Anchimeric assistance by the anomeric phenylthio group in the nucleophilic substitution of a 6-O-trifluoromethanesulfonyl-β-D-galactopyranoside

Article abstract of DOI:10.1016/S0008-6215(99)00048-8

Nucleophilic displacement by the cyanide anion of the 6-O-triflyl group in phenyl 6-O-triflyl-2,3-di-O-benzyl-4-O-p-methoxybenzyl-1-thio-β-D-galactopyranoside takes place via an intermediate 1,6-sulfonium salt resulting from the anchimeric assistance of the C-1 phenylthio group. Copyright (C) 1999 Elsevier Science Ltd.

Full text of DOI:10.1016/S0008-6215(99)00048-8

Carbohydrate Research 316 (1999) 201–205
Note
Anchimeric assistance by the anomeric phenylthio group
in the nucleophilic substitution of a
6-O-trifluoromethanesulfonyl-b-
D-galactopyranoside
Muriel Compain-Batissou, Lamya Mesrari, Daniel Anker, Alain Doutheau *
Laboratoire de Chimie Organique, EA 1844, Institut National des Sciences Applique´es, 20 a6enue Albert Einstein,
F-69621 Villeurbanne, France
Received 17 December 1998; accepted 10 February 1999
Abstract
Nucleophilic displacement by the cyanide anion of the 6-O-triflyl group in phenyl 6-O-triflyl-2,3-di-O-benzyl-4-O-
p-methoxybenzyl-1-thio-b-
D-galactopyranoside takes place via an intermediate 1,6-sulfonium salt resulting from the
anchimeric assistance of the C-1 phenylthio group. © 1999 Elsevier Science Ltd. All rights reserved.
Keywords: -Galactose; Thioglycoside; Anchimeric assistance; C-6 substitution; Cyanide
D
corresponding cyano derivatives 2 that could
result from the displacement by the cyanide
1. Introduction
anion of the 6-O-triflyl group of the
derivatives 3.
D
-galacto
Pectin, a constituent of plant cell walls, is
mainly composed of partially methyl esterified
Starting alcohols 4a and 4b were prepared
oligomers of
D
-galacturonic acid units joined
respectively from phenyl 2,3-di-O-benzyl-1-
together by a-(1“4) glycosidic linkages.
These polysaccharides can be degraded by
enzymes from phytopathogenic fungi or bacte-
ria that cause damage to plants. Among them
are lyases that catalyse a b-elimination reac-
tion involving the glycosidic linkage and the
thio-b-
the corresponding 4,6-O-benzylidene deriva-
tive [1], and benzyl 2,3-di-O-benzyl-b- -galac-
topyranoside 5b [2], by a three-step sequence
involving selective tritylation at O-6, p-
methoxybenzylation at O-4 and detritylation.
When 4a was reacted with triflic anhydride
and then tetrabutylammonium cyanide [3], the
expected phenyl 2,3-di-O-benzyl-6-deoxy-4-O-
D
-galactopyranoside 5a obtained from
D
C-5 proton of
D
-galacturonic acid residues.
With the aim of preparing potential inhibitors
for these enzymes, we recently became inter-
ested in the synthesis of the
D-galacto-hep-
p-methoxybenzyl-1-thio-b- -galacto-heptopyr-
D
topyranuronic acid derivatives 1a and 1b and
planned to obtain these compounds from the
anosidurononitrile 2a was obtained in 66%
yield. In contrast, when the benzyl glycoside
4b was treated under the same conditions, the
expected benzyl 2,3-di-O-benzyl-6-deoxy-4-O-
* Corresponding author. Tel.: +33-4-72438221; fax: +33-
4-72438896.
E-mail address: doutheau@insa.insa-lyon.fr (A. Doutheau)
p - methoxybenzyl - b -
D
- galacto - heptopyran-
0008-6215/99/$ - see front matter © 1999 Elsevier Science Ltd. All rights reserved.
PII: S0008-6215(99)00048-8

202
M. Compain-Batissou et al. / Carbohydrate Research 316 (1999) 201–205
tion product at C-6 and an orthoacid cyanide
involving the O-2 acyl group participation,
was obtained. With sodium methoxide, only
the C-1 position was involved. These authors
rationalized their results on the basis of the
hard and soft acid and bases (HSAB) princi-
ple. In our case, the absence of both an alkyl
substituent on the sulfur atom and of a partic-
ipating group at C-2 ruled out the formation
of the anhydro and the orthoacid derivatives.
