Hydrazulene Ring Systems via Heteroatom-Assisted [1,2]-Shift of Oxonium and Sulfonium Ylides

Cyclic mixed acetals with pendant diazoketone side chains undergo rearrangement to ether-bridged cycloheptane ring systems on treatment with Cu(hfacac)₂. Stevens [1,2]-shift of an oxonium ylide furnishes the major product (7), in some cases accompanied by minor amounts of a product (8) resulting from [1,2]-shift of a sulfonium ylide. In the subsequent sulfur-triggered cleavage of the bridging ether, the desired bicyclo[5.3.0]heptene was obtained, along with the product of novel S_N2‘ attack on the resulting allylic ketal.