Methoxycarbonyl radicals

Interaction of Polymers with Nitric Oxide macromolecules with methoxycarbonyl radicals ... 

As a result of the mobile hydrogen-atom abstraction from oximes by reactive methoxycarbonyl radicals, iminoxyls are formed ... 

A limiting stage of reaction (Equation 3.67) is NO diffusion into a polymeric matrix. The rate of reaction (Equation 3.68) should depend much more on the mobility of macromolecular reagents. Therefore, the distinction in the composition of radicals in PMMA photolysed at room temperature and 383 K is observed. At room temperature, acylalkylaminoxyl radicals are formed due to acceptance of low-molecular methoxycarbonyl radicals COOCHj by nitroso compounds. At 383 K, when molecular mobility essentially grows, dialkylaminoxyl radicals Rj-NO -R are formed because the meeting of two macromolecular particles occurs. The results obtained demonstrate an opportunity of using NO for elucidation of the polymer photolysis mechanism. With the help of this reactant, it was possible to establish the nature and mechanism of formation of intermediate short-lived radicals in photochemical process using ESR spectra of stable ARs. 

In the pyrrole series, ester groups a to nitrogen are more readily hydrolyzed by alkali, but those in a /3 position more readily by acid. A methoxycarbonyl group in the 2-positlon is meta directing thus bromination yields mainly 4-bromo-2-methoxycarbonylpyrrole. Free radical chlorination with f-butylhypochlorite gives the 5-chloro derivative. 

Pyridyl radicals, 4-methoxycarbonyl-l-methyldihydro-ES 2, 145 3-Pyridylmethanol as metabolite... 

The synthetic utility of radical cyclization was used as the key step in a four-step synthesis of the natural product (d,0-epilupinine (134b, a quinolizidine alkaloid) (75CB1043) from methyl nicotinate (146). Thus, l-(4-bromobutyl)-3-methoxycarbonyl-l,4,5,6-tetrahydropyridine (140), obtained from methyl nicotinate (146), was cyclized to 141 (43%), which on reduction with LiAlH4 in THF provided 134b in 95% yield (89T5269). 

Vinyl radicals can also participate in 6-exo cyclizations. In pioneering work, Stork and his group at Columbia University showed that stereoisomeric vinyl bromides 20 and 21 (see Scheme 3) can be converted to cyclohexene 22.7 The significance of this finding is twofold first, the stereochemistry of the vinyl bromide is inconsequential since both stereoisomers converge upon the same product and second, the radical cyclization process tolerates electrophilic methoxycarbonyl groups. The observation that the stereochemistry of the vinyl bromide is inconsequential is not surprising because the barrier for inversion of most vinyl radicals is very low.8 This important feature of vinyl radical cyclization chemistry is also exemplified in the conversion of vinyl bromide 23 to tricycle 24, the key step in Stork s synthesis of norseychellanone (25) (see Scheme 4).9 As in... 

One method of producing penta(methoxycarbonyl)cyclopentadiene radical is by irradiation of [SnBu3(OH2)2]+[C5(C02Me)5]" (reaction 36)58. The radical thus produced is remarkably stable, with the ESR signal not decreasing in intensity one hour after cessation of photolysis. 

Anodic regioselective acetamidosulfeny-lation of alkenes is similarly achieved by oxidation of diphenyldisulfide in acetonitrile [81]. Cyclic enamines, which are intermediates in the oxidation of cyclic N-methoxycarbonyl amines, react in aqueous acetonitrile that contains chloride ions to a-hydroxy- 8-chloro compounds via intermediate chloronium ions [82]. Enolethers undergo a regioselective azidomethoxyla-tion to yield acetals of a-azido carbonyl compounds upon electrolysis in methanol containing sodium azide [83]. The reaction proceeds possibly via addition of an anodicaUy generated azide radical. 

In order to find out whether captodative substitution of a methyl radical can lead to persistency, the rate of disappearance by bimolecular selfreaction was measured for typical sterically unhindered captodative radicals (Korth et al., 1983). The t-butoxy(cyano)methyl radical, t-butylthio(cyano)-methyl radical and methoxy(methoxycarbonyl)methyl radical have rate constants for bimolecular self-reactions between 1.0 x 10 and 1.5 X 10 1 mol s Mn the temperature range —60 to - -60°C. The dilTusion-controlled nature of these dimerizations is supported by the Arrhenius activation parameters. Thus, it has to be concluded that there is no kinetic stabilization for captodative-substituted methyl radicals. On the other hand, if captodative-substituted radicals are encountered which are kinetically stabilized (persistent) or which exist in equilibrium with their dimers, then other influences than the captodative substitution pattern alone must be added to account for this phenomenon. 

