Heterocycles manganese dioxide

N-Acylation, reduction of nitro to amino, and condensation produce dihydrotriazine 5.31. This system is readily dehydrogenated with manganese dioxide to afford the fully aromatic heterocycle 5.32. Note how relatively simple chemistry can be used to form a quite complex heterocycle. 

We can illustrate the synthesis of allylic alcohols from allylic sulfoxides with this synthesis of the natural product nuciferal. We mentioned this route on p. 1257 because it makes use of a heterocyclic allyl sulfide to introduce an alkyl substituent regioselectively. The allyl sulfide is oxidized to the sulfoxide, which is converted to the rearranged allylic alcohol with diethylamine as the thiophile. Nuciferal is obtained by oxidizing the allylic alcohol to an aldehyde with manganese dioxide. 

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). 

Dehydrogenations are very easy in six-membered nitrogen-containing compounds that already contain one or two double bonds in the heterocyclic ring. Such dehydrogenations are carried out by nitric acid [455], arsenic oxide (AsjOj) [480], manganese dioxide [825], potassium ferricyanide [919], nitrobenzene, and others (equations 41 and 42). 

The heterocycle 3-methyl-1,2-benzisoxazole, whilst structurally distinct from the phenolic aryl ethers underwent cleavage at the 0-N bond by slow addition in tetrahydrofuran to 1M lithium diethylamide at -75 C followed by isolation of crude 1,6-dihydropyridazine and its oxidation with manganese dioxide over 1 hour to give 3,5-bis(2-hydroxyphenyl)pyridazine in 61% yield (ref.96). 

Kato and co-workers explored reactions of Mvenes with a variety of mesoionic heterocycles. Unfortunately, reactions of munchnone 42 with fulvenes 290-292 afforded complex mixtures in each case, and no identifiable products were reported (Fig. 4.101). However, Friedrichsen and co-workersreported the reaction between 42 and 6,6-dimethylfulvene 292 in the presence of manganese dioxide to give 293 in low yield. 

Nomura, T., T. Fukai, and M. Katayanagi Oxidative Cyclization of Morusin with Manganese Dioxide. Heterocycles 6, 1847 (1977). 

Enhancing the utility of the allq lation-rearrangenient sequence in synthesis, the Evans group addressed the problems of a-versus-y alkylation as well as low anion reactivity by employing heterocyclic sulfides as the alleviation substrates tScheme IR.IfiE For instance, allylic imidazolyl sulfide 56 could be allqvlated efficiently, reaction at the a-position being favored by a chelated but reactive allyl lithium intermediate. Oxidation of 57 to the allylic sulfoxide and treatment with a secondary amine thiophile provided allylic alcohol 58 in high yield and with excellent stereoselectivity at the trisubstituted alkene. Allylic oxidation with manganese dioxide completed a synthesis of the sesquiterpene nuciferal (59). ... 

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