Benzyl group reduction

With H2/Pd-C, the normal conditions for benzyl group removal, it is difficult to remove the benzyl group on histidine without also causing reduction of other aromatic groups that may be present. ... 

This procedure is representative of a new general method for the preparation of noncyclic acyloins by thiazol ium-catalyzed dimerization of aldehydes in the presence of weak bases (Table I). The advantages of this method over the classical reductive coupling of esters or the modern variation in which the intermediate enediolate is trapped by silylation, are the simplicity of the procedure, the inexpensive materials used, and the purity of the products obtained. For volatile aldehydes such as acetaldehyde and propionaldehyde the reaction Is conducted without solvent in a small, heated autoclave. With the exception of furoin the preparation of benzoins from aromatic aldehydes is best carried out with a different thiazolium catalyst bearing an N-methyl or N-ethyl substituent, instead of the N-benzyl group. Benzoins have usually been prepared by cyanide-catalyzed condensation of aromatic and heterocyclic aldehydes.Unsymnetrical acyloins may be obtained by thiazol1um-catalyzed cross-condensation of two different aldehydes. -1 The thiazolium ion-catalyzed cyclization of 1,5-dialdehydes to cyclic acyloins has been reported. 

Nickel in the presence of ammonia is often used for reduction of nitriles to primary amines. The reaction is done at elevated temperatures and pressures ( 100 C, 1000 psig) unless massive amounts of nickel are used. Cobalt is used similarly but mainly under even more vigorous conditions. Nitriles containing a benzylamine can be reduced over Raney nickel to an amine without hydrogenolysis of the benzyl group (7). A solution of butoxycarbonyl)-3-aminopropyl]-N-<3-cyanopropyl)benzylamine (13.6 g) in 100 ml of ethanol containing 4 g. NaOH was reduced over 3.0 g Raney nickel at 40 psig for 28 h. The yield of A/ -benzyl-Air -(f-butoxycarbonyl)s >ermidine was 95% (7). 

Colorimetric assay with hydroxylamine showed this salt to contain 94% of the anhydrous penicillin. Paper chromatography showed complete reduction of the benzyl group. 

The benzyl group can serve as a hydroxy-protecting group if acidic conditions for ether cleavage cannot be tolerated. The benzyl C—O bond is cleaved by catalytic hydrogenolysis,176 or by electron-transfer reduction using sodium in liquid ammonia or... 

Hydrogenolysis of halides and benzylic groups presumably involves intermediates formed by oxidative addition to the active metal catalyst to generate intermediates similar to those involved in hydrogenation. The hydrogenolysis is completed by reductive elimination.58 Many other examples of this pattern of reactivity are discussed in Chapter 8. 

Hydrogenation removes the benzyl group, whose function was to prevent overalkylation. Next, HBr cleaves the ether and ester groups, and either catalytic or hydride reduction completes the synthesis of 6. Separation of diastereoisomers was achieved by fractional crystallization. 

For polymerizations of butadiene in toluene at 50°C with the Ba-Li catalyst, we have observed a reduction in molecular weight and the incorporation of benzyl groups in chains of polybutadiene. We conclude from this result that proton abstraction from toluene occurs to give benzyl carbanions which are capable of forming new polymer molecules in a chain transfer reaction. 

This compound, previously prepared174 in low yield from alginic acid, has been synthesized128 from 61 by oxidation at C-2, and stereospecific reduction of the intermediate methyl 5-0-benzyl-/3-D-nrab-ino-2-hexulofuranosidurono-6,3-lactone. Hydrogenolysis of the 5-0-benzyl group yielded the title compound in a superior yield. 

Extension (70) of this investigation to the reduction of A-phenylazomethines with 36d-LAH gave optically active secondary amines (eq. [IS]). The products had the S configuration, as predicted by reference to Scheme 9, with hydride transfer of the less shielded H2 occurring preferentially when the phenyl points away from the shielding 3-O-benzyl group of the sugar derivative. 

Benzyltriazoles are readily available through cycloaddition reactions of benzyl azide. The benzyl group can be removed by reduction with... 

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]. 

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