Consequently, the exclusive attack at C-6 was
consistent with the results of Lundt and
Skelbaek-Pedersen.
osidurononitrile 2b was obtained in only 16%
yield. The major product was benzyl 3,6-anhy-
dro-2-O-benzyl-4-O-p-methoxybenzyl-b-
galactopyranoside 6 obtained in 68% yield.
D-
From a literature survey, it appeared that
nucleophilic substitution reactions at the 6-pos-
ition of galactopyranoside derivatives carrying
a 3-benzyloxy or a 3-methoxy group often led
to the exclusive or partial formation of 3,6-an-
hydro-derivatives [4–6]. These compounds are
assumed to result from the competitive inter-
nal nucleophilic displacement of the sulfonyl
group by the C-3 oxygen atom, after a confor-
4
1
The hypothesis that neighbouring-group
participation by the 1-phenylthio group of 3a
accounted for the formation of 2a was confi-
rmed by the following result. When the alco-
hol 4a was reacted with triflic anhydride,
followed by sodium methoxide, methyl 2,3-
di-O-benzyl-4-O-p-methoxybenzyl-6-deoxy-6-
mational change from C1 to C4 chair confor-
mation, leading to an intermediate cyclic
oxonium ion which subsequently suffers nu-
cleophilic attack at the benzyl or methyl
group.
Considering these literature data, an expla-
nation had to be found for the successful
formation of the C-6 substituted compound
2a from 3a instead of an internal displacement
involving the 3-O-benzyloxy group to give a
3,6-anhydro analog of 6. Two hypotheses
could be proposed: (i) due to the presence of
the thiophenyl group, the transition state lead-
ing to the anhydro compound is energetically
disfavoured, or (ii) the nitrile 2a is formed via
the sulfonium cation 7 resulting from the in-
ternal nucleophilic displacement of the triflyl
group by the sulfur atom. Since nucleophilic
cleavage of the S–phenyl bond is most un-
likely, sulfonium cation 7 would subsequently
be further attacked by a cyanide anion at the
C-6 position to give 2a. It seemed to us that
the second hypothesis was more likely, since
the formation of intermediate cyclic sulfonium
ions during nucleophilic displacement reac-
tions performed on carbohydrates containing
an alkylthio group has often been put for-
ward. Though these species were not isolated,
their intermediate formation was necessary to
explain the migration of a thio group from
C-1 to C-6 and/or the formation of 1,6 thioan-
hydrosugars [7]. Furthermore, the reaction of
the sulfonium salt 8 with various nucleophiles
had been investigated by Lundt and Skelbaek-
Pedersen [8]. Depending of the nature of the
nucleophilic reagents, different compounds
were formed. With cyanide anion a mixture of
the 1,6-thioanhydro compound, the substitu-
thiophenyl-a- -galactopyranoside (9a) was
D
formed in 68% yield beside a small amount
(10%) of 9b. Thus, in full agreement with
Lundt and Skelbaek-Pedersen’s observation
[8], the hard nucleophilic methoxide anion
attacked the intermediate sulfonium com-
pound 7 at C-1, mainly in an SN2-type substi-
tution reaction, to give 9a as the major anomer.
In conclusion, we report here that when
a phenyl 1-thio-6-O-triflyl-b- -galactopyran-
D
oside bearing a benzyloxy group at C-3 was
reacted with cyanide anion, it led to the C-6
substituted compound. The reaction proceeds
via the formation of an intermediate 1,6-sulfo-
nium salt resulting from the anchimeric assis-
tance of the anomeric phenylthio group. This
result contrasts with the behaviour of its O-
glycoside analogues which lead predomi-
nantly, or exclusively, to 3,6-anhydro
compounds when reacted with nucleophiles.
To our knowledge, such an observation has
not yet been reported in the literature. Since
the anomeric thio group can be further easily
substituted by water or alcohols, it turns out
that thioglycosides could be valuable interme-
diates for the preparation of some C-6 func-
tionalized O-glycoside derivatives of hexoses
otherwise susceptible to resulting in 3,6-anhy-
dro compounds, such as in the galactose, glu-
cose, mannose or talose series. However, the
functionalization at C-6 may be restricted to
soft nucleophilic species.

