Beilstein J. Org. Chem. 2016, 12, 2523–2534.
Preparative separation procedure
(±)-Naringenin ((±)-1): (2R)-1 (first peak) tR = 5.19 min on a
The sample (500 µL/injection) was loaded to a Kinetex 5 µm Chiralpak IA column (hexane/2-propanol 80:20), λ [nm] (Φ)}:
phenyl-hexyl AXIA packed 21.2 × 150 mm column in the 327 (–6.75), 314sh (–4,02), 289 (28.91), 235sh (–7.96), 215
concentration of 20 mg/mL. The separation was achieved (–28.48); (2S)-1 (second peak): tR = 5.84 min on a Chiralpak IA
with isocratic elution (75% HPLC grade water containing column (hexane/2-propanol 80, λ [nm] (Φ)}: 327 (5.49), 314sh
0.1% formic acid, 25% acetonitrile containing 0.1% (3.49), 289 (–22.65), 235sh (4.69), 215 (21.67).
formic acid) at 25 °C. The flow rate was 21 mL/min. The frac-
Preparation of dracocephins A ((±)-2a–d) and
B ((±)-3a–d)
tions were collected based on UV absorption at 220 nm wave-
length.
To a suspension of racemic naringenin [(±)-1] (200 mg,
Chiral HPLC–ECD analysis
0.735 mmol) in nitromethane (5 mL) was added 5-ethoxypyrro-
Chiral HPLC separations were carried out with a Jasco HPLC lidine-2-one ((±)-9, 114 mg, 0.883 mmol), a catalytic amount of
system on Chiralpak IC column (250 × 4.6 mm i.d.; 5 μm) p-toluenesulfonic acid and the mixture was refluxed for 4 h.
using eluent hexane/acetonitrile 97:3 or 90:10 with 0.1% TFA The solvent was evaporated in vacuo and the solid residue was
additive to set the pH to about 2 at a flow rate of 1.0 mL/min for purified by column chromatography (CH2Cl2/CH3OH = 10:1).
2 and 3. HPLC–UV and OR chromatograms were measured An isomeric mixture of (±)-2a–d and (±)-3a–d (168 mg, 64%)
with Jasco MD-910 multiwavelength and OR-2090Plus chiral was obtained as a white crystalline solid, Rf (CH2Cl2:CH3OH =
detectors, respectively. The HPLC–ECD traces were recorded 10:1) 0.32. A sample of the mixture (47 mg) was subjected to
at the specified wavelength with a Jasco J-810 CD spectropo- further purification by preparative HPLC, which gave title com-
larimeter equipped with a 1 cm HPLC flow cell and the base- pounds (±)-2a–d (16 mg, 99.66% purity) as a white solid, mp
line was zeroed after the start of each run. The on-line ECD and 168–170 °C; IR (KBr) νmax: 3393, 3034, 2970, 1634, 1519,
UV spectra were recorded simultaneously by stopping the flow 1455, 1340, 1310, 1279, 1170, 1090 cm−1; 1H NMR (799.7
at the UV absorption maximum of each peak. ECD ellipticity MHz, CD3OD) δH 2.21–2.27 (m, 1H, Hx-4”), 2.36–2.42 (m,
values (Φ) were not corrected for concentration. For an 1H, Hx-3”), 2.43–2.49 (m, 1H, Hy-4”), 2.59–2.64 (m, 1H,
HPLC–ECD spectrum, three consecutive scans were recorded Hy-3”), 2.71–2.75 (m, 1H, Hx-3), 3.12 and 3.13 [sum 1H, (dd, J
and averaged with 2 nm bandwidth, 1 s response, and standard = 17.0, 12.8 Hz, Hy-3A) and (dd, J = 17.0, 12.6 Hz, Hy-3B), re-
sensitivity. The HPLC–ECD spectrum of the eluent was re- spectively], 5.33–5.36 (buried m, 2H, H-5” and H-2), 5.95 (s,
corded in the same way. The concentration of the injected sam- 1H, H-8), 6.80–6.82 (m, 2H, H-3’ and H-5’), 7.29–7.32 (m, 2H,
ple was set so that the HT (voltage) value did not exceed 500 V H-2’ and H-6’); 13C NMR (201.1 MHz, CD3OD) δC 26.7
in the HT channel.
