Single Stereodifferentiation Associated with Carbon Atom Insertion during the Oxonium Ion-Initiated Pinacol Rearrangement of Dihydrofuranyl and Dihydropyranyl Carbinols

Abstract

The stereoselectivity of the acid-promoted rearrangement of dihydrofuranyl and dihydropyranyl carbinols to spirocyclic ketones has been examined. These kinetically controlled isomerizations result in the ring expansion of the hydroxyl-substituted ring with generation of a newly stereogenic spirocyclic carbon atom. All of the adducts formed from several 4,5-dihydrofurans and cyclobutanone, cyclopentanone, and 2,2-dimethylcyclopentanone proved to be reactive. Of the 5,6-dihydropyrans examined, only the cyclobutanone adducts were sufficiently reactive to warrant study. The product distributions arising from the furanoid systems were characterized by modest discrimination in most cases. The more stereodifferentiating pyranoid systems show product distributions as high as 30:1. These results are explained in terms of transition state geometries (conformationally more flexible in the five-membered examples) while also taking into account the principle of stereoelectronic control.