1.Stereoselective direct glycosylation with anomeric hydroxy sugars by activation with phthalic anhydride and trifluoromethanesulfonic anhydride involving glycosyl phthalate intermediates.
Kim KS1, Fulse DB, Baek JY, Lee BY, Jeon HB. J Am Chem Soc. 2008 Jul 2;130(26):8537-47. doi: 10.1021/ja710935z. Epub 2008 Jun 4.
An efficient direct one-pot glycosylation method with anomeric hydroxy sugars as glycosyl donors employing phthalic anhydride and triflic anhydride as activating agents has been developed. Thus, highly stereoselective beta-mannopyranosylations were achieved by the reaction of 2,3-di-O-benzyl-4,6-O-benzylidene-D-mannopyranose (2) with phthalic anhydride in the presence of DBU at room temperature followed by sequential addition of DTBMP and Tf2O and glycosyl acceptors to the reaction mixture at -78 degrees C in one-pot. Stereoselective alpha-glucopyranosylations with 2,3-di-O-benzyl-4,6-O-benzylidene-D-glucopyranose (25) and other glycosylations with glucopyranoses and mannopyranoses having tetra-O-benzyl- and tetra-O-benzoyl protecting groups were also possible by utilizing the present one-pot glycosylation protocol. The possible mechanism for the beta-mannosylation with 2 was proposed based on the NMR study, in which alpha-mannosyl phthalate 55alpha and alpha-mannosyl triflate 59 were detected as intermediates.
2.Synthetic galactomannans with potent anti-HIV activity.
Budragchaa D1, Bai S1, Kanamoto T2, Nakashima H2, Han S1, Yoshida T3. Carbohydr Polym. 2015 Oct 5;130:233-42. doi: 10.1016/j.carbpol.2015.04.047. Epub 2015 May 22.
Ring-opening polymerization of a new 1,6-anhydro disaccharide monomer, 1, 6-anhydro-2, 3-di-O-benzyl-4-O-(2', 3', 4', 6'-tetra-O-benzyl-α-d-galactopyranosyl)-α-d-mannopyranose, was carried out using PF5 as a catalyst under high vacuum at -60°C to give galactose branched mannopyranan (synthetic galactomannan), 4-O-α-d-galactopyranosyl-(1→6)-α-d-mannopyranan, after debenzylation with Na in liquid NH3. The ring-opening copolymerization with 1, 6-anhydro-tri-O-benzyl-α-d-mannopyranose in various feeds was also performed to give synthetic galactomannans with various proportions of galactose branches. After sulfation, sulfated synthetic galactomannans were found to have anti-HIV activity and cytotoxicity as high and low as those of standard curdlan and dextran sulfates, respectively, which are potent anti-HIV sulfated polysaccharides with low cytotoxicity. The anti-HIV mechanism of sulfated synthetic galactomannans used by poly-l-lysine as a model peptide of the HIV surface protein was estimated by using SPR, DSL, and zeta potential measurements, revealing the electrostatic interaction between negatively charged sulfate groups and positively charged amino groups.
3.Is donor-acceptor hydrogen bonding necessary for 4,6-O-benzylidene-directed beta-mannopyranosylation? Stereoselective synthesis of beta-C-mannopyranosides and alpha-C-glucopyranosides.
Crich D1, Sharma I. Org Lett. 2008 Nov 6;10(21):4731-4. doi: 10.1021/ol8017038. Epub 2008 Oct 1.
2,3-Di-O-benzyl-4,6-O-benzylidene-thiohexopyranosides, on activation with 1-benzenesulfinyl piperidine and triflic anhydride, react with allyl silanes and stannanes, and with silyl enolethers to give C-glycosides. In the mannose series the beta-isomers are formed selectively whereas the glucose series provides the alpha-anomers. This selectivity pattern parallels that of O-glycoside formation and eliminates the need to consider donor-acceptor hydrogen bonding in the formation of the O-glycosides.
4.Regioselective ring opening of benzylidene acetal protecting group(s) of hexopyranoside derivatives by DIBAL-H.
Tanaka N1, Ogawa I, Yoshigase S, Nokami J. Carbohydr Res. 2008 Oct 13;343(15):2675-9. doi: 10.1016/j.carres.2008.07.017. Epub 2008 Aug 3.
The reductive ring-opening reaction of benzylidene-protected glucosides and mannosides such as methyl 2,3-di-O-benzyl-4,6-O-benzylidene-alpha-d-glucoside (1) and methyl 2,3-di-O-benzyl-4,6-O-benzylidene-alpha-d-mannoside (4) by using a toluene stock solution of DIBAL-H and a dichloromethane stock solution of DIBAL-H gives mainly or selectively the corresponding 2,3,4-tri-O-benzyl derivatives (2, 5) and 2,3,6-tri-O-benzyl derivatives (3, 6), respectively, although that of methyl 2,3-di-O-benzyl-4,6-O-benzylidene-alpha-d-galactoside (7) gives methyl 2,3,6-tri-O-benzyl-alpha-d-galactoside (9) selectively irrespective of the solvent of the stock solution of DIBAL-H. Similarly, the reaction of the exo-isomer of methyl 2,3:4,6-di-O-benzylidene-alpha-d-mannopyranosides (exo-10) with a toluene stock solution and dichloromethane stock solutions of DIBAL-H selectively gives methyl 3-O-benzyl-4,6-O-benzylidene-alpha-d-mannoside (12) and methyl 2-O-benzyl-4,6-O-benzylidene-alpha-d-mannoside (11), respectively, although that of endo-10 selectively affords 11, irrespective of the solvent of the stock solution of DIBAL-H.