Gold-Catalyzed [3 + 2] Annulation, Carbenoid Transfer, and C–H Insertion Cascade: Elucidation of Annulation Mechanisms via Benzopyrylium Intermediates

It
has been established that a cationic gold­(I)/PCy3 complex
catalyzes the intramolecular cascade reaction of enediyne-carbonyls
via benzopyrylium intermediates giving pentacyclic compounds, bearing
five or six stereocenters with high yields and diastereoselectivity.
In this cascade reaction, [3 + 2] annulation, carbenoid transfer,
and C–H insertion proceed in one-pot. The DFT calculation revealed
that both the [3 + 2] and [4 + 2] annulations proceed via the same
gold­(I) carbenoid, produced by the [3 + 2] cycloaddition, and the
reaction between the gold­(I) carbenoid and the additional alkyne unit
promotes the present cascade reaction.