Benzyl ethers, deprotection

The acetate group is cleaved without benzyl ether deprotection under mild basic conditions using sodium methanolate in methanol. In this so-called Zemplen saponification, AMBERLITE IR120 is used, which is a gel-type strongly acidic cation-exchange resin of the sulfonated polystyrene type. 

In our synthesis, iterative aldol reactions of dipropionate reagent (R)-18 allowed for the control of the C3-C10 stereocenters (Scheme 9-72) [89]. Hence, a tin-mediated, syn aldol reaction followed by an anti reduction of the aldol product afforded 270. Diol protection, benzyl ether deprotection and subsequent oxidation gave aldehyde 271 which reacted with the ( )-boron enolate of ketone (/ )-18 to afford anti aldol adduct 272. While the ketone provides the major bias for this reaction, it is an example of a matched reaction based on Felkin induction from the... 

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