Divergent photochemical ring-replacement of isoxazoles

Domain: neuroscience
Published: 2026-01-29
Editorial outcome: major_revision

Abstract
Isoxazoles, oxazoles, and other five-membered heteroaromatics are prevalent motifs in core structure of pharmaceuticals and agrochemicals. In early-stage drug discovery, it is common practice to prepare libraries of analogues featuring different heterocyclic cores and this generally requires a de novo synthesis for each scaffold. A valuable but currently unavailable strategy would involve the possibility for direct heterocycle “ring-replacement”. Here we report a photochemical platform for the selective conversion of isoxazoles into oxazoles, pyrazoles, pyrroles, and isothiazoles by exploiting excited-state reactivity. Starting from a successful isoxazole-to-oxazole transformation, we uncover position-sensitive reactivity that prompted computational investigation. These insights guide a systematic reactivity survey and reveal a solvent-controlled deconstruction–reconstruction pathway via α-ketonitrile intermediates. This approach enables scaffold diversification without de novo synthesis, affording access to five distinct azole classes under mild conditions. The method’s selectivity, functional group tolerance, and late-stage applicability suggest broad utility in heterocyclic library design for pharmaceutical research.

Introduction
The ability to modify complex molecules at late stages, without the need for de novo synthesis, has become a central objective in modern synthetic chemistry1. Considering (hetero)aromatic molecules, significant advances have been made in the way we perform peripheral modifications (e.g., cross-coupling and C–H activation)2,3. However, direct replacement or remodeling of the core (hetero)aromatic scaffold remains a largely unsolved challenge, despite its potential to accelerate chemical space exploration in pharmaceuticals, agrochemicals, and functional materials4,5.

Among five-membered heteroaromatics, isoxazoles and oxazoles occupy a privileged position, frequently appearing in marketed drugs and advanced materials (Fig. 1a)6,7. These heterocycles can serve as bioisosteres for ketone, ester, and (hetero)aryl groups, often offering benefits such as improved metabolic stability, reduced lipophilicity, and enhanced potency8,9. Their privileged status makes them invaluable tools in modern drug discovery and development.

A classic approach in early structure-activity relationship (SAR) studies involves the preparation of substrate libraries in which a specific heterocyclic system is replaced by closely related bioisosteres10–12. A representative case is the development of the isoxazole-based SETD2 inhibitor 1, where oxazole, imidazole, and pyrazole analogs were individually synthesized and evaluated (Fig. 1b)13,14. While effective for lead identification, such strategies typically necessitate the de novo synthesis of each variant, which becomes especially challenging when the heterocycle is embedded within the molecular core15,16. Consequently, a late-stage method for heterocycle “ring-replacement” would offer significant value by enabling rapid access to bioisosteric analogs from a common lead derivative17,18.

Isoxazoles offer a compelling platform for this strategy. Early reports demonstrated their capacity to rearrange photochemically to oxazoles and α-ketonitriles, but the reactions were typically unselective, low-yielding, and limited to a narrow set of derivatives19–30. Due to these challenges, the photochemistry of isoxazoles has remained unexplored, and no general framework exists to harness their reactivity in a selective or synthetically useful way.

Here we report a photochemical platform for the selective conversion of isoxazoles into other azoles, including oxazoles, pyrazoles, pyrroles, and isothiazoles (Fig. 1c). Beginning with the efficient isoxazole-to-oxazole rearrangement of 5-phenylisoxazole (2a), we observed unexpectedly different behavior across closely related analogs. This prompted a mechanistic investigation using DFT, which revealed how small changes in the isoxazole substitution pattern drastically influence excited-state reactivity. Leveraging these insights, we constructed a comprehensive reactivity map encompassing 24 substituted isoxazoles and identified structural features that govern productive rearrangement. Beyond rearrangement, we discovered that solvent modulation could redirect excited-state reactivity away from C–C bond fragmentation, yielding α-ketonitrile intermediates that serve as branch points for further heterocycle formation. Using this principle, we developed a divergent ionic deconstruction–reconstruction platform capable of generating five distinct heterocyclic families from readily available isoxazole precursors31.

Results
In developing a strategy for the conversion of isoxazoles into other heteroaromatics, we drew inspiration from the early photochemical experiments from Ullman and Singh26,27. These pioneering studies demonstrated that simple photoexcited isoxazoles can undergo a series of structural rearrangements leading to either azirine intermediates or α-ketonitriles32. However, pioneering studies from Pavlik et al. demonstrated its synthetic potential in isoxazole-to-oxazole isomerization, although their conditions often led to complex reaction outcomes, low chemical yields, and mixtures of isomeric products19–25,28–30. More recently, Baumann has translated the isoxazole-to-oxazole conversion under photo-flow settings using high-energy Hg-lamps, but this reactivity was applied to a small subset of derivatives that do not capture the full chemical space options around the isoxazole core33. As a result, the synthetic scope and selectivity of isoxazole photochemistry remain essentially unknown.

We initiated our study with 5-phenylisoxazole (2a) and explored its conversion into either the corresponding oxazole (2b) or α-ketonitrile (2c), depending on the reaction conditions (Fig. 2a). Under irradiation at λ = 310 nm, we observed some remarkable differences in the reaction outcome on the basis of the solvent and additive used. Specifically, irradiation in MeOH led to the selective formation of 2b in good yield (entry 1). Interestingly, other polar protic solvents lead to considerably lower conversions and significant decomposition (entries 2 and 3). We then evaluated a few additives, and while both standard acid and base did not improve the reaction efficiency (entries 4 and 5), the use of 20 mol% of 2,6-lutidine delivered 2b in 85% yield (entry 7)34. These conditions could be scaled up in a single batch condition (8 mmol) using the same irradiating apparatus to provide 2b in 80% yield (see the Supplementary Information Section 7 for more details).

The use of non-polar solvents (e.g., trifluorotoluene and EtOAc, entries 8 and 9) had a dramatic switch in the reaction, now providing nitrile 2c as the major product. Pleasingly, the use of DCE completely suppressed the formation of 2b and provided access to 2c in 72% yield.

Encouraged by these results, we explored how changes to the position of the Ph substituent on the isoxazole ring might influence reactivity (Fig. 2b). In general, all isoxazoles discussed below have been subjected to six photochemical conditions (A–F), and the results reported represent the highest yielding ones. Strikingly, the reactivity of 2a was not preserved on its constitutional isomers. Indeed, the 4-phenylisoxazole (3a) did not undergo any reactivity when irradiated (λ = 310 nm) in the presence of 2,6-lutidine as the additive and MeOH as the solvent. However, partial conversion was achieved with more energetic irradiation (λ = 254 nm), yielding oxazole 3b in modest yield (23%, conditions E). Furthermore, the 3-phenyl analog (4a) proved photochemically inert under all tested conditions, even under prolonged irradiation.

This sharp change in photochemical reactivity is difficult to rationalize based on the limited knowledge in the area, and therefore, we have run full reaction analysis by DFT. The proposed mechanism leading to the isoxazole-to-oxazole (A-to-E) isomerization or isoxazole-to-α-ketonitrile (A-to-G) ring-opening has generally been approached considering that irradiation populates the isoxazole π,π* singlet excited state (S-A) (step i), subsequently evolving to a vinyl nitrene (B) via fast N–O bond homolysis (step ii) (Fig. 2c)35,36. This high-energy intermediate can undergo either a ring-contraction by 2π-electrocyclization to azirine C (step iii)26,32,37 or, if R = H, a 1,2-H shift to ketenimine F (step vi)38. Previous work has suggested that azirine C can undergo further photoexcitation to deliver upon isomerization and C–C fragmentation (step iv), the nitrile ylide D38–40. At this point, ionic cyclization (step v) can complete the isoxazole-to-oxazole isomerization. Regarding the formation of α-ketonitriles G, this is most likely arising from a 1,3-H shift of F (step vii). The strong difference in reaction performance depending on the position of the Ph-group across the isoxazole core indicates a more complex reaction manifold than previously appreciated.

