Cycloadditions with vinylcyclobutanone

Initial attempts to effect the [6+2] cycloaddition with simple vinylcyclobutanes were unsuccessful. Either the substrates were unreactive or complex mixtures resulted. The work of Liebeskind (Scheme 13.9) [20] suggested that a vinylcyclobutanone might be a preferred 6 C component. Such systems are more strained and would be expected to encounter heteroatom-facilitated insertion. 

With vinylketene adducts (2-vinylcyclobutanones) possessing an enolizable hydrogen, the presence of triethylamine used for generating the ketene will occasionally cause double-bond isomerization resulting in alkylidenecyclobutanones 22.138,139 These derivatives are not readily prepared by alkylideneketene to alkene cycloadditions because of the extreme reactivity towards dimerization of these ketenes. 

Vinylcyclobutanones (51) can be prepared by cycloaddition of vinylketenes such as 49 (generated in situ by elimination of hydrogen chloride with triethylamine from a,/ -unsaturated acid chlorides) to / ,/ -dialkyl enamines such as 50 (equation 8)40. This type of reaction was originally reported by Hickmott and coworkers41. 

In 2000, inspired by the work of Liebeskind [31], Wender and co-workers reported [32] an intramolecular Rh-catalyzed [6-1-2] cycloaddition reaction between vinylcyclobutanone and terminal aUcenes (Scheme 5). In this transformation 5 mol% [Rh(CO)2Cl]2, 10 mol% PPhs, and 10 mol% AgOTf were employed and cyclooctenone 24 was afforded in 92% yield as a single diastereoisomer. Besides sulfonamides, other linkers such as ether and geminal diesters were also found to be compatible with this reaction condition using specified catalyst precursors. 

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