A facile procedure for the preparation of pyrrolo[2,1-c][1,4]benzodiazepine(PBD) imines via ethanethiol deprotective cyclization by using a mild and efficient clay supported ammonium nitrate catalyst is described. A significant improvement in yield over the customary HgCl2/HgO deprotective cyclization method is observed and the reaction proceeds with no detectable racemization.