Enoate triol

Elaboration of triol 88b to bryostatin 7 requires chemoselective hydrolysis of the Cl methyl ester in the presence of the C7 and C20 acetates, macrolide formation, installation of the C13 and C21 methyl enoates, and, finally, global deprotection. The sequencing of these transformations is critical, as attempts to introduce the C21 methyl enoate to form the fully functionalized C-ring pyran in advance of macrolide formation resulted in lactonization onto the C23 hydroxyl. In the event, trimethyltin hydroxide promoted hydrolysis [73] of the Cl carboxylate of triol 88b, and subsequent trie thy lsilylation of the C3 and C26 hydroxyls each occurs in a selective fashion, thus providing the seco-acid 89. Yamaguchi macrolacto-nization [39] proceeds uneventfully to provide the macrolide 67 in 66 % yield (Scheme 5.14). 

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