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.