ZnCl₂‐catalysed cycloadditions between ketene acetals and α,β‐unsaturated carbonyl compounds. A simple route to 2,2‐dialkoxy‐3,4‐dihydropyrans

Abstract

Abstract 2,2‐Dialkoxy‐3,4‐dihydropyrans (3) can be obtained under mild conditions and in good yields from ketene acetals (1) and α,β‐unsaturated carbonyl compounds ( 2 ) in the presence of ZnCl 2 . At low temperatures (< −20°C) the reaction between 1 and 2 proceeds as a (2 + 2)‐cycloaddition, leading to an oxetane ( 4 ), which can sometimes be isolated by neutralisation of the Lewis acid. At higher temperatures, however, the oxetanes decompose into the starting compounds, eventually leading to the thermodynamically more stable dihydropyrans. The cycloadditions of tetramethoxyethene ( 1a ) with 2 , having no substituents at the β‐carbon atom, yield a cyclobutane derivative as the low temperature product; in cycloadditions of α,β‐unsaturated esters this is the final product. Cycloadditions of 2‐chloroketene acetal ( 1e ) lead in some cases to 2:1 adducts. The deviating behaviour of 1a and 1e is discussed.