Sequential 1,2-Addition−Electrocyclic Ring Closures Involving Acyclic α,β-Unsaturated Iminiums:  A Formal [3 + 3] Cycloaddition Strategy to Unique Pyranyl Spirocycles

Sequential 1,2-Addition−Electrocyclic Ring Closures Involving Acyclic α,β-Unsaturated Iminiums:  A Formal [3 + 3] Cycloaddition Strategy to Unique Pyranyl Spirocycles