The deamination of methyl 4-amino-4,6-dideoxy-2,3-O-isopropylidene-α-L-talo- and -manno-pyranosides with nitrous acid

Abstract

The deamination of methyl 4-amino-4,6-dideoxy-2,3-O-isopropylidene-α-L-talopyranoside (5) with nitrous acid in 90% acetic acid gives methyl 6-deoxy-2,3-O-isopropylidene-α-L-mannopyranoside (9) and the corresponding 4-acetate (10) as final products. Similar deamination of methyl 4-amino-4,6-dideoxy-2,3-O-isopropylidene-α-L-mannopyranoside (2) gives compounds (9) and (10) together with the ring-contracted products methyl 6-deoxy-2,3-O-isopropylidene-β-D-allofuranoside (11) and the corresponding 5-acetate (12). The stereospecificity observed in the latter deamination is ascribed to solvent attack on an intermediate bicyclic oxonium ion (18) arising from participation of the ring-oxygen atom during heterolysis of the intermediate diazonium ion. Comparison is made with certain displacement reactions of methyl 6-deoxy-2,3-O-isopropylidene-α-L(D)-mannopyranoside 4-sulphonates.