To understand the differences in photochemical reactivity observed among these three isoxazoles, we carried out computational studies on compounds 2a, 3a and 4a (Fig. 3). Upon photoexcitation to the singlet (S), all three species undergo N–O bond cleavage via rapid population of a πσ* state35, independent of the position of the Ph substituent on the isoxazole core. Following internal conversion to the ground state (S0), a singlet biradical is generated (L2–4). Previous analysis on this reactivity invoked the formation of a vinyl nitrene B (see above). While B and L can be considered as resonance structures, our calculations suggest L as the best way of representing this intermediate on the basis of their spin density. From this intermediate, regeneration of the isoxazole or formation of the azirine can occur. For L4 (blue line), the pathway leading to isoxazole 4a regeneration is favored, with a significantly lower activation barrier (6.1 kcal mol−1, via TS1) compared to that for azirine C3 formation (18.4 kcal mol−1, via TS2). In contrast, for both L3 (yellow line) and L2 (red line) derivatives, azirine C3 and C2 formation is preferred, as the activation barriers for C–N bond development (C3: 2.0 and C2: 3.5 kcal mol−1, respectively) are lower than those for isoxazole reformation (3a: 7.2 and 2a: 6.0 kcal mol−1). The conversion of the azirine intermediates C2 and C3 to the corresponding oxazoles 2b and 3b via a thermal process is associated with prohibitively high activation barriers (TS3), supporting the hypothesis that this transformation is also photochemically driven. Indeed, our calculations demonstrate that photoexcitation of C2 and C3 to the singlet states (S1) enables a facile conversion into the nitrile ylides N2 and N3 that can evolve into 2b and 3b via low-barrier processes. To account for the experimental difference in reactivity efficiency between the two starting isoxazoles 2a and 3a, we speculated that differences in the absorption profile of their corresponding azirines might impact the overall photochemical behavior. Notably, C3 exhibits minimal absorption in the λ = 200–300 nm range, while C2 shows strong absorption and should readily undergo photoexcitation. Overall, these computational findings might be used to rationalize the experimental outcomes: 4a (complete recovery of starting material) undergoes N–O cleavage upon photoexcitation, but the high barrier to azirine formation favors reversion to the starting isoxazole. 3a (~20% product, ~20% starting material, ~60% decomposition) and 2a (85% yield) undergo N–O cleavage, with azirine formation being slightly favored. However, due to poor UV absorption, the azirine coming from 3a, C3, cannot efficiently re-enter the photochemical manifold, leading to competing thermal degradation pathways. In contrast, the azirine intermediate coming from 2a, C2, strongly absorbs in the UV range, enabling a second photoexcitation that drives conversion to the oxazole product. The solvent-controlled conversion of 2a into either 2b (MeOH) or 2c (DCE) is more difficult to rationalize. We currently propose that the divergence arises from differential stabilization of reactive nitrile ylide intermediates, controlled by solvent polarity and proticity. A plausible explanation for this pronounced solvent effect involves the differential stabilization of the nitrile ylide intermediate. In methanol (MeOH), a polar protic solvent, the nitrile ylide is likely significantly stabilized, shifting the equilibrium toward its formation. This, in turn, facilitates an intramolecular cyclization, ultimately affording the oxazole product. In contrast, 1,2-dichloroethane (DCE), being an apolar aprotic solvent, likely disfavors the formation of the nitrile ylide and instead promotes two consecutive [1,n]-hydrogen shifts, resulting in the formation of the α-ketonitrile product. Overall, these results suggest that the observed experimental selectivity arises not only from the initial photoactivation but also from downstream differences in the photophysical behavior of high-energy intermediates.

Given the strong dependence of reactivity on changes in substitution, we recognized the need for a comprehensive and systematic evaluation of isoxazole substitution patterns (Fig. 4). We sought to explore relevant mono- and disubstituted isoxazoles across a diverse range of aryl (Ph), alkyl (Me), and electron-withdrawing (CO2Me) groups. We therefore selected a set of 24 isoxazole derivatives categorized into five structural classes: (1) Ph-monosubstituted (2a–4a); (2) ester-monosubstituted (5a–7a); (3) Ph, Me-disubstituted (8a–13a); (4) ester, Me-disubstituted (14a–19a); and (5) Ph, CO2Me-disubstituted (20a–25a). Each derivative was subjected to six photochemical conditions (A–E) varying solvent (MeOH vs 1,2-DCE), additive (with or without 2,6-lutidine), and light source (λ = 254 vs 310 nm). In Fig. 4, only the result leading to the highest yield/lowest decomposition was reported for each substrate; the remaining results can be found in the Supplementary Information Section 4.

Our systematic analysis revealed a highly sensitive reactivity profile dictated by the nature and position of substituents on the isoxazole ring. In many cases, small structural changes dramatically altered photochemical outcomes, ranging from clean isomerization to complete decomposition or photostability, highlighting our inherent low understanding of isoxazole photochemistry. We have performed detailed 1H NMR spectroscopy analyses of the crude mixtures for representative low-yielding examples. These spectra reveal that aside from the desired oxazoles, the major components typically consist of decomposition products manifesting as broad, unresolved signals in the aromatic and aliphatic regions, consistent with polymeric or oligomeric byproducts. Unfortunately, these byproducts could not be cleanly isolated or fully characterized due to their complexity and instability. In some cases, minor amounts of starting isoxazole were also detected.

For instance, within the Ph-substituted series, only the 5-Ph isomer (2a) underwent efficient and selective rearrangement. The 3-Ph (4a) was photostable, while 4-Ph (3a) gave a low yield of the corresponding oxazole. For ester-substituted isoxazoles, the 3- and 4-substituted variants (5a and 6a) were unstable and decomposed across all tested conditions, precluding rearrangement. In contrast, the 5-substituted isomer (7a) cleanly afforded oxazole 7b in 38% yield, underscoring the privileged reactivity of the 5-position. These findings support the hypothesis that productive rearrangement correlates with the favorability of azirine formation and subsequent re-excitability, as seen in our computational studies.

In the Ph, Me-disubstituted series, most isomers were accessible and evaluated, with the exception of 11a, whose synthesis remains elusive, highlighting the synthetic limitations even for apparently simple heterocycles. Here, introducing a Me group at C4 (8a) enabled reactivity from an otherwise inert 3-Ph core (see the results for 4a), forming 8b in moderate yield. However, the 5-Me (9a) and 3-Me (10a) analogs decomposed entirely. Notably, 12a and 13a underwent efficient rearrangement to 12b and 13b, respectively, with wavelength sensitivity influenced by the methyl group’s position (310 nm for 12a vs 254 nm for 13a). The ester, Me-disubstituted isoxazoles, posed greater challenges. Several substrates (14a, 16a, and 19a) decomposed under irradiation, while 15a, 17a, and 18a yielded the corresponding oxazoles with variable efficiency. In the case of 18a, careful reaction timing (2 h irradiation time) was key to maximizing yield.

Finally, in the Ph, CO2Me-disubstituted series, both 3-Ph derivatives (20a and 21a) successfully underwent rearrangement to give oxazoles 20b and 21b in moderate yields. Although 22a and 23a were synthetically inaccessible, the 5-Ph analogs 24a and 25a rearranged smoothly to 24b and 25b in good yield.

Taken together, this survey identified 5-substituted isoxazoles bearing aryl or electron-withdrawing groups as privileged substrates for selective ring-replacement, while reactivity at other positions proved highly variable and substrate-specific. We therefore focused subsequent synthetic efforts on this class. In terms of synthetic potential, it is interesting to note that this photochemical strategy uses inexpensive and easy-to-make isoxazoles to access 2,5-disubstituted oxazoles (12b, 15b, and 24b) that in turn are generally difficult to make or very expensive (see the Supplementary Information Section 9 for more details).

We next evaluated the generality of the transformation using a broad range of 5-arylisoxazoles, which our initial screen identified as robust substrates (Fig. 5a). The reaction conditions (typically λ = 310 nm, MeOH, 2,6-lutidine) were broadly applicable, with minor tuning required in isolated cases. Overall, the transformation proved tolerant to diverse functional groups.

The method tolerated a wide range of aryl substituents, including electron-donating (Me, OMe, thioether, and NH2; see 26a–29a) and electron-withdrawing (halides, CF3, CN, and B(pin); see 30a–34a) groups. Remarkably, sensitive functionalities such as vinyl and C(sp2)–Br groups (see 31b and 35b) remained intact, highlighting the method’s chemoselectivity.

Meta- and ortho-substituted arenes (36a, 37a) as well as polyfunctional systems (38a–40a) also performed well, demonstrating broad positional tolerance. While the reaction for 41a was initially low-yielding under standard conditions, extending the reaction time to 72 h at λ = 254 nm improved the yield to 59%. Importantly, heteroaromatic groups were compatible, including benzoxazole (42a), benzothiazole (43a), thiophene (44a), and furan (45a) derivatives, which underwent clean isomerization with no damage to the auxiliary heterocycles.

We also examined polysubstituted isoxazoles (46a–52a), which required individual optimization. Trisubstituted systems featuring combinations of aryl and Me groups rearranged selectively to their oxazoles, provided that substitution patterns aligned with those favoring productive excited-state behavior. The high yield observed for 49b (from 3-Ph, 5-Me isoxazole) contrasts with the decomposition seen for simpler analogs (9a, 20a), again underlining the complex interplay between sterics, electronics, and photophysical properties.

