An ingenious method for the determination of the relative and absolute configurations of compounds containing aryl-glycerol fragments by ¹H NMR spectroscopy

Abstract

A concise method was established to determine the relative and absolute configurations of aryl-glycerols that depend on the chemical shift differences (Δδ) of the diastereotopic methylene protons (H-3) by ¹H NMR spectroscopy. When using DMSO-d₆ as the preferred solvent, the threo configuration corresponded to a larger Δδ_H3a–H3b value (>0.15 ppm), whereas the erythro configuration (<0.07 ppm) corresponded to a smaller value. Furthermore, the absolute configurations were determined with the aid of a simple acylation reaction through camphanoyl chloride. In the threo enantiomers, the Δδ value of the 1R,2R configuration was <0.15 ppm, and that of the 1S,2S configuration was >0.20 ppm. In the erythro enantiomers, the Δδ value of 1R,2S was >0.09 ppm, and that of 1S,2R was <0.05 ppm. Remarkably, this empirical rule is invalid in CDCl₃. In addition, this method was also verified by a quantum ¹H NMR calculation.

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Introduction

Chiral alcohols, such as chiral secondary alcohols, prim,sec-diols, sec,sec-diols and 1,2,3-prim,sec,sec-triols, are ubiquitous in natural products and synthetic products. The assignment of their relative and absolute configurations is always a challenging task. Because of the difficulty of growing crystals of these compounds and the lack of distinct Cotton effects in their electronic circular dichroism (ECD) spectra, NMR spectroscopy is suitable for this purpose and is readily available compared with other methods. For instance, the absolute configurations of chiral secondary alcohols were determined using Mosher's method by ¹H NMR spectroscopy;¹ the relative configurations of some sec,sec-diols were assigned using a JBCA (J-based configuration analysis) method² or by comparison of their ¹³C-Δδ behaviors in a chiral bidentate NMR solvent.³ We have also developed two approaches to (1) discriminate the threo and erythro configurations of vicinal diol in polyacetylene glycosides by the ³J_HH value using acetic acid-d₄/D₂O as the solvent;⁴ and (2) determine the absolute configurations of 2-hydroxy phenylethanoid glycosides (prim,sec-diol derivatives) by the chemical shift differences (Δδ) of the diastereotopic methylene protons.⁵

Aryl-glycerols (AGs), a kind of 1,2,3-prim,sec,sec-triols, are frequent in natural products such as phenylpropanoids and lignans^6–15 and are important intermediates in many chemical syntheses.^16–21 Due to the conformational flexibility of the triol chain in AGs, determining the relative and absolute configurations is significantly challenging. Recently, Mosher's method was developed to assign the absolute configuration of AGs by comparing the ¹H NMR data of the tris-(R)- and the tris-(S)-MPA ester derivatives with those of the AGs.^22,23 However, the inability to prepare tris-MPA ester derivatives for trace compounds and the complex rules governing the comparison of the Δδ^RS(H-1) and |Δδ^RS_H(2) − Δδ^RS_H(1)| of AGs, to some extent, have limited the application of this method. For some naturally occurring AGs, the regular Δδ_C1–C2 parameters have been used to differentiate only the threo and erythro relative configurations.^9,15 However, we found that this rule is not applicable to AGs without substituent groups on the benzene ring. Therefore, a new reliable method to determine the relative and absolute configurations of AGs is a worthwhile task.

Results and discussion

Our recent study found that the threo and erythro configurations of H-7 and H-8 in 8,4′-oxyneolignans can be discriminated by the values of Δδ_H9a–H9b of methylene protons in ¹H NMR spectroscopy.²⁴ When we determined the configurations of a pair of threo syringylglycerol enantiomers (1 and 2) obtained from the Arnebia euchroma, a careful comparison of the ¹H NMR spectroscopic data of 1 in methanol-d₄ with the corresponding data in erythro syringylglycerol in the literature showed that there were significant differences.⁹ The value of Δδ_H3a–H3b of methylene protons in 1 is 0.13 ppm, while the value is only 0.07 ppm in erythro syringylglycerol. Surprisingly, this result is similar to the rule in 8,4′-oxyneolignans. For AGs without large steric hindrance groups, can the Δδ_H3a–H3b values of the methylene groups reflect their relative stereochemistry? To explore and summarize this phenomenon, two pairs of 1-phenylpropane-1,2,3-triol enantiomers were prepared (M1–M4) through the hydrolysis of (2R,3R)-3-phenyl-2-oxiranemethanol and (2S,3S)-3-phenyl-2-oxiranemethanol (Fig. 1). The ¹H NMR spectra of compounds M1 and M4 in methanol-d₄ were recorded. Obviously, the Δδ_H3a–H3b (0.14 ppm) value of methylene protons in the threo configuration was remarkably larger than the corresponding value (0.06 ppm) in the erythro configuration (Fig. 3). Considering that deuterated solvents have a major impact on the chemical shifts of methylene protons, the ¹H NMR spectra of compounds M1 and M4 were recorded in other deuterated solvents, acetone-d₆, pyridine-d₅, DMSO-d₆, and CDCl₃, as shown in Fig. 3. Similarly, in the threo configurations, the Δδ_H3a–H3b values of methylene protons were 0.12 in acetone-d₆, 0.15 in pyridine-d₅, and 0.21 in DMSO-d₆. In the erythro configurations, the Δδ_H3a–H3b values of methylene protons were 0.00 in acetone-d₆, 0.00 in pyridine-d₅, and 0.06 in DMSO-d₆. Remarkably, there was no obvious difference in CDCl₃ (threo: 0.09 ppm; erythro: 0.07 ppm). In addition, the chemical shift values of methylene protons in the erythro configuration were obviously larger than those in the threo configuration.

Fig. 1 Structures of compounds M1–M4.

Fig. 2 Structures of compounds 1–11.

Fig. 3 Influence of different deuterated solvents on the Δδ_H3a–H3b values of M1 and M4 (¹H NMR 500 MHz).

To further confirm the accuracy and reliability of the method and investigate the influence of different substitution groups on the benzene ring, 1-(4-bromophenyl)propane-1,2,3-triol (3–6) and 1-(4-nitrophenyl)propane-1,2,3-triol (7 and 8) were synthesized through the Sharpless asymmetric epoxidation and hydrolysis reaction (Fig. 2).²⁵ As expected, larger Δδ_H3a–H3b values of methylene protons appeared in the threo configurations (3–4), and smaller Δδ_H3a–H3b values of methylene protons appeared in the erythro configurations (5–8). Additionally, three natural products, threo-1-C-guaiacylglycerol (9 and 10), obtained from bamboo juice, and compound 11, obtained from Cortex Lycii, were also in accordance with the general trends in methanol-d₄ and DMSO-d₆ (Fig. 2). Thus, the relative configuration of AGs can be determined from the Δδ_H3a–H3b values of methylene protons of the ¹H NMR spectra in methanol-d₄, acetone-d₆, pyridine-d₅, and DMSO-d₆; DMSO-d₆ is the preferred solvent (Table 1).

Table 1 Δδ_H3a–H3b values (ppm) of the AGs 1–11 in different solvents (¹H NMR 500 MHz)

No.	δH3a	δH3b	ΔδH3a–H3b	Solvent	Relative conf.
1 & 2	3.50	3.37	0.13	Methanol-d4	Threo
	3.49	3.40	0.09	Acetone-d6
3 & 4	3.55	3.39	0.16	Methanol-d4	Threo
	3.56	3.42	0.14	Acetone-d6
	4.27	4.09	0.18	Pyridine-d5
	3.39	3.15	0.24	DMSO-d6
5 & 6	3.64	3.59	0.05	Methanol-d4	Erythro
	3.61	3.61	0.00	Acetone-d6
	4.39	4.39	0.00	Pyridine-d5
	3.42	3.37	0.05	DMSO-d6
7 & 8	3.68	3.65	0.03	Methanol-d4	Erythro
	3.67	3.63	0.04	Acetone-d6
	3.42	3.42	0.00	DMSO-d6
9 & 10	3.47	3.34	0.13	Methanol-d4	Threo
	3.31	3.14	0.17	DMSO-d6
11	3.52	3.38	0.14	Methanol-d4	Threo
	3.35	3.18	0.17	DMSO-d6

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Then, a more challenging task is how to determine the absolute configurations of a pair of enantiomers. Considering the conformational flexibility of the triol chain in AGs, the introduction of a chiral auxiliary agent bearing a larger steric hindrance group may be an efficient approach. Camphanoyl chloride, a acylating agent containing two special stereogenic centers, is frequently utilized to determine enantiomeric purity in organic synthesis and its derivatives was also used to assign absolute configurations of chiral alcohols by NMR.^26,27 It made molecules more rigid due to its steric hindrance in conformational studies. These features inspired us to thoroughly investigate the relationship between the chemical shift differences of methylene protons and the absolute configurations of mono-camphanoyl AGs.

