Substituent effects unsaturated ketenes

Density functional and semiempirical AMI molecular orbital calculations have been used to investigate substituent effects on site selectivity in heterocumulene-hetero-diene4 + 2-cycloadditions between ketene imines and acroleins.The new and novel heterocumulenes a, /3-unsaturated thioaldehyde S -oxides (97) behave as both diene... 

The Meerwein-Eschenmoser-Claisen rearrangement is one of the most useful pericyclic reactions. In its basic form, it involves the conversion of an allylic alcohol 1 to a ketene N, 0-acetal 2, which undergoes rapid [3,3]-sigmatropic rearrangement to yield a y,d-unsaturated amide 3 (Scheme 7.1). In accordance with the general electronic effects observed in Claisen rearrangements, the presence of an electron-donating amino substituent on the ketene acetal intermediate substantially increases the rate of the pericydic step. 

With some cyclic ketones, the a-cleavage can also be followed by intramolecular hydrogen abstraction that eventually leads to an unsaturated ring-opened aldehyde. An alternative reaction path involves formation of a ketene. The competition between these two reactions is determined by the effect of substituents on the conformation and reactivity of the diradical intermediate. ... 

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