A particularly impactful application was the two-step synthesis of oxazole 50b, a derivative of a friulimicin B lipopeptide, previously accessed via an eight-step route41. Our method reduced this to a simple esterification followed by photochemical rearrangement, showcasing the synthetic streamlining potential of this approach.

Finally, we demonstrated late-stage applicability by converting two pharmaceutically relevant isoxazoles—parecoxib (51a) and a Hepatitis C antiviral (52a)—into their oxazole analogs (51b, 52b). These transformations illustrate the direct replacement of heterocyclic cores without de novo synthesis, which is otherwise required for SAR studies.

Interestingly, certain substrates exhibited divergent or tandem reactivity, opening new avenues for discovery. Isoxazoles bearing two aryl groups at C4 and C5 (53a–55a) underwent both oxazole formation and 6π-electrocyclization, yielding fused polycyclic products with potential applications in organic electronics (Fig. 5b). The selectivity and modularity of this cascade merit further exploration in materials-oriented contexts42,43.

Even more striking was the behavior of 12a, which could be selectively diverted to three different products depending on conditions (Fig. 5c). While conditions B (DCE, 2,6-lutidine, λ = 254 nm) gave oxazole 12b, reactions in MeOH yielded 12c via trapping of a ketenimine intermediate44, and lower-energy irradiation in DCE afforded the azirine 12d. These conditional divergences from a single scaffold underscores the unique power of photochemical control and further validates the need for detailed reactivity mapping.

As discussed above, while investigating the isoxazole-to-oxazole rearrangement, we discovered that solvent tuning can redirect the reaction pathway toward C–C bond cleavage, yielding α-ketonitrile intermediates. Notably, α-ketonitriles are valuable intermediates in condensation chemistry and can also be accessed by conventional routes. However, their direct formation from isoxazoles offers a streamlined and synthetically powerful “ring-replacement” opportunity to directly convert them into pyrroles (H), pyrazoles (I), and isothiazoles (K).

It is important to note that, at present, the photochemical isoxazole-to-α-ketonitrile conversion is only accessible using 5-Ar-substituted derivatives. The inclusion of other substituents on the isoxazole core leads mostly to the formation of oxazoles, photostability, or photodecomposition (see Figs. 3 and 4).

Recognizing the synthetic potential of α-ketonitriles, we designed a modular platform for isoxazole-based scaffold remodeling, converting these intermediates into five heterocyclic frameworks via one-pot transformations (Fig. 6a, b). Specifically, in order to demonstrate the applicability of this approach in generating a large library of heteroaromatic derivatives, we identified conditions for converting 5-aryl-isoxazoles into five distinct heterocyclic systems: amino-pyrazoles, pyrazoles, pyrroles, amino-isoxazoles, and isothiazoles. This methodology was applied to a selection of seven commercially available starting materials (2a, 27a, 29a, 35a, 37a, 45a, and the 4-Cl-Ph not represented in Fig. 6), delivering a library of 34 new derivatives without resorting to de novo individual syntheses.

Each substrate was first subjected to the optimized photochemical conditions (DCE, λ = 310 nm), generating the corresponding α-ketonitriles, which were immediately used in subsequent condensation reactions.

For instance, treatment with benzenesulfonylhydrazide in EtOH followed by heating under reflux enabled isoxazole-to-3-amino-pyrazole “ring-replacement” (56a–56g). The 5-amino-pyrazole motif is highly versatile, with widespread applications in drug discovery, particularly in anticancer, antibacterial, antimalarial, and anti-inflammatory agents7,45.

Alternatively, DIBAL-H reduction of the α-ketonitriles, followed by condensation with H2N–NH2·H2O, converted 5-aryl-isoxazoles into 3-aryl-pyrazoles 57a–57g46. This transformation represents a formal oxygen-to-nitrogen transmutation, and while related strategies are known, they typically require nickel catalysts and high-pressure hydrogenation, making our method a milder alternative47–49.

Further heterocyclic diversity was achieved by treating the crude α-ketonitriles with tosyl chloride and then diethylaminomalonate to form 3-amino-pyrroles (58a–58g)50.

Additionally, refluxing the crude α-ketonitriles with HO–NH2·H2O in EtOH led to the conversion of 5-aryl-isoxazoles into 3-aryl-5-amino-isoxazoles (59a–59g)51. This transformation effectively moves the aromatic substituent from C5 to C3 while introducing an amino group at C5.

Lastly, the isoxazole-to-isothiazole conversion was realized as a formal O-to-S atom transmutation. This was achieved by converting the α-ketonitriles into an alkyne using the Mukaiyama reagent, followed by reaction with sodium sulfide and chloramine-T, furnishing the desired isothiazoles (60a–60g) in moderate yields52.

In summary, we reported a photochemical strategy that transforms isoxazoles into structurally distinct heterocycles, including oxazoles, pyrazoles, pyrroles, and isothiazoles. This platform exploits both ring-contraction and deconstruction–reconstruction pathways, enabled by selective control of excited-state reactivity. The approach provides a rare example of scaffold editing for aromatic heterocycles and offers a practical route to heterocycle diversification without de novo synthesis. Its operational simplicity, broad functional group tolerance, and successful application to drug-like scaffolds suggest strong potential for adoption in medicinal chemistry.

Methods
General procedure for the permutation of isoxazole
A dry tube equipped with a stirring bar was charged with the corresponding isoxazole (1.0 equiv.). The tube was capped with a Supelco aluminum crimp seal with septum (PTFE/butyl), evacuated and refilled with N2 (×3). The corresponding anhydrous and degassed solvent and the corresponding additive (0.2 equiv.) were added. The tube was placed into a Helios photoreactor. The photoreactor and a fan were switched on, and the mixture was stirred under irradiation for the specified time. The solvent was evaporated, and the residue was purified by column chromatography on silica gel to give the desired product. All modifications regarding the solvent and/or additive used are detailed in the Supplementary Information in Section 2, page S6.

