Synthesis and glycosylation of pyrimidin-2-yl 1-thio-α-D-manno- and -α-L-rhamnopyranoside

Article abstract of DOI:10.1016/S0008-6215(98)00164-5

Pyrimidin-2-yl 2,3,4,6-tetra-O-benzyl-1-thio-α-D-mannopyrano side (9), pyrimidin-2-yl 2,3,4-tri-O-benzyl-α-L-rhamnopyranoside (10), pyrimidin-2-yl 2-O-acetyl-3,4,6-tri-O-benzyl-1-thio-α-D-mannopyranoside (7), and pyrimidin-2-yl 2-O-acetyl-3,4-di-O-benzyl-1-thio-α-L-rhamnopyranoside (8) were prepared almost quantitatively from the corresponding protected 1,2-O-methoxyethylidene-β-D-manno-or-β-L-rhamnopyranose with 2-mercaptopyrimidine in the presence of mercuric bromide. Coupling reactions of the thioglycosides promoted by silver triflate with suitable glycosyl acceptors afforded 1,2-trans linked disaccharides. Copyright (C) 1998 Elsevier Science Ltd.

Full text of DOI:10.1016/S0008-6215(98)00164-5

Carbohydrate Research 310 (1998) 135±139
Note
Synthesis and glycosylation of pyrimidin-2-yl
1-thio-ꢀ-d-manno- and -ꢀ-l-rhamnopyranoside
Xianglan Ding, Guangbin Yang, Fanzuo Kong *
Research Center for Eco-Environmental Sciences, Academia Sinica, P.O. Box 2871, Beijing 100085,
People's Republic of China
Received 20 February 1998; accepted 5 June 1998
Abstract
Pyrimidin-2-yl 2,3,4,6-tetra-O-benzyl-1-thio-ꢀ-d-mannopyrano side (9), pyrimidin-2-yl 2,3,4-tri-
O-benzyl-ꢀ-l-rhamnopyranoside (10), pyrimidin-2-yl 2-O-acetyl-3,4,6-tri-O-benzyl-1-thio-ꢀ-d-
mannopyranoside (7), and pyrimidin-2-yl 2-O-acetyl-3,4-di-O-benzyl-1-thio-ꢀ-l-rhamnopyrano-
side (8) were prepared almost quantitatively from the corresponding protected 1,2-O-methoxy-
ethylidene-ꢁ-d-manno-or-ꢁ-l-rhamnopyranose with 2-mercaptopyrimidine in the presence of
mercuric bromide. Coupling reactions of the thioglycosides promoted by silver tri¯ate with sui-
table glycosyl acceptors a€orded 1,2-trans linked disaccharides. # 1998 Published by Elsevier
Science Ltd. All rights reserved
Keywords: Pyrimidin-2-yl; 1-Thio-ꢀ-d-mannopyranoside; 1-Thio-ꢀ-l-rhamnopyranoside
Thioglycosides have attracted much attention
recently as versatile glycosyl donors, particularly
for synthesis of complex carbohydrates, since they
are stable under a variety of reaction conditions
but can be activated with appropriate thiophilic
reagents [1]. However, an addition to the require-
ment of more than equimolar amounts of catalysts
which are generally sensitive to moisture and are
expensive, relatively low glycosyl-donor properties
were observed with these systems [2]. These pro-
blems were partly overcome with the use of het-
erocyclic thioglycosides [3]. Hanessian [3a] studied
the activity of basic heterocyclic thioglycosides and
reported that, with suitable catalysts, unprotected
pyrimidin-2-yl 1-thio-ꢁ-d-glucopyranoside could
e€ectively glycosylate simple alcohols. Our lab [4]
also reported that glycosylation using pyrimidin-2-
yl 1-thio-ꢁ-d-gluco-, -ꢁ-d-galacto-, -ꢁ-d-xylo-, and
-ꢀ-d-arabinopyranoside benzyl ethers as glycosyl
donors gave 1,2-cis linked disaccharides pre-
dominantly. Here, we report the synthesis and
glycosylation of pyrimidin-2-yl 1-thio-ꢀ-d-manno-
and -ꢀ-l-rhamnopyranoside.
