The presence of water improves reductive openings of benzylidene acetals with trimethylaminoborane and aluminium chloride

Article abstract of DOI:10.1016/S0008-6215(03)00015-6

The acidic reagent formed in situ from anhydrous AlCl₃ and H₂O in 3:1 ratio is much more efficient for the reductive openings of the cyclic benzylidene acetals with Me₃N·BH₃ in tetrahydrofurane than the AlCl

Full text of DOI:10.1016/S0008-6215(03)00015-6

Carbohydrate Research 338 (2003) 697–703
www.elsevier.com/locate/carres
The presence of water improves reductive openings of benzylidene
acetals with trimethylaminoborane and aluminium chloride
Andrei A. Sherman,a Yuri V. Mironov,b Olga N. Yudina,a Nikolay E. Nifantieva,*
aN.D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, Leninsky prospect 47, Moscow 119991, Russia
bHigher Chemical College, Russian Academy of Science, Miusskaya sq. 9, Moscow, Russia
Received 27 August 2002; accepted 4 January 2003
Abstract
The acidic reagent formed in situ from anhydrous AlCl3 and H2O in 3:1 ratio is much more efficient for the reductive openings
of the cyclic benzylidene acetals with Me3N·BH3 in tetrahydrofurane than the AlCl3 alone. Under proposed conditions, the
dioxane-type 4,6-O-bezylidene acetals of hexopyranosides give regioselectively the corresponding 4-hydroxy,6-O-benzyl deriva-
tives in excellent yields. Reductive openings of the dioxolane-type 3,4-O-benzylidene acetals of galactopyranoside are also very
efficient and regioselective and give either 3-O-benzyl derivative (from 3,4-O-exo-benzylidene acetal) or 4-O-benzyl derivative
(from 3,4-O-endo-benzylidene acetal) depending on the configuration of the acetal carbon atom. © 2003 Elsevier Science Ltd. All
rights reserved.
Keywords: Protecting group; Benzylidene acetal; Reductive opening; Hexopyranosides; Water
1. Introduction
determined not by the nature of reagent, but by the
7
configuration of the acetal carbon atom.6 Reductive
The cyclic benzylidene acetals can be transformed into
the monobenzyl ethers of the corresponding diols by a
number of reagents1 including NaBH3CN–HCl,2
Et3SiH–CF3CO2H,3 Cl2AlH,4 i-Bu2AlH.5 Reductive
opening of the dioxane ring of 4,6-O-benzylidenated
hexopyranosides with equatorial phenyl group can be
performed regioselectively to give either 4-O- or 6-O-
benzyl derivatives depending on the reaction condi-
tions. For the dioxolane ring of both 3,4-O- and
2,3-O-benzylidene acetals the direction of cleavage is
benzylidene acetal opening was shown to be a useful
technique for the protecting group manipulations and
has found wide application.1
One of the most convenient procedures for reductive
benzylidene acetal opening employs Me3N·BH3 and
AlCl3 in THF.8 Such treatment of 4,6-O-benzylidenated
derivatives of
D-glucose and D-galactose have been
described8 to give 4-hydroxy,6-O-benzyl ethers in 62–
86% yields. Under the same conditions, methyl 4-O-
benzyl-exo-2,3-O-benzylidene-a-L-rhamnopyranoside
gave the 3,4-di-O-benzyl ether with the 2-OH free in
85% yield.
* Corresponding author. Fax: +7-095-1358784
E-mail address: nen@ioc.ac.ru (N.E. Nifantiev).
0008-6215/03/$ - see front matter © 2003 Elsevier Science Ltd. All rights reserved.
doi:10.1016/S0008-6215(03)00015-6

698
A.A. Sherman et al. / Carbohydrate Research 338 (2003) 697–703
10
However, as we have described,9
when we at-
desired products 3 and 4, respectively, and even after 2
tempted reductive opening of 4,6-O-benzylidene acetals
in fully substituted compounds 19 and 210 with
Me3N·BH3 (6 equiv) and AlCl3 (6 equiv) in THF, the
reactions stopped after 10–15% of conversion into the
days at rt the unreactive starting materials were the
main components of the reaction mixtures (Table 1,
Entries 1, 3). The use of large (ꢀ20-fold) excess of the
reagents did not improve the yields, while heating of
Table 1
Reductive openings of the benzylidene acetals rings in compounds 1, 2, 5, 6, 7, 8, 14, 16, 19, 20
a
Entry
Acetal
Procedure
Time (h)
Product
Isolated yield (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
1
1
2
2
5
5
6
6
7
7
8
8
14
16
19
20
20
A
B
A
B
A
B
A
B
A
B
A
B
B
B
B
B
A
72
6
48
6
48
5
48
8
18
5
8
5
7
3
3
4
4
9
9
10
10
10
89
12
87
13
91
15
82
28
87
66
77
82
62
92
88
85
b
12
11
13
13
15
17
21
22
22
7
5
5
7
a Procedure A: Me3N·BH3 (4 equiv), AlCl3 (6 equiv) in THF at rt. Procedure B: Me3N·BH3 (4 equiv), AlCl3 (6 equiv), and H2O
(2 equiv) in THF at rt.
