1. Departments of Medicinal Chemistry and Pharmacology - Istituto Di Ricerche Di Biologia Molecolare, P. Angeletti S.p.A. (IRBM-MRL Rome), Via Pontina Km 30,600, 00040 Pomezia, Italy, Department of Medicinal Chemistry, Merck Research Laboratory, West Point, Pennsylvania, Department of Process Research, Merck Research Laboratory, Rahway, New Jersey, and Department of Antiviral Research, Merck Research Laboratory, West Point, Pennsylvania