Furanosides from carbonates

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]. 

The ring contraction of certain pyranosides to give furanosides may be considered as resulting from intramolecular displacement by a neighboring carbon atom. 

Very interestingly, D-ribose complexed within the resorcinol-dodecylidene cyclotetramer (4) in carbon tetrachloride underwent highly efficient and selective conversion to methyl 3-D-ribofuranoside. Several methyl 0-furanoside derivatives of L-e/yt/jfo-pentulose have been described following synthesis from the commercially available sugar. The a- and /3-septanosides (5) were obtained in 43% combined yield on treatment of methyl 4,6-0-isopropylidene-a-D-glucopyranoside with anhydrous pyridinium chloride. ... 

The macrocycle (1) complexes with free sugars (frivouring 2-deoxyribose, ribose > erythrose, arabinose > glucose, mannose > xylose, lyxose) and enables them to be solubilized in carbon tetrachloride from aqueous solution and hence converted to their glycosides with the furanosides being favoured. The preparation of methyl a-D-[U C]glucopyranoside (from the labelled free sugar) has been reported. ... 

A number of 2,6-dideoxy-6,6,6-trifluorohexoses were prepared from a six-carbon acetylenic precursor via selective hydroxylation of the derived alkene diastereomers, and differences in stereoselectivity of reduction of a trifluoro-methyl ketone compared with the corresponding methyl ketone are outlined in Chapter 18. Oxidation of methyl 5-0-benzyl-3(2)-deoxy-3(2)-fluoro-a-D-pento-furanosides (DMSO/TFAA) was accompanied by epimerization at the fluori-nated carbon atom a- to the ketone resulting in formation of the corresponding 2-(or 3-) keto derivatives as mixtures of two epimers. Reduction then afforded various mixtures of 2- and 3- fluoro compounds. ... 

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