Amino acids manganese dioxide

One of the most common approaches to pyrazine ring construction is the condensation of diaminoethane and 1,2-dicarbonyI compounds such as 206 to provide pyrazines 207 after aromatization. Aromatization was accomplished by treating the dihydropyrazines with manganese dioxide in the presence of potassium hydroxide <00JCS(P1)381>. The N-protected 1,2-dicarbonyl compounds 206 were prepared from L-amino acids by initial conversion into diazoketones followed by oxidation to the glyoxal. 

Chemical manganese dioxide (CMD). This form of Mn02 is used for batteries it is available from I. C. Sample office (Cleveland, Ohio, 44101). Shioiri et al. report it is superior to commercial activated Mn02 (Aldrich) and more convenient than freshly prepared activated Mn02 for dehydrogenation of 2-(l-ami-noalkyl)thiazolidine-4-carboxylic acids to the corresponding thiazoles (thiazole amino acids). 

The specific synthesis of 1,4- and 1,5-disubstituted imidazoles in 70% yields has been achieved by cyclizing 2-amino-3-methylaminopropionic acid with triethyl orthoformate. The products are isolated after dehydrogenation with active manganese dioxide of the 2-imidazoline (69 Scheme 37) (70AHC(12)103). 

A specific synthesis of 1,4- and 1,5-disubstituted imidazoles has been accomplished in about 70% yields by cyclization of 2-amino-3-methylaminopropanoic acid and 3-amino-2-methylaminopropanoic acid, respectively, with tricthyl orthoformate (Scheme 3.1.2). The initial product is the 2-imidazoline, which needs to be aromatized by treatment with active manganese dioxide [10]. 

The isomeric 1-suhstituted imidazole-5-carboxylates are made by cyclization of 3-amino-2-alkylaminopropanoic acids with triethyl orthoformate followed by active manganese dioxide oxidation of the imidazoline product (see Section 3.1.1), or from IV-substituted glycine esters, which are formylated, converted into the enolates and then condensed with potassium thiocyanate... 

With a-di ketones 1- and 2-amino- and 1,2- and 1,5-diaminoimidazoles condense to give fused-ring heterocycles, with initial attack at the C-amino function. An N-amino group can be removed by treatment with nitrous acid. Oxidation of quaternary salts of 1-aminoimidazoles gives azoimidazolium salts, while manganese dioxide converts 1,2-di-aminoimidazoles into triazoles and triazines (see Section IV,D). 

Coupling between aromatic amines and either phenols or other aromatic amino compounds is known and it is important in the production of azo polymers, and in the formation of azo dyes. Most of this work is outside the scope of this chapter a number of reviews are cited in ref. 20. One or two examples will suffice here to illustrate the area. Thus anthranilic acid (330) forms phenazine-l,6-dicaiboxylic acid (331) on treatment with manganese dioxide or lead dioxide in dry benzene or chloroform. The mechanistic details are obscure. Excellent yields could be obtained in intramolecular cases, e.g. amine (332)... 

Routes via o-aminophenylpyrroles present the most convenient syntheses of a wide variety of pyrrolo[l,2-a]quinoxalines. Thus reaction of the amino compound 6 with acetic anhydride in acetic acid gave the acetamido derivative which was cyclized with phosphoryl chloride to give the 4-methyl compound 7 (R = Me) in 56% yield. The 4-phenyl compound 7 (R = Ph) has been prepared similarly. An even more convenient synthesis of 4-aryl compounds is achieved by reaction of compound 6 with aromatic aldehydes to give the 4,5-dihydro derivatives These are readily oxidized to 4-arylpyrrolo[l,2-a]quinoxalines 9 with manganese dioxide. This approach may be carried out in one step by reaction of compound 6 with aromatic aldehydes (e.g., benzaldehyde) in the presence of cupric acetate. Reaction of the aminophenylpyrrole 6 with 90% formic acid gave pyrrolo[l,2-a]quinoxaline (7, R = H) directly in 98% yield. Pyrrolo[l,2-a]quinoxalines substituted in the l-position and the 7-position have also been prepared from appropriately substituted... 

Manganese dioxide Preferential dehydrogenation to carbon-carbon double bonds -Amino-a,/ -ethylenesulfonic acid fluorides... 

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