b In addition to 12, the product of its O-debenzoylation 13 was also obtained in 31% yield.

A.A. Sherman et al. / Carbohydrate Research 338 (2003) 697–703
699
the reaction mixtures lead to degradation. The same
treatment of fully benzoylated or acetylated galac-
tosides 511 and 612 gave also mainly recovered starting
materials and the products 9 and 10, respectively, were
obtained in low yields (Entries 5, 7). In this communi-
cation we describe our investigation of the reductive
opening of benzylidene acetals with Me3N·BH3 and
AlCl3 in THF.
because the acetal 8 contains the requisite amount of
protons in itself.
Thus, catalysis of the reductive opening of the diox-
ane-type 4,6-O-benzylidene acetals of hexapyranosides
by the acidic species formed from AlCl3 and H2O in 3:1
ratio is much more efficient than that by anhydrous
AlCl3.
The investigation of reductive openings of the diox-
olane-type benzylidene acetals was performed with the
use of 3,4-O-benzylidenated substrates 19 and 20. The
endo-acetal 19 (lPhCH 5.98 ppm) was obtained in 80%
yield, together with 8% of its exo isomer 20 (lPhCH 6.07
ppm), by benzylidenation of diol 1815 with Ph-
CH(OMe)2 and TsOH in THF as described for its
lactose analogue.16
In contrast to the dioxanes 1, 2, 5, 6, the completely
substituted dioxolanes 19 and 20 underwent reductive
opening into 21 and 22, respectively, both with and
without water (Entries 15–17). Although in the pres-
ence of water the reactions were faster, the result of the
anhydrous experiment (Entry 17) shows that the acidity
of AlCl3 alone is sufficient in the case of more reactive5
2-phenyl-1,3-dioxolanes as compared to 2-phenyl-1,3-
dioxanes. Similar conditions have already been
recommended8 for the reduction of methyl 4-O-benzyl-
2. Results and discussion
Our study of the reaction revealed that addition of
water to the reaction mixture favored dramatically the
efficiency of benzylidene opening. After some experi-
mentation, 2 equiv of water proved to be the optimal
amount. Thus treatment of the acetals 1, 2, 5, 6 with
Me3N·BH3 (4 equiv), anhydrous AlCl3 (6 equiv), and
H2O (2 equiv) in THF (freshly distilled from Na–
Ph2CO) for 6–8 h at rt afforded 4-hydroxy,6-O-Bn
derivatives 3, 4, 9, 10 in 82–91% yields (Entries 2, 4, 6,
8) without formation of the isomeric 4-O-benzyl deriva-
tives. No 3-O- “ 4-O-acyl migration in the galactose
derivatives 9 and in particular 10 was observed. More
complex di- and tetrasaccharide substrates 14 and 16
(their preparation will be reported elsewhere) gave also
regioselectively 4-hydroxy derivatives 15 and 17 in good
yields (Entries 13 and 14), and no cleavage of the
exo-2,3-O-benzylidene-a-
L
-rhamnopyranoside
into
methyl 3,4-di-O-benzyl-a-
L-rhamnopyranoside. Com-
pound 21 was successfully applied for the synthesis of
aminoethyl glycosides of the carbohydrate chains of
glycolipides Gb3, Gb4 and Gb5.17
acid-labile a-L-fucose linkage occurred. Thus the reac-
tion conditions tolerate the typical set of glycosidic
bonds and carbohydrate protecting groups, including
acyl, allyl (as in 6), and azido (as in 16) ones.