M. Compain-Batissou et al. / Carbohydrate Research 316 (1999) 201–205
203
Phenyl 2,3-di-O-benzyl-1-thio-i-
D-galac-
topyranoside (5a).—To a solution of phenyl
2,3-di-O-benzyl-4,6-O-benzylidene-1-thio-b- -
D
galactopyranoside [1] (7 g, 13.0 mmol) in ace-
tone (80 mL) was added water (18 mL) and
HCl 1 N (5.2 mL). The reaction mixture was
refluxed for 4 h and BaCO3 (2 g, 10.4 mmol)
and water (40 mL) were added. After stirring
for 10 min, acetone was evaporated under
reduced pressure and the aq phase extracted
with CH2Cl2 (3×40 mL). Organic layers were
pooled and dried and, after solvent evapora-
tion under reduced pressure, the solid residue
was washed with pentane (2×20 mL) and
then dissolved in hot EtOH (50 mL). Com-
pound 5a (5.58 g, 95%) was precipited by
addition of water (100 mL) filtered and dried
(P2O5); mp (from cyclohexane) 120–122 °C;
1
[h]2D5 −4.7° (c 1.0, CHCl3); H NMR: l 7.57–
7.52 (m, 2 H, H aromatic), 7.41–7.20 (m, 13
H, H aromatic), 4.82, 4.73 (2d, 2 H, J 11.2
Hz, CH2Ph), 4.70 (brs, 2 H, CH2Ph), 4.64 (d,
1 H, J1,2 9.6 Hz, H-1), 4.05 (pt, 1 H, J 3 Hz,
H-4), 3.95 (ddd, 1 H, J6,OH 4.4, J6,6% 11.7, J6,5
6.7 Hz, H-6), 3.8 (ddd, 1 H, J6%OH 8.3, J6%,5 4.5
Hz, H-6%), 3.75 (dd, 1 H, J2,1 9.6, J2,3 9.0 Hz,
H-2), 3.56 (dd, 1 H, J3,2 9.0, J3,4 3.2 Hz, H-3),
3.46 (brpt, 1 H, H-5), 2.74 (brs, 1 H, CHOH),
2.4 (dd, 1 H, CH2OH). Anal. Calcd for
C26H28O5S: C, 69.03; H, 6.24. Found: C,
69.13; H, 6.11.
2. Experimental
General methods.—Solvents were distilled
and dried before use. Reaction mixtures were
magnetically stirred. The reactions were moni-
tored by TLC on silica gel (60 F254, E. Merck)
and detection was carried out by UV exami-
nation and then charring with a 5% phospho-
molybdic acid solution in ethanol containing
10% H2SO4. Column chromatography was
performed on silica gel (Kieselgel 60, 0.04–
0.06 mm, E. Merck). The following solvent-
systems (v/v) were used: (A1) 3:1, (A2) 4:1, (A3)
5:1 pentane–acetone; (B) 5:1 pentane–EtOAc;
(C) CH2Cl2; (D) 5:1 CH2Cl2–Et2O. Organic
solutions were dried using MgSO4. Melting
points were determined with a Kofler hot-
stage melting-point apparatus. Optical rota-
tions were measured with a Perkin–Elmer 241
Phenyl 2,3,-di-O-benzyl-4-O-p-methoxy-
benzyl-1-thio-i- -galactopyranoside (4a).—To
D
a solution of 5a (1.5 g, 3.3 mmol) in pyridine
(24 mL), were added trityl chloride (1.57 g,
5.65 mmol) and a catalytic amount of 4-
dimethylaminopyridine. The reaction mixture
was stirred at room temperature for 3 days
(TLC, solvent A1). The reaction mixture was
concentrated under reduced pressure and Et2O
(200 mL) was added to the residue. The solu-
tion was washed with a saturated aq NaHCO3
solution (2×30 mL), dried and concentrated
under reduced pressure to give a solid residue
which was dried (P2O5) and then dissolved in
10 mL of dry DMF. The resulting solution
was added to NaH (0.27 g of a 60% suspen-
sion in oil, ꢀ6.6 mmol, and washed three
times with pentane) in 10 mL of dry DMF. A
catalytic amount of imidazole was added to
the reaction mixture stirred at 20 °C. After the
1
polarimeter. H NMR and 13C NMR spectra
were recorded, in CDCl3, using a Bruker AM
200 spectrometer. Chemical shifts are given in
ppm downfield from internal Me4Si. Splitting
patterns abbreviations are: s, singlet; d, dou-
blet; t, triplet; m, multiplet; p, pseudo. Ele-
mental analyses were performed by the
‘Service Central de Microanalyses du CNRS’
Solaize (France).

