The spongipyran macrolides, a new class of marine natural products, exhibit subnanomolar antitumor activities against a number of human cancer cell lines. Independent bioassayguided isolation and structure elucidation efforts by the Pettit, Fusetani, and Kitagawa groups have recently identified the structurally related macrolides spongistatins (Spongia family),['] cinachyrolide A (Cinachyra family),'*] and the altohyrtins (Hyrtios and Spirastrella The structures of the major constituents in each study, spongistatin 1, cinachyrolide A, and altohyrtin A, were determined on the basis of NMRI41 spectroscopic investigations. While each of these structures has been assigned the same carbon skeleton, the proposed structures differ in the relative stereochemical relationships among the AB and CD spiroketals, rings E and F, and at the CI5 and C16 stereocenters (Figure 1). Since only the Kitagawa altohyrtin structure elucidation included