Neighboring group effect of pyridazine and pyrazine rings for π-facial selectivity in the reactions of fused isopropylidenenorbornene systems with electrophilic reagents

Abstract

A series of pyridazine- and pyrazine-fused isopropylidenenorbornenes have been synthesized and the π-facial selectivity of the electrophilic reactions with 4-phenyl-1,2,4-triazole-3,5(4H)-dione (PTAD), m-chloroperbenzoic acid (MCPBA) and N-bromosuccinimide (NBS) has been investigated. The ene reactions with PTAD exhibited exclusive syn selectivity to the heteroaromatic rings except for an isopropylidenenorbornene fused with a pyridazine N-oxide ring. The epoxidations with MCPBA and the ene reactions with NBS afforded mixtures of syn and anti isomers depending on the heteroaromatic rings and substituents. The predominant syn selectivity compared with that of a benzene-fused congener may be attributed to the presence of a strong positive electrostatic potential field over the heteroaromatic ring to stabilize a polar transition state by the electrostatic interaction.