Retroviral protease inhibiting compounds

A compound of the formula:

• wherein P3 and P4 are independently selected from hydrogen and an N-protecting group;
• R1 and R2 ae independently selected from the group consisting of loweralkyl, cycloalkylalkyl and ary lalkyl;
• and
• R3 is loweralkyl, hydroxyalkyl or cycloalkylalkyl;
• and
• R5 is
• wherein n is 1, 2 or 3, X is O, S, or NH and Y is —CH2—, —O—, —S— or —N(R6)— wherein R6 is hydrogen, loweralkyl, cycloalkyl, cycloalkylakyl, aryl or arylalkyl;
• or a salt thereof.