Keteniminium salts are powerful electrophilic heterocumulene reagents well-known for their selectivity and stereocontrol in [2+2] cycloadditions to olefins and carbonyl derivatives. Furthermore, they are readily accessible from stable and simple precursors under a variety of different conditions. Herein, we present the chemistry of keteniminium salts with the hindsight of time, describe preparation methods, recent [2+2] chemistry, mechanistic studies, and assorted applications that significantly extend the scope of utility of these unique electrophilic heterocumulenes.

Surprise, surprise! An unexpected skeletal rearrangement was developed into a chemo‐ and stereoselective synthesis of α‐allyl and allenyl lactones with challenging substitution patterns (see scheme; EWG=electron‐withdrawing group). The generality, unique features, and synthetic potential of this reaction were probed and a mechanism was proposed.

Last‐minute deal: A direct lactonisation of ethers and alcohols onto amides that proceeds at room temperature under mild conditions is reported (see scheme). This allows the effective saving of up to two unproductive, sequential deprotection operations in synthetic sequences. Mechanistic studies are described, and a new “amide strategy” that exploits the dual robustness/late‐stage selective activation properties of this functional group is outlined.