The thesis is dedicated to the study of heterocyclization reactions of 3-amino and 5-aminoisoxazoles, unsubstituted in the position C4, aromatic aldehydes, and their derivatives.
The reactions of 5-aminoisoxazoles, aldehydes, and Meldrum`s acid or N,N`-dimethylbarbituric acid without heating result in the formation of 4,6-diaryl-6,7-dihydroisoxazolo[5,4-b]pyridines. At the same time refluxing in n-butanol the starting mixture of 5-amino-3-methyl(tert-butyl)isoxazole, aromatic aldehydes, and Meldrum`s acid leads to the 4,7-dihydroisoxazolo[5,4-b]pyridine-6(5H)ones. 3-Amino-5-methyl¬isoxazole, in the reactions with the same reagents, does not lead to the formation of new heterocyclic compounds.
It is shown that isoxazole moiety decomposes in condensations of pyruvic acids and 5-aminoisoxazoles due to its low stability in acidic media. On the other hand, domino-reactions with ethyl 4-aryl-2-oxobut-3-enoates in the presence of scandium triflate in acetonitrile open route to the isoxazolo[5,4-b]pyridines and their heteroaromatic analogs in case of 3-methyl derivatives. The unstable products of β-addition of the amine involving C4 nucleophilic center to α,β-unsaturated ketoester as well as 6-hydroxytetrahydropyridine have been isolated as intermediates when the condensation reactions have been carried out at lower temperatures. It is shown that the hydroxyl group in 6-hydroxypyridines undergoes nucleophilic substitution reactions: application of the ethanol as a solvent leads to the formation of the 6-ethoxy derivatives, while treating starting ethyl 4-aryl-2-oxobut-3-enoates or 6-hydroxytetrahydropyridines with an excess of 5-amino-3-methyl(phe¬nyl)isoxazole in acetonitrile/scandium triflate system results in the substitution involving C4 nucleophilic center of isoxazole moiety.
In the reactions of 3-amino-5-methylisoxazole with pyruvic acid`s derivatives, the only exocyclic amino group is involved in condensations. Depending on the substituents in α,β-unsaturated pyruvic acid`s derivatives, different furan-2(5H)ones, and 1,5-2H-pyrrole-2-ones have been formed.
A comparative analysis of the electronic structures of 5-amino-3-methyl and
3-amino-5-methylisoxazole, according to X-ray diffraction, 13C and 15N NMR spectroscopy data has been carried out.
