The invention relates to compounds of formula (I) R 1 is -C(O)O-lower alkyl, cyano or is hetaryl; hetaryl is a five-or six membered heteraryl group, optionally substituted by R'; R' is halogen, lower alkyl, lower alkyl substituted by halogen, lower alkoxy or lower alkoxy substituted by halogen; R 2 is hydrogen, lower alkoxy, lower alkyl, halogen or cyano; R 3 is -(CH 2 ) n -C(O)O-lower alkyl, lower alkyl, lower alkoxy, hydroxy, -O-Si(CH 3 ) 2 -lower alkyl, -C(O)-N(lower alkyl) 2 , -O-S(O) 2 -lower alkyl, C 3-7 -cycloalkyl, S(O) 2 -aryl, heterocyclyl, -C(O)-heterocyclyl, or is aryl or hetaryl, which aryl or hetaryl rings are optionally substituted by one or more R'; R 4 is hydrogen, lower alkyl, hydroxy or CH 2 CN; X is S or -N=C(R 5 )-; R 5 is hydrogen, lower alkyl or hydroxy, is a bond, -O-, -CH 2 -, -CH 2 -CH 2 -, formula (II) or or -N(R)-; R is hydrogen, lower alkyl, C(O)O-lower alkyl, C(O)-lower alkyl, S(O) 2 -lower alkyl, or benzyl; n is 0 or 1 or to pharmaceutically active acid addition salts. The compounds may be used for the treatment of Alzheimer's disease, cerebral amyloid angiopathy, hereditary cerebral hemorrhage with amyloidosis, Dutch-type (HCHWA-D), multi-infarct dementia, dementia pugilistica or Down syndrome.