The present invention relates to a compound of formula I, 1-1 or 1-2 wherein R1 is hydrogen, lower alkyl, lower alkyl substituted by halogen, halogen, lower alkoxy, or lower alkoxy substituted by halogen; R1 may be different if n=2 or 3 n is 1, 2 or 3 Ar is a six membered heteroaryl group, selected from wherein R2 is hydrogen, lower alkyl, lower alkyl substituted by halogen, halogen or lower alkoxy; R3 is hydrogen or halogen; or to a pharmaceutically active acid addition salt thereof. 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.