Recently, Kim and colleagues have described a new efficient method for the preparation of a-keto esters 48 via a free-radical acylation approach using (phenylsulfonyl) methoxycarbonyl oxime ether 46 as carbonyl equivalent radical acceptor (Scheme 28). The oxime 46 was conveniently prepared from readily available methylphenylsulfonyl acetate 44 by a two-step sequence (via oxime 45) as shown in Scheme 28. Nitrosation of 44 with isoamyl nitrite in the presence of sodium methoxide gave oxime 47 in 78% yield. 

Fragmentation of cyclopropylcarbinyl radicals has been incorporated into several synthetic schemes.260 261 262 263 For example, 2-dienyl-l,l-bis(methoxycarbonyl)cyclopropanes undergo ring expansion to cyclopentenes. 

The elusive radical cation of pyridine (140) has been obtained by irradiation of pyridine in CFCb at 4 K (79MI20403) and g values and hyperfine coupling constants have been measured for the parent molecule and deuterated derivatives. This species is of cr-type, the odd electron spending most of its time in the N sp2 lone pair orbital. Radical cations and anions of pyridinium bis(alkoxycarbonyl)methylides have been produced in the former case (78CC817) as a cyclopropenone complex, and in the latter by reduction of pyridinium bis(methoxycarbonyl)methylide with sodium (79JMR(35)l7l). The coupling constants in the ESR spectrum of both the radical cation and the anion agree to some extent with simple Huckel MO calculations. 

However, a few additive procedures are still employed for specific purposes. Thus the prefix hydro, although normally treated as a substituent prefix, does in fact represent addition of a hydrogen atom. Also, in forming complex radical prefixes for substitutive nomenclature, an additive operation is often necessary [e.g., methoxy-(MeO—) + carbonyl ( C=0) = methoxycarbonyl (Me02C—)]. 

Appropriate bromides, made by photobromination procedures, have been used as sources for the e.s.r. study of acetylated 1-cyano- and 1-chloro-hexopyranos-l-yl radicals and acetylated pentopyranos-5-yl radicals and their 5-acetoxymethyl and 5-methoxycarbonyl analogues.101... 

METHOXYCARBONYL-1,1,6-TRIMETHYL-1,4,4a,5,6,7,8,8a-OCTAHYDRO-2,3-BENZOPYRONE, an intramolecular Diels-Alder reaction is responsible for the diastereoselectivity. The stereoselective 1,4-functionalization of 1,3-dienes is exemplified by a two-step process leading to cis- and trans-1-ACETOXY-4-(DICARBOMETHOXYMETHYL)-2-CYCLOHEXENE. The effectiveness of a silyl hydride in providing a means for erythro-directed reduction of a p-keto amide is applied in a route to ERYTHRO-1 -(3-HYDROXY-2-METHYL-3-PHENYL-PROPANOYLJPIPERIDINE. This is followed by an asymmetric synthesis based on a chiral bicyclic lactam leading to (R)-4-ETHYL-4-ALLYL-2-CYCLOHEXEN-1-ONE. The stereoselectivity with which acetoxy migration can operate to an adjacent radical center is reflected in the one-step reaction that gives rise to 1,3,4,6-TETRA-O-ACETYL-2-DEOXY-a-D-GLUCOPYRANOSE. 

Benzophenone (Amax = 340 nm, log e = 2.5, n-ir electronic transition) can be used as a photochemical reagent and eq. 4.25 shows a radical Michael-addition reaction with benzophenone. The formed benzophenone biradical (triplet state, Tx) abstracts an electron-rich a-hydrogen atom from methyl 3-hydroxypropanoate (62) to generate an electron-rich a-hydroxy carbon-centered radical [III], then its radical adds to the electron-deficient (3-carbon of a, (3-unsaturated cyclic ketone (63) through the radical Michael addition. The electrophilic oxygen-centered radical in the benzophenone biradical abstracts an electron-rich hydrogen atom from methyl 3-hydroxypropanoate (62) [70]. So, an a-hydroxy carbon-centered radical [III] is formed, and an electron-deficient a-methoxycarbonyl carbon-centered radical [III7] is not formed. 

Theoretical calculation (UB3LYP/6-311+G , UB3LYP/6-31G , UMPW1K/ 6-31+G ) for the addition of 2-methoxycarbonyl-2-propyl radical to MMA gave predicted isotope effects on Cl(1.0378-1.0388), C2(l.0070-1.0076), C3(1.0008-1.0009) and C4(l. 0018-1.0023) in good agreement with experimental values. Thus, observed by Harrison et al. isotope effect of 1.002 on C2 in the free-radical polymerization should be experimentally verified, particularly that association of vinyl monomers and the [CuI(PMDETA)]BPh4 was proofed not... 

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