Pyrimidine derivatives of formula I, wherein R.sup.1, R.sup.2, R.sup.3, R.sup.4, R.sup.5, R.sup.6, R.sup.7, R.sup.8 and R.sup.9 are as defined in the specification, as a base or a pharmaceutically acceptable salt thereof in the manufacture of pharmaceutical compositions and in the treatment or prophylaxis of Alzheimer's Disease.