Nucleophilic-Addition-Initiated Ring Expansion and Selectivity in Anionic Fragmentation.

The expansion of rings into larger ones by a mechanism of anionic fragmentation is a potent tool for exploring new structural space in a concise manner. As the concept has been known and applied for decades now, going a step further calls for selectivity in the production of new scaffolds. In this regard, recent results demonstrated promising levels of regio-, dia- and enantioselectivity which give impulses to the concept of ring expansion involving the fragmentation of anionic species generated from unstrained cycles. Initiated by addition of a nucleophile, the energy received in the form of electrons triggered skeletal rearrangements of simple starting materials while revealing new stereocenters and/or regioisomers. Disrupting the symmetry of smaller rings in some cases, the strategy gives access to complex larger rings.