Kaken Pharmaceutical Company, Ltd.

Yasuhito Iida

Nobuo Ishiyama

Kenji Seri

Kazuko Sanai

Katuyuki Ohtuka

Yoshihiro Ishizaka

Takeshi Murai

A compound of the formula (I): ##STR1## or a pharmaceutically acceptable salt thereof, wherein: R.sup.1 is a hydrogen atom, a substituted or unsubstituted C.sub.1-6 alkyl group, a C.sub.2-6 alkenyl group, a C.sub.2-6 alkynyl group or a R.sup.7 --CO-- group (wherein R.sup.7 is a C.sub.1-6 alkyl group, a phenyl group, a C.sub.2-6 alkenyl group which may be substituted by a phenyl group, or a C.sub.1-6 alkoxy group);

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Otsu, Japan

Yasuhito Iida