The Lossen rearrangement is a chemical reaction that converts hydroxamic acids into isocyanates, which are highly versatile intermediates for building amines, ureas, carbamates, and other nitrogen-containing compounds. First described by the German chemist Wilhelm Lossen in 1872, the reaction has experienced a quiet renaissance over the past two decades as chemists have discovered milder, more efficient ways to trigger it. What once required harsh conditions and stoichiometric amounts of strong electrophiles can now proceed with catalytic base at moderate temperatures, and the reaction has been successfully scaled to tens of kilograms for pharmaceutical manufacturing.
What the Reaction Actually Does
At its core, the Lossen rearrangement takes a hydroxamic acid and rearranges its molecular skeleton so that a carbon-nitrogen bond forms where a carbon-carbon bond used to be. The hydroxamic acid loses carbon dioxide in the process, and the nitrogen atom migrates from a side position to the center of the molecule, producing an isocyanate. That isocyanate is a reactive species: it readily combines with water to give an amine, with an alcohol to give a carbamate, or with another amine to give a urea. This flexibility is what makes the Lossen rearrangement useful. A single starting material class, hydroxamic acids, can be funneled into several different product families depending on what the isocyanate meets next.
Hydroxamic acids themselves are easy to prepare from carboxylic acids, one of the most abundant and diverse classes of molecules in organic chemistry. So the Lossen rearrangement effectively lets chemists convert a carboxylic acid (a very common functional group) into a nitrogen-containing product (a much more valuable functional group in drug design). That carboxylic-acid-to-amine or carboxylic-acid-to-urea transformation is the practical headline.
Why Hydroxamic Acids Need a Push
Hydroxamic acids do not spontaneously undergo the Lossen rearrangement at room temperature. The oxygen-hydrogen bond on the hydroxamic acid’s nitrogen-oxygen unit needs to be converted into a better leaving group before the nitrogen can migrate. In classical versions of the reaction, chemists attached an electrophilic group to that oxygen, turning it into something the molecule could kick out as it rearranged. Strong electrophiles like sulfonyl chlorides, carbonyl diimidazole, or phosphorus reagents were the traditional tools for this activation step.
The need for a strong activating agent was the reaction’s main limitation for over a century. These reagents are often expensive, generate a lot of waste, and can be incompatible with sensitive functional groups elsewhere in the molecule. Much of the modern research on the Lossen rearrangement has focused on finding gentler or more efficient ways to coax the hydroxamic acid into rearranging.
Modern Activation Strategies
A range of milder activating agents have been developed that make the Lossen rearrangement practical for a wider set of substrates. Cyanuric chloride, an inexpensive triazine-based reagent, acts as an efficient promoter that converts hydroxamic acids directly into carbamates, thiocarbamates, and ureas in good yields.1Tetrahedron Letters. Cyanuric chloride: an efficient reagent for the Lossen rearrangement Another approach uses ethyl 2-cyano-2-(4-nitrophenylsulfonyloxyimino)acetate, known by the shorthand 4-NBsOXY, which can convert a carboxylic acid all the way to a urea in a single reaction vessel without causing the scrambling of stereocenters that plagues many classical methods.2PubMed. Ethyl 2-cyano-2-(4-nitrophenylsulfonyloxyimino)acetate-mediated Lossen rearrangement: single-pot racemization-free synthesis of hydroxamic acids and ureas from carboxylic acids
A closely related reagent called Boc-Oxyma achieves the same kind of one-pot conversion while being more environmentally friendly and highly selective about which part of the molecule it reacts with. That selectivity matters in complex molecule synthesis, where a drug candidate might contain half a dozen reactive sites, and you only want the Lossen rearrangement to happen at one of them. Boc-Oxyma has been used to build peptide-derived ureas without racemization, keeping the stereochemistry of each amino acid building block intact.3Advanced Synthesis & Catalysis. Ethyl 2‐(tert‐Butoxycarbonyloxyimino)‐2‐cyanoacetate (Boc‐Oxyma): An Efficient Reagent for the Racemization Free Synthesis of Ureas, Carbamates and Thiocarbamates via Lossen Rearrangement
