Isatin tautomerism

Monosubstituted hydrazones of isatin can exist in at least three tautomeric forms 124-126. According to spectroscopic data, only hydrazone 124 is present in solution (81JOC2764). 

Preliminary studies with a thio derivative of isatin (indole-2-thione-3-phenylhydrazone) also showed the hydrazone structure to be the favored tautomeric form (81JOC2764). 

Thianaphtheneqiiinone - (109) and diazomethane give a different reaction from that found with isatin, A -methylisatin and coumarandione. In as far as crystalline products could be isolated, the ring expansion occurs here between the sulfur and the carbonyl group in the 2-position. Depending on the solvent, there are formed 3-hydroxy-thiochromone (110), its 0-methyl derivative (111), or (presumably by attack on the 3-keto group of the tautomeric 3,4-diketo form of 110), 3,3 -epoxy-3-methylthiochromone (108). 

The decomposition of 2-nitrobenzoylacetone (72a) to isatin (74a) is considered to proceed via 2-acetylisatogen (73a).4 2-Acylisatogens were unknown until recently, when Robertson8 reported the preparation of 73b from the corresponding acetylene. 1-Hydroxyisatin (74b), formulated as the tautomeric 2-hydroxisatogen (73c) (Section II,A,l,a) is postulated as an intermediate in the decomposition of 2-nitrophenacyl chloride (72b)41 and 2-nitrobenzoyldiazomethane (72c).42... 

The present volume encompasses a wide range of heterocyclic chemistry. Syntheses of heterocycles from thioureas are reviewed by T. S. Criffin, T. S. Woods, and 1). L. Klayman, while S. W. Schneller describes the chemistry of benzothiins and their derivatives (thiochromans, thiochromones, and thio-chromanones). Developments in chrom-3-ene chemistry are reviewed by L. Merlini. F. D. Popp contributes a chapter on the isatins. A discussion of theoretical aspects of the tautomerism of pyrimidines, by J. S. Kwiatkowski and B. Pullman, follows up a corresponding earlier contribution (Vol. 13) on tautomeric purines. In the final chapter P. and D. Cagniant describe the natural occurrence and synthesis of the benzofurans. 

Isatin Chloride.— Now isatin which is a di-ketone of di-hydro indole, or the tautomeric form, a mixed ketone and hydroxyl derivative of indole, yields a chloride when treated with phosphorus pentachloride. 

Deamination of amino acids by carbonyl compounds has long been known in organic chemistry. Classical instances are the Strecker reaction of amino acids with alloxan and rimilar reactions with o-quinones, isatin, ninhydrin, methylglyoxal, and the like. In such cases deamination is usually associated with decarboxylation of the amino acid. Condensation of the amino and carbonyl groups to yield SchiiTs bases, and tautomeric transformation of these, are generally assumed as intermediary steps. 

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