Preparation of some fully chloroacetylated glycopyranosyl bromides: Useful intermediates for the synthesis of base‐ and hydrogenolysis‐sensitive glycosides

Chloroacetylation of alkyl glycopyranosides of β‐ D ‐galactose, β‐ D ‐glucose and α‐ D ‐mannose as well as of free D ‐glucose, D ‐galactose, D ‐mannose and D ‐xylose, using chloroacetic anhydride as acylating reagent, gave the corresponding fully chloroacetylated pyranose derivatives in excellent yields. From D‐glucose, 1,2,3,6‐tetra‐ O ‐chloroacetyl‐α‐ D ‐glucopyranose ( 9 ) was also obtained. Subsequent treatment of the fully chloroacetylated α‐ D ‐glycopyranoses 3 , 8 , 13 and 15 of galactose, glucose, mannose, and xylose, respectively, with HBr resulted in 2,3,4,6‐tetra‐ O ‐chloroacetyl‐α‐ D ‐galactopyranosyl bromide ( 4 , 98%), ‐glucopyranosyl bromide ( 10 , 92%), ‐mannopyranosyl bromide ( 14 , 72%) and 2,3,4‐tri‐ O ‐chloroacetyl‐α‐ D ‐xylopyranosyl bromide ( 16 , 68%). 10 reacted with D ‐pantholactone ( 17 ) and silver trifluoromethanesulfonate under acidic conditions to give the corresponding α‐linked ( 18 , 45%) and β‐linked ( 20 , 38%) glucopyranosides. Under basic conditions the corresponding ortho ester product 21 was formed (92%) which upon treatment with BF 3 gave the glucosides 18 and 20 . Silver silicate as the catalyst gave only traces of 18 . Thus, 20 was isolated in 80% yield. Reaction of 10 with silver crotonate and silver laurate gave 1‐ O ‐crotonoyl‐ ( 22 ) and 1‐ O ‐lauroyl‐2,3,4,6‐tetra‐ O ‐chloroacetyl‐β‐D‐glucopyranose ( 24 ), respectively. 18 and 22 were dechloroacetylated with thiourea, affording the corresponding deblocked glucose derivatives 19 and 23 , respectively.