The present invention provides compounds of the general formula wherein R<;SUP>;4 <;/SUP>;is one of the following groups and R<;SUP>;1<;/SUP>;, R<;SUP>;2<;/SUP>;, R<;SUP>;3<;/SUP>;, R<;SUP>;7<;/SUP>;, R<;SUP>;8<;/SUP>;, and R<;SUP>;9 <;/SUP>;are as defined in the specification and pharmaceutically acceptable salts, optically pure enantiomers, racemates or diastereomeric mixtures thereof. The compounds are useful for the treatment of Alzheimer's disease.