Direct Synthesis of 5-Methyl-3-aryl-1,2,4-oxadiazoles from Aryl Aldehydes, Nitroethane, and Ammonium Acetate

Article abstract of DOI:10.1021/jo00208a008

The condensation of 2,5-dimethoxybenzaldehyde (1b) with nitroethane and ammonium acetate in glacial acetic acid has been found to give three diffrent products, depending on reactant ratio and reaction time.At an aldehyde:nitroethane:ammonium acetate ratio of 1:1.5:0.8 a normal Knoevenagel condensation occured, yielding 1-(2,5-dimethoxyphenyl)-2-nitropropene.At a reactant ratio of 1:3:2 (same reactant sequence), the primary product was 2,5-dimethoxybenzonitrile, and at a reactant ratio of 1:40:8, with extended reflux time, the major product was 3-(2,5-dimethoxyphenyl)-5-methyl-1,2,4-oxadiazole (8b).This last reaction served as a prototype for a new oxadiazole synthesis which was then extended to include six additional 5-methyl-3-aryl-1,2,4-oxadiazoles (8a,c-g; where aryl = Ph, 2,5-dimethoxyphenyl, 2,4-dichlorophenyl, m-chlorophenyl, p-tolyl, 3,5-dimethoxyphenyl, and p-carboxyphenyl), whose structures were assigned on the basis of 13C NMR characteristics of known reference compounds.Benzonitrile also reacted with excess nitroethane and ammonium acetate to yield 5-methyl-3-phenyl-1,2,4-oxadiazole (8a).The overall mechanism of oxadiazole formation is shown to be dependent on a preliminary reaction wherein the nitroalkane, in the presence of ammonium acetate and acetic acid, is first transformed into the corresponding alkanoic acid and hydroxylamine.Hydroxylamine then converts the aromatic aldehyde, via the intermediary nitrile, to the oxadiazoles following reactions of established precedent.