Mixed & Other

Heterocycles containing sulfur, boron, phosphorus, or combinations of these with nitrogen and oxygen represent a structurally and pharmacologically rich class of scaffolds in drug discovery. Among cyclic sulfur-containing architectures, thiazoles and thiophenes are the most frequently approved, and many commonly used medications are based on the thiazole nucleus, a five-membered ring containing both sulfur and nitrogen. Beyond sulfur, boron-containing heterocycles have emerged as a compelling class in their own right. The Lewis acidity of the boron atom allows dynamic transition between trigonal planar and tetrahedral geometries upon binding to nucleophilic enzyme residues, enabling a form of covalent reversible pharmacology that is difficult to access with purely carbon-based scaffolds. Based on a benzoxaborole framework, the FDA has approved boron-based heterocycle drugs tavaborole and crisaborole. Mixed heteroatom systems such as thiadiazoles, benzothiazoles, and thienopyridines provide additional diversity in terms of electronic properties, metabolic stability, and binding geometry, spanning indications from oncology and cardiovascular disease to CNS disorders and antibacterial therapy.

Clopidogrel (Plavix, Sanofi/Bristol-Myers Squibb) is one of the defining commercial successes among sulfur-heterocycle-containing drugs. It belongs to the thienopyridine chemical class, incorporating a dihydrothieno[3,2-c]pyridine ring system that undergoes metabolic activation to generate the reactive thiol metabolite responsible for irreversible P2Y12 receptor blockade. Clopidogrel (Plavix) was a blockbuster antiplatelet agent used globally to prevent thrombotic events including myocardial infarction and stroke, with annual sales reaching multi-billion dollar levels before patent expiry in 2012. Rivaroxaban (Xarelto, Bayer/Janssen) provides a second illustration of the complementary roles that different heteroatom ring systems can play within a single drug molecule. Rivaroxaban is a morpholine and thiophene derivative that functions as a direct factor Xa inhibitor, with the thiophene carboxamide engaging the S1 and S4 binding pockets and the morpholine directing solubility and selectivity.

Among first-in-class approvals since 2023, three drugs exemplify the range of mixed heteroatom heterocycles entering the clinic. Xanomeline/trospium chloride (Cobenfy, Bristol-Myers Squibb), approved in September 2024, is the first-in-class M1/M4 muscarinic agonist for schizophrenia, representing the first new pharmacological approach to that indication in several decades by selectively targeting muscarinic receptors without blocking dopamine D2 receptors. The xanomeline component carries a 1,2,5-thiadiazole ring, a five-membered aromatic ring with one sulfur and two nitrogen atoms, which projects a hexyloxy substituent that is essential for receptor selectivity and membrane penetration. Sulbactam/durlobactam (Xacduro, Entasis/Innoviva), approved in May 2023, is the first antibacterial combination specifically approved for hospital-acquired pneumonia caused by Acinetobacter baumannii–calcoaceticus complex, an organism classified by the WHO as a critical-priority pathogen. Sulbactam's antibacterial activity derives from its penam scaffold, a β-lactam ring fused to a thiazolidine, the classical S/N bicyclic system shared by all penicillin-derived antibacterials, where the sulfur lone pairs contribute to ring conformational rigidity and resist enzymatic hydrolysis.

Mixed heteroatom heterocycles are prominent substrates and targets in emerging synthetic methodology. Photochemical irradiation of thiazoles and isothiazoles populates their π,π* singlet excited states and triggers selective structural rearrangements, an overall permutation of the cyclic system and its substituents, providing a conceptually distinct approach where an initial heteroaromatic scaffold serves as an entry point to access otherwise inaccessible derivatives under mild conditions. This photopermutation strategy removes the need for case-by-case de novo synthesis of closely related thiazole isomers, a recurring challenge in lead optimisation campaigns. In photoredox organocatalysis, organocatalysed photoredox reactions have been applied to the synthesis of thiazolines and oxazolines from amides and thioamides with alkenes, generating heterocyclic products with selectivity for anti-configured products, and the direct aromatisation of the initial adducts to thiazoles provides rapid access to medicinally relevant mixed-heteroatom motifs.

Electrochemistry has also expanded access to sulfur heterocycles. Recent advances in electrochemical synthesis of N-, S- and O-heterocycles demonstrate that anodic oxidation can replace stoichiometric chemical oxidants in heterocycle-forming reactions, with the electrode potential tunable to control selectivity in ways that are not achievable using traditional reagents. For boron heterocycles, the benzoxaborole scaffold has attracted particular attention as an organocatalytic platform. Cyclic hemiboronic acids including benzoxaboroles, composed of an exocyclic B-OH group and endocyclic B-C and B-O bonds, have been employed in organocatalysis, bioconjugation, drug discovery, and as components of dynamic materials, with the reversible Lewis acid-base chemistry at boron enabling novel modes of substrate activation. Taken together, heterocycles bearing sulfur, boron, and mixed heteroatom combinations occupy a distinct and growing niche in medicinal chemistry, one where the unique electronic and binding properties of these atoms create pharmacological opportunities that conventional N- and O-heterocycles are unable to access.