Process for the separation of (S,S) diastereoisomers

Claim 1

A process for separating the (S,S) diastereoisomer of a compound of the formula ##STR7## from a solution containing said (S,S) isomer and one or both of the corresponding (R,S) and (S,R) diastereoisomers in a non-polar, water-immiscible organic solvent selected from the group consisting of toluene, methylene chloride, diethyl ether, chloroform, ethyl acetate, and mixtures thereof, wherein R 6 is t-butyl or benzyl, comprising (i) reacting said diastereoisomers with an acid, selected from the group consisting of maleic, tartaric, p-nitrocinnamic, cinnamic, hippuric, citric, and 2,4-dichlorobenzoic acids, which forms acid addition salts with said diastereoisomers which salts are characterized in that the salts of the (R,S) and (S,R) diastereoisomers are soluble in said solution and said salt of the (S,S) diastereoisomer is not, under conditions wherein the salt of the (S,S) diastereoisomer selectively precipitates from said solution, and (ii) converting said precipitated salt to the (S,S) compound of formula (A).