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The natural amino acid L-cysteine, commercially available aldehydes, and 3-(acetylsulfanyl)propionyl chloride were used for the synthesis of 2-substituted (2R,4R)-3-(3-mercaptopropionyl)thiazolidine-4-carboxylic acids, potential antihypertensive drugs with angiotensin converting enzyme inhibiting action.

The products of the condensation of aliphatic aldehydes with N-(2-aminobenzoyl)-N-methylhydrazine exist in DMSO-d6 solution as tautomeric mixtures of linear aldohydrazone and cyclic benzo-1,3,4-triazepine forms. The linear tautomer predominates for 2-aminobenzoyl-N-methylhydrazones of aromatic aldehydes. A tautomeric equilibrium is observed in DMSO-d6 for the products of the condensation of the hydrazide of 2-aminobenzoic acid with a series of aldoses. This equilibrium exists between α,β-isomeric pyranose forms and the open aldosohydrazone form. Isomeric conversion to the seven-membered benzo-1,3,4-triazepine form is observed for the products of the condensation of aldoses with N-(2-aminobenzoyl)-N-methylhydrazine.

It has been shown by 1H NMR spectroscopy that 2-mercaptobenzoylhydrazones of aromatic aldehydes 2-HSC6H4CONHN=CHC6H4X (X = 4-NO2, 3-NO2, 4-Br, H, 4-Me, 4-MeO, 4-Me2N) exist in DMSO-d6 solution as tautomeric mixtures of linear and cyclic benzo-1,3,4-thiadiazepine forms. The linear hydrazone form is represented by (E,Z′)-conformational isomers, differing in the disposition relative to the amide C–N bond. It was shown that the logarithm of the tautomeric equilibrium constant K T correlates with the σ-constants of the substituents in the aromatic nucleus.

The isomeric transition of 2-aminobenzoylhydrazones of aliphatic aldehydes 2-H2NC6H4CONHN=CHAlk (Alk = Me, Et, Pr, Bu) in chloroform-d1 solution into the cyclic 1,2-dihydroquinazolin-4-one form has been studied. The structure of the obtained compounds was demonstrated by methods of 1H and 13C NMR spectroscopy.

1H and 13C NMR spectroscopy showed that the previously unknown thiosalicyloylhydrazones of aliphatic aldehydes, 2-HSC6H4CONHN=CHAlk (Alk = Me, Et, Pr, Bu, i-Pr, i-Bu) exist in solution as a tautomeric mixture of the linear and cyclic 1,3,4-benzothiadiazepine forms.

Mercaptoacetylhydrazones of methyl alkyl ketones (Alk = Me, Et, Pr, i-Pr, i-Bu, s-Bu, t-Bu) exist in solutions as a tautomeric mixture of linear and cyclic 1,3,4-thiadiazine forms.The linear hydrazone form exists as a set of conformers caused by restricted rotation of the amide group relative to the C–N bond. It is shown that tautomeric equilibrium constants correlate with the steric constants of the alkyl substituents, ES.

We have studied the reaction of 5-hydroxy-3,3,5-trimethylisoxazolidine and 5-hydroxy-3,5-dimethyl-Δ2-isoxazoline with derivatives of thiosemicarbazide and also thiocarbonohydrazine. Both reactions serve as a method for synthesis of the previously unknown 5-thiosemicarbazido(thiocarbonohydrazino)isoxazolidines and -Δ2-isoxazolines. 1H and 13C NMR spectroscopy revealed a tendency of the indicated compounds toward ring-chain and ring-ring tautomeric conversions in solutions involving the 1,2,4-triazolidine, Δ2-pyrazoline, and 1,2,4,5-tetrazine rings.

The coexistence in solution of tautomeric isoxazoline and pyrazoline forms of 1,3-alkanoylhydrazonoximes of acetylacetone has been detected and investigated by1H and13C NMR spectroscopic methods. The compounds indicated eliminate hydroxylamine under the action of acid catalysts, forming 1-acyl-3,5-dimethylpyrazoles.

It has been shown by 1H NMR spectroscopic methods that the previously unknown N-methyl-2-amino-benzoyl- and 2-mercaptobenzoylhydrazones of phenylacetic and 3-phenylpropionic aldehydes have cyclic benzo-1,2,4-triazepine and benzo-1,3,4-thiadiazepine structures respectively.

The reaction of 5-hydroxy-3,3,5-trimethylisoxazolidine with methyl, ethyl, and tert-butyl acrylates, proceeds as Michael addition, and produces previously, unknown 5-hydroxy-3,3,5-trimethylisoxazolidine-2-propionic acid esters. The structures of the obtained products are proved by1H and13C NMR spectroscopy and by their subsequent transformation into 5-hydroxy-3,3,5-trimethylisoxazolidine-2-propionic acid.

The reaction of N-benzylhydroxylamine with α,Β-unsaturated carbonyl compounds provides a method of synthesis of 5-hydroxy-2-benzyloxazolidines, nucleophilic replacement of the hydroxyl group in which gives the corresponding 5-amino- and 5-hydrazinoisoxazolidines.

The corresponding previously unknown 5-amino-2-isoxazolines were obtained by the action of ammonia and primary amines on acetylacetone monoxime and on 3,4,4-trimethyl-5-methylene-2-isoxazoline. Their structure was confirmed by NRM spectroscopy.

The reaction of methyl vinyl ketone with N-Phenyl-hydroxylamine leads to a tautomeric mixture of 5-hydroxy-2-phenyl-5-methylisoxazolidine and its linear form, while the reaction with benzohydroxamic acid leads to linear products of addition to the oxygen or nitrogen atoms, depending on the reaction conditions. The first representatives of 5-hydroxyisoxazolidines with the residue of an aliphatic carboxylic acid attached to the nitrogen atom were obtained by acylation of 5-hydroxy-3,3,5-trimethylisoxazolidine.

Reactions of N-phenylhydroxylamine with the simplest alkanals leads initially to the formation of N-phenylnitrones, dimerization of which gives 5-substituted isoxazolidine. Taking the reactions of N-phenylnitrones, benzaldehyde, and acetone as an example, the possibility of a crossover reaction has been established.

Nucleophilic substitution of the hydroxy-group in 3- (or 5-)hydroxyisoxazolidines affords the aryl(alkyl)amino-, alkoxy-, and hydrazino-compounds. 5-Dimethylhydrazinooxazolidines exist preferentially in the linear form.

It was established by the methods of 1H and 13C NMR spectroscopy that the oximohydrazones and bis(hydrazones) of aroylacetic aldehydes exist in the form of the corresponding 5-hydrazino- and 5-hydroxyamino-3-aryl-2-pyrazolines and -2-isoxazolines. The features of the aromatization of these compounds to the corresponding pyrazoles and isoxazoles were studied. Data on the mass-spectral study of the 5-hydroxyamino-2-pyrazoline-5-hydrazino-2-isoxazoline tautomerism in the gas phase are presented.

The reactions of N-substituted hydroxylamines with alkenals serve as a method for the synthesis of the corresponding 2-substituted 3(5)-hydroxyisoxazolidines. The reaction pathway is determined by the nature of the substituent attached to the nitrogen atom. Ring-chain isomerism has been detected in these newly obtained compounds