Accordingly, acylation reactions (Scheme 1) were carried out using (−)-1S,4R-camphanoyl chloride and two pairs of 1-phenyl glycerol enantiomers M1–M4 as raw materials, and four products (M1a–M4a) were successfully obtained. Their ¹H NMR spectra were measured in several deuterated solvents, including methanol-d₄, DMSO-d₆, acetone-d₆, pyridine-d₅, and CDCl₃. To our delight, in methanol-d₄ and DMSO-d₆, the Δδ_H3a–H3b values of methylene protons showed obvious differences in the threo and erythro configurations. In the threo enantiomers, the Δδ_H3a–H3b values of methylene protons in M1a (1R,2R) were 0.15 and 0.15, respectively, while the Δδ_H3a–H3b values in M2a (1S,2S) were 0.28 and 0.32, respectively (Fig. 4). In the erythro system, the Δδ_H3a–H3b values of methylene protons in M3a (1R,2S) were 0.09 and 0.15, respectively, while the Δδ_H3a–H3b values in M4a (1S,2R) were 0.03 and 0.00, respectively. The chemical shift differences were high enough to discriminate the absolute configuration, especially when using DMSO-d₆ as the solvent. However, in the other three deuterated solvents, acetone-d₆, CDCl₃, and pyridine-d₅, the Δδ_H3a–H3b values were irregular. These results suggested that the anisotropic effect of camphanoyl could be efficiently and selectively space oriented toward the methylene protons of mono-camphanoyl AGs in methanol-d₄ and DMSO-d₆. To further verify the reliability of the method, derivatives 1a–8a, and 10a were also successfully synthesized. As expected, the rule still applies in methanol-d₄ and DMSO-d₆ (Fig. 5). Furthermore, the theoretical ¹H NMR calculation of M1a–M4a was carried out by GIAO NMR calculation method in DMSO (the detailed procedure see ESI†).²⁸ The calculated results were better match with the experimental data (R² ≥ 0.995) (Fig. 6). The calculated Δδ_H3a–H3b values were M1a: 0.08, M2a: 0.37, M3a: 0.72, and M4a: 0.06, respectively. This result verified our method was also reliable by quantum chemical calculation. These results confirmed that camphanoyl chloride is a suitable pure chiral derivatizing agent to assign the absolute configuration of AGs.

Scheme 1 The acylation reaction.

Fig. 4 Influence of different deuterated solvents on the Δδ_H3a–H3b values of M1a–M4a (¹H NMR 500 MHz).

Fig. 5 Δδ_H3a–H3b values (ppm) of the acylation products.

Fig. 6 Linear correlation plots of predicted versus experimental ¹H NMR chemical shifts.

Experimental section

General experimental procedures

The optical rotations were recorded with a RUDOLPH automatic V polarimeter (RUDOLPH, Hackettstown, NJ, USA). The NMR spectra were recorded with a Bruker 500 MHz (Bruker-Biospin, Billerica, MA, USA). HRESIMS reports were obtained from an Agilent 6520 HPLC-Q-TOF (Agilent Technologies, Waldbronn, Germany) and Q Exactive Focus LCMSMS (Thermo Scientific, MA, USA). Preparative HPLC separations were performed using a Shimadzu LC-10AT with an ODS-A column (250 mm × 10 mm, 5 μm; YMC Corp., Kyoto, Japan). An Agilent 1200 series system with an Apollo C 18 column (250 mm × 4.6 mm, 5 μm; Alltech Corp., KY, USA) was used for HPLC-DAD analysis. All reactions were magnetically stirred with a DF-101S magnetic stirrer (Hengfengchangwei Technology Co. Ltd, Beijing, China). Commercially available reagents were used without further purification unless otherwise stated. All reactions were monitored by TLC with silica gel pre-coated GF254 plates and HPLC-DAD.

Structure characterization and synthesis procedures

(2S,3S)-3-Phenyl-2-oxiranemethanol and (2R,3R)-3-phenyl-2-oxiranemethanol. The asymmetric epoxidation was carried out according to the Sharpless asymmetric Epoxidation. Colorless oil; the specific rotations were [α]²⁰_D −47 (c 0.1 methanol) and [α]²⁰_D +70 (c 0.1 methanol), respectively; ¹H-NMR (500 MHz, methanol-d₄): δ 7.27–7.35 (5H, one mono-substituted benzene ring system), 3.87 (1H, dd, J = 12.5, 3.0 Hz), 3.83 (1H, d, J = 2.0 Hz), 3.67 (1H, dd, J = 12.5, 5.5 Hz), 3.15 (1H, m); ¹³C-NMR (125 MHz, CDCl₃): δ 136.8, 128.7, 128.7, 128.5, 125.9, 125.9, 62.5, 61.3, 55.7; HRESIMS m/z 151.0754 [M + H]⁺ (calcd for C₉H₁₁O₂ 151.0753).

A solution of (2R,3R)-3-phenyl-2-oxiranemethanol (100 mg, 0.6 mmol) and acetic acid (0.5 ml) in acetonitrile was stirred by rotary evaporator at 55 °C until the solution was dry. Through the analysis of HPLC, the (2R,3R)-3-phenyl-2-oxiranemethanol was completely hydrolyzed. The residue was separated by preparative HPLC (MeOH/H₂O, 25 : 75, v\v, HOAc, 0.1%) to afford M1 (45 mg) and M4 (50 mg). The hydrolysis procedure of (2S,3S)-3-phenyl-2-oxiranemethanol was same as the above. The residue was separated by preparative HPLC (MeOH/H₂O, 25 : 75, v\v, HOAc, 0.1%) to afford M2 and M3. Compounds M1–M4 are known compounds and they were identified by comparing their spectral data with those of the known compounds.²²

(1R,2R)-1-Phenylpropane-1,2,3-triol and (1S,2S)-1-phenylpropane-1,2,3-triol (M1 and M2). Colorless oil; the specific rotations were [α]²⁰_D −39 (c 0.1 methanol) and [α]²⁰_D +33 (c 0.1 methanol), respectively. ¹H-NMR (500 MHz, methanol-d₄): δ 7.25–7.40 (5H, one mono-substituted benzene ring system), 4.63 (1H, d, J = 5.5 Hz), 3.69 (1H, m), 3.50 (1H, dd, J = 11.5, 4.0 Hz), 3.36 (1H, dd, J = 11.5, 6.0 Hz). ¹H-NMR (500 MHz, acetone-d₆): δ 7.23–7.41 (5H, one mono-substituted benzene ring system), 4.68 (1H, d, J = 5.5 Hz), 3.65 (1H, m), 3.51 (1H, dd, J = 11.0, 3.5 Hz), 3.39 (1H, dd, J = 11.5, 5.5 Hz). ¹H-NMR (500 MHz, pyridine-d₅): δ 7.84 (2H, d, J = 7.5 Hz), 7.40 (2H, t, J = 7.5 Hz), 7.30 (1H, t, J = 7.5 Hz), 5.39 (1H, d, J = 5.5 Hz), 4.39 (1H, m), 4.24 (1H, dd, J = 11.0, 4.5 Hz), 4.09 (1H, dd, J = 11.0, 6.0 Hz). ¹H-NMR (500 MHz, DMSO-d₆): δ 7.19–7.34 (5H, one mono-substituted benzene ring system), 4.53 (1H, t, J = 5.0 Hz), 3.48 (1H, m), 3.36 (1H, m), 3.15 (1H, m). ¹H-NMR (500 MHz, CDCl₃): δ 7.28–7.33 (5H, one mono-substituted benzene ring system), 4.66 (1H, d, J = 5.5 Hz), 3.76 (1H, brs), 3.54 (1H, brd, J = 11.0 Hz), 3.46 (1H, brd, J = 11.0 Hz). ¹³C-NMR (125 MHz, CDCl₃): δ 140.6, 128.6, 128.6, 128.2, 126.8, 126.8, 76.2, 74.8, 63.2; HRESIMS refer to compounds M3 and M4.

