Syntheses of Cyclopropylcarbonyl Compounds

There are three important direct routes to cyclopropylcarbonyl compounds: 1. Cyclization of chains of three carbon atoms, the first or third of which is adjacent to a carbonyl or potential carbonyl carbon atom (this type includes syntheses by intramolecular alkylation of γ‐halogeno ketones or related compounds in alkaline media); 2. insertion of a methylene group or substituted methylene group into the olefinic double bond of an α,β‐unsaturated carbonyl compound; and 3. introduction of an acetonyl group into the double bond of an olefin. However, cyclopropylcarbonyl compounds can also be obtained from 1,2‐epoxycyclobutane and 2‐bromocyclobutanone derivatives by ring contraction. Another possibility is the dehalogenation of α,α‐bis(bromomethyl) cycloalkanones. This review concludes with a discussion of these little known routes and of a particularly suitable method which involves the reaction of methylene iodide, a ZnCu couple, and α,β‐unsaturated ketones.