General methods. All reagents used in chemical synthesis were purchased from Aldrich (Milwaukee, WI). The structures of all compounds investigated were confirmed by using 1H and 31P NMR spectroscopy at 400 MHz or 500 MHz on Varian (Palo Alto, CA) Unity spectrometers. Cellulose TLC plates were visualized by using iodine or a sulfosalicylic acid-ferric chloride stain.