Synthesis of aryl-fused 1,4-oxathiepines from pyridinium 1,4-zwitterionic thiolates and vinylidene ortho-quinone methides

The synthesis of aryl-fused 1,4-oxathiepines from pyridinium 1,4-zwitterionic thiolates with ortho-alkynyl aromatic phenols has been achieved in good to excellent yields under mild conditions. The key reactive intermediates (i.e., vinylidene ortho-quinone methides) are readily generated in situ from alpha-alkynylnaphthalen-2-ols and they can also be generated from 2-alkynylphenols successfully by rationally varying the substituent of the phenyl ring according to computational results. Replacing vinylidene ortho-quinone methides (VQMs) with common ortho-quinone methides (QMs) was also investigated, and benzooxathiepine can be obtained in a moderate yield with (2-(bromomethyl)phenoxy)(tert-butyl)dimethylsilane as the QM precursor. Moreover, a preliminary study of the catalytic asymmetric version of this novel (3 + 4) reaction was attempted, affording a unique axially chiral arene-acyclic alkene scaffold with moderate enantioselectivity.