Barton

Barton J 1996 Free-radical polymerization in inverse microemulsions Prog. Polym. Sc/. 21 399-438... 

Lewis F D, Wu T F, Zhang Y F, Letsinger R L, Greenfieid S R and Wasieiewski M R 1997 Science 277 673-6 Keiiey S O and Barton J K 1999 Eiectron transfer between bases in doubie heiicai DNA Science 283 375-81 Beratan D N, Priyadashy S and Risser S M 1997 Chemistry and Biology 4 3-8... 

Barton G J1996. Protein Sequence Alignment and Database Scanning. In Sternberg M E (Editor) Prote Structure Prediction - A Practical Approach. Oxford, IRL Press, pp. 31-63. 

Barton G J 1998. Protein Sequence Aligrunent Techniques. Acta Crystallographica 054 1139-1146. Blundell T L, B L Sibanda, M J E Sterbnerg and J M Thornton. Knowledge-based Prediction of Prote Structures and the Design of Novel Molecules. Nature 326 347-352. 

Cuff IA and G J Barton 1999. Evaluation and Improvement of Multiple Sequence Methods for P Secondary Structure Prediction. Proteins Structure, Function and Genetics 34 508-519. 

The experimental detciia were kindly supplied by Professor D. H. R. Barton, F.R.S. and Dr. W. Rigby. 

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

The problem of the synthesis of highly substituted olefins from ketones according to this principle was solved by D.H.R. Barton. The ketones are first connected to azines by hydrazine and secondly treated with hydrogen sulfide to yield 1,3,4-thiadiazolidines. In this heterocycle the substituents of the prospective olefin are too far from each other to produce problems. Mild oxidation of the hydrazine nitrogens produces d -l,3,4-thiadiazolines. The decisive step of carbon-carbon bond formation is achieved in a thermal reaction a nitrogen molecule is cleaved off and the biradical formed recombines immediately since its two reactive centers are hold together by the sulfur atom. The thiirane (episulfide) can be finally desulfurized by phosphines or phosphites, and the desired olefin is formed. With very large substituents the 1,3,4-thiadiazolidines do not form with hydrazine. In such cases, however, direct thiadiazoline formation from thiones and diazo compounds is often possible, or a thermal reaction between alkylideneazinophosphoranes and thiones may be successful (D.H.R. Barton, 1972, 1974, 1975). 

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. 

C—C double bonds may be protected against electrophiles by epoxidation and subsequent removal of the oxygen atom by treatment with zinc and sodium iodide in acetic acid (J.A. Edwards, 1972 W. Kndll, 1975). Halogenation has often been used for protection, too. The C—C double bond is here also easily regenerated with zinc (see p. 138, D.H.R. Barton, 1976). 

A similar intramolecular oxidation, but for the methyl groups C-18 and C-19 was introduced by D.H.R. Barton (1979). Axial hydroxyl groups are converted to esters of nitrous or hypochlorous acid and irradiated. Oxyl radicals are liberated and selectively attack the neighboring axial methyl groups. Reactions of the methylene radicals formed with nitrosyl or chlorine radicals yield oximes or chlorides. 

The reaction conditions applied are usually heating the amine with a slight excess of aldehyde and a considerable.excess of 2d-30hydrochloric acid at 100 °C for a few hours, but much milder ( physiological ) conditions can be used with good success. Diols, olefinic double bonds, enol ethers, and glycosidic bonds survive a Pictet-Spengler reaction very well, since phenol and indole systems are much more reactive than any of these acid sensitive functional groups (W.M. Whaley, 1951 J.E.D. Barton, 1965 A.R. Battersby, 1969). 

Fleming, I. 1979, Organic Silicon Chemistry, in Barton, D. Ollis, W. D. (eds.). Comprehensive Organic Chemistry, Vol. 3, p. 539, Pergamon Press Oxford... 

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