Organic synthesis, radicals

Recently, radical species have attracted much attention as useful intermediates for carbon-carbon bond formations. By contrast, in organic synthesis, radicals centered on heteroatoms have not been widely utilized for construction of molecular skeletons with carbon-heteroatom bond formation. In this section we will discuss the generation of alkylideneaminyl radicals, conventionally called iminyl radicals from oxime derivatives by electron transfer reactions. AU -lideneaminyl radicals thus created are utilized for making nitrogen-containing heterocycles. 

W. B. Motherwell, D. Crich Free Radical Chain Reactions in Organic Synthesis (Academic Press 1992)... 

B. Giese Radicals in Organic Synthesis Formation of Carbon-Carbon Bonds (Pergamon Press NY) 1986 Bull. Soc. Chirn. Fr. 1990, 127,675 Tetrahedron 1981, 37, 3073 Tetrahedron 1987, 43, 3541 Advances in Free Radical Chemistry 1990, 1, 121. 

Giese, B Radicals in Organic Synthesis Formation of Carbon-Carbon Bonds, Pergamon Press New York, 1986... 

Thus, radical denitradon has developed as a reliable tool in organic synthesis and has been mainly carried out using dn hydnde in total syntheses of natural products. There is one report in which NaTeH was used for removing the nitro group. Norslanadione, a biologically acdve... 

Other applications of radical denitration in organic synthesis are summarized in Table 7.3. 

Application of radical reactions to organic synthesis has recently received much atrendon, and various important reacdons have been discovered in this field. Alkyl halides, sulfides, seleniJes, and thiocarbonyl compounds have been used as precursors to alkyl radicals. Some examples are illustrated in Scheme 7.18. ... 

The wide variety of methods available for the synthesis of orga-noselenides,36 and the observation that the carbon-selenium bond can be easily cleaved homolytically to give a carbon-centered radical creates interesting possibilities in organic synthesis. For example, Burke and coworkers have shown that phenylselenolactone 86 (see Scheme 16), produced by phenylselenolactonization of y,S-unsaturated acid 85, can be converted to free radical intermediate 87 with triphenyltin hydride. In the presence of excess methyl acrylate, 87 is trapped stereoselectively, affording compound 88 in 70% yield 37 it is noteworthy that the intramolecular carbon-carbon bond forming event takes place on the less hindered convex face of bicyclic radical 87. 

On the basis of the examples addressed thus far, it is clear that radical reactions can accomplish manifold transformations in organic synthesis. One of the outstanding achievements of synthetic radical chemistry is the development of synthetic strategies based on controlled, tandem radical cyclizations. The efficiency of such strategies is exemplified in the substantial and elegant synthetic work of D. P. Curran and his group.54 The remainder of this chapter will address the concise total syntheses of ( )-hirsutene [( )-1]55 and ( )-A9(12)-capnellene [( )-2]56 by the Curran group. 

There is an excellent, if non critical, compilation of absolute and relative rate data for reactions of oxygen-centered radicals covering the literature through 1982389 and for 1982-1992.39 1 Selected data from these and other sources are summarized in Table 3.7 and Table 3.8. The reactions of oxygen-centered radicals and their use in organic synthesis has been recently reviewed by Hartung el uIS )]... 

In 1988 a paper by Zard and coworkers4(, reported that xanlhates were a convenient source of alkyl radicals by reversible addition-fragmentation and used the chemistry for the synthesis of a monoadduct to monomer (a maleimide). Many applications of the chemistry in organic synthesis have now been described in papers and reviews by the Zard group.406 407... 

Although the free-radical chemistry of organocobaloximes is an interesting and useful reaction of some potential in organic synthesis, the validity of the label SH2 and SHi for... 

Table 1 shows the kinetic data available for the (TMSjsSiH, which was chosen because the majority of radical reactions using silanes in organic synthesis deal with this particular silane (see Sections III and IV). Furthermore, the monohydride terminal surface of H-Si(lll) resembles (TMSjsSiH and shows similar reactivity for the organic modification of silicon surfaces (see Section V). Rate constants for the reaction of primary, secondary, and tertiary alkyl radicals with (TMSIsSiH are very similar in the range of temperatures that are useful for chemical transformations in the liquid phase. This is due to compensation of entropic and enthalpic effects through this series of alkyl radicals. Phenyl and fluorinated alkyl radicals show rate constants two to three orders of magnitude... 

Giese B (1986) Radicals in organic synthesis, Pergamon, Oxford... 

A number of synthetic procedures are available (Ai2). (2) For precisely defined stoichiometries, the isobaric, two-bulb method of Herold is preferred H5, H6, H2). (2) To generate compounds suitable for organic synthesis work, graphite and alkali metal may be directly combined, and heated under inert gas (Pl, lA). (5) Electrolysis of fused melts has been reported to be effective iN2). 4) Although alkali metal -amine solutions will react with graphite, solvent molecules co-inter-calate with the alkali metal. Utilization of alkali metal-aromatic radical anion solutions suffers the same problem. 

For a summary of methods of radical formation, see Giese, B. Radicals in Organic Synthesis Formation of Carbon-Carbon Bonds Pergamon Elmsford, NY, 1986, p. 267. For a review on formation of free radicals by thermal cleavage, see Brown, R.F.C. Pyrolytic Methods in Organic Chemistry Academic Press NY, 1980, p. 44. 

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