Two-step synthesis of imidazoles from activated alkynes

Abstract

Conjugate addition of 2-(tri-n-butylstannyl) tetrazoles (1) to activated alkynes gives 1-alkenyltetrazoles (4) and (5) predominantly. Use of the N-tributylstannyl derivatives, rather than the parent tetrazole, gives a high ratio of 1- to 2-alkenyl isomers and avoids the complication of further addition of the tetrazole or the alkyne to the initial adduct. Irradiation of the (Z)- and (E)-1-alkenyltetrazoles (4) and (5) at 254 nm then gives the expected imidazoles in moderate yield. However, 5-phenyl-2-(tri-n-butylstannyl)tetrazole (1a) reacted only slowly with ethyl phenylpropiolate to give ethyl 3,5-diphenylpyrazole-4-carboxylate (8), presumably via the 2-alkenyltetrazole (9) formed in preference to the 1-isomer for steric reasons.