There are disclosed various compounds of the formula below, ##STR1## where X is O, S, NH, H-loweralkyl, or N-arylloweralkyl;Y is hydrogen, fluorine, chlorine, bromine, nitro, loweralkyl, loweralkoxy, or triloweralkylsilyl;R.sub.1 is loweralkyl, halogen-substituted loweralkyl, aryl, arylloweralkyl, cycloalkyl, heteroaryl or heteroarylloweralkyl;R.sub.2 is hydrogen, loweralkyl or arylloweralkyl;R.sub.3 is loweralkyl or arylloweralkyl;R.sub.4 is hydrogen, loweralkyl, loweralkenyl, loweralkynyl, arylloweralkyl, formyl, loweralkylcarbonyl, arylloweralkylcarbonyl or loweralkoxycarbonyl;R.sub.5 is hydrogen or loweralkyl; andm is 0 or 1, with the proviso that when m is O, R.sub.1 may also be hydrogen,which compounds are useful for alleviating various memory dysfunctions characterized by a cholinergic deficit such as Alzheimer's disease.