Sequential 1,2-Addition−Electrocyclic Ring Closures Involving Acyclic α,β-Unsaturated Iminiums: A Formal [3 + 3] Cycloaddition Strategy to Unique Pyranyl SpirocyclesRichard P. Hsung, Letitia J. Yao, Christopher D. Morgan et al.|The Journal of Organic Chemistry|1999Cited by 89