1. Department of Chemistry, Roy and Diana Vagelos Laboratories, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States
2. U.S. Army Edgewood Chemical Biological Center, RDCB-DRC-P/Building E3400, Aberdeen Proving Ground, Maryland 21010-5424, United States
3. Department of Process Research and Development, Merck Research Laboratories, Rahway, New Jersey 07065, United States