Heterodiene cycloadditions of C2 symmetric 4,5-disubstituted ketene acetals: the nett asymmetric conjugate addition of recyclable acetic ester enolate equivalents to an activated enone

Abstract

Heterodiene cycloadditions of 3-formylchromone 2 to a series of ketene acetals 1 derived from C₂ symmetric 1,2-diarylethane-1,2-diols are diastereoselective. From (S,S)-1,2-di(o-tolyl)ethane-1,2-diol 6c the major cycloadduct 3c was isolated by crystallisation and transformed by acid-catalysed methanolysis into (S)-methyl 4-oxo-3,4-dihydro-2H-1-benzopyran-2-ylacetate 5, together with the original 1,2-diol 6c which could be recycled. The structures of two cyclic carbonates 22a and 22c were determined by X-ray diffraction and used as models in seeking a mechanistic rationale for the stereoselective cycloadditions of the analogous ketene acetals 1a and 1c.