A process for manufacturing a compound of Formula (I) which has cis-conformation and wherein R1 represents a 1-phenyl-C1-C4alkyl or 1-naphthyl-C1-C4alkyl group, wherein the phenyl or naphthyl moiety of R1 is unsubstituted or substituted with one or more C1-C4alkoxy groups and the carbon atoms in 2-, 3-, and/or 4-position of the alkyl part of R1 are, independently of the phenyl or naphthyl moiety of R1 and independently of one another, unsubstituted or substituted with C1-C4alkoxy and/or silyloxy or, preferably, are unsubstituted or substituted with one C1-C4alkoxy group and/or silyloxy group per carbon atom, and R2 represents a C1-C6alkyl group or an unsubstituted or substituted benzyl group, in which process a compound of Formula (II) wherein R3 represents a C1-C6alkyl group or an unsubstituted or substituted benzyl group, and R1 and R2 have the same meaning as in formula (I); is treated with a base at a temperature of 0° C. or less in a liquid aprotic solvent for a time period sufficient to obtain the compound of formula (I).