Synthesis of 2,3-dihydrobenzofurans by Mn(OAc)(3)-based oxidative cycloaddition of 2-cyclohexenones with alkenes. Synthesis of (+/-)-conocarpan

Oxidative cycloaddition of a 2-cyclohexenone or alpha-tetralone and an alkene with dried Mn(OAc)(3) in benzene at 80-140 degrees C provides a general route to dihydrobenzofurans 15 and dihydronaphthofurans 17. Although the yields are modest, this one-pot reaction provides simple access to these compounds, which have previously been prepared by multistep routes. Oxidative cycloaddition of 2-cyclohexenones with beta-methylstyrenes provides a new route to benzofuranoid neolignans, which was applied to the synthesis of conocarpan (22). The formation of 2-acetoxyhexanedioic acids 27 and 47 from acetoxylation of 2-cyclohexenones in HOAc, but not in benzene, opens up a new class of Mn(OAc)(3) reactions and explains Watt and Demir's discovery that much higher yields of alpha'-acetoxy enones are obtained in benzene than in HOAc.