Formation of a new ring system: Tetrazolo[5,1-f][1,2]azaborinin

Phenothiazinyldienes obtained from tetrazolopyridinium salts and phenothiazine derivatives were subjected to reduction by borane–dimethyl sulfide in THF. Structure elucidation of the products revealed that one of the olefinic bond underwent reduction and, furthermore, borane addition at the double bond took place to yield derivatives of tetrazolo[5,1-f][1,2]azaborinin as a new fused ring system involving a bridge-head nitrogen atom. The new products have been synthesized and characterized by X-ray analysis, solution and solid-state NMR.