The invention relates to compounds of the formula I ##STR00001## wherein R, R.sup.1, R.sup.2, R.sup.3, X, and n are defined in the specification. The invention also provides pharmaceutically suitable acid addition salts thereof and all forms of optically pure enantiomers, recemates or diastereomers and diastereomeric mixtures thereof. Compounds of the invention are useful for the treatment of Alzheimer's disease.