Hydrogenation, catalytic benzyl ethers

The methods discussed above all require chemically aggressive reagents. Freudenberg and his co-workers,41 however, found that benzyl ethers can also be cleaved by catalytic hydrogenation alkyl benzyl ethers can be converted into toluene and an alcohol by hydrogenation under very mild conditions in presence of platinum metals or Raney nickel reductive cleavage of aryl benzyl ethers is best carried out in presence of palladium-charcoal. 

Catalytic hydrpgenation in acetic anhydride-benzene,- moves the aromatic benzyl ether and forms a monoacetate hydrogenation in ethyl acetate re-moves the aliphatic benzyl ether to give, after acetylation, the diacetate. ... 

Cr03/AcOH, 25°, 50% yield, ROCOPh ROH + PhC02H)]. This method was used to remove benzyl ethers from carbohydrates that contain functional groups sensitive to catalytic hydrogenation or dissolving metals. Esters are stable, but glycosides or acetals are cleaved. 

The catalytic system employing (2 - Fur)3P as ligand was applied to the coupling of methyl vinyl ketone and ethyl vinyl ketone to aromatic, aliphatic, acetylenic, and olefinic aldehydes (Scheme 23) [37]. Despite the hydrogenation conditions, alkyne and alkene moieties, as well as benzylic ether and nitro functional groups all remained intact. Furthermore, extremely high lev-... 

The Delft synthesis makes use of an acid-catalyzed ring closure - in fact an intramolecular aromatic alkylation - of a l-(3,5-dihydroxy-4-methoxybenzyl) isoquinoline derivative that is prepared starting from (natural) gallic acid. One of the hydroxyl groups is removed via a Pd/ C hydrogenation of the benzyl ether. Other catalytic steps play an important role some steps were improved recently [27]. The crucial step in the Rice synthesis makes use of a l-(2-bromo-5-hydroxy-4-methoxybenzyl)isoquinoline derivative that is also cyclized in an acid-catalyzed ring closure to the morphinan skeleton, followed by catalytic removal of the bromo substituent (Scheme 5.8). 

Hydroxy aldehydes are reduced to hydroxy alcohols without complications. In benzyl ethers of phenol aldehydes the benzyl group was hydrogenolyzed in preference to the reduction of the carbonyl by catalytic hydrogenation over 10% palladium on charcoal at room temperature and 2 atm to give phenol aldehyde [791]. 

Addition of palladium on carbon to this system markedly enhances the reactivity and permits reduction of nonactivated carbon carbon double and triple bonds. One advantage of this system over catalytic hydrogenation is that cycloptopyl groups and benzylic ethers and alcohols are not affected. Yields are generally higher than... 

Cleavage of benzyl ethers Benzyl ethers are cleaved in high yield by catalytic transfer hydrogenation with 20% palladium on carbon (1, 782)4 as catalyst and cyclohexcne as hydrogen donor. 

Methoxyphenyl ethers can be cleaved by mild oxidants (Entry 10, Table 7.8). Because many acid-labile linkers are also readily oxidized, care must be taken when applying this deprotection strategy. Benzyl ethers have been removed from Tentagel-or PEGA-bound carbohydrates by catalytic hydrogenation using palladium nanoparticles [112],... 

Catalytic hydrogenation is a standard method for cleaving benzylic ethers. The resulting secondary alcohol constitutes the nucleophilic reaction center in a subsequent Michael addition to the acceptor-substituted triple bond. Compound 5 is obtained quantitatively with a double bond in the trans configuration. 

Benzyl esters, cyclization-hydrosilylation, 11, 386-387 Benzyl ethers, cyclization-hydrosilylation, 11, 386-387 Benzyl groups, C-H bond silylation, 10, 240 Benzylic alcohols, catalytic alkylation, 11, 146 Benzylic carbon-hydrogen bonds borylation, 9, 174... 

Catalytic hydrogenolysis using Pd—C, Pd(OH)2 or Pd(OAc)2 is the most commonly employed method for the removal of benzyl ethers, and yields are often quantitative. Cyclohexene, cyclohexadiene, formic acid and ammonium formate can also be used as hydrogen sources rather than hydrogen. Benzyl ethers can also be removed by Birch reduction with lithium or sodium dissolved in liquid ammonia, but this procedure is not often applied in carbohydrate chemistry. 

An excellent method for cleaving benzylic ethers, esters, carbamates, and amines uses hydrogen In the presence of a transition metal catalyst such as Pd. Alternatively a process known as catalytic transfer hydrogenation can be employed which uses 1,4-cyclohexadiene, cyclohexene, formic acid or ammonium formate as the source of hydrogen24 The method Is exceptionally mild and compatible with most functional groups devoid of unsaturation. Hydrogenolysis of benzyloxycarbonyl (Z or Cbz) groups of amines was a major advance in the... 

Direct 2 - or 3 -0-benzylation of nucleosides has been reported,71,95,96 but removal of the benzyl group by catalytic hydrogen-olysis is not straightforward. The deprotection of methyl-substituted-benzyl ethers of nucleosides has been effected by treatment with acid. An improvement here has been the use of the photosensitive o-nitro-... 

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