The present invention relates to a compound of formula (I), wherein R1 is hydrogen, lower alkyl, lower alkyl substituted by halogen, halogen, lower alkoxy or lower alkoxy substituted by halogen; R may be the same or different, if n=2 or 3; n is 1, 2 or 3; m is 1, 2 or 3; Ar is a five or six membered heteroaryl group, selected from (II), (III), (IV) or (V) wherein R2 is hydrogen, lower alkyl, lower alkyl substituted by halogen, halogen or lower alkoxy; R3 is hydrogen or halogen; -( ) m - is —(CH2)m— or to pharmaceutically active acid addition salts 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-in-farct dementia, dementia pugilistica or Down syndrome.