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 (S S )-(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 1a − 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 1i, j, with 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, (S S )-(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 π-facial diastereoselectivities at both partners.