STERIC CONTROL IN RING-OPENING POLYMERIZATION OF 1,6-ANHYDRO GALACTOSE DERIVATIVES BY NEIGHBORING GROUP PARTICIPATION

Four 1,6-anhydro-beta-D-galactopyranoses having benzoyl derivatives in position 2 and benzyl derivatives in positions 3 and 4 were synthesized and polymerized. (1-->6)-Beta-D-galactopyranan oligosaccharide derivatives (DP(n)BAR < 7.6) were obtained with PF5 as initiator in dichloromethane at 0 to -40-degrees-C from 1,6-anhydro-2-O-benzoyl-3,4-di-O-benzyl-beta-D-galactopyranose (1a), 1,6-anhydro-2-O-benzoyl-3,4-di-O-p-bromobenzyl-beta-D-galactopyranose (1c), and 1,6-anhydro-2-O-p-bromobenzoyl-3,4-di-O-p-bromobenzyl-beta-D-galactopyranose (1d). 1,6-Anhydro-2-O-p-nitrobenzoyl-3,4-di-O-benzyl-beta-D-galactopyranose (1b) gave a stereoirregular product of lower molecular weight in lower yield. The apparent rate of polymerization increased in the order of 1b < 1a < 1c < 1d. The formation of beta-(1-->6)-linked configuration is explained by the mechanism in which the benzoyl group in position 2 took part in the steric retention of the anomeric center during the ring-opening polymerization. Substituent effects on the reactivities of the monomers and growing species are discussed on the basis of the mechanism.