Barton s reaction

The photolysis of nitrites which do not contain g-hydrogen atoms usually results in the elimination of nitric oxide and the formation of hydroxy and carbonyl compounds. When g-hydrogen atoms are present than the product is an oxime or the corresponding nitroso dimer. This reaction is known as Barton S reaction. 

Carbon-centered radicals generated by Barton s thiohydroxamate method can also participate in ring-forming reactions (see Scheme 26).52b,s3 For example, irradiation of 129 results in the formation of compound 130 (82% yield). The outcome of this transformation is reminiscent of Stork s elegant radical cyclization/trapping processes (see Schemes 7 and 8), in that/botn alkene carbon atoms have become functionalized. / I... 

The addition of silyl radicals to thiocarbonyl derivatives is a facile process leading to a-silylthio adducts (Reaction 5.37). This elementary reaction is the initial step of the radical chain deoxygenation of alcohols or Barton McCombie reaction (see Section 4.3.3 for more details). However, rate constants for the formation of these adducts are limited to the value for the reaction of (TMS)3Si radical with the xanthate c-C6HuOC(S)SMe (Table 5.3), a reaction that is also found to be reversible [15]. Structural information on the a-silylthio adducts as well as some kinetic data for the decay reactions of these species have been obtained by EPR spectroscopy [9,72]. 

Columbia University in 1979. She did subsequent postdoctoral work at both AT T Bell Laboratories and Yale University. Barton s research areas are biophysical chemistry and inorganic chemistry. She has focused on stndies of recognition and reaction of nncleic acids by transition metal complexes, and particularly upon DNA-mediated charge transport chemistry. She is a member of the Board of Directors of the Dow Chemical Company and the National Academy of Sciences. 

J. E. Forbes and S. Z. Zard, A novel radical chain reaction of xanthic anhydrides. Further observations on the intermediacy of alkoxythiocarbonyl radicals in the Barton-McCombie reaction. Tetrahedron Lett. 30 4367 (1989). 

Thiocarbonyl compounds for which the C=S group is linked to two heteroatoms, as in thionocarbonates, thionocarbamates, thioureas and trithiocarbonates, have been in general less directly involved as intermediates in synthesis, and we have considered here only xanthates which intervene in the Chugaev reaction (see Section 3.1) and Barton radical reactions (see Section 3.2). They are traditionally prepared by reaction of an alkoxide anion, generated from the alcohol and a strong... 

Sequences in which addition precedes cyclization are not as straightforward to conduct as the reverse however, they are very important because a net annulation results (that is, a new ring is formed by the union of two acyclic precursors in one experimental step). The intermediate radical is differentiated from the other radicals provided that the cyclization reaction is rapid, but it can be difficult to differentiate the initial radical from the final radical. As illustrated in Scheme 57, this is particularly true in the tin hydride method because many different types of radicals react with tin hydride at similar rates. Reaction of (69) under standard radical addition conditions produces (70), which results from a sequence of addition/cy-clization/addition.233 That the last C—C bond is formed actually results from a lack of selectivity the initial and final radicals are not differentiated and they must undergo the same reaction. Of course, this lack of selectivity is of no consequence if the product contains the desired skeleton and the needed functionality for subsequent transformations. Such sequences are very useful for forming three carbon-carbon bonds, and they can also be conducted by Barton s thiohydroxamate method.234 Structural modifications are required to differentiate the initial and final radicals, and, as illustrated by the conversion of (71) to (72), phenyl groups can provide the needed differentiation (probably by retarding the rate of addition more than they retard the rate of hydrogen abstraction). Clive has demonstrated that phenyl-substituted vinyl radicals also provide the needed selectivity, as illustrated by the second example in Scheme 57.233... 

Barton, S. D. Ollis, V. D. Comprehensive Organic Chemistry - The Synthesis and Reactions of Organic Compounds, Vol. 1, Pergamon Press Oxford, 1979. 

Radical deoxygenation of alcohols is important, and the reduction of xanthates prepared from alcohols, with Bu3SnH in the presence of AIBN is called the Barton-McCombie reaction (eq. 2.13) [37-51]. The driving force for the reaction is the formation of a strong C=0 bond from the C=S bond, approximately 10 kcal/mol stronger. This reaction can be used for various types of substrates such as nucleosides and sugars. Though methyl xanthates, prepared from alcohols with carbon disulfide and methyl iodide under basic conditions are very frequently used, other thiocarbonates, as shown in eq. 2.14, can also be employed. 

Both Bu3SnH and (Me3Si)3SiH are able to reduce alkyl iodides or bromides but not alcohols. However, in the Barton-McCombie reaction, they reduce certain alcohol derivatives, namely, ones that contain a C=S double bond (e. g., thiocarboxylic esters or thiocarbonic esters). Figure 1.39 shows how the OH group of cholesterol can he removed by means of a Barton-McCombie reaction. The C=S-containing alcohol derivative used there is a xanthate. 

The book explores the invention of new chemical reactions for use in the synthesis of biologically and economically important compounds. It begins with a mechanistic study of the industrial importance of the pyrolysis of chlorinated alkanes. It continues with a theory on the biosynthesis of phenolate derived alkaloids involving phenolate radical coupling. Included in the book is a description of the work on nitrite photolysis (the Barton Reaction) which involved the invention of new radical chemistry leading to a simple synthesis of the hormone, aldosterone. In two final chapters Dr Shyamal Parekh views Professor Barton s pioneering work from the modern perspective, with a review of recent applications in industry and research. 

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