Radicals heterocycles

Free radical heterocyclization of unsaturated compounds using carbonyl compounds in the presence of Mn(III) acetate 89UK475. 

Ease of Use and User Friendliness. Once a chemical stmcture has been entered, the user must select a specific mechanistic module (i.e., Carbenoid, Basic/Nucleophilic, Acidic/Electrophilic, Electrophilic Aromatic, Radical, Heterocyclic, Oxidative/Reductive, or Pericyclic) for evaluation of the reactants and define the reaction conditions, reagent, and solvent. Then the user instructs CAMEO to run the reaction. Within approximately 10 seconds, the results are displayed on the screen. 

Ease of Use and User Friendliness. CAMEO was found to be easy to use. CAMEO S menu screens are well designed and easy to follow, and greatly facilitate use and operation of the program, particularly structure entry. To explore fully all potential reactions and the products of these reactions, however, the user must separately evaluate a given set of starting materials and reaction conditions under several if not all of the available reaction modules (i.e., Carbenoid, Radical, Heterocyclic, Basic/Nucleophilic, Acidic/Electrophilic, Electrophilic Aromatic, Oxidative/Reductive, and Pericyclic). Thus, the user can enter reactants and reaction conditions, and, depending upon which module is selected, CAMEO may predict different results. For example, CAMEO correctly predicted carbaryl as the product from the reaction of methyl isocyanate with 1-naphthol only if the Acidic/Electrophilic mechanistic module was selected no product was predicted when the Basic/Nucleophilic module was selected. 1-Naphthol is clearly the nucleophile in this reaction, and it seems that CAMEO should have recognized it as such. 

The free-radical reactivity of thiazoles has been well studied with various radicals such as methyl, phenyl, substituted phenyl, cyclohexyl, and aromatic-heterocyclic, in nonpolar solvent or strong acids (180-182). 

In Table III-33 results for the methylation of thiazoles in acetic acid are given (lead tetraacetate is used as radical source), but in this case some discrepancies appear, the acidic medium being too weak, and the heterocyclic base not fully protonated. Thiazole has also been methylated by the DMSO-H2O2 method (201), and the results are in agreement with those described previously. 

Radicals derived from heterocyclic compounds by removal of hydrogen from a ring are named by adding -yl to the names of the parent compounds (with elision of the final e, if present). These exceptions are retained ... 

Cyclic ethers are named either as heterocyclic compounds or by specialist rules of heterocyclic nomenclature. Radicofunctional names are formed by citing the names of the radicals R and R followed by the word ether. Thus methoxyethane becomes ethyl methyl ether and ethoxyethane becomes diethyl ether. 

Sulfonium Compounds. Sulfonium compounds of the type R R R S X are named by citing in alphabetical order the radical names followed by -sulfonium and the name of the anion. For heterocyclic compounds, -ium is added to the name of the ring system. Replacement of > CH by sulfonium sulfur is denoted by the prefix thionia-, and the name of the anion is added at the end. 

An interesting method for the substitution of a hydrogen atom in rr-electron deficient heterocycles was reported some years ago, in the possibility of homolytic aromatic displacement (74AHC(16)123). The nucleophilic character of radicals and the important role of polar factors in this type of substitution are the essentials for a successful reaction with six-membered nitrogen heterocycles in general. No paper has yet been published describing homolytic substitution reactions of pteridines with nucleophilic radicals such as alkyl, carbamoyl, a-oxyalkyl and a-A-alkyl radicals or with amino radical cations. 

The major internal comparisons to be made within this chapter are between (13) pyrrole (1), furan (2), thiophene (3), selenophene (4) and tellurophene (5) b) pyrrole (1) and indole (6) (c) indole (6), benzo[6 Jfuran (7) and benzo[6]thiophene (8) d) indole (6), isoindole (9) and indolizine (10) and (e) benzo[6] and benzo[c] fused systems. The names of relevant heterocyclic radicals are given with the structures of the parent heterocycle. 

The direct combination of selenium and acetylene provides the most convenient source of selenophene (76JHC1319). Lesser amounts of many other compounds are formed concurrently and include 2- and 3-alkylselenophenes, benzo[6]selenophene and isomeric selenoloselenophenes (76CS(10)159). The commercial availability of thiophene makes comparable reactions of little interest for the obtention of the parent heterocycle in the laboratory. However, the reaction of substituted acetylenes with morpholinyl disulfide is of some synthetic value. The process, which appears to entail the initial formation of thionitroxyl radicals, converts phenylacetylene into a 3 1 mixture of 2,4- and 2,5-diphenylthiophene, methyl propiolate into dimethyl thiophene-2,5-dicarboxylate, and ethyl phenylpropiolate into diethyl 3,4-diphenylthiophene-2,5-dicarboxylate (Scheme 83a) (77TL3413). Dimethyl thiophene-2,4-dicarboxylate is obtained from methyl propiolate by treatment with dimethyl sulfoxide and thionyl chloride (Scheme 83b) (66CB1558). The rhodium carbonyl catalyzed carbonylation of alkynes in alcohols provides 5-alkoxy-2(5//)-furanones (Scheme 83c) (81CL993). The inclusion of ethylene provides 5-ethyl-2(5//)-furanones instead (82NKK242). The nickel acetate catalyzed addition of r-butyl isocyanide to alkynes provides access to 2-aminopyrroles (Scheme 83d) (70S593). 

The polarographic half-wave reduction potential of 4-nitroisothiazole is -0.45 V (pH 7, vs. saturated calomel electrode). This potential is related to the electron affinity of the molecule and it provides a measure of the energy of the LUMO. Pulse radiolysis and ESR studies have been carried out on the radical anions arising from one-electron reduction of 4-nitroisothiazole and other nitro-heterocycles (76MI41704). 

Radicals of most small and many large heterocycles are known, but their chemistry has not always been explored in depth. The ESR spectra of small ring radicals have been measured and generally found to be in good agreement with theoretical predictions. Table 1 gives some data for three-membered heterocyclics. Noteworthy is the close similarity of the ESR spectra of 1-aziridinyl, 1-azetidinyl and dimethylaminyl (71TL2247). The radicals in the table are all tt-radicals. 

Large ring heterocyclic radicals are not particularly well known as a class. Their behavior often resembles that of their alicyclic counterparts, except for transannular reactions, such as the intramolecular cyclization of 1-azacyclononan-l-yl (Scheme 1) (72CJCH67). As is the case with alicyclic ethers, oxepane in the reaction with r-butoxy radical suffers abstraction of a hydrogen atom from the 2-position in the first reaction step (Scheme 2) (76TL439). 

Surprisingly little is known about the attack of radicals on small and large heterocycles. Hydrogen abstraction from the heteroatom of small rings leads to ring opening, and in the... 

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