Polysubstituted 7-oxabicyclo

Enantiomerically pure 7-oxanorbornenyl derivatives 319, 320, 321, their saponification products (recovery of the chiral auxiliary in the aqueous phase) and ketones (+)-322 and (—)-322 are coined naked sugars of the first generation because they are chirons (= enantiomerically pure synthetic intermediates) like those derived from natural hexoses. Like natural sugars, they are enantiomerically pure. However, unlike natural sugars, they possess three unsubstituted (naked) carbon centers, the substitution of which follows highly stereoselective methods giving polysubstituted 7-oxabicyclo[2.2.1]heptane-2-ones that can be oxidized into the corresponding uronolactones as illustrated below. 

Oxyallyl carbocations can be generated efficiently using diethylzinc together with polybromoketones. These intermediates can be trapped with fitran-alcohols to give good yields of polysubstituted 8-oxabicyclo[3.2.1]oct-6-en-3-ones <96S31, 96JCS(P1)1101>. A route to... 

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