Intramolecular [4 + 2] cycloaddition of nitroalkenes for construction of vicinal quaternary stereocenters

chemical reaction: see text]. Nitroalkene (E)-1 has been synthesized to test the feasibility of an intramolecular 4 + 2] cycloaddition in a planned synthesis of daphnilactone B. This nitro olefin contains two unique structural features, a nitromethylene lactone and a pendant diene, that combine under the action of SnCl4 in a highly selective fashion to afford nitronates 2a and 2b. These products represent the correct relationship for the vicinal quaternary stereogenic centers in the core of daphnilactone B.