Labdane-type diterpenoids from Croton laevigatus

Abstract

Sixteen new labdane-type diterpenoids (1–9, 11–17) were isolated from the twigs and leaves of Croton laevigatus, along with 15-hydroxylabda-7,13(E)-diene-17,12-olide (10), which is reported here as a natural product for the first time. Their structures were elucidated on the basis of extensive spectroscopic data interpretation, including UV, IR, NMR, and MS, and comparison with literature data. The structures of compounds 7, 8, 10, 14, 15, 16, and 17 were further confirmed by single-crystal X-ray diffraction analysis. The absolute configurations of 10–17 were determined by the CD exciton chirality method and supported by the single-crystal X-ray diffraction analysis of 10, 14, and 15. Crotonlaevins A (1) and B (2) are the reported first labda-type diterpenoids with a dodecahydronaphtho [1,2-c] furan moiety, and compounds 10–17 are the first reported labda-17,12-olide derivatives isolated from nature.

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Introduction

Plants of the genus Croton (Euphorbiaceae) are rich sources of structurally diverse diterpenoids, which have brine shrimp lethality,¹ antitumor,² antimycobacterial,³ antifungal,⁴ and anti-inflammatory⁵ activities. Within this genus, 21 species mainly grow in the southern Provinces of China,⁶ and some of them have been used in traditional Chinese medicine to treat dysmenorrhea, gastric ulcers, malaria, gastric cancer, and dysentery.^7–11 Croton laevigatus Vahl. is an arbor mainly distributed in the Yunnan, Guangdong and Hainan provinces of China. The twigs and leaves of Croton laevigatus, locally known as Bao-Long in Chinese, is a traditional Chinese medicine used for the treatment of bone fracture, malaria, and stomach ache.¹² In prior reports, six cembranoids and a neocrotocembraneic acid were isolated from this species by Zou's group, among which laevigatlactone B exhibited modest cytotoxicity against HeLa cells.⁸ Moreover, two diterpenoids with an unprecedented backbone,¹¹ and clerodane diterpenoids have also been discovered from this plant.¹³ As a continuation of the work on structurally interesting and biologically significant secondary metabolites of plant origin, we collected twigs and leaves of C. laevigatus from Yunnan Province and investigated the secondary metabolites of the EtOH extract. As a result, 16 new labdane-type diterpenoids (1–9, 11–17) and an additional new natural product (10) were obtained (Fig. 1). Herein, the isolation and structure elucidation of these compounds are reported.

Fig. 1 The structures of compounds1–17.

Results and discussion

Compound 1 was obtained as a colorless oil. Its molecular formula was established to be C₁₈H₃₀O₄ from the [M + Na]⁺ peak at m/z 333.2043 (calcd 333.2036) in the positive HRESIMS, indicating four degrees of unsaturation. The IR absorptions suggested the presence of hydroxyl (3398 cm⁻¹) and carbonyl (1709 cm⁻¹) functionalities. Its ¹H NMR spectroscopic data (Table 1) showed three tertiary methyls signals at δ_H 0.79, 0.96, and 2.18; two oxymethylene signals at δ_H 3.09 and 3.43 (d, J = 11.2 Hz, H₂-18) as well as 3.68 and 3.76 (d, J = 9.6 Hz, H₂-17); and one oxymethine signal at δ_H 4.43 (td, J = 5.2, 2.4 Hz, H-11). The ¹³C NMR spectroscopic data (Table 2) indicated 18 carbon signals, including two quaternary carbon signals at δ_C 35.9 and 37.5, two oxymethylene carbon signals at δ_C 71.5 and 78.5, one oxymethine carbon signal at δ_C 77.3, one oxygenated quaternary carbon signal at δ_C 80.1, and one ketone carbon signal at δ_C 208.1. The above observations indicated that 1 may be a nor-labdane-type diterpene, which was confirmed by the following analysis. The ¹H–¹H COSY experiment showed connectivity of three moieties: I: ¹CH₂–²CH₂–³CH₂; II: ⁵CH–⁶CH₂–⁷CH₂; and III: ⁹CH–¹¹CH(O)–¹²CH₂. The HMBC correlations from CH₃-19 (δ_H 0.79, 3H, s) to C-3 (δ_C 35.1), C-5 (δ_C 44.4), and C-18 (δ_C 15.7); from H₂-18 (δ_H 3.09 and 3.43, each 1H, d, J = 11.2 Hz) to C-3, C-5, and C-19 (δ_C 17.7); and from H₂-2 (δ_H 1.50 and 1.60, each 1H, m) to C-4 (δ_C 37.5) indicated that C-4 connected units I and II together. The HMBC correlations from CH₃-20 (δ_H 0.96, 3H, s) to C-1 (δ_C 41.4), C-5, and C-9 (δ_C 66.0) and from H₂-1b (δ_H 0.96, 1H, m) to C-9 suggested that C-10 (δ_C 35.9) connected units I and III together. Furthermore, the HMBC correlations from H-9 (δ_H 1.57, 1H, d, J = 2.0 Hz) to C-17 (δ_C 78.5), from H-11 (δ_H 4.43, 1H, td, J = 5.2, 2.4 Hz) and H₂-6 (δ_H 1.32 and 1.60, each 1H, m) to C-8 (δ_C 80.1), and from H₂-17 (δ_H 3.68 and 3.76, each 1H, d, J = 9.6 Hz) to C-7 (δ_C 33.3), C-9, and C-11 (δ_C 77.3) implied that C-8 connected units II and III together. The above spectra delineated some structural fragments corresponding to AB rings of labdane-type diterpene. Moreover, the established molecular formula suggested the presence of an excess of epoxy group functionality. Considering the HMBC correlations from H₂-17 to C-11, this epoxy group was positioned between C-11 and C-17, forming a tetrahydrofuran ring. The HMBC correlations from H₃-16 (δ_H 2.18, 3H, s) to C-12 (δ_C 50.9) and C-13 (δ_C 208.1) implied that an acetyl group bonded to C-12. The relative configuration of 1 was determined by the NOE difference spectra and ROESY experiments. In the ROESY and NOE difference spectra, the presence of correlations of H-11, H₂-17 and H₃-19 to H₃-20; H₂-18 to H-5, and H-9 to H-5 and H₂-12 demonstrated that H-5, 8-OH, H-9 and H₂-18 are α-oriented whereas H-11, H₂-17, H₃-19 and H₃-20 are β-oriented. Consequently, the structure of 1 was deduced as crotonlaevin A. Compound 2 was isolated as a white, amorphous powder. It was assigned to have a molecular formula of C₂₀H₃₂O₅ by HRESIMS, which gave a pseudo-molecular ion at m/z 370.2594 [M + NH₄]⁺ (calcd 370.2588). Spectroscopic data (Experimental section) of 2 were closely similar in structure to those of 1. The major difference was the presence of additional acetyl group signals (δ_H 2.02, 3H, s; δ_C 22.3, 170.0). Correspondingly, the oxygenated quaternary carbon signal C-8 was shifted downfield to δ_C 90.3 in 2, which indicated that 2 was an acetylated derivative of 1 at C-17; moreover, the 2D NMR experiments confirmed the proved structure. Therefore, the structure of 2 was identified as crotonlaevin B.

