Benzyne-Mediated Nonconcerted Pathway toward Synthesis of Sterically Crowded [5]- and [7]Oxahelicenoids, Stereochemical and Theoretical Studies, and Optical Resolution of Helicenoids

Synthesis
of [5]- and [7]­oxahelicenoids via Diels–Alder
reaction of sterically crowded bichromenes with benzyne is presented.
Studies carried out on Diels–Alder addition product establish
the unusual preference for a stepwise mechanism over the concerted
reaction pathway. This high yielding general synthetic protocol affords
unexpected anti cycloadducts [5]- and [7]­oxahelicenoids, as confirmed
by crystallographic analysis. To rationalize these intriguing antiaddition
products, the reaction mechanism was elucidated by means of DFT analysis.
Additionally, hydroxy-functionalized [7]­oxahelicenoid has been resolved
in its optically pure forms.