Pyrimidin-2-yl 1-thio-ꢀ-d-manno- and -ꢀ-l-
rhamnopyranoside were not available by the PTC
method [4] or Helferich procedure [5]. However, we
found that (5, 6, 7 and 8) were prepared almost
quantitatively by re¯uxing protected 1,2-O-meth-
oxyethylidene-ꢁ-d-manno- (1,3) [6] and -ꢁ-l-rham-
nopyranose (2,4) [7] with 2-mercaptopyrimidine
in dry acetonitrile in the presence of mercuric
bromide.
The pyrimidin-2-yl 1-thio group was suciently
stable to allow chemical modi®cation at the other
* Corresponding author. Fax: 00-8690-629-23563.
0008-6215/98/$Ðsee front matter # 1998 Published by Elsevier Science Ltd. All rights reserved
PII S0008-6215(98)00164-5

136
X. Ding et al./Carbohydrate Research 310 (1998) 135±139
the coupling reaction did not occur, and the main
product was compound 16.
Condensation of 7, 9, and 15 with methyl 2,3,4-
tri-O-benzyl-ꢀ-d-mannopyranoside (12) was also
carried out, giving 17 (92%), 18 (97%), and 19
(83%), respectively, indicating that the acetyl
group in the thiomannopyranoside lowered its
reactivity.
Coupling of the thioglycoside 10 with methyl 3-
O-allyl-4,6-di-O-benzyl-ꢀ-d-glucopyranoside (13)
[10] a€orded the sole ꢀ-linked disaccharide deriva-
tive (25) in high yield (87%). However, under the
same conditions, condensation of 9 or 10 with
1,2:5,6-di-O-isopropylidene-ꢀ-d-glucofuranose (14)
[9] gave a non-separable ꢀ; ꢁ anomeric mixture (4:1
1
for 21, 1:1 for 24, estimated from H NMR spec-
tra) in relatively low yields (64±57%).
For comparison of the reactivity, phenyl 2,3,4-
tri-O-benzyl-1-thio-ꢀ-l-rhamnopyranoside (26) [11]
was reacted with 11 in dichloromethane in the pre-
sence of 2 equivalents of silver tri¯ate at room
temperature, but no coupling product was
observed, indicating that as a leaving group the
pyrimidin- 2-yl-1-thio group is much more active
than the phenylthio group. Compound 26 was
readily obtained via the Helferich procedure [5]
from treatment of 1,2,3,4-tetra-O-acetyl-l-rham-
nopyranose with phenylthiol followed by deacety-
lation and benzylation, but the same procedure
failed to yield the corresponding pyrimidin-2-yl 1-
thio-manno- or -rhamnopyranoside. This result is
also a clear indication of the di€erence between the
phenylthio group and pyrimidin-2-yl-1-thio group.
In summary, protected pyrimidin-2-yl 1-thio-ꢀ-
d-manno- and -ꢀ-l-rhamnopyranoside were read-
ily synthesized, and their stability and relatively
high reactivity should prove bene®cial in the
synthesis of complex oligosaccharides.
hydroxyl groups of the thioglycosides. For exam-
ple, selective 6-O-debenzylation was successfully
carried out by treating 7 with trimethylsilyl tri¯ate
ꢀ
[8] in acetic anhydride at 50 C, a€ording pyr-
imidin-2-yl 2,6-di-O-acetyl-3,4-di-O-benzyl-1-thio-
ꢀ-d-mannopyranoside (15) in 76% yield. In the
1H NMR spectrum of 15, the H-6, 60 signals
shifted from 3.70±3.90 to 4.25±4.39 ppm, indicat-
ing 6-O-acetylation.
The activity of the described thioglycosides was
examined by coupling with sugar acceptors of
diverse structure. Condensation of thioglycosides
8, 9, and 10 with 1,2:3,4-di-O-isopropylidene-ꢀ-d-
galactopyranose (11) [9] using 2 equivalents of sil-
ver tri¯ate as the promoter in dichloromethane at
room temperature for 30 min gave the sole ꢀ-linked
disaccharide derivatives 22 (95%), 20 (95%) and 23
(93%), respectively, in spite of the presence or
absence of neighboring group participation at C-2.
Reducing of the amount of silver tri¯ate to 1
equivalent lowered the disaccharide yield (65%)
for coupling of 8 with 11, and a byproduct, benzyl
2-O-acetyl-3,4-di-O-benzyl-ꢀ-l-rhamnopyranoside
(16), formed. Using 0.4 equivalents of silver tri¯ate,
1. Experimental
General methods.ÐSee reference [6a]. Com-
pounds 17, 19, 20, 22, and 23 have been reported
1
previously, and their H NMR data were in agree-
ment with those previously published.
