nthesis, for example according to the process illustrated in the attached FIG. 1.As indicated in FIG. 1, a commercially available phenethylic acid derivative A, in which Ra denotes a hydrogen atom or a methyl, ethyl, isopropyl, tert-butyl or benzyl group, is reacted with a compound B in which Rb denotes a protecting group such as a benzyl group, in the presence of a base in an anhydrous organic me