General procedure for the preparation of α-ketonitrile
A dry tube equipped with a stirring bar was charged with the corresponding isoxazole (0.2 mmol, 1.0 equiv.). The tube was capped with a Supelco aluminum crimp seal with septum (PTFE/butyl), evacuated and refilled with N2 (×3). Degassed DCE (4 ml) was added. The tube was placed into a Helios photoreactor equipped with 310 nm lamps, and the fan was switched on. The mixture was stirred under irradiation for 24 h. Upon completion, the solvent was evaporated under reduced pressure, and the crude product was used without any further purification.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Leonori and co-workers report in this manuscript a comprehensive study pertaining to the photochemical isomerisation of isoxazoles into oxazoles and related products which typically proceeds via azirines, keto nitriles or ketenimine intermediates. These reactions are well described in the literature for both isoxazoles and analogous heterocycles possessing a weak X-Y bond (for instance as reviewed by Hoffmann: doi.org/10.1002/ejoc.201901190). A recent paper by the authors in Nature highlights the underlying reactivity for related azoles (Nature volume 637, pages860–867 (2025)). While recent years have witnessed a renaissance of this classical transformation in view of 'molecular editing' approaches, the underlying chemistry is well documented for almost 50 years. The authors provide thus a comprehensive summary of the current understanding of these isoxazole-based transformations by varying the substitution patterns as well as electronic properties of the resulting substrates which shows that some building blocks are very suited to undergoing this photo-process whereas low yields as well as significant amounts of decomposed material are observed in other cases. Provision of DFT data aids in understanding this behaviour which follows on from previous computations though these may not have been as comprehensive. As a consequence, the authors report instances where oxazoles are favoured as well as other circumstances/conditions where keto nitriles or transient ketenimines are favoured. This diverse reactivity is very appealing, but recent studies by Bracken and Baumann have already reported these outcomes and provided mechanistic reasoning governing these outcomes (ref 31/43). An interesting addition provided by the authors concerns the use of aryl isoxazoles that can be directed to a variety of different azoles exploiting the ability to bias the formation of a major photo-product that can be converted into pyrazoles, pyrroles as well as isothiazoles/isoxazoles using appropriate reagents and conditions as known in the literature.\nIt would have been useful if the authors had discussed in more detail what the crude product composition is for cases where only modest amounts of oxazoles are formed, i.e. what other species (and in which amounts) were observed. Does the analysis of these crude mixtures provide new insights in this chemistry? Also, it appears that these reactions are often rather slow (sometimes 72 hours are needed) - why is this? Is it related to the low optical power of the light sources used or are the reaction set-ups photon-limited? Please discuss/demonstrate if this chemistry can be scaled to make gram quantities of valuable products as this would be required to validate the chosen argument that this method is relevant to medicinal and industrial chemists. As a minor point, I didn't understand why several of the UV-Vis spectra (SI45 etc.) show areas of negative absorbance - is that a measurement issue?\nAltogether, this is a well-executed study that highlights the diverse reactivity of isoxazoles under photochemical conditions. The manuscript and SI are very informative. My main concern would be in relation to the novelty of this chemistry where I encourage the authors to make it clear to the reader where new understanding on this intriguing reactivity has been found.", "reviewer_2": "Skeletal editing has emerged as a highly promising area in synthetic organic chemistry, offering substantial potential across various fields, particularly in medicinal chemistry. This strategy provides a shortcut to late-stage core modifications, avoiding de novo synthesis and thereby significantly reducing resource and time consumption. In this context, Leonori and coworkers present an excellent example by remodeling isoxazoles into diverse azole derivatives, with potential applications in lead optimization and drug discovery. Although the photochemical transformation of isoxazoles into corresponding oxazoles has been previously reported by Ulman, Singh, and others, the present study explores the influence of substituents, irradiation wavelengths, and solvent effects, enhancing reaction control and broadening the applicability of the methodology. Overall, I recommend the publication of this manuscript in Nature Communications after addressing the following points:\n1) The computational section is brief and lacks sufficient mechanistic detail. The authors are encouraged to provide full information on the computed transition states, particularly TS3 (Fig. 2d). Is the conversion from azirine to oxazole a concerted single-step process, or does it involve intermediates? The methodology for obtaining the absorption profiles of intermediates C2 and C3 should also be clarified (experimental or TD-DFT calculations?). The explanation for the differential reactivity of 3a and 2a, based primarily on absorption profiles, appears insufficient, especially considering the lower yield observed for 2a at 254 nm. A more comprehensive computational analysis would significantly strengthen the mechanistic insight.\n2) The observed solvent effects are intriguing but insufficiently rationalized. The authors are encouraged to propose plausible mechanistic explanations or hypotheses, supported by experimental or computational evidence, to account for solvent-dependent reactivity. In particular, the behavior of substrates 2a and 12a under different solvents and substitution patterns deserves further discussion.\n3) The mechanistic rationale behind the effect of 2,6-lutidine on yield improvement remains unclear. Although the authors cite related work by Taniguchi et al., a more detailed discussion is needed to explain the observed increase in yield from 55% to 85%. It would also be helpful to explore the effect of lower equivalents (e.g., 0.1 eq), given that 0.2 eq outperformed higher loadings.\n4) While the substrate scope is broad, discussing the method’s limitations would enhance its practical relevance. Specifically, it would be valuable to explore common functional groups in drug-like molecules that may not tolerate the reaction conditions (e.g., amides, lactones) and to include examples of suboptimal or unsuccessful transformations.\n5) If feasible, applying the divergent isoxazole ring placement (Fig. 5) to more complex molecules such as substrates 50, 51, or 52 would provide better evidence of the method's generality and synthetic utility.\n6) The introduction would benefit from better contextualizing this work within the field of skeletal editing, which has seen significant recent advances. The authors should consider citing key recent reviews, such as:\nSarpong and Levin, Nat. Synth., 2022, 1, 352–364; Ball, Helv. Chim. Acta, 2023, 106, e202200182.\n7) Although a minor issue, the current compound numbering system may cause confusion. It may be clearer to assign unique numbers to distinct skeletons and use letters (a, b, c, etc.) for derivatives.\n8) Please consider the commented PDF attached.", "reviewer_3": "Leonori et al. present a photochemical platform for the selective conversion of isoxazoles into five-membered heteroaromatics by leveraging excited-state reactivity. The authors systematically investigated the effects of substitution sites during the isoxazole-to-oxazole transformation using DFT calculations. Concurrently, they conducted a scope analysis of differentially substituted isoxazoles. For the divergent \"ring replacement\" of isoxazoles, the key lies in the transient formation of common intermediate α-ketonitriles. By subjecting these intermediates to distinct reaction conditions, the authors observed divergent outcomes, enabling the construction of five heterocyclic frameworks through a one-flask operation.\nMy main concerns with this manuscript arise from several critical scientific considerations, which cumulatively cast doubt on its readiness for publication in Nature Communications. The concerns can be systematically categorized as follows:\n1. The manuscript's core focus on the isoxazole-to-oxazole transformation, while methodologically sound, is circumscribed by critical precedents. As detailed in References 24 and 25 (Ullman, Singh et al.), this rearrangement class has established synthetic precedence, with Reference 31 recently reporting an isoxazole-derived oxazole synthesis under Hg-lamp irradiation. These prior disclosures inherently diminish the work's claimed novelty, despite the authors' contributions to substrate scope expansion and mechanistic elaboration. The study thus manifests as a synthetic and mechanistic refinement rather than an innovation meeting the elevated standards expected of Nature Communications.\n2. On page 4, line 13, the authors note that \"the 3-phenyl analogue (4a) proved photochemically inert under all tested conditions, even upon prolonged irradiation,\" with DFT calculations attributing this to a higher energy barrier for aziridine formation. However, a discrepancy arises: substrates 8a, 20a, 21a, 48a, and 49a—all 3-phenyl-substituted isoxazoles—react smoothly under the protocol. This inconsistency necessitates mechanistic clarification. The authors are expected to perform additional DFT calculations to rationalize the reactivity differences between di-substituted and tri-substituted isoxazoles.\n3. Additionally, the observed substrate specificity warrants concern. As illustrated in Figure 3, minor substituent variations on the isoxazole scaffold lead to divergent reaction outcomes. Even for privileged aryl substrates bearing 5-substituted isoxazole moieties (e.g., 33a, 38a, 41a), reaction conditions required case-by-case optimization to enable the transformation. Such results undermine the synthetic utility of the reported method.