric esters.25
Considering the practical aspects, the kinetic enzymatic
resolution was chosen although enzyme-catalyzed reactions
have not been fully exploited in nucleoside chemistry.26
Thus, compounds rac-8 and rac-17 were subjected to
enzymatic transesterification using vinyl acetate as an acyl
donor and organic solvent in the presence of different
lipases. The behavior of six different enzymes (Candida
rugosa lipase, novozym 435 lipase, pork liver esterase,
porcine pancreatic lipase, Geotricum candida lipase and
Pseudomas sp. lipase) was screened. Porcine pancreatic
lipase presented the best selectivity and enzymatic activity
(Table 2).
0
bond (like the C9 –O9 bond):
0
ꢀ
ꢁ
Dc
3 N0dCðOÞ–H4 a
0
Ds Z
ð1K3 cos23CðOÞ–H4
Þ
0
where Dc and N0 are, respectively, the difference between
the susceptibility parallel to the axis and the transverse
susceptibility and the Avogadro number, the contribution
Ds is positive (diamagnetic effect) for 3C(O)–H4 O548450.
0
0
Thus, both H4 a and H4 b protons of rac-8 should receive an
0
0
0
upfield shielding, a high one for H4 b (3C(O)–H4 bz908) and a
very low one for H4 a (3C(O)–H4 bz548450). Conversely, both
0
0
0
0
0
H4 a and H4 b protons of rac-17, for which 3C(O)–H4 aZ268
and 3C(O)–H4 bZ158, would receive a negative downfield
0
Table 2. Lipase screening for the resolution of alcohols rac-8 and rac-17 (conditions: 0.2 mg of rac-8 (or rac-17) in 200 mL of vinyl acetate and 3 mg of
enzyme powder, CCC means that the conversion reached 50% at the time indicated)
Enzyme
tZ2 h
rac-17
tZ18 h
rac-17
tZ24 h
rac-17
tZ30 h
rac-17
tZ72 h
rac-17
rac-8
rac-8
rac-8
rac-8
rac-8
C. rugosa lipase
C
KC
CC
K
C
K
CC
CCC
K
C
C
C
CCC
K
CC
C
CC
CCC
K
C
C
CC
CCC
K
CC
CC
CC
CC
CC
CC
CCC
Novozym 435 lipase
Pork liver esterase
Porcine pancreatic lipase
G. candida lipase
CCC
K
K
K
K
K
C
C
K
K
K
K
CC
CC
CC
CC
CC
K
CCC
CCC
CCC
Pseudomas sp. lipase
KC
K
C
K

C. Len et al. / Tetrahedron 61 (2005) 10583–10595
10587
Among the enzymes used, novozym 435 lipase gave the
highest reaction rates with both rac-8 and rac-17, but did
not seem to present a high enantiomeric discrimination
factor since the transesterification took place to a great
extent beyond 50% conversion. We discarded Pseudomonas
sp. lipase because it induced very low reaction rates,
particularly with rac-17. Thus, two enzymes were selected:
C. rugosa lipase (AY 30 Amano) and porcine pancreatic
lipase (PPL). The results, determined by means of chiral
HPLC, showed that a better enantiomeric discrimination
was obtained when using PPL.
Thus, starting from rac-8, the ee’s of ester and of remaining
substrate were 0.98 (the major being 10R, 30S) and 0.74 (the
major being 10S, 30R), respectively, after 33% of conversion
(reaction time, 7 h). Starting from rac-17, the ee’s 0of ester
and residual substrate were 00.740(the major being 1 R, 30R)
and 0.78 (the major being 1 S, 3 S), respectively, after 51%
of conversion (reaction time, 48 h), (Fig. 5). The absolute
configurations were determined as described later in this
paper. Considering these results, enzyme-catalyzed trans-
esterification was not adopted as a method for the separation
of the racemic nucleosides rac-8 and rac-17.
Scheme 3. Reagents and conditions: (i) BuLi, THF then 2,3-O-
isopropylidene-D-glyceraldehyde; (ii) PhO(CS)Cl, DMAP, EtOAc; (iii)
Bu3SnH, AIBN, toluene, 100 8C; (iv) HCl, MeOH; (v) Ac2O, pyridine; (vi)
silylated thymine, C2H4Cl2, SnCl4; (vii) NH3, MeOH.
(ratio 1:1) in 51% yield. Esterification of the secondary
hydroxyl of compound 20 as phenylthionocarbonate esters
21 followed by standard Barton deoxygenation27 yielded the
desired enantiomerically pure compound 22 in 69% yield.
The intermediate 22 was then converted to epimeric
methoxide 13S as major anomer by treatment with HCl
10% in methanol, followed by acetylation of the primary
hydroxyl group to afford acetate 14S in 72% yield. As
described in the racemic synthesis, the formation of the
thymine derivatives by standard Vorbruggen chemistry21
resulted in a mixture of anomers 15S and 16S was formed
due to the lack of anchimeric assistance. Removal of the
acetyl group by treatment with saturated methanolic
ammonia produced the desired free nucleosides 8S and
17S in quantitative yield.
2.3. Single enantiomer synthesis using 2,3-O-isopropyl-
idene-D-glyceraldehyde
A selective synthetic strategy avoiding the need for
resolution procedures was shown to afford the enantiomeri-
cally pure target nucleosides 8S and 17S. The strategy used
for this purpose is shown in Scheme 3. The starting material
was enantiomerically pure 2,3-O-isopropylidene-D-glycer-
aldehyde, prepared from the oxidative cleavage of 1,2:5,6-
di-O-isopropylidene-D-mannitol. The chiral glyceraldehyde
derivative was reacted with the metalated aromatic ring of
10 to give the diastereoisomeric mixture of the alcohols 20
Figure 5. Chiral HPLC chromatograms. A: rac-15 and rac-8 (50/50). B: 15 and 8 from the enzymatic reaction without purification. C: 8 from the enzymatic
reaction after purification. D: 15 from the enzymatic reaction after purification. E: rac-16 and rac-17 (50/50). F: 16 and 17 from the enzymatic reaction without
purification. G: 17 from the enzymatic reaction after purification. H: 16 from the enzymatic reaction after purification. Conditions: Chiralcel OJ (250!46 mm),
1 mL/min, 30 8C, EtOH–hexane (30/70), lZ200 nm.

10588
C. Len et al. / Tetrahedron 61 (2005) 10583–10595
The positive ion ESI mass spectra of nucleosides 8S and 17S
indicated a high level of purity (Fig. 6). Indeed, the target
molecules 8S and 17S showed abundant cationic mono-
charged ions: [MCNa]C m/z 311.08 and [MCK]C m/z
327.06. The other ions observed at m/z 163.07, 149.03 and
117.07 were attributed to fragments of the sodium adduct by
an MS/MS experiment.
anthryl groups, have been proposed.29,30 Thus, a number of
rules concerning the configuration of aliphatic chiral
primary alcohols were drawn from a molecular modeling
study that had shown a conservation of the conformational
preference in this series.31 Unfortunately, these rules could
not be applied to primary alcohols whose vicinal chiral
centres were included in a cycle, which is the case for
isochromans 8 and 7. Meanwhile, it was shown that for
Mosher’s esters [a-methoxy-a-(trifluoromethyl)phenyl-
acetyl or MTPA esters], the absolute configuration of the
vicinal carbon can be deduced from the relative magnitude
of the lanthanide-induced chemical shift (LIS) of the
methoxy group for each diastereoisomer.32 In that case, it
was assumed that complexation of europium salts with both
oxygens of the ester and of methoxy groups induced the
existence of the conformers indicated in Scheme 4.