Table 1 ¹H NMR spectroscopic data of compounds 1, 3, 4, 7, 12, and 15

	1^a	3^a	4^a	7^b	12^a	15^a
Position	δH mult (J in Hz)	δH mult (J in Hz)	δH mult (J in Hz)	δH mult (J in Hz)	δH mult (J in Hz)	δH mult (J in Hz)
a Data were measured in CDCl3.b Data were measured in CD3OD.^1H assignments aided by DEPT,^1H–^1H COSY, HSQC, HMBC and ROESY experiments. (400 MHz).
1	0.96 m	0.94 m	0.89 td (13.2, 3.6)	0.96 td (13.2, 3.6)	1.09 td (12.8, 4.4)	0.96 td (12.8, 4.0)
	1.67 m	1.53 m	1.36 m	1.63 m	1.77 d (12.8)	1.72 m
2	1.50 m	1.54 m	1.48 m	1.55 m	1.56 m	1.50 m
	1.60 m	1.58 m	1.60 m	1.70 m	1.61 m	1.56 td (13.6, 3.2)
3	1.28 m	1.27 m	1.23 m	1.25 m	1.21 td (13.2, 4.4)	1.19 td (12.8, 4.0)
	1.45 m	1.50 m	1.45 m	1.51 m	1.51 m	1.46 m
5	1.43 m	1.44 m	1.34 m	1.43 m	1.33 dd (12.8, 4.8)	1.34 dd (5.2, 4.8)
6	1.32 m	1.62 m	1.31 m	1.29 m	2.12 m	2.08 m
	1.60 m	1.64 m	1.62 m	1.59 m	2.41 m	2.35 m
7	1.69 m	1.39 m	1.75 m	1.32 m	7.39 m	7.03 m
	2.05 m	2.09 dt (13.2, 3.2)	1.89 m	1.99 m
9	1.57 d (2.0)	1.72 t (5.2)	2.00 d (10.8)	2.01 m	2.32 m	2.47 m
11	4.43 td (5.2, 2.4)	2.35 m	6.83 dd (15.6, 15.6)	1.67 m	1.48 m	1.77 m
		2.66 dt (16.4, 4.4)		1.83 td (13.6, 2.8)	1.95 m	1.88 m
12	2.78 dd (16.0, 5.2)	6.61 t (6.8)	6.18 d (15.6)	4.09 t (3.5)	4.75 d (11.2)	3.98 dd (4.0, 4.0)
	3.10 dd (16.0, 5.2)
14		9.38 s		6.10 dd (18.2, 18.2)	6.13 d (7.6)	5.87 dd (17.2, 17.2)
15				4.98 d (11.5)	10.08 d (7.6)	5.21 dd (10.8, 0.8)
				5.08 d (18.2)		5.40 dd (17.2, 0.8)
16	2.18 s	1.77 s	2.27 s	1.19 s	2.22 s	1.40 s
17	3.68 d (9.6)	3.99 dd (11.2, 1.2)	1.26 s	4.28 d (12.4)
	3.76 d (9.6)	4.38 d (11.2)		4.44 d (12.4)
18	3.09 d (11.2)	3.12 d (10.8)	3.09 d (10.8)	3.00 d (11.1)	0.91 s	0.88 s
	3.43 d (11.2)	3.43 d (10.8)	3.42 d (10.8)	3.36 d (11.1)
19	0.79 s	0.76 s	0.76 s	0.73 s	0.93 s	0.90 s
20	0.96 s	0.93 s	1.02 s	0.85 s	0.77 s	0.75 s
17-OAc		2.13 s		2.10 s

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Table 2 ¹³C NMR spectroscopic data of compounds 1, 3, 4, 7, 8, and 11–17

carbon	1^a	3^a	4^a	7^b	8^a	11^a	12^a	13^c	14^d	15^a	16^a	17^a
a Data were measured in CDCl3.b Data were measured in CD3OD.c Data were measured in acetone-D6.d Data were measured in C5D5N. The assignments were based on DEPT, HSQC, and HMBC experiments. (100 MHz).
1	41.4, t	39.6, t	40.5, t	39.9, t	38.7, t	38.6, t	38.7, t	39.5, t	36.9, t	38.1, t	38.6, t	38.6, t
2	17.6, t	17.7, t	17.6, t	19.2, t	17.9, t	18.5, t	18.5, t	19.4, t	19.5, t	18.4, t	18.5, t	18.5, t
3	35.1, t	34.9, t	35.0, t	36.5, t	35.1, t	41.9, t	41.8, t	44.3, t	41.8, t	41.8, t	41.9, t	41.8, t
4	37.5, s	37.7, s	48.0, s	38.8, s	37.6, s	32.8, s	32.8, s	33.9, s	33.7, s	32.8, s	32.8, s	32.8, s
5	44.4, d	49.1, d	48.4, d	51.1, d	49.5, d	48.8, d	48.8, d	56.2, d	45.1, d	49.1, d	48.9, d	48.8, d
6	18.8, t	19.6, t	19.8, t	20.2, t	19.3, t	25.2, t	25.2, t	68.5, d	29.0, t	24.8, t	25.2, t	25.1, t
7	33.3, t	37.7, t	42.7, t	37.8, t	37.1, t	143.4, d	144.2, d	144.6, d	62.3, d	141.1, d	143.4, d	143.5, d
8	80.1, s	73.8, s	72.3, s	77.9, s	75.0, s	125.6, s	125.2, s	127.0, s	126.3, s	127.4, s	125.9, s	125.8, s
9	66.0, d	60.6, d	65.8, d	50.7, d	56.6, d	49.2, d	49.1, d	49.5, d	162.4, s	43.6, d	48.6, d	48.6, d
10	35.9, s	37.6, s	35.2, s	37.5, s	36.7, s	34.7, s	34.8, s	33.9, s	40.0, s	35.4, s	34.8, s	34.8, s
11	77.3, d	24.7, d	144.7, d	24.8, t	24.0, t	27.0, t	27.5, t	27.9, t	27.3, t	22.3, t	22.6, t	22.6, t
12	50.9, t	158.2, d	69.3, d	69.3, t	74.8, d	77.0, d	81.8, d	84.4, d	81.2, d	82.0, d	84.6, d	84.6, d
13	208.1, s	137.3, s	197.8, s	78.1, s	77.7, s	137.2, s	157.5, s	135.3, s	134.1, s	74.6, s	74.0, s	74.1, s
14		195.4, s		147.3, d	145.2, d	127.8, d	126.6, d	129.4, d	131.0, d	139.9, d	140.6, d	139.7, d
15				112.5, t	113.6, t	58.1, t	191.0, s	58.8, t	59.0, t	114.5, t	114.9, t	114.9, t
16	30.9, q	9.3, q	27.6, q	27.6, q	19.1, q	18.3, q	13.4, q	12.1, q	13.0, q	25.6, q	23.4, q	24.2, q
17	78.5, t	65.7, t	24.8, q	63.9, t	63.8, t	165.8, s	164.6, s	165.5, s	166.7, s	168.3, s	165.5, s	165.7, s
18	71.5, t	71.8, t	71.6, t	22.7, q	71.6, t	32.9, q	32.9, q	36.8, q	33.5, q	32.7, q	32.9, q	32.9, q
19	17.7, q	17.3, q	17.5, q	17.7, q	17.0, q	21.4, q	21.4, q	22.6, q	22.2, q	21.4, q	21.4, q	21.4, q
20	15.7, q	16.4, q	16.3, q	17.5, q	16.9, q	13.4, q	13.4, q	15.0, q	18.9, q	12.9, q	13.4, q	13.4, q
17-OAc		171.1, s		173.2, s	171.8, s
		20.9, q		21.1, q	21.2, q

