Sulfonation, Barton esters

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]. 

Barton and Crich reported the first examples of the uses of 2-substituted allylic sulfur compounds [53]. Their initial experiments with additions of simple alkyl radicals to allyl sulfides, sulfoxides and sulfones were relatively unsuccessful. This failure was largely due to the fact that the nucleophilic alkyl radicals, which were generated by photolysis of the corresponding Barton ester, underwent addition to a second equivalent of Barton ester faster than they added to the allyl transfer agent. Reactions were much more successful with the electron-deficient acrylate reagent 93 (Fig. 4). Crich was later able to show that this same reagent underwent addition reactions with an acyl radical derived from an acyl phenyl telluride [54]. 

W. H. Daly and T. S. Evenson. Grafting of vinyl polymers to carboxylated poly(arylene ether sulfone) utilizing barton ester intermediates and nitrox-ide mediation. Polymer, 41(13) 5063-5071, June 2000. 

Interestingly, alkyl radicals can be generated along the polymer backbone in the presence of monomer and nitroxide via a bimolecular process. The preparation of polypropylene (PP) functionalized by hydroperoxide by means of y-irradiation was reported. The resulting macroperoxide was then heated up to 125 °C in the presence of styrene or a styrene/butyl methacrylate mixture to produce the desired PP-g-PS or PP-g-P(S-co-BMA), respectively. Daly et a . and Daly and Evenson used esters of N-hydroxypyridine-2-thione or Barton esters to produce radicals on a polymer backbone under UV irradiation. This was successfully applied to poly (arylene ether sulfone) or hydroxypropyl cellulose backbones. ... 

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