Compounds having the formula (I): ##STR00001## wherein A represents a group such as a cyclic group, R.sup.1 and R.sup.2 represent groups such as alkyl groups or hydroxymethyl groups, and n represents 1 or 2, or pharmacologically acceptable salts thereof or pharmacologically acceptable esters thereof have superior activity and stability, and are useful for the treatment and/or prevention of diabetes mellitus, or the like.