Barton ester

Radical Decarboxylation Barton esters AJdrichirnica Acta 1987, 20 (2), 35... 

The photochemical reduction of Barton ester 40 is depicted in Scheme 12. A series of hydrogen atom donors were screened. A stoichiometric amount of benzenethiol at - 78 °C provided the product in 86% ee (entry 3). This implies that, in the presence of an efficient hydrogen atom donor, radical trapping is competitive with the ring/radical inversion, generating an enantiomeri-... 

Scheme 12 Memory of chirality reduction of Barton esters...

Caution Barton esters are light sensitive therefore all procedures should be carried out in the absence of light. 

Note 2), and 40 mL of methylene chloride (Note 3). The resulting homogeneous solution is cooled to 0°C and 4.99 g (27.1 mmol) of 10-undecenoic acid (1) is added dropwise (Note 4). Following the addition of the undecenoic acid, the ice bath is removed and the reaction mixture is allowed to warm to room temperature and stirred for a further 8 hr. The bright yellow suspension that results is filtered through a bed of silica gel (Note 5). The solvent is removed under reduced pressure to give 8.06 g of the crude Barton ester (3) (Notes 6 and 7). 

Bis(tributylstannyl)benzopinacolate PTOC ester or Barton ester... 

Examples of radical-mediated C-alkylations are listed in Table 5.4. In these examples, radicals are formed by halogen abstraction with tin radicals (Entries 1 and 2), by photolysis of Barton esters (Entry 3), and by the reduction of organomercury compounds (Entry 4). Carbohydrate-derived, polystyrene-bound a-haloesters undergo radical allylation with allyltributyltin with high diastereoselectivity (97% de [41]). Cleavage from supports by homolytic bond fission with simultaneous formation of C-H or C-C bonds is considered in Section 3.16. 

Thioethers have also been prepared on cross-linked polystyrene by radical addition of thiols to support-bound alkenes and by reaction of support-bound carbon radicals (generated by addition of carbon radicals to resin-bound acrylates) with esters of l-hydroxy-l,2-dihydro-2-pyridinethione ( Barton esters Entry 6, Table 8.5). Additional methods include the reaction of metallated supports with symmetric disulfides (Entries 7-9, Table 8.5) and the alkylation of polystyrene-bound, a-lithiated thioani-sole [65],... 

Another way that has potential for the generation of perfluoroalkyl radicals from carboxylic acids is the use of Barton esters. However, unlike the situation for their hydrocarbon analogs, fluorinated thiohydroxamate esters have thus far only been able to be prepared in situ [65]. 

The radical decarboxylation of a Barton ester proceeds to the corresponding alkane after treatment with tributyltin hydride or t-butylmercaptan ... 

The indolizidine alkaloid skeleton can be easily synthesized using the Barton-Ester method (Scheme 25) <94TL(35)9157>. 

A variety of different sources of radicals have been used in several heteroaromatic substitution reactions [2] these include acyl peroxides, oxaziridines, thiohydroxa-mic Barton esters, the Gif reaction, alkyl xanthates, and ketones/H202 (Scheme 8). 

Further acid-catalyzed reactions include the use of />-toluene sulfonic acid-DMF in a cyclization of the protected amino acid 17 (DMF = dimethylformamide Scheme 15) <20040L4941>. This was the key step in the stereoselective synthesis of 5-hydroxypipecolic acid. A similar acid-catalyzed ring closure of a hemiacetal yielded the fused piperidine 18 <2004JOC1872> (Equation 32). The indolizidine alkaloid can be accessed by a Barton-Ester method utilizing a polyphosphoric acid (PPA) cyclization (Scheme 16) <1994T19157>. 

Reaction of the Barton ester (124) with vinyl sulfone can be carried out under irradiation with a tungsten lamp (eq. 4.43) [123]. 

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]. 

The best-known reactions belonging to this class are based either on the photo-NOCAS process [32] or on the photochemistry of Barton esters [69]. In the first case, a three-component reaction involving a cyanoarene, an olefin, and a nucleophile (usually the solvent) occurs. The reaction is generally initiated by a PET process between the aromatic and the olefin. The examples presented below are chosen from among the most representative and most recent. A typical reaction is illustrated in Scheme 3.26, where an enolized P-dicarbonyl compound acts as an added nucleophile. 

The photolysis of Barton esters (N-hydroxy-2-thiopyridone esters, 43) proved to be an efficient method for generating carbon-centered radicals that are exploited for the regioselective alkylation of electron-deficient olefins a thiopyridyl unit is likewise incorporated into the end products. In a recent application, Barton esters were found useful in the synthesis of natural and unnatural disubstituted maleimides or maleic anhydrides by way of two consecutive radical addition steps, as described in Scheme 3.27 [72]. 

Dalko, P.I. (2004) The photochemistry of Barton esters, in Handbook of Organic Photochemistry and Photobiology, 2nd edn (eds W. Horspool and F. Lenci), CRC Press, Boca Raton, pp. 67-1-67-23. 

Musso has reported the synthesis of diasterane (tricyclo-[3.1.1.I2 4]octane) 15. For this first member of the series of asteranes, the decarboxylation of 16b -> 16c was best achieved via the photolysis of the Barton ester of 16a in the presence of BuSH, as shown in Scheme 5.14 Fukumoto has accomplished asymmetric total synthesis of atisine 17, where the bridged pentacyclic intermediate 18, a precursor for atisine, was synthesized via an intramolecular double Michael reaction starting with 19, Scheme 6.15 Barton protocol was favored during the late stages of the synthesis and the presence of various functionalities was easily accommodated. 

Vogel and coworkers have described the synthesis of anti-1,6 7,12-/ wmethano[ 14]annulene 44 for studies of its -electron structure.32 Incorporation of AIBN into decarboxylative bromination of vinylogous carboxylic acid 45 via Barton esters increased the efficiency of this reaction, Scheme 16b. Harvey... 

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