204
M. Compain-Batissou et al. / Carbohydrate Research 316 (1999) 201–205
(dd, 1 H, H-3), 3.45 (m, 1 H, H-6%), 3.36 (m, 1
H, H-5). Anal. Calcd for C35H38O7: C, 73.66;
H, 6.71. Found: C, 73.43; H, 6.50.
evolution of H2 ceased, p-methoxybenzyl bro-
mide (0.614 mL, 4.53 mmol) and a catalytic
amount of Bu4NI were added. After 4 h, the
starting material had disappeared (TLC, sol-
vent B). The solvent was evaporated under
reduced pressure and Et2O (100 mL) was
added to the residue. The excess of NaH was
destroyed by careful addition of water (5 mL).
The mixture was washed with aq 3 N HCl
(2×20 mL) dried, concentrated and the
residue was dissolved in 1:1 MeOH–CHCl3
(40 mL). To this solution was added Am-
berlyst 15 (H+, 1.5 g) and the reaction mix-
ture was gently stirred (rotatory evaporator)
and heated at 60 °C. After 1 h, the reaction
was complete (TLC, solvent A1). The resin
was removed by filtration and washed with
MeOH (3×30 mL). The filtrate was concen-
trated under reduced pressure in the presence
of a small amount of triethylamine at 40 °C
giving a residue that was dissolved in Et2O
(100 mL). To this solution was added water
(50 mL) and the organic phase was separated,
washed with water (3×10 mL) dried and
concentrated under reduced pressure. Column
chromatography of the crude product (solvent
A1) gave 4a (1.62 g, 85% from 5a) as a colour-
less solid; mp (cyclohexane) 109–111 °C; [h]D25
Phenyl 2,3-di-O-benzyl-6-deoxy-4-O-p-me-
thoxybenzyl-1-thio-i- -galacto-heptopyran-
D
osidurononitrile (2a).—To a stirred solution of
4a (1.0 g, 1.75 mmol) in CH2Cl2 (25 mL) at
−78 °C, under a dry nitrogen atmosphere,
were added 2,6-di-tert-butylpyridine (1 mL,
3.68 mmol) and, dropwise (syringe), triflic an-
hydride (0.50 mL, 3 mmol). The reaction mix-
ture was warmed to −25 °C for 30 min
allowing the reaction to be complete (TLC,
solvent A1). After cooling to −78 °C, a solu-
tion of dried (P2O5; 24 h) tetrabutylammon-
ium cyanide (2.35 g, 8.75 mmol) in dry
CH2Cl2 (15 mL) was added and the reaction
mixture was allowed to warm to room temper-
ature. After 3 h, the reaction was complete
(TLC, solvent A2). After concentration under
reduced pressure, the residue was purified by
column chromatography (solvent A2) to give
2a (0.672 g, 66%) as a solid; mp 112–114 °C
(from cyclohexane). [h]2D5 −3.9° (c 1.1,
1
CHCl3); H NMR: l 7.58–7.53 (m, 2 H, Ar),
7.37–7.22 (m, 15 H, Ar) 6.88 (pd, 2 H, Ar),
4.99 (d, 1 H, J 11.1 Hz, CH2Ph), 4.86–4.73
(m, 4 H, 2 CH2Ph), 4.61 (d, 1 H, J
9.3 Hz,
1
−11.3° (c 1.0, CHCl3); H NMR: l 7.60–7.21
H-1), 4.56 (d, 1 H, J 11.1 Hz, CH12,P2 h), 3.88
(m, 17 H, H aromatic), 6.94–6.88 (m, 2 H, H
aromatic), 4.79 (s, 2 H, CH2Ph), 4.93–4.61
(m, 4 H, 2 CH2Ph), 4.68 (d, 1 H, J1,2 9.6 Hz,
H-1), 3.98 (pt, 1 H, H-2), 3.87 (brd, 1 H, J4,3
2.8 Hz, H-4), 3.84 (s, 3 H, OCH3), 3.85–3.74
(m, 1 H, H-6), 3.62 (dd, 1 H, J3,2 9.2 Hz, H-3),
3.52–3.39 (m, 2 H, H-6%, H-5). Anal. Calcd
for C34H36O6S: C, 71.30; H, 6.34. Found: C,
71.54; H, 6.31.