(1R,2S)-1-Phenylpropane-1,2,3-triol and (1S,2R)-1-phenylpropane-1,2,3-triol (M3 and M4). Colorless oil; the specific rotations were [α]²⁰_D −47 (c 0.1 methanol) and [α]²⁰_D +27 (c 0.1 methanol), respectively. ¹H-NMR (500 MHz, methanol-d₄): δ 7.24–7.41 (5H, one mono-substituted benzene ring system), 4.61 (1H, d, J = 6.5 Hz), 3.75 (1H, m), 3.66 (1H, dd, J = 11.5, 3.5 Hz), 3.60 (1H, dd, J = 11.5, 6.5 Hz); ¹H-NMR (500 MHz, acetone-d₆): δ 7.21–7.41 (5H, one mono-substituted benzene ring system), 4.67 (1H, d, J = 4.0 Hz), 3.73 (1H, m), 3.61 (2H, overlap). ¹H-NMR (500 MHz, pyridine-d₅): δ 7.88 (2H, d, J = 8.0 Hz), 7.40 (2H, t, J = 8.0 Hz), 7.30 (1H, t, J = 8.0 Hz), 5.35 (1H, d, J = 6.0 Hz), 4.50 (1H, m), 4.42 (2H, overlap). ¹H-NMR (500 MHz, DMSO-d₆): δ 7.19–7.34 (5H, one mono-substituted benzene ring system), 4.42 (1H, overlap), 3.51 (1H, m), 3.44 (1H, m), 3.38 (1H, m). ¹H-NMR (500 MHz, CDCl₃): δ 7.30–7.39 (5H, one mono-substituted benzene ring system), 4.89 (1H, d, J = 5.0 Hz), 3.85 (1H, m), 3.74 (1H, dd, J = 11.0, 5.5 Hz), 3.67 (1H, dd, J = 11.0, 3.5 Hz). ¹³C-NMR (125 MHz, CDCl₃): δ 140.4, 128.8, 128.8, 128.3, 126.3, 126.3, 76.1, 74.6, 63.1; HRESIMS m/z 191.0679 [M + Na]⁺ (calcd for C₉H₁₂O₃Na 191.0679).

(2R,3R)-2,3-Dihydroxy-3-phenylpropyl(1S,4R)-4,7,7-trimethyl-3-oxo-2-oxabicyclo[2.2.1]heptane-1-carboxylate (M1a). Compound M1 (10 mg, 0.06 mmol) solution in dry CH₂Cl₂ or THF was added to a solution of camphanic acid chloride (13 mg, 0.06 mmol), EDCI (11.4 mg, 0.06 mmol) and DMAP (3.6 mg, 0.03 mmol) at 0 °C with the protection of argon gas. The reaction was stirred at 0 °C for 1 h and was detected by TLC. Then, the reaction was quenched by dropwise addition of H₂O. The suspension was extracted with CH₂Cl₂. Removal of the solvent under reduced pressure and purification of the residue by preparative HPLC, eluting with 40% acetonitrile/H₂O, gave the M1a 8.7 mg. Colorless oil; ¹H-NMR (500 MHz, methanol-d₄): δ 7.27–7.42 (5H, one mono-substituted benzene ring system), 4.65 (1H, d, J = 5.5 Hz), 4.21 (1H, dd, J = 11.5, 3.5 Hz), 4.06 (1H, dd, J = 11.5, 7.0 Hz), 3.95 (1H, m), 2.47 (1H, m), 2.03 (1H, m), 1.96 (1H, m), 1.62 (1H, m), 1.09 (3H, s), 1.09 (3H, s), 0.94 (3H, s). ¹H-NMR (500 MHz, DMSO-d₆): δ 7.23–7.38 (5H, one mono-substituted benzene ring system), 4.56 (1H, t, J = 5.0 Hz), 4.10 (1H, dd, J = 11.0, 3.5 Hz), 3.95 (1H, dd, J = 11.0, 7.5 Hz), 3.80 (1H, m), 2.35 (1H, m), 1.97 (1H, m), 1.90 (1H, m), 1.54 (1H, m), 1.01 (3H, s), 1.00 (3H, s), 0.83 (3H, s). ¹H-NMR (500 MHz, acetone-d₆): δ 7.26–7.45 (5H, one mono-substituted benzene ring system), 4.73 (1H, t, J = 5.0 Hz), 4.24 (1H, dd, J = 11.5, 3.5 Hz), 4.11 (1H, dd, J = 11.5, 7.0 Hz), 3.98 (1H, m), 2.46 (1H, m), 2.01 (1H, m), 1.95 (1H, m), 1.60 (1H, m), 1.09 (3H, s), 1.05 (3H, s), 0.92 (3H, s). ¹H-NMR (500 MHz, pyridine-d₅): δ 7.80 (2H, d, J = 7.5 Hz), 7.42 (2H, d, J = 7.5 Hz), 7.32 (1H, t, J = 7.5 Hz), 5.22 (1H, d, J = 5.0 Hz), 4.75 (1H, dd, J = 11.0, 7.5 Hz), 4.70 (1H, dd, J = 11.0, 4.0 Hz), 4.52 (1H, m), 2.47 (1H, m), 1.99 (1H, m), 1.82 (1H, m), 1.57 (1H, m), 1.06 (3H, s), 1.05 (3H, s), 1.02 (3H, s). ¹H-NMR (500 MHz, CDCl₃): δ 7.30–7.37 (5H, one mono-substituted benzene ring system), 4.68 (1H, d, J = 7.0 Hz), 4.25 (1H, dd, J = 11.5, 3.5 Hz), 4.11 (1H, dd, J = 11.5, 6.5 Hz), 3.98 (1H, m), 2.41 (1H, m), 2.02 (1H, m), 1.92 (1H, m), 1.69 (1H, m), 1.11 (3H, s), 1.05 (3H, s), 0.96 (3H, s). ¹³C-NMR (125 MHz, methanol-d₄): δ 180.3, 168.7, 142.9, 129.3, 129.3, 128.8, 127.9, 127.9, 92.9, 75.6, 74.5, 67.6, 56.0, 55.4, 31.5, 29.9, 17.1, 17.0, 9.9; HRESIMS m/z 371.1465 [M + Na]⁺ (calcd for C₁₉H₂₄O₆Na 371.1458).

