1. For reviews of spiroacetals see: (a) Vaillancourt, V.; Praft, N. E.; Perron, F.; Albizati, K. F. InThe Total Synthesis of Natural Products; ApSimon, J., Ed.; Wiley: New York, 1992; Vol. 8, pp 533−691. (b) Perron, F.; Albizati, K. F.Chem.Rev.1989,89, 1617−1661. (c) Boivin, T. L. B.Tetrahedron1987,43, 3309. (d) Kluge, A. F.Heterocycles1986,24, 1699−1740.