Incorporation of Peptide Isosteres into Enantioselective Peptide-Based Catalysts as Mechanistic Probes This research is supported by the U.S. National Science Foundation (CHE-9874963). We are also grateful to the U.S. NIH (GM-57595), DuPont, Eli Lilly, Glaxo-Wellcome, and Merck for research support. S.J.M. is a Fellow of the Alfred P. Sloan Foundation, a Cottrell Scholar of Research Corporation, and a Camille Dreyfus Teacher-Scholar.