1 -(1 -Pyrrolidinyl)-1,3-butadienes (2, 3, 5, and 7), all having an electron-withdrawing substituent at C-3, undergo a thermal rearrangement to pyrrolizine derivatives (1, and 10-14). The rate of reaction generally increases with increasing polarity of the solvent; in aprotic solvents Lewis acids act as catalysts. The rate of reaction is also dependent on the configuration of the l-( 1-pyrrolidinyl)-1,3-butadiene (k^y¡/k(Z)-5 = 5.2). These thermal rearrangements take place in two steps. The first step comprises a concerted antarafacial [1,6] hydrogen shift that generates a 1,5-dipolar species, e.g., 15a, that may undergo stereomutation to 15b; in the second step a disrotatory electrocyclization takes place to give the corresponding pyrrolizines

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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 &,ciscycloalkadienes 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-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.

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