- Regioselective methylation of methyl glycopyranosides with diazomethane in the presence of transition-metal chlorides and of boric acid
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Partial methylation of the methyl pyranosides of a number of pentoses, hexoses, 6-deoxyhexoses, methyl uronates and their methyl ethers with diazomethane in the presence of transition-metal chlorides and boric acid was studied. It was found for methyl glycosides of pentoses and 6-deoxyhexoses that tin(II), antimony(III), and titanium(IV) chlorides as well as boric acid promoted substitution mainly of OH-3, but with cerium(III) and zinc(II) salts mainly substitution of OH-2 was observed. Methylation of methyl β-l-rhamnopyranoside demonstrated higher reactivity of OH-2 in all cases. The methylation of methyl glycosides of hexoses in the presence of tin(II), antimony(III) and cerium(III) chlorides gave mainly 3-methyl ethers. The 3-methyl ethers, which are not involved in further complexation, accumulated up to 50-80% of the reaction mixture (95-100% of monomethyl ether fraction). Convenient preparative syntheses of methyl ethers for a number of sugars are suggested. Copyright (C) 1999 Elsevier Science Ltd.
- Evtushenko, Evgeny V.
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p. 187 - 200
(2007/10/03)
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- REGIOSELECTIVE ENHANCEMENT OF THE NUCLEOPHILICITY OF THE HYDROXYL GROUPSIN METHYL α-L-RHAMNOPYRANOSIDE BY COMPLEXATION WITH TIN(II) CHLORIDE
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A tentative mechanism for complexanation, and a possible model of a tin(II)chloride-methyl glycoside intermediate complex, have been established largely from analysis of methyl ethers formed on methylation of methyl α-L-rhamnopyranoside and its monomethyl ethers by diazomethane in the presence of a catalytic amount of tin(II) chloride in selected solvents.The complex is mainly formed through displacement of molecules of the donor solvent coordinated to a tin(II) atom by the favorably cis-disposed, hydroxyl groups of the sugar moiety.The spatiel arrangement of the hydroxyl groups plus the distribution of atomic charges at the individual oxygen atoms of hydroxyl groups of the methyl glycoside were found to be the main factors responsible for the selectivity observed.The effect of selected solvents on the stability and/or ability to participate in the formation of the foregoing intermediate complex could not be satisfactorily clarified.
- Toman, Rudolf,Janecek, Frantisek,Tvaroska, Igor,Zikmund, Miroslav
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- Steroid Saponins and Sapogenins of Underground Parts of Trillium kamtschaticum PALL. III. On the Structure of a Novel Type of Steroid Glycoside, Trillenoside A, an 18-Norspirostanol Oligoside
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A glycoside, mp 209-220 deg C (decomp), D -142 degree, C47H70O24, named trillenoside A (I), was isolated from the rhizomes of Trillium kamtschaticum PALL (Liliaceae).The structure of its aglycone (designated trillenogenin, II), mp 250-251 deg C, D -198 degree, C26H36O8, was determined by X-ray crystallographic analysis of the tetraacetyl monobrosyl derivative, mp 242-244 deg C (dec.), D -112 degree, C40H47BrO14S, and I was characterized as 15-oxo-18-nor-25R-spirosta-5,13-diene-1β,3β,21,23α,24β-pentaol-1-O-β-D-apiofuranosyl-(1->3)-α-L-rhamnopyranosyl-(1->2)-3)>-α-L-arabinopyranoside.I is thus a novel type of steroid glycoside, noteworthy in that the aglycone is an 18-nor-spirostane derivative having an enone system in the D-ring and hydroxyl groups at C21 and in the F-ring, and also in that the sugar moiety is a branched-chain tetrasaccharide containing apiose.Keywords - trillenoside A; 15-oxo-18-nor-spirostadiene-pentaol tetraglycoside; Trillium kamtschaticum PALL; structure determination; trillenogenin; X-ray analysis
- Nohara, Toshihiro,Komori, Tetsuya,Kawasaki, Toshio
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p. 1437 - 1448
(2007/10/02)
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