= Solution
Tin tetrachloride coordinates the enone <carbonyl>, lowering the dienophile pi-star energy. Butadiene reacts in an endo-oriented Diels–Alder <cycloaddition>: its terminal <carbons> bond to the enone's two <alkene> <carbons>, its central bond becomes the new cyclohexene double bond, and the enone C=C pi <electrons> become a new <sigma bond>. All three bond reorganizations occur in a <suprafacial> six-electron cycle.
Approach from the face \b[opposite the methyl substituent] avoids steric crowding and gives the indicated relationship between methyl and the non-enolizable junction <hydrogen>. The immediate stereospecific <cycloaddition> gives a cis-fused junction relationship. The junction <carbon> next to the <ketone> can subsequently enolize; reprotonation on either face epimerizes this centre without breaking either newly formed C–C bond. This accounts for the mixture at the wavy-bond position while the other junction and methyl relationship remains fixed. The modest 1.5:1 ratio should not be mistaken for failure of <suprafaciality> in the <cycloaddition> itself. Later oxime formation removes the <ketone>'s ordinary <enolization> pathway and furnishes the depicted isolated junction <diastereomer>.
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