Photoinduced Carboborative Ring Contraction Enables Regio- and Stereoselective Synthesis of Multiply Substituted Five-Membered Carbocycles and Heterocycles

We report herein a photoinduced carboborative
ring contraction
of monounsaturated six-membered carbocycles and heterocycles. The
reaction produces substituted five-membered ring systems stereoselectively
and on preparative scales. The products feature multiple stereocenters,
including contiguous quaternary carbons. We show that the reaction
can serve as a synthetic platform for ring system alteration of natural
products. The reaction can also be used in natural product synthesis.
A concise total synthesis of artalbic acid has been enabled by a sequence
of photoinduced carboborative ring contraction, Rauhut–Currier
reaction, and nitrilase-catalyzed hydrolysis. The synthetic utility
of the reaction has been further demonstrated by converting the intermediate
organoboranes to alcohols, amines, and alkenes.