The present invention relates to a compounds of formula I
wherein R
1
, R
1′
, R
2
, R
3
, R
4
, X, Ar, and m are as defined in the specification and claims and pharmaceutically active acid addition salts thereof. Compounds of the invention have Asp2 (β-secretase, BACE 1 or Memapsin-2) inhibitory activity and are useful for the treatment of diseases characterized by elevated β-amyloid levels or β-amyloid deposits, particularly Alzheimer's disease.