Radical ribofuranosyl

Guanine, 9-/3-D-ribofuranosyl-, 5, 536 Guanine, 6-thio-tautomerism, 5, 509 toxicity, 1, 141 Guanine, 8-trifluoromethyl-synthesis, 5, 574 Guanines, thio-synthesis, 5, 572 Guanosine arylation, 5, 538 dipole moment, 5, 522 free radical alkylation, 5, 543 hydrobromide... 

Y. Araki, T. Endo, M. Tanji, J. Nagasawa, and Y. Ishido, Additions of a ribofuranosyl radical to olefins. A formal synthesis of showdomycin. Tetrahedron Lett. 29 351 (1988). 

Access to 1,4-anhydro-D-alditols by radical reduction has received only limited attention, but the most detailed work concerns the reduction of protected glycofuranosyl isocyanides on heating for 2 h at 80 °C in toluene in the presence of /J-BU3S11H (2 eq) and AIBN.188 In particular, 2,3,5-tri-0-benzoyl-0-D-ribofuranosyl isocyanide (53) was converted in 78% yield into l,4-anhydro-2,3,5-tri-0-benzoyl-D-ribitol (54) also accessible from phenyl 2,3,5-tri-0-benzoyl-1 -thio-D-ribofuranoside (55).189... 

Ethylene and thioformaldehyde are the products of irradiation of matrix-isolated thietane at lOK. Sulphur-carbon bond homolysis has also been observed on irradiation of the nucleoside membrane transport inhibitor, 6-[ (4-nitrobenzyl) thio] -9- (/8-D-ribofuranosyl) -purine (94), and the oxazolidin-2-one (95) has been converted into the allyl derivative (96) by photochemically induced radical allylation.Efficient conversion of cyclic thioacetals into the corresponding carbonyl compounds under neutral conditions has been achieved by 2,4,6-triphenylpyrylium tetrafluoroborate-sensitised irradiation in moist dichloromethane,and diaryl sulphides and the corresponding sulphoxides and sulphones have been reported to undergo anion-promoted carbon-sulphur bond photocleavageboth processes appear to involve an initial electron transfer. Sulphur-hydrogen bond horaolysis has been reported in t-butanethiol and is also responsible for the photoinitiated thiylation of fluorobromoethylenes and of trialkylethynylsilanes and t-butylacetylene. 

Benzothiazol-2-yl C-nucleosides were also obtained by a different mode of synthesis that involved the free radical coupling of the sugar subunit with the heterocycle residue. The 2-deoxy-D-ribofuranosyl free radical 170, formed by photolysis of the 2-thiopyridone-iV-(2,5-anhydro-3-deoxy)-D-allonoate 169, reacted with benzothiazolium camphorsulfonate to form a mixture of 2-(a and 3-2-deoxy-D-ribofuranosyl) benzothiazoles 171 (92CL1673) (Scheme 50). 

Togo, Yokoyama, and their co-workers prepared the lepidin-2-yl homo C-nucleoside 254 by C—C bond formation between the o-ribofuranosyl-methyl free radical 253, generated by photolytic or thermolytic decarboxylation of the D-ribofuranosylacetic acid derivative 252, in the presence of HVIC, with lepidinium trifluoroacetate (91TL6559) (Scheme 75). 

Stereoselective addition of ribofuranosyl radicals to a, 9-unsaturated carbonyl compounds was exploited by Araki [22] in the formal synthesis of the antiobiotic Showdomycin 16 (Scheme 4). Barton [23] provided a more direct synthesis of 16 by addition of the ribofuranosyl radical, generated using the sugar telluride 17, to maleimide. Ribofuranosyl radicals also undergo addition to Tt-systems of electron-deficient aromatics such as 18, which provides a direct route to C-nucleosides [24], Addition of glycosyl radical donors to allyltributyltin was introduced by Keck... 

Free radical coupling of 5-methyl l-(/3-D-ribofuranosyl)dithiocarbonate (97), as ribofuranosyl radical precursor, and A-ethylmaleimide (98) gave a mixture of the a and /3-dihydro-A-ethyl showdomycins 99 (88TL351) (Scheme 18). 

The /3-anisyltelluro-D-ribofuranoside (101) photolytically generated the corresponding ribofuranosyl free radical, which stereoselectively coupled with maleimide to give, after dehydrogenation and de-O-protection, show-domycin (4) (90JA891) (Scheme 19). 

Recently, C-nucleoside synthesis by glycosyl free radical coupling with protonated nitrogen heterocycles started to gain impetus. Thus, photoirradiation of the l-(2,5-anhydro-D-allonoyloxy)pyridine-2-thione derivative 798 gave the D-ribofuranosyl free radical 799 that couples with substituted pyridines to give a mixture of the two anomers of 2-pyridyl C-nucleosides... 

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