Potentially Tautomeric Groups

111 the case of four-membered rings where the eiidocyclic double bond leads to an aromatic tautomer, this possibility was never observed for functional tautomers like l,2-diazetidin-3-ones 12 and 1,2-dioxetanone 13. Tlie geometry of this last compound (essentially planar) has been calculated at the 4-31G level (81JA1292). 

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Modern concepts have been extended to the chemistry of heterocyclic compounds more slowly than to the chemistry of aromatic and aliphatic systems, but efforts are now being made to classify and explain the properties and reactions of heterocyclic compounds in terms of these newer ideas (cf. reference 11). However, many of the most important heterocyclic compounds are potentially tautomeric, and elucidation of their tautomeric composition must precede a logical treatment of their properties. Further, many natural products such as the nucleic acids and alkaloids contain potentially tautomeric groups and information of this type is needed for a detailed explanation of th reactions which they undergo,... 

D. Azoles with T vo and More Potentially Tautomeric Groups. 239... 

E. Azoles with Chelated Potential Tautomeric Groups. 246... 

B. Compounds with Potentially Tautomeric Groups 1. Azepines... 

Studies on the tautomerism of pyridazines with potential tautomeric groups have continued. On the basis of recorded ionization constants and UV spectra of 3,4,5-trimercaptopyridazine and its derivatives, it is concluded that the compound exists as 3,4-dimercaptopyridazine-5(2f/)-thione. Tautomerism of hydroxy- and mercaptopyridazines was investigated with the aid of the corresponding anhydro bases as reference compounds. They exist predominantly in the -one or -thione forms. Similarly, the 3-mercapto-pyridazine-6(l/f)-thione structure in aqueous solution has been redeter-... 

J. Compounds with Three Different Potentially Tautomeric Groups. 108... 

Detailed reviews on methylene tautomerism of monocyclic azines and their benzo-and heterocycle-fused derivatives have been published in 1995 (95H(41)2057, 95KG816). The effects of the mesomeric-type substituents (88ZOR1806), the position of the potentially tautomeric group and benzannulation and the solvent effect (94JCS(P2)2461) on the position of the tautomeric equilibrium has been investigated in detail. Studies of the mechanism of methylene tautomeric interconversion in azines have also been undertaken (83IZV1032, 86IZV1166). 

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