\n4. The SI is well-prepared, but ¹H NMR spectra of 43b and 58d show impurities. Please re-purify and resubmit clean spectra.", "reviewer_4": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts.", "reviewer_5": "The manuscript presents a novel photochemical platform for the transformation of isoxazoles into structurally diverse heterocycles, including oxazoles, pyrazoles, pyrroles, and isothiazoles. This work provides significant value for the derivatization of five-membered nitrogen-containing heterocycles in pharmaceutical development. Although a robust methodological design, comprehensive data collection, and detailed DFT calculations are done to investigate the complex interplay between sterics, electronics, and photophysical properties, certain aspects of the mechanistic analysis and discussion require further clarification.\n(1) The proposed mechanism leading to the isoxazole-to-oxazole (A-to-E) isomerization includes the photoexcitation of isoxazole A, N–O bond homolysis, 2(cid:1)-electrocycliczation, and second photoexcitation to produce the corresponding oxazoles. To better understanding the selectivity，the energies of the excited singlet state (S₁-A) and the corresponding conical intersection (CI, S1/S0) points should be determined in the energy profiles.\n(2) According to the mechanism in Figure 2c, the conversion from C to E involves second photoexcitation and C-N bond cleavage to obtain nitrile ylide (D), and then through an electrocycliczation transition state to obtain the oxazole product. However, the second photoexcitation process and the detailed structure and energy information of the nitrile ylide (D) are missing in Figure 2d. As reported in Su’s work (J. Phys. Chem. A 2015, 119, 9666−9669), it is separated processes for C-N bond cleavage (C to D) and C-O bond formation (D to E). Is TS3 a concerted process for these two steps or just C-O bond formation from nitrile ylide?\n(3) The potential for intersystem crossing (ISC) from singlet to triplet along the reaction pathways should be considered, including the triplet states of isoxazole (T₁-A), and the triplet states of other key intermediates, just in case the involvement of triplet-state mechanisms.\n(4) The NMR spectral data require the following corrections: The 1H NMR integrals need calibration, as exemplified by the unlabeled integrations for peaks 8a and 18a. Additionally, the notation '1§C NMR' for compound 52a (151 MHz, CDCl3) should be revised to '13C NMR.' Furthermore, the 1H NMR integrals for peaks 39a/b must be properly adjusted for accuracy.\nMajor Revision The authors should address the above concerns, particularly the mechanistic clarifications and the explanation for the energy barrier/yield discrepancy, before the manuscript can be reconsidered for publication. The revisions are essential for ensuring the robustness and clarity of the proposed mechanism."}, "author_response": "We thank the Reviewer for their thorough assessment and for raising important points regarding the novelty of our work. We would like to bring some clarifications what we hope will be taken into consideration.\n• Historical context. We agree that the photochemistry of isoxazoles has some important precedents. However, these reports are fragmented, often limited to a handful of substrates, and lack mechanistic or predictive guidelines. For example, Pavlik’s study (ref. 27) examined only five substrates, with preparative data for just two. The remaining cases were based on GC analysis only, without product isolation or characterization, limiting broader conclusions.\n• Selectivity. In their work, Pavlik also used substrate 2a and reported a mixture of oxazole 2b (42%) along a-ketonitrile 2c (27%). By contrast, in our study, we can selectively obtain either 2b or 2c in >80% yield by changing the reaction conditions (see table below). We believe this represents a significant advance with strong synthetic implications. A similar situation applies to other examples where Pavlik's outcomes are mixtures – please note that their data (e.g. 12a to 12b) is not based on synthetic experiments but rather on GC conversion based on recovered starting material.\nState-of-the-art\n2b 2c rsm\nJ. Het. Chem. 2005, 273 42 27 4\nOur work\nConditions C – 84 –\nConditions F 85 – –\n• Scope and mechanism. Our work systematically surveys a wide range of substitution patterns, revealing dramatic and unexpected reactivity differences depending on each substituent and, more remarkably, their position on the isoxazole core. This illustrates that reactivity trends established for a single class of substrates (e.g., Bracken/Baumann with C3-ester,C5-aryl isoxazoles) cannot be generalized across the isoxazole family and do not represent the complexity of the chemistry.\nWe show that variations in reactivity often stem from changes in the absorption profiles of reactive intermediates – an effect not previously considered. Our combined experimental/DFT analysis provides mechanistic rationales missing from earlier studies.\nIn summary, while we build our work upon important prior contributions, these earlier reports feature limited scope, often non-preparative conditions, low or unreported yields, and no general strategy for controlling product distribution. I hope the Reviewer and the Editor will consider our study delivering the first systematic framework that enables condition-dependent, selective access to divergent products from some isoxazole substrates. We believe this constitutes a substantive advance with strong implications for synthetic chemistry and photochemistry.\nIt would have been useful if the authors had discussed in more detail what the crude product composition is for cases where only modest amounts of oxazoles are formed, i.e. what other species (and in which amounts) were observed. Does the analysis of these crude mixtures provide new insights in this chemistry?\nWe have performed detailed 1H NMR spectroscopy analyses of the crude mixtures for representative examples featuring low-yields (e.g. 3a, 8a, 9a and 10a in Figure 3). These spectra reveal that aside from the desired oxazoles, the major components typically consist of decomposition products manifesting as broad, unresolved signals in the aromatic and aliphatic regions, consistent with polymeric or oligomeric byproducts. Unfortunately, these byproducts could not be isolated and characterized so we cannot propose a tentative structure and decomposition pathway. In some cases, minor amounts of starting isoxazole were detected.\nAlso, it appears that these reactions are often rather slow (sometimes 72 hours are needed) - why is this? Is it related to the low optical power of the light sources used or are the reaction set-ups photon-limited?\nWe do not think our reaction set up to have low optical power as the light bulbs we use are powerful. We believe the second option mentioned by the Reviewer to be more likely. Unfortunately, we do not have the equipment for measuring quantum yields for these reactions.\nPlease discuss/demonstrate if this chemistry can be scaled to make gram quantities of valuable products as this would be required to validate the chosen argument that this method is relevant to medicinal and industrial chemists.\nWe have been able to scale-up the preparation of 2b on 8.0 mmol scale without any significant change in yield. This result has been added to the revised manuscript and SI.\nAs a minor point, I didn't understand why several of the UV-Vis spectra (SI45 etc.) show areas of negative absorbance - is that a measurement issue?\nWe tried several times to rerun the measurements, but we have not been able to remove the areas of negative absorbance, which upon discussion with the technical support of our UV/Vis instrument are believed to be artifacts.\nReviewer #2: Skeletal editing has emerged as a highly promising area in synthetic organic chemistry, offering substantial potential across various fields, particularly in medicinal chemistry. This strategy provides a shortcut to late-stage core modifications, avoiding de novo synthesis and thereby significantly reducing resource and time consumption. In this context, Leonori and coworkers present an excellent example by remodeling isoxazoles into diverse azole derivatives, with potential applications in lead optimization and drug discovery. Although the photochemical transformation of isoxazoles into corresponding oxazoles has been previously reported by Ulman, Singh, and others, the present study explores the influence of substituents, irradiation wavelengths, and solvent effects, enhancing reaction control and broadening the applicability of the methodology.\nOverall, I recommend the publication of this manuscript in Nature Communications after addressing the following points:\n1) The computational section is brief and lacks sufficient mechanistic detail. The authors are encouraged to provide full information on the computed transition states, particularly TS3 (Fig. 2d).\nWe have performed additional computational work to fully elucidate the reaction path. The various TS3s correspond to the thermal isomerization of azirines C2–4 to oxazoles 2b–4b, and do not reflect any potential photochemical pathway. These thermal reactions are concerted yet asynchronous: the C–C cleavage occurs first, followed by C–O bond formation. Since these barriers are energetically costly, we propose that photoexcitation of the azirines might overcome them and lead to the oxazoles via the nitrile ylides N2–N4. The energy barriers that separate these ylides from their corresponding oxazoles are low, so these species should evolve fast to the formation of the final products. This pathway helps also rationalise the difference in reactivity of isoxazoles 2a and 3a, since their corresponding azirines C2 and C3 have drastically different absorption profiles. We revised our discussion in the revised manuscript. Furthermore, we have included the energies of the S /S conical intersections 1 0 for all species and for both photochemical pathways.\nIs the conversion from azirine to oxazole a concerted single-step process, or does it involve intermediates? The methodology for obtaining the absorption profiles of intermediates C2 and C3 should also be clarified (experimental or TD-DFT calculations?).\nThe absorption spectra were calculated using TD-DFT, specifically at the TD-CAM-B3LYP/cc-pVTZ/SMD level of theory. This was already indicated in the original version of the SI and we have also added it to the caption for Figure 2d.\nThe explanation for the differential reactivity of 3a and 2a, based primarily on absorption profiles, appears insufficient, especially considering the lower yield observed for 2a at 254 nm. A more comprehensive computational analysis would significantly strengthen the mechanistic insight.