Radical-mediated deoxygenation

3-Deoxy and 4-deoxy-p-D-hexopyranosides were prepared from a galactoside precursor by the same strategy, but by employing imidazol-l-ylthiocarbonyl 

Reagents based on the cleavage of a PH bond have been used with success for radical deoxygenation. Hypophosphorous acid and its salts proved to be excellent reducing agents.25 It has the advantage of nontoxicity, low price, and ease of removal from the reaction mixture. 

4-0-Benzylidene derivatives led to comparable amounts of 3- and 4-deoxy derivatives,26 whereas 4,6-O-benzylidene derivatives of glucose suffer deoxygenation at C-6 via the (less stabilized) primary radical, while the analogous galactose derivative led mainly to the 4-deoxy derivatives. The different regioselectivity has been explained considering the relative strain in the fused bicyclic structures.27 

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Difluorospirocyclopropane) analogues (77) of nucleosides have been synthesized by addition of difluorocarbene to (76).60 Radical-mediated deoxygenation of the 2 -position leaves the difluorocyclopropane untouched whereas deoxygenation of the 3 -position induces a ring opening to (78). 

Miscellaneous Reactions. DTBP has been used as a hydrosi-lylation catalyst, even though catalysis by transition metal complexes have largely replaced the radical methods. DTBP has also been used as an oxidant for silanes. Other applications of DTBP include its use as an initiator for radical mediated deoxygenation of alcohols via the corresponding chloroformate or acetate ester (eq 20). It has also been used as an initiator for the reduction of lactones and esters to ethers using trichlorosilane. In a rare example of a nonradical reaction, DTBP has been used in conjunction with titanium(IV) chloride for the formation of chlorohydrin from alkenes (eq 21). ... 

Deoxygenation of Alcohols. A new application of DTBP is its use as an initiator for radical-mediated deoxygenation of alcohols via the corresponding xanthate using diphenylphosphinic acid as the hydrogen source (eq 27). ... 

Domino reactions are not only useful for the construction of molecules, but also for their degradation. This concept is often encountered in nature. Thus, ribonucleotide reductases (RNRs) are enzymes that catalyze the formation of DNA monomers from ribonucleotides by radical mediated 2 -deoxygenation. This process has also been studied... 

Tri-/ -butyltin hydride also serves as a hydrogen-atom donor in radical-mediated methods for reductive deoxygenation of alcohols.131 The alcohol is converted to a thiocarbonyl derivative. These thioesters undergo a radical reaction with tri-w-butyltin... 

Several other types of polymer-bound stannanes have since been reported, including a polymer-bound distannane used in radical-mediated cyclizations and radical addition to acetylenes [87], and stannanes with longer tethers to the polymer [88, 89] applied in the Barton-McCombie deoxygenation of alcohols [90, 91] and... 

A distinctly different reaction cascade leading to tetrahydrofurans is illustrated in Scheme 40 [74]. It was proposed that these reactions proceed by iodine-mediated deoxygenation of the peroxyl radical to an alkoxyl radical, which undergoes intramolecular C-H abstraction to form a y9-keto radical [74], Elimination of an a-hydrogen atom generates an enone, which is ultimately captured intramolecularly by conjugate hydroxyl addition to give a tetrahydrofuran (Scheme 40). 

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