The present invention provides compounds having the formula: ##STR00001## wherein R.sup.1, R', R.sup.2, R.sup.3, R.sup.3', R.sup.4, X.sup.1, X.sup.2 and X.sup.3 are as defined herein, and pharmaceutical compositions thereof. The present invention also provides methods of inhibiting proteases, more specifically aspartyl proteases. In certain embodiments, compounds inhibit BACE (.beta.-site APP-cleaving enzyme), and thus are useful in the treatment or prevention of a disease characterized by .beta.-amyloid deposits in the brain (including, but not limited to, Alzheimer's Disease). The present invention also provides methods for preparing compounds of the invention.