Transaminase-mediated synthesis of enantiopure drug-like 1-(3',4'-disubstituted phenyl)propan-2-amines

Abstract

Transaminases (TAs) offer an environmentally and economically attractive method for the direct synthesis of pharmaceutically relevant disubstituted 1-phenylpropan-2-amine derivatives starting from prochiral ketones. In this work, we report the application of immobilised whole-cell biocatalysts with (R)-transaminase activity for the synthesis of novel disubstituted 1-phenylpropan-2-amines. After optimisation of the asymmetric synthesis, the (R)-enantiomers could be produced with 88-89% conversion and >99% ee, while the (S)-enantiomers could be selectively obtained as the unreacted fraction of the corresponding racemic amines in kinetic resolution with >48% conversion and >95% ee.

Fig. 1. APIs containing the 1-phenylpropan-2-amine building block.

Scheme 1. Chemical methods for the synthesis of (R)-1-phenylpropan-2-amine.

Scheme 2. Biocatalytic syntheses of (R)-1-phenylpropan-2-amine.

Scheme 3. Transaminase-catalysed biotransformations leading to the (R)- and (S)-enantiomers of 1-arylpropan-2-amines 8a–d.

Scheme 4. Synthesis of racemic disubstituted 1-phenylpropan-2-amines (8b–d). Reaction conditions: (i) A: 10 (20 mmol), K₂CO₃ (1 equiv.), Me₂SO₄ (2 equiv.) in 75 mL acetone, reflux; B: 10 (9.9 mmol), K₂CO₃ (1.5 equiv.), EtI (1.25 equiv.) in 15 mL DMF; C: 10 (6.6 mmol), K₂CO₃ (1.5 equiv.), iPrBr (1.5 equiv.) in 6.6 mL DMF, 80 °C; (ii) 9b–d (6 mmol), (±)-2-chloropropionic acid methyl ester (1.2 equiv.), NaOMe/MeOH (1.15 mmol, 25 wt%) in toluene (10 mL); 1N NaOH in toluene at 50 °C; 5N HCl in toluene, reflux; (iii) 7b–d (1 mmol), ammonium formate (10 equiv.), 10% Pd/C (0.04 equiv.) in 5 mL methanol.

Fig. 2. Various amine donors facilitating the displacement of the equilibrium of transamination.

Fig. 3. Screening the amine donors in ArR-TA-mediated transamination of 1-phenylpropan-2-one (7a). Reaction conditions: immobilised whole-cell TA biocatalyst (20 mg), 7a (10 mM), amine donor (10–100 mM), PLP (1 mM), sodium phosphate buffer (100 mM, pH 7.5), DMSO (5 v/v%), 30 °C, 24 h.

Fig. 4. The effect of initial substrate concentration on the conversion of the transamination of 1-phenylpropan-2-one (7a) mediated by ArR-TA or AtR-TA (panel (a)); or ArR_m-TA (panel (b)). Reaction conditions: immobilised whole-cell TA biocatalyst (20 mg), 7a (10–100 mM), sec-butylamine 12 (0.1–1 M), PLP (1 mM), sodium phosphate buffer (100 mM, pH 7.5), DMSO (5 v/v%), 30 °C, 24 h.

Fig. 5. Cosolvent effect on conversion of the ArR-TA-mediated transamination of 1-phenylpropan-2-one (7a). Reaction conditions: immobilised whole-cell TA biocatalyst (20 mg), 7a (10 mM), sec-butylamine 12 (100 mM), PLP (1 mM), sodium phosphate buffer (100 mM, pH 7.5), DMSO (0–25 v/v%), 30 °C, 24 h.

Fig. 6. AtR-TA-mediated kinetic resolution of the racemic amines 8a–d. Reaction conditions: immobilised whole-cell TA biocatalyst (20 mg), 8a–d (10 mM), sodium pyruvate (5 mM), PLP (1 mM), sodium phosphate buffer (100 mM, pH 7.5), DMSO (5 v/v%), 30 °C, 24 h.