Intramolecular Benzannulation of 3‑Sulfonyl-2-benzylchromen-4-ones: Synthesis of Sulfonyl Dibenzooxabicyclo[3.3.1]nonanes

In this work, a concise route for
the synthesis of sulfonyl dibenzo-oxabicyclo[3.3.1]­nonanes
by a two-step route is described, including (i) NaBH4/LiCl-mediated
reduction of 3-sulfonyl-2-benzylchromen-4-ones and (ii) sequential
BF3·OEt2-mediated intramolecular annulation
of the resulting 3-sulfonyl-2-benzylchroman-4-ols. A plausible mechanism
is proposed and discussed herein. This protocol provides a highly
effective stereocontrolled aryl-hydroxyl Friedel–Crafts-type
cross-coupling to construct the tetra- or pentacyclic bridged framework.
The use of various reaction conditions is investigated for an efficient
transformation.