Chiral cycloalkylidene α,β-unsaturated iminium approach to stereoselective formal [3+3] cycloaddition reaction in spiroheterocycle synthesis

An approach to stereoselective formal [3+3] cycloaddition reaction using chiral cycloalkylidene α,β-unsaturated iminiums is described here leading to preparations of spiroheterocycles in modest stereoselectivity. The reversibility of 6π-electron electrocyclic ring closure did not always lead to the thermodynamically more favored product. These observations led to other useful mechanistic understanding of electrocyclic ring closure of 1-oxatrienes.