(2S,3S)-2,3-Dihydroxy-3-phenylpropyl(1S,4R)-4,7,7-trimethyl-3-oxo-2-oxabicyclo[2.2.1]heptane-1-carboxylate (M2a). The synthesis procedure of M2a was same with the M1a. Colorless oil; ¹H-NMR (500 MHz, methanol-d₄): δ 7.27–7.41 (5H, one mono-substituted benzene ring system), 4.67 (1H, d, J = 6.0 Hz), 4.27 (1H, dd, J = 11.0, 3.5 Hz), 3.99 (1H, dd, J = 11.0, 6.5 Hz), 3.94 (1H, m), 2.49 (1H, m), 2.04 (1H, m), 1.97 (1H, m), 1.63 (1H, m), 1.11 (3H, s), 1.09 (3H, s), 0.94 (3H, s). ¹H-NMR (500 MHz, DMSO-d₆): δ 7.23–7.37 (5H, one mono-substituted benzene ring system), 4.56 (1H, d, J = 5.0 Hz), 4.18 (1H, dd, J = 11.0, 3.5 Hz), 3.85 (1H, dd, J = 11.0, 7.0 Hz), 3.80 (1H, m), 2.38 (1H, m), 1.97 (1H, m), 1.90 (1H, m), 1.54 (1H, m), 1.01 (3H, s), 1.00 (3H, s), 0.83 (3H, s). ¹H-NMR (500 MHz, acetone-d₆): δ 7.25–7.45 (5H, one mono-substituted benzene ring system), 4.73 (1H, d, J = 5.5 Hz), 4.29 (1H, dd, J = 11.0, 4.0 Hz), 4.03 (1H, dd, J = 11.0, 6.5 Hz), 3.97 (1H, m), 2.49 (1H, m), 2.02 (1H, m), 1.96 (1H, m), 1.60 (1H, m), 1.10 (3H, s), 1.05 (3H, s), 0.92 (3H, s). ¹H-NMR (500 MHz, pyridine-d₅): δ 7.80 (2H, d, J = 8.0 Hz), 7.42 (2H, d, J = 8.0 Hz), 7.32 (1H, t, J = 8.0 Hz), 5.24 (1H, d, J = 5.0 Hz), 4.80 (1H, dd, J = 11.0, 3.0 Hz), 4.62 (1H, dd, J = 11.0, 7.0 Hz), 4.53 (1H, m), 2.50 (1H, m), 2.01 (1H, m), 1.84 (1H, m), 1.58 (1H, m), 1.06 (3H, s), 1.06 (3H, s), 1.02 (3H, s). ¹H-NMR (500 MHz, CDCl₃): δ 7.30–7.38 (5H, one mono-substituted benzene ring system), 4.68 (1H, d, J = 6.5 Hz), 4.25 (1H, dd, J = 11.5, 3.5 Hz), 4.08 (1H, dd, J = 11.5, 6.5 Hz), 3.98 (1H, m), 2.42 (1H, m), 2.02 (1H, m), 1.92 (1H, m), 1.68 (1H, m), 1.11 (3H, s), 1.06 (3H, s), 0.97 (3H, s). ¹³C-NMR (125 MHz, methanol-d₄): δ 180.3, 168.8, 142.8, 129.3, 129.3, 128.8, 127.9, 127.9, 92.9, 75.6, 74.5, 67.6, 56.0, 55.4, 31.5, 30.0, 17.1, 17.0, 9.9; HRESIMS refer to compound M1a.

(2S,3R)-2,3-Dihydroxy-3-phenylpropyl(1S,4R)-4,7,7-trimethyl-3-oxo-2-oxabicyclo[2.2.1]heptane-1-carboxylate (M3a). The synthesis procedure of M3a was same with the M1a. Colorless oil; ¹H-NMR (500 MHz, methanol-d₄): δ 7.26–7.42 (5H, one mono-substituted benzene ring system), 4.61 (1H, d, J = 6.5 Hz), 4.42 (1H, dd, J = 11.5, 3.0 Hz), 4.32 (1H, dd, J = 11.5, 7.0 Hz), 3.95 (1H, m), 2.49 (1H, m), 2.04 (1H, m), 1.97 (1H, m), 1.62 (1H, m), 1.10 (3H, s), 1.09 (3H, s), 0.94 (3H, s). ¹H-NMR (500 MHz, DMSO-d₆): δ 7.23–7.37 (5H, one mono-substituted benzene ring system), 4.46 (1H, dd, J = 6.5, 4.5 Hz), 4.35 (1H, dd, J = 11.5, 3.0 Hz), 4.20 (1H, dd, J = 11.5, 7.0 Hz), 3.74 (1H, m), 2.39 (1H, m), 1.98 (1H, m), 1.90 (1H, m), 1.55 (1H, m), 1.03 (3H, s), 1.00 (3H, s), 0.83 (3H, s). ¹H-NMR (500 MHz, acetone-d₆): δ 7.24–7.44 (5H, one mono-substituted benzene ring system), 4.72 (1H, dd, J = 6.5, 4.5 Hz), 4.39 (1H, dd, J = 11.5, 3.0 Hz), 4.33 (1H, dd, J = 11.5, 7.0 Hz), 4.01 (1H, m), 2.48 (1H, m), 2.00 (1H, m), 1.94 (1H, m), 1.59 (1H, m), 1.09 (3H, s), 1.05 (3H, s), 0.92 (3H, s). ¹H-NMR (500 MHz, pyridine-d₅): δ 7.82 (2H, d, J = 7.5 Hz), 7.41 (2H, d, J = 7.5 Hz), 7.31 (1H, t, J = 7.5 Hz), 5.22 (1H, d, J = 6.5 Hz), 5.06 (1H, dd, J = 11.5, 3.0 Hz), 4.98 (1H, dd, J = 11.5, 7.0 Hz), 4.55 (1H, m), 2.49 (1H, m), 1.99 (1H, m), 1.82 (1H, m), 1.57 (1H, m), 1.07 (3H, s), 1.06 (3H, s), 1.05 (3H, s). ¹H-NMR (500 MHz, CDCl₃): δ 7.29–7.38 (5H, one mono-substituted benzene ring system), 4.83 (1H, d, J = 5.5 Hz), 4.34 (1H, dd, J = 11.5, 7.5 Hz), 4.25 (1H, dd, J = 11.5, 3.0 Hz), 4.09 (1H, m), 2.40 (1H, m), 2.00 (1H, m), 1.92 (1H, m), 1.67 (1H, m), 1.10 (3H, s), 1.04 (3H, s), 0.95 (3H, s). ¹³C-NMR (125 MHz, methanol-d₄): δ 180.3, 168.9, 143.2, 129.2, 129.2, 128.6, 128.1, 128.1, 93.0, 75.8, 74.1, 67.8, 56.0, 55.4, 31.5, 30.0, 17.1, 17.0, 9.9; HRESIMS refer to compound M1a.

(2R,3S)-2,3-Dihydroxy-3-phenylpropyl(1S,4R)-4,7,7-trimethyl-3-oxo-2-oxabicyclo[2.2.1]heptane-1-carboxylate (M4a). The synthesis procedure of M4a was same with the M1a. Colorless oil; ¹H-NMR (500 MHz, methanol-d₄): δ 7.26–7.42 (5H, one mono-substituted benzene ring system), 4.62 (1H, d, J = 7.0 Hz), 4.38 (1H, dd, J = 11.5, 6.5 Hz), 4.35 (1H, dd, J = 11.5, 3.5 Hz), 3.96 (1H, m), 2.49 (1H, m), 2.03 (1H, m), 1.97 (1H, m), 1.62 (1H, m), 1.10 (3H, s), 1.09 (3H, s), 0.94 (3H, s). ¹H-NMR (500 MHz, DMSO-d₆): δ 7.22–7.36 (5H, one mono-substituted benzene ring system), 4.47 (1H, dd, J = 6.5, 4.5 Hz), 4.26 (2H, overlap), 3.75 (1H, m), 2.38 (1H, m), 1.97 (1H, m), 1.90 (1H, m), 1.54 (1H, m), 1.01 (3H, s), 1.00 (3H, s), 0.84 (3H, s). ¹H-NMR (500 MHz, acetone-d₆): δ 7.24–7.44 (5H, one mono-substituted benzene ring system), 4.72 (1H, dd, J = 6.0, 4.5 Hz), 4.38 (1H, dd, J = 11.5, 7.0 Hz), 4.33 (1H, dd, J = 11.5, 3.5 Hz), 4.01 (1H, m), 2.47 (1H, m), 2.00 (1H, m), 1.94 (1H, m), 1.59 (1H, m), 1.09 (3H, s), 1.05 (3H, s), 0.92 (3H, s). ¹H-NMR (500 MHz, pyridine-d₅): δ 7.82 (2H, d, J = 7.5 Hz), 7.41 (2H, d, J = 7.5 Hz), 7.31 (1H, t, J = 7.5 Hz), 5.22 (1H, d, J = 7.0 Hz), 5.06 (1H, dd, J = 11.5, 7.0 Hz), 4.98 (1H, dd, J = 11.5, 3.0 Hz), 4.55 (1H, m), 2.50 (1H, m), 1.99 (1H, m), 1.83 (1H, m), 1.57 (1H, m), 1.07 (3H, s), 1.06 (3H, s), 1.04 (3H, s). ¹H-NMR (500 MHz, CDCl₃): δ 7.30–7.38 (5H, one mono-substituted benzene ring system), 4.84 (1H, d, J = 5.5 Hz), 4.38 (1H, dd, J = 11.5, 7.5 Hz), 4.24 (1H, dd, J = 11.5, 3.0 Hz), 4.08 (1H, m), 2.41 (1H, m), 2.01 (1H, m), 1.92 (1H, m), 1.68 (1H, m), 1.11 (3H, s), 1.04 (3H, s), 0.95 (3H, s). ¹³C-NMR (125 MHz, methanol-d₄): δ 180.3, 168.9, 143.2, 129.2, 129.2, 128.6, 128.1, 128.1, 93.0, 75.8, 74.2, 67.8, 56.0, 55.4, 31.5, 30.0, 17.1, 17.0, 9.9; HRESIMS refer to compound M1a.

