Ketones

The ketone group is among the most chemically versatile functional groups in drug discovery, appreciated both as a structural feature influencing physicochemical properties and as an electrophilic warhead in targeted covalent inhibitor design. As a hydrogen bond acceptor, the ketone carbonyl contributes to binding affinity through direct interactions with active-site residues, while its polar character can modulate solubility and membrane permeability in a context-dependent manner. In covalent drug design, ketones activated by adjacent electron-withdrawing groups, particularly alpha-ketoamides and alpha,beta-epoxyketones, form reversible or irreversible tetrahedral adducts with nucleophilic serine, threonine, and cysteine residues at enzyme active sites, mimicking the transition state of substrate hydrolysis and delivering high potency with mechanistic selectivity. This reactivity is especially well exploited in protease inhibitor programmes, where peptidomimetic ketone warheads have served as effective design tools for targeting viral and oncological protease targets. Beyond covalent applications, ketones appear widely in approved drug scaffolds as metabolically stable carbonyl elements, and they are ubiquitous as synthetic intermediates throughout pharmaceutical manufacture.

Two FDA-approved drugs illustrate the pharmacological utility of the ketone group. Boceprevir (Victrelis, Schering-Plough/Merck), the first-in-class direct-acting antiviral approved in 2011, was the first HCV NS3/4A serine protease inhibitor approved by the FDA for the treatment of chronic hepatitis C genotype 1 infection. Its alpha-ketoamide warhead reacts reversibly with the catalytic serine of the protease, forming a covalent hemiketal that mimics the transition state of amide bond hydrolysis and delivers sub-nanomolar inhibitory potency. Carfilzomib (Kyprolis, Amgen), approved in 2012 for relapsed or refractory multiple myeloma, employs an alpha,beta-epoxyketone warhead that irreversibly acylates the N-terminal threonine of the 20S proteasome’s beta-5 subunit, achieving prolonged target occupancy through the high selectivity of the epoxyketone for proteasomal threonine over other cellular nucleophiles.

In organic synthesis, ketones are central to a broad range of fundamental transformations. The aldol condensation allows construction of beta-hydroxy ketones and their dehydrated enone products, and is the basis of the Robinson annulation, which combines a Michael addition with an intramolecular aldol to assemble six-membered carbocyclic rings found throughout natural product and steroid synthesis. The Baeyer-Villiger oxidation converts ketones to esters or lactones using peracids, providing a regioselective route to oxygen-containing heterocycles. The Beckmann rearrangement converts ketone oximes to amides under acidic activation. Grignard and organolithium additions to ketones afford tertiary alcohols, and the Clemmensen and Wolff-Kishner reductions enable net deoxygenation of the carbonyl. Asymmetric transfer hydrogenation, as in Noyori’s catalysis, provides enantioselective ketone reduction, widely applied in the preparation of chiral alcohol drug intermediates.

Emerging synthetic techniques are significantly expanding access to ketone-containing building blocks and drug-like molecules. In photochemistry, the Norrish type I and type II reactions of ketones under UV irradiation have long provided routes to radical intermediates and cyclobutanol products, and modern photoredox catalysis has reinvented this reactivity: visible-light photocatalysis enables radical addition to ketones for asymmetric C-C bond formation and allows the generation of acyl radicals from alpha-oxo acids, directly building ketone-containing fragments in a modular fashion. Electrochemically, cathodic reduction of ketones generates radical-anion intermediates that participate in reductive cross-coupling and pinacol-type C-C bond formation, and recent anodic alpha-oxygenation strategies have enabled the direct installation of oxo groups adjacent to carbonyl-containing fragments under mild, scalable conditions, avoiding the stoichiometric oxidants that historically limited these transformations.