Introduction

The synthesis of chiral primary amines from ketones is a highly valued transformation, given the significance of the amine products and the ready availability and low cost of the ketone substrates (Fig. 1a)1,2,3. Synthetic chemists have utilized the versatile reactivity of imines, such as reduction4,5, nucleophilic addition6,7,8,9,10,11, and 1,3-hydrogen shift12,13,14,15,16,17,18,19,20,21, to access chiral primary amines after deprotection. In nature, transamination of carbonyl compounds, enabled by transaminases via two 1,3-proton shift processes, represents an attractive strategy, as it directly yields N-unprotected chiral amines22,23,24. This approach avoids the condensation, protection, and deprotection steps required in conventional chemical routes. Thus, enzymatic transamination has been extensively studied since the 1950s25,26,27,28. Today, the enzymatic synthesis of chiral amines catalyzed by transaminases, particularly ω-transaminases (ω-TAs), has been industrialized and successfully applied in the production of various pharmaceuticals29,30. Although enzymatic transamination has emerged as a powerful tool for asymmetric synthesis of chiral amines, biomimetic transamination has also attracted considerable interest as a promising approach for chiral amine synthesis12,13,14,15,16,17,18,19,20,21,31,32,33,34,35,36,37,38. By emulating the structural and mechanistic features of transaminase enzymes, biomimetic catalysts not only retain key aspects of enzymatic efficiency but also benefit from the flexibility and tunability of small-molecule systems, enabling mild and enantioselective synthesis of diverse amines39,40,41. Our group has employed chiral pyridoxamines (1), biomimetic catalysts that mimic the structure and function of vitamin B6-dependent transaminases42, to achieve the asymmetric transamination of α-keto acids43,44 and α-keto amides45. This platform creates opportunities to develop new transformations by leveraging the unique structural and electronic features of these biomimetic pyridoxamine catalysts.

Fig. 1: Asymmetric biomimetic transamination of α-keto phosphonates enabled by chiral pyridoxamine catalysts.
Fig. 1: Asymmetric biomimetic transamination of α-keto phosphonates enabled by chiral pyridoxamine catalysts.
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a Different approaches to N-unprotected chiral amines from ketone compounds and the transaminase-inspired chiral pyridoxamine catalysts. b Drug-like α-aminophosphonic acid compounds. c Representative approaches to N-protected chiral α-aminophosphonates. d This work: biomimetic transamination of α-keto phosphonates. PG protecting groups, PYR diphenylpyrimidine, TFE trifluoroethanol.

Chiral α‑aminophosphonates stand as an important class of organophosphorus compounds with broad relevance in medicinal and synthetic chemistry46,47,48,49. Their unique structural and electronic properties enable diverse biological activities, including antibacterial fosfomycin derivative Alafosfalin (hFPPs)50, the inhibitor of the human farnesyl pyrophosphate synthase51, imidazoline I2 receptors (I2-IRs) ligand in Alzheimer’s disease therapy52, and the inhibitor of matrix metalloproteinases (MMPs)53 (Fig. 1b). Given their pharmacological potential, structural versatility, and synthetic adaptability, a variety of synthetic strategies have been developed for the preparation of chiral α‑aminophosphonates54,55,56. Among these, one of the most classical and widely utilized methods is the hydrophosphonylation57,58, represented by the Kabachnik–Fields reaction59,60,61 and Pudovik reaction62,63,64,65. In addition, chiral α‑aminophosphonates also can be efficiently accessed via asymmetric hydrogenation66,67 or nucleophilic addition68,69 of iminophosphonates (Fig. 1c, above). In recent years, significant efforts have been directed toward the development of organocatalyzed enantioselective 1,3-proton shift of Schiff bases70,71, a transformation that mimics the mechanism of biological half-transamination (Fig. 1c, below)15,42,72. In 2015, Albrecht and coworkers demonstrated this approach using a cupreine-derived catalyst to facilitate the 1,3-proton shift of in situ generated α-iminophosphonates (from α-ketophosphonates and benzylamines), yielding chiral α-aminophosphonates with excellent enantioselectivity70. More recently, Deng’s group reported a related strategy employing a betaine-derived quaternary ammonium salt catalyst, which exhibited remarkable efficiency and broad substrate compatibility (Fig. 1c, box)71. Despite being the most efficient approach to chiral α-aminophosphonates, enzymatic transamination has rarely been applied to non-proteinogenic α-keto phosphonates, likely due to the inherent substrate specificity of enzymes. Biomimetic asymmetric transamination offers a promising alternative for synthesizing unnatural chiral α-aminophosphonates. However, a general and efficient method for directly accessing N-unprotected derivatives remains scarce65. Herein, we report a chiral pyridoxamine-catalyzed biomimetic transamination of α-keto phosphonates that affords structurally diverse chiral α-aminophosphonates in up to 86% yield and 98% ee (Fig. 1d). Mechanistic studies reveal that both the catalyst structure and the solvent microenvironment play decisive roles in controlling the stereoselectivity.