As expected,18 the openings of dioxolane rings pro-
ceeded regioselectively: the endo isomer 19 afforded the
3-OH derivative 21 (lC-3 75.6, lC-4 76.2 ppm) solely,
while the 4-hydroxy derivative 22 (lC-3 82.5, lC-4 66.9
ppm) was obtained as the only product from the exo
isomer 20. Furthermore, both yield and regiochemical
outcome of the reduction of 20 under anhydrous condi-
tions (Entry 17) were the same as those in the presence
of water (Entry 16). Thus, the direction of cleavage is
determined by configuration of the acetal carbon atom
solely, and independent on the nature of the acidic
catalyst. The same independence can be concluded from
the comparison between the dioxolane openings with
NaBH3CN–HCl and those with LiAlH4–AlCl3, both
of which give the same regioselectivity.18
The regioselectivity of the dioxolane reductions sup-
ports the previous assumption1 that the formation of
the oxo-carbenium ion from the initial acetal is a slow
rate-determining process, while further reduction of the
cation with Me3N·BH3 occurs very rapidly. Otherwise,
racemization of the acetal carbon atom leading to the
mixture of 3-O- and 4-O-benzyl ether products could
be expected,19 which actually is not observed.
We also studied the reductive opening of mono- and
dihydroxylated compounds in order to investigate
whether substrate’s free hydroxy groups may favor
benzylidene opening by serving as proton donors neces-
sary to catalyze the reaction. The reaction of monohy-
droxy substrate 713 in the presence of 2 equiv of H2O
afforded rapidly a single product 11 in 87% yield (Entry
10). On the contrary, in the absence of H2O the sub-
strate 7 reacted slowly (Entry 9) and only traces of 11
were obtained. Instead, the major compounds formed
after 18 h were the benzoyl group migration product 12
(28%) and debenzoylated derivative 13 (31%). Other
components of the reaction mixture were unidentified
degradation products of very low chromatographic mo-
bility. Hence, the presence of only 1 equiv of protons in
the reaction media is not enough for the efficient reduc-
tive opening.
The reactions of dihydroxylated benzylidene deriva-
tive 811 14 in the presence and absence of water showed
small difference in results (Entries 11 and 12) of reduc-
tive opening into 6-O-benzyl derivative 13.14 Although
in the presence of water the reaction was faster and the
yield of 13 was higher, its influence was not so crucial,
In none of the water-assisted reactions investigated
the products of benzylidene acetal hydrolysis were
formed in appreciable amounts. Furthermore, when the

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A.A. Sherman et al. / Carbohydrate Research 338 (2003) 697–703
acetal 2 was treated with 4 equiv of Me3N·BH3, 6 equiv
of AlCl3, and 20 equiv of H2O (in THF for 24 h at rt),
the reduced derivative 4 was still the major product
(43% yield), but the corresponding 4,6-diol 23 was also
obtained in 20% yield. Hence, the reaction of oxo-car-
benium ion with the hydride donor is likely to occur
faster then that with water.
(0.036 mL, 2 mmol) is added dropwise and stirring is
continued at rt for the time period shown in the Table
1 until the complete consumption of the starting acetal
(TLC control). Workup of the reaction mixture as in
Section 3.2 gives the product(s) in the yield(s) shown in
the Table 1.
In conclusion, further investigation of the known
protocol8 for the reductive openings of the cyclic ben-
zylidene acetals with Me3N·BH3 and anhydrous AlCl3
in THF have shown that the scope of the method can
be greatly broadened by changing the acidic catalyst to
that formed in situ from anhydrous AlCl3 and H2O in
3:1 ratio.
3.4. Ethyl 3,6-di-O-benzyl-2-deoxy-1-thio-2-trichloroac-
etamido-b-D-glucopyranoside (3)
Rf 0.33 (4:1 toluene–EtOAc); [h]D −9° (c 1, CHCl3);
1
NMR (CDCl3): H, see Table 2 for carbohydrate ring
protons; l 7.42–7.31 (m, 10 H, Ph), 6.82 (d, 1 H, JNꢀH,2
8 Hz, NꢀH), 4.79 (s, 2 H, PhCH2), 4.62 (d, 1 H, J 12.1
Hz, PhCH2), 4.55 (d, 1 H, PhCH2), 2.89 (broad s, 1 H,
OH), 2.71 (m, 2 H, SꢀCH2), 1.37 (t, 3 H, J 7.5 Hz,
SꢀCH2CH3); 13C, see Table 2 for carbohydrate ring
carbons; l 161.8 (NꢀC(O)CCl3), 137.8, 137.4 (2 ipso
Ph), 128.6–127.8 (Ph), 92.4 (CCl3), 74.8 (PhCH2), 73.8
(PhCH2), 24.4 (SꢀCH2), 15.1 (SꢀCH2CH3). Anal. Calcd
for C24H28Cl3NO6S: C, 52.52; H, 5.14; Cl, 19.38; N,
2.55; S, 5.84. Found: C, 52.49; H, 5.03; Cl, 18.99; N,
2.64; S, 5.71.
