Abstract: The present invention relates to substantially pure stereoisomers of a compound of the formula: ##STR1## wherein n.sub.1 and n.sub.2 are the same or different and are 1 or 2; X is alkyl (C.sub.1 -C.sub.10), aryl (C.sub.6 -C.sub.10) or aralkyl (C.sub.7 -C.sub.12); and wherein R, R.sub.1, R.sub.2 and R.sub.3 are the same or different and are chosen from H, alkyl (C.sub.1 -C.sub.10), aryl (C.sub.6 -C.sub.10), aralkyl (C.sub.7 -C.sub.12), or a heterocyclic group, and addition salts thereof with pharmaceutically acceptable acids. The invention also relates to a method for the inhibition of blood platelet aggregation in a blood supply comprising administering to said blood supply a blood platelet aggregation inhibiting amount of the compounds of the present invention.