Tautomerism Rings

Photochromism in solution and in the solid state can be the result oi intramolecular changes, e.g. tautomerism, ring opening, cis-trans isomerization, free radical formation, stereoisomenc transition, formation of dimers nd such similar reversible reactions. Some examples are given below ... 

The first of these decarboxylations is catalyzed by the cytoplasmic uroporphyrinogen decarboxylase, which removes the carboxylate groups of the four acetate side chains sequentially from the D, A, B, and C rings.395-3963 A possible mechanism, utilizing a tautomerized ring, is illustrated in the accompanying structural formula. 

Meyers et al.221-222 showed that tetrahydro-l,3-oxazines exist in tautomeric ring-chain forms [Eq. (62)]. When a 5,6-dihydro-l,3-oxazine is reduced to a tetrahydro-l,3-oxazine, some 3-aminoalcohol can also be formed through the reduction of the open-chain imino form [cf. Eq. (62)]. To avoid this the reduction should be carried out with sodium borohydride at - 40°C. 

The last major section deals with tautomerism, including prototropic tautomerism, ring-chain tautomerism, and valence tautomerism. 

J. Elguero, C. Marzin, A. R. Katritzky, and P. Linda, Advances in Heterocyclic Chemistry, Suppl. 1. Academic Press, New York, 1976. Tautomerism, ring-chain ... 

Thermal and photochemical reactions formally involving no other species (0 Tautomerism ring-chain tautomerism... 

It will be seen that the cleavage of the hypothetical hydrazo derivatives to the hypothetical imino compounds obviates the necessity of assuming direct oxidation by phenylhydrazine. Fieser and Fieser inquire why osazone formation ceases with the introduction of two phenylhydrazine residues, and seek to explain this fact by the postulation of chelate tautomeric ring structures XLIII or to the existence of a resonance... 

Structural tautomerism breaks down into several categories, which include the most common prototropic tautomerism, ring-chain and valence tautomerism, and charge mesomerism. Only the first and the last have been successfully treated in software - everyone is familiar with the CAS Registry III tautomer normalisation conventions. It quickly became apparent, if not obvious, that in the case of a data handbook such as Beilstein, it was imperative that different tautomeric forms of a compound for which different data have been published must be registered differently the situation is analogous to that of registering stereoisomers as separate compounds. ... 

Exhibits ring-chain tautomerism. Ring form predominates. Used for photometric detn. of amines. Cryst. (CgH /pet. ether). Mp 105°. 

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