\nWe have modelled the full pathway from isoxazole to oxazole considering both thermal and photochemical steps and our proposed explanation for the reactivities of 2a and 3a is based on the combination of all these computational results. It is difficult to consider other aspects to compute beyond the ones we have already discussed. Furthermore, both compounds 2a and 3a have the same thermal barriers for the N–O cleavage/regeneration (TS1), for the formation of the azirine from the vinylnitrene (TS2), and for the formation of the oxazole from the arizine (TS3). The only difference we have been able to find is in the absorption spectra of their azirine intermediates; C2 (the azirine form of 2a) has a strong absorption while C3 (the azirine form of 3a) does not. If the Reviewer has some specific aspects or suggestion in mind, we would be very happy to consider them.\n2) The observed solvent effects are intriguing but insufficiently rationalized. The authors are encouraged to propose plausible mechanistic explanations or hypotheses, supported by experimental or computational evidence, to account for solvent-dependent reactivity. In particular, the behavior of substrates 2a and 12a under different solvents and substitution patterns deserves further discussion.\nWe agree with the Reviewer this is one of the most fascinating yet puzzling aspect of permutation chemistry. Indeed, this is not restricted to the isoxazole-to-oxazole conversion but also it played a prominent role in the development of the thiazole-isothiazole permutations. However, dissecting the role(s) of the various solvents on such complex reaction profiles where multiple species are involved in both thermal and photochemical steps is very challenging.\nWe have attempted the calculation of the reaction pathways for the formation of 2b and 2c (stopping at the ketenimine intermediate) using an implicit solvation model based on MeOH and DCE solvents. The results, shown below, indicate that the relative barriers for the formation of the azirine and ketenimine intermediates do not change substantially with the solvent polarity, which does not explain the experimental observations. This might be a limitation in the use of implicit solvation. Capturing these effects would require explicit solvation (microsolvation) models, which are highly demanding and, we believe, beyond the scope of the present study. We therefore restrict ourselves here to presenting the experimental observations and noting this solvent effect as an open question for future dedicated computational investigations.\nRegarding 2a, we could propose the participation of MeOH, trapping the ketenimine F and preventing the last hydrogen shift forming the α-ketonitrile. Regarding 12a, the absence of hydrogen at C3 precludes the formation of the α-ketonitrile and therefore, in the presence of MeOH, the unstable ketenimine intermediate is trapped and forms product 12c (see Org. Lett. 2023, 25, 6593–6597 for the formation of this ketenimine intermediate). In the absence of such nucleophile, the ketenimine could revert to the vinylnitrene B and then follow the other mechanistic pathway leading to either azirine C at 310 nm (given azirines C1–C3 were shown not to absorb above 280 nm, see Figure 2d) or oxazole E at 254 nm. Since we do not have any support for these effects and in light of what discussed above, I would prefer to keep the manuscript as it is.\n3) The mechanistic rationale behind the effect of 2,6-lutidine on yield improvement remains unclear. Although the authors cite related work by Taniguchi et al., a more detailed discussion is needed to explain the observed increase in yield from 55% to 85%. It would also be helpful to explore the effect of lower equivalents (e.g., 0.1 eq), given that 0.2 eq outperformed higher loadings.\nThe reaction indeed proceeds in MeOH without additives, affording oxazole 4b in 55% yield. Addition of 2,6-lutidine significantly enhances the outcome, with 0.2 equiv. giving the optimal 85% yield. When we tested 0.1 equiv., the yield decreased to 72%, whereas higher loadings were less effective. We agree that the precise mechanistic rationale remains elusive. Based on the precedent by Taniguchi et al. and our own observations, a plausible explanation is that 2,6-lutidine transiently buffers against acid build-up or engages in specific interactions with reactive intermediates, thereby suppressing side pathways. The non-linear dependence on the amount of additive suggests that its beneficial effect is subtle and may involve a balance between proton management and undesired coordination. Unfortunately we do not see any clear interaction between the lutidine and either the starting material or the product and we cannot detect the transient intermediates so at the moment we cannot provide any more conclusive rationale.\n4) While the substrate scope is broad, discussing the method’s limitations would enhance its practical relevance. Specifically, it would be valuable to explore common functional groups in drug-like molecules that may not tolerate the reaction conditions (e.g., amides, lactones) and to include examples of suboptimal or unsuccessful transformations.\nI agree with the reviewer on this but I do not think the key aspect is which functional groups are tolerate, but rather how they impact the reactivity based on where they are located on the isoxazole core. There are some non-working examples in Figure 4 as the work was designed to capture this aspect. As an example, even a Me-group, which is not consider a functionality, can have a dramatic impact on the photochemistry depending on where it is placed. We do not have examples that did not work as we evaluated the scope on the basis of the guidelines that emerged from the initial screening of all substitution pattern with Me, Ph and ester substituent.\n5) If feasible, applying the divergent isoxazole ring placement (Fig. 5) to more complex molecules such as substrates 50, 51, or 52 would provide better evidence of the method's generality and synthetic utility.\nUnfortunately, compounds bearing substituents at positions C3 or C4 of the isoxazole core cannot be deconstructed to the corresponding a-ketonitrile, and various side reactions are observed depending on the conditions employed (see Figure 4c). Specifically, when a substituent is present at C3, the a-ketonitrile does not form; instead, only product 13c is observed. Similarly, when a substituent is introduced at C4, we were unable to detect the a-ketonitrile product under any of the conditions tested. As correctly pointed out by the reviewer, this represents a limitation of our methodology: only “terminal” isoxazoles can currently undergo the deconstruction–reconstruction sequence to access diverse new heteroaromatic frameworks. Surprisingly, we were not able to obtain the corresponding α-ketonitrile in the case of 52a and we currently do not have any clear explanation for this behaviour. I have discussed this aspect clearly stating the limitation in the revised manuscript.\n6) The introduction would benefit from better contextualizing this work within the field of skeletal editing, which has seen significant recent advances. The authors should consider citing key recent reviews, such as:\nSarpong and Levin, Nat. Synth., 2022, 1, 352–364; Ball, Helv. Chim. Acta, 2023, 106, e202200182.\nThese have been added\n7) Although a minor issue, the current compound numbering system may cause confusion. It may be clearer to assign unique numbers to distinct skeletons and use letters (a, b, c, etc.) for derivatives.\nI am not convinced this assignment will avoid confusions about the numbering. I believe with this numbering system, it is easier for the reader to identify which product comes from which substrate.\n8) Please consider the commented PDF attached.\nWe modified the main text accordingly. See the modifications below:\npage 3:\n• \"isothiazole\" → isoxazole\n• check \"H-bond\" and \"π-stacking\" (there are symbol letters that should not be there)\n• \"However, pioneering studies from Pavlik and other demonstrated its potential for application in isoxazole-to-oxazole isomerization although their conditions often led to complex reaction outcomes, low chemical yields, and mixtures of isomeric products.\" → However, pioneering studies from Pavlik and other demonstrated its synthetic potential in isoxazole-to-oxazole isomerization although their conditions often led to complex reaction outcomes, low chemical yields, and mixtures of isomeric products.\npage 6:\n• An image has been added to Section 8 of the SI showing the spin densities of the vinylnitrene intermediates L2-4.\npage 7:\n• Regarding the synthesis of 11a, I am not sure which reported procedure you are mentioning. On SciFinder, we can indeed find three procedures, one with permutation (4% yield, cited in the manuscript), one forming a mixture of isomers and using a tailored titanium catalyst (Tetrahedron 2012, 68, 807-812), and a last one where the starting material needs to be synthesized and almost no experimental details are given.\n• Regarding the synthesis of 23a, there is indeed a reported two-step synthesis in ACS Medicinal Chemistry Letters so we have cited it. This does not alter the conclusions of our work.\npage 8:\n• Add note for \"E\" in Figure 3.\n• The yields obtained for all conditions are reported in the SI. If the reviewer believes it is necessary, we can add a note or a sentence in the main text highlighting this.\n• Add numbers for oxazoles with vinyl and C(sp²)–Br\npage 11:\n• Figure 2 → Figure 3 for 2a and Figure 4 for the others\n• 4a → 2a\npage 12:\n• Figure 2 → Figure 3 & Figure 4\nReviewer #3: Leonori et al. present a photochemical platform for the selective conversion of isoxazoles into five-membered heteroaromatics by leveraging excited-state reactivity. The authors systematically investigated the effects of substitution sites during the isoxazole-to-oxazole transformation using DFT calculations. Concurrently, they conducted a scope analysis of differentially substituted isoxazoles. For the divergent \"ring replacement\" of isoxazoles, the key lies in the transient formation of common intermediate α-ketonitriles. By subjecting these intermediates to distinct reaction conditions, the authors observed divergent outcomes, enabling the construction of five heterocyclic frameworks through a one-flask operation.\nMy main concerns with this manuscript arise from several critical scientific considerations, which cumulatively cast doubt on its readiness for publication in Nature Communications. The concerns can be systematically categorized as follows:\n1. The manuscript's core focus on the isoxazole-to-oxazole transformation, while methodologically sound, is circumscribed by critical precedents. As detailed in References 24 and 25 (Ullman, Singh et al.), this rearrangement class has established synthetic precedence, with Reference 31 recently reporting an isoxazole-derived oxazole synthesis under Hg-lamp irradiation. These prior disclosures inherently diminish the work's claimed novelty, despite the authors' contributions to substrate scope expansion and mechanistic elaboration. The study thus manifests as a synthetic and mechanistic refinement rather than an innovation meeting the elevated standards expected of Nature Communications.