Results

Optimization of reaction conditions

The transamination of α-keto phosphonate 2a was initially conducted with diphenylglycine (4a) as the nitrogen donor and chiral pyridoxamine 1a as the catalyst at 20 °C (Table 1). The effects of the solvent, ester group, additive, amino donor, and catalyst structure were systematically investigated. Solvent screening revealed that alcohols generally provided higher enantioselectivities, with trifluoroethanol (TFE) proving optimal (entries 1–6, for more details see Supplementary Table S1). Evaluation of the ester group of α-keto phosphonates suggests that both methyl ester and isopropyl ester are suitable for this reaction (Supplementary Table S1, entries 7–10 and 31). Various additives were examined, such as Brønsted acids (Table 1, entries 7–12) and buffer systems (e.g., NaOAc/AcOH, Supplementary Table S1, entries 26 and 27). Among the additives tested, HCOOH markedly accelerated the transamination, affording the product in 83% yield with 97% ee (Table 1, entry 12). In contrast, stronger acids such as CF3COOH (Table 1, entry 7) or bases like NaOAc (Table 1, entry 9) led to significant decreases in both yield and enantioselectivity. These results suggest that this biomimetic transamination operates optimally under neutral to weakly acidic conditions, which is consistent with the behavior of biological transamination systems. Several alternative amino donors, including N,N-dimethylglycine (4b), methylphenylglycine (4c), phenylalanine (4d), and benzylamine (4e), all afforded <5% yield under the optimized conditions (Supplementary Table S1, entries 32–35). These donors likely fail to promote the essential pyridoxal-to-pyridoxamine conversion required for the catalytic cycle44,45, whereas diphenylglycine readily undergoes decarboxylation to efficiently convert the pyridoxal to the pyridoxamine catalyst. Catalyst structure has a vital impact on the reactivity and enantioselectivity of this transamination (Table 1, entries 13–18). Catalyst 1a-e bearing an amine side chain all perform well for the transamination in terms of enantioselectivity (Table 1, entries 13–16), implying that the OH group in pyridoxamine 1d likely doesn’t participate in the catalysis via hydrogen bonding (Table 1, entry 15). Catalyst 1e, which has a primary amine side chain, showed slightly reduced reactivity (Table 1, entry 16). In contrast, catalysts 1f and 1g, featuring amide side chains, exhibited a dramatic decrease in both yield and enantiomeric excess (Table 1, entries 17 and 18). These results highlight that the basicity of the amine side chain is crucial for efficient transamination, thereby rendering a secondary amine indispensable for catalysis. Although α-ketophosphonates can act as acylating agents due to the good leaving ability of the phosphonate group, no such side products were observed under our optimized conditions, likely because the acidic environment effectively suppresses this pathway.

Table 1 Reaction conditions optimizationa
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Substrate scope study

