Synthesis of Two Dipeptide Isosteres Containing Di- and Trisubstituted E-Configured Double Bonds

The stereoselective syntheses of a Tyr-Tyr and a Pro-Pro E-alkene isostere are described. While the Tyr-Tyr isostere was synthesized following a convergent olefination strategy, the trisubstituted E-configured double bond of the Pro-Pro isostere was generated by an Ireland-Claisen rearrangement. The configuration of all key intermediates containing new stereocenters was determined by X-ray crystallography.