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[摘要]：A systematic study of the suitability of alpha,alpha'-dibromo-o-xylene as a reagent for cyclic o-xylylene protection of vic-diols in different monosaccharide substrates is reported. The installation of this protecting group, formally equivalent to a di-O-benzylation reaction, proceeds with good regioselectivity toward 1,2-trans-diequatorial diol systems in pyranose and furanose rings. Initially, the benzyl ether-type derivative of the more acidic hydroxyl is preferentially formed. Subsequent intramolecular etherification toward the equatorial-oriented vicinal OH is kinetically favored. The methodology has been implemented for the simultaneous protection of the secondary O-2 and O-3 positions of a single D-glucopyranosyl unit in cyclic oligosaccharides of the cyclodextrin (CD) family (cyclomaltohexa-, -hepta-, and -octaose; alpha, beta, and gamma CD). |
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