An Efficient 4 + 2 + 1 Entry to Seven-Membered Rings

A new nickel-catalyzed procedure for the 4 + 2 + 1 cycloaddition of trimethylsilyl diazomethane with alkynes tethered to dienes has been developed. A broad range of unsaturated substrates participate in the sequence, and stereoselectivities are generally excellent. Three possible mechanisms are proposed, and each involves the generation of a transient nickel carbene species.