Described herein is a process for the preparation of a mixture of compounds of formula having a weight ratio of the cis-diastereomers to trans-diastereoisomers higher than 1:1, where R1 represents a C1-8 alkyl group, a C2-8 alkenyl group or a C2-8 alkynyl group, each optionally substituted with one or two of a C1-4 alkyl alkoxy ether group and/or a C1-4 alkyl carboxylester group and R2 represents a C1-6 alkyl, a C2-6 alkenyl or a C2-6 alkynyl group, each optionally substituted with a C1-4 alkyl alkoxy ether group, a carboxylic acid group or a C1-4 alkyl carboxylester group and compounds suitable in the process.