Pyrimidin-2-yl 2,3,4,6-tetra-O-acetyl-1-thio-a-d-
mannopyranoside (5) and pyrimidin-2-yl 2,3,4-tri-O-
acetyl-1-thio-a-l-rhamnopyranoside (6).ÐTo a stir-
red solution of 1 or 2 (4 mmol) and 2-mercapto-
pyrimidine (5.6 mmol) in dry acetonitrile (30 mL)
was added mercuric bromide (0.4 mmol). The mix-

X. Ding et al./Carbohydrate Research 310 (1998) 135±139
137
ture was stirred and re¯uxed for 30 min. TLC (1:2
ethyl acetate±petroleum ether) indicated that the
reaction was complete. The mixture was ®ltered,
washed with acetonitrile and concentrated. The
residue was chromatographed on silica gel with 1:2
ethyl acetate±petroleum ether (v/v) to give the syr-
upy products 5 (95%) and 6 (95.6%). For com-
CHCl3); 1H NMR: ꢂ 8.58, 7.00 (s, t, 3 H, Pyrim-H),
7.45±7.20 (m, 15 H, Ph), 6.59 (d, 1 H, J1,2 1.7 Hz,
H-1), 4.98, 4.65 (2 d, 2 H, J 11 Hz, CH2Ph), 4.87,
4.71 (2 d, 2 H, J 12 Hz, CH2Ph), 4.56 (s, 2 H,
CH2Ph), 4.00 (dd, J2,3 2.9 Hz, H-2), 3.95±3.85 (m, 1
H, H-5), 3.82 (dd, J3,4 9.0 Hz, H-3), 3.70 (dd, 1 H,
J4,5 9.2 Hz, H-4), 1.35 (d, 3 H, J 4.4 Hz, CH3);
Anal. Calcd for C31H32N2O4S: C, 70.45. H, 6.06.
Found: C, 69.98; H, 6.04.
pound 5 [ꢀ]d +42.7ꢀ (c 4.5, CHCl3); H NMR: ꢂ
1
8.58, 7.10 (s, t, 3 H, Pyrim-H), 6.54 (d, 1 H, J1,2
1.7 Hz, H-1), 5.55 (dd, 1 H, J2,3 3.2 Hz, H-2), 5.40
(dd, 1 H, J3,4 9.0 Hz, J4,5 9.1 Hz, H-4), 5.26 (dd, 1
H, H-3), 4.36±4.04 (m, 3 H, H-5, 6, 60), 2.24±2.00 (4
s, 12 H, 4 COCH3); Anal. Calcd for C18H22N2O9S:
C, 48.87; H, 4.98. Found: C, 49.02; H, 5.10. For
compound 6 [ꢀ]d 79.7ꢀ (c 12.5, CHCl3); 1H
NMR: ꢂ 8.58, 7.10 (s, t, 3 H, Pyrim-H), 6.46 (d, 1
H, J1,2 1.7 Hz, H-1), 5.55 (dd, 1 H, J2,3 3.0 Hz, H-
2), 5.37 (dd, 1 H, J3,4 10 Hz, H-3), 5.20 (dd, 1 H,
J4,5 10 Hz, H-4), 4.20±4.05 (m, 1 H, H-5), 2.20±2.00
(3 s, 9 H, 3 COCH3), 1.24 (d, 3 H, J 4.4H2, CH3);
Anal. Calcd for C16H20N2O7S: C, 50.00; H, 5.21,
Found: C 50.04; H, 5.32.
Pyrimidin 2-yl 2-O-acetyl-3,4,6-tri-O-benzyl-1-
thio-a-d-mannopyranoside (7) and pyrimidin 2-yl 2-
O-acetyl-3,4-di-O-benzyl-1-thio-a-l-rhamnopyrano-
side (8).ÐThe syrupy products 7 (97%) and 8
(97%) were obtained using the same procedure as
described in the preparation of 5 and 6. For com-
ꢀ
1
pound 7 [ꢀ]d +46.7 (c 4.5, CHCl3); H NMR: ꢂ
8.58, 7.02 (s, t, 3 H, Pyrim-H), 7.44±7.10 (m, 15 H,
Ph), 6.62 (d, 1 H, J1,2 1.7 Hz, H-1), 5.65 (dd, 1 H,
J2,3 3.2 Hz, H-2), 4.89, 4.59 (2 d, 2 H, J 11 Hz,
CH2Ph), 4.76, 4.52 (2 d, 2 H, J 11 Hz, CH2Ph).
