Deoxygenation reactions, functionalization

Addition of carbon radicals to carbon-carbon double bonds is an important reaction that can be carried out under hydrogen transfer conditions [27], Peroxides are usually used as radical precursors and an application of this chemistry is presented in Scheme 3 (Eq. 3.1). More recently, reduction of alkyl radical by C-H hydrogen donor has been examined in order to find an environmentally friendly alternative to tin hydride. Zard has reported a simple and cheap alternative to tin hydride for Barton-McCombie deoxygenation reactions [80]. Heating of xanthates derived from carbohydrates in 2-propanol in the presence of dilauroyl peroxide affords the de-oxygenated products in good yields (Scheme 17, Eq. 17.1). 2-Propanol functions as... 

As with other alkyllithiums, 1 can react with epoxides in an alkylative deoxygenation reaction to provide an allyl alcohol. At least 2 equiv of the alkyllithium are required as ring opening occurs from the a-lithioepoxide, whose formation requires the first equivalent of the alkyllithium to act as a base. With functionalized epoxides such as 4, subsequent reactions can also occur as illustrated in eq 9. An analogous reaction is observed with 3,4-epoxytetrahydrofurans to give 1,2-diols. ... 

Olefins can be prepared by the deoxygenation of epoxides usually in good to excellent yields. However, the reaction is of synthetic use only if the epoxides employed as starting materials are prepared from other functional groups than olefins or if they can be isolated or readily accessed from natural sources. The issue of regioselectivity of deoxygenation is critical, however. 

Pd-catalyzed carbonylation of 2-chloropyrazine 1-oxide failed, whereas that of 3-chloropyrazine 1-oxide (40) proceeded without deoxygenation of the IV-oxide function to give 3-methoxycarbonylpyrazine 1-oxide (73). This observation was in accord with the failure of Stille reactions of 2-chloropyrazine 1-oxide [9,18]. 

Electron-withdrawing groups decrease the rate of the fluoro Pummerer reaction, which, in certain cases,7 allows a DAST-mediated deoxygenation to compete with the introduction of fluorine alpha to sulfur. The reaction is compatible with a number of functional groups and can readily be carried out with nucleosides. Robins and coworkers4 reported the synthesis of a 5 -fluoro-5 -S-phenyladenosine analog using antimony trichloride as catalyst at room temperature. It should be noted that a-fluoro sulfoxides provide a convenient entry to terminal fluoroalkenes.3 8 9 10... 

Another method of functionalizing 72 was initiated by catalytically hydrogenating 78 to 79. Thermolysis of 79 at 220 °C under vacuum resulted in an ethylene extrusion to generate furan 80. Diels-Alder reaction of 80 with dimethyl acetyle-nedicarboxylate (DMAD) provided endoxide 81, which on subsequent deoxygenation with low-valent titanium gave ester 82 (Scheme 17). ... 

Until 1995 the only A-substituted 1,2,5-oxadiazoles for which the chemistry had been examined in any depth were the furoxans, and the first section is therefore devoted to the principal reaction of their A-oxide function, i.e. deoxygenation to the furazan other aspects of their chemistry... 

To demonstrate the problems associated with ex-chiral-pool syntheses, some typical examples are given in this section. Three start with n-glucose, which is by far the most popular substrate in ex-chiral-pool synthesis, and illustrate the key transformations, in the first example, D-glu-cose is transformed into the mannosidase inhibitor iV-acetyl-4-deoxymannosamine by 4-deoxygenation and a SN2 displacement reaction of nitrogen introducing an amino function in place of a 2-hydroxy function 5. 

---