Comparison of the proton NMR spectra of 8S and 17S with
those of rac-8 and rac-17 allowed the respective trans and
cis re0lative configurations to be ascribed. Moreover, due to
the 3 -S carbon configuration afforded by D-mannitol, t0he
absolute configuration of 8S and 17S were 10R, 30S and 1 S,
30S, respectively. As a consequence, it was possible to
ascribe by comparison the absolute configurations of the
stereoisomers obtained via lipase kinetic resolution.
2.4. Confirmation of the absolute configuration deter-
mination of 8S, 8R and 17S and 17R isochromans by
means of NMR study of their Mosher’s esters 23–26
Thus, starting from the isochroman derivatives 8R, 8S, 17R
and 17S, the corresponding (S)-MTPA esters 23S, 23R, 24R
and 24S (Fig. 7) were prepared using classical methodology
A review about the determination of the absolute
configuration of chiral alcohols and amines by means of
NMR spectroscopy has recently been published.28 Thus, the
derivatization of the titled enantiomers into diastereoiso-
meric esters or amides by means of enantiopure acid
derivatives (like Mosher’s esters, for example) and the study
of their chemical shifts induced by an aromatic group of the
chiral center of the auxiliary compound was shown to be
particularly efficient in the case of secondary alcohols (or
amines). The application of this technique to primary
alcohols having the chiral center beside the hydroxymethyl
group was not so powerful. The reasons were: (i) lower
substituent effects due to the greater distance between the
perturbing groups of the auxiliary chiral center and the
methylene protons; (ii) the presence of supplementary C–C
bonds reduces the conformational preference and thus, can
make difficult the interpretation of the chemical shift
variations usually induced by phenyl substituents. In order
to avoid these drawbacks and to enhance the intensity of the
perturbing magnetic fields, larger aromatic substituents, like
Scheme 4. Application of the lanthanide-induced shifts (LIS) for OCH3
groups of (S)-MTPA esters for the determination of the absolute
configuration of alcohols (K1 and K2 are the equilibrium constants between
the complexed and the free forms of diastereoisomeric Mosher’s esters;
DdOMeZdifference between the chemical shift for the methoxy group of the
esters with and without the Europium salt).
Figure 6. Positive ion ESI mass spectra of compounds 8S (top) and 17S (bottom).

C. Len et al. / Tetrahedron 61 (2005) 10583–10595
10589
Figure 7. (S)-MTPA esters 23S, 23R, 24R and 24S.
and the classical shifts induced by Eu(fod)3 on the methoxy
group were studied.
this simple rule, established in a series of steroids for which
conformational features remained constant throughout the
series, could not be applied in our case. Moreover, the low
0
and similar DdH9 values measured for each (S)-MTPA
esters 23S, 23R, 24S, and 24R (ca. 0.04 ppm) (Figs. 8 and 9)
have precluded any reliable interpretation.
Thus, the lanthanide induced shifts on the methoxy groups
of (S)-MTPA esters 23 and 24 of the remaining alcohol
obtained after lipase resolution (Table 3) gave a clear
confirmation of the absolute configurations already deter-
mined by the use of 2,3-O-isopropylidene-D-glyceraldehyde
as an optically pure precursor.
3. Conclusion
Table 3. Lanthanide-induced shifts on the proton chemical shifts of (S)-
MTPA esters 23 and 24 and absolute configuration of the major remaining
alcohol obtained after lipase resolution
In this work, we have described an efficient route, starting
from 2-bromobenzaldehyde, to synthesize racemic diastereo-
isomeric thymine derivatives of isochroman. The relative
configurations were determined by NOE proton NMR
experiments in connection with molecular modeling.
Following the separation of the latter diastereoisomers,
kinetic resolution was achieved via a transesterification
reaction catalyzed by lipases. Using this method, moderate
ee’s were obtained (0.75–0.98). Thus, an alternative
strategy starting from D-mannitol was proposed to provide
pure enantiomers. The attribution of absolute configurations
was made by chemical filiation on the basis of the
configurations obtained from D-mannitol. The structural
attributions were confirmed by studying the proton NMR
LIS of the methoxy group of the corresponding isochroman
Mosher’s esters 23 and 24. An attempt to correlate the
(S)-MTPA
esters
Remaining
DdOCH3
*
DdOCH3**
Configuration
10S,30R
10R,30S
10S,30S
10R,30R
23
24
major
minor
major
minor
C0.31
C0.24
C0.11
C0.15
C0.52
C0.44
C0.29
C0.36
DdOCH3 (ppm)ZdOCH3 [with Eu(fod)3/CDCl3]KdOCH3 (CDCl3). * And **:
molar ratio between MTPA esters and Eu(fod)3 were, respectively, 0.5 and 1.
Previous NMR studies of MTPA esters of primary alcohols
showed that there was a correlation between the shape of the
signals due to the methylenic protons of the CH2O group of
(R)- and (S)-MTPA esters and the absolute configuration of
the vicinal asymmetric carbon. Thus, in a series of 24-
methyl-26-hydroxy steroids, the chemical shift difference
between the two OCH2 protons of (S)-MTPA esters was
larger when the configuration of C-26 was S than when this
carbon presented the R spatial arrangement.33 Obviously,
0
0
chemical shifts of O9 –C9 H2 protons of the Mosher’s esters
23 and 24 with the absolute configuration was unsuccessful
because the diastereoisomeric esters exhibited a too small
0
DdH9 value. The isochroman derivatives were evaluated for
their inhibitory effect on the replication of HIV-1 in human
0
Figure 8. Proton NMR spectra (CDCl3) of H9 protons of Mosher’s esters 23S (6) and 23R (B). (a) spectra of pure enantiomers prepared from D-mannitol, (b)
spectra of enantiomerically enriched enantiomers obtained via lipase resolution, (c) spectra of racemic mixture.

10590
C. Len et al. / Tetrahedron 61 (2005) 10583–10595
0
Figure 9. Proton NMR spectra (CDCl3) of H9 protons of Mosher’s esters 24S (6) and 24R (B). (a) spectra of pure enantiomers prepared from D-mannitol, (b)
spectra of enantiomerically enriched enantiomers obtained via lipase resolution, (c) spectra of racemic mixture.
T4-lymphoblastoid cells, CEM-SS and MT-4. Unfortu-
nately, these compounds were found inactive against HIV-1
replication at concentrations up to 10 mM.
a single internal lock mass correction, using characteristic
ions of reference components, was applied. For both EI-
HRMS and ESI-HRMS measurements, data acquisition and
processing were performed with MassLynx 4.0 software.
4. Experimental
4.1. General
Chromatographic analysis was performed on a Waters 600
HPLC system equipped with a Waters 996 photodiode array
spectrophotometer. The sample loop was 20 mL (Rheodyne
7125 injector). Chromatographic data were collected and
processed on a computer running with Millennium 2010
(Waters). The stainless steel column Chiralcel OJ (cellulose
tris-methylbenzoate; 250!4.6 mm i.d.; 10 mm) was pur-
chased from Daicel Chemical Industries. Lyophilized
enzymes were kindly donated by Amano Enzyme Inc.
(Nagoya, Japan), apart from porcine pancreatic lipase,
which was supplied by Sigma (PPL type II). The
enantiomeric excesses (ee) of the substrates and of the
acetates were determined using chiral high performance
liquid chromatography (HPLC, Chiralcel OJ) with the
following experimental conditions: sample concentration
0.21 mM; eluent: n-hexane/ethanol, 70:30; flow-rate: 1 mL/
min; temperature: 30 8C; wavelengths: 200, 226 nm.
