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,S-azadienes, which in some cases have been isolated and charact erised. A conrotatory ring closure of these compounds gave rise to the form ation of the azetidine-2-ones, thus allowing a formal two-step Staudinger r eaction. Exclusive trans diastereoselectivity was observed. A less stringen t diastereofacial selectivity was obtained. A set of experiments has been p erformed in order to evaluate the influence of the structural parameters as well as reaction renditions. Ab initio studies at MP2/6-31G* and QCISD(T)/ 6-311G** levels on model compounds provide a rationalization of the experi mental results obtained. From the experimental as well as theoretical data it is clear that the presence of the silyl enol group in the intermediate a zadiene 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.