(S)-2-{(S)-2-[(2R,4S,5S)-5-((S)-2-Benzyloxycarbonylamino-propanoylamino)-4-hydroxy-2-methyl-6-phenyl-hexanoylamino]-3-methyl-butanoylamino}-3-methyl-butyric acid methyl ester AIDS-050376 AIDS050376 Hydroxyethylene isostere