Melting points were determined on a digital melting-point
apparatus (Electrothermal) and were uncorrected. Optical
rotations were recorded in CHCl3 or MeOH solutions with a
digital polarimeter DIP-370 (JASCO) using a 1 dm cell. 1H
and 13C NMR spectra were recorded in CDCl3 or Me2SO-d6
(internal Me4Si), respectively, at 300.13 MHz (Bruker
Avance-300) and at 500 MHz (Bruker AX500). TLC was
performed on Silica F254 (Merck) and detection was by UV
light at 254 nm or by charring with phosphomolybdic-
H2SO4 reagent. Column chromatography was carried out on
Silica Gel 60 (Merck, 230 mesh). EtOAc, diethyl ether and
petroleum ether were distilled before use. Bases and
solvents were used as supplied. MeOH–NH3 was methanol
saturated with ammonia gas at rt.
Molecular modeling calculations were made on a silicon
graphics (SGI) computer. Molecular building and energy
calculations were produced by means of InsightII, Bio-
polymer, Discover and CFF91 (force fields) software from
Accelrys (San Diego, CA, USA)
Electron impact analysis (EI) was performed on a Waters-
Micromass AUTOSPEC Ultima (EBE geometry) high-
resolution mass spectrometer (HRMS). Electron energy,
emission current and accelerating voltage values were
70 eV, 200 mA and 8 kV, respectively. Accurate mass
measurements of molecular and fragment ions were
performed using perfluoro kerosene (PFK) as the internal
reference. High-resolution electrospray experiments (ESI-
HRMS) were performed on a Waters-Micromass Q-TOF
Ultima Global hybrid quadrupole time-of-flight instrument,
equipped with a Z-spray ion source. The source and
desolvation temperatures were kept at 80 and 150 8C,
respectively. Nitrogen was used as drying and nebulizing
gas at flow rates of 350 and 50 L/h, respectively. The
capillary voltage was 2.5 kV, the cone voltage 100 V and
the RF lens1 energy 50 V. For accurate mass measurements,
4.2. Synthesis of racemic nucleosides rac-8 and rac-17
4.2.1. 2-(2-Bromophenyl)-1,3-dioxane (10). A stirred
mixture of 2-bromobenzaldehyde (10.0 g, 53.9 mmol),
propan-1,3-diol (4.9 g, 53.9 mmol), toluene-4-sulfonic
acid (0.4 g) and toluene (15 mL) was refluxed for 4 h
using a Dean-Stark condenser. Et3N (2 mL) was added and
the reaction mixture cooled and extracted with diethyl ether
(8 mL). The extract was worked up and the crude product
was purified by flash chromatography (diethyl ether/
petroleum ether, 5:95) to yield compound 10 (11.2 g,

C. Len et al. / Tetrahedron 61 (2005) 10583–10595
10591
1
84%) as a white solid: mp 54–55 8C; H NMR (CDCl3,
and the resulting mixture was stirred for 2 h at rt. Et3N
(4 mL) was added, the mixture was stirred for 30 min at rt
and then extracted with EtOAc (100 mL). The extract was
worked up and the crude product was co-evaporated
successively with toluene (3!10 mL) and pyridine
(10 mL). The residue was dissolved in anhydrous pyridine
(8.5 mL) and Ac2O (1.2 mL, 13.0 mmol) was added. The
resulting mixture was stirred overnight at rt. MeOH (10 mL)
was added and the mixture was evaporated to dryness. The
residue was co-evaporated with toluene and purified by flash
chromatography (EtOAc/petroleum ether, 30:70) to yield
rac-14 (1.78 g, 75%) as an oil. The major isomer rac-14 had
the (10R,30S) and (10S,30R) configuration; 1H NMR (CDCl3,
300 MHz) d 7.07 (m, 3H, H aromatic), 6.91 (m, 1H, H
aromatic), 5.36 (s, 1H, H-1), 4.16 (m, 1H, H3), 4.08 (m, 2H,
300 MHz) d 7.72 (m, 1H, H aromatic), 7.36 (m, 3H, H
aromatic), 5.97 (s, 1H, H-2), 4.29, 4.05 (m, 4H, H-4, H-6),
2.09, 1.47 (m, 2H, H-5); 13C NMR (CDCl3, 75 MHz) d
137.8 (C), 133.0 (CH), 130.7 (CH), 128.5 (CH), 128.0 (CH),
122.7 (CBr), 101.3 (C-2), 68.0 (2C, C-4, C-6), 26.1 (C-5);
EI-HRMS: MC% m/z 242, found 241.9931, C10H11O2Br
requires 241.9942 and [MKH%]C m/z 241, found 240.9863,
C10H10O2Br requires 240.9864. Anal. Calcd for
C10H11BrO2 (241.99 g/mol): C, 49.41; H, 4.56. Found: C,
49.39; H, 4.58.
4.2.2. 2-(2-Allylphenyl)-1,3-dioxane (11). To a solution of
bromide 10 (2.0 g, 8.3 mmol) in THF (32 mL) was added n-
BuLi (10 mL, 1.6 M solution) in THF at K10 8C under
argon. After 1 h, a solution of allyl bromide (2.15 mL,
12.3 mmol) was added dropwise to the mixture at the same
temperature and the mixture was stirred overnight at rt. The
solution was poured into saturated aqueous NH4Cl solution
and extracted with EtOAc (100 mL). The combined organic
layers were washed with brine, dried over anhydrous
Na2SO4 and concentrated in vacuo. The crude product
was purified by flash chromatography (EtOAc/petroleum
ether, 5:95) to yield compound 11 (1.47 g, 87%) as an oil;
1H NMR (CDCl3, 300 MHz) d 7.76 (m, 1H, H aromatic),
7.34 (m, 3H, H aromatic), 6.03 (m, CH allyl), 5.73 (s, 1H, H-
2), 5.18 (m, CH allyl), 4.29, 4.01 (m, 4H, H-4, H-6), 3.64
(m, 2H, CH2 allyl), 2.26, 1.41 (m, 2H, H-5); 13C NMR
(CDCl3, 75 MHz) d 137.9 (CH allyl), 137.8 (C), 136.8 (C),
130.1 (CH), 129.3 (CH), 126.6 (CH), 126.5 (CH), 116.1
(CH2 allyl), 10.3 (C-2), 68.0 (2C, C-4, C-6), 37.1 (CH2
allyl), 26.1 (C-5); EI-HRMS: [MKH%]C m/z 203, found
203.1072, C13H15O2 requires 203.1072. Anal. Calcd for
C13H16O2 (204.12 g/mol): C, 76.44; H, 7.90. Found: C,
76.51; H, 7.86.
0
H9a, H-9b), 3.39 (s, 3H, OCH3), 2.57 (dd, 1H, J4 a,4 b
0
Z
Z
0
0
0
0
0
0
16.5 Hz, J3 ,4 b Z11.3 Hz, H4 b), 2.39 (dd, 1H, J3 ,4 a
2.6 Hz, H40a), 1.94 (s, 3H, COCH3); 13C NMR (CDCl3,
75 MHz) d 170.7 (COCH3), 134.8, 134.1 (2C, C-10, C-11),
128.8 (CH), 128.5 (CH), 127.8 (CH), 126.6 (CH), 98.5
(C-10), 66.4 (C-3), 65.4 (C-9), 55.2 (OCH3), 29.9 (C-4), 20.9
(CH3CO). Anal. Calcd for C13H16O4 (236.10 g/mol): C,
66.09; H, 6.83. Found: C, 66.10; H, 6.90.
