The present invention relates to a compound of formula I, HetAr is a five or six membered heteroaryl group, selected from wherein R1 is hydrogen, halogen, lower alkyl, lower alkoxy or lower alkyl substituted by halogen, and may be the same or different, if two R1 occur; R2 is lower alkyl or a mono- or polydeutered derivative thereof, lower alkyl substituted by halogen or lower alkoxy, lower alkenyl unsubstituted or substituted by halogen, cycloalkyl or CH2-cycloalkyl unsubstituted or substituted by halogen, heterocycloalkyl or CH2-heterocycloalkyl unsubstituted or substituted by lower alkyl; R3 is halogen, lower alkyl, lower alkyl substituted by halogen, lower alkoxy substituted by halogen or S(O)2-lower alkyl; n is 1, 2 or 3; if n is >1, then R3 may be the same or different; or to a pharmaceutically active acid addition salt thereof, to a racemic mixture or to its corresponding enantiomer and/or an optical isomer and/or stereoisomer 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.