Reactions of Enamines of Cyclic Ketones with Methyl Propiolate. Reinvestigation and Application in a Tandem Ring Expansion with Four C Atoms

Enamines of cyclic ketones 1 with ring sizes ranging from seven to twelve react with methyl propiolate via (2 + 2) cycloaddition and subsequent conrotatory ring opening of the cyclobutene moiety in 2. The resulting cis,trans-cycloalkadienes 3 rearrange further via a thermal [1,5] hydrogen shift to the corresponding cis,cis-cycloalkadienes 4. The structures previously described in the literature of several of these cycloalkadienes are shown to be incorrect. For a representative cis,trans-cycloalkadiene (3g, R = H) and a cis,cis-cycloalkadiene (5a, R = H) the structures were proven by single-crystal X-ray analysis. The reactive “enamine” moiety in 4 allows a tandem ring-expansion reaction of enamines of cyclic ketones with four C atoms. Reaction of 4 with a second electron-deficient acetylenic ester gives the cis,cis,trans-cycloalkatrienes 6.