A. H. C. Chang et al. / Tetrahedron: Asymmetry 11 (2000) 595–606
605
Commercially available zinc dust was activated by successive washes with 10% HCl, water, acetone,
and ether. To a solution of the foregoing bromide in THF (50 mL) was added air-dried activated
zinc powder (12.4 g, 12 equiv.), the mixture was heated and 1-methylimidazole (1.2 mL, 1 equiv.)
was added when the inner temperature reached 45°C. The heating was continued for 45 min when
TLC (5:1 toluene:acetone) showed that the reaction was complete and two products had been formed.
Chromatography gave the faster moving, desired glycal 25 (3.6 g, 63%), mp 86.5–87.5°C, [α]D −54.8 (c
0.8, CHCl3); CIMS: m/z 357 ([M+18]+). Anal. calcd for C16H23NO8: C, 53.78; H, 6.49; N, 3.92. Found:
C, 53.93; H, 6.58; N, 3.92.
3.9. Methyl
3-O-acetyl-2,4,6-trideoxy-4-(2,4-di-O-acetyl-3-deoxy-L-glycero-tetronamido)-2-fluoro-
α-D-glucopyranoside 26, methyl 2,4,6-trideoxy-4-(3-deoxy-L-glycero-tetronamido)-2-fluoro-α-D-
mannopyranoside 15, and methyl 3-O-acetyl-2,4,6-trideoxy-4-(2,4-di-O-acetyl-3-deoxy-L-glycero-
tetronamido)-2-fluoro-β-D-glucopyranoside 27
To a mixture of glycal 25 (2.94 g, 8.2 mmol) and 4 Å molecular sieves (2 g) in acetonitrile (9 mL)
were added MeOH (3 mL), 2,6-di-t-butyl-4-methylpyridine (2.54 g, 12.4 mmol) and Selectfluor™(4.4 g,
12.4 mmol). The mixture was stirred in an argon atmosphere overnight, when TLC (3:2 toluene:EtOAc)
showed that all starting material was consumed and that, in addition to some base-line material, five
products had been formed. After filtration through a pad of Celite, the filtrate was concentrated and the
residue was chromatographed.
Eluted first were two compounds (∼30 mg each) whose CI mass spectra showed a peak at m/z 413,
indicating that each of these compounds contained two fluorine atoms. These were not investigated
further.
Eluted next was material enriched in a compound whose NMR data confirmed that it was the α-gluco
derivative 26. Crystallization (twice) from ethanol gave material (120 mg, 3.6%), melting at 154–155°C,
[α]D +121.5 (c 1.1, CHCl3); CIMS: m/z 425 ([M+18]+). Anal. calcd for C17H26FNO9: C, 50.12; H, 6.43;
N, 3.44. Found: C, 49.97; H, 6.43; N, 3.47.
Eluted next was material whose NMR data confirmed that it was the α-manno derivative 15 (0.234 g,
7%); [α]D +57.7 (c 0.9, CHCl3); CIMS: m/z 425 ([M+18]+).
Eluted last was material whose NMR data confirmed that it was the β-gluco derivative 27 (0.458 g,
14.7%), mp 96–98°C (from ether), [α]D +16.2 (c 1.4, CHCl3); CIMS: m/z 425 ([M+18]+). Anal. calcd
for C17H26FNO9: C, 50.12; H, 6.43; N, 3.44. Found: C, 50.26; H, 6.37; N, 3.36. Intermediate, mixed
fractions were also obtained.
3.10. Methyl 2,4,6-trideoxy-4-(3-deoxy-L-glycero-tetronamido)-2-fluoro-α-D-mannopyranoside
A solution of 15 in methanol was deacetylated (Zemplén) to give the desired 2-fluoro derivative 17
in virtually theoretical yield, mp 128–130°C (from EtOAc), [α]D +40.5C (c 0.8, H2O). Anal. calcd for
C11H26FNO6: C, 46.97; H, 7.17; N, 4.98. Found: C, 47.03; H, 7.26; N, 4.9.
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