A compound represented by the following formula or a salt thereof: wherein A is a hydrogen atom or a fluorine atom, R.sup.1 is --CONH.sub.2 or --CN and R.sup.2 is a hydrogen atom, a tert-butoxycarbonyl group, a trityl group, an o-nitrobenzenesulfenyl group, a benzyloxycarbonyl group, a fluorenyloxycarbonyl group, an allyloxycarbonyl group or --C(.dbd.O)--CH.sub.2-Rc wherein Rc is a halogen atom, a methanesulfonyloxy group, a p-toluenesulfonyloxy group or a hydroxyl group.