4.68±4.46 (2 d, 2 H, J 11 Hz, CH2Ph), 4.10±4.02
(m, 2 H, H-5, 6), 3.92 (dd, 1 H, J3,4 9.3 Hz, H-3),
3.84 (dd, 1 H, J4,5 7.3 Hz, H-4), 3.70 (dd, 1 H, J5,6
Pyrimidin 2-yl 2,3,4,6-tetra-O-benzyl-1-thio-a-d-
mannopyranoside (9) and pyrimidin 2-yl 2,3,4-tri-O-
0.7 Hz, J6,6 11 Hz, H-60), 2.22 (s, 3 H, COCH3);
0
benzyl-1-thio-a-l-rhamnopyranoside (10).ÐTo
a
Anal. Calcd for C33H34N2O6S: C, 67.58; H, 5.80.
Fouꢀnd: C, 67.47. H, 5.72. For compound 8 [ꢀ]d
solution of 5 or 6 (10 mmol) in anhydrous metha-
nol (15 mL) was added sodium methoxide (11 mg,
0.2 mmol), and the solution was stirred at room
temperature for 3 h. Concentration of the solution
gave a solid residue which was dissolved in N,N-
dimethylformamide (15 mL) and subjected to ben-
zylation with sodium hydride (80% in oil,
66 mmol) and benzyl bromide (8.0 mL, 64 mmol).
The mixture was stirred at room temperature for
5 h, at the end of which time TLC (1:2 ethyl acet-
ate±petroleum ether) indicated that the reaction
was complete. The mixture was poured into water,
the solution was extracted repeatedly with dichlor-
omethane, and the combined extracts were con-
centrated to a syrup. Puri®cation by column
chromatography with 1:3 ethyl acetate±petroleum
ether as the eluent a€orded syrupy 9 (95.3%) and
10 (96%). For compound 9 [ꢀ]d +54.8ꢀ (c 2.5,
CHCl3); 1H NMR: ꢂ 8.54, 7.00 (s, t, 3 H, Pyrim-H),
7.46±7.18 (m, 20 H, Ph), 6.70 (d, 1 H, J1,2 1.7 Hz,
H-1), 4.91, 4.54 (2 d, 2 H, J 11 Hz, CH2Ph), 4.65±
4.47 (m, 7 H, H-2, 3 CH2Ph), 4.41 (dd, 1 H, J3,4
9.3 Hz, J4,5 9.2 Hz, H-4), 4.02±3.96 (m, 1 H, H-5),
3.90±3.79 (m, 2 H, H-3, 60), 3.71 (dd, 1 H, J5,6
1
58.4 (c 2.5, CHCl3); H NMR: ꢂ 8.58, 7.00 (s, t, 3
H, Pyrim-H), 7.46±7.20 (m, 10 H, Ph), 6.44 (d, 1 H,
J1,2 2.0 Hz, H-1), 5.64 (dd, 1 H, J2,3 2.7 Hz, H-2),
4.94, 4.62 (2 d, 2 H, J 11 Hz, CH2Ph), 4.69 (s, 2 H,
CH2Ph), 4.00 (m, 1 H, H-5), 3.85 (dd, J3,4 9.3 Hz,
H-3), 3.70 (dd, 1 H, J4,5 9.3 Hz, H-4), 2.22 (s, 3 H,
COCH3), 1.36 (d, 3 H, J 4.4 Hz, CH3); Anal. Calcd
for C26H28N2O5S: C, 65.00; H, 5.83. Found: C,
64.88; H, 5.97.
