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.