Transfer-hydrogenation benzyl ethers

Pd/CaC03 is also used for the hydrogenolysis of benzyl-oxygen bonds.152 Hydrogenation and hydrogenolysis of an unsaturated benzyl ether over 5% Pt/ C and Pd(OH)2 gave the saturated and deprotected product.153 In contrast, transfer hydrogenolysis with 1,4-cylohexadiene or ammonium formate failed to provide the product cleanly, rapidly, or dependably. 

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. 

After debenzylation (Pd-C, cyclohexene) of the hypercores (e.g., 25) by transfer hydrogenation, they were treated with aryl branched, benzyl ether dendrons (Scheme 5.6) that were prepared by similar iterative transformations,1271 i.e., benzylic bromination and phenolic O-alkylation (See Section 5.4.2). Thus, hexaphenol core 27 was reacted with six equivalents of the benzylic bromide building block 28 to give the benzyloxy terminated dendrimer 29. Key features of these dendrimers include cores with flexible alkyl spacers and a three-directional, quaternary carbon branching center. 

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... 

In the early 1970s, Barton et al. published the results of their woik on the oxidation of acetals and ethers by hy de transfer. o They observed that substituted benzyl ethers and benzyloxy carbonates, on brief exposure to trityl tetrafluoroborate in dichloromethane at 0 C followed by aqueous work-up, afforded go( yields of the parent alcohols together with the corresponding bennldehydes. Undw the same conditions, the tetrahydropyranyl ether of cholesterol was also efficiently deprotected. A mechanism was proposed which involved an initial hydrogen abstraction, followed by quenching of the resulting stabilized cation by water (Scheme 6). 

Transfer hydrogenation. Hydrazine is apparently superior to cyclohexene for transfer hydrogenation with palladium black as catalyst for hydrogenolysis of various protective groups of peptides. It can be used for cleavage of CBZ groups, benzyl esters, and benzyl ethers it is particularly useful for removal of nitro groups. 

A new cortisone derivative, 3,20-dioxo-lljS,17a,21-trihydroxypregna-l,4,7-triene (298), has been prepared from the intermediate (297) by a classical combination of chemical and microbiological reactions.The chemistry of 17,20-and 20,21-acetonides, epimeric at C(20), has been described. A deprotection of masked steroidal alcohols by hydride transfer has been reported.Several benzyl ethers and bismethylenedioxy-steroids have been prepared and treated with trityl fluoroborate. The results demonstrated that the benzyl hydrogen atoms are sufficiently basic to give a benzyloxonium ion, as shown in the conversion of the 17,20 20,21-bismethylenedioxy-corticoid (299) into its free analogue (302) (see Part II, Chapter 1, refs. 120 and 323). 

Petrini and co-workers used the bis(methoxymethyl)-protected nitrone 150, also derived frern L-tartrate, as an electrophile rather tfaiui as a 1,3-dipole (Scheme 21, top line) (89). In their key step, reaction with 4-ben loxybutylmagnesium bromide gave the cyclic hydroxylamine 151 in 82% yield (de 90%). Transfer hydrogenation with ammonium formate and a palladium catalyst cleaved both the hydroxylamine and the benzyl ether, affording the aminoalcohol 152. Cyclization via the corresponding primary chloride created the protected indolizidine 153, acidic hydrolysis of t ch completed this short synthesis of ( + )-132 in 16%... 

Transfer hydrogenation has been employed to effect debenzylation. In this way the benzyl ether of 2,4-dicarboxyphenol in ethanol solution with 10% palladium-carbon, and aqueous sodium hypophosphite after refluxing for 2... 

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