N-Methylimidazole offers yet another option. It catalyzes the conversion of the isocyanate intermediate into carbamates and helps suppress a common side reaction in which the isocyanate reacts with the starting hydroxamic acid to form unwanted dimers. Using an arylsulfonyl chloride alongside N-methylimidazole allowed researchers to run the reaction at lower temperatures, broadening the range of sensitive substrates that survive the process.4PubMed Central. N-Methylimidazole-catalyzed synthesis of carbamates from hydroxamic acids via the Lossen rearrangement
The Pseudo-Catalytic Process
One of the most striking modern developments is a version of the Lossen rearrangement that does away with strong electrophilic activators entirely. Researchers found that common nitriles, particularly acetonitrile (the same solvent many labs already use), can activate hydroxamic acids and kick off the rearrangement. The isocyanate produced in the first pass then acts as an activator for additional hydroxamic acid molecules, creating a self-sustaining cycle. The isocyanate is “pseudo-catalytic” because each molecule of it facilitates the rearrangement of another hydroxamic acid, regenerating a new isocyanate in the process.5PubMed. Revisiting a Classic Transformation: A Lossen Rearrangement Initiated by Nitriles and “Pseudo-Catalytic” in Isocyanate
The practical upshot is remarkable. The reaction requires only a catalytic amount of a mild organic base (DBU) and runs at just 60 °C in acetonitrile. It works on both aliphatic and aromatic hydroxamic acids. And it scales: the process was demonstrated at 55 kilograms of product in a single batch, with an isolated yield of 95.5%.6PubMed. Revisiting a Classic Transformation: A Lossen Rearrangement Initiated by Nitriles and “Pseudo-Catalytic” in Isocyanate That kind of scale and efficiency is exactly what pharmaceutical process chemistry demands, where a reaction needs to be safe, reproducible, and economical in batches of tens or hundreds of kilograms.
Building Unsymmetrical Ureas
Ureas, molecules with two nitrogen atoms flanking a central carbonyl group, are found in many biologically active compounds and drug candidates. Symmetrical ureas (the same group on each nitrogen) are straightforward to make, but unsymmetrical ureas (different groups on each nitrogen) pose a selectivity challenge. The Lossen rearrangement has become one of the go-to methods for this problem because the isocyanate intermediate naturally pairs with just one amine partner.
A recent approach starts from Nms-protected amines, a class of nitrogen-containing compounds bearing a specific sulfonyl protecting group. Under optimized conditions, the protecting group is removed and the Lossen rearrangement is triggered in a single operation, producing unsymmetrical ureas directly. The best results achieved 96% yield, and a range of substrates including alkyl, benzyl, and alkenyl hydroxamic acids all performed well, giving yields between 75% and 97%.7PubMed Central. Deprotective Lossen rearrangement: a direct and general transformation of Nms-amides to unsymmetrical ureas Combining a deprotection and a rearrangement in one step saves time and avoids the need to isolate a sensitive intermediate.
Palladium-Catalyzed Variants for Making Amines
The Lossen rearrangement traditionally produces isocyanates, which then react with nucleophiles. But a palladium-catalyzed version reported in the Journal of the American Chemical Society opened an entirely different product space: secondary amines. In this transformation, palladium orchestrates both the Lossen rearrangement of a hydroxamic acid and a cross-coupling with an aryl halide, stitching two fragments together through a new carbon-nitrogen bond in a single operation.8Journal of the American Chemical Society. Pd-Catalyzed Arylative Lossen Rearrangement: Synthesis of Secondary Amines from Aryl/Alkyl Carboxylic Acids and Aryl Halides
This is a big deal because it means carboxylic acid derivatives can serve as stand-ins for primary amines in cross-coupling chemistry. Primary amines are sometimes difficult to handle (they can coordinate too strongly to palladium and poison the catalyst), so having an alternative entry point via hydroxamic acids expands the toolbox. It also means chemists can tap into the enormous diversity of commercially available carboxylic acids to build amine-containing products they might otherwise struggle to access.
