Alkenylidenes in Organic Synthesis

Abstract

Abstract Alkenylidenes R 2 CC: (= alkylidene carbenes) undergo regio‐ and stereoselective intramolecular CH insertion reactions that are excellently suited for the synthesis of cyclopentenes. The 1, 2‐shifts occurring with RH and RAr are useful for the preparation of alkynes. Alkenylidenes are efficiently generated from carbonyl compounds by diazomethylation, from vinyl halides by α‐elimination, from alkynyliodonium salts by addition of nucleophiles, and from alkynes by retro‐1, 2‐shifts. Specific applications of the various methods, particularly in the synthesis of natural products, are discussed.