Abstract
The chemistry of the highly substituted cyclobutanone derivative 2, which is easily accessible from 1, is dominated by the steric hindrance caused by the substitutents: Reduction with hydride reagents and nucleophilic additions with methyllithium are less stereoselective than corresponding reactions with less substituted bicyclo[3.2.0]hept‐2‐en‐6‐one derivatives, but under certain conditions, the exo product predominates, with methyl‐, benzyl‐, ethenyl‐, and ethynylmagnesium halides no nucleophilic addition at the carbonyl group is observed, presumably as a result of the steric hindrance. Instead, an unexpected rearrangement to the bicyclo[4.1.0]heptane carbon skeleton occurs with complete diastereoselectivity and high yields. The rearrangement is rationalized by an increase of the carbocationic character of the carbonyl carbon atom as a result of a coordination of the carbonyl oxygen atom at the Lewis acidic magnesium. This facilitates rearrangement of a cyclobutyl carbenium ion to a cyclopropylmethyl carbenium ion, the latter being stabilized by the amino group. Remarkably, with Grignard reagents derived from propargyl or allyl halides, no rearrangement is observed. The constitutions and configurations of the products were determined by ^1^H‐ and ^13^C‐NMR spectroscopy using techniques including NOESY and 2D‐INADEQUATE.