Enantioselective Cascades
Making molecules with defined three-dimensional shape (chirality) is critical in pharmaceutical chemistry because the two mirror-image forms of a drug often have very different biological effects. Researchers have developed an enantioselective version of the Lossen rearrangement by combining it with a desymmetrization step in a cascade sequence. Starting from symmetrical succinimide substrates bearing a sulfonate leaving group on nitrogen, a chiral thiourea-based organocatalyst guides an alcohol nucleophile to attack one of two identical carbonyl groups selectively. The resulting intermediate then undergoes the Lossen rearrangement, giving cyclic beta-amino acid derivatives with high enantioselectivity (up to 96:4 ratio of the two mirror-image forms) and yields ranging from 61% to 91%.9The Journal of Organic Chemistry. Synthesis of Cyclic β-Amino Acid Derivatives by Desymmetrization and Lossen Rearrangement of N-Sulfoxy Meso-Succinimides
Cyclic beta-amino acids are themselves interesting targets: they appear as building blocks in peptide mimics, antibiotics, and other bioactive molecules. Getting them in high optical purity through a cascade that starts from cheap, symmetrical starting materials is an efficient shortcut compared to traditional multi-step routes that might require chiral auxiliaries or resolution.
Use in Pharmaceutical Process Chemistry
The pseudo-catalytic Lossen rearrangement described earlier was not an academic curiosity. It was developed specifically as a key step in the synthesis of BMS-955176, an HIV drug candidate. The reaction converted a hydroxamic acid intermediate directly to an amine in a single step under mild conditions, replacing what would otherwise have been a multi-step sequence involving hazardous reagents.10PubMed. Revisiting a Classic Transformation: A Lossen Rearrangement Initiated by Nitriles and “Pseudo-Catalytic” in Isocyanate The fact that the reaction ran in acetonitrile with only catalytic base, produced minimal waste, and scaled to 55 kilograms made it attractive from both a safety and an economic standpoint.
Pharmaceutical companies care intensely about the scalability and safety profile of each reaction in a manufacturing sequence. Isocyanates, the intermediates generated during a Lossen rearrangement, are toxic and moisture-sensitive. Having a process where the isocyanate is generated transiently and immediately consumed (as in the pseudo-catalytic cycle) reduces the hazard considerably. You never accumulate a large quantity of the dangerous intermediate.
Applications in Polymer Chemistry
The Lossen rearrangement is not limited to small-molecule synthesis. Researchers at Carleton University explored a polymer chemistry application where a precursor monomer containing an acetohydroxamic acid group (AA group) could be thermally converted to an isocyanate group within a polymer chain. Heating these copolymers to 110–130 °C in the presence of a base catalyst triggered the in situ Lossen rearrangement, generating reactive isocyanate groups along the polymer backbone. When a polyol was present during this process, polyurethanes could be formed directly.11Carleton University Institutional Repository (CURVE). Lossen Reaction, Polymer Synthesis and Potential Applications of Latent Isocyanates
The concept of a “latent isocyanate” is appealing because free isocyanates are hazardous to handle and sensitive to moisture. By embedding the hydroxamic acid precursor in a polymer and then unmasking the isocyanate only when needed (by applying heat), you avoid storing and transporting the reactive species. This approach could find use in coatings, adhesives, and foams where polyurethane formation at the point of application is desirable.
How the Lossen Rearrangement Relates to Curtius, Hofmann, and Beckmann Reactions
The Lossen rearrangement belongs to a family of reactions that share a common theme: a group migrates from carbon to nitrogen (or vice versa) as part of a molecular rearrangement. Its closest relatives are the Curtius rearrangement, which converts acyl azides to isocyanates, and the Hofmann rearrangement, which converts primary amides to isocyanates using a strong oxidant like bromine. All three produce the same key intermediate (an isocyanate) but start from different precursors: hydroxamic acids for Lossen, acyl azides for Curtius, and primary amides for Hofmann.
The practical trade-offs differ. The Curtius rearrangement uses azides, which can be explosive and require careful handling, especially at scale. The Hofmann rearrangement requires harsh oxidizing conditions that not all substrates tolerate. The Lossen rearrangement avoids azides and strong oxidants but historically required electrophilic activators. With modern mild conditions, the Lossen version has become increasingly competitive, particularly when the hydroxamic acid starting material is easy to prepare. Continuous-flow technology has also been explored for some of these rearrangements as a way to improve safety by generating and consuming hazardous intermediates in small volumes at any given time.12ACS Medicinal Chemistry Letters. Continuous-Flow Technology for Chemical Rearrangements: A Powerful Tool to Generate Pharmaceutically Relevant Compounds
The Beckmann rearrangement, often mentioned alongside these others, is mechanistically distinct: it converts oximes to amides (or lactams) and does not produce isocyanates. It gets lumped into the same discussion because it also involves nitrogen migration, but the product classes and applications are quite different.
