This invention relates to a compound of formula I: ##STR1## or a pharmaceutically acceptable salt thereof; in which preferably R.sup.3, R.sup.4 and R.sup.6 are each hydrogen; X is C.dbd.O or CH.sub.2 ; and R.sup.7 and R.sup.8 are each independently selected from the group consisting of hydrogen, (C.sub.1 -C.sub.12)alkyl, (C.sub.3 -C.sub.8)cycloalkyl and (C.sub.1 -C.sub.12)alkyl (C.sub.6 -C.sub.14)aryl; or R.sup.7 and R.sup.8 when taken together form a (C.sub.2 -C.sub.7)alkylene group; or --NR.sup.7 R.sup.8 together forms a (C.sub.2 -C.sub.14)heterocyclic or substituted (C.sub.2 -C.sub.14)heterocyclic. Such compounds modulate the activity of serine hydrolases and can be used in pharmaceutical compositions for the treatment of Alzheimer's disease.