(pt, 1 H, J
4,3
1 H, J 5,6 =J
H-3), 2.69 (dd, 1 H, J
H-6), 2.46 (dd, 1 H, J6,6% 7 Hz, H-6%); 13C
9.3 Hz, H-2), 3.82 (brd, 1 H, J
2,3
3.2 Hz, H-4), 3.81 (s, 3 H, OCH3), 3.66 (pt,
6.9 Hz, H-5), 3.61 (dd, 1 H,
5,6%
16.7, J
7.0 Hz,
6,5
6%,5
NMR: l 159.3, 138.1, 137.8, 133.6, 129.9,
129.8, 129.1, 128.8, 128.4, 128.3, 128.2, 127.8,
127.7, 127.5, 127.4 (C Ar), 117.1 (CN), 113.8
(C Ar), 87.8 (C-1), 83.5, 76.8, 73.6, 73.5 (C-
2,3,4,5) 75.5, 74.2, 73.0 (3 CH2Ph), 55.2
(OCH3), 20.0 (C-6). Anal. Calcd for
C35H35O5NS: C, 72.27; H, 6.06. Found: C,
72.01; H, 6.20.
Benzyl 2,3-di-O-benzyl-4-O-p-methoxybenz-
yl-i- -galactopyranoside (4b).—This com-
D
pound was prepared from 5b [2] (2 g, 4.4
mmol) as described for 4a. After column chro-
matography (solvent A3), 4b (1.23 g, 49%) was
obtained as a colourless solid; mp 109–110 °C
(from cyclohexane); [h]2D5= −48.8° (c 0.93,
Benzyl 2,3-di-O-benzyl-6-deoxy-4-O-p-me-
thoxybenzyl - i -
D
- galacto - heptopyranosidur-
ononitrile(2b)andbenzyl3,6-anhydro-2-O-benz-
yl-4-O-p-methoxybenzyl-i- -galactopyran-
1
D
CHCl3); H NMR: l 7.37–7.23 (m, 17 H, H
oside (6).—A solution of 4b (0.5 g, 0.88
mmol) in 20 mL of dry CH2Cl2 was treated
according to the above mentionned condi-
tions, using 0.50 mL of 2,6-di-tert-
butylpyridine, 0.25 mL of triflic anhydride and
aromatic), 6.85 (brd, 2 H, J 8.7 Hz, H aro-
matic), 4.98–4.58 (4d, 8 H, 4 CH2Ph), 4.47 (d,
1 H, J
7.7 Hz, H-1), 3.90 (dd, 1 H, J
9.7
1,2
2,3
Hz, H-2), 3.79 (s, 3 H, OCH3), 3.76 (brd, 1 H,
2.4 Hz, H-4), 3.70 (m, 1 H, H-6), 3.52
J
4,3

M. Compain-Batissou et al. / Carbohydrate Research 316 (1999) 201–205
205
give sequentially pure 9a (0.040 g), a mixture
of 9a and 9b (0.110 g), pure 9b (0.010 g) and
recovered starting material (0.040 g) (overall
yield: 78%; a/b 7/1). 9a: Syrup; [h]2D5 −27° (c
1.2 g of tetrabutylammonium cyanide. After
solvent evaporation under reduced pressure,
the residue was chromatographed on a
column (solvent A2) to afford sequentially 6
(0.270 g, 68%) and 2b (0.08 g, 16%). 2b:
1
2, CHCl3); H NMR: l 7.5–6.8 (m, 19 H, H
1
Ar), 5.0–4.4 (6d, 6 H, 3 CH2Ph), 4.64 (par-
tially hidden d, 1 H, H-1), 4.02 (dd, 1 H, J2,3
Syrup; H NMR: l 7.34–7.30 (m, 17 H, H
Ar), 6.86 (pd, 2 H, J 8.5 Hz, H Ar), 5.0–4.54
10.0 J
3.5 Hz, H-2), 3.93 (brd, 1 H, H-4),
(4d, 8 H, 4 CH2Ph), 4.45 (d, 1 H, J
7.6 Hz,
2,1
H-1), 3.86 (dd, 1 H, J
9.5, H-2),13,2.81 (s, 3
3.90 (dd, 1 H, J
2.6 Hz, H-3), 3.78 (s, 3 H,
3,4
H, OCH3), 3.73 (brd, 12,H3 , J
2.5 Hz, H-4),
