Polycyclic oxonium ylides — Use of cyclic acetals as convenient scaffolds in the construction of fused bicyclic compounds containing a medium ring

Cyclic mixed acetals with pendant diazoketone side chains undergo efficient rearrangement to ether-bridged cyclooctanoid and cycloheptanoid systems upon treatment with Cu(hfacac)2. Stevens [1,2]-shift of an oxonium ylide furnishes the major product, in some cases accompanied by minor amounts of a product resulting from [1,2]-shift of a sulfonium ylide. These results demonstrate that hetero-substituted carbons are suitable migrating groups for the Stevens [1,2]-shift of oxonium ylides. In cases employing a mixed thioacetal, the resulting sulfide served as a trigger for cleavage of the bridging ether through one of two complementary strategies. In the hydrazulene series, the desired bicyclo[5.3.0]heptene was accompanied by the product of novel transannular SN2′ attack on the resulting allylic ketal.Key words: [1,2]-shift, diazo, medium-sized ring, oxonium ylide, ring expansion.