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Compound 3 possessed a molecular formula of C₂₁H₃₄O₅ with five degrees of unsaturation based on the [M + NH₄]⁺ peak at m/z 384.2749 (calcd 384.2744) in the positive HRESIMS. The IR spectrum showed absorption bands for hydroxyl (3422 cm⁻¹), ester carbonyl (1735 cm⁻¹) and olefin (1676 cm⁻¹) functionalities. The ¹H and ¹³C NMR spectroscopic data (Tables 1 and 2) of 3 resembled those of 1 and 2, sharing the same AB rings, and also belongs to nor-labdane-type diterpene series. The structural feature was further corroborated by the 2D NMR experiments. A ¹H–¹H COSY experiment showed connectivity of a moiety: ⁹CH–¹¹CH₂–¹²CH=¹³C–¹⁶CH₃. The HMBC spectrum showed correlations of H-9 (δ_H 1.72, 1H, t, J = 5.2 Hz) to C-17 (δ_C 65.7) and C-20 (δ_C 16.4) and CH₃-20 (δ_H 0.93, s, 3H) to C-9 (δ_C 60.6), which revealed that the moiety was connected to the B ring at C-9. The ¹H NMR spectrum displayed one aldehyde group signal at δ_H 9.38 (s, CHO-14). The HMBC experiment established correlations from CHO-14 to C-12 (δ_C 158.2), C-13 (δ_C 137.3) and C-16 (δ_C 9.3), which indicated the presence of an aldehyde group at C-13. Moreover, the ¹H and ¹³C NMR spectroscopic data also showed an extra acetyl group signal at δ_H 2.13 (s, 3H; δ_C 20.9, 171.1). The HMBC experiment demonstrated correlations of H₂-17 (δ_H 3.99 dd, J = 11.2, 1.2 Hz; 4.38, d, J = 11.2 Hz) to C-21 (δ_C 171.1), which indicated the acetoxyl group attached to C-17 in 3. The NOESY spectrum showed correlations of H-9 to H-5 and H₂-17 to CH₃-20, which indicated that the H₂-17 was β-oriented, and the 8-OH and H-9 were α-oriented. The NOESY correlations of H₂-11 to CH₃-16 and CHO-14 to H-12 indicated that the olefinic bond at C-12 was E-type. Finally, the structure of 3 was assigned as crotonlaevin C.

Compound 4 had the molecular formula of C₁₈H₃₀O₃ with four degrees of unsaturation as determined by HRESIMS at m/z 312.2526 [M + NH₄]⁺ (calcd 312.2533). The IR absorptions revealed the presence of hydroxyl (3372 cm⁻¹), carbonyl (1693 cm⁻¹) and olefin (1658 cm⁻¹) functionalities. Analysis of the NMR spectroscopic data (Tables 1 and 2) of 4 established that 4 shared a closely similar structure with (11E)-14,15-bisnor-8α-hydroxy-11-labden-13-one.^15,16 The major difference was that the methyl group CH₃-18 signals (δ_H 1.02, 3H, s; δ_C 34.3) in the known compound were replaced by oxymethylene group CH₂-18 signals (δ_H 3.09 and 3.42, each 1H, d, J = 10.8 Hz; δ_C 71.6) in 4. The above observations identified 4 as an 18-oxygenated derivative of the known compound. Considering the established molecular formula and the diagnostic chemical shift value of C-18, a hydroxyl group should be bonded to C-18. The relative configuration and the conformation of the olefinic system Δ^11,12 provided by the NOE difference spectra and ROESY experiment were the same as those of (11E)-14,15-bisnor-8α-hydroxy-11-labden-13-one. In particular, the NOE enhancements of H-11 to H₃-16 and H₂-17 to H₃-20 indicated the β-orientation of H-17 and the E-type of Δ.^11,12 Thus, the structure of 4 was determined as crotonlaevin D.

Compound 5 was isolated as a colorless oil. It was assigned to have a molecular formula of C₂₀H₃₂O₅ by HRESIMS at m/z 370.2579 [M + NH₄]⁺ (calcd 370.2588). Comparison of its ¹H and ¹³C NMR spectroscopic data (Experimental section) with those of 4 indicated that the two compounds were structurally similar. The major difference was that the C-17 signal at δ_C 24.8 in 4 was replaced with an oxymethylene carbon signal at δ_C 67.9 in 5. Correspondingly, oxygenated methylene proton signals at δ_H 4.17 and 4.29 (each 1H, d, J = 12.0 Hz) in 5 were observed in the ¹H NMR spectrum. These observations suggested that the Me-17 in 4 was oxidized to an oxymethylene group in 5. The ¹H and ¹³C NMR spectra also showed additional acetyl group signals at δ_H 2.09 (3H, s) and δ_C 20.9, 171.2, indicating that the acetoxyl group was attached to C-17. This conclusion was confirmed by the HMBC correlation from H₂-17 to C-21. The above information indicated that 5 was a 17-acetylated derivative of 4, crotonlaevin E.

Compound 6 was obtained as a white, amorphous powder. Its molecular weight was determined by the peak at m/z 412.2690 in the positive HRESIMS ([M + NH₄]⁺, calcd 412.2694), corresponding to a molecular formula of C₂₂H₃₄O₆, which required six degrees of unsaturation. Compared with 5, the ¹H and ¹³C NMR spectroscopic data (Experimental section) of 6 showed extra acetyl group signals at δ_H 2.10 (3H, s) and δ_C 21.0, 171.1, indicating that it was an acetylated derivative of 5. In addition, the chemical shifts of CH₂-18 changed from δ_H 3.09 and 3.41 (each 1H, d, J = 10.8 Hz; δ_C 71.5) in 5 to δ_H 3.62 and 3.88 (each 1H, d, J = 10.8 Hz; δ_C 72.3) in 6, which were interpreted to represent an acetoxyl group at C-18. This conclusion was supported by the HMBC correlations from H₂-18 to the carbonyl carbon signal of the acetyl group. Therefore, 6 was a 18-acetylated derivative of 5, crotonlaevin F.