3. Experimental
3.1. General methods
Trimethylaminoborane and anhyd AlCl3 were pur-
chased from Fluka. THF was always freshly distilled
from dark-blue Na–Ph2CO. Column chromatography
was performed on silica gel 60 (Fluka, 70–230 mesh),
and TLC was performed on silica gel 60 F254 (E.
Merck, Darmstadt, Germany). Optical rotations were
measured with JASCO DIP-360 digital polarimeter at
26–30 °C. NMR spectra were recorded at 27 °C with
3.5. 3-Trifluoroacetamidopropyl 2-acetamido-3,6-di-O-
benzyl-2-deoxy-b-D-glucopyranoside (4)
Rf 0.3 (3:2 toluene–acetone); [h]D −12° (c 1, acetone);
1
Bruker DRX-500 instrument (500 MHz for H and 125
1
NMR (acetone-d6): H, see Table 2 for carbohydrate
MHz for 13C) using Me4Si as internal standard, assign-
ments were aided by APT, 1H–1H, and 1H–13C correla-
tion spectroscopies. Melting points were determined
with a Kofler apparatus and are uncorrected.
ring protons; l 7.35–7.18 (m, 10 H, Ph), 4.83 (d, 1 H,
J 11.6 Hz, PhCH2), 4.72 (d, 1 H, PhCH2), 4.59 (m, 4 H,
H-1, 2 PhCH2, HNC(O)CF3), 3.87 (m, 2 H, OCH2-
(CH2)2NHC(O)CF3, H-6a), 3.58 (m, 2 H, H-4, OCH2-
(CH2)2NHC(O)CF3,), 3.48 (m, 2 H, H-5, O(CH2)2CH2-
3.2. Procedure A
NHC(O)CF3), 3.32 (m,
1 H, O(CH2)2CH2NHC-
(O)CF3), 1.89 (s, 3 H, NꢀAc), 1.81 (m, 2 H, OCH2CH2-
CH2); 13C, see Table 2 for carbohydrate ring carbons; l
170.8 (NꢀC(O)CH3), 140.5, 140.0 (2 ipso Bn), 129.2–
128.1 (Ph), 74.9, 73.9 (2 PhCH2), 66.9 (OCH2(CH2)2-
NHC(O)CF3), 37.9 (CH2NHC(O)CF3), 29.6 (OCH2-
CH2CH2), 23.5 (NꢀC(O)CH3); Anal. Calcd for
C27H33F3N2O7: C, 58.48; H, 6.00; N, 5.05. Found: C,
58.33; H, 5.92; N, 4.88.
Me3N·BH3 (292 mg, 4 mmol) is added with stirring
under Ar to a solution of benzylidene acetal (1 mmol)
in abs THF (20 mL), followed by anhyd AlCl3 (800 mg,
6 mmol) and the reaction mixture is stirred at rt for the
time period shown in the Table 1. The reaction is
terminated by addition of water (10 mL) followed by 1
M aq HCl (10 mL) and the mixture is extracted with
EtOAc (3×20 mL), the extracts are washed with brine,
dried, and concentrated. Chromatography of the
residue on a column of silica gel (50 g) by gradient
elution with the solvent system in which the Rf value is
specified gives the product(s) in the yield(s) shown in
the Table 1.
3.6. Ethyl 2,3-di-O-benzoyl-6-O-benzyl-1-thio-b-D-
galactopyranoside (9)
Rf 0.48 (6:1 toluene–EtOAc); [h]D 58° (c 1, EtOAc); mp
97–99 °C (from Et2O–petroleum ether); lit.11 [h]D 69°
1
3.3. Procedure B
(c 0.5, CHCl3); mp 98–99 °C; NMR (CDCl3): H, see
Table 2 for carbohydrate ring protons; l 7.98 (d, 4 H,
J 7.5 Hz, 4 ortho protons of 2 Bz), 7.55–7.31 (m, 11 H,
Ph), 4.61 (s, 2 H, PhCH2), 2.73 (m, 3 H, SꢀCH2, OH),
1.31 (t, 3 H, J 7 Hz, SꢀCH2CH3). Anal. Calcd for
C29H30O7S: C, 66.65; H, 5.79. Found: C, 66.78; H, 5.87.