Pyrimidin-2-yl 2,6-di-O-acetyl-3,4-di-O-benzyl-1-
thio-a-d-mannopyranoside (15).ÐTo a solution of
7 (100 mg, 0.17 mmol) in acetic anhydride (2 mL)
was added 1:1 (v/v) trimethyisilytri¯uoromethane
sulfonate±dichloromethane (0.2 mL) at 50 ꢀC,
and the solution was stirred for 1 h at the same
temperature. The mixture was poured into 1:1 (v/v)
dichloromethane±saturated NaHCO3 solution
(50 mL) and stirred for 0.5 h, and the organic layer
was washed with water, dried, and concentrated
under reduced pressure. The residue was applied to
a column of silica gel and 15 (62 mg, 76%) was
obtained as a syrup. [ꢀ]d +67.6ꢀ (c 5.0, CHCl3); 1H
NMR: ꢂ 8.55, 7.05 (s, t, 3 H, Pyrim-H), 7.40±7.20
(m, 10 H, Ph), 6.55 (d, 1 H, J1,2 1.7 Hz, H-1), 5.63
(dd, 1 H, J2,3 2.7 Hz, H-2), 4.94, 4.58 (2 d, 2 H, J
11 Hz, CH2Ph), 4.75, 4.52 (2 d, 2 H, J 11 Hz,
0
1.7 Hz, J6,6 12 Hz, H-6); Anal. Calcd for
C38H38N2O5S: C, 71.92; H, 5.99. Found: C, 72.07;
H, 6.03. For compound 10 [ꢀ]d 47.3ꢀ (c 2.5,

138
X. Ding et al./Carbohydrate Research 310 (1998) 135±139
0
CH2Ph), 4.39 (dd, 1 H, J5,6 4.4 Hz, J6,6 12 Hz, H-
0
Table 1
Optical rotation data for isolated compounds
6), 4.25 (dd, 1 H, J5,6 12 Hz, H-60), 4.10±4.02 (m, 2
H, H-5), 3.95 (dd, 1 H, J3,4 7.3 Hz, H-3), 3.88 (dd, 1
H, J4,5 7.1 Hz, H-4), 2.22, 2.00 (2 s, 6 H, 2
COCH3); Anal. Calcd for C28H30N2O7S: C, 62.45;
H, 5.58. Found: C, 62.79; H, 5.70.
Compound
Yield (%)
[ꢀ]d (c, CHCl3)
Lit: [Ref.]
17
19
20
21
22
23
24
92
83
95
64
95
93
57
+29.1ꢀ (5.5)
+32.6ꢀ (3.5)
+36.0ꢀ (1.5)
2.4ꢀ (2.5)
35.7ꢀ (14.0)
38.7ꢀ (3.5)
0.10ꢀ (3.5)
+29ꢀ [6a]
+34ꢀ [12]
+39ꢀ [3b]
[13]a
[14]a
47ꢀ [3b]
[3b]a
Coupling reaction using pyrimidin-2-yl 1-thio-a-
d-manno- and -l-rhamnopyranosides as donors.Ð
To a solution of a donor (7, 8, 9, or 10)
(0.5 mmol) and an acceptor (11, 12, 13, or 14)
(0.45 mmol) in dry dichloromethane (5 mL) was
added 4 A molecular sieves (500 mg), and the mix-
ture was stirred at room temperature. To the mix-
ture was added silver tri¯ate (0.9 mmol) and the
mixture was stirred for 30 min to 2 h. The reaction
was monitored by TLC (1:2 ethyl acetate±petro-
leum ether). After completion of the reaction, the
reaction mixture was ®ltered, and the ®ltrate was
concentrated under diminished pressure. The resi-
due was chromatographed on silica gel to give dis-
accharide.
a
Optical rotation data were not reported for this com-
pound in the cited article.
from the condensation of 9 and 10 with 14 respec-
tively. For 21 1H NMR: ꢂ 7.40±7.20 (m, 20 H, Ph),
5.94 (d, 0.8 H, J1,2 3.7 Hz, H-1), 5.80 (d, 0.2 H, J1,2
3.7 Hz, H-1), 4.90±3.60 (m, 21 H, H-10, 2, 20, 3, 30,
4, 40, 5, 50, 6a, 60a, 6b, 60b, 4 CH2Ph), 1.50, 1.48,
1.44, 1.40, 1.38, 1.36, 1.33, 1.26 (8 s, 12 H, 4
1
CCH3). For 24 H NMR: ꢂ 7.40±7.22 (m, 15 H,
Ph), 5.95 (d, 0.5 H, J1,2 3.7 Hz, H-1), 5.75 (d, 0.5 H,
J1,2 3.7 Hz, H-1), 4.94±3.60 (m, 20 H, H-10, 2, 20, 3,
30, 4, 40, 5, 50, 6a, 6b, 3 CH2Ph), 1.52, 1.48, 1.44,
1.36, 1.36, 1.32, 1.32, 1.30 (8 s, 12 H, 4 CCH3),
1.26, 1.24 (2 d, 3 H, J 4.4 Hz, CH3).