Under the optimized reaction conditions, substrate scope of the transamination was investigated (Fig. 2). Benzyl and phenethyl substituted α-keto phosphonates all proceeded the amino transfer in the presence of pyridoxamine catalyst 1a to produce α-amino phosphonates 3b-d in excellent enantioselectivity. Substituents on arylethyl were then examined (for 3e-n). Both electron-donating groups (for 3e, 3f, 3g, and 3n) and electron-deficient groups (for 3h-m) generally gave moderate to high reactivity with 92-97% ee. Notably, in contrast with methyl phosphonate, isopropyl phosphonate produced the corresponding chiral α-amino phosphonates with significantly enhanced yields (3f vs 3g and 3h vs 3i). In addition, naphthylethyl (3q) and thiophenethyl (3r) α-amino phosphonates were respectively obtained in 96% ee with good to high yields. Aliphatic chain substituted α-keto phosphonates underwent the transamination smoothly, delivering a wide range of aliphatic α-amino phosphonates 3t-af in 66-86% yields and 85-98% ee. Various functional groups, such as hetero-atom (as in 3u and 3ac), ester (as in 3v and 3z), double bond (as in 3aa), triple bond (as in 3ab) and alicyclic substituent (as in 3ad) were all well tolerated. The biomimetic transamination could be also applied to α-keto phosphonates containing complicated moieties. For example, citronellal- and deoxycholic acid-derived α-aminophosphonate 3ae and 3af were, respectively, obtained in good yields with high diastereomeric ratios by this transamination. Biologically active molecule (androgen receptor antagonist)73-tethered α-amino phosphonate 3ag was delivered with 68% yield and 90% ee. More importantly, aromatic α-keto phosphonate, a challenging substrate for transamination, was able to produce the desired product 3ah, albeit with moderate yield and enantioselectivty. Among these products, several chiral aminophosphonates corresponding to natural amino acids, such as 3b (Phe), 3c (Tyr), 3t (Ala), 3u (Met), and 3v (Glu), were successfully obtained, demonstrating the method’s broad applicability to the synthesis of chiral aminophosphonic acid analogs of proteinogenic amino acids. Unfortunately, Ile- and Val-derived α-keto phosphonates are not effective substrates for the asymmetric transamination, likely due to the steric effect of the β-substituents. The biomimetic reaction completes two half-transamination steps, thereby mimicking the full catalytic cycle of biological transamination22,23,24, and directly furnishes a broad range of chiral N-unprotected α-aminophosphonates without requiring any additional hydrolytic manipulation. Taken together, this reaction establishes an alternative platform to the previously reported enantioselective 1,3-proton-shift reactions of Schiff bases70,71, further expanding the accessible methodological scope for constructing chiral α-aminophosphonates.

Fig. 2: Substrate scope study.
Fig. 2: Substrate scope study.
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aAll the reactions were carried out with α-keto phosphonate 2 (0.1 mmol), diphenylglycine (4a) (45.4 mg, 0.2 mmol), catalyst (R)−1a (2.6 mg, 0.005 mmol) and HCO2H (16.1 mg, 0.35 mmol) in TFE/H2O (0.51 mL/0.09 mL) at 20 °C for 72 h unless otherwise stated. b10 mol% catalyst loading (5.2 mg, 0.01 mmol) was used. cCatalyst (S)−1a instead of catalyst (R)−1a. dProduct 3 v was isolated as its lactam. eCatalyst (R)−1c (0.01 mmol) instead of catalyst (R)-1a. Yield: isolated yield based on α-keto phosphonate 2. The absolute configuration of product 3a has been determined as R by the X-ray analysis and those of 3b-3ah were tentatively assigned by analogy. Ph phenyl, Me Methyl, iPr isopropyl.

Synthetic applications of α-aminophosphonates

The synthetic utility of the transamination was tentatively demonstrated (Fig. 3). The scaled-up reaction was investigated using the (S)-1a catalyzed transamination of 3.0 mmol of α-ketophosphonate 2a, affording (S)-α-aminophosphonate 3a in 80% yield and 97% ee, which are also identical to those obtained in the reaction in a small scale (83% yield and 97% ee), demonstrating good scalability for this process. Hydrolysis of (S)-3a gave chiral improving- aminophosphoric acid 5 in a 96% yield (Fig. 3a). The transamination can also be applied to efficient synthesis of bioactive molecules, such as MMPs inhibitor 8 (Fig. 3b). Transamination of 6-fluorohexanoyl phosphonate (2ai) under biomimetic standard conditions afforded chiral α-aminophosphonate 3ai in 71% yield with 96% ee. Subsequent protection of the NH2 group using sulfonyl chloride 6 yielded compound 7 in 75% yield while maintaining 95% ee. Hydrolysis of this intermediate delivered the MMPs inhibitor 8. In contrast, the previous synthetic route started from 6-tetrahydropyranyl-protected 1,6-hexanediol (9) and required seven steps, including a chiral resolution, to furnish the corresponding N-protected α-amino phosphonate (8) in a mere 4% overall yield53 (Fig. 3b). In addition, this transamination also can be employed to the rapid synthesis of the enantiomeric analog (12) of proteasome inhibitor Ixazomib—a drug approved for the treatment of multiple myeloma (MM)74. α-Aminophosphonate 3aj, which was obtained from 2aj with 60% yield and 92% ee via catalyst (S)-1a, was condensed with (2,5-dichlorobenzoyl)glycine (10) followed by hydrolysis with TMSBr to produce chiral dipeptide phosphoric acid 12 with 92% ee (Fig. 3c).

