Chemistry of ketene acetals. Part 9. A simple ‘one-pot’ synthesis of 4-hydroxy-δ-lactones and 5,6-dihydro-2-pyrones from 1,1-dimethoxypropene and β-oxy aldehydes

Abstract

Protected β-oxy aldehydes react easily with 1,1-dimethoxypropene (1a) in the presence of ZnCl₂ to give 2,2-dimethoxyoxetanes. Hydrolysis of these oxetanes and deprotection of the latent hydroxy function in a one-pot procedure gives 4-hydroxy-δ-lactones in moderate to good yields. Starting with β-oxy aldehydes having an α-branched side chain and defined stereochemistry, δ-lactones with completely defined stereochemistry can be synthesized. Dehydration of the hydroxy lactones with concentrated sulphuric acid gives easy access to 5,6-dihydro-2-pyrones. The usefulness of this route is demonstrated in the synthesis of a simple, optically active 5,6-dihydro-2-pyrone (37).