Baeyer-Villiger Oxidation of Bridged endo-Tricyclic Ketones with Engineered Escherichia coli Expressing Monooxygenases of Bacterial Origin

Abstract

Whole-cell biotransformations using engineered strains of Escherichia coli expressing cyclopentanone (CPMO) and cyclohexanone monooxygenases (CHMO) of various bacterial origins have been tested for substrate acceptance on tricyclic ketones. Based on the stereopreference of the biocatalytic Baeyer-Villiger oxidation, our recent clustering of this library of enzymes into two distinct groups based on protein sequence was confirmed. Together with short and facile reaction sequences for the production of the substrate ketones, microbial biooxidation enables access to antipodal product lactones as versatile building blocks in natural product and bioactive compound synthesis.

Key words

biocatalysis - biooxidation - recombinant whole-cell biotransformation - Baeyer-Villiger oxidation - monooxygenase - stereoselectivity

References

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21

endo -Tricyclo[6.2.1.0 ^²,7 ]undecan-11-one ( 1a): yellow oil. ¹H NMR (200 MHz, CDCl₃): δ = 0.81-1.89 (m, 14 H), 2.06-2.14 (m, 2 H). ¹³C NMR (50 MHz, CDCl₃): δ = 17.1 (t), 18.1 (t), 19.5 (t), 32.9 (d), 43.4 (d), 216.7 (s).
endo -Tricyclo[5.2.1.0 ^²,6 ]decan-10-one ( 1b): yellow oil. ¹H NMR (200 MHz, CDCl₃): δ = 1.63-1.75 (m, 12 H), 2.46 (m, 2 H). ¹³C NMR (50 MHz, CDCl₃): δ = 27.7 (t), 29.2 (t), 29.9 (t), 38.2 (d), 43.6 (d), 214.9 (s).
endo -Tricyclo[5.2.1.0 ^²,6 ]dec-8-en-4-one ( 1c): beige amorphous solid; mp = 97-99 °C. ¹H NMR (200 MHz, CDCl₃): δ = 1.30-1.55 (m, 2 H), 1.70-1.90 (m, 2 H), 2.05-2.35 (m, 2 H), 2.70-2.95 (m, 4 H), 6.00-6.15 (m, 2 H). ¹³C NMR (50 MHz, CDCl₃): δ = 39.7 (d), 41.0 (t), 47.0 (d), 49.7 (t), 136.1 (d), 219.9 (s).
endo -Tricyclo[5.2.1.0 ^²,6 ]decan-4-one ( 1d): colorless amorphous solid; mp = 97-100 °C. ¹H NMR (200 MHz, CDCl₃): δ = 1.16-1.59 (m, 6 H), 2.03-2.40 (m, 6 H), 2.51-2.74 (m, 2 H). ¹³C NMR (50 MHz, CDCl₃): δ = 22.1 (t), 38.6 (d), 39.3 (t), 40.6 (t), 41.3 (d), 221.0 (s).

23

Typical Procedure for Screening Experiment in Multi-Well Dishes (12 or 24 Wells).
Each well was charged with LB-amp medium (2 mL/12-well format or 1 mL/24-well format) and inoculated with 1% of an overnight preculture of recombinant E. coli strains. A plate was incubated at 120 rpm at 37 °C on an orbital shaker for 2 h. IPTG was added (final concentration of 0.025 mM) together with substrate (1 mg or 0.5 mg) and β-cyclodextrin (1 equiv). The plate was shaken at r.t. for 24 h and then analyzed by chiral phase GC after extraction of the sample with EtOAc.

25

Physical and Spectroscopic Data of Lactones 2a-d.
endo -9-Oxatricyclo[6.2.2.0 ^²,7 ]dodecan-10-one ( 2a): beige crystals, mp 76-78 °C. ¹H NMR (200 MHz, CDCl₃): δ = 1.09-2.06 (m, 14 H), 2.39-2.41 (q, J = 2.7 Hz, 1 H), 4.36-4.41 (q, J = 3.3 Hz, 2 H). ¹³C NMR (50 MHz, CDCl₃): δ = 16.9 (t), 19.3 (t), 19.5 (t), 20.0 (t), 20.5 (t), 20.8 (t), 32.3 (d), 36.2 (d), 39.8 (d), 78.9 (d), 177.3 (s). Specific optical rotation (CHMO _Brachy ): [α]_D ²⁰ -31.8 (c 0.95, CHCl₃); 99% ee.
endo -8-Oxatricyclo[5.2.2.0 ^²,6 ]undecan-9-one ( 2b): colorless crystals, mp 78-80 °C. ¹H NMR (200 MHz, CDCl₃): δ = 1.66-1.90 (m, 12 H), 2.55 (q, J = 3.1 Hz, 1 H), 4.55 (q, J = 3.7 Hz, 1 H). ¹³C NMR (50 MHz, CDCl₃): δ = 16.7 (t), 20.6 (t), 27.7 (t), 27.9 (t), 28.1 (t), 37.7 (d), 39.2 (d), 41.2 (d), 78.9 (d), 177.3 (s). Specific optical rotation (CHMO _Rhodo2 ): [α]_D ²⁰ -17.3 (c 1.70, CHCl₃); 99% ee.
endo -5-Oxatricyclo[6.2.1.0 ^²,7 ]undec-9-en-4-one ( 2c): colorless oil. ¹H NMR (200 MHz, CDCl₃): δ = 1.45 (d, J = 8.0 Hz, 1 H), 1.64 (d, J = 8.0 Hz, 1 H), 1.93-2.98 (m, 6 H), 3.70 (m, 1 H), 4.30 (m, 1 H), 6.05-6.28 (m, 2 H). ¹³C NMR (50 MHz, CDCl₃): δ = 33.4 (t), 35.9 (d), 38.6 (d), 44.2 (d), 46.0 (d), 50.6 (t), 69.9 (t), 134.9 (d), 136.3 (d), 173.7 (s). Specific optical rotation (CHMO _Brevi2 ): [α]_D ²⁰ -12.7 (c 1.72, CHCl₃); 74% ee.
endo -5-Oxatricyclo[6.2.1.0 ^²,7 ]undecan-4-one ( 2d): colorless solid, mp 78-80 °C. ¹H NMR (200 MHz, CDCl₃): δ = 1.30-1.59 (m, 6 H), 2.12-2.55 (m, 6 H), 4.02-4.32 (m, 2 H). ¹³C NMR (50 MHz, CDCl₃): δ = 21.9 (t), 23.3 (t), 30.4 (t), 35.9 (d), 37.0 (d), 39.0 (d), 40.7 (d), 41.2 (t), 68.2 (t), 174.3 (s). Specific optical rotation (CHMO _Rhodo1 ): [α]_D ²⁰ +32.2 (c 3.53, CHCl₃); 95% ee.