Disclosed are novel compounds of the formula or a pharmaceutically acceptable salt or solvate thereof, wherein R1, R2, R3, R4 and X 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 methods of treating a cognitive or neurodegenerative disease comprising administering to a patient I need of such treatment a combination of at least one compound of formula I and at least one compound selected from the group consisting of β-secretase inhibitors other than those of formula I, HMG-CoA reductase inhibitors, gamma-secretase inhibitors, non-steroidal anti-inflammatory agents, N-methyl-D-aspartate receptor antagonists, cholinesterase inhibitors and anti-amyloid antibodies.