Condensation products of aliphatic aldehydes with 3-sulfanylpropionic acid hydrazide exist in solution as mixtures of linear hydrazone and cyclic 1,3,4-thiadiazepine tautomers. Hydrazones derived from 3-sulfanylpropionic acid hydrazide and aromatic aldehydes and ketones have mostly linear structures of different stereoisomers arising from Z-E isomerism with respect to the double C=N bond and restricted rotation about the C(O)-N bond. Condensation products of 3-sulfanylpropionohydrazide with a series of aldoses give rise to ring-chain-ring tautomeric equilibrium between α,β-isomeric pyranose structures, open-chain aldose hydrazone tautomer, and two diastereoisomeric seven-membered cyclic 1,3,4-thiadiazepine forms.We previously showed that condensation products of carbonyl compounds with sulfanylacetic and 2-sulfanylbenzoic acid hydrazides in solution give rise to ring-chain tautomerism between linear acylhydrazone and cyclic 1,3,4-thiadiazine or 1,3,4-benzothiadiazepine structures, respectively [1][2][3][4][5]. In both cases, the cyclic tautomer is formed via intramolecular nucleophilic addition of the sulfanyl group at the C=N bond in the hydrazone fragment. While continuing studies in this line, we examined the ability of condensation products formed by aldehydes, ketones, and aldoses with 3-sulfanylpropionic acid hydrazide to undergo ring-chain isomerism (Schemes 1, 2).We initially studied 3-sulfanylpropanoylhydrazones IIIa-IIIj derived from aliphatic and aromatic aldehydes and ketones. These compounds were synthesized in 40-85% yield by reaction of 3-sulfanylpropionohydrazide (II) with an equimolar amount of the corresponding carbonyl compounds Ia-Ij in methanol either at room temperature or on heating under reflux for a short time.3-Sulfanylpropionylhydrazones IIIa-IIIj can exist as linear tautomer A and seven-membered cyclic 1,3,4-thiadiazepine structure B. The latter could be formed via intramolecular addition of the SH group at the C=N bond in the hydrazone fragment. Furthermore, open-chain tautomer A may be represented by four stereoisomers with different configurations with respect to the C=N bond (Z-E isomerism) and C-N bond in the amide fragment (conformational Z′-E′ isomerism) [1, 2] (Scheme 1).Let us consider condensation products of hydrazide II with acetaldehyde, propionaldehyde, and butyraldehyde (compounds IIIa-IIIc). As follows from variation of the 1 H NMR spectra with time, these compounds in the crystalline state have structure A. The 1 H NMR spectrum of acetaldehyde hydrazone IIIa in DMSO-d 6 , recorded immediately after dissolution, contained two sets of signals assignable to two stereoisomers of linear tautomer A. The major stereoisomer (its fraction was 73% in the initial moment) was characterized by a quartet signal from the CH=N proton at δ 7.29 ppm, a triplet signal from the SH proton at 2.43 ppm, and a broadened singlet from the NH proton at δ 10.91 ppm. The corresponding signals of the minor stereoisomer (27%) were located at
