Tautomeric intramolecular reversible addition

Ring-chain tautomerism via intramolecular reversible addition reactions to the C=0 group 95AHC(64)251. 

Recent Developments in Ring-Chain Tautomerism L Intramolecular Reversible Addition Reactions to the C = O Group ... 

This chapter is the first part of a review that updates an earlier work by R. E. Valters and W. Flitsch, Ring-Chain Tautomerism (A. R. Katritzky, ed.). Plenum Press, New York and London, 1985. The second part of this review, to be published in a subsequent volume of Advances in Heterocyclic Chemistry, includes data (1982-1993) for intramolecular reversible addition reactions to C = N, C=N, C = C, and C=C groups. 

In the second group of ring-chain tautomeric interconversions, an open-chain system is transformed into a cyclic system through the intramolecular reversible addition of a functional group to a polar multiple bond lA IB 2A 2B 3A 3B and 4A 4B. The book (I) and this article deal with... 

Ring-chain tautomeric interconversions proceeding by intramolecular reversible addition reactions to the C=0 group (Scheme 1) have been well studied, particularly with respect to the 3- and 4-oxocarboxylic acids containing five- or six-membered rings, respectively. Relatively few new investigations have appeared in the literature. 

Valters, R. E., Ffilop, F., Korbonits, D., Recent Developments in Ring-Chain Tautomerism I. Intramolecular Reversible Addition Reaction of the C = 0 Group,... 

Volume 66 of our serial consists of five chapters. Ring-chain tautomerism was covered in an authoritative monograph by Valters and Flitch, which appeared in 1985. Now Professor Valters (Latvia), together with Professor Fiilop and Dr. Korbonits (Hungary), has updated this monograph in two chapters for our serial. The first of these has already appeared in Volume 64. The second, covering intramolecular reversible addition reactions to C=N and other groups, constitutes the first chapter of this volume. 

Ring-chain tautomerism established by intramolecular reversible N — H group addition to the hydrazone C=N bond has been studied in a wide series of derivatives with different structures containing hydrazone N — H groups [for a review, see (88KGS3)]. The cyclic tautomers mostly involve five- or six-membered rings. 

Oxazolidines 288 are subject to ring-chain tautomerism. The process can be considered as a reversible intramolecular nucleophilic addition to the C=N bond. A variety of substituted oxazolidines exist in the open-chain form 289 in the solid state <1985T5919, 1992T4979>. In solution, the two forms are in equilibrium, the position of which depends on the solvent and the substituents. 

The ring-chain tautomerism of 1,3-thiazolidines and 1,3-benzothiazolines can be considered as a reversible intramolecular addition to the C=N bond. The system highly prefers the ring form (27a) and only in some cases the open-chain tautomer (27b) can detected (Equation (4)). 

The nucleophilic properties of enamines uncovered by Stork have found a wide application in Michael additions. Secondary enamines are usually in equilibrium with the corresponding imines. These imines are generally more stable, unless the tautomeric enamine is stabilized by conjugation (Figure 7.71). The primary product of the reaction of an enamine with an a,P-unsaturated carbonyl compound is a dipolar intermediate 7.108. This intermediate is converted to a 1,5-dicarbonyl compound on exposure to aqueous add. Proton transfers can take place before hydroysis to the ketone occurs, and the stereoselectivity of the process may be determined by such steps. Moreover, the enamine addition reaction can be reversible. These problems notwithstanding, the use of chiral amines to generate imines or enamines for use as Michael donors has been widely developed. The chiral imine/enamine can be preformed or, espedally in the case of intramolecular reactions, the amine can be added to the reaction medium in stoichiometric amounts. 

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