A) [Pi]-facial diastereoselectivity in Diels-Alder reactions of 2,5-dimethylthiophene oxide and related compounds ; B) A synthetic approach to a chiral ketene equivalent

Abstract

The reactivity and diastereofacial selectivity of some thiophene oxides particularly 2,5-dimethylthiophene oxide in Diels-Alder reactions are reported. The sulphoxides were generated "in situ" by peracid oxidation (m-chloroperoxybenzoic acid). In all successful cases of cycloaddition, the dienophiles benzoquinone, naphthoquinone, tetracyanoethylene, N-phenylmaleimide and 2-chloroacrylonitrile afforded exclusively the syn adduct with respect to the sulphoxide oxygen. The structures were confirmed by X-ray crystallographic analysis. -- This facial preference is consistent with the relative donor ability of the lone pair versus the sulphoxide oxygen. In accord with the concept of transition state stabilization by ϭ electron donation into the vacant ϭ†∙ orbital, associated with the incipient bond, addition should occur anti to the better donor, (i.e.) the lone pair, as observed. Thiophene oxides thus represent latent butadiene equivalents that react with complete facial control. -- The second and third parts of this thesis describe unsuccessful synthetic studies directed toward intramolecular thiophene oxide cycloadditions and the preparation of a sulphoxide based ketene equivalent.