Benzyl protecting groups, use

BW Erickson, RB Merrifield. Acid stability of benzylic protecting groups used in solid-phase peptide synthesis. Rearrangement of O-benzyltyrosine to 3-benzylty-rosine. J Am Chem Soc 95, 3750, 1970. 

Section 27 16 Carboxyl groups are normally protected as benzyl methyl or ethyl esters Hydrolysis m dilute base is normally used to deprotect methyl and ethyl esters Benzyl protecting groups are removed by hydrogenolysis... 

Catalytic hydrogenolysis of an O-benzyl protective group is a mild, selective method introduced by Bergmann and Zervas to cleave a benzyl carbamate (>NC0-0CH2C6H5 —> >NH) prepared to protect an amino group during peptide syntheses. The method has also been used to cleave alkyl benzyl ethers, stable compounds prepared to protect alkyl alcohols benzyl esters are cleaved by catalytic hydrogenolysis under neutral conditions. 

Ni prior to use by refluxing it for 3 h with acetone, in order to suppress hydrogenolysis of the benzylic protecting groups (51). 

As seen in the retro-synthetic Scheme 5.3, intermediate 15 is useful for both routes. The choice of benzyl protection group was made based on the robust stability of benzyl phenol ethers toward most reactions and several possible avenues to remove it, although it was reported from Medicinal Chemistry that benzyl group removal via hydrogenolysis posed challenges in this compound. The choice of iodide substitution was born out of the known high reactivity of iodides in the Ullmann-type coupling reaction with alcohols and the robust stability of aryl iodides in many other common reactions. 

Benzyl esters of phosphoric acid have been employed also with success in organic synthesis. Usually palladium catalysts are used to remove the benzyl protecting group (Scheme 4.50). 

D. P. Curran, R. Ferritto, Y. Hua, Preparation of a Eluor-ous Benzyl Protecting Group and its Use in a Fluorous Synthesis Approach to a Disaccharide , Tetmzhedron Lett. 1998, 39, 4937. 

Raney Nickel was also used as the catalyst for the reduction of nitro sugars to amino sugars by hydrogenation.87 Under these conditions, nitro sugar 127 was reduced to amino sugar derivative 128, without removal of the benzyl protecting group. Compound 128 was finally converted into azepane 129 (Scheme 39).88... 

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