Ring-chain tautomerism review

Ring transformations heterocyclic compounds reviews, 1, 70 Ring-chain tautomerism polyheteroatom six-membered rings, 3, 1056 Ripariochromene A synthesis, 3, 751, 755 Robinson-Gabriel synthesis oxazoles, 6, 216... 

Section 4.2.1 will be devoted to heterocycles, section 4.2.2 will cover other kinds of protomeric tautomeric equilibria (e.g., enol/ketone, formic acid, formamidine, etc.), and section 4.2.3 will discuss an example of a ring/chain tautomeric equilibrium. The order of presentation will be approximately by increasing molecular weight within each section. A review by Kwiatkowski et al. [267] covers work on formamide, pyridines, pyrimidines, purines, and nucleic... 

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. 

The ring-chain tautomerism of tetrahydro-1,3-oxazines, which involves the reversible addition of a hydroxy group to a C = N bond, is a well-established process and was reviewed recently [94ACH(131)697 95AHC(64)251 96AHC(66)1]. In this context, only the most important characteristics of the ring-chain tautomerism of the alicycle-fused 1,3-oxazines is briefly discussed. 

For a monograph, sec Valters Flitsch Ring-Chain Tautomerism Plenum New York, 1985. For reviews, see Valters Russ. Chem. Rev. 1973, 42, 464-476, 1974. 43, 665-678 Escale Vcrducci Bull. Soc. Chim. Fr. 1974, 1203-1206. 

The ring-chain tautomerism of the hydrazone derivatives has been discussed in a monograph (79MI1) and more thoroughly in a review (88KGS3). 

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. 

Reviews have appeared that in part cover recent advances in the ring-chain tautomerism of the nitrogen-containing derivatives of 1,3-dicarbonyl compounds (95ZOB705) of the reaction products of alkenals, alkenones, and alkane-1,3-diones with hydrazines and hydroxylamines (950PP519) and of other compounds [95H(41)1805 95H(41 )2057]. 

Many examples of ring-chain tautomerism in phenols are described in a recent review . It should be mentioned in conclusion that tautomerism can also take place in the substituents at the phenolic ring. 

The ring-chain equilibrium 42A 42B [Kr = 0.67 in (CD3)2SO, H-NMR] has been observed (91KGS912). A similar tautomerism of a condensed imidazole derivative (43A 43B) had been reported previously [86JPR(328)589, 86JPR(328)597]. For a review on earlier investigations in this field, see (87MI2). 

The tautomerism exhibited by 87 is another example of ring-ring tautomerism (81ZOR2451 for a review, see 92KGS851) because the open-chain tautomer could not be detected by means of H- and 13C-NMR... 

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