Compound 7 was obtained as colorless crystals (mixed solvents: petroleum ether/MeOH/CH₃CN), with a molecular formula of C₂₂H₃₆O₅ as determined by the [M + H]⁺ peak at m/z 381.2638 (calcd 381.2636) in the positive HRESIMS, indicating five degrees of unsaturation. The IR spectrum showed absorption bands due to hydroxyl (3328 cm⁻¹) and ester carbonyl (1735 cm⁻¹) functionalities. The ¹H and ¹³C NMR spectroscopic data (Tables 1 and 2) of 7 resembled those of 18-hydroxy-13-epi-manoyl oxide.¹⁷ The major difference was that the methyl H₃-17 signals and methylene group H₂-12 signals in the known compound were replaced by the oxymethylene group H₂-17 signals (δ_H 3.00 and 3.36, d, J = 11.1 Hz, each 1H; 4.28 and 4.44, d, J = 12.4 Hz, each 1H; δ_C 63.9) and the oxymethine group H-12 signals (δ_H 4.09, t, J = 3.5 Hz, 1H; δ_C 69.3) in 7. These observations suggested that 7 was a 12- and 17-oxygenated derivative of the known compound. Considering the established formula, a hydroxyl group should be present and bonded to C-12. Moreover, extra acetyl group signals (δ_H 2.10, s, 3H; δ_C 21.1, 173.2) were observed in the ¹H and ¹³C NMR spectra. H₂-17 showed HMBC correlations to C-7 (δ_C 37.8), C-8 (δ_C 77.9), and C-21 (δ_C 173.2, s), indicating that the acetoxyl group was located at C-17. The NOE difference spectra and ROESY experiments provided the correlations of H-9 to H-5, H-12 to H-14 and H₃-22, and H₂-17 to H-14 and H₃-20, indicating that H-12, H-14 and H₂-17 were β-oriented, whereas H-9 and 12-OH were α-oriented. This conclusion was in accordance with the known compound. However, there were no correlations of H₃-16 to H-9 or OH-12 observed in NOE difference spectrum and NOESY experiments. To confirm the structure and the stereochemistry, 7 was crystallized in petroleum ether/MeOH/CH₃CN to obtain a crystal of the monoclinic space group, which was analyzed by X-ray crystallography. Single-crystal X-ray diffraction analysis (Fig. 2A) showed that CH₃-16 was α-oriented. The absolute configuration was determined by the measurement of the Flack parameter, which is calculated during the structural refinement.^18,19 The final refinement on the Cu Kα data of the crystal of 7 resulted in a Flack parameter of 0.03 (6),¹⁸ allowing an unambiguous assignment of the absolute structure as shown. The seven chiral centers, C-4, C-5, C-8, C-9, C-10, C-12, and C-13, were thus determined to be R, R, R, R, S, S, and R, respectively. Thus, 7 was determined to be a manoyl oxide derivative and named crotonlaevin G.

Fig. 2 X-ray crystal structures of compounds 7 and 8.

HRESIMS, IR, and NMR spectroscopic data (Experimental section and Table 2) indicated that 8 has the same planar structure as 7, and the difference may be in the configuration. This conclusion was supported by NOE difference experiments. The NOE difference spectra showed NOE enhancements of H-9 to H-5 and H-12 as well as H₃-16 to H₂-17, which indicated that the OH-12 and H₃-16 were β-oriented and H-12 and H-14 were α-oriented. The structure was confirmed by single-crystal X-ray diffraction analysis (Fig. 2B). Considering their closely similar NMR patterns and their coexistence in the same plant, compounds 7 and 8 may possess the same absolute configurations at corresponding stereocenters, except for C-12 and C-13. Finally, the structure of 8 was elucidated as crotonlaevin H.

Compound 9, isolated as a colorless oil, possessed a molecular formula of C₂₀H₃₄O₄ as established by HRESIMS ([M + NH₄]⁺ at m/z 356.2793). The IR spectrum showed absorption bands due to hydroxyl (3414 cm⁻¹) and olefin (1709 cm⁻¹) functionalities. 9 shared similar ¹H and ¹³C NMR patterns (Experimental section) to those of 8. However, the acetyl group signal in 9 was missing. Hence, the structure of 9 was a 17-deacetylated derivative of 8 and was assigned as crotonlaevin I.

Compound 10, a white amorphous powder, presented a molecular formula of C₂₀H₃₀O₃ as determined by HRESIMS at m/z 341.2083 [M + Na]⁺ (calcd 341.2087) with six degrees of unsaturation. The ¹H and ¹³C NMR spectroscopic data (Experimental section) of 10 were in good agreement with literature data of 15-hydroxylabda-7,13(E)-diene-17,12-olide,²⁰ the semisynthesis of which has been already described. Here 10 was isolated as a new natural product, and its relative configuration was determined by NOE difference experiments. The NOE enhancements of H-9 to H-5 and H-12, H-12 to H-14, and H₂-15 to H₃-16 indicated that H-9 and H-12 were α-oriented and the olefinic bond at C-13 was E-type, which supported the previously reported relative configuration.

The absolute configuration of 10 was determined by applying the CD exciton chirality method.²¹ The UV spectrum of 10 showed a strong absorption at λ_max 229 nm (log ε 6.87) attributable to the α,β-unsaturated lactone (π–π*, Woodward Rule: 229 nm).²² Corresponding to this UV maximum, the CD spectrum of 10 (Fig. 3B) showed a positive Cotton effect at λ_max 234 nm (Δε + 0.21) and a negative Cotton effect at λ_max 214 nm (Δε − 0.0095) due to the transition interaction between two different chromophores of the α,β-unsaturated lactone and Δ^13,14 double bond. The above information demonstrated a positive chirality for 10 and the two aforementioned chromophores should be oriented clockwise in space²³ (Fig. 3B). Thus, the absolute configurations of 10 were established as 5S, 9R, 10S and 12S.

Fig. 3 X-ray crystal structures of compound 10a. CD and UV spectra of compounds 10–17 (in MeOH). The arrow denotes the electronic transition dipole of the chromophores for compounds 10 and 14.

To confirm the structure and the CD exciton chirality rule used to determine the absolute configuration, 10 was esterified with p-bromobenzoyl chloride, and the product 10a was crystallized in petroleum ether/EtOAc/MeOH to afford a crystal of the monoclinic space group P2(1), which was analyzed by X-ray crystallography (Fig. 3A). A bromine atom in the single-crystal determined the absolute configuration of 10 on the basis of Flack's method with Flack's parameter determined to be 0.008 (14).¹⁸ Single-crystal X-ray analysis confirmed the structure of 10.

Compound 11 gave a molecular formula of C₂₀H₃₀O₃, as determined by the observed pseudo-molecular ion at 319.2267 [M + H]⁺ (calcd 319.2268) in the HRESIMS. In the ¹H and ¹³C NMR spectra (Experimental section and Table 2), 11 showed similar signals to those observed for 10, except for changes at CH-12 (δ_H 5.16, 1H, dd, J = 11.2, 2.0 Hz; δ_C 77.0), C-13 (δ_C 137.2), CH-14 (δ_H 5.61, 1H, t, J = 6.8 Hz; δ_C 127.8), and CH₃-16 (δ_H 1.80, 3H, s; δ_C 18.3). These data suggested that 10 and 11 were a pair of cis-trans isomers at Δ,^13,14 and the olefinic bond in 11 at C-13 was Z-type. This inference was supported by the NOE enhancements of H-12 to H₂-15 and H-14 to H₃-16. The CD spectrum of 11 (Fig. 3B) also showed a split Cotton effect (λ_max 235 nm Δε + 0.078, λ_max 215 nm Δε − 0.079), which arose from the exciton coupling of the chromophores of the α,β-unsaturated lactone and Δ^13,14 double bond with positive chirality. These split CD signals suggested the absolute configurations of 11 were 5S, 9R, 10S and 12S. Accordingly, the structure of 11 was established to be crotonlaevin J.

