The present invention relates to compounds of formula I ##STR00001## wherein R.sup.1 is as defined in the specification and to esters thereof which are hydrolyzable under physiological conditions and to the pharmaceutically acceptable salts thereof. The compounds of the invention are inhibitors of COMT and, thus, are useful for the treatment of diseases for which COMT inhibition is beneficial. The invention further relates to the treatment, control, or prevention of diseases such as depression, schizophrenia, Parkinson's disease, and to improve cognition.