Fig. 3: Derivatization of α-aminophosphonates.
Fig. 3: Derivatization of α-aminophosphonates.
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a Scaled-up transamination and hydrolysis of chiral α‑aminophosphonate 3a. b Biomimetic synthesis of biologically active molecule 8. c Biomimetic synthesis of drug-like compound 12. THP tetrahydropyranyl, HATU 2-(7-Azabenzotriazol-1-yl)-N,N,N’,N’-tetramethyluronium hexafluorophosphonate, DIPEA diisopropylethylamine, TMSBr trimethylsilyl bromide.

Mechanistic study

The reaction follows a typical pyridoxamine catalyzed transamination mechanism42,72,75, which mimics the two half reactions of enzymatic transamination43. The first half-reaction (from α-keto phosphonate 2 to α-amino phosphonate 3) involves 1,3-proton shift of Schiff base 13 formed between α-keto phosphonate 2 and chiral pyridoxamine (R)-1a, which is promoted by the intramolecular amine side chain of catalyst43,44. The second half-reaction transfers the amine group of diphenyl glycine (4a) to internal imine 16, regenerating chiral pyridoxamine (R)-1a via the isomerization between Schiff bases 17 and 19 (Fig. 4). Although the internal ammonium species, such as intermediate 14 or 18, could in principle act as a proton donor for the 1,3-proton shift, in a reaction system containing multiple proton sources (HCOOH, CF3CH2OH and H2O), proton transfer mediated by external proton donors or via a proton-shuttle pathway also represents a plausible mechanism.

Fig. 4: Proposed mechanism for transamination of α-keto phosphonates.
Fig. 4: Proposed mechanism for transamination of α-keto phosphonates.
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For clarity, the iodide and chloride anions of the catalyst 1a are omitted.

Density functional theory (DFT) calculations were performed to elucidate the detailed mechanism of transamination of α-keto phosphonate (2a) to chiral α-aminophosphonate (3a) and to investigate the origin of chiral induction. The potential energy surface (PES) for the step of the condensation between the 2a and (R)-1a is illustrated in Fig. 5. The reaction initiates with the nucleophilic attack of the amine group of pyridoxamine (R)-1a onto α-keto phosphonate 2a. This initial step proceeds via transition state TS1 (ΔG‡ = 15.2 kcal/mol) to generate intermediate M1. Assisted by a formic acid-mediated hydrogen-bonding network, a proton is transferred from N1 to O1 through TS2 to form intermediate M2. Subsequently, M2 undergoes a ligand exchange wherein a formic acid molecule dissociates as a formate ion enters, coordinating with the phenolic hydroxyl group to generate M3. This is followed by an intermolecular proton transfer from the phenolic hydroxyl group to the formate species via TS3. This stage generates intermediate M4, which undergoes C1–O1 bond cleavage and the elimination of a water molecule via TS4 to afford intermediate M5. A subsequent proton transfer from N2 to the formate ion proceeds via TS5, generating intermediate M6 (Fig. 5b). The entry of a TFE molecule into the system facilitates hydrogen bonding interaction with the side chain N2–H group, leading to the formation of intermediate M7. Subsequently, the benzylic C2–H undergoes deprotonation by the side-chain amine through TS6 (ΔG‡ = 14.4 kcal/mol) to produce intermediate M8. A stereoselective protonation step occurs via TS7R (ΔG‡ = 19.7 kcal/mol), furnishing the chiral intermediate M9R and ultimately the (R)- product. In contrast, the competing transition state TS7S is energetically unfavorable, characterized by a significantly higher barrier (ΔG‡ = 25.1 kcal/mol). Geometric analysis reveals that the N–H···O and O–H···O hydrogen bonds are markedly shorter in TS7R than in TS7S. These shorter distances indicate that the hydrogen-bonding network provides superior stabilization in TS7R, thereby lowering its activation barrier relative to TS7S. These findings clearly indicate a strong kinetic preference for the R-configurational pathway. Further transformation, including intramolecular cyclization and the subsequent release of α-aminophosphonate 3a, are discussed in the Supplementary Information (Fig. S2). Overall, the computational results identify the protonation of M8 via TS7R is the rate-determining and stereo-determining step for the initial half-transamination.

