Pentenyl mannosides in the synthesis of N-acylmannopyranosyl amides: conformational analysis of intermediates
Abstract
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-α-D-mannopyranosylimide, 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 α-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. 1H 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 1S5 twist boat.