3.7- Dioxa-2-azabicyclo octanes

S, 4S, 5Sw,85 )-2-Benzyl-8-methyl-4-plienyl-3,7-dioxa-2-azabicyclo[3.3.0 octan-6-one121 ... 

Recently, an intramolecular 1,3-dipolar cycloaddition of transient enantiomerically pure oxa-alkenyl nitrones illustrated a synthesis of enantiomerically pure 3,7-dioxa-2-azabicyclo-[3.3.0]octanes. Treatment of (5)-112 with allyl bromide or cinnamyl chloride in diethyl ether in the presence of silver(I) oxide affords the alkylated esters 173 and 174, respectively. No racemization occurs in this process, as determined by proton nmr. Reduction of the esters with DIBAL at — 72 °C provides the corresponding aldehydes, which are immediately reacted with JV-alkylhydroxylamines in order to minimize racemization. The resulting nitrones 175 cannot be isolated, but undergo spontaneous intramolecular 1,3-dipolar cycloaddition to the enantiomerically pure 3,7-dioxa-2-azabicylo[3.3.0]octanes 176a-d (Scheme 40) [48]. 

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