4.2.5. (10R,30S)- and (10S,30R)-1-(3-Acetyloxymethyl-iso-
chroman-1-yl)thymine (rac-15) and (10S,30S)- and
(10R,30R)-1-(3-acetyloxymethyl-isochroman-1-yl(thymine)
(rac-16). A mixture of thymine (534 mg, 4.23 mmol),
anhydrous HMDS (50 mL) and (NH4)2SO4 (10 mg) was
refluxed under N2 until a clear solution was obtained (ca.
12 h). Then the solvent was removed under reduced
pressure to give a colorless syrup that was dissolved in
anhydrous dichloroethane (10 mL). To the solution of
silylated thymine in anhydrous dichloroethane were added
acetate rac-14 (500 mg, 2.12 mmol) in anhydrous dichlor-
oethane and SnCl4 (578 mL, 4.24 mmol) at K10 8C. After
stirring the mixture for 3 h at rt, saturated NaHCO3 (50 mL)
was added, the mixture was stirred for 30 min and then
extracted with CH2Cl2. This extract was worked up and the
crude product was purified by flash chromatography
(EtOAc/petroleum ether, 40:60) to give compound rac-15
(276 mg, 39%) as a foam as first eluting compound and
compound rac-16 (259 mg, 37%) as a foam as second
4.2.3. 3-(2-(1,3)-Dioxan-2-ylphenyl)-propan-1,2-diol
(rac-12). N-Methylmorpholine-N-oxide (6.9 g, 58.8 mmol),
pyridine (0.5 mL), H2O (0.7 mL) and OsO4 (32 mL of a
2.5% solution in tert-BuOH, 0.1 mmol) were added to a
solution of alkene 11 (2.0, 9.80 mmol) in tert-BuOH
(10 mL). The mixture was stirred at rt overnight then
treated with 20% aqueous sodium bisulfite solution (4 mL)
and evaporated to dryness. Saturated aqueous NaCl (5 mL)
was added and the mixture was extracted twice with EtOAc.
The combined extracts were worked up and the crude
product was purified by flash chromatography (EtOAc/
petroleum ether, 50:50) to yield compound rac-12 (1.75 g,
75%) as an oil; 1H NMR (CDCl3, 300 MHz) d 7.35 (m, 1H,
H aromatic), 6.99 (m, 3H, H aromatic), 5.44 (s, 1H, H-2),
3.94 (m, 5H, H-4, H-6, CHOH, 2OH), 3.67 (m, 2H, H-4,
H-6), 3.32 (dd, JZ11.3, 3.5 Hz, 1H, CH2OH), 3.22 (dd, JZ
6.1 Hz, 1H, CH2OH), 2.68 (d, JZ6.6 Hz, CH2 allyl), 1.87,
1.13 (m, 2H, H-5); 13C NMR (CDCl3, 75 MHz) d 137.0 (C),
136.8 (C), 131.0 (CH), 129.0 (CH), 126.9 (CH), 126.5 (CH),
100.6 (C-2), 73.4 (CHOH), 67.6 (2C, C-4, C-6), 66.1
(CH2OH), 30.0 (CH2 allyl), 25.8 (C-5). Anal. Calcd for
C13H18O4 (238.12 g/mol): C, 65.53; H, 7.61. Found: C,
65.60; H, 7.62.
1
eluting compound. rac-15: H NMR (CDCl3, 300 MHz) d
8.58 (s, 1H, NH), 6.62 (d, JZ1.3 Hz, 1H, H-6), 7.30, 7.23,
7.20, 6.97 (4H, H aromatic), 7.00 (s, 1H, H-10), 4.22 (m,
J3 ,9 bZ6.1 Hz, J9 a,9 bZ11.7 Hz, 1H, H-90b), 4.14 (m,
0
0
0
0
J3 ,9 aZ4.1 Hz, 1H, H-90a), 4.09 (m, J3 ,4 aZ5.0 Hz,
0
0
0
0
J3 ,4 bZ8.7 Hz, 1H, H-30), 2.85 (m, J4 a,4 bZ16.3 Hz, 1H,
H-40a), 2.80 (m, 1H, H-40b), 2.00 (s, 1H, CH3CO), 1.73 (d,
1H, CH3 thymine); 13C NMR (CDCl3, 75 MHz) d 170.8
(COCH3), 163.60 (C-4), 0 151.1 (C-2), 137.0 (C-6),0 134.5,
130.8 (2C, C-100, C-11 ), 110.4 (C-5), 79.8 (C-1 ), 68.0
(C-30), 65.6 (C-9 ), 29.8 (C-40), 20.8 (CH3CO), 12.5 (CH3
thymine). Anal. Calcd for C17H18N2O5 (330.12 g/mol): C,
61.81; H, 5.49; N, 8.48. Found: C, 61.79; H, 5.53; N, 8.45.
0
0
0
0
1
rac-16: H NMR (CDCl3, 300 MHz) d 8.85 (s, 1H, NH),
6.68 (d, JZ1.3 Hz, 1H, H-6), 7.25, 7.18, 7.14, 6.91 (4H, H
aromatic), 7.04 (s, 1H, H-10), 4.14-4.23 (m, J3 ,9 bZ6.7 Hz,
0
0
0
0
0
0
0
0
J9 a,9 bZ11.5 Hz, 1H, J3 ,9 aZ3.5 Hz, J3 ,4 aZ2.4 Hz,
J3 ,4 bZ11.5 Hz, 3H, H-30, H-90a, H-90b), 2.89 (m,
0
0
4.2.4. (1R,3S)- and (1S,3R)-3-Acetyloxymethyl-1-meth-
oxyisochroman (rac-14). Compound rac-12 (2.4 g,
10.0 mmol) was dissolved in methanol HCl (1%, 60 mL)
J4 a,4 bZ16.3 Hz, 1H, H-40b), 2.70 (m, 1H, H-40a), 2.05 (s,
0
0
1H, CH3CO), 1.75 (d, 1H, CH3 thymine); 13C NMR (CDCl3,

10592
C. Len et al. / Tetrahedron 61 (2005) 10583–10595
75 MHz) d 170.9 (COCH3), 163.7 (C-4), 151.2 (C-2), 136.7
(C-6), 134.5, 132.0 (2C, C-100, C-110), 111.9 (C-5), 81.1 (C-
10), 72.9 (C-30), 65.9 (C-90), 29.7 (C-40), 20.9 (CH3CO),
12.4 (CH3 thymine). Anal. Calcd for C17H18N2O5
(330.12 g/mol): C, 61.81; H, 5.49; N, 8.48. Found: C,
61.81; H, 5.51; N, 8.47.
