. . . . "wherein R' and B are as defined above, and X is a leaving group such as fluorine, chlorine, bromine, iodine, methanesulfonyloxy, p-toluenesulfonyloxy, trifluoromethanesulfonyloxy or the like, in the presence of a base in an inert solvent (e.g. N,N-dimethylformamide, dimethyl sulfoxide or tetrahydrofuran)." .