(1R,2R)-1-(4-Hydroxy-3,5-dimethoxyphenyl)propane-1,2,3-triol and (1S,2S)-1-(4-hydroxy-3,5-dimethoxyphenyl)propane-1,2,3-triol (1 and 2). Colorless oil; the specific rotations were [α]²⁰_D −100 (c 0.1 methanol) and [α]²⁰_D +70 (c 0.1 methanol), respectively. ¹H-NMR (500 MHz, methanol-d₄): δ 6.68 (2H, brs), 4.53 (1H, d, J = 6.0 Hz), 3.67 (1H, m), 3.50 (1H, dd, J = 11.5, 4.0 Hz), 3.37 (1H, dd, J = 11.5, 6.5 Hz). ¹H-NMR (500 MHz, acetone-d6): δ 6.69 (2H, brs), 4.56 (1H, d, J = 6.0 Hz), 3.62 (1H, m), 3.49 (1H, dd, J = 11.0, 4.0 Hz), 3.40 (1H, dd, J = 11.0, 6.0 Hz). ¹³C-NMR (125 MHz, methanol-d₄): δ 149.1, 149.1, 135.9, 134.1, 105.0, 105.0, 77.6, 75.6, 64.2, 56.7, 56.7; HRESIMS m/z 267.0839 [M + Na]⁺ (calcd for C₁₁H₁₆O₆Na 267.0837). Compounds 1 and 2 are known compounds and they were identified by comparing their spectral data with those of the known compounds.⁸

4-((1R,2R)-1,2-Dihydroxy-3-(((1S,4R)-4,7,7-trimethyl-3-oxo-2-oxabicyclo[2.2.1]heptane-1-carbonyl)oxy)propyl)-2,6-dimethoxyphenyl(1S,4R)-4,7,7-trimethyl-3-oxo-2-oxabicyclo[2.2.1]heptane-1-carboxylate (1a). Compound 1 (6 mg, 0.02 mmol) solution in dry CH₂Cl₂ or THF was added to a solution of camphanic acid chloride (9 mg, 0.04 mmol), EDCI (7.6 mg, 0.04 mmol) and DMAP (2.4 mg, 0.02 mmol) at 0 °C with the protection of argon gas. The reaction was stirred at 0 °C for 1 h and was detected by TLC. Then, the reaction was quenched by dropwise addition of H₂O. The suspension was extracted with CH₂Cl₂. Removal of the solvent under reduced pressure and purification of the residue by preparative HPLC, eluting with 40% acetonitrile/H₂O, gave the 1a 2.9 mg. Colorless oil; ¹H-NMR (500 MHz, methanol-d₄): δ 6.84 (2H, brs), 4.68 (1H, d, J = 5.0 Hz), 4.33 (1H, dd, J = 11.5, 4.0 Hz), 4.16 (1H, dd, J = 11.5, 6.5 Hz), 3.97 (1H, m), 2.67 (1H, m), 2.48 (1H, m), 2.12 (2H, overlap), 2.04 (1H, m), 1.97 (1H, m), 1.70 (1H, m), 1.63 (1H, m), 1.22 (3H, s), 1.14 (3H, s), 1.10 (3H, s), 1.09 (3H, s), 1.08 (3H, s), 0.97 (3H, s); HRESIMS m/z 605.2593 [M + H]⁺ (calcd for C₃₁H₄₁O₁₂ 605.2593).

4-((1S,2S)-1,2-Dihydroxy-3-(((1S,4R)-4,7,7-trimethyl-3-oxo-2-oxabicyclo[2.2.1]heptane-1-carbonyl)oxy)propyl)-2,6-dimethoxyphenyl(1S,4R)-4,7,7-trimethyl-3-oxo-2-oxabicyclo[2.2.1]heptane-1-carboxylate (2a). Compound 2 (4.4 mg, 0.018 mmol) solution in dry CH₂Cl₂ or THF was added to a solution of camphanic acid chloride (7.8 mg, 0.036 mmol), EDCI (6.8 mg, 0.036 mmol) and DMAP (2.2 mg, 0.018 mmol) at 0 °C with the protection of argon gas. The reaction was stirred at 0 °C for 1 h and was detected by TLC. Then, the reaction was quenched by dropwise addition of H₂O. The suspension was extracted with CH₂Cl₂. Removal of the solvent under reduced pressure and purification of the residue by preparative HPLC, eluting with 55% methanol/H₂O, gave the 2a 2.0 mg. Colorless oil; ¹H-NMR (500 MHz, methanol-d₄): δ 6.82 (2H, brs), 4.69 (1H, d, J = 5.0 Hz), 4.36 (1H, dd, J = 11.5, 4.0 Hz), 4.13 (1H, dd, J = 11.5, 6.5 Hz), 3.97 (1H, m), 2.67 (1H, m), 2.49 (1H, m), 2.12 (2H, overlap), 2.02 (2H, overlap), 1.70 (1H, m), 1.63 (1H, m), 1.22 (3H, s), 1.14 (3H, s), 1.11 (3H, s), 1.09 (3H, s), 1.08 (3H, s), 0.94 (3H, s); HRESIMS m/z 605.2593 [M + H]⁺ (calcd for C₃₁H₄₁O₁₂ 605.2600).

((2R,3R)-3-(4-Bromophenyl)oxiran-2-yl)methanol and ((2S,3S)-3-(4-bromophenyl)oxiran-2-yl)methanol. The asymmetric epoxidation was also carried out according to the Sharpless asymmetric Epoxidation. Colorless oil; the specific rotations were [α]²⁰_D +45 (c 0.1 methanol) and [α]²⁰_D −25 (c 0.1 methanol), respectively. ¹H-NMR (500 MHz, methanol-d₄): δ 7.49 (2H, d, J = 8.5 Hz), 7.22 (2H, d, J = 8.5 Hz), 3.85 (1H, dd, J = 12.5, 3.0 Hz), 3.83 (1H, d, J = 1.5 Hz), 3.67 (1H, dd, J = 12.5, 4.5 Hz), 3.13 (1H, m); ¹³C-NMR (125 MHz, methanol-d₄): δ 138.2, 132.6, 132.6, 128.7, 128.7, 122.8, 63.9, 62.6, 56.2; HRESIMS m/z 228.9859 [M + H]⁺ (calcd for C₉H₁₀O₂Br 228.9858).

A solution of ((2R,3R)-3-(4-bromophenyl)oxiran-2-yl)methanol (65 mg, 0.3 mmol) and acetic acid (0.5 ml) in acetonitrile was stirred by rotary evaporator at 55 °C until the solution was dry. Through the analysis of HPLC, the ((2R,3R)-3-(4-bromophenyl)oxiran-2-yl)methanol was completely hydrolyzed. The residue was separated by preparative HPLC (MeOH/H₂O, 45 : 55, v\v) to afford 3 (27 mg) and 6 (38 mg). The hydrolysis procedure of ((2S,3S)-3-(4-bromophenyl)oxiran-2-yl)methanol was same as the above. The residue was separated by preparative HPLC (MeOH/H₂O, 45 : 55, v\v) to afford 4 and 5. Compounds 3–6 are known compounds and their absolute configurations were identified by comparing the optical rotations of M1–M4.

