Cyclopentenone Annulation via Cyclobutanones

Placing the electron deficient migration terminus within the original carbonyl partner converts the 1,1-cyclobutanone annulation into a 1,2- or lateral cyclo-pentenone annulation as summarized in Eq. 77. In the ring enlargement of 65 to 66, 

Not too many examples of this process exist. Protonation of the vinyl cyclobutanones derived from a,P-unsaturated ketones creates just such a reactive intermediate. 

Indeed, exposure of the vinylcyclobutanone 67 to acid produces the perhydro-azulene skeleton 68 as summarized in Eq. 78 n2). With more ready access to the types of substituted cyclobutanones required for this lateral cyclopentannulation (see substitutive spiroannulation), milder conditions and thus better processes will probably ultimately be developed. 

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