This invention relates to compounds of the formulae:
wherein
A1 is O, S, N—R1 or CHR1;
A4 is N—R4 or CHR4;
R2 is a sidechain containing an acid or ester group;
R1, R4 and R5 are substituents such as H, alkyl and aryl alkyl, and
R6 is a sidechain containing a nitrogen group; and
pharmaceutically acceptable salts thereof,
which are effective for inhibiting platelet aggregation, pharmaceutical compositions for effecting such activity, and a method for inhibiting platelet aggregation.