There is provided a series of tetrahydroisoquinoline diaminopropane compounds of Formula (I) or a stereoisomer; or a pharmaceutically acceptable salt thereof, wherein R, R8 and R9 are as defined herein, their pharmaceutical compositions and methods of use. These compounds inhibit the processing of amyloid precursor protein (APP) by β-secretase and, more specifically, inhibit the production of Aβ-peptide. The present disclosure is directed to compounds useful in the treatment of neurological disorders related to β-amyloid production, such as Alzheimer's disease and other conditions affected by anti-amyloid activity.