The invention relates to compounds of general formula ##STR00001## wherein R.sup.1, R.sup.2, R.sup.3/R.sup.3', R.sup.4/R.sup.4' and R.sup.5/R.sup.5' are as defined in the specification and to pharmaceutically acceptable acid addition salts, optically pure enantiomers, racemates or diastereomeric mixtures thereof. The compounds are .gamma.-secretase inhibitors which can be useful in the treatment of Alzheimer's disease or common cancers including, but not limited to, cervical carcinomas and breast carcinomas and malignancies of the hematopoietic system.