Intramolecular [4 + 2] Cycloaddition of Nitroalkenes for Construction of Vicinal Quaternary Stereocenters

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.