There are provided compounds of the formula: ##STR1## wherein R.sub.1 is a hydrogen or C.sub.1 -C.sub.5 alkyl; R.sub.2 is an C.sub.1 -C.sub.10 alkyl, aryl, aralkyl, or pyridyl, where the aryl, aralkyl, or pyridyl are unsubstituted or are substituted by C.sub.1 -C.sub.5 alkyl, C.sub.1 -C.sub.5 alkoxy, nitro, hydroxy, carboxy, amino, C.sub.1 -C.sub.5 alkylamino, C.sub.1 -C.sub.5 dialkyamino whose two alkyl groups may be different from each other, C.sub.2 -C.sub.3 alkanoylamino, or halogen and X is an C.sub.1 -C.sub.7 alkylene, C.sub.2 -C.sub.7 alkenylene, --CH(F)CH.dbd.CH(CH.sub.2).sub.3 --, --(CH.sub.2).sub.3 --S--(CH.sub.2).sub.2 --, --(CH.sub.2).sub.2 --S--(CH.sub.2).sub.3 --, --C.sub.6 H.sub.4 -O--CH.sub.2 -- or --CH.sub.2 CH.dbd.CH--m--C.sub.6 H.sub.4 --, or their pharmaceutically accepatble salts. The compounds are useful as antithrombotic, anti-vasconstricting and anti-bronchoconstricting drugs.