Catalytic Enantioselective Synthesis of β-Lactams: Intramolecular Kinugasa Reactions and Interception of an Intermediate in the Reaction Cascade ( Support has been provided by the National Institutes of Health (National Institute of General Medical Sciences, R01-GM066960), Merck, and Novartis. Funding for the MIT Department of Chemistry Instrumentation Facility has been furnished in part by NSF CHE-9808061 and NSF DBI-9729592. We thank Ivory D. Hills for an X-ray crystallographic study. )