(C-4”), 32.07 and 32.10 (C-3”), 43.96 and 43.99 (C-3), 49.4
(C-5”), 80.52 and 80.53 (C-2), 96.04 and 96.06 (C-8), 103.1
Dracocephins A1–A4 (2ª–d): 2b (first eluted stereoisomer): (C-10), 109.44 and 109.46 (C-6), 116.4 (C-3’ and C-5’), 129.09
tR = 7.26 min, λ [nm] (Φ)}: 330 (4.43), 288 (–15.34), 249sh and 129.10 (C-2’ and C-6’), 131.0 (C-1’), 159.12 and 159.13
(2.48), 230 (11.60), 213 (27.01). 2a (second eluted stereoiso- (C-4’), 163.47 and 163.50 (C-5), 163.90 and 163.91 (C-9),
mer): tR = 7.49 min, λ [nm] (Φ)}: 329 (–3.95), 289 (13.60), 166.8 (C-7), 181.60 and 181.63 (C-2”), 198.1 (C-4); HRMS [M
250sh (–2.18), 227 (–7.35), 202 (15.98). 2c (third eluted stereo- + H] calcd for C19H18O6N: 356.11286; found: 356.11284;
isomer): tR = 11.49 min, λ [nm] (Φ)}: 329 (1.92), ESI–MS–MS (CID = 35%) (rel. int. %): 339(100); 338(4);
289 (–7.86), 250sh (1.09), 227 (4.56), 202 (–10.49). 2d 311(2); 236(3); and title compounds (±)-3a–d (21 mg, 99.92%
(fourth eluted stereoisomer): tR = 12.38 min, λ [nm] (Φ)}: 330 purity), also a white solid, mp 238–240 °C; IR (KBr) νmax:
(–5.71), 288 (17.65), 249sh (–3.54), 230sh (–17.68), 213 3416, 3034, 2970, 1630, 1519, 1447, 1378, 1343, 1257, 1176,
(–33.89).
1081 cm−1; 1H NMR (799.7 MHz, CDCl3:CD3OD = 2:1) δH
2.15–2.29 (buried m, 2H, Hx-3” and Hx-4”), 2.30–2.37 (m, 1H,
Dracocephins B1–B4 (3a–d): 3b (first peak): tR = 4.87 min, λ Hy-3”), 2.38–2.46 (m, 1H, Hy-4”), 2.73–2.79 (m, 1H, Hx-3),
[nm] (Φ)}: 327 (–2.15), 288 (8.40), 252sh (–0.88), 234sh 3.09 and 3.14 [sum 1H, (dd, J = 17.0, 13.1 Hz, Hy-3A) and (dd,
(–2.24), 221 (–5.07), 201 (19.79). 3a (second peak): tR = 4.87 J = 17.1, 13.2 Hz, Hy-3B), respectively], 5.28–5.37 (buried m,
min, λ [nm] (Φ)}: 332 (0.65), 290 (–2.74), 254 (0.13), 240sh 2H, H-5” and H-2), 6.00 (s, 1H, H-6), 6.87–6.90 (m, 2H, H-3’
(0.90), 203 (18.81). 3c (third peak): tR = 5.53 min, λ [nm] (Φ)}: and H-5’), 7.28–7.31 (m, 2H, H-2’ and H-6’); 13C NMR (201.1
328 (–1.02), 288 (6.25), 254 (–0.42), 235sh (–2.14), 203 MHz, CD3OD) δC 25.8 and 25.9 (C-4”), 31.1 (C-3”), 42.8 and
(–11.47). 3d (fourth peak): tR = 7.56 min, λ [nm] (Φ)}: 328 43.5 (C-3), 48.4 and 48.6 (C-5”), 79.7 and 80.0 (C-2), 96.6
(1.02), 288 (–4.68), 252sh (0.17), 234sh (0.42), 221 (1.46), 201 (C-6), 102.6 (C-10), 107.9 (C-8), 115.9 (C-3’ and C-5’), 128.2
(–9.10).
(C-2’ and C-6’), 129.3 (C-1’), 158.0 (C-4’), 161.9 (C-9), 163.4*
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