215F07-notes-11-30-07 - Chem 215 F07 Notes Dr Masato...

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Chem 215 F07 Notes – Dr. Masato Koreeda - Page 1 of 3. Date: November 30, 2007 (3) Protection of Carboxylic Acid Groups Usually as methyl, ethyl, tert -butyl, or benzyl esters. F o r m a t i o n : With an alcohol (CH 3 OH, C 2 H 5 OH, or PhCH 2 OH) and an acid catalyst (H 2 SO 4 or p -TsOH) O O H 3 N H O O H 3 N H p -TsOH PhCH 2 OH (excess) ! quantitative With an alkyl iodide or benzyl bromide and Cs 2 CO 3 (base). With an alcohol and DCC (to be discussed later). With isobutylene and H 2 SO 4. O H 3 N O O O O H 2 N O O O C CH 3 CH 3 CH 3 H 3 C C H 3 C CH 2 isobutylene H 2 SO 4 (0.2 mol equiv) ( excess ) CH 2 Cl 2 , rt, 48 h 1. 2. NaHCO 3 92% OTs D e p r o t e c t i o n : Methyl, ethyl, and benzyl esters: easily hydrolyzable with H 2 O/HO - . Benzyl esters: hydrogenolysis with H 2 , Pd-C; deprotected with HBr in CH 3 COOH. O O H 2 N H OH O H 2 N H O O H 3 N H H 2 , Pd-C + toluene No decarboxylation here! tert -Butyl esters : Readily deprotected in trifluoroacetic acid (neat or in CH 2 Cl 2 solution) at room temperature with loss of isobutylene (essentially the reverse of the acid-catalyzed method of preparation).
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