Barton decarboxylation reaction

The allyl alcohol trapping reaction was further studied by Crich and coworkers, who applied the Barton decarboxylation reaction as radical source,... 

Barton Decarboxylation Reaction with A/-Hydroxy-2-thiopyridone... 

Taddei [7] has studied the use of the Barton decarboxylation reaction in solid-phase chemistry. FeasibiUty studies began with the preparation of simple immobihzed Barton ester 37 from Wang resin-derived acid 36 (Scheme 8). Irradiation of 37 followed by acid cleavage gave 38 in good overall yield. Attempts to prepare bromide 39 by interception of the intermediate radical... 

Conversion of a carboxylic acid to a thiohydroxamate ester, followed by heating the product in the presence of a suitable hydrogen donor such as tri-n-butyltin hydride, produces a reductive decarboxylation. This sequence of reactions is called the Barton decarboxylation reaction and may be used to remove a carboxylic acid and replace it with other functional groups. 

A cyclopropyl to allyl radical rearrangement could also be induced in 20 % yield by the thermal decomposition (in carbon tetrachloride at 32 °C with sonication) of the A-hy-droxypyridine-2-thione ester of l-fluoro-2,2-diphenylcyclopropane-l-carboxylic acid (Barton decarboxylation reaction). ... 

The initiation of the Barton Decarboxylation ( Bu3Sn-H -> Bu3Sn-) is effected with a radical initiator, and as with the Barton-McCombie Deoxygenation, the driving force for the reaction itself is the formation of the stable S-Sn bond. 

From the practical point of view, alkylation of heteroaromatics with Barton decarboxylation of A-acyloxy-2-thiopyridones (17), prepared from carboxylic acids and Af-hydroxy-2-thiopyridone, is very useful, since it can be used for various kinds of carboxylic acids such as sugars and nucleosides [23-26]. This reaction comprises of the initial homolytic cleavage of the N-0 bond in A-acyloxy-2-thiopyridone to form an acyloxyl radical and PyS , (3-cleavage of the acyloxyl radical to generate an alkyl radical and C02, addition to the electron-deficient position of heteroaromatics by the alkyl radical to form the adduct, and finally, abstraction of a hydrogen atom from the adduct by PyS , as shown in eq. 5.10. 

Stereochemistry in radical reactions for organic synthesis has not been studied very extensively, because mild or low temperature-promoted radical reaction methods are extremely limited and the stereoselectivity in radical reactions is generally rather poor. Recently, however, stereoselective organic synthesis with radical reactions has become popular, since mild radical reaction methods such as the Barton decarboxylation, Et3B-initiated Bu3SnH reaction, etc. have been developed. Normally, low temperature-initiated radical reactions induce high stereoselectivity. 

Since Barton decarboxylation can be performed under mild conditions, thermal or photolytic treatment of the Barton ester (13) of propionic acid with TV-hydroxy-2-thiopyridone in the presence of chiral menthyl acrylates generates addition products (14). However, diastereoselectivity is rather poor, since the chiral menthyl center is too far away from the C-C bond-forming position, as shown in eq. 10.7. When the chiral center is adjacent to the reaction position, stereocontrol is significantly affected, as shown in eq. 10.8 [9-12]. 

Disciplined radicals and disciplinary radical reactions (Barton decarboxylation)... 

All compounds of the arachidonic acid cascade as well as many peptides and biotin contain the carboxyl function. After seeing the value of the Barton-McCombie reaction, it was logical to consider if similar chemistry could be carried out with the carboxyl function.1 In particular, decarboxylation to the corresponding radical seemed a promising way to replace -C02H by -H. 

During their studies on the stereochemistry of intramolecular 1,3-diyl trapping reaction as depicted in Scheme 13, Little and collaborators utilized the reductive decarboxylation in preparing the required compound 31.24 The usefulness of Barton decarboxylation was also realized by Braekman and colleagues during their studies on ichthyotoxic sesterterpenoids in providing the needed methyl ketone 32 from the carboxylic acid 33.25 Helmchen has developed an easy route to a stable... 

Piperidine 141 was synthesized from the Barton-McCombie reaction <75JCSP11574> of 142 which gave the expected amido-ester (96 %) as a 3 2-mixture of diastereomers. The mixture was hydrolyzed to the corresponding carboxylic acid which, upon thermal decarboxylation, gave the desired /V-bcnzyl lactam (85% overall yield) as a single diastereomer whose structure was unequivocally established by a single-crystal X-ray analysis. Reduction of the lactam with LiAlH4 (81%) followed by debenzylation via... 

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