A series of cyclic urea HIV protease inhibitors containing alkynyl- and allrenyl-tethered heterocycles in the P2 region were synthesized The alkynyl compounds exhibited poor activity against the HN protease enzyme. However, reduction of the triple bond afforded trans-alkenyl derivatives (XU348 and XU430) which displayed superb potency (sub-nanomolar K-i). Copyright (C) 1996 Elsevier Science Ltd