Shiro Ikegami

Novel biological function of sialic acid (N-acetylneuraminic acid) as a hydrogen peroxide scavenger

We have found that N‐acetylneuraminic acid (NANA) consumes toxic hydrogen peroxide (H2O2) under physiological conditions. Close investigation of this finding revealed that NANA was oxidized by an equimolar amount of H2O2 to provide its decarboxylated product, 4‐(acetylamino)‐2,4‐dideoxy‐D‐glycero‐D‐galacto‐octonic acid (ADOA). To date, there have been little data on this reaction, and its physiological significance has not been discussed. Examining the detoxification of H2O2 in cultured cells with NANA, we were able to confirm that the cell death caused by H2O2 was suppressed by NANA in a dose‐dependent manner. These results revealed a novel role for NANA as a reactive oxygen scavenger. It is known that terminal NANA residues are removed by neuraminidase and that free NANA molecules are recycled or degraded by enzymes. We propose that released monomeric NANA is the potent defense molecule against oxidative damage.

An efficient heterogeneous Heck reaction promoted by a new assembled catalyst of palladium and non-cross-linked amphiphilic polymer

Abstract A newly assembled insoluble catalyst 1b composed of palladium and non-cross-linked amphiphilic polymer was developed. It was named PdAS-V: a supramolecular complex of (NH 4 ) 2 Pd Cl 4 ( 2 ) and poly[( N -isopropylacrylamide) 5 - co -(4-diphenylstyrylphosphine)] ( 3b ). The use of 8.0×10 −7 –5.0×10 −5 mol equiv. of PdAS-V efficiently catalyzed the Heck reaction of aryl iodides with acrylates and styrenes. The turnover number of PdAS-V reached up to 1,150,000, and the catalyst was recycled five times without any decrease in its activity.

An efficient synthesis of new 1′-C-methyl-α-O-disaccharides using 1-methylenesugars as the glycosyl donors

A series of new 1′-C-methyl-α-disaccharides were firstly synthesized by the directly Lewis acid-catalyzed O-glycosidation of 1-methylenesugars used as glycosyl donors. The O-glycosidation afforded stereospecifically the corresponding 1′-methyl-α-O-glycosides whose configurations were tentatively assigned by the NMR spectra, COSY and the C–H coupling constant 3JH−2′,Me−1′. The O-glycosidation of the acetyl protected 1-methylenesugars showed that the acetyl group at the C-2-O position was not effective to control the stereochemistry of the product by neighboring group participation because of the formation of a tertiary oxycarbenium ion as a stable intermediate.

Stereoselective synthesis and structure elucidation of spiro-ketodisaccharides

Abstract Cycloglycosylation of 3,4,5,7-tetra-O-benzyl-α- d -hept-2-ulopyranoses (2a–c) was carried out stereoselectively under the catalysis of Lewis acid to afford two spiro-cyclodisaccharides 3a–c and 4a–c in good yields. The reaction provided the kinetic products 3a–c or the thermodynamic products 4a–c as the predominant products under different conditions, respectively. The unprotected disaccharides 5a–c and 6a–c and the acetylated derivatives 7a–c and 8a–c were prepared by catalytic hydrogenation and followed by acetylation. The structures of compounds 4a–c, 6a–c and 8a–c were confirmed to be α,β-anomeric configuration with chair–chair–chair form for the tri-cycles based on the X-ray crystallographic analysis of 6a–c. The α,α-anomeric configurations of compounds 3a–c, 5a–c and 7a–c were determined based on the measurements of the three bond coupling constants 3JC,H between the C-1 and the H-3 of 7a–c.

Efficient synthesis of 1-deoxy-azasugars as useful synthetic tools

1-Deoxy-azasugars are efficiently prepared from sugar-lactones using a stereoinversion process and they are applied to the synthesis of a natural product.

Synthesis of cyclophellitol utilizing a palladium chloride mediated-Ferrier-II rearrangement.

Cyclophellitol and its C3-epimer have been synthesized from 5-enoglucopyranoside and 5-enomannopyranoside, respectively. The carbocyclic skeleton was constructed through a Ferrier-II reaction meditated by PdCl2.

Short-step synthesis of 5-thio-L-hexopyranoses from D-glyconothio-O-lactones

We developed a synthetic method for 5-thio-L-hexopyranose from D-glyconothio-O-lactone using the initial ring-opening reaction and subsequent recyclization under Mitsunobu conditions. Using this method, only a three-step transformation is required from D-glyconothio-O-lactone to 5-thio-L-glyconolactone.

Catalytic Decarboxylative Glycosylation Promoted by Hafnium(IV) ­Trifluoromethanesulfonate: β-Selective Glycosylation via a Mixed Carbonate of an Acyl-Protected Donor Sugar

Acyl-protected mixed carbonates were effectively decarboxylated by catalytic amounts (2-5 mol%) of hafnium(IV) trifluoromethanesulfonate in dichloromethane at room temperature to give the corresponding β-glycosides in excellent yields.