\nWe thank the Reviewer for their thorough assessment and for raising important points regarding the novelty of our work. We hope the following clarifications will be taken into consideration.\n• We agree that the photochemical reactivity of isoxazoles has historical precedent. I am very sorry if the way I wrote the original manuscript gave the impression of dismissing prior contributions, especially the seminal work in Ref. 24, 25 & 27. However, despite several reports, the photochemistry of isoxazoles (as well as the one of many other heteroaromatics) has neither been really explored nor thoroughly understood and lacks clear predictive guidelines. We believe this work attempts to fill some of these gaps.\n• Singh reported only two substrates, outlining the yield only for the first one which was diphenylisoxazole (see scheme below). Despite a relatively in-depth mechanistic analysis, the authors did not study the possible scope, substituent or solvent effect of this photoreactivity.\n• Regarding Pavlik’s work (Ref. 27), despite being a crucial contribution to the field, this study examined only five isoxazole substrates, and only two were irradiated on a preparative scale. The remaining were evaluated under short irradiation times, limiting the ability to understand their true reactivity. See the answer to Reviewer #1 for more details.\n• See the answer to Reviewer #1 for more details about Ref. 31.\n• Our central contribution also lies in framing isoxazoles as programmable, masked precursors to access directly multiple heteroarenes. This one-pot deconstruction–reconstruction aligns well with emerging needs in medicinal chemistry, particularly in late-stage diversification and bioisosteric replacement strategies (see Nature Chem 2018, 10, 383–394. https://doi.org/10.1038/s41557-018-0021-z). As illustrated by recent work from the McNally group (Nature 2024 631, 87–93. https://doi.org/10.1038/s41586-024-07474-1; ref. 9a in the manuscript), there is strong and growing interest in modular frameworks that enable rapid access to chemical diversity from common building blocks.\nI hope to believe that our work goes beyond revisiting known transformations. We conducted a systematic and comprehensive exploration of >50 substrates, aiming to establish general reactivity trends and operationally simple guidelines that are relevant to modern synthetic applications. Indeed, aspects like the impact of Ph-substitution around the isoxazole ring or the impact of additional functionalities have never been demonstrated and reveal a complex photochemical scenario that cannot be appreciated looking at the reactivity reported in Ref. 24, 25 & 27.\n2. On page 4, line 13, the authors note that \"the 3-phenyl analogue (4a) proved photochemically inert under all tested conditions, even upon prolonged irradiation,\" with DFT calculations attributing this to a higher energy barrier for aziridine formation. However, a discrepancy arises: substrates 8a, 20a, 21a, 48a, and 49a—all 3-phenyl-substituted isoxazoles—react smoothly under the protocol. This inconsistency necessitates mechanistic clarification. The authors are expected to perform additional DFT calculations to rationalize the reactivity differences between di-substituted and tri-substituted isoxazoles.\nWe have calculated the energy profiles for the thermal isomerization pathways of two additional isoxazoles, 20a (disubstituted, 3-Ph-4-CO Me) and 49a (trisubstituted, 3-Ph-4-CO Me-5-Me). These profiles are shown below. In both cases we could not locate any nitrene intermediate, with a key difference. For 20a, the transition state for the N–O cleavage leads to a bifurcation that promotes the formation of two intermediates, the expected azirine C20 and a new diradical intermediate, I20, in which the N undergoes ipso addition to the Ph ring. In contrast, 49a leads to the exclusive formation of azirine C49. We believe that these computational results are in good agreement with the experimental observations. Specifically, for 4a (monosubstituted 3-Ph-isoxazole,) N–O bond cleavage forms the vinylnitrene intermediate, for which the energy barrier to generate the azirine is prohibitively high and prevents any reactivity (see the discussion around Figure 2d). For 20a, N–O cleavage forms either the azirine C20, which could evolve into the oxazole, or the biradical I20, which cannot and might lead to decomposition explaining the loss of ca. 40% of starting material. Finally, for 49a, exclusive formation of the azirine C49 promotes the formation of the oxazole without significant degradation, in accordance with <10% starting material loss observed experimentally. These results have been added to Section 8 of the SI.\n3. Additionally, the observed substrate specificity warrants concern. As illustrated in Figure 3, minor substituent variations on the isoxazole scaffold lead to divergent reaction outcomes. Even for privileged aryl substrates bearing 5-substituted isoxazole moieties (e.g., 33a, 38a, 41a), reaction conditions required case-by-case optimization to enable the transformation. Such results undermine the synthetic utility of the reported method.\nThe photochemistry of heteroaromatic compounds such as isoxazoles is inherently sensitive to electronic and steric effects. Small changes in substitution patterns (e.g., replacing a Ph with a Me or COMe) can dramatically alter excited-state behaviour, reaction pathways, and product distributions. This is not a shortcoming of the chemistry; it is a defining feature of photochemical reactivity. Expecting uniformly identical outcomes across diverse substrates ignores this core principle. Nevertheless, compounds rather diverse but belonging to the class of 5-aryl isoxazole display rather general reactivity with a single reaction condition (condition F).\n4. The SI is well-prepared, but ¹H NMR spectra of 43b and 58d show impurities. Please re-purify and resubmit clean spectra.\nWe have attempted to re-purify these two samples but I am sorry we do not manage. However, the impurities are very minor peaks. Unfortunately, our prep-HPLC is out of order so we cannot address this comment any better.\nReviewer #4: I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts.\nWe thank the reviewer for their positive evaluation of our work.\nReviewer #5: The manuscript presents a novel photochemical platform for the transformation of isoxazoles into structurally diverse heterocycles, including oxazoles, pyrazoles, pyrroles, and isothiazoles. This work provides significant value for the derivatization of five-membered nitrogen-containing heterocycles in pharmaceutical development. Although a robust methodological design, comprehensive data collection, and detailed DFT calculations are done to investigate the complex interplay between sterics, electronics, and photophysical properties, certain aspects of the mechanistic analysis and discussion require further clarification.\nWe thank the reviewer for their positive evaluation of our work.\n(1) The proposed mechanism leading to the isoxazole-to-oxazole (A-to-E) isomerization includes the photoexcitation of isoxazole A, N–O bond homolysis, 2𝜋-electrocycliczation, and second photoexcitation to produce the corresponding oxazoles. To better understanding the selectivity，the energies of the excited singlet state (S₁-A) and the corresponding conical intersection (CI, S1/S0) points should be determined in the energy profiles.\nWe included an extended version of Figure 2d including additional information regarding the photochemical steps. Specifically, this revised image includes the energies of the S /S conical intersections, the nitrile ylide intermediates that arise following excitation of the azirines, and the energy barriers that separate them from the oxazoles.\n(2) According to the mechanism in Figure 2c, the conversion from C to E involves second photoexcitation and C-N bond cleavage to obtain nitrile ylide (D), and then through an electrocycliczation transition state to obtain the oxazole product. However, the second photoexcitation process and the detailed structure and energy information of the nitrile ylide (D) are missing in Figure 2d. As reported in Su’s work (J. Phys. Chem. A 2015, 119, 9666−9669), it is separated processes for C-N bond cleavage (C to D) and C-O bond formation (D to E). Is TS3 a concerted process for these two steps or just C-O bond formation from nitrile ylide?\nPlease refer to our response to the first comment of reviewer #2. In a nutshell, the azirine → oxazole thermal isomerization pathway (corresponding to TS3 in Figure 2d) is concerted, the photochemical pathway (now shown in the revised Figure 2d) is stepwise.\n(3) The potential for intersystem crossing (ISC) from singlet to triplet along the reaction pathways should be considered, including the triplet states of isoxazole (T₁-A), and the triplet states of other key intermediates, just in case the involvement of triplet-state mechanisms.\nRegarding the reviewer’s suggestion that a triplet excited state could be involved, we had already conducted some EnT catalysis screening experiments using a range of known triplet photosensitizers. These efforts, detailed below and in the revised Supporting Information, showed no productive reactivity – only full recovery of starting material. This outcome supports our choice to focus on direct excitation of the isoxazole core and a singlet excited state mechanism, which ultimately proved more effective under the current conditions.\nentry PC solvent wavelength (nm) rsm (%) 2b (%) 2c (%)\n1 [Ru(bpz) ][PF ] DCE 440 nm > 95% n.d. n.d.\n3 62\n2 [MesAcr]ClO DCE 440 nm > 95% n.d. n.d.\n4\n3 4CzIPN DCE 440 nm > 95% n.d. n.d.\n4 [Ir{dF(CF )ppy} (dtbpy)]PF DCE 390 nm > 95% n.d. n.d.\n3 2 6\n5 fac-Ir(ppy) DCE 390 nm > 95% n.d. n.d.\n3\n6 [Ru(bpz) ][PF ] CH CN 440 nm > 95% n.d. n.d.\n3 62 3\n7 [MesAcr]ClO CH CN 440 nm > 95% n.d. n.d.\n4 3\n8 4CzIPN CH CN 440 nm > 95% n.d. n.d.\n3\n9 [Ir{dF(CF )ppy} (dtbpy)]PF CH CN 390 nm > 95% n.d. n.d.\n3 2 6 3\n10 fac-Ir(ppy) CH CN 390 nm > 95% n.d. n.d.\n3 3\n(4) The NMR spectral data require the following corrections: The 1H NMR integrals need calibration, as exemplified by the unlabeled integrations for peaks 8a and 18a. Additionally, the notation '13C NMR' for compound 52a (151 MHz, CDCl3) should be revised to '13C NMR.' Furthermore, the 1H NMR integrals for peaks 39a/b must be properly adjusted for accuracy.\nSee above – I am sorry but we have not managed to remove these very minor impurities from the samples.