Furst-Plattner rule epoxides

On nucleophilic attack, the epoxide ring opens according to the Furst-Plattner rule, with the formation of the di-axial derivative, because the pyranoid rings in the frons-linked derivatives (98) and (99) are completely fixed and the cfs-linked derivatives (100) and (101) are fixed to a considerable extent in the Cl form. (The corresponding members of the L series belong to the 1C form ) The 2-deoxy derivatives are prepared by the mercaptal method, and also, more easily, by direct reduction of the anhydro derivatives (98) and (100) with lithium aluminum hydride. Only the riho and xylo configurations, (102) and (104), are accessible (X = H), but these are obtained in very good yield. This... 

The observation that the methylative ring opening of the cis-epoxide 56 did not, according to the Furst-Plattner rule, take place at C3 (trans-diaxial process) but at C4 to provide 58 could be explained by the less stable but chelation-supported 56a. To prove this effect, the reaction was reran but this time in the presence of a crown ether to sweep all the cations from the solution. 

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