Stereoselective Synthesis of Spirocyclic Ketones by Nazarov Reaction

The Suzuki-Miyaura cross-coupling reaction between -ethoxydienyl boronates and lactone-derived vinyl triflates affords functionalized 6-(1-ethoxy-1,3-butadienyl)dihydropyran derivatives which 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 -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.