Metal-Free Electrochemical Construction of Oxygen Heterocycles.

Oxygen heterocycles are ubiquitous across chemistry─from epoxides that underpin bulk materials and industrial manufacturing to five- and six-membered rings that populate natural products, fragrances, agrochemicals, dyes, and modern medicines. Yet, compared with N-heterocycles, O-heterocycles have historically received less systematic attention in synthetic-method development. One reason is that oxygen's higher electronegativity often renders these frameworks more reactive and less straightforward to control under conventional conditions. Organic electrosynthesis provides a compelling platform to address this challenge: by using electrons as traceless redox reagents and controlling reactivity through potential or current, electrochemistry enables access to high-energy intermediates (radical-anions, radical-cations, "active halogen", superoxide) under mild conditions and often without stoichiometric oxidants or reductants. This Account summarizes electrosynthetic conceptions that repeatedly enable the construction─and, in several cases, editing─of oxygen heterocycles spanning three-, five-, and six-membered rings. We first discuss three-membered oxiranes, emphasizing cathodic manifolds that resemble Darzens-type closures initiated by C-X bond reduction and electrogenerated bases (EGBs), as well as solvent-enabled radical pathways that become accessible only upon reaching the dianion state of 1,2-dicarbonyls. These mechanistic thresholds are conveniently diagnosed by cyclic voltammetry (CV), which serves throughout the Account as a fingerprint for identifying when productive electron-transfer pathways switch on. Complementary anodic epoxidations proceed via mediator-driven oxygen transfer, including iodide-derived "active iodine". We then survey five-membered O-heterocycles (furans, dihydrofurans, butenolides, and mixed O,N/O,S rings), highlighting how cathodic and anodic routes deliver distinct synthetic leverage. Representative advances include anodic dimerizations to bioactive butenolides, dearomative spirocyclizations via chalcogen radicals, oxidative [3 + 2] annulations to dihydrofurans, and mediator-enabled cyclizations (TEMPO, triarylamines, iodide) that convert simple feedstocks into densely functionalized heterocycles with high selectivity. Finally, we address six-membered O-heterocycles-coumarins/δ-lactones, dioxane/dioxine frameworks, and O,N/O,S systems, where electrochemistry enables both ring construction and ring remodeling. Cathodic pathways involving electrogenerated superoxide can trigger Baeyer-Villiger/Dakin-type lactone formation and ring expansion to isocoumarins, while anodic oxidation in terpenoid settings can proceed through carbocationic pathways that induce rearrangement, ring contraction, or lactonization. Beyond ring construction, electrochemistry uniquely enables skeletal editing─most notably redox-triggered ring expansions to isocoumarins and related lactones─by accessing rearrangement-prone radical anions and cationic intermediates under mild conditions. Across ring sizes, a recurring theme is that solvents and electrolytes are often reactive partners: chlorinated media (e.g., dichloromethane, 1,2-dichloroethane) can be electroactivated to generate radical or electrophilic equivalents that unlock otherwise inaccessible cyclizations, an idea that points toward constructive upgrading of chlorinated starting materials into higher-value heterocycles. Collectively, the examples herein position electrosynthesis as a sustainable design paradigm for oxygen-heterocycle synthesis: it replaces hazardous reagents with electrons, enables mechanistically informed selectivity via CV, and is naturally compatible with flow electrochemistry for improved safety, scalability, and precise control of short-lived intermediates. These features should accelerate future reaction discovery, late-stage ring editing, and practical deployment of O-heterocycles in medicinal and materials chemistry.