The structures and reactions of stannylene acetals from carbohydrate-derived trans-diols. Part I. In the absence of added nucleophiles

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Abstract
Di-n-butylstannylene acetals of benzyl 4,6-O-benzylidene-α- and -β-D-glucopyranoside and galactopyranoside have been prepared and studied in solution by 1H, 13C, and 119Sn nuclear magnetic resonance spectroscopy. The species present in solution have been identified from the 119Sn nmr spectral data, by comparison of the 13C nmr chemical shifts of the stannylene acetals and their precursor diols and also by analysis of the products of reactions performed without added nucleophiles. The orientations of the two substituents on the carbons in the pyranose ring attached to the carbons in the stannylene ring determine the structures adopted by the stannylene acetal in solution. If one substituent is axial and the other equatorial, the stannylene acetal exists as a single symmetrical dimer in which the two oxygen atoms in the two 1,3,2-dioxastannolane rings adjacent to the axial substituents are dicoordinate. A stannylene acetal with two adjacent equatorial substituents exists as a non-interconverting mixture of dimers; one with two adjacent axial substituents is present as a rapidly interconverting mixture of dimers, trimers, and tetramers. Benzoylation and benzylation of the latter two types of stannylene acetals have been performed and have been shown to be only slightly regioselective in contrast to the known highly regioselective reactions of the first type. Only when single dimers are present are regiospecific or highly regioselective reactions obtained. The causes of the variation in the species present and of the reaction regioselectivity for different stannylene acetals are discussed. Keywords: stannylene acetals, 1,3,2-dioxastannolanes, 119Sn NMR spectroscopy, regioselective reactions, carbohydrates.

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