Gas-phase conversion of tetrazoles to oxadiazoles: isolation and characterization of the N-acylated intermediate.

The 5-substituted tetrazole ring was reacted in the gas phase with an acyl ion generated as the secondary reactive chemical ionization plasma in the mass spectrometer. The product ions obtained, among others, were proposed as the rearranged 2,5-disubstituted-1,3,4-oxadiazole nucleus. Its structure was demonstrated by comparison of the product ion spectrum of the 2,5-disubstituted-1,3,4-oxadiazole derivative obtained by condensed-phase reaction and the product ion spectrum of the oxadiazole derivative generated in situ by reaction of the 5-substituted tetrazole derivative with the acyl plasma. It was proposed that the mechanism of this transformation involved the presence of an acylated tetrazole intermediary, which could not be isolated in the condensed phase, followed by the rearrangement, with nitrogen loss, to afford the oxadiazole derivative. Under our conditions we were able to isolate the intermediate ion in the first field free region and demonstrate its structure by collision induced dissociation-tandem mass spectrometry.