(1R,2R)-1-(4-Bromophenyl)propane-1,2,3-triol and (1S,2S)-1-(4-bromophenyl)propane-1,2,3-triol (3 and 4). Colorless oil; the specific rotations were [α]²⁰_D −20 (c 0.1 methanol) and [α]²⁰_D +16 (c 0.1 methanol), respectively. ¹H-NMR (500 MHz, methanol-d₄): δ 7.48 (2H, d, J = 8.5 Hz), 7.33 (2H, d, J = 8.5 Hz), 4.65 (1H, d, J = 5.0 Hz), 3.65 (1H, m), 3.55 (1H, dd, J = 11.5, 4.5 Hz), 3.39 (1H, dd, J = 11.5, 6.5 Hz). ¹H-NMR (500 MHz, acetone-d₆): δ 7.49 (2H, d, J = 8.5 Hz), 7.37 (2H, d, J = 8.5 Hz), 4.72 (1H, t, J = 4.5 Hz), 3.64 (1H, m), 3.56 (1H, m), 3.42 (1H, m). ¹H-NMR (500 MHz, pyridine-d₅): δ 7.71 (2H, d, J = 8.5 Hz), 7.55 (2H, d, J = 8.5 Hz), 5.38 (1H, d, J = 5.0 Hz), 4.35 (1H, m), 4.28 (1H, dd, J = 11.0, 4.5 Hz), 4.09 (1H, dd, J = 11.0, 6.0 Hz). ¹H-NMR (500 MHz, DMSO-d₆): δ 7.48 (2H, d, J = 8.5 Hz), 7.29 (2H, d, J = 8.5 Hz), 4.55 (1H, t, J = 5.0 Hz), 3.46 (1H, m), 3.39 (1H, m), 3.15 (1H, m). ¹³C-NMR (125 MHz, methanol-d₄): δ 143.0, 132.2, 132.2, 129.8, 129.8, 122.0, 77.1, 74.4, 64.1; HRESIMS m/z 268.9784 [M + Na]⁺ (calcd for C₉H₁₁O₃BrNa 268.9778).

(1R,2S)-1-(4-Bromophenyl)propane-1,2,3-triol and (1S,2R)-1-(4-bromophenyl)propane-1,2,3-triol (5 and 6). Colorless oil; the specific rotations were [α]²⁰_D −8 (c 0.1 methanol) and [α]²⁰_D +12 (c 0.1 methanol), respectively. ¹H-NMR (500 MHz, methanol-d₄): δ 7.48 (2H, d, J = 8.5 Hz), 7.33 (2H, d, J = 8.5 Hz), 4.58 (1H, d, J = 6.5 Hz), 3.70 (1H, m), 3.64 (1H, dd, J = 11.5, 4.0 Hz), 3.59 (1H, dd, J = 11.5, 6.5 Hz). ¹H-NMR (500 MHz, acetone-d₆): δ 7.48 (2H, d, J = 8.5 Hz), 7.37 (2H, d, J = 8.5 Hz), 4.65 (1H, d, J = 6.5 Hz), 3.69 (1H, m), 3.62 (2H, overlap). ¹H-NMR (500 MHz, pyridine-d₅): δ 7.74 (2H, d, J = 8.5 Hz), 7.55 (2H, d, J = 8.5 Hz), 5.30 (1H, dd, J = 5.5, 3.0 Hz), 4.42 (1H, m), 4.39 (2H, overlap). ¹H-NMR (500 MHz, DMSO-d₆): δ 7.47 (2H, d, J = 8.5 Hz), 7.28 (2H, d, J = 8.5 Hz), 4.41 (1H, t, J = 5.0 Hz), 3.47 (1H, m), 3.42 (1H, m), 3.37 (1H, m). ¹³C-NMR (125 MHz, methanol-d₄): δ 142.9, 132.0, 132.0, 130.3, 130.3, 122.0, 76.5, 75.3, 64.3; HRESIMS refer to compounds 3 and 4.

(2R,3R)-3-(4-Bromophenyl)-2,3-dihydroxypropyl(1S,4R)-4,7,7-trimethyl-3-oxo-2-oxabicyclo[2.2.1]heptane-1-carboxylate (3a). The synthesis procedure of 3a was same with the M1a. Colorless oil; ¹H-NMR (500 MHz, methanol-d₄): δ 7.50 (2H, d, J = 8.5 Hz), 7.35 (2H, d, J = 8.5 Hz), 4.66 (1H, d, J = 5.0 Hz), 4.26 (1H, dd, J = 11.5, 4.0 Hz), 4.11 (1H, dd, J = 11.5, 7.0 Hz), 3.92 (1H, m), 2.46 (1H, m), 2.03 (1H, m), 1.97 (1H, m), 1.63 (1H, m), 1.09 (6H, overlap), 0.94 (3H, s). ¹H-NMR (500 MHz, DMSO-d₆): δ 7.51 (2H, d, J = 8.5 Hz), 7.31 (2H, d, J = 8.5 Hz), 4.56 (1H, d, J = 3.5 Hz), 4.12 (1H, dd, J = 11.0, 3.5 Hz), 3.97 (1H, dd, J = 11.0, 8.0 Hz), 3.79 (1H, m), 2.35 (1H, m), 1.97 (1H, m), 1.90 (1H, m), 1.54 (1H, m), 1.00 (6H, overlap), 0.82 (3H, s). ¹³C-NMR (125 MHz, methanol-d₄): δ 180.3, 168.7, 142.4, 132.3, 132.3, 129.9, 129.9, 122.3, 92.9, 74.7, 74.2, 67.6, 56.1, 55.4, 31.5, 29.9, 17.1, 17.0, 9.9; HRESIMS m/z 449.0570 [M + Na]⁺ (calcd for C₁₉H₂₃O₆BrNa 449.0562).

(2S,3S)-3-(4-Bromophenyl)-2,3-dihydroxypropyl(1S,4R)-4,7,7-trimethyl-3-oxo-2-oxabicyclo[2.2.1]heptane-1-carboxylate (4a). The synthesis procedure of 4a was same with the M1a. Colorless oil; ¹H-NMR (500 MHz, methanol-d₄): δ 7.50 (2H, d, J = 8.5 Hz), 7.34 (2H, d, J = 8.5 Hz), 4.67 (1H, d, J = 5.0 Hz), 4.31 (1H, dd, J = 11.5, 4.0 Hz), 4.04 (1H, dd, J = 11.5, 7.0 Hz), 3.91 (1H, m), 2.48 (1H, m), 2.04 (1H, m), 1.98 (1H, m), 1.63 (1H, m), 1.10 (3H, s), 1.09 (3H, s), 0.93 (3H, s). ¹H-NMR (500 MHz, DMSO-d₆): δ 7.51 (2H, d, J = 8.0 Hz), 7.31 (2H, d, J = 8.0 Hz), 4.56 (1H, d, J = 3.5 Hz), 4.19 (1H, dd, J = 11.5, 3.5 Hz), 3.88 (1H, dd, J = 11.5, 7.0 Hz), 3.79 (1H, m), 2.36 (1H, m), 1.97 (1H, m), 1.90 (1H, m), 1.54 (1H, m), 1.00 (6H, overlap), 0.82 (3H, s). ¹³C-NMR (125 MHz, methanol-d₄): δ 180.3, 168.7, 142.4, 132.3, 132.3, 129.9, 129.9, 122.3, 92.9, 74.7, 74.1, 67.5, 56.0, 55.4, 31.5, 30.0, 17.1, 17.0, 9.9; HRESIMS refer to compounds 3a.

