Benzyl selective removal

NBS, CH3CN, H2O, 62-90% yield.The POM group has been selectively removed in the presence of an ethoxy ethyl ether, TBDMS ether, benzyl ether, p-methoxybenzyl ether, an acetate, and an allyl ether. Because the hydrolysis of a pentenyl 2-acetoxyglycoside was so much slower than a pentenyl 2-benzyloxyglycoside, the 2-benzyl derivative could be cleaved selectively in the presence of the 2-acetoxy derivative. The POM group is stable to 75% AcOH, but is cleaved by 5% HCl. 

Benzyl, allyl, methyl, THP, TBDMS, and TBDPS ethers are all stable to these conditions. A primary MEM group could be selectively removed in the presence of a hindered secondary MEM group. 

Me3SiI, CH3CN, 25-50°, 100% yield. Selective removal of protective groups is possible with this reagent since a carbamate, =NCOOCMe3, is cleaved in 6 min at 25° an aryl benzyl ether is cleaved in 100% yield, with no formation of 3-benzyltyrosine, in 1 h at 50°, at which time a methyl ester begins to be cleaved. 

Aiyl esters, prepared from the phenol and an acid chloride or anhydride in the presence of base, are readily cleaved by saponification. In general they are more readily cleaved than the related esters of alcohols, thus allowing selective removal of phenolic esters. 9-Fluorenecarboxylates and 9-xanthenecarboxylates are also cleaved by photolysis. To permit selective removal, a number of carbonate esters have been investigated aryl benzyl carbonates can be cleaved by hydrogenolysis aryl 2,2,2-trichloroethyl carbonates, by Zn/THF-H20. 

H2, Pd(OH)2." When the cleavage is performed in dioxane, the DEIPS group is stable and benzyl ethers are selectively removed, whereas if MeOH is use as solvent, both the DEIPS and the benzyl ether are cleaved. 

Pd-C, 4.4% HCOOH, CH3OH, 25°, 10 h, 80-90% yield. The cleavage of benzylamines with H2/Pd-C is often very slow. Note in example 1 above that one of the benzyl groups can be selectively removed from a dibenzyl derivative. 

Primary 1,2-diamines can be obtained by hydrogenolysis of the benzylic N-auxiliary bond over Pd/C, e.g., the preparation of 249d. The cleavage is not selective in the presence of other benzylic bonds in the molecule, e.g., aryl substituents at Cl - C2. However, in the presence of methoxyphenyl or dimethoxyphenyl C substituents, the N substituents can be selectively removed, as exemplified by the preparation of the primary 1,2-diamine... 

Quebrachitol was converted into iL-c/j/roinositol (105). Exhaustive O-isopropylidenation of 105 with 2,2-dimethoxypropane, selective removal of the 3,4-0-protective group, and preferential 3-0-benzylation gave compound 106. Oxidation of 106 with dimethyl sulfoxide-oxalyl chloride provided the inosose 107. Wittig reaction of 107 with methyl(triphenyl)phos-phonium bromide and butyllithium, and subsequent hydroboration and oxidation furnished compound 108. A series of reactions, namely, protection of the primary hydroxyl group, 0-debenzylation, formation of A-methyl dithiocarbonate, deoxygenation with tributyltin hydride, and removal of the protective groups, converted 108 into 7. 

Selective removal of the benzyl ether-protecting group without hydrogenolysis of the ketal C-O bonds was performed on Ra-Ni in EtOH.154 155 A solution of benzyloxy acetal in MeOH with 5% Pd/C was stirred at room temperature with a H2 pressure from a balloon until the reaction was complete (Scheme 4.30).156... 

Selective removal of the benzyl ether-protecting group in compound 9 (Fig. 4.1) was made through both transfer hydrogenolysis and the conventional catalytic hydrogenolysis. 

The benzyl—nitrogen bond is not so easily cleaved as the benzyl—oxygen bond, unless the O-benzyl group is sterically hindered. This difference in activity allows the selective removal of the O-benzyl function in a molecule containing both N-benzyl and O-benzyl protecting groups. The selectivity can be reversed if the amine is protected by the Cbz group. If a small amount of amine (e g., butylamine) is added, then the selective removal of N-benzyl amines can be achieved.292... 

Benzyl and triphenylmethyl ethers of carbohydrates are preferred over methyl ethers when selective removal of protecting groups is important. The relatively high nucleophilic activity of the 5-hydroxyl group in glycosides and 1,2-O-alkylidene derivatives of 4 permits its benzylation and triphenylmethylation under mild conditions. Thus, treatment of 33 (Ref. 34) and 36 (Ref. 57) with benzyl bromide and... 

The Pyoc group is stable to acids and bases, but may be selectively removed after conversion into the jV-methylpyridinium form and subsequent treatment with morpholine, to give 146. Treatment of 146 with acetic anhydride in pyridine gave 147. Hydrogenolysis of the benzyl ester group of 147 followed by methanolysis catalyzed by hydrazine afforded (44,73) the de-protected derivative 148. 

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