Stereoselective Synthesis of Spirocyclic Ketones by Nazarov Reaction

The Suzuki−Miyaura cross-coupling reaction between α-ethoxydienyl boronates and lactone-derived vinyl triflates affords functionalized 6-(1-ethoxy-1,3-butadienyl)dihydropyran derivatives that undergo a Nazarov electrocyclic reaction under mild acidic conditions to give functionalized
spirocyclic ketones. The product distribution and the stereoselectivity of the process are strongly dependent on the substitution of both the
α-ethoxydiene and dihydropyran moieties. High stereoselectivity is observed in the presence of a C2-substituent on the dihydropyran moiety.
The results are explained in terms of transition state geometries.