Compound 12, obtained as an amorphous powder, was shown to have a molecular formula of C₂₀H₂₈O₃ on the basis of the HRESIMS at m/z 317.2112 [M + H]⁺ (calcd 317.2111). The UV spectrum showed absorption at λ_max 234 nm due to an α,β-unsaturated lactone, while the IR spectrum showed absorption bands attributable to ester carbonyl (1711 cm⁻¹) and olefin (1674 and 1638 cm⁻¹) functionalities. A combined analysis of ¹H and ¹³C NMR (Tables 1 and 2), HSQC, and HMBC spectra of 12 indicated that it was a closely related analogue of 10 sharing the same ABC rings and Δ.^13,14 The only structural difference was the aldehyde group (δ_H 10.08, d, J = 7.6 Hz; δ_C 191.0) in 12 instead of the oxymethylene group (δ_H 4.17, t, J = 3.0 Hz; δ_C 58.7) in 10. This conclusion was further confirmed by the HMBC observations, in which the key correlations were from CHO-15 to C-13 (δ_C 157.5) and C-16 (δ_C 13.4). The relative configuration of 12 supported by the NOE difference experiment was in accordance with that of 10. The CD spectra of 10 and 12 (Fig. 3B) showed a similar CD split pattern. Based on a biogenetic analogy and comparisons with molecular models, the absolute configurations of 12 were established as 5S, 9R, 10S and 12S. Consequently, the structure of 12 was identified as crotonlaevin K.

Compound 13 was isolated as a white, amorphous powder. Its positive HRESIMS gave a [M + Na]⁺ peak at m/z 357.2025 (calcd 357.2034), consistent with a molecular formula of C₂₀H₃₀O₄, which required six degrees of unsaturation. The UV spectrum showed the characteristic absorption maxima of α,β-unsaturated lactone at λ_max 226 nm. The IR spectrum revealed the presence of hydroxyl (3385 cm⁻¹), ester carbonyl (1705 cm⁻¹) and olefin (1647 cm⁻¹) functionalities. The ¹H and ¹³C NMR spectroscopic data (Experimental section and Table 2) proved that compounds 13 and 10 were similar in structure, with the only difference being the presence of an excess of oxymethine signals for CH-6 (δ_H 4.35 dt, J = 10.4, 2.8 Hz; δ_C 68.5) in 13. Considering the established formula, a hydroxyl group should be present and bonded to C-6 based on the HMBC correlations from H-6 to C-4 (δ_C 33.9) and C-8 (δ_C 127.0). The coupling constant and the NOE enhancements of H-6 to H₃-19 and H₃-20 indicated the β-orientation of H-6 and the α-orientation of HO-6. With a similar CD spectrum (Fig. 3B) to that of 10, the absolute configurations of 13 were determined to be 5S, 6S, 9R, 10R and 12S. 13 was assigned as crotonlaevin L.

Compound 14 showed the molecular formula of C₂₀H₃₀O₄, obtained from the [M + Na]⁺ peak at m/z 357.2034 (calcd 357.2036) in the positive HRESIMS. The IR spectrum exhibited absorption bands due to hydroxyl (3381, 3296 cm⁻¹), conjugated carbonyl (1683 cm⁻¹) and olefin (1625 cm⁻¹) functionalities. The ¹H and ¹³C NMR spectroscopic data of 14 (Experimental section and Table 2) were similar to those of 13, except for changes at CH₂-6 (δ_H 1.68, 1H, dd, J = 13.6, 4.4 Hz, δ_H 2.11, 1H, d, J = 13.6 Hz; δ_C 29.0), CH-7 (δ_H 5.25, 1H, d, J = 4.0 Hz; δ_C 62.3), C-8 (δ_C 126.3), and C-9 (δ_C 162.4). These data suggested that a hydroxyl group should be bonded to C-7 and a double bond was at C-8. This conclusion was confirmed by the HMBC correlations of H-7 to C-5 (δ_C 45.1) as well as C-9 and C-17 (δ_C 166.7). In the NOE difference spectra and ROESY experiment, the presence of the correlations between H-12 and H-14 and between H₂-15 and H₃-16 demonstrated the olefinic bond at C-13 to be E-type. Unfortunately, the NMR spectra could not give useful information to determine the relative configuration of H-7 and H-12. A single-crystal X-ray diffraction analysis (Fig. 4A) of 14 showed that H-5, OH-7, H-12, and H₃-18 were α-oriented, whereas H-7, H₃-19, and H₃-20 were β-oriented. The UV spectrum of 14 showed a strong absorption at λ_max 221 nm, which resulted from the α,β-unsaturated lactone (π–π*, Woodward rule: 229 nm).²² Corresponding to this UV absorption, the CD spectrum (Fig. 3B) of 14 showed a negative Cotton effect centering at λ_max 217 nm (Δε − 0.79), which indicated that the α,β-unsaturated lactone and the Δ^13,14 double bond should be oriented counterclockwise in space and a negative chirality for 14.²² The above information demonstrated that the absolute configurations of 14 were 5S, 7R, 10S and 12S. The final refinement on the Cu Kα data of the crystal of 14 resulted in a Flack parameter of −0.03(6),¹⁸ which confirmed the structure. Thus, the structure of 14 was characterized as crotonlaevin M.

Fig. 4 X-ray crystal structures of compounds 14–17.

