Abstract: Compounds of formula (I): ##STR1## wherein R.sup.1 is hydrogen, halogen or alkyl; R.sup.2 is hydrogen, alkyl, optionally substituted phenyl or a heterocycle; or R.sup.1 and R.sup.2 together form a group of formula --CR.sup.4 .dbd.CR.sup.5 --CR.sup.6 .dbd.CR.sup.7 --, where R.sup.4, R.sup.5, R.sup.6 and R.sup.7 are hydrogen, halogen, alkyl group, alkoxy, halomethyl, alkylamino, dialkylamino, hydroxy, nitro, aliphatic carboxylic acylamino or amino; and R.sup.3 is piperidyl, substituted piperidyl or quinuclidinyl. The compounds are useful in the treatment and prophylaxis of cognitive disorders, notably senile dementia, including Alzheimer's disease. Processes for preparing the compounds are also provided.