Acetoxy groups, analysis

The use of lead tetraacetate for adding acetoxy groups to a double bond, as here used for preparing the pentaacetate (76), has been widely applied in synthetic organic chemistry, but there have been only a few applications to carbohydrates (see Perlin s review in this Series, for other examples). The conversion of (76) to the dicarbonyl derivative (77) (in 65% yield and in the crystalline state) was made by autohydrolysis in water at room temperature for 3 hr. The structure was deduced from elemental analysis, acetyl analysis, and the absence of methoxyl groups, and from its mode of synthesis. The acetyl determination could be made by direct titration with... 

H-NMR spectroscopic analysis of purine-like C-nucleosides revealed the presence of a hydrogen bond between N-1 and OH-5, an indicator of syn- over a/i//-conformational preference, in compound 6, but not in 1 The bicyclic deoxynucleoside derivatives 8 have been subjected to conformational analysis by H-NMR spectroscopy, to show that a r-exo/2 -emfo-arrangement is preferred and that the acetoxy group at C-5 is axially disposed, ie., the torsion angle y is in the unusual jjn-clinal range. ... 

Vedejs et al., on the basis of both DFT calculations and of X-ray diffraction analysis of compound 6, proposed a qualitative model for the stereochemical induction observed with this catalyst [18b]. These authors conjectured that in the transition state of the Steglich and related rearrangements, to minimize the steric interactions, the endocyclic oxygen of the azlactone is oriented towards the acetoxy group and that the C4-substituent is placed between the two oxygens of the phe-noxycarbonyl group (Figure 40.1). This model would account for the preferred C4 Re-face attack observed with catalyst (R)-6. >... 

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