Enantioselective Diels-Alder cycloadditions with (SS)-2-(p-tolylsulfinyl)-1,4-naphthoquinone:: Efficient kinetic resolution of chiral racemic vinylcyclohexenes

Chiral racemic vinylcyclohexenes 1a-m, bearing allylic and nonallylic oxygenated substituents on the cyclohexene ring, were submitted to Diels-Alder reactions with enantiomerically pure (SS)(2-p-tolylsulfinyl)-1,4-naphthoquinone. The tandem cycloaddition/pyrolytic sulfoxide elimination led to the formation of enantiomerically enriched angularly tetracyclic quinones (+)-8a and (+)-12, arising from the kinetic resolution of the racemic diene. Dienes la-c, bearing an alkoxy substituent at the allylic position C-3, and 1d-g, with C-3 alkoxy and C-5 methyl groups, gave exclusively quinones (+)-8a-g, resulting from the anti approach to the dienophile. A similar anti selectivity occurred in cycloadditions of dienes 1ij, viith the alkoxy group situated at the allylic position C-6. Nonallylic 4-substituted vinylcyclohexenes 1k-m evolved to ca. 75:25 mixtures of (+)-anti-8k-m and (+)-syn-12k-m derivatives. In all cases, (SS)-(2-p-tolylsulfinyl)-1,4-naphthoquinone reacted from the less hindered face of the more reactive s-cis conformation, giving rise to products in moderate to excellent enantiomeric excesses. Steric effects and torsional interactions in the corresponding approaches account for the observed pi-facial diastereoselectivities at both partners.