Chemistry of ketene acetals. Part 8. Stereochemistry of the reaction of 1,1-dimethoxypropene with aldehydes

Abstract

The Lewis acid-catalysed (2 + 2)cycloadditions of 1,1-dimethoxypropene with various aldehydes RCHO have been studied. These reactions, which are supposed to proceed via dipolar intermediates, yield 2,2-dimethoxyoxetanes, the formation of which is reversible in the presence of the catalyst used. The cis:trans ratio of the oxetanes was determined after short reaction times at low temperature (‘kinetic’ conditions) and after prolonged reaction times at room temperature (thermodynamic conditions). This was performed by stopping the reaction with triethylamine or by transforming the oxetanes at low temperature into β-hydroxy esters. The influence of the reaction conditions and the size of R on the stereochemistry of the reaction has been discussed. It appeared that under ‘kinetic’ conditions the cis:trans ratio of the oxetanes is not only determined by the most favourable transoid approach of the cycloaddends, but also by the rotation of the dipolar intermediate to a cisoid gauche conformation.