Fig. 5: Energy profiles and geometries of key transition states for the transamination of α-keto phosphonate.
Fig. 5: Energy profiles and geometries of key transition states for the transamination of α-keto phosphonate.
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a Energy profile of the condensation step. b Energy profile of the proton transfer step and geometries of key transition states. Relative Gibbs free energies are given in kcal/mol. Hydrogen bonds are shown as green dashed lines in the geometries of key transition states. Distances are reported in angstroms (Å). For clarity, the iodide and chloride counterions of catalyst 1a were excluded from the DFT calculations and are not shown. TFE trifluoroethanol.

To further probe the critical role of TFE in the asymmetric 1,3-proton shift process, a series of control experiments were conducted with different alcohol solvents (Fig. 6). Gradual removal of fluorine substituents from the alcohols (from TFE to difluoroethanol and monofluoroethanol) led to a progressive decrease in enantioselectivity (entries 1–3), while ethylene glycol and ethanol afforded substantially lower ee values (entries 4 and 5). These results indicate that the electron-withdrawing effect at the β position of alcohol solvent is crucial for stereochemical induction during the 1,3-proton-shift step. To further rationalize this solvent effect, transition-state calculations were also performed using ethanol as an alternative hydrogen-bonding solvent for TS7S and TS7R. The calculated difference in Gibbs free energy of activation in ethanol is smaller than that in TFE, capturing the experimentally observed reduction in enantioselectivity (see Supplementary Fig. S3). Moreover, in the TS7R transition states in TFE and EtOH (Fig. 6 box), more acidic solvent forms stronger hydrogen-bonding interaction with formate, stabilizing this species and enhancing its proton-donating ability, which in turn improves both enantioselectivity and catalytic activity. In contrast, solvents with weaker acidity display diminished stereocontrol and catalytic efficiency (entries 2–5). Overall, TFE plays a decisive role in enabling both high activity and enantioselectivity, which is critical for the success of this biomimetic transamination.

Fig. 6: Control experiments on alcohol solvents and TS7R transition states in TFE and EtOH.
Fig. 6: Control experiments on alcohol solvents and TS7R transition states in TFE and EtOH.
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aAll the reactions were carried out with α-keto phosphonate 2a (31.8 mg, 0.1 mmol), diphenylglycine (4a) (45.4 mg, 0.2 mmol), catalyst (R)−1a (2.6 mg, 0.005 mmol) and HCO2H (16.1 mg, 0.35 mmol) in solvent/H2O (0.51 mL/0.09 mL) at 20 °C for 72 h. bIsolated yield.

Discussion

In conclusion, we have developed a highly efficient enantioselective biomimetic transamination of α-keto phosphonates catalyzed by a chiral pyridoxamine derivative with synergistic assistance from the solvent. This method provides direct access to a broad range of biologically active and synthetically valuable chiral α-aminophosphonates in good to excellent yields and with up to 98% ee under mild conditions, highlighting the potential of vitamin B6-inspired catalysts in asymmetric synthesis. The strategy features direct transfer of an unprotected amino group, high atom economy, broad substrate scope, and compatibility with complex molecular architectures. Mechanistic studies, supported by DFT calculations, reveal the origin of stereocontrol and identify the rate-determining step of biomimetic transamination. In addition, the solvent environment, exemplified by the decisive role of TFE, was found to cooperate with the catalyst in stabilizing key transition states, thereby contributing to the high enantioselectivity and reflecting the cooperative catalysis principles inherent to enzymatic systems.

Methods

General procedure for the asymmetric transamination

To a 5 mL vial equipped with a magnetic stirrer bar was added 2 (0.10 mmol), (R)-1a (0.0026 g, 0.005 mmol), diphenylglycine (4a) (0.0454 g, 0.2 mmol), trifluoroethanol (TFE, 0.51 mL), H2O (0.09 mL) and HCO2H (0.0161 g, 0.35 mmol) under N2 atmosphere. Upon stirring at 20 ˚C for 72 h, the reaction mixture was evaporated to remove most of the solvent and submitted to flash column chromatography on silica gel (DCM:MeOH:saturated NH3 solution in ethanol = 50:1:1) to give product 3. For some specific substrates, the reaction conditions were slightly changed, the detailed information of which can be found in the Supplementary Information.