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Barton Reaction

This photochemical reaction was demonstrated first by D. H. R. BARTON in 1960. In a narrow sense, the BARTON reaction is the photolysis of an alkyl nitrite to form a δ-nitroso alcohol.[1]

The alkyl nitrite can be formed from a hydroxy group by reaction with NOCl or nitrosyl hydride. After the actual BARTON reaction, the nitroso alcohol can be dehydrated to a carbonyl group. The mechanism is shown in Figure 1.

 Barton

Figure 1: Reaction mechanism of the BARTON reaction with preceding akyl nitrite formation and consecutive hydrolysis.

The photochemical RO–NO cleavage yields a nitrosy radical and an alkoxide radical. Within a six-membered transition state, the alkoxide radical abstracts the δ-hydrogen. After free radical recombination of nitrogen mono oxide and the alkyl radical, the resulting nitroso compound undergoes tautomerization to give a δ-nitroso alcohol.[2] The formation of aldosterone acetate oxime by BARTON et. al is an example for the application of this synthesis. Figure 2 shows the reaction scheme. The nitrite can be obtained by adding nitrosyl chloride and consecutive irradiation for 75 min under nitrogen with a 200 W Hg lamp. The oxime is received by treatment with a 5% sodium nitrite solution. By hydrolysis the hemiacetal can be obtained.[3]

Barton2


Figure 2: Reaction scheme of the synthesis of aldosterone acetate. 

 

  1. BARTON, D. H. R.; BEATON, J. M.; GELLER, L. E.; PECHET, M. M.: A NEW PHOTOCHEMICAL REACTION, 1960, 82, 2640–2641, DOI: 1021/ja01495a061.
  2. BARTON, D. H. R.; HESSE, R. H.; PECHET, M. M.; SMITH, L. C.: The mechanism of the barton reaction, 1979, 1159, DOI: 1039/p19790001159.
  3. BARTON, D. H. R.; BEATON, J. M.: A SYNTHESIS OF ALDOSTERONE ACETATE, 1960, 82, 2641, DOI: 1021/ja01495a062.


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