Enantiomerically pure decalinic structures from carbohydrates using intramolecular Diels-Alder and Ferrier carbocyclization

The synthesis of enantiomerically pure decalinic structures, advanced intermediates for the synthesis of the hexahydronaphtalen part of mevinic acids, is described. The key steps are the intramolecular Diels-Alder cycloaddition of a suitably substituted sugar enone obtained via the Ferrier rearrangement of tri-O-acetyl-D-glucal with the appropriate alcohol representing the diene part of the system. Chemical manipulation of the resulting, diastereomerically pure, cycloadduct led to a 5,6-unsaturated carbohydrate which was submitted to the Ferrier carbocylization. This reaction proved difficult likely because of the embedding of the aglycon in a cycle, the hydration product of the double bond being the main product. This compound was in turn transformed into the decalinic structures by treatment in basic medium. The inverse strategy, Ferrier carbocyclization followed by IMDA of an enone and a diene linked via an ester bond was unsuccessful.