Iodonium ion-assisted synthesis of a haptenic tetrasaccharide fragment corresponding to the inner cell-wall glycopeptidolipid of Mycobacterium avium serotype 4.

Condensation of ethyl 2,4-di-O-benzoyl-1-thio-alpha-L-rhamnopyranoside with ethyl 3-O-benzyl-4-O-chloroacetyl-2-O-methyl-1-thio-beta-L-fucopyranoside in the presence of iodonium di-sym-collidine perchlorate afforded exclusively ethyl 2,4-di-O-benzoyl-3-O-(3-O-benzyl-4-O- chloroacetyl-2-O-methyl-alpha-L-fucopyranosyl)-1-thio-alpha-L-rhamnop yra noside. This disaccharide derivative was extended at C-1 with 3-benzyloxycarbonylaminopropyl 6-deoxy-3,4-O-isopropylidene-alpha-L- talopyranoside, using N-iodosuccinimide and triflic acid as the catalyst, to furnish 3-benzyloxycarbonylaminopropyl 6-deoxy-2-O-[2,4-di-O-benzoyl-3-O-(3-O-benzyl-4-O-chloroacetyl-2-O-me thy l- alpha-L-fucopyranosyl)-alpha-L-rhamnopyranosyl]-3,4-O-isopropylidene-alp ha-L- talopyranoside (20). Selective removal of the chloroacetyl group from 20, followed by condensation with ethyl 2,3-di-O-benzoyl-4-O-methyl-1-thio-alpha-L- rhamnopyranoside in the presence of the same thiophilic promoter, yielded a fully protected tetrasaccharide derivative. Deprotection of the latter gave the target compound 3-aminopropyl 6-deoxy-2-O-[3-O-[2-O- methyl-(4-O-methyl-alpha-L-rhamnopyranosyl)-alpha-L-fucopyranosyl]-alpha -L- rhamnopyranosyl]-alpha-L-talopyranoside (1).