(R)-OHEturea isostere, cbz deriv. AIDS-004523 AIDS004523 N-tert-Butyl-N'-(cyclohexylmethyl)-N'-[[2(R)-hydroxy-4-phenyl-3(S)-[4-amino-1,4-dioxo-2(S)-[(benzyloxycarbonyl)amino]butyamino]butyl]urea