Me3N·BH3 (292 mg, 4 mmol) is added with stirring
under Ar to a solution of benzylidene acetal (1 mmol)
in abs THF (20 mL), followed by anhyd AlCl3 (800 mg,
6 mmol). After the dissolution of the reagents, H2O

A.A. Sherman et al. / Carbohydrate Research 338 (2003) 697–703
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A.A. Sherman et al. / Carbohydrate Research 338 (2003) 697–703
3.7. Allyl 2,3-di-O-acetyl-6-O-benzyl-b-
oside (10)
D
-galactopyran-
PhCH2), 4.32 (s, 1 H, 3-OH), 3.83 (s, 1 H, 2-OH), 3.61
(s, 1 H, 4-OH), 2.62 (m, 2 H, SꢀCH2), 1.15 (t, 3 H, J 7.1
Hz, SꢀCH2CH3); 13C, see Table 2 for carbohydrate ring
carbons; l 138.0 (ipso Ph), 128.5 and 127.8 (Ph), 73.5
(PhCH2), 24.7 (SꢀCH2), 15.3 (SꢀCH2CH3). Anal. Calcd
for C15H22O5S: C, 57.30; H, 7.05. Found: C, 57.24; H,
7.09.
Rf 0.28 (3:1 toluene–EtOAc); [h]D −1° (c 1, EtOAc);
1
NMR (CDCl3): H, see Table 2 for carbohydrate ring
protons; l 7.37–7.29 (m, 5 H, Ph), 5.86 (m, 1 H,
OCH2CHꢁCH2), 5.26 (dd, 1 H, Jtrans 17.1 Hz, Jgem
1
Hz, OCH2CHꢁCH2), 5.17 (dd, 1 H, Jcis 10.6 Hz,
OCH2CHꢁCH2), 4.36 (dd, 1 H, Jgem 13.3 Hz, Jvic 4.9
Hz, OCH2CHꢁCH2), 4.10 (dd, 1 H, Jvic 6.2 Hz,
OCH2CHꢁCH2), 2.69 (broad s, OH), 2.11, 2.06 (2 s, 3
H each, 2 OC(O)CH3); 13C, see Table 2 for carbohy-
drate ring carbons; l 170.3, 169.6 (2 OC(O)CH3), 137.6
(ipso Ph), 133.6 (OCH2CHꢁCH2), 128.5–127.7 (Ph),
3.11. Ethyl O-(3,4-di-O-benzoyl-2-O-benzyl-a-
ranosyl)-(1“3)-6-O-benzyl-2-deoxy-1-thio-2-trichloro-
acetamido-b- -glucopyranoside (15)
L-fucopy-
D
Rf 0.28 (4:1 toluene–EtOAc); [h]D −118° (c 1, EtOAc);
1
NMR (CDCl3): H, see Table 2 for carbohydrate ring
protons; l 7.93, 7.78 (2 d, 2 H each, J 7.7 Hz, ortho
protons of 2 Bz), 7.65–7. 18 (m, 16 H, Ph), 7.30 (d, 1
H, JNꢀH,2 9 Hz, NꢀH), 4.61–4.70 (m, 5 H, 2 PhCH2,
H-5Fuc), 4.10 (broad s, 1 H, OH), 2.77 (m, 2 H,
SꢀCH2), 1.30 (t, 3 H, J 7.4 Hz, SꢀCH2CH3); 13C, see
Table 2 for carbohydrate ring carbons; l 73.6 (PhCH2),
73.2 (PhCH2), 24.6 (SꢀCH2), 15.1 (SꢀCH2CH3). Anal.
Calcd for C44H46Cl3NO11S: C, 58.50; H, 5.13; N, 1.55.
Found: C, 58.39; H, 5.08; N, 1.75.
117.3
(OCH2CHꢁCH2),
73.7
(PhCH2),
69.7
(OCH2CHꢁCH2), 20.9, 20.8 (2 OC(O)CH3). Anal.
Calcd for C20H26O8: C, 60.90; H, 6.64. Found: C, 60.89;
H, 7.18.
