Disclosed are novel compounds of the formula ##STR00001## or a pharmaceutically acceptable salt or solvate thereof, wherein R.sup.1 is ##STR00002## X is --O--, --C(R.sup.14).sub.2-- or --N(R)--; Z is --C(R.sup.14).sub.2-- or --N(R)--; t is 0, 1, 2 or 3; each R and R.sup.2 is independently H, alkyl, cycloalkyl, cycloalkylalkyl, aryl, heteroaryl, heterocycloalkyl, arylalkyl, heteroarylalkyl, heterocycloalkylalkyl, alkenyl or alkynyl; each R.sup.14 is H, alkyl, alkenyl, alkynyl, halo, --CN, haloalkyl, cycloalkyl, cycloalkylalkyl, aryl, heteroaryl, heterocycloalkyl, arylalkyl, heteroarylalkyl, heterocycloalkylalkyl, --OR.sup.35, --N(R.sup.24)(R.sup.25) or --SR.sup.35; R.sup.41 is alkyl, cycloalkyl, --SO.sub.2(alkyl), --C(O)-alkyl, --C(O)-cycloalkyl or -alkyl-NH--C(O)CH.sub.3; and the remaining variables are as defined in the specification. Also disclosed are pharmaceutical compositions comprising the compounds of formula I and methods of treating cognitive or neurodegenerative diseases with compounds of formula I.Also disclosed are pharmaceutical compositions and methods of treatment comprising compounds of formula I in combination with other agents useful in treating cognitive or neurodegenerative diseases.