A. Bongini et al., Stereochemical aspects of a two-step Staudinger reaction - Asymmetric synthesis of chiral azetidine-2-ones, EUR J ORG C, (13), 2000, pp. 2379-2390
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.