Provided is a compound represented by the general formula (I) or a salt thereof: ##STR00001## wherein, --XY-- represents --CH.dbd.CH-- and the like, between R.sup.2 and the carbon atom at 5-position represents a single bond or a double bond, R.sup.1 represents a lower alkyl group, a formyl group, a carboxyl group, a lower alkoxycarbonyl group (wherein a lower alkyl moiety of said lower alkoxycarbonyl group may be substituted with a heterocyclic group) and the like, and R.sup.1a represents a hydrogen atom, provided when R.sup.1 represents a lower alkoxycarbonyl group and the like, R.sup.1a may further represents a lower alkoxycarbonyl group and the like, when between R.sup.2 and the carbon atom at 5-position is a single bond, R.sup.2 represents a hydroxyl group and the like, and when between R.sup.2 and the carbon atom at the 5-position is a double bound, R.sup.2 combines with the carbon atom at 5-position to form a hydroxime group (--C(.dbd.NOH)) and the like, and R.sup.3 represents a hydroxyl group or a tri(lower alkyl)silyloxy group.