Methyl Xylopyranosides and 5-Thioxylopyranosides
J. Am. Chem. Soc., Vol. 123, No. 44, 2001 10891
400.13 and 100.6 MHz, respectively. Chemical ionization mass spectra
were measured using a Hewlett-Packard 5985 mass spectrometer.
Tetrahydrofuran was dried by distillation from sodium/benzophenone
under a nitrogen atmosphere, CH2Cl2 and pyridine were distilled from
CaH2, and DMSO was dried over molecular sieves (4 Å) and then
distilled.
All solutions used in the kinetic experiments were made with Milli-Q
water (18.2 MΩ cm-1) containing the appropriate volume of AnalaR
60% (w/w) perchloric acid and amount of NaClO4 such that the ionic
strength was maintained at µ ) 1.0. The acid concentration of all
solutions was measured by titration against standardized 0.100 M NaOH
solution (Fluka). Perchloric acid-d (99+ at. % D) was purchased from
Sigma, and deuterium oxide (99+ at. % D) was purchased from Isotec.
Sodium (18O2)acetate24 and (18O)methanol26 were prepared according
to literature procedures using (18O)water (Isotec, 98.5 atom-% 18O) as
the starting material.
Synthesis of Labeled Xylosides. (i) General. Fischer glycosylation
was used to make all methyl xylopyranosides except both anomers
containing a (1-18O)-label. Most of the â-anomer (mp 156-157 °C)
was readily separated from the anomeric mixture produced by Fischer
glycosylation (R-anomer, mp 89-90 °C) by crystallization from
2-propanol. Subsequent evaporation of the mother liqueur followed by
benzoylation (benzoyl chloride in pyridine) gave a mixture, highly
enriched in the R-anomer, of the two anomeric 2,3,4,6-tetra-O-benzoyl
derivatives that could be separated by flash chromatography (silica gel;
eluent 1:6 v/v EtOAc:hexane). Zemple´n debenzoylation27 of the purified
2,3,4,6-tetra-O-benzoyl precursors, R-anomer (mp 92-93 °C), and
â-anomer (mp 109-111 °C) gave the desired glycosides, R-anomer
(EtOH/Et2O) and â-anomer (2-propanol), that were carefully recrystal-
lized. Methyl R- and â-L-xylopyranosides were made from com-
mercially available L-xylose by Fischer glycosylation as described
above. All methyl xylopyranosides, both labeled and unlabeled, gave
elemental analyses (C and H) that were within acceptable error limits
(( 0.4% C, ( 0.3% H), and all had melting point ranges of 1 °C or
less. In addition, all R- and â-anomers had uncorrected melting points
in the range 88-91 °C and 154-157 °C, respectively.
111-113 °C (lit.29 114-115 °C). Subsequent Zemple´n deacetylation
of the purified methyl 2,3,4-tri-O-acetyl xylosides gave the desired
methyl xylopyranosides, R-anomer (EtOH/Et2O) and â-anomer (2-
propanol), which were carefully recrystallized.
(v) 18O at C5. Sodium (18O2)acetate (2.0 g) was added to a solution
of 1,2-O-isopropylidene-5-O-p-toluenesulfonyl-R-D-xylofuranose30 (8.0
g) in dry DMSO (200 mL). The resulting solution was heated at 85 °C
for 24 h. After cooling the reaction mixture to room temperature, H2O
(50 mL) was added, and the resultant solution was extracted with ether
(3 × 100 mL). The combined organic extracts were dried over
anhydrous Na2SO4, filtered, and evaporated under vacuum. The resulting
syrup was purified by flash chromatography (silica gel, 1:2 v/v ethyl
acetate:toluene) to give 1,2-O-isopropylidene-5-O-(18O)acetyl-R-D-(5-
18O)xylofuranose (2.5 g, 43%), mp 97-98 °C (lit.30 100.0-100.5 °C).
Treatment of the labeled 1,2-O-isopropylidene-5-O-acetyl R-D-xylo-
furanose with 1% (w/v) HCl in methanol at reflux temperature overnight
gave a mixture of the two anomeric methyl xylopyranosides that were
separated and purified as detailed above.
Synthesis of Labeled 5-Thioxylosides. (i) General. All labeled and
unlabeled methyl 5-thioxylopyranosides, except for the two (1-18O)-
labeled anomers, were made from the corresponding 1,2-O-isopropyli-
dene-R-D-xylofuranose by the following general procedures. 1,2-O-
Isopropylidene-R-D-xylofuranose was monotosylated using the method
of Levene and Raymond30 to give 1,2-O-isopropylidene-5-O-p-tolu-
enesulfonyl-R-D-xylofuranose in a yield of 80%. Subsequent nucleo-
philic displacement of the tosylate by sodium benzylthiolate gave 1,2-
O-isopropylidene-5-thiobenzyl-R-D-xylofuranose (80%).31 Lithium-
liquid ammonia reduction of the thioether gave an 85% yield of 1,2-
O-isopropylidene-5-thio-R-D-xylofuranose.31 Methyl 5-thio-R-D-xylo-
pyranoside was made from 1,2-O-isopropylidene-5-thio-R-D-xylo-
furanose via standard Fischer glycosylation conditions (1% HCl in meth-
anol).31 Hydrolysis of 1,2-O-isopropylidene-5-thio-R-D-xylofuranose
(0.2 N H2SO4, 65 °C for 4 h) gave 5-thioxylose, which was transformed
into methyl 5-thio-â-D-xylopyranoside in four steps according to the
method of Whistler and Van Es.10 1,2-O-Isopropylidene-R-L-xylofura-
nose was made from commercially available L-xylose.
