An advanced ABCD fragment (see picture) constructed on the way to the total synthesis of the potent antitumor agent (+)-spongistatin 2 has also been used in a formal total synthesis of (+)-spongistatin 1. In both cases the critical step was an acid-mediated epimerization in the presence of Ca2+ ions to stabilize the axial–equitorial CD spiroketal.
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Smith et al. (2001) studied this question.
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