Alkenylidenes in Organic Synthesis

摘要

Aikenylidenes R_2C=C: (=alkylidene carbenes) undergo regie- and stereo-selective 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 dia-zomethylation, from vinyl halides by alpha-elimination, from, alkynyliodonium salts by addition of nucleophiles, and from alkynes by retro-1,2-shifts. Specificapplications of the various methods, particularly in the synthesis of natural products, are discussed.