C16H22O5 (294.15 g/mol): C, 65.29; H, 7.53. Found: C,
65.32; H, 7.50. First stereoisomer: 1H NMR (CDCl3,
300 MHz) d 7.62 (m, 2H, H aromatic), 7.31 (m, 2H, H
aromatic), 5.73 (s, 1H, H-2), 5.22 (dd, JZ2.0, 5.6 Hz,
CHOH), 4.49 (q, JZ6.1 Hz, CHO), 4.28, 4.00 (m, 4H, H-4,
H-6), 4.13 (dd, JZ6.5, 8.4 Hz, CHO), 3.96 (dd, JZ6.8 Hz,
CHO), 3.24 (d, OH), 2.26, 1.41 (m, 2H, H-5), 1.49 (s, CH3),
1.39 (s, CH3); 13C NMR (CDCl3, 75 MHz) d 138.6 (C),
136.9 (C), 129.7 (CH), 128.3 (CH), 217.1 (CH), 126.9 (CH),
109.7 (Ciso), 100.8 (C-2), 78.4 (CH), 69.8 (CHOH), 67.9
(2C, C-4, C-6), 66.0 (CH2O), 27.0 (CH3), 26.0 (C-5), 25.7
(CH3). Second stereoisomer: 1H NMR (CDCl3, 300 MHz) d
7.56 (m, 1H, H aromatic), 7.45 (m, 1H, H aromatic), 7.26
(m, 2H, H aromatic), 5.72 (s, 1H, H-2), 5.10 (dd, JZ3.0,
7.3 Hz, CHOH), 4.44 (q, JZ6.7 Hz, CHO), 4.22, 3.95 (m,
4H, H-4, H-6), 3.74 (d, JZ6.6 Hz, CHO), 3.39 (d, OH),
2.20, 1.41 (m, 2H, H-5), 1.50 (s, CH3), 1.40 (s, CH3); 13C
NMR (CDCl3, 75 MHz) d 138.6 (C), 136.7 (C), 129.7 (CH),
128.5 (CH), 128.0 (CH), 127.5 (CH), 110.2 (Ciso), 101.2
(C-2), 80.2 (CH), 71.5 (CHOH), 67.8 (2C, C-4, C-6), 66.4
(CH2O), 27.2 (CH3), 26.1 (2C, C-5, CH3).
4.2.6. (10R,30S)- and (10S,30R)-1-(3-Hydroxymethyl-iso-
chroman-1-yl)thymine (rac-8). The protected compound
rac-15 (500 mg, 1.51 mmol) was dissolved in methanolic
ammonia (5 mL) and the mixture stirred for 24 h. After
evaporation of the solvent the crude product was purified by
flash chromatography (MeOH/CH2Cl2, 2:98) to yield
compound rac-8 (414 mg, 95%) as a foam, 1H NMR
(Me2SO-d6, 300 MHz) d 11.39 (s, 1H, NH), 6.98 (d, JZ
1.1 Hz, 1H, H-6), 7.34, 7.30, 7.25, 7.07 (4H, H aromatic),
6.89 (s, 1H, H-10), 4.82 (d, JZ5.8 Hz, 1H, OH), 3.96 (m,
1H, H-30), 3.52 (dd, J9 a,9 bZ11.3 Hz, J3 ,9 aZ4.8 Hz, 1H,
0
0
0
0
H-90a), 3.47 (dd, J3 ,9 bZ5.4 Hz, 1H, H-90a), 2.86 (dd,
0
0
0
0
0
0
0
0
J4 a,4 bZ16.6 Hz, J3 ,4 aZ3.2 Hz, J3 ,4 bZ10.7 Hz, 1H,
H-40a), 2.70 (dd, 1H, H-40b), 1.64 (d, 1H, CH3 thymine);
13C NMR (CDCl3, 75 MHz) d 163.8 (C-4), 151.1 (C-2),
137.6 (C-06), 135.5, 130.2 (2C, C-100, C-110), 0108.8 (C-5),
78.6 (C-1 ), 70.2 (C-30), 63.5 (C-90), 29.5 (C-4 ), 11.9 (CH3
thymine). Anal. Calcd for C15H16N2O4 (288.11 g/mol): C,
62.49; H, 5.59; N, 9.72. Found: C, 62.53; H, 5.65; N, 9.70.
4.3.2. ((S)-2,2-Dimethyl-(1,3)-dioxolan-4-yl)-(2-(1,3)-
dioxan-2-ylphenyl)-methane (22). Phenoxythiocarbonyl
chloride (282 mg, 2.04 mmol) was added to a solution of
alcohol 20 (400 mg, 1.36 mmol) and DMAP (498 mg,
4.08 mmol) in dry AcOEt (20 mL) at rt under argon. The
mixture was stirred for 12 h at rt and then diluted with
AcOEt (30 mL). The whole was washed with H2O (3!
20 mL) and the extract was worked up. The residue was co-
evaporated twice with toluene then dissolved in toluene
(3 mL). Bu3SnH (3.28 mL, 12.2 mmol) was added to the
above solution containing AIBN (44.7 mg, 0.27 mmol) at
100 8C under argon atmosphere. After being heated for
45 min, the solvent was removed in vacuo and the crude
product was purified by flash chromatography (ethylacetate/
petroleum ether, 10:90) to yield compound 22 (261 mg,
4.2.7. (10S,30S)- and (10R,30R)-1-(3-Hydroxymethyl-iso-
chroman-1-yl)thymine (rac-17). The protected compound
rac-16 (500 mg, 1.51 mmol) was dissolved in methanolic
ammonia (5 mL) and the mixture stirred for 24 h. After
evaporation of the solvent, the crude product was purified by
flash chromatography (MeOH/CH2Cl2, 2:98) to give
compound rac-17 (410 mg, 94%) as a foam, 1H NMR
(Me2SO-d6, 300 MHz) d 11.45 (s, 1H, NH), 6.96 (d, JZ
1.2 Hz, 1H, H-6), 7.29, 7.24, 7.22, 6.95 (4H, H aromatic),
6.93 (s, 1H, H-10), 4.87 (d, JZ5.8 Hz, 1H, OH), 4.03 (m,
69%) as an oil; [a]D22 C8 (c 0.05 in CHCl3); H NMR
1
1H, H-30), 3.56 (dd, J3 ,9 bZ5.8 Hz, 1H, H-90a), 3.52 (dd,
0
0
J9 a,9 bZ11.6 Hz, J3 ,9 aZ4.5 Hz, 1H, H-90a), 2.86 (dd, 1H,
(CDCl3, 300 MHz) d 7.62 (m, 1H, H aromatic), 7.25 (m, 3H,
H aromatic), 5.70 (s, 1H, H-2), 4.39 (m, CHO), 4.26, 3.99
(m, 4H, H-4, H-6), 3.96 (dd, JZ5.9, 8.2 Hz, CHO), 3.70
(dd, JZ6.9 Hz, CHO), 3.18 (dd, JZ6.0, 3.9 Hz, CH), 2.95
(dd, JZ3.2, 13.9 Hz, CH), 2.26, 1.41 (m, 2H, H-5), 1.48 (s,
CH3), 1.37 (s, CH3); 13C NMR (CDCl3, 75 MHz) d 137.3
(C), 135.7.9 (C), 130.7 (CH), 129.3 (CH), 127.2 (CH), 127.1
(CH), 109.3 (Ciso), 100.6 (C-2), 77.0 (CHO), 69.3 (CH2O),
67.9 (2C, C-4, C-6), 36.7 (CH2), 27.5 (CH3), 26.2 (CH3),
26.1 (C-5); ESI-HRMS: [MCNa]C m/z 301, found
301.1418, C16H22O4Na requires 301.1416. Anal. Calcd for
C16H22O4 (278.15 g/mol): C, 69.04; H, 7.97. Found: C,
69.15; H, 8.02.
0
0
0
0
H-40b), 2.72 (dd, J4 a,4 bZ16.3 Hz, J3 ,4 aZ2.8 Hz, J3 ,4 b
Z
0
0
0
0
0
0
11.57 Hz, 1H, H-40a), 1.68 (d, 1H, CH3 thymine); 13C NMR
(CDCl3, 75 MHz) d 163.7 (C-4), 151.1 (C-2), 136.8 (C-6),
135.7, 132.7 (2C, C-100, C-110), 110.3 (C-5), 80.3 (C-10),
75.5 (C-30), 63.7 (C-90), 29.5 (C-40), 11.9 (CH3 thymine).
