Aza-[4 + 2] annulations of α-vinyl Morita–Baylis–Hillman adducts for regiodivergent synthesis of exomethylene-bearing azatricyclic compounds

Abstract

Aza-[4 + 2] annulation reactions between α-vinyl Morita–Baylis–Hillman (MBH) adducts and cyclic imines have been developed, achieving regiodivergent synthesis of two distinct types of azatricyclic products. The process initiates with an S_N2′′ addition of triphenylphosphine to the MBH adducts, forming a pentadienyl phosphonium intermediate. This intermediate then undergoes aza-[4 + 2] annulation with saccharin-derived ketimines to yield sultam-fused tetrahydropyridines with exomethylene groups in moderate to good yields. Alternatively, employing DABCO and trifluoroacetic acid facilitates an aza-[4 + 2] annulation with 3,4-dihydroisoquinoline, resulting in the formation of exomethylene-bearing benzoquinolizidines with reversed orientational regioselectivity.