Ammonium acid formate cerium nitrate

Oxidation of 1-(4-methoxyphenyl)-2-( -substituted phenyl) et hands, 6, by Cerium(IV). Dehydration prevented the oxidative-cleavage study of l-(4-hydroxyphenyl)-2-(4,-substituted phenyl)ethanols, 4, (8). As an alternative study, the oxidation of these phenolic compounds using the homolytic oxidant ceric ammonium nitrate (CAN) in an acidic environment was initiated. However, preliminary oxidations of these compounds were unsuccessful due to the apparent formation of complexes of cerium(IV) with the phenolic hydroxyl groups. 

Acetyl- and 3-benzoylisoxazoles 389 (and isoxazolines) have been prepared by one-pot reactions of alkynes (and alkenes) with ammonium cerium(iv) nitrate (CAN(lv)) or ammonium cerium(lll) nitrate tetrahydrate (CAN(m))-formic acid, in acetone or acetophenone. These processes probably involve 1,3-dipolar cycloaddition of nitrile oxides produced via nitration of the carbonyl compound by cerium salts. The existence of nitrile oxides as reaction intermediates was proved by the formation of the dimer furoxan 390 when the above reaction was carried out in absence of any dipolarophile (Scheme 95) <2004T1671>. An analogous improved procedure has been applied to alkynyl glycosides as dipolarophiles for the preparation of carbohydrate isoxazoles <2006SL1739>. 

Initially, sulfuric acid was used to initate the Borsche—Drechsel cyclization. However, Perkin found that acetic acid provided cleaner products. Recently, other reagents—such as cerium ammonium nitrate (CAN) and the more environmentally friendly acidic ionic liquids —have been used to catalyze the ring formation. It should be noted that people have found it difficult to promote the cyclization when there are substituents at the ortho-position of the phenylhydrazine. This difficulty becomes more pronounced when both the hydrazine and the cyclohexanone have a-substitution This difficulty usually leads to low yields compared with other methods of forming the carbazole. 

In a transition-metal-free approach to indolo[2,3-d]quinolizines, our group has shown that the four-component reaction between tryptamines, a,p-unsaturated aldehydes, p-dicarbonyl compounds, and ethanol in the presence of cerium(IV) ammonium nitrate as a Lewis acid catalyst gives direct access to indolo[2,3-a]quinolizines 93 (Scheme 3.29) [72], On the basis of previous work on related multicomponent reactions [73], this domino transformation was proposed to comprise the initial formation of a p-enaminone by reaction between the primary amine and dicarbonyl components, its Michael addition to the unsaturated aldehyde, a hemiaminalization reaction, and a final Pictet-Spengler reaction initiated by the formation of a vinylo-gous acyliminium species. This method could also be extended to the preparation of benzo[fl]quinolizines. 

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