Further immunochemical studies on the combining sites of Lotus tetragonolobus and Ulex europaeus I and II lectins

A series of 2-O-benzoyl-4,6-di-O-benzyl-α-d-galactopyranosyl halides carrying either a second benzoyl group (8a, 12a) or a selectively removable, temporary protecting group (8b–d, 12b) at position 3 was synthesized from allyl α-d-galactopyranoside (1). The key intermediate was 1-propenyl 4,6-di-O-benzyl-α-d-galactopyranoside (5), prepared from 1 via the 4,6-O-benzylidene-2,3-di-O-crotyl derivative 2. The successive incorporation of the 2-O-benzoyl group, by selective acylation at low temperature, and of various 3-substituents gave fully substituted 1-propenyl α-d-galactopyranosides 6a–d. These were converted into the glycosyl halides by published methods. An improved preparation of allyl 2,6-di-O-benzyl-(15) and 2,4,6-tri-O-benzyl-(19) α-d-galactopyranoside was achieved. The direct acetonation of 1 to the 3,4-O-isopropylidene derivative 13, followed by benzylation and mild acid hydrolysis, gave 15 in 56% yield. The transient protection of O-3 in 15 was accomplished by the alkylation of the dibutylstannylene derivative 16 with (2-methoxyethoxy)methyl chloride. Successive benzylation and mild acid hydrolysis of the product 17 efficiently furnished 19.