This invention discloses and claims a compound conforming to the general formula (I): ##STR00001## Wherein R.sub.1, R.sub.2, R'.sub.2, R.sub.3, R.sub.4 and R.sub.5 are as described herein. The compounds of the present invention exhibit an inhibitory effect on the production of .beta.-amyloid peptide (.beta.-A4) by inhibition of gamma protease. Therefore, the compounds of the present invention are useful in the treatment of pathologies such as senile dementia, Alzheimer's disease, Down's syndrome, Parkinson's disease, amyloid angiopathy and/or cerebrovascular disorders.