OCH3), 3.75 (partially hidden brpt, 1 H, H-5),
3.31 (s, 3 H, OCH3), 3.09 (dd, 1 H, J 6.6,
4,3
3.61 (brt, 1 H, H-5), 3.53 (dd, 1 H, J
2.8
6,5
3,4
J 6,6% 13.4 Hz, H-6), 2.85 (dd, 1 H, J 6%,5 6.9 Hz,
H-6%). Anal. Calcd for C35H38O6S: C, 71.65; H,
6.53; S, 5.46. Found: C, 71.65; H, 6.57; S,
Hz, H-3), 2.70 (dd, 1 H, J 6,5 7.8, J 6,6% 16.7 Hz,
H-6), 2.32 (dd, 1 H, J 6.0 Hz, H-6%). 6:
6%,5
1
Syrup; H NMR: l 7.32–7.12 (m, 12 H, H
Ar), 6.8 (brd, 2 H, J 8.7 Hz, H Ar), 4.80–4.40
(6d, 6 H, 3 CH2Ph), 4.71 (s, 1 H, H-1), 4.39
(brd, 1 H, H-3), 4.32 (brs, 1 H, H-5), 4.24 (d,
1
5.49. 9b: Syrup; H NMR l 7.5–6.8 (m, 19 H,
H Ar), 5.0–4.5 (6d, 6 H, 3 CH2Ph), 4.20 (d, 1
H, J
7.6 Hz, H-1), 3.91 (brd, 1 H, H-4),
1,2
3.79 (s, 3 H, OCH3), 3.78 (partially hidden dd,
1 H, J
8.9 Hz, H-6), 4.23 (d, 1 H, J
1.9
6,6%
4,5
1 H, J 2,1+J
17.3 Hz, H-2), 3.50 (s, 3 H,
2,3
Hz, H-4), 3.94 (dd, 1 H, J
3.1 Hz, H-6%),
6%,5
OCH3), 3.48 (partially hidden dd, 1 H, J 3,4 2.9
3.85 (d, 1 H, J
4.7 Hz, H-2), 3.81 (s, 3 H,
2,3
OCH3); 13C NMR: l 160.0, 138.2, 138.0,
130.6, 130.2, 129.1, 128.6, 128.5, 128.4, 128.3,
114.5 (C Ar), 99.0 (C-1), 80.1, 78.2, 77.9, 76.8
(C-2,3,4,5), 73.0, 71.4, 71.3, 70.0 (3 CH2Ph,
C-6), 55.9 (OCH3). This compound was not
pure enough for elemental analysis but its
NMR data are closely related to those of the
4-O-benzyl analogue [6].
Hz, H-3), 3.38 (brpt, 1 H, H-5), 3.17 (dd, 1 H,
J
6.0 Hz, H-6), 2.94 (dd, 1 H, J
7.3 Hz,
6,5
H-6%). This compound was not obt6a%,i5ned in a
sufficient amount for further analysis.
Acknowledgements
The authors thank the European Commu-
nity (Contract AIR2-CT94-1345) for financial
support.
Methyl 2,3,-di-O-benzyl-6-deoxy-4-O-p-me-
thoxybenzyl-6-phenylthio-h-
oside (9a) and methyl 2,3,-di-O-benzyl-6-de-
oxy-4-O-p-methoxybenzyl-6-phenylthio-i-
D
-galactopyran-
D-
galactopyranoside (9b).—To a solution of 4a
(0.2 g, 0.36 mmol) at −78 °C were added a
solution of di-tert-butylpyridine (0.2 mL, 0.73
mmol) in anhyd CH2Cl2 (10 mL) and triflic
anhydride (0.1 mL, 0.6 mmol) and the reac-
tion mixture was allowed to warm to −50 °C.
At this temperature were added a solution of
NaOMe (0.162 g, 3 mmol) in 10 mL of a 1:1
mixture of anhyd MeOH–MeCN. After stir-
ring at room temperature for 3 h, Et2O (50
mL) and pentane (5 mL) were added and the
organic phase was washed with water until
neutrality and dried. After concentration to
dryness under reduced pressure, the residue
was chromatographed (solvent C, then D) to
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