The present invention relates to novel compounds of formula (I) and their pharmaceutical compositions. In addition, the present invention relates to therapeutic methods for the treatment and/or prevention of A.beta.-related pathologies such as Downs syndrome, .beta.-amyloid angiopathy such as but not limited to cerebral amyloid angiopathy or hereditary cerebral hemorrhage, disorders associated with cognitive impairment such as but not limited to MCI ("mild cognitive impairment"), Alzheimer Disease, memory loss, attention deficit symptoms associated with Alzheimer disease, neurodegeneration associated with diseases such as Alzheimer disease or dementia including dementia of mixed vascular and degenerative origin, pre-senile dementia, senile dementia and dementia associated with Parkinson's disease, progressive supranuclear palsy or cortical basal degeneration. ##STR00001##