This invention provides compounds of formula I having the structure ##STR00001## wherein: R.sub.1 and R.sub.2 are H, alkyl, cycloalkyl, alkoxy, halogen, fluorinated alkyl, --CN, --NH--SO.sub.2-alkyl, --SO.sub.2--NH-alkyl, alkyl amide, amino, alkylamino, dialkylamino, fluorinated, acyl, or aroyl; R.sub.3, R.sub.4 are H, alkyl, cycloalkyl or --CH.sub.2-cycloalkyl; R.sub.5 is H or alkyl; R.sub.6 is H or alkyl; and wherein the dashed line indicates an optional double bond; or a pharmaceutically acceptable salt thereof, as well as methods for using these compounds to treat central nervous system disorders, including obsessive-compulsive disorder, depression, anxiety, generalized anxiety disorder, schizophrenia, migraine, sleep disorders, eating disorders, obesity, epilepsy, and spinal cord injury.