Stereoelectronic control in acetal formation

Abstract

The cyclization under acidic conditions of bicyclic and monocyclic hydroxypropyl methoxytetrahydropyrans 1 and 4 is reported. At room température, compound 1 gives only cis tricyclic acetal 2. At reflux, compound 2 equilibrates to yield a mixture of cis and trans tricyclic acetals 2 (45%) and 3 (55%). Compound 4 gives directly the equilibrium mixture of cis and trans bicyclic acetals 5 (55%) and 6 (45%) at room température. The specific formation of 2 from 1 constitutes strong experimental evidence that acetal formation is controlled by stereoelectronic effects. The anomeric effect can be estimated [Formula: see text] from the equilibrium study of 2 and 3. This value confirms the result previously obtained by Descotes (9).