Benzyl 2-O-acetyl-3,4-di-O-benzyl-a-l-rhamno-
pyranoside (16).ÐCompound 16 was obtained as a
syrupy byproduct in the coupling of 8 with 11
when the quantity of promoter was decreased to 1
equivalent or less. [ꢀ]d 25.0ꢀ (c 2.0, CHCl3); H
Methyl 3-O-allyl-4,6-di-O-benzyl-2-O-(2,3,4-tri-
O-benzyl-a-l-rhamnopyranosyl)-a-d-glucopyrano-
side (25).ÐDisaccharide 25 was obtained as a
syrup from the coupling of 10 with 13 in a yield of
87%. [ꢀ]d +28.7ꢀ (c 3.0, CHCl3); 1H NMR: ꢂ 7.42±
7.10 (m, 15 H, Ph), 5.80 (m, 1 H, CH ˆ CH2),
5.30±5.00 (m, 2 H, CH ˆ CH2), 5.00±4.40 (m, 12
H, 5 CH2Ph, CH2 CH ˆ CH2), 4.80 (d, 1 H, J1,2
1
NMR: ꢂ 7.41±7.22 (m, 15 H, Ph), 5.43 (dd, J1,2
2.0 Hz, J2,3 3.4 Hz, H-2), 4.92, 4.61 (2 d, 2 H, J
10.8 Hz, CH2Ph), 4.82 (d, 1 H, H-1), 4.70, 4.52 (2
d, 2 H, J 11.2 Hz, CH2Ph), 4.68, 4.47 (2 d, 2 H, J
11.6 Hz, CH2Ph), 3.98 (dd, J3,4 9.5 Hz, H-3), 3.80
(m, 1 H, H-5), 3.45 (dd, 1 H, J4,5 9.5 Hz, H-4), 2.16
(s, 3 H, COCH3), 1.32 (d, 3 H, J 4.4 Hz, CH3);
Anal. Calcd for C29H32O6: C, 73.11; H, 6.72.
Found: C, 73.33; H, 6.80.
2.0 Hz, H-1), 4.72 (d, 1 H, J1 ,2 1.7 Hz, H-10), 4.16
0
0
(dd, 1 H, J2 ,3 2.0 Hz, 4, 40, 7.8 Hz, H-30), 3.90±3.50
(m, 9 H, H-2, 20, 3, 4, 40, 5, 50, 6, 60), 3.34 (s, 3 H,
OCH3), 1.30 (d, 3 H, J 4.4 Hz, CH3); Anal. Calcd
for C51H58O10: C, 73.73; H, 6.99. Found: C, 73.43;
H, 6.89.
0
0
Methyl 2,3,4-tri-O-benzyl-6-O-(2,3,4,6-tetra-O-
benzyl-a-d-mannopyranosyl)-a-d-mannopyranoside
(18).ÐCompound 18 was obtained as a syrup from
the condensation of 9 with 12 in a yield of 97%.
ꢀ
1
[ꢀ]d +28.7 (c 4.7, CHCl3); H NMR: ꢂ 7.40±7.10
(m, 30 H, Ph), 5.15 (d, 1 H, J1,2 2.0 Hz, H-1), 4.70
(d, 1 H, J1 ,2 2.2 Hz, H-10), 4.95±4.42 (m, 14 H, 7
CH2Ph), 4.10±3.60 (m, 12 H, H-2, 20, 3, 30, 4, 40' 5,
50, 6a, 6b, 60a, 60b), 3.24 (s, 3 H, OCH3); Anal.
Calcd for C62H66O11: C, 75.46. H, 6.69. Found: C,
76.07; H, 6.89.
Acknowledgement
0
0
Project 39740022 was supported by The
National Natural Science Foundation of China.
O-(2,3,4,6-Tetra-O-benzyl-d-mannopyranosyl)-
(1!3)-1,2:5,6-di-O-isopropylidene-a-d-glucofuranose
(21) and O-(2,3,4-tri-O-benzyl-l-rhamnopyrano-
syl)-(1!3)-1,2:5,6-di-O-isopropylidene-a-d-gluco-
furanose (24).ÐCompounds 21 (64%) and 24
(57%) were obtained as non-separable ꢀ; ꢁ mixtures
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