Ch_322b_16.07

Ch_322b_16.07 - Chapter 16.7 lecture note

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While acetals readily hydrolyze back to the carbonyl compound in the presence of acid as a catalyst, they are stable to bases, even strong bases. Acetals as Protecting Groups C R R' OR'' OR'' H + H 2 O C=O R R' HO - H 2 O no reaction Acetals often are used to protect aldehyde and ketone functions from undesired reactions in the presence of strong bases during syntheses. The acetal function is a geminal (1,1) diether and is not reactive towards nucleophiles, and the hydrogens are very non-acidic.
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An Example: The Selective Reduction of an Ester in the Presence of a Ketone O = COC 2 H 5 O O CH 2 OH To selectively reduce the ester function, it is necessary to protect (or mask) the ketone function. O COC 2 H 5 O H + HOCH 2 CH 2 OH addition of protecting group Remember, ketones form cyclic acetals. COC 2 H 5 O O O ketal-ester (i) LiAlH 4 . ether (ii) H 2 O CH 2 OH O O selective reduction H + H 2 O O CH 2 OH hydrolysis: removal of protecting group
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This note was uploaded on 10/20/2009 for the course CHEM 322BL at USC.

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Ch_322b_16.07 - Chapter 16.7 lecture note

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