Amine catalyzed ring opening of (2 + 2) cycloadducts derived from enamines of cyclic ketones and methyl propynoate

The (2 + 2) cycloadducts 3, 6, 10 and 13, derived from pyrrolidine enamines of cyclic ketones and methyl propynoate 2, react, under the influence of pyrrolidine via the α‐[(1‐pyrrolidinyl)methylene]cycloalkeneacetates (Z)‐4, (Z)‐7, (Z)‐11 and (Z)‐14, to give the corresponding E isomers. The structure of (E)‐7 has been unequivocally established by X‐ray analysis. The formation of (E)‐4, (E)‐7, (E)‐11 and (E)‐14 can be explained by the sequence: Michael addition, elimination, conrotatory ring opening, followed by Z/E isomerization. In the cases involving a relatively slow reaction (10, 13) and, in addition, the presence of an excess of pyrrolidine, the enamines 9, 16 and the propenoate 12 could also be detected. The formation of 12 can be rationalized by a Michael addition of pyrrolidine to methyl propynoate (2), formed by a retro (2 + 2) cycloaddition.