Benzyl ethers selectivity

As noted in Section 3.11.1, some of the earliest mechanistic work on the 2,3-Wittig rearrangement was carried out with allyl benzyl ethers. Selective benzyl deprotonation is typically effected with n-butyl-lithium in THF or THF-TMEDA at low temperature. Rautenstrauch s studies, summarized in Table 8, clearly showed that lower temperature favors 2,3- over 1,2-rearrangement, and established the potential... 

Cleavage of benzyl ethers. Selectivity is observed, as exemplified in the following. ... 

Palladium-carbonIhydrogen chloride Protection of alcohol groups as benzyl ethers Selective removal of the protective group... 

Six protective groups for alcohols, which may be removed successively and selectively, have been listed by E.J. Corey (1972B). A hypothetical hexahydroxy compound with hydroxy groups 1 to 6 protected as (1) acetate, (2) 2,2,2-trichloroethyl carbonate, (3) benzyl ether, (4) dimethyl-t-butylsilyl ether, (5) 2-tetrahydropyranyl ether, and (6) methyl ether may be unmasked in that order by the reagents (1) KjCO, or NH, in CHjOH, (2) Zn in CHjOH or AcOH, (3) over Pd, (4) F", (5) wet acetic acid, and (6) BBrj. The groups may also be exposed to the same reagents in the order A 5, 2, 1, 3, 6. The (4-methoxyphenyl)methyl group (=MPM = p-methoxybenzyl, PMB) can be oxidized to a benzaldehyde derivative and thereby be removed at room temperature under neutral conditions (Y- Oikawa, 1982 R. Johansson, 1984 T. Fukuyama, 1985). 

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. 

Lithium diphenyphosphide (THF, 25°, 2 h HCl, H2O, 87% yield) selectively cleaves an aryl methyl ether in the presence of an ethyl ether.It also cleaves a phenyl benzyl ether and a phenyl allyl ether to phenol in 88% and 78% yield, respectively. ... 

NaAlH2(0CH2CH20CH3)2, PhCH3, reflux, 10 h, 62% yield/ An aryl allyl ether is selectively cleaved by this reagent (which also cleaves aryl benzyl ethers) in the presence of an A-allylamide. 

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. 

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. 

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. 

Hydrogenolysis of the C-7 benzyl ether, followed sequentially by selective triethylsilylation of the newly liberated C-7 hydroxyl and mesylation of the C-5 secondary hydroxyl, provides compound 34 in 60% overall yield (see 33—>34, Scheme 6). On the basis of Potier s studies,35 it was hoped that the C 20 hydroxyl group,... 

Lewis acids are also used in conjunction with acyl halides. The reagent Nal—BF3 etherate selectively cleaves ethers in the order benzylic ethers > alkyl methyl ethers > aryl methyl ethers. 

The disadvantages of the general method using supported palladium and hydrogen are a lack of selectivity and overreduction. Where selectivity is not an important requirement, Pd/C is a widely used catalyst for the hydrogenolysis of the C-O bond in benzyl ethers. 

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. 

Selective hydrogenolysis of the benzyl ether group was performed in the presence of an N-Boc protecting group. In this case, 10 Pd/C (0.45 mol/mol) and HC02NH4 (6.5 mol/mol) was warmed at reflux in acetone for 30 minutes (Scheme 4.55).258... 

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