4-0-Allyl-2,3-di-O-benzoyl-a-D-xylopyranosyl bromide ( 8), the D-xylopyranosyl donor for synthesis of L-serine-linked carbohydrate units in the glycopeptide linkage region of several mammalian proteoglycans, was prepared from D-arabinose in ten steps .
'3' The most laborious step involved in the sequence'*2 for practical preparation of 4-0-allyl-D-xylopyranose
(5, precursor to 8) appears to be the opening of the anhydro ring in benzyl2,3-anhydro-/I-D-ribopyranoside by extended treatment with concentrated alkali at high temperature.
Here we report an alternative, facile synthesis of 5, along with 8, starting from the commercially available methyl fl-D-xylopyranoside
(1). Also described is the preparation of xylobiose ( 18), xylotetraose (20), and the xylohexaose derivative 22 suitable for complete synthesis of carbohydrate chains of trehalosecontaining lipooligosaccharide antigens from Mycobacteriwn kansasii 3, using 8 as the key intermediate.
Acetonation of 1 with 2-methoxypropene in NJV-dimethylformamide in the presence of a catalytic amount of methanolic hydrogen chloride, by a slight mod-ification4s5 of the original procedure6, gave a mixture which, without isolation of the 2,3-0-isopropyhdene derivative' 2, was alkylated with ally1 bromide+sodium hydride in ZVJV-dimethylformamide', and then treated with dilute acid in acetone. Fractional crystallization of the product provided 47Oh of pure methyl 4-O-allyl+D-xylopyrano-side8 (3). Acid hydrolysis of 3, followed by acetylation with acetic anhydride-sodium acetateg, gave 1,2,3-tri-O-acetyl-CO-allyl-~xylopyranose (4, 80%), which was O-deacetylated to afford crystalline 5 (94%). The overall yield of 5, based on 1, was 35%. Benzoylation of 5 gave 4-O-allyl-1,2,3-tri-O-beuzoyl-D-xylopyranose (6,93% ), which was a mixture of the a and /3 anomer 7 in the ratio 1:2.4 ("C-n.m.r.). Fractional crystallization of 6 afforded 54% of 7, the m.p. (89") of which differed significantly from that reported' (98"). The benzoate 6 was converted into 8 (94%) by treatment with hydrogen bromide in acetic acid.
Acetonation of ethyl l-thio-p-D-xylopyranoside'O (9) with 2-methoxypropene, as