Electrophilic Cyclization in Organic Synthesis

Electrophilic cyclization encompasses a suite of reactions in which an electrophile activates a pi-bond or functional group to initiate intramolecular ring closure. This strategy enables rapid assembly of carbocyclic and heterocyclic frameworks under mild conditions, often with high regio- and stereochemical control. Central to its appeal is the ability to orchestrate cascade or tandem sequences, where a single activation event triggers multiple bond-forming steps. Key mechanistic variants include halonium-induced cyclizations of alkynes and alkenes, Lewis and Brønsted acid-mediated cycloisomerisations, and metal-catalysed electrophilic activation of unsaturated substrates. Control of ring size and substitution pattern frequently invokes Baldwin’s rules, guiding endo- versus exo-dig pathways. Electrophilic cyclizations have underpinned the synthesis of natural products, agrochemicals and organic electronic materials, delivering complex scaffolds in one pot and often obviating protecting-group manipulations. Recent advances focus on green reagents, photochemical electrophilic activation and flow-based technologies, broadening the method’s scope and sustainability. A growing interplay between mechanistic insight and computational design has further refined catalyst selection, enabling bespoke cyclisations to access previously elusive ring systems.

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Electrophilic Cyclization in Organic Synthesis publication trend

The graph below shows the total number of articles in electrophilic cyclization in organic synthesis across all publications each year (not limited to Nature Index journals).

Technical terms

Electrophilic cyclization: Intramolecular ring-closure triggered by an electrophile attacking a pi bond or heteroatom.

6-endo-dig cyclization: A ring-closure mode in which an sp-hybridised centre attacks an electrophile to form a six-membered ring following Baldwin’s rules.

Cycloisomerisation: Rearrangement of a linear or polyunsaturated substrate into a cyclic structure under acid or metal catalysis.

Halonium ion: A positively charged intermediate formed when a halogen electrophile adds to a pi bond, often guiding regioselective cyclization.

π-expansion: Extension of conjugated aromatic systems by incorporating additional rings or heteroatoms via cyclization reactions.

References

1. 6-Endo-dig cyclization: Flexible enforce to develop synthetic route in organic syntheses. Results in Chemistry (2023).
2. Synthesis and properties of thienonaphtho[ bc ]pyridines and thienonaphtho[ bc ]quinolines. Organic & Biomolecular Chemistry (2024).
3. π-Expanded azaullazines: synthesis of quinolino-azaullazines by Povarov reaction and cycloisomerisation. Organic & Biomolecular Chemistry (2024).