Provided is a method of resolving a racemic mixture of a compound of formula I to obtain a desired enantiomer: wherein Ar is C6 or C10 aromatic group that can be substituted with H, C1 to C6 alkyl, trifluoromethyl or halo, R5 is halo or —S—R1, wherein R1 is H or acetyl, and R2 is H or C1 to C6 alkyl, the method comprising: reacting a compound of formula I wherein the compound is an ester whereby R2 is C1 to C6 alkyl with a lipase derived from Mucor meihei to stereoselectively hydrolyze the ester bond to produce an acid; and isolating the acid, wherein the reaction is conducted in a solvent comprising 80% to 98% v/v % organic phase and a residue of water phase (which can be buffered).