(2S,3R)-3-(4-Bromophenyl)-2,3-dihydroxypropyl(1S,4R)-4,7,7-trimethyl-3-oxo-2-oxabicyclo[2.2.1]heptane-1-carboxylate (5a). The synthesis procedure of 5a was same with the M1a. Colorless oil; ¹H-NMR (500 MHz, methanol-d₄): δ 7.50 (2H, d, J = 8.5 Hz), 7.34 (2H, d, J = 8.5 Hz), 4.57 (1H, d, J = 7.0 Hz), 4.42 (1H, dd, J = 11.5, 3.5 Hz), 4.31 (1H, dd, J = 11.5, 7.0 Hz), 3.89 (1H, m), 2.48 (1H, m), 2.03 (1H, m), 1.98 (1H, m), 1.63 (1H, m), 1.10 (3H, s), 1.09 (3H, s), 0.94 (3H, s). ¹H-NMR (500 MHz, DMSO-d₆): δ 7.51 (2H, d, J = 8.5 Hz), 7.30 (2H, d, J = 8.5 Hz), 4.42 (1H, d, J = 6.0 Hz), 4.36 (1H, dd, J = 11.0, 3.0 Hz), 4.18 (1H, dd, J = 11.0, 6.5 Hz), 3.69 (1H, m), 2.38 (1H, m), 1.98 (1H, m), 1.91 (1H, m), 1.55 (1H, m), 1.02 (3H, s), 1.00 (3H, s), 0.83 (3H, s). ¹³C-NMR (125 MHz, methanol-d₄): δ 180.3, 168.9, 142.7, 132.2, 132.2, 130.2, 130.2, 122.3, 93.0, 75.0, 74.0, 67.7, 56.0, 55.4, 31.5, 30.0, 17.1, 17.0, 9.9; HRESIMS refer to compounds 3a.

(2R,3S)-3-(4-Bromophenyl)-2,3-dihydroxypropyl(1S,4R)-4,7,7-trimethyl-3-oxo-2-oxabicyclo[2.2.1]heptane-1-carboxylate (6a). The synthesis procedure of 6a was same with the M1a. Colorless oil; ¹H-NMR (500 MHz, methanol-d₄): δ 7.50 (2H, d, J = 8.5 Hz), 7.34 (2H, d, J = 8.5 Hz), 4.57 (1H, d, J = 6.5 Hz), 4.37 (1H, dd, J = 11.5, 4.0 Hz), 4.35 (1H, dd, J = 11.5, 6.0 Hz), 3.90 (1H, m), 2.48 (1H, m), 2.03 (1H, m), 1.97 (1H, m), 1.62 (1H, m), 1.10 (3H, s), 1.09 (3H, s), 0.94 (3H, s). ¹H-NMR (500 MHz, DMSO-d₆): δ 7.51 (2H, d, J = 8.5 Hz), 7.30 (2H, d, J = 8.5 Hz), 4.43 (1H, d, J = 6.5 Hz), 4.28 (1H, dd, J = 11.5, 3.5 Hz), 4.23 (1H, dd, J = 11.5, 6.5 Hz), 3.69 (1H, m), 2.37 (1H, m), 1.97 (1H, m), 1.91 (1H, m), 1.54 (1H, m), 1.00 (6H, overlap), 0.84 (3H, s). ¹³C-NMR (125 MHz, methanol-d₄): δ 180.3, 168.9, 142.7, 132.2, 132.2, 130.1, 130.1, 122.3, 93.0, 75.1, 74.0, 67.7, 56.0, 55.4, 31.5, 30.0, 17.1, 17.0, 9.9; HRESIMS refer to compounds 3a.

((2R,3R)-3-(4-Nitrophenyl)oxiran-2-yl)methanol and ((2S,3S)-3-(4-nitrophenyl)oxiran-2-yl) methanol. The asymmetric epoxidation was also carried out according to the Sharpless asymmetric Epoxidation. Colorless oil; the specific rotations were [α]²⁰_D +44 (c 0.1 methanol) and [α]²⁰_D −46 (c 0.1 methanol), respectively. ¹H-NMR (500 MHz, methanol-d₄): δ 8.21 (2H, d, J = 9.0 Hz), 7.53 (2H, d, J = 9.0 Hz), 4.00 (1H, d, J = 2.0 Hz), 3.89 (1H, dd, J = 13.0, 3.0 Hz), 3.71 (1H, dd, J = 13.0, 4.5 Hz), 3.17 (1H, m); ¹³C-NMR (125 MHz, methanol-d₄): δ 149.1, 146.6, 127.7, 127.7, 124.6, 124.6, 64.5, 62.3, 55.7; HRESIMS m/z 196.0604 [M + H]⁺ (calcd for C₉H₁₀O₄N 196.0608).

(1R,2S)-1-(4-Nitrophenyl)propane-1,2,3-triol and (1S,2R)-1-(4-nitrophenyl)propane-1,2,3-triol (7 and 8). A solution of ((2R,3R)-3-(4-nitrophenyl)oxiran-2-yl)methanol (80 mg, 0.25 mmol) (which was dissolved by THF) and acetic acid (0.5 ml) in water was stirred at 80 °C until the solution was dry. Through the analysis of HPLC, the ((2R,3R)-3-(4-nitrophenyl)oxiran-2-yl)methanol was completely hydrolyzed. The residue was separated by chiral column on preparative HPLC (n-hexane/isopropanol, 80 : 20, v\v) to afford 7 (50 mg) and 8 (30 mg). The hydrolysis of ((2S,3S)-3-(4-nitrophenyl)oxiran-2-yl)methanol was also produce compounds 7 and 8. Compounds 7 and 8 are known compounds and they were identified by comparing their spectral data with those of the known compounds.²⁹ Colorless oil; the specific rotations were [α]²⁰_D −14 (c 0.3 acetone) and [α]²⁰_D +15 (c 0.3 acetone), respectively. ¹H-NMR (500 MHz, methanol-d₄): δ 8.23 (2H, d, J = 8.5 Hz), 7.66 (2H, d, J = 8.5 Hz), 4.74 (1H, d, J = 7.0 Hz), 3.74 (1H, m), 3.68 (1H, dd, J = 11.5, 4.5 Hz), 3.65 (1H, dd, J = 11.5, 6.0 Hz). ¹H-NMR (500 MHz, DMSO-d₆): δ 8.17 (2H, d, J = 8.5 Hz), 7.60 (2H, d, J = 8.5 Hz), 4.72 (1H, d, J = 5.5 Hz), 3.52 (1H, m), 3.42 (2H, overlap). ¹H-NMR (500 MHz, acetone-d₆): δ 8.19 (2H, d, J = 8.5 Hz), 7.70 (2H, d, J = 8.5 Hz), 4.82 (1H, t, J = 6.0 Hz), 3.75 (1H, m), 3.67 (1H, dd, J = 11.0, 5.0 Hz), 3.63 (1H, dd, J = 11.0, 5.0 Hz). ¹³C-NMR (125 MHz, DMSO-d₆): δ 151.8, 146.4, 128.5, 128.5, 122.6, 122.6, 75.2, 72.9, 62.9; found 258.0619. HRESIMS m/z 258.0619 [M + HCOO]⁺ (calcd for C₁₀H₁₂O₇N 258.0620).

(2S,3R)-2,3-Dihydroxy-3-(4-nitrophenyl)propyl(1S,4R)-4,7,7-trimethyl-3-oxo-2-oxabicyclo[2.2.1]heptane-1-carboxylate (7a). The synthesis procedure of 7a was same with the M1a. Colorless oil; ¹H-NMR (500 MHz, methanol-d₄): δ 8.23 (2H, d, J = 8.5 Hz), 7.66 (2H, d, J = 8.5 Hz), 4.72 (1H, d, J = 7.0 Hz), 4.46 (1H, dd, J = 11.5, 3.5 Hz), 4.35 (1H, dd, J = 11.5, 6.5 Hz), 3.91 (1H, m), 2.50 (1H, m), 2.03 (1H, m), 1.98 (1H, m), 1.63 (1H, m), 1.11 (3H, s), 1.09 (3H, s), 0.94 (3H, s). ¹H-NMR (500 MHz, DMSO-d₆): δ 8.20 (2H, d, J = 9.0 Hz), 7.62 (2H, d, J = 9.0 Hz), 4.58 (1H, dd, J = 6.5, 3.5 Hz), 4.39 (1H, dd, J = 11.5, 3.0 Hz), 4.22 (1H, dd, J = 11.5, 6.5 Hz), 3.73 (1H, m), 2.39 (1H, m), 1.98 (1H, m), 1.91 (1H, m), 1.55 (1H, m), 1.03 (3H, s), 1.00 (3H, s), 0.83 (3H, s). ¹³C-NMR (125 MHz, methanol-d₄): δ 180.3, 168.9, 151.4, 148.8, 129.3, 129.3, 124.1, 124.1, 92.9, 74.8, 74.0, 67.6, 56.0, 55.4, 31.5, 29.9, 17.0, 17.0, 9.9; HRESIMS m/z 394.1496 [M + H]⁺ (calcd for C₁₉H₂₄O₈N 394.1494).

