Design and synthesis of a conformationally restricted trans peptide isostere based on the bioactive conformations of saquinavir and nelfinavir

The design and synthesis of a new peptide isostere which contains a trans alkene core is described. The key step involves a Wadsworth-Emmons reaction between chiral aldehyde (2S)-9a and chiral phosphonate 7 under base-sensitive conditions to give a chiral enone (2R)-24a which was reduced to afford the desired trans alkene isosteres (2R,5R)-6a and (2R:5S)-6b (Scheme 6). A potential application of this isostere in the synthesis of HIV protease inhibitors is also discussed.