Karoly Horvath

Ring transformation of 3-(2-aminoaryl)-1,2,4-oxadiazoles into 3-acylaminoindazoles; extension of the Boulton–Katritzky scheme

The ring transformation of 3-(2-aminoethyl)-1,2,4-oxadiazoles (1) into acylaminopyrazolines (2) reported earlier has been extended to 5-substituted 3-(2-aminoaryl)-1,2,4-oxadiazoles (3), (5), (7), and (9). Depending on the reaction conditions and the substitution at the amino-group, the isomerization of 3-(2-aminophenyl)oxadiazoles (3) to 3-acylaminoindazoles (4) follows two different mechanisms (A and B), but is invariably promoted by electron-attracting substituents at C-5. Mechanism A differs from those proposed earlier for azole–azole rearrangements, and resembles the transformation (1)→(2). An extended general scheme is suggested.

Theoretical considerations on the extended Boulton—Katritzky scheme

Abstract It has been shown that in the rearrangement of 3-(2-aminoethyl)- and 3-(2-aminoaryl)-1,2,4-oxadiazoles the saturation of the side chain plays no decisive role from a mechanistic point of view, but the length of the side chain is the determining factor. The π-electronic system of the azole ring is also a prerequisite for a successful ring transformation. A linear correlation has been found between reactivity and CNDO/2 net charges on the N2 atom of the oxadiazole ring. The relative stabilities of the reacting species were assessed by CNDO/2 and ab initio geometry optimization calculations. The conformation of the side chains in two homologous molecules was analysed by X-ray diffraction.

Nitrogen bridgehead compounds. Part 75. Study of tautomerism of 9-oxalyl derivatives of condensed pyrimidinones by 1H and 13C nuclear magnetic resonance and ultraviolet spectroscopy

The tautomerism of the title compounds has been investigated by 1H and 13C n.m.r. and u.v. spectroscopy. The effect of ring size, substituents, and various annellations of the tautomerism have also been studied. The ring size has been found to be decisive from the point of view of tautomerism.

Crystal and molecular structure of 5-diacetylamino-3,4-diphenylisoxazole

The structure of (IV) the acetylation product of 5-amino-3,4-diphenylisoxazole has been investigated by spectroscopic methods and determined from three-dimensional X-ray diffraction data. Crystals are orthorhombic, space group P212121 with Z= 4 in a unit cell of dimensions: a= 9·216(4), b= 9·324(5), and c= 19·043(8)A. The crystal structure has been solved by direct methods and refined by block-diagonal least-squares to R 0·063 (1282 independent reflections). The bond lengths of the isoxazole and of the phenyl rings are as expected. The two bonds linking the phenyl rings to the central isoxazole ring, however, are significantly shorter (1·449 and 1·450 A) than expected for the C(sp2)–C(sp2) bond (1·482 A). The planes of the phenyl rings and of the two acetyl groups are twisted differently about the bonds linking them to the isoxazole ring.