(2R,3S)-2,3-Dihydroxy-3-(4-nitrophenyl)propyl(1S,4R)-4,7,7-trimethyl-3-oxo-2-oxabicyclo[2.2.1]heptane-1-carboxylate (8a). The synthesis procedure of 8a was same with the M1a. Colorless oil; ¹H-NMR (500 MHz, methanol-d₄): δ 8.23 (2H, d, J = 8.5 Hz), 7.66 (2H, d, J = 8.5 Hz), 4.71 (1H, d, J = 7.5 Hz), 4.42 (1H, dd, J = 11.5, 4.0 Hz), 4.39 (1H, dd, J = 11.5, 5.5 Hz), 3.92 (1H, m), 2.50 (1H, m), 2.03 (1H, m), 1.97 (1H, m), 1.62 (1H, m), 1.10 (3H, s), 1.09 (3H, s), 0.95 (3H, s). ¹H-NMR (500 MHz, DMSO-d₆): δ 8.20 (2H, d, J = 9.0 Hz), 7.63 (2H, d, J = 9.0 Hz), 4.59 (1H, d, J = 7.0 Hz), 4.31 (1H, dd, J = 11.5, 3.5 Hz), 4.26 (1H, dd, J = 11.5, 6.5 Hz), 3.74 (1H, m), 2.38 (1H, m), 1.97 (1H, m), 1.91 (1H, m), 1.54 (1H, m), 1.01 (3H, s), 1.00 (3H, s), 0.84 (3H, s). ¹³C-NMR (125 MHz, methanol-d₄): δ 180.3, 168.9, 151.4, 148.8, 129.2, 129.2, 124.2, 124.2, 92.9, 74.9, 74.1, 67.6, 56.0, 55.4, 31.5, 30.0, 17.0, 17.0, 9.9; HRESIMS refer to compound 7a.

(1R,2R)-1-(4-Hydroxy-3-methoxyphenyl)propane-1,2,3-triol and (1S,2S)-1-(4-hydroxy-3-methoxyphenyl)propane-1,2,3-triol (9 and 10). Colorless oil; the specific rotations were [α]²⁰_D −19 (c 0.1 methanol) and [α]²⁰_D +23 (c 0.1 methanol), respectively. ¹H-NMR (500 MHz, methanol-d₄): δ 6.99 (1H, d, J = 1.5 Hz), 6.80 (1H, dd, J = 8.5, 1.5 Hz), 6.76 (1H, d, J = 8.5 Hz), 4.52 (1H, d, J = 6.5 Hz), 3.86 (3H, s), 3.66 (1H, m), 3.47 (1H, dd, J = 11.0, 4.0 Hz), 3.34 (1H, dd, J = 11.0, 6.5 Hz). ¹H-NMR (500 MHz, DMSO-d₆): δ 6.89 (1H, brs), 6.69 (2H, overlap), 4.38 (1H, d, J = 5.0 Hz), 3.74 (3H, s), 3.44 (1H, m), 3.31 (1H, dd, J = 11.0, 4.0 Hz), 3.14 (1H, dd, J = 11.0, 6.0 Hz). ¹³C-NMR (125 MHz, DMSO-d₆): δ 147.0, 145.3, 134.3, 119.1, 114.7, 111.0, 75.9, 72.9, 62.6, 55.6; HRESIMS m/z 237.0733 [M + Na]⁺ (calcd for C₁₀H₁₄O₅Na 237.0719). Compounds 9 and 10 are known compounds and they were identified by comparing their spectral data with those of the known compounds.⁷

4-((1S,2S)-1,2-Dihydroxy-3-(((1S,4R)-4,7,7-trimethyl-3-oxo-2-oxabicyclo[2.2.1]heptane-1-carbonyl)oxy)propyl)-2-methoxyphenyl(1S,4R)-4,7,7-trimethyl-3-oxo-2-oxabicyclo[2.2.1]heptane-1-carboxylate (10a). Compound 10 (3 mg, 0.014 mmol) solution in dry CH₂Cl₂ or THF was added to a solution of camphanic acid chloride (6 mg, 0.028 mmol), EDCI (5.3 mg, 0.028 mmol) and DMAP (1.7 mg, 0.014 mmol) at 0 °C with the protection of argon gas. The reaction was stirred at 0 °C for 1 h and was detected by TLC. Then, the reaction was quenched by dropwise addition of H₂O. The suspension was extracted with CH₂Cl₂. Removal of the solvent under reduced pressure and purification of the residue by preparative HPLC, eluting with 55% methanol/H₂O, gave the 10a 1.0 mg. Colorless oil; ¹H-NMR (500 MHz, methanol-d₄): δ 7.23 (1H, d, J = 1.5 Hz), 7.08 (1H, d, J = 8.0 Hz), 7.02 (1H, dd, J = 8.0, 1.5 Hz), 4.71 (1H, d, J = 5.0 Hz), 4.34 (1H, dd, J = 11.5, 4.0 Hz), 4.11 (1H, dd, J = 11.5, 6.5 Hz), 3.96 (1H, m), 3.86 (3H, s), 2.65 (1H, m), 2.49 (1H, m), 2.16 (2H, overlap), 2.02 (2H, overlap), 1.70 (1H, m), 1.63 (1H, m), 1.21 (3H, s), 1.14 (3H, s), 1.11 (3H, s), 1.09 (3H, s), 1.09 (3H, s), 0.94 (3H, s); HRESIMS m/z 575.2487 [M + H]⁺ (calcd for C₃₀H₃₉O₁₁ 575.2490).

Compound 11. White powder; ¹H-NMR (500 MHz, DMSO-d₆): δ 6.88 (2H, overlap), 6.64 (2H, brs), 5.41 (2H, overlap), 4.56 (1H, d, J = 6.0 Hz), 4.40 (1H, d, J = 7.5 Hz), 3.77 (3H, s), 3.73 (6H, s), 3.68 (2H, m), 3.64 (1H, m), 3.59 (1H, m), 3.46 (1H, m), 3.38 (2H, overlap), 3.17 (2H, overlap), 3.06 (3H, overlap). ¹³C-NMR (125 MHz, methanol-d₄): δ 147.9, 147.9, 146.6, 142.9, 135.3, 133.5, 131.3, 128.7, 115.7, 112.2, 104.2, 103.7, 103.7, 87.3, 82.0, 76.9, 76.5, 75.2, 74.2, 70.0, 62.7, 61.9, 61.0, 56.0, 56.0, 55.7, 53.1; HRESIMS m/z 607.2026 [M + Na]⁺ (calcd for C₂₇H₃₆O₁₄Na 607.2000).

Conclusions

For natural AGs, there was previously no efficient method to determine their absolute configurations. Our present study presents a convenient and quick method for determining the relative and absolute configurations that only depends on the chemical shift difference (Δδ_H3a–H3b) of the diastereotopic methylene protons (H-3) in ¹H NMR spectroscopy. In particular, the method is a good way to determine the limited amounts of natural AGs; such limited amounts prevent the use of other strategies. Remarkably, the empirical rule is invalid in CDCl₃. In addition, the introduction of an economic camphanoyl group with larger steric hindrance in this method may provide a reference for solving the stereochemistry problems of other flexible conformer compounds.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This project was funded by the National Natural Science Foundation of China (No. 81973194) and the Drug Innovation Major Project (No. 2018ZX09711001-008). We appreciate Ms. Y. H. Wang (Institute of Materia Medica, Chinese Academy of Medical Sciences and Peking Union Medical College) for testing the NMR spectra.

References

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Footnote

† Electronic supplementary information (ESI) available: 1D NMR, 2D NMR, and HRMS spectra. See DOI: 10.1039/d0ra09712h

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