The present invention relates to compounds of formula I ##STR1## wherein R.sup.1 is a group selected from 2-, 3-, and 4-hydroxyphenyl, 3-nitrophenyl, and 3-fluoro-4-hydroxyphenyl; as well as readily hydroyzable esters thereof, pharmaceutically acceptable salts of said compounds and hydrates of the compounds of formula I and of their esters and salts.