Methods of Reactivity Umpolung

The past decade of organic chemistry may be charcterized as a period of violent development of new synthetic methods. This was accompained by a systematization of the analysis of synthetic problems (synthetic strategy). The planning of the synthesis of an organic target molecule is greatly facilitated by distinguishing between reagents X(C) n … with normal reactivity (acceptor properties at C 1,3,5… , donor properties at X, C 2,4… ) and with reactivity umpolung (acceptor properties at X, C 2,4… , donor properties at C 1,3,5… ). In this context, reactivity umpolung turned out to be useful as a heuristic principle, as a classification scheme, and as an aid for locating so‐called strategic bonds (synthon, transform, and antithesis according to E. J. Corey ). There are six principal methods of umpolung: 1 2 n ‐oxidation, heteroatom exchange and modification, homologation and its reversal, the cyclopropane “trick”, use of acetylenes, and redox reactions; under certain circumstances none of these techniques is necessary in cases where direct umpolung is possible. Throughout the article, normal reactivity is indicated by green print; reactivity umpolung by red print.