3.8. Ethyl 3-O-benzoyl-6-O-benzyl-1-thio-b-D-
galactopyranoside (11)
Rf 0.42 (4:1 toluene–acetone); [h]D 9° (c 1, EtOAc);
1
NMR (CDCl3): H, see Table 2 for carbohydrate ring
3.12. 2-Azidoethyl O-(3,4-di-O-benzoyl-2-O-benzyl-a-
fucopyranosyl)-(1“3)-O-(6-O-benzyl-2-deoxy-2-
L-
protons; l 8.09 (d, 2 H, J 7.2 Hz, ortho protons of Bz),
7.59–7.22 (m, 8 H, Ph),4.57 (s, 2 H, PhCH2), 2.95
(broad s, 2 H, 2 OH), 2.74 (m, 2 H, SꢀCH2), 1.31 (t, 3
H, J 7.4 Hz, SꢀCH2CH3); 13C, see Table 2 for carbohy-
drate ring carbons; l 166.3 (PhC(O)O), 137.7 (ipso Ph),
133.3, 129.9–127.8 (Ph), 73.4 (PhCH2), 24.3 (SꢀCH2),
15.2 (SꢀCH2CH3). Anal. Calcd for C22H26O6S: C,
63.14; H, 6.26. Found: C, 63.11; H, 6.28.
trichloroacetamido-b-
tri-O-benzoyl-b- -galactopyranosyl)-(1“4)-2,3,6-tri-O-
benzoyl-b- -glucopyranoside (17)
D-glucopyranosyl)-(1“3)-O-(2,4,6-
D
D
Rf 0.45 (5:1 toluene–EtOAc); [h]D −45° (c 1, EtOAc);
1
NMR (CDCl3): H, see Table 2 for carbohydrate ring
protons; l 8.21–7.70 (8 d, 2 H each, J 7.6 Hz, ortho
protons of 8 Bz), 7.60–6.88 (m, 35 H, Ph, NꢀH),
4.50–4.46 (m, 4 H, 2 PhCH2), 3.94, 3.63 (2 m, 1 H
each, OCH2CH2N3), 3.37 (m, 1 H, CH2N3), 3.24 (m, 2
H, H-6bGal, CH2N3), 1.91 (s, 1 H, OH); 13C, see Table
3.9. Ethyl 4-O-benzoyl-6-O-benzyl-1-thio-b-D-
galactopyranoside (12)
Rf 0.63 (1:1 toluene–acetone); [h]D 7° (c 1, EtOAc);
2
for carbohydrate ring carbons; l 165.8–164.6
1
NMR (CDCl3): H, see Table 2 for carbohydrate ring
(PhCO), 161.6 (Cl3CCO), 138.4 (ipso Bn), 137.8 (ipso
Bn), 133.3–127.5 (Ph), 73.4 (PhCH2), 73.1 (PhCH2),
68.3 (OCH2CH2N3), 50.5 (CH2N3). Anal. Calcd for
C98H89Cl3N4O28: C, 62.71; H, 4.78; N, 2.98. Found: C,
62.95; H, 4.92; N, 2.76.
protons; l 8.07 (d, 2 H, J 7.4 Hz, ortho protons of Bz),
7.56 (t, 1 H, J 7.3 Hz, para proton of Bz), 7.43 (t, 1 H,
J 7.7 Hz, meta protons of Bz), 7.29–7.15 (m, 5 H, Bn),
4.41 and 4.30 (2 d, 1 H each, J 11.8 Hz, PhCH2), 3.18
(s, 1 H, 3-OH), 2.91 (s, 1 H, 2-OH), 2.67 (m, 2 H,
SꢀCH2), 1.23 (t, 3 H, J 7.4 Hz, SꢀCH2CH3); 13C, see
Table 2 for carbohydrate ring carbons; l 166.6
(PhC(O)O), 137.7 (ipso Ph), 133.4, 130.2–127.8 (Ph),
73.6 (PhCH2), 24.7 (SꢀCH2), 15.4 (SꢀCH2CH3). Anal.
Calcd for C22H26O6S: C, 63.14; H, 6.26. Found: C,
63.15; H, 6.27.
