Synthesis of α‑Substituted Vinylsulfonium Salts and Their Application as Annulation Reagents in the Formation of Epoxide- and Cyclopropane-Fused Heterocycles

The
discovery of new methods for the synthesis of classes of potentially
bioactive molecules remains an important goal for synthetic chemists.
Vinylsulfonium salts have been used for the synthesis of a wide variety
of small heterocyclic motifs; however, further developments to this
important class of reagents has been focused on reaction with new
substrates rather than development of new vinylsulfonium salts. We
herein report the synthesis of a range of α-substituted vinylsulfonium
tetraphenylborates (10 examples) in a 3 step procedure from commercially
available styrenes. The important role of the tetraphenylborate counterion
on the stability and accessibility of the vinylsulfonium salts is
also detailed. The α-substituted vinylsulfonium tetraphenylborates
gave good to excellent yields in the epoxyannulation of β-amino
ketones (15 examples) and the cyclopropanation of allylic amines (4
examples). Hydrogenation of an epoxyannulation product proceeded with
good diastereoselectivity.