Pentenyl mannosides in the synthesis of N-acylmannopyranosyl amides: conformational analysis of intermediates

The reaction of a pentenyl mannopyranoside with N-bromosuccinimide in the presence of acetonitrile and a carboxylic acid leads to an N-acetyl-N-acyl-alpha-D-mannopyranosyl imide, the acetyl group being derived from acetonitrile, and the acyl group from the carboxylic acid. The latter can be aromatic or aliphatic as exemplified with a protected aspartic acid from which an alpha-mannopyranosyl asparagine is obtained. Chemoselective deacetylation can be effected either with sodium methoxide or piperidine depending on whether the other acyl group is aromatic or aliphatic, respectively. H-1 NMR data, conformational and molecular mechanics analyses show that the N,N-diacyl derivatives exist in a conformation lying between a B2,5 boat and a S-1(5) twist boat.