Protecting group benzyl ester

Show the steps involved in the synthesis of Ala-Leu from alanine and leucine using benzyloxycarbonyl and benzyl ester protecting groups and DCCI-promoted peptide bond formation. 

The benzyl ester protecting group and oxazolidinone ring were hydrogeno-lyzed on 5% Pd/C in MeOH for 16 hours (Scheme 4.16).34... 

During the preparation of 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitors, the benzyl ester protecting group was removed by catalytic hydrogenolysis (Scheme 4.42). 

PREPARATION OFACETONIDE PROTECTED [G2J-C00H [12] AND GENERAL PROCEDURE FOR THE REMOVAL OF THE BENZYL ESTER PROTECTING GROUP... 

The oxime 71d has several advantages over other resins, since the protected peptide segment can be cleaved by aminolysis under conditions which do not affect benzyl ester protecting groups. Moreover, the whole procedure is compatible with the Boc group employed for a-amino protection. The synthesis of several peptides using 71d has been... 

Both the benzyloxycarbonyl protecting group and the benzyl ester protecting group may be removed by hydrogenolysis over palladium. This step completes the synthesis of Ala-Leu. 

Self-catalyzed transesterification with 2-dimethylaminoethanol in DMF has been found effective in cleaving protected peptides from the Mer-rifield resin, in the absence of side-chain benzyl ester protecting groups (Barton et al., 1973). This mild, virtually racemization-free procedure may prove extremely useful for fragment condensation because it yields N-Z-and iV-Boc-protected peptides. Since the reagent, 2-dimethyl-amino-ethanol, specifically cleaves of the benzylic ester group, the benzyl esters of glutamic acid and aspartic acid, if present, will also be hydrolyzed (Barton et al., 1973 Savoie and Barton, 1974). 

Cleavage of Phosphonate Esters. Compared to the reactivity of TMS-I with phosphonate and phosphate esters, TMS-Br is more selective and will cleave phosphonate esters even in the presence of carboxylic esters and carbamates. Benzyl ester protecting groups on aryl phosphates are selectively removed with TMS-Br. The reaction of phosphonate esters with TMS-Br proceeds through a mechanism similar to ester cleavage, providing a silyl ester which is subsequently hydrolyzed with MeOH or H2O (eq 14). 

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