he same or different represent a hydrogen atom or a lower alkyl group or, when taken together, may form a 3- to 6-membered ring, Ar is an optionally substituted 5- or 6-membered aromatic heterocyclic group, and L is a leaving group such as a chlorine atom, a bromine atom, an iodine atom, a trifluoromethanesulfonyloxy group, a p-toluenesulfonyloxy group or a methanesulfonyloxy group.