Experimental
General information Unless otherwise indicated all common reagents and solvents were used as obtained from commercial suppliers without further puri®cation. Tetrahydrofuran (THF) and diethylether were distilled from sodium/benzophenone;methylene chloride was distilled from calcium hydrideprior to use. All reactions were carried out under either an argon or nitrogen atmosphere. Flash chromatography was carried out on Merck silica gel grade 60, 230±400 mesh,
60 A Ê . Elemental analyses were performed by Galbraith Laboratories in Knoxville, Tennessee.