(OHEt)amide isostere AIDS-050403 AIDS050403 [(S)-1-((1S,2R)-1-Benzyl-3-{(R)-2-[(1-ethyl-propylcarbamoyl)-methyl]-3,3-dimethyl-butanoylamino}-2-hydroxy-propylcarbamoyl)-2-hydroxy-propyl]-carbamic acid benzyl ester