Reactions of Electron-Rich Olefins with Proton-Active Compounds

Proton‐active substances react with certain electron‐rich olefins with cleavage of the central CC double bond to give compounds that can be formally regarded as insertion products of nucleophilic carbenes. If they satisfy certain structural conditions, they isomerize with β elimination to give open‐chain compounds. Both CH‐acidic compounds and compounds containing NH or OH groups can undergo this reaction. The mechanisms are discussed, and the importance of the intermediate products to biochemistry (thiamine, tetrahydrofolic acid) is indicated.