Zur Reaktionsweise von Enaminen mit Cyclopropenonen IV. Einsatz von Ketenacetalen

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Abstract
A new interpretation – based on a reevaluation of the spectroscopic properties of products 16 to 27 – is proposed for the reaction of diphenyl‐cyclopropen‐one 14 and ‐thione 15 with ketene‐A, N‐diacetals 8 to 13 (A  R2N, RO and RS) originally reported by Sauer & Krapf. It is concluded that the previous structural assignments (see the a‐structures), made on the assumption of a prevailing “C,C‐insertion” reaction, must be rcplaced as follows: (1) All the “secondary adducts” are, in fact, derivatives (amides and lactams) of 2,3‐diphenyl‐penta‐2, 4‐dienoic acid and thioacid (structures 16b to 24b); (2) the “isomerization products”, differ from the latter only in the configuration of the α,β‐double bond (structures 25b and 26b); (3) the common “hydrolysisproduct” is α,β‐diphenyl‐γ‐methyl‐γ‐hydroxy‐Δα‐butenolide (27b), The above cyclopropenone‐ketcneacetal reactions represent, therefore, cases of “C, N‐insertion”.This is rationalized with a reaction scheme, in which the “acylide” structure of the “primary adducts” plays a role.