A novel optically active cis-4,5-disubstituted imidazolidin-2-one derivative of the formula: ##STR1## wherein R.sup.1 is a C.sub.1 -C.sub.4 alkyl group or benzyl and R.sup.2 is a chiral aralkyl group optionally having at least one of C.sub.1 -C.sub.4 alkyl, C.sub.1 -C.sub.4 alkoxy and hydroxyl groups is produced asymmetrically by the reaction of 1,3-dibenzyl-cis-4,5-dicarboxy-imidazolidin-2-one or its anhydride with an optically active secondary amine of the formula: ##STR2## wherein R.sup.1 and R.sup.2 are each as defined above and is transformed into the lactone of 1,3-dibenzyl-cis-4-carboxy-5-hydroxymethyl-imidazolidin-2-one, which is a key intermediate in the synthesis of d-biotin.