Ring-chain tautomerism addition, 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 ... 

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

V. INTERNAL ADDITIONS, ELIMINATIONS AND RING-CHAIN TAUTOMERISMS A Intramolecular Additions to Double Bonds... 

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. 

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. 

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)). 

Various methods are available for the preparation of the foregoing precursors to V-acyliminium ions. The most straightforward synthesis of V-(l-hydroxyalkyl)-amides or -carbamates (2 X = OH) involves the addition of primary or secondary amides (carbamates) to aldehydes or ketones. This reaction is an equilibrium process and usually disfavors the adduct except for two special cases. The first involves very reactive aldehydes like formaldehyde, chloral and glyoxylic acid and its esters (equation 4) compounds (4)-(6) are fairly stable compounds that are frequently used in amidoalkylation reactions. The second special case is the intramolecular variant (ring-chain tautomerism), if it leads to five- or six-membered rings (equations 5 and 6). Thus, for n = 1 or 2, (7) and (8) cyclize to (9) and (10), respectively, although the nature of R has an influence on this process. " ... 

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