The invention provides PDE9-inhibiting compounds of Formula (I), ##STR00001## and pharmaceutically acceptable salts thereof, wherein R, R.sub.1, R.sub.2 and R.sub.3 are as defined herein. Pharmaceutical compositions containing the compounds of Formula I, and uses thereof in treating neurodegenerative and cognitive disorders, such as Alzheimer's disease and schizophrenia, are also provided.