Nucleophilic displacement-reactions of the 4-sulfonyloxy group in derivatives having the D-manno configuration

Methyl 2,3-di-O-benzoyl-6-deoxy-4-O-(p-tolylsulfonyl) and 4-O-[(p-nitrophenyl)sulfonyl]-α-d-mannopyranosides (9 and 10) were prepared by three different routes from methyl α-d-mannopyranoside (1). The analogous 4-sulfonyloxy derivatives having HO-2 and HO-3 free (14 and 15) were also synthesized from 1. Nucleophilic substitution of the sulfonyloxy group of 9, 10, 14, and 15 by potassium thiocyanate in N,N-dimethylformamide was attempted. Compounds 9 and 10 gave a mixture of solvolysis products: methyl 2,3-di-O-benzoyl-6-deoxy-α-d-talopyranoside (17), methyl 3,4-di-O-benzoyl-6-deoxy-α-d-talopyranoside (18), and methyl 2,4-di-O-benzoyl-6-deoxy-α-d-talopyranoside (19), which are evidently formed by intramolecular displacement of the 4-sulfonate by backside attack of the C-2 benzoyloxy substituent, followed by benzoyl migration. The structure of compounds 17–19 was established by spectroscopic analysis, and then chemically confirmed. Although compound 14 decomposed during the substitution reaction, the 4-p-nitrophenylsulfonyl derivative 15 gave a 2:1 mixture of the 4-thiocyano derivatives with inversion [methyl 4,6-dideoxy-4-thiocyano-α-d-talopyranoside (22)] and retention [methyl 4,6-dideoxy-4-thiocyano-α-d-mannopyranoside (23)] of the C-4 configuration.