Type I Intramolecular Cycloadditions of Vinylketenes

Article abstract of DOI:10.1021/jo00244a002

The scope of type I intramolecular <2 + 2> cycloadditions of alkenes with α,β-unsaturated ketenes (see eq 1) has been explored.These reactions generally produce bicyclo<3.2.0>heptan-6-ones containing an unsaturated substituent at position 5.Ketenes 4, 7, 13, and 30 undergo the expected cycloaddition to give 5, 8, 14, and 31 in 50-80percent yield.Ketenes 10 and 16 undergo a 1,5-sigmatropic hydrogen shift to give dienals 11 and 17.Ketenes 23 and 24 undergo a reversible electrocyclic ring closure instead of a cycloaddition to give cyclobutenones 25 and 26.At higher temperatures electrocyclic ring closure is reversible and cyclobutenones 25 and 26 can be converted into 27 and 28 in 75percent yield.Type I intramolecular cycloadditions cannot be used to produce bicyclooctanones such as 38.These vinyl cyclobutanones are versatile synthetic intermediates.Treatment of 32 with potassium hydride gives 34 and 35, suggesting that this approach will be useful for steroid synthesis.Treatment of 8 with boron trifluoride gives 39.