Compound 15 was obtained as colorless crystals (mixed solvents: petroleum ether/EtOAc/acetone) and had the molecular formula of C₂₀H₃₀O₃ with six degrees of unsaturation as determined by HRESIMS at m/z 319.2257 [M + H]⁺ (calcd 319.2268). The UV absorption at λ_max 223 nm was ascribed to the α,β-unsaturated lactone. The IR absorptions revealed the presence of hydroxyl (3427 cm⁻¹), ester carbonyl (1707 cm⁻¹) and olefin (1641 cm⁻¹) functionalities. Detailed analysis of the NMR spectroscopic data (Tables 1 and 2) indicated that 15 was similar in structure to 10 bearing a α,β-unsaturated lactone. The ¹H NMR spectrum showed one monosubstituted vinyl group signal for H-14 (δ_H 5.87, 1H, dd, J = 17.2, 17.2 Hz) and H₂-15 (δ_H 5.21, 1H, dd, J = 10.8, 0.8 Hz; 5.40, dd, J = 17.2, 0.8 Hz). In the HMBC spectrum, H-14 showed correlations to C-12 (δ_C 82.0) and C-16 (δ_C 25.6), and H₂-15 showed the correlation to C-13 (δ_C 74.6), indicating that the monosubstituted vinyl group was at C-13. The H₃-16 (δ_H 1.40, 3H, s) showed HMBC correlations to C-12 and C-13, indicating that CH₃-16 bonded to C-13. Considering the diagnostic chemical shift value of C-13 and the established molecular formula, a hydroxyl group was at C-13. Moreover, the ¹H and ¹³C NMR data also showed one oxymethine signal for CH-12 (δ_H 3.98, dd, J = 4.0, 4.0 Hz; δ_C 82.0). The H-12 and H₃-20 had the same orientation, as supported by the NOE enhancements of H-12 to H₃-20. However, the relative configurations of OH-13 and CH₃-16 could not be determined from the NOE difference spectra. A single-crystal X-ray diffraction analysis (Fig. 4B) showed that H-5, H-9, CH₃-16, and CH₃-18 were α-oriented, and H-12, OH-13, CH₃-19, and CH₃-20 were β-oriented. The similar CD spectra (Fig. 3) of 15 and 14 suggested that the absolute configurations of 15 were 5S, 9R, 10S, 12R and 13S (a negative Cotton effect centering at λ_max 219 nm, Δε −0.44).²² The final refinement based on the Cu Kα data of the crystal of 15 resulted in a Flack parameter of 0.0(2),¹⁸ which confirmed the structure. Therefore, 15 was elucidated to be crotonlaevin N.

HRESIMS, UV, IR, CD, and NMR spectroscopic data (Experimental section and Table 2) of compounds 16 and 17 indicated that 16 shares a closely similar structural pattern as 17 and differs in the side chain. The changed proton and carbon signals of CH-14 and CH₃-16 were observed in the ¹H and ¹³C NMR spectra. Single-crystal X-ray diffraction analysis (Fig. 4C and D) showed that CH₃-16 was α-oriented and OH-13 was β-oriented in 16 and that CH₃-16 was β-oriented and OH-13 was α-oriented in 17, indicating that 16 and 17 were a pair of epimers at C-13. Compounds 16 and 17 showed similar CD spectra (Fig. 3B) to that of 10. Therefore, the absolute configurations of 16 were determined as 5S, 9R, 10S, 12S, and 13S, and 16 was named as crotonlaevin O. The absolute configurations of 17 were determined as 5S, 9R, 10S, 12S, and 13R, and 17 was named as crotonlaevin P.

Conclusions

In this study, sixteen new labdane-type diterpenoids (1–9, 11–17) were obtained. To the best of our knowledge, crotonlaevins A (1) and B (2) are the first labda-type diterpenoids with a dodecahydronaphtho[1,2-c] furan moiety. Although a few labda-17,12-olide derivatives have been semi-synthetized,^20,24 compounds 10–17 are relatively uncommon, and they are the first labda-17,12-olide derivatives to be isolated from nature and reported in the literature.

Experimental

General experimental procedures

Melting points were determined on an X-4 Digital Display micro-melting point apparatus, uncorrected. Optical rotations were measured on a Perkin-Elmer Model 341 polarimeter with a 1 dm cell. UV spectra were measured on a Shimadzu UV-260 spectrophotometer. CD spectra were recorded on an Olis DSM 1000 circular dichroism spectrophotometer. IR spectra were recorded on a Nicolet NEXUS 670 FT-IR spectrometer over the range of 400–4000 cm⁻¹. NMR spectra were conducted on a Bruker AM-400BB (400 MHz) spectrometer (δ in ppm rel. to TMS, J in Hz). HRESIMS determinations were run on a Bruker APEX II mass spectrometer. Single-crystal X-ray diffractions were recorded on Bruker Smart Apex CCD diffractometer using graphite-monochromated Mo Kα or Cu Kα radiation. Sephadex LH-20 (Amersham Pharmacia Biotech), RP-C18 silica gel (150–200 mesh, Merck), and silica gel (200–300 mesh, Qingdao Marine Chemical Factory) were used for column chromatography (CC). Thin-layer chromatography (TLC): silica gel GF₂₅₄ (10–40 μm; Qingdao Marine Chemical Factory); detection under UV light and visualized by spraying with 5% H₂SO₄ in C₂H₅OH (v/v), followed by heating. Analytical TLC was provided to follow the separation and check the purity of isolated compounds.

Plant material

The twigs and leaves of Croton laevigatus Vahl. were collected from Yunnan Province, P. R. China, on October 2009, and authenticated by Prof. Guo-Da Tao of Xishuangbanna Tropical Botanical Garden, Chinese Academy of Sciences. A voucher specimen (no. 200910CL) was deposited in the Xishuangbanna Tropical Botanical Garden, Chinese Academy of Sciences.

Extraction and isolation

The air-dried leaves and twigs of Croton laevigatus (8.0 kg) were crushed into a powder by a wood crusher, then extracted with EtOH (95%, v/v) three times (each for 3 hours) at 40 °C and concentrated in vacuo to give a crude extract (809 g). The crude extract was suspended in H₂O (3.0 L) and then extracted with CHCl₃ (3 × 2.0 L) and EtOAc (3 × 2.0 L), successively. The CHCl₃-soluble and EtOAc-soluble fractions were combined due to similar constituents on TLC. The combined extract (244.7 g) was subjected to silica gel column chromatography (CC) (1.5 kg), eluting with a CHCl₃–MeOH (1 : 0–2 : 1, CH₃OH) gradient system to afford fractions 1–6 under the aid of TLC examination. Fraction 1 was not further purified due to the presence of fatty materials as the main constituents on the basis of TLC analysis. Fraction 2 (43.4 g) was applied to silica gel eluting with petroleum ether–acetone (40 : 1–1 : 1) to provide fractions 2a–2d. Fraction 2c (23.5 g) was repeatedly chromatographed on silica gel (petroleum ether–EtOAc 20 : 1–1 : 1) to give fractions 2c1–2c3. Fraction 2c1 (2.6 g) was separated by CC over a silica gel (petroleum ether–acetone, 10 : 1, 0 : 1), MCI gel (MeOH–H₂O, 0 : 1–2 : 1), RP-18 silica gel (MeOH–H₂O, 2 : 1–1 : 1), and silica gel (petroleum ether–EtOAc, 9 : 1–1 : 1) to give 10 (13 mg) and 11 (16 mg). The fraction 2c2 (4.6 g) was chromatographed over a MCI gel (MeOH–H₂O, 0 : 1–2 : 1), RP-18 silica gel (MeOH–H₂O, 1 : 1–1.5 : 1), and silica gel (300–400 mesh, petroleum ether–EtOAc, 10 : 1) to afford 15 (35 mg), 16 (27 mg), and 17 (21 mg). Fraction 3 (51.1 g) was rechromatographed over a Sephadex LH-20 column (CH₂Cl₂–CH₃OH 1 : 1) and silica gel (CHCl₃–acetone 40 : 1–2 : 1, 0 : 1) to provide fractions 3a–3f. Fraction 3b (6.7 g) was subjected to a column of MCI gel (MeOH–H₂O, 0 : 1–2 : 1), RP-18 silica gel (MeOH–H₂O, 1 : 2–2 : 1), and silica gel CC (petroleum ether–EtOAc, 1 : 1, 0 : 1) to yield 6 (15 mg). A similar isolation procedure as used for fraction 3b was adopted for fraction 3d (8.1 g) to afford 2 (4 mg), 13 (8 mg) and 14 (16 mg). 5 (24 mg) was isolated from fraction 3e (3.4 g) after preparative TLC (petroleum ether–acetone, 1.5 : 1). Fraction 4 (11.3 g) was fractionated using silica gel (petroleum ether–acetone, 10 : 1–2 : 1, 0 : 1), then subjected to a column of RP-18 silica gel (MeOH–H₂O, 1 : 1–2 : 1), and further purified on Sephadex LH-20 (CH₂Cl₂–MeOH, 1 : 1) to obtain 12 (21 mg). After CC over silica gel developed with CHCl₃–acetone (10 : 1–2 : 1, 0 : 1), fraction 5 (93.4 g) afforded fractions 5a–5c. 3 (5 mg), 4 (18 mg) and 8 (8 mg) were obtained from fraction 5a (5.1 g) after repeated silica gel CC eluting with petroleum ether–acetone (3 : 1, 0 : 1). Fraction 5b (8.9 g) was chromatographed on silica gel (petroleum ether–acetone, 6 : 1–2 : 1, 0 : 1), followed by RP-18 silica gel (MeOH–H₂O, 1 : 1.5) and silica gel (300–400 mesh, petroleum ether–acetone, 3 : 1, 0 : 1), to give 7 (29 mg) and 9 (9 mg). Fraction 6 (11.2 g) was purified on a column of MCI gel (MeOH–H₂O 1 : 2–2 : 1), RP-18 silica gel (MeOH–H₂O, 1 : 1.5), and silica gel (300–400 mesh, petroleum ether–EtOAc, 1 : 3, 0 : 1) to afford 1 (6 mg).

