Disclosed are novel compounds of the formula or a pharmaceutically acceptable salt or solvate thereof, wherein R1 is X is —O—, —C(R14)2— or —N(R)—; Z is —C(R14)2— or —N(R)—; t is 0, 1, 2 or 3; each R and R2 is independently H, alkyl, cycloalkyl, cycloalkylalkyl, aryl, heteroaryl, heterocycloalkyl, arylalkyl, heteroarylalkyl, heterocycloalkylalkyl, alkenyl or alkynyl; each R14 is H, alkyl, alkenyl, alkynyl, halo, —CN, haloalkyl, cycloalkyl, cycloalkylalkyl, aryl, heteroaryl, heterocycloalkyl, arylalkyl, heteroarylalkyl, heterocycloalkylalkyl, —OR35, —N(R24)(R25) or —SR35; R41 is alkyl, cycloalkyl, —SO2(alkyl), —C(O)-alkyl, —C(O)-cycloalkyl or -alkyl-NH—C(O)CH3; 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.