Synthesis and structure of 4-O,6-O-glycosylidene glycosides

Radical-Mediated C-H Functionalization: A Strategy for Access to Modified Cyclodextrins

A simple and efficient radical C-H functionalization to access modified cyclodextrins (CDs) has been developed. The well-defined conformation of glycosidic and aglyconic bonds in α-, β-, and γ-CDs favors the intramolecular 1,8-hydrogen atom transfer (HAT) promoted by the 6^I-O-yl radical, which abstracts regioselectively the hydrogen at C5^II of the contiguous pyranose. The C5^II-radical evolves by a polar crossover mechanism to a stable 1,3,5-trioxocane ring between two adjacent glucoses or alternatively triggers the inversion of one α-d-glucose into a 5-C-acetoxy-β-l-idose unit possessing a ¹C₄ conformation. The 6^I,IV- and 6^I,III-diols of α- and β-CDs behave similarly to the monoalcohols, forming mostly compounds originating from two 1,8-HAT consecutive processes. In the case of 6^I,II-diols the proximity of the two 6-O-yl radicals in adjacent sugar units allows the formation of unique lactone rings within the CD framework via a 1,8-HAT-β-scission tandem mechanism. X-ray diffraction carried out on the crystalline 1,4-bis(trioxocane)-α-CD derivative shows a severe distortion toward a narrower elliptical shape for the primary face.

Total synthesis of periploside A, a unique pregnane hexasaccharide with potent immunosuppressive effects

Periploside A is a pregnane hexasaccharide identified from the Chinese medicinal plant Periploca sepium, which features a unique seven-membered formyl acetal bridged orthoester (FABO) motif and potent immunosuppressive activities. Here, we show the synthesis of this molecule in a total of 76 steps with the longest linear sequence of 29 steps and 9.2% overall yield. The FABO motif is constructed via a combination of Sinaÿ's and Crich's protocol for the formation of orthoester and acetal glycosides, respectively. The 2-deoxy-β-glycosidic linkages are assembled stereoselectively with judicious choice of the glycosylation methods. The epimer at the spiro-quaternary carbon in the FABO motif has also been elaborated in a stereo-controlled manner. This epimer, as well as the synthetic analogues bearing the FABO motif, retain largely the inhibitory activities of periploside A against the proliferation of T-lymphocyte, indicating the importance of the chemical connection of the FABO motif to their immunosuppressive activity.

Synthesis of Anemoclemoside B, the First Natural Product with an Open-Chain Cyclic Acetal Glycosidic Linkage

Anemoclemoside B (1), the first natural product containing a brand-new glycosidic linkage, the open-chain cyclic acetal linkage, was synthesized.

A highly stereoselective construction of beta-glycosyl linkages by reductive cleavage of cyclic sugar ortho esters

The preparation of beta-glycosides by the reductive cleavage of spiro sugar ortho esters is described in this report. This procedure is based on a concept completely different from those of other methods for glycosylation. Twelve sugar ortho esters that commonly possess perhydrospiro[2H-pyran-2,2'-pyrano[3,2-d][1,3]dioxin] ring systems in their molecules were reduced by LiAlH(4)/AlCl(3) or NaCNBH(3)/AlCl(3). Among these ortho esters, those (9a-12a) prepared from the D-sugar lactones (1-4) and 2, 3-di-O-benzyl-alpha-D-glucopyranoside (7) or those (19a, 20a) prepared from the L-sugar lactones (5, 6) and 2, 3-di-O-benzyl-alpha-D-galactopyranoside (8) were selectively converted into beta-(1 --> 4)-glycosides (9b-12b or 19b, 20b) in excellent yields by the treatment of LiAlH(4)/AlCl(3). In contrast, the ortho esters (13a-16a or 17a, 18a) that were prepared from combinations of the D-sugar lactones and 8 or those of the L-sugar lactones and 7 were efficiently reduced with NaBH(3)CN/AlCl(3) to afford beta-(1 --> 6)-glycosides (13b-16b or 17b, 18b) selectively. It was remarkable that the resulting disaccharides were obtained with extremely high beta-selectivity even in the cases with mannosyl or rhamnosyl glycosides. Moreover, these products would be useful units for the construction of branched saccharides, because the newly formed hydroxy groups could be again glycosylated without further deprotection procedures. The high regio- and stereoselectivity was totally explained by considering the structures and the conformations of these ortho ester molecules and the stereoelectoronic effects of their spiro ring systems. In addition, the preparation of the sugar ortho esters with glucosamine derivatives and the reactivity of these ortho esters are described in this report. N-Phthaloyl glucosamine derivatives (21, 22) were efficiently reacted with the benzyl-protected gluconolactone (1) in the presence of TMSOMe and TMSOTf to afford ortho esters (23a-c). After the conversion of the phthalimido functionality to the dibenzyl amino group, glucosylideneglucosamine (25) was reduced with LiAlH(4)/AlCl(3) to afford beta-(1 --> 4)-glycoside (26) selectively.