the right of carbon in the Periodic Table. By contrast, a novel approach involving an analysis of the steric requirements inherent in the structure of the transition state of a reactive center and an examination of the stereodynamic possibilities has given rise to a target-oriented molecular design of compounds capable of rapid and reversible intramolecular migration of the type indicated. The implementation of this ap­ proach, which is the subject of the present book, has already led to the preparation of new tautomeric compounds in which such heavy organic migrants as acyl, aryl, sulfinyl, phosphoryl, arsinyl, and other groups migrate in molecules at a frequency 6 9 of 10 -10 S-I at ambient temperature, i. e. , at the rates comparable with protonic migrations.