Disclosed are compounds of the formula ##STR00001## or a pharmaceutically acceptable salt or solvate thereof, wherein R.sup.1 is ##STR00002## R is --C(O)--N(R.sup.27)(R.sup.28) or ##STR00003## and the remaining variables are as defined in the specification.Also disclosed are pharmaceutical compositions comprising the compounds of formula I.Also disclosed are methods of treating cognitive or neurodegenerative diseases such as Alzheimer's disease.Also disclosed are pharmaceutical compositions and methods of treating cognitive or neurodegenerative diseases comprising the compounds of formula I in combination with a .beta.-secretase inhibitor other than those of formula I, an HMG-CoA reductase inhibitor, a gamma-secretase inhibitor, a non-steroidal anti-inflammatory agent, an N-methyl-D-aspartate receptor antagonist, a cholinesterase inhibitor or an anti-amyloid antibody.