Molecular orbital calculations relating to the mechanism of the reactions of sugar orthoesters: 1,2,4-orthoacetyl-α-D-xylopyranose

Abstract

Quantum chemical studies of 1,2,4-orthoacetyl-α-D-xylopyranose and three protonated forms, four acyloxonium ions, and the glycosyl cation by means of the MINDO/2 method are reported. The protonated forms (oxonium ions) should not be considered as product-determining intermediates in acid-catalysed reactions of ortho-esters due to their fast rearrangement into isomeric acyloxonium ions. Of the latter, only 1,2- and 1,4-acyloxoniums, adopting a conformation close to that of starting orthoester (i.e. a distorted boat), were found to be relatively stable and reactive and so are considered to be the main product-determining intermediates. The distribution of the positive charge in these ions was interpreted as evidence of preferred nucleophilic attack on C-1 rather than on other centres of these ions. The isomerisation of the 1,2-acyloxonium ion into the glycosyl cation was found to be energetically very unlikely and so would be product determining only in fast, especially intramolecular, reactions. The results obtained were in good agreement with qualitative data on the chemistry of sugar orthoesters.