Discussion
The major products P and Q formed in the following reactions
respectively, are:
- A.
- B.
- C.
- D.
Answer: B
P is the spiro-epoxide and Q the cyclopropyl ketone, with the isopropenyl group untouched: option (B).
Both reagents are Corey-Chaykovsky sulfur ylides, but they differ in stability. Dimethylsulfonium methylide is reactive and unstabilised; it adds quickly and irreversibly to the carbonyl carbon (1,2-addition), and the alkoxide then displaces dimethyl sulfide to close an epoxide. Dimethylsulfoxonium methylide is stabilised by the S=O group; its 1,2-addition is reversible, so it ends up in the conjugate (1,4) position of the enone, and ring closure gives a cyclopropane next to the ketone.
Neither ylide reacts with the isolated isopropenyl double bond, which is not electron-poor, so (C) and (D) are wrong. (A) swaps the two products.
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