Provided is a method of preparing an optically pure compound having formula 1 or its salts. The method includes: reacting (R)-2-(4-methoxy-3-aminosulfonyl-phenyl)-1 -methylethylamine or its salts with a compound selected from the group consisting of chloroacetic acid, bromoacetic acid, fluoroacetic acid, iodoacetic acid, .alpha.-halogenoacetic acid anhydride, and .alpha.-halogenoacetyl halide in the presence of a base or an acylating agent.