\nMajor Revision The authors should address the above concerns, particularly the mechanistic clarifications and the explanation for the energy barrier/yield discrepancy, before the manuscript can be reconsidered for publication. The revisions are essential for ensuring the robustness and clarity of the proposed mechanism."}, {"round": 2, "reviewer_comments": {"reviewer_2": "The manuscript has improved, and I appreciate the authors’ efforts. However, I still have two major concerns: (i) the synthetic value/generalizability remains uncertain, as small substituent changes seem to cause large outcome shifts and performance on complex, drug-like substrates appears limited; and (ii) the mechanistic basis of the differential reactivity of 3a and 2a leans mainly on TD-DFT absorption profiles of intermediates (not a soild evidence and doesn't explain the whole reaction profile), and does not yet offer a solid explanation for solvent/additive effects.", "reviewer_3": "The authors have addressed most of my concerns, yet some aspects still require further refinement.\nBelow are my comments on the resubmitted manuscript.\nThe authors stated that “Small changes in substitution patterns (e.g., replacing a Ph with a Me or CO₂Me) can dramatically alter excited-state behaviour, reaction pathways, and product distributions.” While it is reasonable to anticipate the need for reaction condition adjustments when a phenyl group is substituted with a methyl or ester group—given the distinct electronic and steric properties of these moieties—the fact that 33a, 38a, and 41a all fall within the 5-aryl isoxazole scaffold undermines the persuasiveness of this explanation.\nIn the ¹H NMR spectrum of compound 58d, both solvent peaks and impurity peaks are non-negligible, as they can significantly interfere with the accurate determination of the compound's purity—and thus indirectly affect the reliable calculation of its yield. If further purification proves unfeasible, this substrate should be excluded from subsequent studies.\nAdditionally, after carefully reviewing the authors' responses to Reviewer #1, this reviewer confirms that the latter’s technical requests have been properly addressed.", "reviewer_4": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts.", "reviewer_5": "In the new version, the authors did a more detailed mechanistic study, both in calculation and experimentally. As shown in Figure 3, the second photoexcitation process and the detailed structure and energy information of the nitrile ylide are calculated. The energies of the S1/S0 conical intersections are also added to support the proposed mechanism. The triplet-state mechanisms are excluded by EnT catalysis screening experiments using a range of known triplet photosensitizers, but if the authors have calculated the energies of triplet states of the key intermediates. Are they also consistent with the experimental results?\nAfter the author's efforts, most of my concerns have been addressed in the new revisions. The revised manuscript could be considered for publication."}, "author_response": "Reviewer #2 (Remarks to the Author):\nThe manuscript has improved, and I appreciate the authors’ efforts. However, I still have two major concerns: (i) the synthetic value/generalizability remains uncertain, as small substituent changes seem to cause large outcome shifts and performance on complex, drug-like substrates appears limited; and (ii) the mechanistic basis of the differential reactivity of 3a and 2a leans mainly on TD-DFT absorption profiles of intermediates (not a soild evidence and doesn't explain the whole reaction profile), and does not yet offer a solid explanation for solvent/additive effects.\nResponse to point 1.\nOur study includes 55 examples, covering mono-, di-, and tri-substituted isoxazoles, thus encompassing all types of substitution classes. We demonstrate compatibility with a wide range of substituents, including alkyl, electron-rich and electron-poor aryl, heteroaryl (both electron rich and electron poor – indole, thiophene, pyridine…), and ester substituents directly attached to the isoxazole ring. Furthermore, we show tolerance of diverse functional groups such as Br, Cl, B(pin), cyclopropane, ether, carboxylic acid, and sulfonamide. We have illustrated the synthetic relevance of this methodology through:\n• The two-step synthesis of a bioactive compound that previously required an eight-step sequence.\n• Successful permutation of two marketed drugs.\nPhotochemical sensitivity to substitution is an intrinsic property of excited-state processes and does not, per se, imply lack of generality. To address this explicitly, we systematically evaluated substitution at all three positions of the isoxazole core across 55 examples and identified well-defined structural domains where the reaction is robust and high-yielding. Once these domains were established, extended scope studies were performed within these regions of reactivity. The successful permutation of two marketed drugs and the streamlined synthesis of a bioactive target further demonstrate that the method is applicable to structurally complex, medicinally relevant substrates within these defined boundaries.\nResponse to point 2.\nThe key intermediates likely involved in the reactivity are transient and cannot be isolated or detected, even when probed using laser flash photolysis. TD-DFT calculations therefore represent the only viable approach. While TD-DFT alone cannot provide a complete kinetic profile, it successfully reproduces the relative absorption trends and rationalizes the distinct photochemical fates of 3a and 2a.\nExperimental observations (quenching, selectivity, and control experiments) are consistent with these theoretical insights.\nRegarding solvent and additive effects, this remains a complex and open field in photochemistry. The interplay between polarity, hydrogen-bonding, and solvation of excited states is often non-linear and system-dependent. Even in well-studied systems (e.g., Pd-catalyzed C–H activation in HFIP, see “Hexafluoroisopropanol: the magical solvent for Pd-catalyzed C–H activation” Chem. Sci. 2021, 12, 15773), the mechanistic origins of solvent effects remain incompletely understood. I started a collaboration with Prof. Davide Avagliano (Chimie ParisTech) to model these phenomena in detail; however, this is a substantial computational challenge that will require dedicated future work and goes beyond the scope of the present manuscript.\nReviewer #3 (Remarks to the Author):\nThe authors have addressed most of my concerns, yet some aspects still require further refinement.\nBelow are my comments on the resubmitted manuscript.\nThe authors stated that “Small changes in substitution patterns (e.g., replacing a Ph with a Me or CO₂Me) can dramatically alter excited-state behaviour, reaction pathways, and product distributions.” While it is reasonable to anticipate the need for reaction condition adjustments when a phenyl group is substituted with a methyl or ester group—given the distinct electronic and steric properties of these moieties—the fact that 33a, 38a, and 41a all fall within the 5-aryl isoxazole scaffold undermines the persuasiveness of this explanation.\nI am sorry but I do not understand what they mean with this comment and how these 3 compounds undermine the claim. The sentence cited by the reviewer is related to substituents directly attached to the scaffold.\nIn the ¹H NMR spectrum of compound 58d, both solvent peaks and impurity peaks are non-negligible, as they can significantly interfere with the accurate determination of the compound's purity—and thus indirectly affect the reliable calculation of its yield. If further purification proves unfeasible, this substrate should be excluded from subsequent studies.\nThe NMR spectrum of compound 58d has been re-acquired after additional purification and now meets standard purity and reporting criteria.\nAdditionally, after carefully reviewing the authors' responses to Reviewer #1, this reviewer confirms that the latter’s technical requests have been properly addressed.\nThank you.\n[Note from the Editor: Reviewer #3 was asked to assess also the response given to Reviewer #1 who was unable to look over the revision]\nReviewer #4 (Remarks to the Author):\nI co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts.\nReviewer #5 (Remarks to the Author):\nIn the new version, the authors did a more detailed mechanistic study, both in calculation and experimentally. As shown in Figure 3, the second photoexcitation process and the detailed structure and energy information of the nitrile ylide are calculated. The energies of the S1/S0 conical intersections are also added to support the proposed mechanism. The triplet-state mechanisms are excluded by EnT catalysis screening experiments using a range of known triplet photosensitizers, but if the authors have calculated the energies of triplet states of the key intermediates. Are they also consistent with the experimental results?\nAfter the author's efforts, most of my concerns have been addressed in the new revisions. The revised manuscript could be considered for publication.\nThank you.\nI am very sorry not to have responded to the first point from Reviewer 5. We had calculated all values and not sure why I didn’t include it… Here it is everything that supports the lack of reactivity we observed using triplet energy transfer catalysis.\nReviewer #5 (Remarks to the Author):\nIn the new version, the authors did a more detailed mechanistic study, both in calculation and experimentally.\nAs shown in Figure 3, the second photoexcitation process and the detailed structure and energy information\nof the nitrile ylide are calculated. The energies of the S1/S0 conical intersections are also added to support\nthe proposed mechanism. The triplet-state mechanisms are excluded by EnT catalysis screening experiments\nusing a range of known triplet photosensitizers, but if the authors have calculated the energies of triplet states\nof the key intermediates. Are they also consistent with the experimental results?\nWe have calculated the ET for several isoxazoles:\nIsoxazoles 2a (5-Ph) 3a (4-Ph) 4a (3-Ph)\nE (kcal mol-1) 66.7 74.2 74.0\nT\nThese ET values are significantly higher than the ones of the standard EnT photocatalysts commonly\nemployed in photocatalysis.\nPhotocatalysts [Ru(bpz) ]2+ [MesAcr]+ 4CzIPN [Ir{dF(CF )ppy} (dtbpy)]+ fac-Ir(ppy)\n3 3 2 3\nE (kcal mol-1) 48.4 [1] 44.7 [2] 58.3 [1] 61.8 [1] 58.1 [1]\nT\n[1] 10.1039/D3CS00190C\n[2] 10.1021/ja052967e"}, {"round": 3, "reviewer_comments": {"reviewer_5": "I am ok with the manuscript and don't have further comments."}, "author_response": ""}]

Decision letter