C-Furanosides

By using either one of these photosystems, one-electron (3-activation of a,(3-unsaturated carbonyl compounds produced carbon-centered radical precursors which cyclize efficiently and stereoselectively to tethered activated olefins or carbonyl groups. The 1,2-anti-stereochemistry observed contrasts with the general trend of syn-stereochemistry expected in 5-hexenyl radical cyclizations. Application of this methodology was successfully demonstrated by the stereoselective synthesis of optically pure C-furanoside, starting from L-tartaric acid (Scheme 38) [57,58]. 

Scheme 38 PET-supported synthesis of optically pure C-furanosides.

The generation of ketyl radicals and their intramolecular addition to tethered olefin to obtain cyclopentanol derivatives have also been achieved by Pandey et al. [102] by using such as those shown in Figures 8.2. An interesting application of this strategy is demonstrated by the synthesis of C-furanosides 231 (Scheme 8.64). 

Pandey, G., Hajra, S., Ghorai, M.K., and Kumar, K.R. (1997) Visible light initiated photosensitized electron transfer cyclizations of aldehydes and ketones to tethered a,p-unsaturated esters stereoselective synthesis of optically pure C-furanosides. Journal of Organic Chemistry, 62, 5966-5973. 

C-glycosides or C-furanosides (2), but without stereoselectivity. However, the reaction with a chlorodimethylsilane shows considerable selectivity. The difference... 

Although technically ether formations, the displacement of halides by hydroxyl groups differs from the examples presented above in that these reactions are base mediated. As such, under these conditions, C-glycosidations complement the previously described cyclization strategies given that they allow reactions with acid sensitive substrates. As shown in Scheme 7.100, Schmid and Whitesides [244] used this approach in the preparation of C-furanosides. Specifically, the illustrated... 

The methods described thus far represent only a small subset of the available techniques for the formation of C-furanosides and C-pyranosides. Other useful methods include cyclizations of halo olefins and ene-ynes [224] and the direct modification of pyran ring systems [251]. Although not discussed further here, these examples are mentioned to illustrate the breadth of work accomplished in this area. 

Photooxygenation of furanosyl furans can lead to either O- or C-furanosides, depending on the transformation of the intermediate endoperoxides <05JOC6503>. Photosensitized oxidation of 2-trimethylsilylfuran was used to construct the fcw-spiroketal core of prunolides. As shown below, both the (Z)- and ( -isomers provided the same 2 1 mixture of the trans and cis products <05OL2357>. 

Although the above results appear useful for the preparation of C-furanosides, these reactions are not always this specific. As shown in Scheme 2.3.31, Martin, et al.,60 attempted a similar reaction with the simpler furanose shown. Although a good yield was achieved for this reaction, the stereochemical outcome presented a 1 1 mixture of anomers. However, as shown in Scheme 2.3.32, when intramolecular delivery of the allyl group was effected, the reaction yielded a 5 1 mixture favoring the p anomer. Thus, this approach is a potentially useful route to p-C-glycosides. 

As shown in Scheme 7.3.1, Frick, etal.,13 formed the perbenzylated polyol on treatment of the lithiated flavonoid with the ring opened form of penta-O-benzylglucose. As illustrated, catalytic hydrogenation in acetic acid produced a C-furanoside after three hours and a C-pyranoside after two days. Additionally,... 

This overall process is also an efficient route to C-furanoside in the ribosc scries equations I and 11). 

Furanoid Compounds.—As has become usual, a range of methods have been applied to the synthesis of compounds of this series. Scandium perchlorate, an unusual Lewis acid catalyst, catalysed the reaction of 2,3,5-tri-O-benzoyl-P-D-ribopyranosyl acetate and a-(trimethylsilyloxy)styrene to give the benzoylmethyl C-furanosides in very high yield with 94 6 selectivity in favour of the a-anomer. In parallel work the analogous benzylated ribofuranosyl... 

Note that the reaction is under thermodynamic control when performed at 90 °C. In contrast, at 10 °C, ketone C-furanosides were formed as the main products. Interestingly, in the presence of CeCls instead of NaHCOg, polyhydroxyallq l- and C-glycosylfurans were formed (Scheme 16). Such dichotomy was also observed with the use of indium chloride as the catalyst. ... 

C. Coiffier, Top-down approach for synthesis of new biologically active substrates analysis of preferred conformations of C-furanosides by means of theoretical chemistry , PhD Thesis, University of Reims Champagne-Ardenne, Reims, 2009, 297 pages. 

SCHEME 13.18 (a) Acid-catalyzed ring-opening of glycal derived gem-dibromo-cyclopropane derivative 58 and (b) rearrangement of halo-oxepines to C-furanosides 61 and 62 [30],... 

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