(R)-OHEturea isostere, cbz deriv. AIDS-004521 AIDS004521 N-tert-Butyl-N-isopentyl-N-[[2(R)-hydroxy-4-phenyl-3(S)-[4-amino-1,4- dioxo-2(S)-[(benzyloxycarbonyl)amino]butylamino]butyl]urea