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

Abstract

This paper is dedicated to Professor Arthur S. Perlin on the occasion of his 67th birthday T. BRUCE GRINDLEY and RASIAH THANGARASA. Can. J. Chem. 68, 1007 (1990). Di-n-butylstannylene acetals of benzyl 4,6-0-benzylidene-a-and -P-D-glucopyranoside and galactopyranoside have been prepared did studied in solution by 'H, I3c, and nuclear magnetic resonance spectroscopy. The species present in solution have been identified from the '19sn nrnr spectral data, by comparison of the l3C 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.Key words: stannylene acetals, 1,3,2-dioxastannolanes, IL9sn NMR spectroscopy, regioselective reactions, carbohydrates.T. BRUCE GRINDLEY et RASIAH THANGARASA. Can. J. Chem. 68, 1007 (1990). On a prtpart les di-n-butylstannylkne acttals des 4,6-0-benzylidene-a-et -P-glucopyranosides et galactopyranosides de benzyle et on les a ttudits, en solution, en faisant appel h la rtsonance magnttique nucltaire du IH, du 13C et du 119Sn. On a identifit les espkces prtsentes en solution h partir des donntes de la rmn du Il9Sn, par comparaison des dtplacements chimiques en rmn du 13C des stannylkne acttals et des diols qui leur ont donnt naissance ainsi que parune analyse des produits des rtactions rtalistes en l'absence de nucltophiles. Les orientations des deux substituants attachts aux carbones du cycle pyranosique qui font partie du cycle stannylkne dtterminent les structures adopttes par le stannylkne acttal en solution. Si l'un des substituants est axial alors que I'autre est tquatorial, le stannylene acttal existe sous la forme d'un dimkre symttrique unique dans lequel les deux atomes d'oxygkne des deux cycles 1,3,2-dioxastannolanes adjacents aux substituants axiaux sont doublement coordonnts. Un stannylkne acCtal comportant deux substituants tquatoriaux adjacents existe sous la forme...

Control of regioselectivity in reactions of dialkylstannylene acetals. Part II. NMR results and mechanistic analysis

“…The preceding paper outlines the large effects of changing the alkyl groups in dialkylstannylene acetals on the regioselectivity obtained in reactions on dialkylstannylene derivatives of carbohydrate-derived terminal 1,2-diols with ptoluenesulfonyl chloride (2). In a previous publication, we related the regioselectivity obtained in reactions on dibutylstannylene acetals of tmns-1,2-diols to the species present in solution (4). The current results demonstrate that the previous conclusions were oversimplified; this publication outlines a more complete description of the causes of the regioselectivity obtained in reactions of dialkylstannylene acetals.…”

Control of regioselectivity in reactions of dialkylstannylene acetals. Part I. A dramatic reversal of regioselectivity in mono-p-toluenesulfonation reactions

“…For trans-1,2-diols on pyranose rings, reaction regioselectivity of the derived dibutylstannylene acetals was related to the dimeric structures assumed by these compounds in solution (9). For stannylene acetals obtained from diols in which the oxygen atoms are diastereotopic, three dimers are possible.…”

“…The nature of the dimeric mixtures present in solution has been shown to be controlled mainly by steric effects (9). The most populated dimer has the tricoordinate oxygen atoms as far away as possible from the side of the diol that has the most bulky substituents.…”

“…The most populated dimer has the tricoordinate oxygen atoms as far away as possible from the side of the diol that has the most bulky substituents. Dicoordinate oxygen atoms in the dimer are more nucleophilic than tricoordinate oxygen atoms (9). Dibutylstannylene acetals from trans-1,2-diols that exist as single symmetric dimers gave highly regioselective reactions on the dicoordinate oxygen atoms in the dimers.…”

“…Reversal of regioselectivity to direct reaction at secondary Can (9). hydroxyl oxygen atoms in the presence of primary hydroxyls is often highly desirable.…”