Dramatic Diastereoselectivity Differences in the Asymmetric Ene Reactions of Triazolinediones and Singlet Oxygen with Chiral 2,2-Dimethyloxazolidine Derivatives of Tiglic Acid

Abstract

The diastereoselectivity in the ene reactions of triazolinedinone and singlet oxygen with chiral oxazolidine or oxazolidinone derivatives of tiglic acid depends both on the chiral auxiliary used and the size of the attacking enophile. While chiral 2,2-dimethyloxazolidine amides of tiglic acid give with triazolinediones TAD only one single ene adduct, the corresponding chiral oxazolidinone derivatives (Evans′ auxiliaries) display a very low diastereoselectivity. No selectivity is observed in the singlet-oxygen ene reaction with dimethyloxazolidine amides of tiglic acid. These stereochemical results are rationalized in terms of the differences in the steric demand of the singlet oxygen and TAD enophiles rather than electronic factors.