3.13. Ethyl 2,6-di-O-benzyl-3,4-O-endo-benzylidene-1-
thio-b- -galactopyranoside (19) and ethyl 2,6-di-O-ben-
D
zyl-3,4-O-exo-benzylidene-1-thio-b-D-galactopyranoside
(20)
A solution of 18 [15] (1.43 g, 3.54 mmol), p-TsOH
monohydrate (215 mg, 1.13 mmol), and PhCH(OMe)2
(2.6 mL, 17.1 mmol) in abs THF (37 mL) was stirred
for 12 h at rt, Et3N (0.2 mL) was added, and the
reaction mixture was concentrated. Chromatography
(petroleum ether“3:2 petroleum ether–Et2O) of the
residue on a column of silica gel afforded endo-acetal
19 (1.4 g, 80%) and exo-acetal 20 (140 mg, 8%).
3.10. Ethyl 6-O-benzyl-1-thio-b-D-galactopyranoside
(13)
Rf 0.31 (1:1 toluene–acetone); [h]D −25° (c 1, EtOAc);
1
NMR (CDCl3): H, see Table 2 for carbohydrate ring
protons; l 7.37–7.22 (m, 5 H, Ph), 4.46 (s, 2 H,

A.A. Sherman et al. / Carbohydrate Research 338 (2003) 697–703
703
Compound 19: Rf 0.37 (3:2 petroleum ether–Et2O);
[h]D −20° (c 1, EtOAc); NMR (CDCl3): H, see Table
2 for carbohydrate ring carbons; l 138.2, 137.9, 137.8
(3 ipso Ph), 128.5–127.8 (Ph), 75.9, 73.8, 72.1 (3
PhCH2), 24.8 (SꢀCH2), 15.2 (SꢀCH2CH3). Anal. Calcd
for C29H34O5S: C, 70.42; H, 6.93. Found: C, 70.54; H,
7.05.
1
2 for carbohydrate ring protons; l 7.45–7.28 (m, 15 H,
Ph), 5.98 (s, 1 H, PhCH), 4.82 and 4.74 (2 d, 1 H each,
J 11.4 Hz, PhCH2), 4.68 and 4.62 (2 d, 1 H each, J 11.9
Hz, PhCH2), 2.71 (m, 2 H, SꢀCH2), 1.37 (t, 3 H, J 6.5
Hz, SꢀCH2CH3); 13C, see Table 2 for carbohydrate ring
carbons; l 138.2, 137.7, 137.3 (3 ipso Ph), 129.0–128.0
(Ph), 104.7 (PhCH), 73.6, 73.3 (2 PhCH2), 24.6
(SꢀCH2), 15.0 (SꢀCH2CH3); Anal. Calcd for
C29H32O5S: C, 70.71; H, 6.55. Found: C, 70.89; H, 6.58.
Compound 20: Rf 0.39 (3:2 petroleum ether–Et2O);
Acknowledgements
This work was supported by Russian Foundation for
Basic Research (projects 00-0332815a and 02-03-06477-
MAC).
1
[h]D −5° (c 1, EtOAc); NMR (CDCl3): H, see Table
2 for carbohydrate ring protons; l 7.51–7.30 (m, 15 H,
Ph), 6.07 (s, 1 H, PhCH), 4.96 and 4.90 (2 d, 1 H each,
J 11.5 Hz, PhCH2), 4.67 and 4.59 (2 d, 1 H each, J 11.9
Hz, PhCH2), 2.72 (m, 2 H, SꢀCH2), 1.40 (t, 3 H, J 6.8
Hz, SꢀCH2CH3); 13C, see Table 2 for carbohydrate ring
carbons; l 138.6, 138.2, 137.6 (3 ipso Ph), 129.2–126.9
(Ph), 103.5 (PhCH), 73.6, 73.5 (2 PhCH2), 24.8
(SꢀCH2), 15.0 (SꢀCH2CH3); Anal. Calcd for
C29H32O5S: C, 70.71; H, 6.55. Found: C, 70.83; H, 6.49.
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1
drate ring protons; l 7.49–7.31 (m, 15 H, Ph), 4.98 and
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1
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drate ring protons; l 7.49–7.29 (m, 15 H, Ph), 4.92 and
4.81 (2 d, 1 H each, J 10.3 Hz, PhCH2), 4.77 and 4.72
(2 d, 1 H each, J 11.6 Hz, PhCH2), 4.61 (s, 2 H,
PhCH2), 2.70 (m, 2 H, SꢀCH2), 2.61 (broad s, 1 H,
OH), 1.35 (t, 3 H, J 7 Hz, SꢀCH2CH3); 13C, see Table