Sigmatropic Rearrangement of Uronic Acid Derived Saccharide Allyl Ketene Acetals. A Study of Stereochemistry and Structural Limitations

Allyl ketene acetal Claisen rearrangement of urinoc acid derivatives with furanoside structure was studied. Reproducibility of the observed stereodiscrimination was confirmed in the conversion of enantiomeric starting materials. From X-ray structural data of the major reaction product obtained from a cis-pentenyl uronate, the transition state was proven to preferentially adopt a chair-like geometry. Alterations in the protecting group pattern led to a significant decrease in total yields. Use of β-deoxygenated starting material allowed a greater extent of variation, but was accompanied by the total loss of stereodiscrimination. For the first time, the reaction was applied to pyranoid starting material structures. The conversion of a gluco-configurated compound remaining unsuccessful, with the corresponding galacto and 4-deoxy derivatives an interesting stereoselectivity could be observed. This was lost by further structural simplification to a 1,4-dideoxypyranoid ring. This way, however, a substantial improvement in yield could be achieved.

Claisen rearrangements - allyl ketene acetals - furanosiduronates - pyranosiduronates