Anal. Calcd for C15H16N2O4 (288.11 g/mol): C, 62.49; H,
5.59; N, 9.72. Found: C, 62.55; H, 5.57; N, 9.75.
4.3. Selective synthesis of nucleosides 8S and 17S
4.3.1. ((S)-2,2-Dimethyl-(1,3)-dioxolan-4-yl)-(2-(1,3)-
dioxan-2-ylphenyl)-methanol (20). n-Butyl lithium
(1.6 M in hexane, 23.8 mL, 38 mmol) was added dropwise
to a solution of bromide 10 (4.6 g, 19.0 mmol) in dry THF
(50 mL) at K15 8C under argon. After 1 h, a solution of 1,2-
O-isopropylidene-D-glyceraldehyde28 (500 mg, 3.84 mmol)
in THF (10 mL) was added dropwise at K15 8C under
argon. After being stirred overnight at rt, the reaction
mixture was treated with aqueous NH4Cl and extracted with
ethylacetate. The extract was worked up and the crude
product was purified by flash chromatography (ethylacetate/
petroleum ether, 20:80) to yield compound 20 (577 mg,
51%) as an oil; ESI-HRMS: [MCNa]C m/z 317, found
317.1361, C16H22O5Na requires 317.1365. Anal. Calcd for
4.3.3. (1R,3S)-Acetyloxymethyl-1-methoxyisochroman
(14S). Compound 22 (500 mg, 1.80 mmol) was dissolved
in methanol HCl (1%, 10 mL) and the resulting mixture was
stirred for 2 h at rt. Et3N (2 mL) was added, the mixture was
stirred for 30 min at rt and then extracted with EtOAc
(10 mL). The extract was worked up and the crude product
was co-evaporated successively with toluene (3!5 mL) and
pyridine (5 mL). The residue was dissolved in anhydrous
pyridine (3 mL) and Ac2O (235 mL, 2.5 mmol) was added.
The resulting mixture was stirred overnight at rt. MeOH
(3 mL) was added and the mixture was evaporated to

C. Len et al. / Tetrahedron 61 (2005) 10583–10595
10593
dryness. The residue was co-evaporated with toluene and
was purified by flash chromatography (EtOAc/petroleum
ether, 30:70) to yield the (1R,3S) isomer 14S as the major
isomer; enantiomer 14S had NMR data identical to those for
the racemic compound rac-14; ESI-HRMS: [MCNa]C m/z
259, found 259.0950, C13H16O4Na requires 259.0946.
remaining alcohol (84% yield, eeZ0.78, determined by
means of chiral HPLC tr1Z7.5 min and tr2Z11.2 min).
4.4.3. Enzymatic resolution of isochroman rac-8. rac-8
30 mg (0.10 mmol) in 200 mL of vinyl acetate and 300 mg
of freeze-dried PPL lipase were incubated at rt for 7 h with
magnetic stirring. Then, the lipase was filtered and vinyl
acetate was eliminated under vacuum. NMR analysis of the
mixture indicated a conversion of 33%. The reaction
products were separated by silica gel flash chromatography
(eluent petroleum ether/acetone 1:1), which afforded 9 mg
of ester (90% yield, eeZ0.98, determined by means of
chiral HPLC tr1Z17.2 min and tr2Z20.7 min) and 16 mg of
remaining alcohol (80% yield, eeZ0.74, determined by
means of chiral HPLC tr1Z7.5 min and tr2Z8.8 min).
4.3.4. (10R,30S)-1-(3-Acetyloxymethyl-isochroman-1-yl)
thymine (15S) and (10S,30S)-1-(3-acetyloxymethyl-1-
isochroman-1-yl)thymine (16S). Compound 14S was
converted to the thymine nucleoside analogue by the
procedure employed above for the racemate. The mixture
of stereoisomers was separated by chromatography as above
to give the (1R,3S) isomer 15S [a]2D2 C16 (c 0.05 in CHCl3);
enantiomer 15S had NMR data identical to those for the
racemic compound rac-15; ESI-HRMS: [MCNa]C m/z
353, found 353.1118, C17H18N2O5Na requires 353.1113
and the (1S,3S) isomer 16S [a]2D2 K4 (c 0.05 in CHCl3);
ESI-HRMS: [MCNa]C m/z 353, found 353.1098,
C17H18N2O5Na requires 353.1113.
4.5. Synthesis of the Mosher’s esters 23S, 23R, 24S and
24R
4.5.1. General procedure for the preparation of
Mosher’s esters of isochromans. Purified acetates obtained
by lipase transesterification were hydrolyzed in the presence
of ammoniac methanol solution. Corresponding alcohols as
well as remaining alcohols were derivatized as follows. In
an NMR tube, isochroman 8 or 17 (5 mg, 17 mmol) was
dissolved in 0.7 mL of pyridine d5 and 25 mL of a solution of
(R)-Mosher’s chloride (50 mg, 198 mmol in 0.25 mL of
CCl4) was added. The course of the reaction was followed
by means of 1H NMR spectroscopy. When the alcohol had
completely disappeared, the solvents were removed under
reduced pressure. Then, the residue was dissolved in 20 mL
of ether and the solution was washed twice with Na2CO3
saturated solution, then twice with water. After elimination
of ether, the corresponding (S)-Mosher’s esters were
obtained with sufficient purity for their further analysis by
NMR spectroscopy.
4.3.5. (10R,30S)-1-(3-Hydroxymethyl-isochroman-1-yl)
thymine (8S). Compound 15S was converted to the thymine
nucleoside analogue by the procedure employed above for
the racemate to give the nucleoside 8S; enantiomer 8S had
NMR data identical to those for the racemic compound rac-
8; [a]2D2 C18 (c 0.06 in CHCl3); ESI-HRMS: [MCNa]C
m/z 311, found 311.1017, C15H16N2O4Na requires
311.1008.
4.3.6. (10S,30S)-1-(3-Hydroxymethyl-isochroman-1-yl)
thymine (17S). Compound 16S was converted to the
thymine nucleoside analogue by the procedure employed
above for the racemate to give the nucleoside 17S;
enantiomer 17S had NMR data identical to those for the
racemic compound rac-17; [a]2D2 C3 (c 0.06 in CHCl3);
ESI-HRMS: [MCNa]C m/z 311, found 311.0995,
C15H16N2O4Na requires 311.1008.
4.5.2. Compound 23S. 1H NMR (CDCl3, 500 MHz) d 8.22
(s, 1H, NH), 7.25, 7.19, 7.18, 6.97 (9H, H aromatic), 7.00 (s,
1H, H-10), 6.60 (q, JZ1.2 Hz, 1H, H-6), 4.17 (m, 1H, H-30),
4.41 (dd, J9 a,9 bZ11.7 Hz, J3 ,9 aZ5.1 Hz, 1H, H-90a), 4.41
4.4. Lipase-catalyzedkineticresolutionofrac-8and rac-17
0
0
0
0
(dd, J3 ,9 bZ5.1 Hz, 1H, H-90b), 3.44 (s, 3H, CH3O), 2.77
0
0
(dd, J4 a,4 bZ16.3 Hz, J3 ,4 aZ5.3 Hz, 1H, H-40a), 2.77 (dd,
4.4.1. Lipase screening for the resolution of isochromans
8 and 17. Ten milligrams of each isochroman of was
dissolved in 100 mL of vinyl acetate. Then, 0.2 mL of this
solution was incubated at rt in the presence of 3 mg of each
lyophilized enzymatic preparation. The course of the
transesterification was followed by means of TLC (silica
gel plates, eluent petroleum ether/acetone 1:1, Rf 0.35 and
0.73 for 17 and corresponding acetate, 0.29 and 0.69 for 8
and corresponding acetate). Spots were revealed by UV at
254 nm.
