The present invention provides compounds of the general formula ##STR00001## wherein R.sup.4 is one of the following groups ##STR00002## and R.sup.1, R.sup.2, R.sup.3, R.sup.7, R.sup.8, and R.sup.9 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.