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

Abstract

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.