3.9. Isolation and Characterisation of Chiral Epoxides

Flash column chromatography (FCC) was carried out with a Büchi system (Pump Manager C-615 and Fraction Collector C-660) using Normasil 60 silica gel (0.040–0.063 mm; VWR). Thin layer chromatography (TLC) analysis was carried out using TLC silica gel 60 F₂₅₄ (aluminium sheets, Merck), and plates were visualised with UV light or by treatment with permanganate solution followed by heating. Optical rotations were measured with a JASCO P-2000 digital polarimeter with a Na-D lamp (10 cm cell length). Concentrations (c) are given in grams per 100 mL. NMR spectra were recorded with a Varian INOVA-300 spectrometer (¹H, 299.95 MHz) in CDCl₃ using tetramethylsilane as the internal standard. Data are presented as follows: chemical shift (in ppm), multiplicity (bs = broad singlet, t = triplet, dd = doublet of doublets, m = multiplet), integration and coupling constants (J/Hz). All solvents used were dried and distilled according to conventional methods.

The crude biotransformation mixture (100 mL) was vigorously stirred with ethyl acetate (150 mL) for 20 min. Then, the slurry was filtered through a pad of Celite, the organic phase was separated, and an aqueous layer was extracted with ethyl acetate (2 × 30 mL). The combined organic layers were dried over anhydrous MgSO₄, and the solvent was evaporated under reduced pressure (max 120 mbar in a 25 °C water bath or max 50 mbar in a 35 °C water bath for long-chain aliphatic epoxides). The crude product was purified by FCC using hexanes as the eluent to give (S)-4-chlorostyrene (60 mg, 34% of theoretical yield) oxide as a colourless liquid with spectroscopic data in good agreement with those reported in the literature [36,37].

(S)-4-Chorostyrene Oxide

¹H-NMR (300 MHz, CDCl₃): δ = 2.75 (dd, 1H, J = 5.5, 2.5 Hz, OCH₂), 3.14 (dd, 1H, J = 5.5, 4.1 Hz, OCH₂), 3.83 (dd, 1H, J = 4.1, 2.5 Hz, OCH), 7.18–7.23 (m, 2H, H-Ar), 7.29–7.34 (m, 2H, H-Ar).

αD20= +27.3 (c 1.35, CHCl₃).

R_f = 0.15 (n-hexane/ethyl acetate, 50:1).

Reported rotation for (S)-4-chlorostyrene oxide αD20 = +26 (c 1.29, CHCl₃) for 99% ee [36], αD20 = +26 (c 1.2, CHCl₃) for 99% ee [37].

(S)-Allylbenzene Oxide

Yield: 26 mg (13% of theoretical yield), colourless liquid with spectroscopic data in good agreement with those reported in the literature [38].

¹H-NMR (300 MHz, CDCl₃): δ = 2.55 (dd, 1H, J = 5.0, 2.7 Hz, OCH₂), 2.78–2.85 (m, 2H, PhCH₂, OCH₂), 2.93 (dd, 1H, J = 14.5, 5.6 Hz, PhCH₂), 3.13–3.19 (m, 1H, OCH), 7.21–7.34 (m, 5H, H-Ar).

αD20= –17.1 (c 1.3, EtOH).

R_f= 0.30 (n-hexane/ethyl acetate, 9:1).

Reported rotation for (S)-allylbenzene oxide αD25 = −18.37 (c 2.01, EtOH) for 99% ee [39], αD20 = −17.3 (c 1.0, EtOH) for 98% ee [40].

(2R,5R)-1,2:5,6-Diepoxyhexane

Yield: 157 mg (purified by FCC, hexanes/ethyl acetate, 9:1, 34% of theoretical yield), colourless liquid with spectroscopic data in good agreement with those reported in the literature [32].

¹H-NMR (300 MHz, CDCl₃): δ = 1.57–1.85 (m, 4H, CH₂CH₂), 2.51 (dd, 2H, J = 4.9, 2.7 Hz, OCH₂), 2.78 (dd, 2H, J = 4.9, 4.0 Hz, OCH₂), 2.91–3.01 (m, 2H, OCH).

αD26= +25.7 (c 1.2, CHCl₃).

R_f= 0.18 (n-hexane/ethyl acetate, 7:3).

Reported rotation for (2R,5R)-1,2:5,6-diepoxyhexane αD26 = +26.8 (c 5.03, CHCl₃) [41], αD20 = +20.4 (c 1.3, CHCl₃) [42].

(S)-4-(Oxiran-2-yl)butan-1-ol

Yield: 76 mg (purified by FCC, hexanes/ethyl acetate, 4:1, 33% of theoretical yield), colourless liquid with spectroscopic data in good agreement with those reported in the literature [32].

¹H-NMR (300 MHz, CDCl₃): δ = 1.51–1.68 (m, 6H, CH₂CH₂CH₂), 2.49 (dd, 1H, J = 5.0, 2.7 Hz, OCH₂), 2.76 (dd, 1H, J = 5.0, 4.0 Hz, OCH₂), 2.90–2.96 (m, 1H, OCH), 3.67 (t, 2H, J = 6.2 Hz, CH₂OH).

αD25= −11.2 (c 1.0, CHCl₃).

R_f= 0.15 (n-hexane/ethyl acetate, 1:1).

Reported rotation for (S)-4-(oxiran-2-yl)butan-1-ol αD24.7 = −12.8 (c 1.0, CHCl₃) for >99.9% ee [32].

2-(3-Bromopropyl)oxirane

Yield: 173 mg (26% of theoretical yield) of the crude product (could not be purified by FCC because of its instability on silica gel), orange liquid with spectroscopic data in good agreement with those reported in the literature [43,44].

¹H-NMR (300 MHz, CDCl₃): δ = 1.53–1.66 (m, 1H, CH₂CH₂), 1.76–1.88 (m, 1H, CH₂CH₂), 1.98–2.08 (m, 2H, CH₂CH₂), 2.51 (dd, 1H, J = 4.9, 2.7 Hz, OCH₂), 2.77 (dd, 1H, J = 4.9, 4.0 Hz, OCH₂), 2.92–2.98 (m, 1H, OCH), 3.41–3.53 (m, 2H, CH₂Br).

R_f = 0.19 (n-hexane/ethyl acetate, 7:3).