Adams catalyst acid anhydrides

The aromaticity of the imidazole nucleus ensures stability towards reduction, and when benzimidazole (27) is hydrogenated over Adams catalyst in acetic acid the carbocyclic ring is reduced first to give the tetrahydrobenzimidazole (28). However, if the solvent is changed to acetic anhydride, A(-acylation promotes the reduction of the heterocycle and the 1,3-diacetylbenzimidazoline (29) is then formed (Scheme 1). Imidazole (30) under these conditions gives 1,3-diacetylimidazoline (31). Imidazolium salts (32) are easily reduced and treatment with excess sodium borohydride in 95% aqueous ethanol culminates in the formation of 1,2-diamines, (33) or (34). Either N—C bond may cleave, although if the substituent R is benzyl the major products are benzylamines (33 R = Bn). ... 

The presence of the epoxide moiety at C-3 and C-4 in excelsine explained the interesting chemical reactions observed earlier. On treatment with acetic anhydride and p-toluenesulfonic acid, excelsine yielded a triacetate derivative, while treatment with acetyl chloride afforded a tetraacetate derivative. On reduction with Raney nickel in methanolic base, excelsine yielded lapaconidine (92), but was inert toward other reducing agents, e.g., lithium aluminum hydride, sodium borohydride, and Adams catalyst. Treatment of excelsine with boiling aqueous hydrochloric acid yielded an epimeric mixture of chlorohydrins with molecular formula C22H34NO6CI. These epimers were hydrolyzed to the crystalline compound C22H33NO6 when treated with aqueous sulfuric acid. This compound formed a tetraacetate derivative for which structure 105 was proposed on the basis of spectral data. 

Two diphenylaminophthalides have been synthesized The 3,3-diphenyl-6-aminophthalide [24] was synthesized in 20% yield by the following sequence of reactions which was patterned after early work °. This series involved nitration of phthalimide, hydrolysis and dehydration to 4-nitrophthalic anhydride, Friedel-Crafts reaction with benzene to 2-benzoyl-5-nitrobenzoic acid, cyclization with thionyl chloride to the pseudoacid chloride, Friedel-Crafts reactions with benzene, and, finally reduction of the nitro group to the amino function with Adams catalyst. 

This is carried out by reduction with hydrogen in the presence of Pt02 (Adams catalyst) or palladium on a carrier (26) in ethanol. After filtering off the catalyst, the solution is worked up best by evaporating alcohol after the addition of acetic acid. The residue can be worked up immediately by acetylating it with acetic anhydride. The reduction of nitro groups takes place easily even in the case of dinitro compounds and nitrophenols. 

Hydrolysis of the azlactone leads to the acylaminooinnamic acid the latter may be be reduced catal3rtlcally (Adams PtOj catalyst 40 lb. p.s.i.) and then hydrolysed by hydrochloric acid to the amino acid. Alternatively, the azlactone (say, of a-benzylaminocinnamic acid) may undergo reduction and cleavage with phosphorus, hydriodic acid and acetic anhydride directly to the a-amino acid (d/ p phenylalanine). 

A more basic acetylation catalyst was used by Adams, again after making amino acid n-propyl esters, who used a freshly prepared mixture of acetone, triethyl-amine and acetic anhydride in the proportions of 3 2 1 at 60 °C for 30 seconds. Excess reagents were blown off cautiously to avoid volatilization losses, and the residue was dissolved in ethyl acetate for injection into the gas chromatograph [23]. 

The efficient use of methyltrioxorhenium as a catalyst for this transformation has been described and considerably improved by the groups of Adam and Herrmann. Under optimised conditions, less than 1 mol% of the catalyst is needed, and regios-electivities (1,4- vs 5,8-quinone) of above 85% can be obtained. The use of concentrated (85%) H2O2 in mixtures of acetic acid and acetic anhydride is, however, necessary, and the concentration of the acid is critical for the outcome of the reaction. 

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