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Rotational isomers due to the restricted motion of the formyl group have been unambiguously detected in orthosubstituted benzaldehydes. The conformers with the ortho-group and formyl oxygen anti have been shown, through l H and 13C n.m.r. spectroscopy, to be the more stable species, the relative abundance depending on the bulkiness of the substituents. Thermodynamic activation parameters have been also obtained by computer simulation of the spectral lines, and a rotational model is proposed to rationalize the dependence of AG3 on the steric hindrance of o-alkyl groups. Proof of the existence of cis-and trans-isomers in terephthalaldehyde has been provided by means of liquid crystal n.m.r. spectra as well as by low temperatures n.m.r. measurements. Comparison has been made between this result and that of an e.s.r. conformational investigation on the corresponding radical anion obtained by photolytic as well as by chemical reduction.

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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.