(OHEt)amide isostere (R)-2-tert-Butyl-N1-((2R,3S)-3-{(S)-3-carbamoyl-2-[(1-quinoxalin-2-yl-methanoyl)-amino]-propanoylamino}-2-hydroxy-4-phenyl-butyl)-N4-(1-ethyl-propyl)-succinamide AIDS-050407 AIDS050407