Spectroscopic data of compounds 1–17

Crotonlaevin A (1): colorless oil; [α]²⁶_D −11.43 (c 0.35, in CHCl₃); IR (KBr) ν_max 3398, 2930, 2872, and 1709 cm⁻¹; ¹H NMR and ¹³C NMR data (CDCl₃, see Tables 1 and 2); HRESIMS m/z 333.2043 [M + Na]⁺ (calcd for C₁₈H₃₀O₄Na ⁺, 333.2036).

Crotonlaevin B (2): white amorphous powder; [α]²⁶_D −23.33 (c 0.30, in CHCl₃); IR (KBr) ν_max 3378, 2933, 2873, 1724, and 1700 cm⁻¹; ¹H NMR (CDCl₃, 400 MHz) δ_H: 4.46 (1H, td, J = 5.2, 2.8 Hz, H-11), 4.40 (1H, d, J = 10.8 Hz, H₂-17a), 3.75 (1H, d, J = 10.8 Hz, H₂-17b), 3.40 (1H, d, J = 8.8 Hz, H₂-18a), 3.14 (1H, d, J = 8.8 Hz, H₂-18b), 2.92(1H, dd, J = 15.6, 8.4 Hz, H₂-12a), 2.58 (1H, dd, J = 15.6, 8.4 Hz, H₂-12b), 2.46 (1H, dt, J = 14.0, 5.2 Hz, H₂-7a), 1.99 (1H, m, H₂-7b), 2.19 (3H, s, H₃-16), 2.02 (3H, s, H₃-22), 1.86 (1H, d, J = 2.0 Hz, H-9), 1.69 (1H, m, H₂-1a), 0.98 (1H, m, H₂-1b), 1.60 (1H, m, H₂-2a), 1.52 (1H, m, H₂-2b), 1.60 (1H, m, H₂-6a), 1.32 (1H, m, H₂-6b), 1.50 (1H, m, H-5), 1.44 (1H, m, H₂-3a), 1.32 (1H, m, H₂-3b), 0.99 (3H, s, H₃-20), 0.80 (3H, s, H₃-19); ¹³C NMR (CDCl₃, 100 MHz) δ_C: 207.1 (C-13, s), 170.0 (C-21, s), 90.3 (C-8, s), 76.4 (C-11, d), 74.6 (C-17, t), 71.7 (C-18, t), 63.6 (C-9, d), 50.7 (C-12, t), 43.5 (C-5, d), 41.5 (C-1, t), 37.4 (C-4, s), 35.9 (C-10, s), 35.1 (C-3, t), 29.0 (C-7, t), 30.9 (C-14, q), 22.3 (C-22 q), 18.5 (C-6, t), 17.6 (C-19, q), 17.5 (C-2, t), 15.9 (C-20, q); HRESIMS m/z 370.2594 [M + NH₄]⁺ (calcd for C₂₀H₃₆O₅N⁺, 370.2588).

Crotonlaevin C (3): colorless oil; [α]²⁶_D +12.15 (c 0.11, in CHCl₃); IR (KBr) ν_max 3422, 2926, 2869, 1735, and 1676 cm⁻¹; ¹H NMR and ¹³C NMR data (CDCl₃, see Tables 1 and 2); HRESIMS m/z 384.2749 [M + NH₄]⁺ (calcd for C₂₁H₃₈O₅N⁺, 384.2744).

Crotonlaevin D (4): colorless oil; [α]²⁶_D +16.67 (c 0.24, in CHCl₃); IR (KBr) ν_max 3372, 2933, 2882, 2862, 1693, and 1658 cm⁻¹; ¹H NMR and ¹³C NMR data (CDCl₃, see Tables 1 and 2); HRESIMS m/z 312.2526 [M + NH₄]⁺ (calcd for C₁₈H₃₄O₃N⁺, 312.2533).

Crotonlaevin E (5): colorless oil; [α]²⁶_D +11.11 (c 0.54, in CHCl₃); IR (KBr) ν_max 3443, 2929, 2874, 1736, and 1667 cm⁻¹; ¹H NMR (CDCl₃, 400 MHz) δ_H: 6.84 (1H, dd, J = 15.6, 15.6 Hz, H-11), 6.21 (1H, d, J = 15.6 Hz, H-12), 4.29 (1H, d, J = 12.0 Hz, H₂-17a), 4.17 (1H, d, J = 12.0 Hz, H₂-17b), 3.41 (1H, d, J = 10.8 Hz, H₂-18a), 3.09 (1H, d, J = 10.8 Hz, H₂-18b), 2.25 (3H, s, H₃-16), 2.15 (1H, m, H-9), 2.09 (3H, s, H₃-22), 2.04 (1H, m, H₂-7a), 1.46 (1H, m, H₂-7b), 1.64 (1H, m, H₂-6a), 1.28 (1H, m, H₂-6b), 1.61 (1H, m, H₂-2a), 1.49 (1H, m, H₂-2b), 1.43 (1H, m, H₂-3a), 1.24 (1H, m, H₂-3b), 1.35 (1H, m, H-5), 1.32 (1H, m, H₂-1a), 0.89 (1H, td, J = 12.4, 2.8 Hz, H₂-1b), 1.04 (3H, s, H₃-20), 0.75 (3H, s, H₃-19); ¹³C NMR (CDCl₃, 100 MHz) δ_C: 197.5 (C-13, s), 171.2 (C-21, s), 143.5 (C-11, d), 134.7 (C-12, d), 73.2 (C-8, s), 71.5 (C-18, t), 67.9 (C-17, t), 64.8 (C-9, d), 48.5 (C-5, d), 40.4 (C-1, t), 37.6 (C-4, s), 37.4 (C-10, s), 37.1 (C-7, t), 34.9 (C-3, t), 27.9 (C-16, q), 20.9 (C-22, q), 19.4 (C-6, t), 17.6 (C-2, t), 17.4 (C-19, q), 16.4 (C-20, q); HRESIMS m/z 312.2526 [M + NH₄]⁺ (calcd for C₁₈H₃₄O₃N⁺, 312.2533).