(ii) Deuterium at C1 and C5. Treatment of the required 1,2-O-
isopropylidene-R-D-xylofuranose (Supporting Information) with 1%
(w/v) HCl in methanol at reflux temperature overnight gave a mixture
of the anomeric methyl xylopyranosides. These mixtures were separated
and purified as described above.
All R-anomeric isotopomers had melting points within the range
109-111 °C (lit.32 112-113 °C) and satisfactory elemental analyses
(C ( 0.4%, H ( 0.3%). For the unlabeled D-sugar [R]25 ) +321.8°
D
(c 1.01, H2O) (lit.33 +332°) and for the unlabeled L-sugar [R]25
)
D
-325.7° (c 1.01, H2O). All â-anomeric isotopomers had melting points
within the range 159-161 °C (lit.10 162 °C) and satisfactory elemental
(iii) Deuterium at C2 and 13C at the Anomeric Center. These
isotopomers were made by the standard glycosylation/benzoylation
procedure using either D-(2-2H)xylose (Omicron Biochemicals, 97.5
analyses (C ( 0.4%, H ( 0.3%). For the unlabeled D-sugar, [R]25
)
D
-66.0° (c 1.01, H2O) (lit.10 -66.3°), and for the unlabeled L-sugar,
[R]25 ) +66.0° (c 1.00, H2O).
2
atom-% H) or D-(1-13C)xylose (Omicron Biochemicals, 99 atom-%
D
13C) as the starting material.
(ii) Deuterium at C1, C2, and C5, and 13C at the Anomeric
Center. Treatment of the requisite labeled 1,2-O-isopropylidene-R-D-
xylofuranose (Supporting Information), as described above, gave both
anomers of the labeled methyl 5-thio-D-xylopyranoside.
(iv) 18O in the Leaving Group. Addition of (18O)methanol (1.6 mL)
to a solution of 1,2,3,4-tetra-O-acetyl-â-D-xylopyranose (6.0 g) and BF3‚
Et2O (13 mL) in dry CH2Cl2 (50 mL) was performed at -78 °C under
an atmosphere of dry N2 gas. The resulting solution was stirred
overnight, during which time the temperature was allowed to reach
ambient temperature. The reaction was quenched by the addition of a
saturated NaHCO3 solution (25 mL), and stirring was continued for
another 0.5 h. After the organic layer was separated, it was washed
with H2O (50 mL) and saturated NaCl (50 mL), and then it was dried
over anhydrous Na2SO4. After filtration, the solution was evaporated
under vacuum. The resulting syrup was purified by flash chromatog-
raphy (1:4 v/v EtOAc:hexane) to give pure R-anomer (0.75 g, 14%),
mp 82-84 °C (lit.28 85-86 °C), and the â-anomer contaminated with
∼15% 1,2,3,4-tetra-O-acetyl-R-D-xylopyranose (1.1 g, 24%). Recrys-
tallization of the â-anomer from ether:hexane gave pure material, mp
(iii) 18O in the Leaving Group. A solution of 1,2,3,4-tetra-O-acetyl-
5-thio-R,â-D-xylopyranose (11.2 g, 33.4 mmol)34 and NH2NH2‚AcOH
(4.0 g, 43.5 mmol) in DMF (250 mL) was stirred for 4 h at room
temperature under a N2 atmosphere. The reaction mixture was then
diluted with CH2Cl2 (250 mL), washed with 5% NaCl, and dried over
anhydrous Na2SO4. The filtered solution was evaporated under reduced
pressure to obtain a syrup. This syrup was purified using flash
chromatography (silica gel, 2:1 v/v ethyl acetate:hexane) to give 2,3,4-
tri-O-acetyl-5-thio-R-D-xylopyranose (7.1 g, 72%) and a mixture of the
R- and â-anomers (0.7 g, 7%). Characterization for the R-anomer, mp
118-119 °C, 1H NMR (CDCl3) δ 5.54 (t, 1H, J3,2 + J3,4 ) 20.0, H-3),
5.14 (ddd, 1H, J2,1 ) 2.8, J2,OH ) 1.0, H-2), 5.10 (m, 1H, H-1), 4.09
(24) In our hands, synthesis of sodium (18O2)acetate using the method
of Hutchinson and Mabuni (reference 25) yielded material with a lower
atom % 18O than the reagent (18O)water (presumably because of the presence
of adventitious water).
(29) Schroeder, L. R.; Counts, K. M.; Haigh, F. C. Carbohydr. Res. 1974,
37, 368-372.
(30) Levene, P. A.; Raymond, A. L. J. Biol. Chem. 1933, 102, 317-
330.
(25) Hutchinson, C. R.; Mabuni, C. T. J. Labelled Compd. Radiopharm.
1977, 13, 571-574.
(31) Ingles, D. L.; Whistler, R. L. J. Org. Chem. 1962, 27, 3896-3898.
(32) Hughes, N. A.; Munkombwe, N. M. Carbohydr. Res. 1985, 136,
(26) Sawyer, C. B. J. Org. Chem. 1972, 37, 4225-4226.
(27) Thompson, A.; Wolfrom, M. L. Methods Carbohydr. Chem. 1963,
2, 215-220.
411-418.
(33) Whistler, R. L.; Feather, M. S.; Ingles, D. L. J. Am. Chem. Soc.
1962, 84, 122.
(28) McEwan, T.; McInnes, A. G.; Smith, D. G. Carbohydr. Res. 1982,
104, 161-168.
(34) Al-Masoudi, N. A.; Pfleiderer, W. Tetrahedron, 1993, 49, 7579-
7592.