DIASTEREOFACIAL SELECTIVITY IN REDUCTION OF CHIRAL TETRAMIC ACIDS

The reduction of (5S)-5-alkyl-2,4-dioxopyrrolidines, so-called tetramic acids, by NaBH4 gives only partial diastereofacial selectivity in the case of the N-substituted analogues 9f-i, unlike the carbamate derivatives 9a-3 which give the reduced cis-pyrrolidinones 10betaa-e. Increasing the steric hindrance of either the N- or C-5-substituents enhances the re-face selectivity. On the other hand, reduction of the heterobicyclic compound 9n leads to a dramatic reversal of the stereoselectivity. Preliminary calculations show that the N-atom of the ring is slightly pyramidalized; the direction of hydride addition could be a consequence of this finding.