Tetramic acid chemistry. Part 1. Reinvestigation of racemisation during the synthesis of tetramic acids via Dieckmann cyclisation

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Abstract
Epimerisation during the synthesis of tetramic acids by Dieckmann cyclisation of the corresponding chiral N-acyl-α-amino esters was investigated. In the case of the isoleucine derivative, the extent of epimerisation was directly evaluated by 1H NMR analysis of the 3-acylated tetramic acids (3a, b) and (5a). A chemical correlation was carried out in the case of the 5-benzyl derivative (8h). The moderate overall yield and the partial epimerisation at position C-5 limit the usefulness of this approach for tetramic acid preparation.