Radical induced deoxygenations

Imidazole-A-thionocarboxylates (also called thiocarbonylimidazolides ) are intermediates in a convenient radical-induced deoxygenation of primary and secondary alcohols with tributylstannane (Barton reaction).[1],[2]... 

Primary alcohols (a) D. H. R. Barton, W. B. Motherwell, and A. Stange, Radical-induced deoxygenation of primary alcohols, Synthesis, p. 743 (1981) (b) D. H. R. Barton, P. Blundell, J. Dorchak, D. O. Jang, and J. C. Jaszberenyi, The invention of radical reactions. Part XXI. Simple methods for the radical deoxygenation of primary alcohols, Tetrahedron 47 8969 (1991). 

Barton, D. H. R., Motherwell, W. B., Stange, A. Radical-induced deoxygenation of primary alcohols. Synf/tes/s 1981,743-745. 

Addition of trifluoromethyltrimethylsilane to l,2-0-isopropylidene-5-0-Tbdms-a-D-er f/iro-pentofuranos-3-ulose (prepared from D-xylose) afforded the corresponding 3-C-trifluoromethyl-ribose derivative 33. After de-silylation at 0-3 (NaOMe/MeOH) and radical-induced deoxygenation, the 3-deoxy-3-C-trifluoro-methyl-D-ribose derivative 34 was formed. D-Glucose was also used as starting material. ... 

Leitner, N.K.V. and Dore, M., Hydroxyl radical induced decomposition of aliphatic acids in oxygenated and deoxygenated aqueous solutions, /. Photochem. Photo-biol, A, 99(2-3) 137-143, 1996. 

Janik I, Ulanski P, Flildenbrand K, Rosiak JM, von Sonntag C (2000) Hydroxyl-radical-induced reactions of poly(vinyl methyl ether) a pulse radiolysis, EPR and product study in deoxygenated and oxygenated aqueous solutions. J Chem Soc Perkin Trans 2 2041-2048 Jayson GG, Stirling DA, Swallow AJ (1971) Pulse- and X-radiolysis of 2-mercaptoethanol in aqueous solution. Int J Radiat Biol 19 143-156... 

Schuchmann H-P, von Sonntag C (1988) The oxidation of methanol and 2-propanol by potassium peroxodisulphate in aqueous solution free-radical chain mechanisms elucidated by radiation-chemical techniques. Radiat Phys Chem 32 149-156 Schuchmann H-P, Wagner R, von Sonntag C (1983) y-Radiolysis of 2 -deoxycytidine-5 -phosphate in deoxygenated aqueous solutions. OH radical-induced alternations at the sugar moiety. Z Naturforsch 38b 1213-1220... 

Stelter L, von Sonntag C, Schulte-Frohlinde D (1976) Phosphate ester cleavage in ribose-5-phos-phate induced by OH radicals in deoxygenated aqueous solution. The effect of Fe(ll) and Fe(lll) ions. Int J Radiat Biol 29 255-269... 

Deeble DJ, Schulz D, von Sonntag C (1986) Reactions of OH radicals with poly(U) in deoxygenated solutions sites of OH radical attack and the kinetics of base release. Int J Radiat Biol 49 915-926 Denk O, Washino K, Schnabel W (1983) OH radical-induced chemical reactions of polynucleotide complexes. Makromol Chem 184 165-173... 

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). 

Detailed studies concerning the products that are formed after OH-radical attack (radiolysis of NjO-saturated solutions) have been carried out for uracil [27], thymine [28, 29], cytosine [30] and 1,3-dimethyluracil [26]. In addition, the radiolysis of uracil in deoxygenated solutions (in the absence of N2O) has also found attention [31, 32]. Under such conditions, however, not only do the OH-radical-induced reactions play a role, but also the electron-adduct radical with all the ensuing mechanistic complications contribute to the products. 

Thermolysis ° and radical-induced reactions (such as reductionf or termination ) offer a solution and can provide complete desulfurization. Radical-induced reduction of low molecular weight thiocarbonylthio compounds is well known.P Radical-induced reduction of xanthates is the basis of the Barton- McCombie reaction for deoxygenation of secondary alco-hols. ° Stannanes are the most efficient reagents for use in this process but are toxic and residual reagent and the devived reaction bydroducts can be difficult to remove. Hypophosphite salts/ including N-ethylpiperidine hjrpophosphite (17)/ have been recommended as an alternative to stannanes in the radical induced reductions. 

Interest continues in the synthesis of deoxydisaccharides. A simple route to 2 -deoxv-D-arabino-hexosyl compounds proceeds from l,2-di-Q-acetyl-3,4,6-tri-Q-benzyl- -D-glucose, the glycosylation being catalysed by trimethylsilyl triflate and the deoxygenation at c-2 effected by radical induced reduction of derived xanthate esters. Compounds (46) and (47), obtainable... 

Since the deoxygenation of compound 38 was well characterized using classical Barton-McCombie conditions, Roberts chose this same model substrate to illustrate the viability of silane/thiol reduction protocol. Reduction with this system gave a good yield of 39, however, it was observed that minor diasteromer 40, resulting from radical-induced epimerization at C-5, could also be isolated. The rationale for the formation of 38 was potentially due to the thiyl catalyst abstracting H from the substrate faster than from the silane. The authors propose that the use of a better donor, such as diphenylsilane, could prevent such side-products from occurring. 

Two-photon absorption chemistry of 2AP, specifically photoionization processes, can be induced by intense nanosecond 308-nm XeCl excimer laser pulses [10]. Typical transient absorption spectra of 2AP in deoxygenated neutral aqueous solutions are shown in Fig. 1. The stronger (385 nm) and weaker (510 nm) absorption bands were assigned to 2AP radicals derived from the ionization of 2AP (bleaching near 310 nm) [10], whereas a structureless absorption band from -500 to 750 nm corresponds to the well-known spectrum of the hydrated electron (eh ) [41]. 

Scheme 8.6 Cp2TiCl induced the deoxygenation and reduction of epoxide ring opening by radical chemistry.

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