Ring-chain tautomerism intramolecular

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. 

Ring-chain tautomerism in which a ring closure occurs via intramolecular addition of an OH group to a C=0 bond (Scheme 4) is observed widely in organic chemistry. This type of tautomerism is sometimes called ketolo-lactolic, keto-lactolic, or oxo-cyclo tautomerism. The ring tautomers are usually referred to as cyclic hemiacetals or hemiketals. 

In the course of the catalytic hydrogenation of a,us dinitriles over Raney nickel, by-products are obtained from C-N and C-C bond formation. The mechanism of the formation of these compounds was investigated. Cyclic and linear secondary amines can result from the same secondary imine through a transimination process involving a ring-chain tautomerism. Stereochemical results for 2-aminomethyl-cyclopentylamine (AMCPA) are in accordance with a specific cyclisation pathway favored by an intramolecular hydrogen bond giving rise to the cis isomer from aminocapro-nitrile, unfavored in the case of adiponitrile which leads to the trans AMCPA as the major isomer. 

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

Several examples of ring-chain tautomerism are known where the cyclic tautomer is formed as a result of an intramolecular hydroxy group addition to the C = N bond of nitrones, yielding hydroxylamine derivatives. 

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. 

The products of formal [4 + 2]-cycloaddition between enamines and ( )-l-phenyl-2-nitropropane display the ring-chain tautomeric equilibrium 113A 113B (Table XXI), where the ring form is obtained by intramolecular nucleophilic addition of the N02 group to the enamine C=C bond (86CCA165) (see also 1-222). 

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. 

In the crystalline state, salts of benzamidrazones (34A, R1 = Ph, R2 = H) also exist in the linear form, but with a fixed (Zj-configuration. Increased steric interactions relative to the C—N2 bond from phenyl substitution causes a syn orientation of N and N stabilized by intramolecular hydrogen bonding (82KGS1264). In solution, a ring-chain tautomeric... 

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)). 

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