The present invention provides HIV protease inhibitors of formulas I, IA, IB, Ib or II, or pharmaceutically acceptable salts thereof, wherein R2 may be, for example, 2-pyridyl-CH2—, 3 -pyridyl-CH2—, 4-pyridyl-CH2—, a sulfonyl group as described in the formulas herein including benzenesulfonyl or thiophenesulfonyl groups, R2a—CO—, R2a being selected from the group consisting of piperonyl, 2-pyrazinyl (unsubstituted or substituted with H, or an alkyl of 1 to 4 carbon atoms) or a picolylamine group as described herein, wherein R3 may be, for example, a phenyl group or diphenylmethyl group as described herein, and wherein Cx may be, for example, COOH, CONR5R6, CH2OH or CH2OR7.