0
0
0
0
J3 ,4 bZ8.4 Hz, 1H, H-40b), 1.71 (d, 1H, CH3 thymine); 13C
NMR (CDCl3, 100 MHz) d 166.4 (COO), 163.4 (C-4), 151.0
(C-2), 136.9 (C-6), 123.2 (CF3), 110.6 (C-5), 79.7 (C-10),
67.9 (C-30), 67.1 (C-90), 29.8 (C-40), 12.5 (CH3 thymine).
0
0
4.5.3. Compound 23R. 1H NMR (CDCl3, 500 MHz) d 8.18
(s, 1H, NH), 7.25, 7.19, 7.18, 6.97 (9H, H aromatic), 6.99 (s,
1H, H-10), 6.57 (q, JZ1.2 Hz, 1H, H-6), 4.12 (m, 1H, H-30),
4.42 (dd, J9 a,9 bZ11.7 Hz, J3 ,9 bZ6.8 Hz, 1H, H-90b), 4.38
0
0
0
0
(dd, J3 ,9 aZ4.0 Hz, 1H, H-90a), 3.45 (s, 3H, CH3O), 2.77
0
0
(dd, J4 a,4 bZ16.3 Hz, J3 ,4 aZ6.8 Hz, 1H, H-40a), 2.77 (dd,
0
0
0
0
4.4.2. Enzymatic resolution of isochroman rac-17. rac-17
31.8 mg (0.11 mmol) in 200 mL of vinyl acetate and
500 mg of freeze-dried PPL lipase were incubated at rt for
48 h with magnetic stirring. Then, the lipase was filtered and
vinyl acetate was eliminated under vacuum. NMR analysis
of the mixture indicated a conversion of 51%. The reaction
products were separated by silica gel flash chromatography
(eluent petroleum ether/acetone 1:1), which afforded 16 mg
of ester (96% yield, eeZ0.74, determined by means of
chiral HPLC tr1Z16.8 min and tr2Z26.8 min) and 14 mg of
J3 ,4 bZ6.8 Hz, 1H, H-40b), 1.69 (d, 1H, CH3 thymine); 13C
NMR (CDCl3, 100 MHz) d 166.5 (COO), 163.4 (C-4), 151.0
(C-2), 136.9 (C-6), 123.3 (CF3), 110.4 (C-5), 79.7 (C-10),
67.8 (C-30), 67.2 (C-90), 29.8 (C-40), 12.4 (CH3 thymine).
0
0
4.5.4. Compound 24S. 1H NMR (CDCl3, 500 MHz) d 8.36
(s, 1H, NH), 7.25, 7.19, 7.12, 6.91 (9H, H aromatic), 7.02 (s,
1H, H-10), 6.62 (q, JZ1.2 Hz, 1H, H-6), 4.25 (m, 1H, H-30),
4.50 (dd, J9 a,9 bZ11.7 Hz, J3 ,9 bZ6.1 Hz, 1H, H-90b), 4.38
0
0
0
0

10594
C. Len et al. / Tetrahedron 61 (2005) 10583–10595
(dd, J3 ,9 aZ3.9 Hz, 1H, H-90a), 3.48 (s, 3H, CH3O), 2.67
References and notes
0
0
(dd, J4 a,4 bZ16.3 Hz, J3 ,4 aZ2.8 Hz, 1H, H-40a), 2.96 (dd,
0
0
0
0
J3 ,4 bZ11.6 Hz, 1H, H-40b), 1.74 (d, 1H, CH3 thymine);
13C NMR (CDCl3, 100 MHz) d 166.4 (COO), 163.3 (C-4),
151.0 (C-2), 136.5 (C-6), 123.2 (CF3), 111.8 (C-5), 81.0
(C-10), 72.2 (C-30), 67.0 (C-90), 29.8 (C-40), 12.4 (CH3
thymine).
0
0
1. (a) Nucleosides and Nucleotides as Antitumor and Antiviral
Agents; Chu, C. K., Baker, D. C., Eds.; Plenum: New York,
1993. (b) Chemistry of Nucleosides and Nucleotides; Town-
send, L. B., Ed.; Plenum: New York, 1988.
2. (a) Bergmann, W.; Feeney, R. J. J. Am. Chem. Soc. 1950, 72,
2809–2810. (b) Bergmann, W.; Feeney, R. J. J. Org. Chem.
1951, 16, 981–987. (c) Bergmann, W.; Burke, D. C. J. Org.
Chem. 1955, 20, 1501–1507.
4.5.5. Compound 24R. 1H NMR (CDCl3, 500 MHz) d 8.36
(s, 1H, NH), 7.25, 7.19, 7.12, 6.92 (9H, H aromatic), 7.04 (s,
1H, H-10), 6.61 (q, JZ1.2 Hz, 1H, H-6), 4.25 (m, 1H, H-30),
3. (a) Takeuchi, S.; Hirayama, K.; Ueda, K.; Sakai, H.; Yonehara,
H. J. Antibiot. 1958, 11, 1–5. (b) Swaminathan, V.; Smith,
J. L.; Sundaralingam, M.; Coutsogeorgopoulos, C.; Kartha, G.
Biochim. Biophys. Acta 1981, 655, 335–341.
4.49 (dd, J9 a,9 bZ11.8 Hz, J3 ,9 bZ3.8 Hz, 1H, H-90b), 4.41
0
0
0
0
(dd, J3 ,9 aZ5.3 Hz, 1H, H-90a), 3.49 (s, 3H, CH3O), 2.67
0
0
(dd, J4 a,4 bZ16.3 Hz, J3 ,4 aZ2.8 Hz, 1H, H-40a), 2.93 (dd,
0
0
0
0
4. Yamaguchi, I. Crop Protection Agents from Nature: Natural
Products and Analogues. Royal Society of Chemistry;
Copping, L. G., Ed.; 1996, 27.
J3 ,4 bZ11.7 Hz, 1H, H-40b), 1.69 (d, 1H, CH3 thymine);
13C NMR (CDCl3, 100 MHz) d 166.4 (COO), 163.3 (C-4),
149.9 (C-2), 136.5 (C-6), 123.2 (CF3), 111.8 (C-5), 81.0
(C-10), 72.1 (C-30), 67.0 (C-90), 29.8 (C-40), 12.3 (CH3
thymine).
0
0
5. Iwasa, T.; Kusuka, T.; Suetomi, K. J. Antibiot. 1978, 31,
511–518.
6. Gumina, G.; Choi, Y.; Chu, C. K. In Antiviral Nucleosides:
Chiral Synthesis and Chemotherapy; Chu, C. K., Ed.; Plenum:
New York, 2003.
4.6. Molecular modeling
7. Lin, T. S.; Schinazi, R. F.; Prusoff, W. H. Biochem.
Pharmacol. 1987, 36, 2713–2718.
4.6.1. Force fields and energy minimization. The
conformational analysis was studied considering molecules
without the presence of solvent and with the hypothesis that
the energies of the conformations were qualitatively
correlated to their existence probability via the Boltzmann
relationship. Considering molecular mechanics calcu-
lations, only the relative energies of identical configurations
were comparable and the lowest energies correspond to the
most stable conformations. Since the molecular weights of
the molecules studied were relatively low, it was possible to
undertake a systematic study of the conformations via a
screening of dihedral angles. In connection with this
systematic conformational research, the different
molecular geometries studied were not obtained from
experimental data but were produced from a builder module
(‘Builder’).
