Alternatively, compound of the formula ##STR21## which can be obtained by the method disclosed in TOKKAI-Sho 62-201843 (Japanese unexamined patent application publication No. 201843/'87), is esterified with a phenol, e.g. with ##STR22## wherein R is an alkyl or alkoxy group to give compound of ##STR23## The compound is then separated by preparative chromatography into its cis and trans forms, and