Common Pitfalls and Side Reactions
The most frequently encountered side reaction during a Lossen rearrangement is the formation of hydroxamate-isocyanate dimers. The isocyanate intermediate, being highly electrophilic, can react with unreacted hydroxamic acid in the flask rather than with the intended nucleophile. This leads to unwanted coupled products and reduces yield. The N-methylimidazole/arylsulfonyl chloride system was developed in part to address this problem: the catalyst accelerates the desired reaction with the nucleophile, outcompeting the dimerization pathway.13PubMed Central. N-Methylimidazole-catalyzed synthesis of carbamates from hydroxamic acids via the Lossen rearrangement
Racemization is another concern when the substrate has a stereocenter adjacent to the reaction site. Several of the modern reagent systems (4-NBsOXY, Boc-Oxyma) were designed specifically to avoid this problem, and both have been validated on peptide-derived substrates where retention of configuration is essential.14PubMed. Ethyl 2-cyano-2-(4-nitrophenylsulfonyloxyimino)acetate-mediated Lossen rearrangement: single-pot racemization-free synthesis of hydroxamic acids and ureas from carboxylic acids 15Advanced Synthesis & Catalysis. Ethyl 2‐(tert‐Butoxycarbonyloxyimino)‐2‐cyanoacetate (Boc‐Oxyma): An Efficient Reagent for the Racemization Free Synthesis of Ureas, Carbamates and Thiocarbamates via Lossen Rearrangement
Moisture sensitivity is a perennial issue. Isocyanates react with water to form carbamic acids, which spontaneously decompose to amines and carbon dioxide. If the goal is to trap the isocyanate with a specific amine or alcohol partner, even trace water in the solvent or atmosphere can divert the reaction. Careful drying of solvents and an inert atmosphere are standard precautions. In the polymer chemistry context, this sensitivity is actually exploited: the latent isocyanate approach deliberately uses the reactivity of the in situ-generated isocyanate with polyols to form polyurethane networks, turning what would be a side reaction in small-molecule chemistry into the desired outcome.
Why the Lossen Rearrangement Was Overlooked for So Long
For much of the twentieth century, the Lossen rearrangement lived in the shadow of the Curtius and Hofmann reactions. Textbooks covered it as a historical curiosity, and working chemists rarely reached for it. The main reasons were practical: hydroxamic acids were not always easy to prepare in high purity, and the activation step added complexity. The Curtius rearrangement, despite its azide hazard, was well-understood and widely used. The Hofmann rearrangement, while harsh, was a one-step process from readily available amides.
What changed was the development of reliable, mild methods for preparing hydroxamic acids from carboxylic acids and, separately, the invention of activation strategies that did not require stoichiometric amounts of expensive or hazardous electrophiles. The one-pot carboxylic-acid-to-urea protocols exemplify this shift: a researcher can now start from a carboxylic acid, form the hydroxamic acid, activate it, and capture the isocyanate with an amine all in a single flask without isolating any intermediate.16PubMed. Ethyl 2-cyano-2-(4-nitrophenylsulfonyloxyimino)acetate-mediated Lossen rearrangement: single-pot racemization-free synthesis of hydroxamic acids and ureas from carboxylic acids That convenience, combined with the mild conditions and functional group tolerance of newer reagent systems, has brought the reaction back into regular use in both academic and industrial labs.
The palladium-catalyzed arylative variant represents a further expansion, merging the Lossen rearrangement with transition-metal catalysis in a way that would have seemed improbable two decades ago.17Journal of the American Chemical Society. Pd-Catalyzed Arylative Lossen Rearrangement: Synthesis of Secondary Amines from Aryl/Alkyl Carboxylic Acids and Aryl Halides Carboxylic acids, the most mundane of organic starting materials, can now be transformed into secondary amines through a single catalytic operation. That kind of step economy is what makes a reaction earn its place in a chemist’s daily toolkit rather than sitting in a textbook footnote.