= Solution
The <cyclohexanone> <enamine> attacks the beta <carbon> of the cyclopentenone. Its <nitrogen> <lone pair> forms an <iminium> C=N bond as the <enamine> <pi bond> forms the new C–C bond; simultaneously the <enone> pi system shifts onto its <oxygen>. <proton transfer> converts the resulting cyclopentanone <enolate> back to the <ketone>.
A final <iminium> <deprotonation> regenerates an <enamine>, but the position of its double bond matters. Label the <nitrogen>-bearing <cyclohexane> <carbon> $h_1$, its initially nucleophilic neighbour $h_2$, and the other neighbour $h_6$. After addition, $h_2$ bears the cyclopentanone <substituent>. \b[<deprotonation> at $h_6$ gives $h_1{=}h_6$, the less substituted, less hindered <enamine> drawn in A.] Merely restoring the original $h_1{=}h_2$ bond would give a different regioisomer. Interconversion through the <iminium ion> permits this <enamine regioisomerization> under the reaction conditions. The methyl and <mesylate> <substituents> are spectators during this first <Michael addition>.
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