This invention relates to a compound of formula (I) ##STR1## wherein R.sub.1 and R.sub.2 are each independently hydrogen, C.sub.1 -C.sub.3 alkyl or C.sub.1 -C.sub.3 alkoxy; and R.sub.3 is a C.sub.7 -C.sub.9 aralkyl, the phenyl nucleus of which can be mono-, di-, or trisubstituted by halogen, C.sub.1 -C.sub.3 alkyl or C.sub.1 -C.sub.3 alkoxy, and nontoxic, pharmaceutically acceptable salts thereof. The compounds have useful antithrombic properties.