A Flexible Approach to (S)-5-Alkyl Tetramic Acid Derivatives: Application to the Asymmetric Synthesis of (+)-Preussin and Protected (3S,4S)-AHPPA

[reaction: see text] A flexible asymmetric approach to 5-alkyl tetramic acid derivatives is described, which is based on the use of 9 as the first synthetic equivalent to chiral nonracemic tetramic acid 5-carbanionic synthon 9b. The existence of the carbanion intermediate 9b was proven by trapping with trimethylchlorosilane. Application of the present method to the synthesis of antifungal alkaloid (+)-preussin, as well as protected (3S,4S)-AHPPA 6, is also described.