1. For recent reviews on cyclic ether syntheses see: (a) Boivin, T. L. B.Tetrahedron1987,43, 3309. (b) Moody, C. J.; Davies, M. InStudies in Natural Product Chemistry; Atta-ur-Rahman, Ed.; Elsevier: Amsterdam, 1992; Vol. 10, pp 201−239. (c) Harmange, J.C.; Figadère, B.Tetrahedron: Asymmetry1993,4, 1711. (d) Alvarez, E.; Candenas, M.L.; Perez, R.; Ravelo, J. L.; Martin, J. D.Chem. Rev.1995,95, 1953.