The invention relates to compounds of the formula ##STR00001## and their pharmaceutically acceptable salts wherein R.sup.1, R.sup.2, R.sup.3, R.sup.4, R.sup.5, n, m, and o are as defined in the specification. The compounds are selective monoamine oxidase .beta. inhibitors and are useful for the treatment and prevention of Alzheimer's disease and senile dementia, as well as other CNS disorders. The invention also relates to processes for preparing such compounds and pharmaceutical compositions containing them.