Formal asymmetric 1,3-dipolar cycloaddition of azomethine ylides to (S)-2-p-tolylsulfinyl-2-cyclopentenone

Azomethine ylides, derived from iminoesters 1 and DBU in the presence of silver salts, react with (S)-2-p-tolylsulfinyl-2-cyclopentenone 2 in a completely regio- and endoselective manner but with a low facial selectivity, affording a mixture of two cycloadducts 3 and 4. When the ylides were prepared with LHMDS, only one diastereoisomer 3 was obtained in an almost quantitative yield. A nucleophilic addition/ring closure process easily accounts for the stereochemical results. Compounds 3 were transformed into optically pure 4-oxocyclopentac]dihydropyrroles and tetrahydropyrroles by elimination of the sulfinyl group.