8. Balzarini, J.; Van Aerschot, A.; Herdewijn, P.; De Clercq, E.
Biochem. Pharmacol. 1989, 38, 869–874.
9. Chu, C. K.; Schinazi, R. F.; Arnold, B. H.; Cannon, D. L.;
Doboszewski, B.; Bhadti, V. B.; Gu, Z. Biochem. Pharmacol.
1988, 37, 3543–3548.
10. Tisdale, M.; Alnadaf, T.; Cousens, D. Antimicrob. Agents
Chemother. 1997, 41, 1094–1098.
11. Faletto, M. B.; Miller, W. H.; Garvey, E. P.; St Clair, M. H.;
Daluge, S. M.; Good, S. S. Antimicrob. Agents Chemother.
1997, 41, 1099–1107.
12. Larder, B. A.; Darby, G.; Richman, D. D. Science 1989, 243,
1731–1734.
13. St Clair, M. H.; Martin, J. L.; Tudor-Williams, G.; Bach,
M. C.; Vavro, C. L.; King, D. M.; Kellam, P.; Kemp, S. D.;
Larder, B. A. Science 1991, 253, 1557–1559.
14. Richman, D.; Shih, C. K.; Lowy, I.; Rose, J.; Prodanovich, P.;
Goff, S.; Griffin, J. Proc. Natl. Acad. Sci. U.S.A. 1991, 88,
11241–11245.
4.6.2. Initial molecular building. Enantiomers of isochro-
mans 8 and 17 were first studied. The central bicyclic
nucleus presented a low conformational flexibility since the
aromatic ring considerably limited the pseudo-rotation of
the pyran ring. Thus, this bicyclic system adopted a quasi-
planar geometry and the only conformational duality was
revealed by the shift of the pyran oxygen atom above or
below the plane. The energy barrier between the two
resulting conformations was sufficient to divide further
energy calculations into two individual classes, unable to
interconvert when energy minimizations were applied. In
the initial molecular building, two dihedral angles were
15. Ewing, D. F.; Fahmi, N. E.; Len, C.; Mackenzie, G.; Ronco,
G.; Villa, P.; Shaw, G. Nucleosides Nucleotides 1999, 18,
2613–2630.
16. Ewing, D. F.; Fahmi, N. E.; Len, C.; Mackenzie, G.; Pranzo, A.
J. Chem. Soc., Perkin Trans. 1 2000, 21, 3561–3565.
17. Egron, D.; Perigaud, C.; Gosselin, G.; Aubertin, A. M.; Faraj,
A.; Selouane, A.; Postel, D.; Len, C. Bioorg. Med. Chem. Lett.
2003, 13, 4473–4475.
18. Kolb, H. C.; Andersson, P. G.; Sharpless, K. B. J. Am. Chem.
Soc. 1994, 116, 1278–1291.
19. Kolb, H. C.; VanNieuwenhze, M. S.; Sharpless, K. B. Chem.
Rev. 1994, 94, 2483–2547.
0
systematically screened with 108 steps: C2N1C1 O2 (a angle
0
0
already defined in text, see Table 1) and O2 C3 C9 O9 .
0
0
0
20. Moitessier, N.; Henry, C.; Len, C.; Chapleur, Y. J. Org. Chem.
2002, 67, 7275–7282.
21. (a) Vorbruggen, H.; Krolikievicz, K.; Bennua, B. Chem. Ber.
1981, 114, 1234–1255. (b) Vorbruggen, H.; Bennua, B. Chem.
Ber. 1981, 114, 1279–1286. (c) Vorbruggen, H.; Ruh-Pohlenz,
C. Org. React. 2000, 55, 1–630.
Acknowledgements
The authors thank L. Hoffmann for technical support in
molecular modeling.
22. (a) Mc Connell, H. J. Chem. Phys. 1957, 27, 226–229. (b)
Pople, J. A. Proc. Roy. Soc. A 1957, 239, 541–549.

C. Len et al. / Tetrahedron 61 (2005) 10583–10595
10595
23. Jacobsen, E. N. In Ojima, I., Ed.; Catalytic Asymmetric
Synthesis; VCH: New York, 1993; pp 159–202.
Zard, S. Z. In Radicals in Organic Synthesis; Renaud, P., Sibi,
M. P., Eds.; Wiley-VCH: Weinheim, 2001; pp 90–108.
28. Seco, J. M.; Quinoa, E.; Riguera, R. Chem. Rev. 2004, 104,
17–118.
24. (a) Johnson, C. R.; Golebiowski, A.; Steensma, D. H. J. Am.
Chem. Soc. 1992, 114, 9414–9418. (b) Tsuji, T.; Onishi, T.;
Sakata, K. Tetrahedron: Asymmetry 1999, 10, 3819–3825.
25. Vastmans, K.; Pochet, S.; Peys, A.; Kerremans, L.; Van
Aerschot, A.; Hendrix, C.; Marliere, P.; Herdewijn, P.
Biochemistry 2000, 39, 12757–12765.
29. Latypov, S. K.; Seco, J. M.; Quinoa, E.; Riguera, R. J. Org.
Chem. 1995, 60, 504–515.
30. Ferreiro, M. J.; Latypov, S. K.; Quinoa, E.; Riguera, R.
Tetrahedron: Asymmetry 1996, 7, 2195–2198.
26. (a) Ferrero, M.; Gotor, V. Chem. Rev. 2000, 100, 4319–4348.
(b) Albert, M.; De Souza, D.; Feiertag, P.; Honig, H. Org. Lett.
2002, 4, 3251–3254. (c) Garcia, J.; Fernandez, S.; Ferrero, M.;
Sanghvi, Y. S.; Gotor, V. Org. Lett. 2004, 6, 3759–3762. (d)
Bondada, L.; Gumina, G.; Nair, R.; Ning, X. H.; Schinazi,
R. F.; Chu, C. K. Org. Lett. 2004, 6, 2531–2534.
31. Latypov, S. K.; Ferreiro, M. J.; Quinoa, E.; Riguera, R. J. Am.
Chem. Soc. 1998, 120, 4741–4751.
32. Yasuhara, F.; Yamaguchi, S. Tetrahedron Lett. 1977, 18,
4085–4088.
33. (a) Bruno, I.; Minale, L.; Riccio, R.; La Barre, S.; Laurent, D.
Gazz. Chim. Ital. 1990, 120, 449–451. (b) D’Auria, M. V.;
Minale, L.; Pizza, C.; Riccio, R.; Zollo, F. Gazz. Chim. Ital.
1984, 114, 469–470. (c) De Rosa, S.; Milone, A.; Crispino, A.;
Jaklin, A.; De Guilio, A. J. Nat. Prod. 1997, 60, 462–463. (d)
Finamore, E.; Minale, L.; Riccio, R.; Rinaldo, G.; Zollo, F.
J. Org. Chem. 1991, 56, 1146–1153.
27. (a) Barton, D. H. R.; McCombie, S. W. J. Chem. Soc., Perkin
Trans. 1 1975, 1574–1585. (b) Crich, D.; Quintero, L. Chem.
Rev. 1989, 89, 1413–1432. (c) Barton, D. H. R.; Ferreira, J. A.;
Jaszberenyi, J. C. In Preparative Carbohydrate Chemistry;
Hanessian, S., Ed.; Marcel Dekker: New York, 1997; p 15. (d)