Barton reduction

Deoxygenation of the partially substituted lactoside derivative 3694 was accomplished by the Barton reduction method to afford the 4 -dcoxy-p-lactosidc 37.6 The 4 -deoxy derivative was not a substrate for the p-D-galactosidase (E.C. 3.2.1.23) of Escherichia coli. 

A similar concept was applied for the synthesis of 2-deoxy- -glycoside where Barton reductive decarboxylation was utilized as the key transformation [117]. 

Crich has reported another example of anomeric radical inversion which involves a Barton reductive decarboxylation [17] of mannoulosonic acid glycosides to generate )5-mannopyranosides (Scheme 9) [16]. Photolysis of the intermediate O-acyl thiohydroxamate in the presence of r-BuSH cleanly affords the requisite jS-anomer. 

Low-valent nitrogen and phosphorus compounds are used to remove hetero atoms from organic compounds. Important examples are the Wolff-Kishner type reduction of ketones to hydrocarbons (R.L. Augustine, 1968 D. Todd, 1948 R.O. Hutchins, 1973B) and Barton s olefin synthesis (p. 35) both using hydrazine derivatives. 

The control of chemical reactions (e.g., esterification, sulfonation, nitration, alkylation, polymerization, oxidation, reduction, halogenation) and associated hazards are an essential aspect of chemical manufacture in the CPI. The industries manufacture nearly all their products, such as inorganic, organic, agricultural, polymers, and pharmaceuticals, through the control of reactive chemicals. The reactions that occur are generally without incident. Barton and Nolan [1] examined exothermic runaway incidents and found that the principal causes were ... 

Reduction of a conjugated enone to a saturated ketone requires the addition of two electrons and two protons. As in the case of the Birch reduction of aromatic compounds, the exact order of these additions has been the subject of study and speculation. Barton proposed that two electrons add initially giving a dicarbanion of the structure (49) which then is protonated rapidly at the / -position by ammonia, forming the enolate salt (50) of the saturated ketone. Stork later suggested that the radical-anion (51), a one electron... 

When saturated steroidal ketones are reduced in ammonia, an alcohol is usually present to act as a proton donor and high yields of steroidal alcohols are obtained. Under these conditions, reduction probably proceeds by protonation of the radical-anion (or ketyl) (61), which results from a one electron addition to the carbonyl group, followed by addition of a second electron and proton. Barton has proposed that reduction proceeds via protonation of the dianion (62) arising from addition of two electrons to the carbonyl group. This proposal implies that the ketyl (61) undergoes addition of a second electron in preference to undergoing protonation by the... 

Lithium-ammonia reductions of most steroidal enones of interest create one or two new asymmetric centers. Such reductions are found to be highly stereoselective and this stereoselectivity constitutes the great utility of the reaction. For conjugated enones of the normal steroid series, the thermodynamically most stable products are formed predominantly and perhaps exclusively. Thus the following configurations are favored 5a, 8/ , 9a, and in certain cases 14a (see page 35). Starr has listed numerous examples illustrating these facts and Smith " and Barton have tabulated similar data. 

The reduction of an asymmetric cyclohexanone (e.g. a steroidal ketone) can lead to two epimeric alcohols. Usually one of these products predominates. The experimental results for the reduction of steroidal ketones with metal hydrides have been well summarized by Barton and are discussed in some detail in a later section (page 76) unhindered ketones are reduced by hydrides to give mainly equatorial alcohols hindered ketones (more accurately ketones for which axial approach of the reagent is hindered " ) are reduced to give mainly axial alcohols. 

Scheme 24. Barton s thiohydroxamate ester chemistry reductive decarboxylation.

The Barton-McCombie process is an important synthetic tool for the generation of radical species. Me3SiO-(SiHMeO)n-SiMe3 works as a stoichiometric reductant in the tin-catalyzed reaction since Bu3Sn(OPh) is reduced to Bu3SnH as shown in Scheme 31 [71]. 

Iron Rat Lead absorption in everted duodenal sac preparation Reduction in intubated iron increases lead absorption increased levels decrease lead uptake Barton et al. 1978b... 

Barton and coworkers have explored the arylation of various nucleophiles including nitroalkanes using bismuth reagents.105 Reaction of 2-nitropropane with triphenylbismuth carbonate gives 2-nitro-2-phenylpropane in 80% yield.106 Recently, this arylation has been used for the synthesis of unusual amino acids. Arylation of a-nitro esters with triphenylbismuth dichloride followed by reduction gives unique a-amino acids (Eq. 5.68).107... 

Cifuentes F.R., Lindemann W.C., Barton L.L. Chromium sorption and reduction in soil with implications to bioremediation. Soil Sci 1996 161 233-241. 

The stereoselective total synthesis of (+)-epiquinamide 301 has been achieved starting from the amino acid L-allysine ethylene acetal, which was converted into piperidine 298 by standard protocols. Allylation of 297 via an. V-acyliminium ion gave 298, which underwent RCM to provide 299 and the quinolizidine 300, with the wrong stereochemistry at the C-l stereocenter. This was corrected by mesylation of the alcohol, followed by Sn2 reaction with sodium azide to give 301, which, upon saponification of the methyl ester and decarboxylation through the Barton procedure followed by reduction and N-acylation, gave the desired natural product (Scheme 66) <20050L4005>. 

In most other cases, however, the diene system simply becomes too unreactive to participate in radical chain reactions. Thus, the reductive decarboxylation of ester 7 by Barton-POC ester methodology20 or as the selenoester21 gives the reduced product 8, cleanly without any trace of product in which the diene system has participated in the reaction (equation 4)20-21. 

Compound 136 (Scheme 35) is classically used as a reagent for Barton s reductive decarboxylation, in the presence of triethylamine and/-BuSH <1995JOC6237, 1996JOC6189, 2003T6797>. 

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