Evidence for Bicyclic Oxonium Ions in the Nitrous Acid Deamination of Benzyl 2-Amino-4,6-O-benzylidene-2-deoxy-D-glucopyranosides

Article abstract of DOI:10.1021/jo01296a005

The nitrous acid deamination of conformationally rigid benzyl amino-4,6-O-benzylidenedeoxy-D-hexopyranosides was studied: benzyl 2-amino-4,6-O-benzylidene-2-deoxy-α-D-altropyranoside (5) and benzyl 3-amino-4,6-O-benzylidene-3-deoxy-α-D-glucopyranoside (4) in aqueous dioxane both gave some benzyl 2,3-anhydro-4,6-O-benzylidene-α-D-allopyranoside (2).Thus, a trans-diaxial arrangement of hydroxyl and amino groups in the ground state does not appear to be an absolute requirement for epoxide formation, although such an arrangement clearly is most conducive to this reaction, as a comparision of yields showed.Benzyl 2-amino-4,6-O-benzylidene-2-deoxy-α-D-mannopyranoside (7) under similar conditions gave, by elimination, benzyl 4,6-O-benzylidene-α-D-erythro-hexopyrano-3-uloside (8) and, by benzyloxy migration, 2-O-benzyl-4,6-O-benzylidene-D-glucopyranose (11).Contrary to expectation, the same products, 8 and 11, were isolated from the deamination of benzyl 2-amino-4,6-O-benzylidene-2-deoxy-α-D-glucopyranoside (10) and its α-D-mannopyranoside analogue (7).A bicyclic oxonium ion is postulated as an intermediate for the deamination of the α-D-gluco isomer.Benzyl 2-amino-4,6-O-benzylidene-2-deoxy-β-D-glucopyranoside (13), with nitrous acid, gave benzyl 4,6-O-benzylidene-2-deoxy-β-D-erythro-hexopyrano-3-uloside (14).Here also, a bicyclic oxonium ion appears to be involved.All products isolated in this or earlier work from the deamination of 2-amino-4,6-O-benzylidene-D-glucopyranosides can be explained by mechanisms involving bicyclic oxonium ion intermediates.Such intermediates are apparently not involved in the deamination of the manno or altro analogues.