compounds having the structure ## equ1 ## and the pharmaceutically acceptable salts thereof , wherein r 1 and r 2 are the same or different and are selected from alkyl , cycloalkyl , aryl and arylalkyl ; r 3 is hydrogen , halogen , trifluoromethyl , alkoxy , aryloxy , or dialkylamidosulfonyl ; r 4 is hydrogen , halogen , alkyl , alkoxy or aryl ; and n is 1 or 2 , are useful central nervous system stimulants .