The present invention relates to a method of preparing a compound of Formula II-a′ or Formula II-b′, wherein RI represents hydrogen or a linear, branched and/or cyclic alkyl or alkenyl group having 1 to 10 carbon atoms;RII represents a linear, branched and/or cyclic alkyl or alkenyl group having 1 to 10 carbon atoms; RIII represents hydrogen or a linear, branched and/or cyclic alkyl or alkenyl group having 1 to 10 carbon atoms or a linear, branched and/or cyclic alkoxy group having 1 to 10 carbon atoms; RIV represents hydrogen or a linear, branched and/or cyclic alkyl group having 1 to 10 carbon atoms or an optionally substituted aryl or alkylaryl group having 6 to 40 carbon atom or acetyl or silyl or a protective group; and RV represents hydrogen or a methyl group; wherein RI and RII are different from each other.