Chemistry of tetra-alkoxyethenes. Part VI. Cycloadditions with αβ-unsaturated carbonyl compounds and chemistry of the resulting tetra-alkoxydihydropyrans

Abstract

Heating tetra-alkoxyethenes (1) at 100°C with α-cyano-αβ-unsaturated carbonyl compounds (2) gives [4 + 2] cycloaddition products in high yields. Mild acidic hydrolysis of the resulting 2,2,3,3-tetra-alkoxy-5-cyano-3,4-dihydro-2H-pyrans (4) gives separable mixtures containing a δ-oxo-ester (11), formed by ring opening, and an α-pyrone (12), which arises by recyclization of the δ-oxo-ester after hydrolysis of its acetal function. In concentrated sulphuric acid, compounds (4) are converted either into 3,3-dialkoxy-5-cyano-3,4-dihydro-2-pyrones (13) or into 3-alkoxy-5-cyano-2-pyrones (14), depending on the reaction temperature. Treatment of (4) with base yields 2,2,3-trialkoxy-5-cyano-2H-pyrans (15) by elimination of alcohol.