Stereochemical Aspects of a Two-Step Staudinger Reaction − Asymmetric Synthesis of Chiral Azetidine-2-ones

Reaction of N-trialkylsilylimines with a variety of glycine-derived ketenes produced 1,3-azadienes, which in some cases have been isolated and characterised. A conrotatory ring closure of these compounds gave rise to the formation of the azetidine-2-ones, thus allowing a formal two-step Staudinger reaction. Exclusive trans diastereoselectivity was observed. A less stringent diastereofacial selectivity was obtained. A set of experiments has been performed in order to evaluate the influence of the structural parameters as well as reaction conditions. Ab initio studies at MP2/6−31G* and QCISD(T)/6−311G** levels on model compounds provide a rationalization of the experimental results obtained. From the experimental as well as theoretical data it is clear that the presence of the silyl enol group in the intermediate azadiene is crucial in its stabilisation and plays a fundamental role in the conrotatory ring closure and, therefore, in the formation of the azetidine-2-one ring.