Crotonlaevin F (6): colorless oil; [α]²⁶_D +23.08 (c 0.39, in CHCl₃); IR (KBr) ν_max 3470, 2933, 2877, 1737, and 1670 cm⁻¹; ¹H NMR (CDCl₃, 400 MHz) δ_H: 6.84 (1H, dd, J = 15.6, 15.6 Hz, H-11), 6.24 (1H, d, J = 15.6 Hz, H-12), 4.29 (1H, d, J = 11.6 Hz, H₂-17a), 4.18 (1H, d, J = 11.6 Hz, H₂-17b), 3.88 (1H, d, J = 10.8 Hz, H₂-18a), 3.62 (1H, d, J = 10.8 Hz, H₂-18b), 2.26 (3H, s, H₃-16), 2.16 (1H, d, J = 11.2 Hz, H-9), 2.10 (3H, s, H₃-24), 2.07 (3H, s, H₃-22), 2.05 (1H, m, H₂-7a), 1.37 (1H, m, H₂-7b), 1.60 (1H, m, H₂-2a), 1.47 (1H, m, H₂-2b), 1.58 (1H, m, H₂-6a), 1.36 (1H, m, H₂-6b), 1.35 (1H, m, H₂-1a), 0.87 (1H, m, H₂-1b), 1.34 (2H, m, H₂-3), 1.29 (1H, m, H-5), 1.05 (3H, s, H₃-20), 0.84 (3H, s, H₃-19); ¹³C NMR (CDCl₃, 100 MHz) δ_C: 197.3 (C-13, s), 171.2 (C-21, s), 171.1 (C-23, s), 143.1 (C-11, d), 134.7 (C-12, d), 73.1 (C-8, s), 72.3 (C-18, t), 67.9 (C-17, t), 64.7 (C-9, d), 49.4 (C-5, d), 40.3 (C-1, t), 37.4 (C-10, s), 37.1 (C-7, t), 36.5 (C-4, s), 35.4 (C-3, t), 28.1 (C-16, q), 21.0 (C-24, q), 20.9 (C-22, q), 19.6 (C-6, t), 17.4 (C-2, t), 17.3 (C-19, q), 16.3 (C-20, q); HRESIMS m/z 412.2690 [M + NH₄]⁺ (calcd for C₂₂H₃₈O₆N⁺, 412.2694).

Crotonlaevin G (7): colorless crystal (mixed solvents: petroleum ether/MeOH/CH₃CN); mp 202–203 °C; [α]²⁶_D +4.69 (c 0.64, in CHCl₃); IR (KBr) ν_max 3328, 2942, 2876, and 1735 cm⁻¹; ¹H NMR and ¹³C NMR data (CD₃OD, see Tables 1 and 2); HRESIMS m/z 381.2638 [M + H]⁺ (calcd for C₂₂H₃₇O₅⁺, 381.2636).

Crotonlaevin H (8): colorless crystal (mixed solvents: petroleum ether/CH₂Cl₂/EtOAc); mp 142–143 °C; [α]²⁶_D +9.09 (c 0.11, in CHCl₃); IR (KBr) ν_max 3399, 2932, 2873, 1733, and 1716 cm⁻¹; ¹H NMR (CDCl₃, 400 MHz) δ_H: 5.91 (1H, dd, J = 17.6, 17.6 Hz, H-14), 5.25 (1H, d, J = 17.6, Hz, H₂-15a), 5.13 (1H, d, J = 10.8 Hz, H₂-15b), 4.32 (2H, d, J = 4.8, Hz, H₂-17), 3.65 (1H, dd, J = 9.2, 8.0 Hz, H-12), 3.43 (1H, d, J = 10.8 Hz, H₂-18a), 3.10 (1H, d, J = 10.8 Hz, H₂-18b), 2.12 (3H, s, H₃-22), 2.14 (1H, m, H₂-7a), 1.28 (1H, m, H₂-7b), 1.91 (1H, dd, J = 10.8, 5.2 Hz, H₂-11a), 1.54 (1H, m, H₂-11b), 1.67 (1H, m, H₂-2a), 1.57 (1H, m, H₂-2b), 1.64 (1H, m, H₂-1a), 0.89 (1H, m, H₂-1b), 1.61 (1H, m, H₂-6a), 1.29 (1H, m, H₂-6b), 1.57 (1H, m, H-9), 1.44 (1H, td, J = 13.2, 4.8 Hz, H₂-3a), 1.25 (1H, m, H₂-3b), 1.32 (1H, m, H-5), 1.30 (3H, s, H₃-16), 0.87 (3H, s, H₃-20), 0.74 (3H, s, H₃-19); ¹³C NMR data (CDCl₃, see Table 2); HRESIMS m/z 381.2638 [M + H]⁺ (calcd for C₂₂H₃₇O₅⁺, 381.2636).

Crotonlaevin I (9): colorless oil; [α]²⁶_D +15.71 (c 0.13, in CHCl₃); IR (KBr) ν_max 3414, 2929, 2873, and 1709 cm⁻¹; ¹H NMR (CDCl₃, 400 MHz) δ_H: 5.88 (1H, dd, J = 17.2, 17.2 Hz, H-14), 5.30 (1H, d, J = 17.2, Hz, H₂-15a), 5.17 (1H, d, J = 10.8 Hz, H₂-15b), 3.95 (1H, dd, J = 8.4, 7.6 Hz, H-12), 3.49 (2H, s, Hz, H₂-17), 3.42 (1H, d, J = 10.8 Hz, H₂-18a), 3.11 (1H, d, J = 10.8 Hz, H₂-18b), 2.31 (1H, m, H₂-7a), 1.24 (1H, m, H₂-7b), 1.72 (1H, m, H₂-6a), 1.30 (1H, m, H₂-6b), 1.69 (1H, m, H₂-11a), 1.66 (1H, m, H₂-11b), 1.67 (1H, m, H-9), 1.66 (1H, m, H₂-2a), 1.51 (1H, m, H₂-2b), 1.47 (1H, m, H₂-3a), 1.27 (1H, m, H₂-3b), 1.42 (1H, m, H₂-1a), 1.05 (1H, td, J = 11.2, 3.6 Hz, H₂-1b), 1.31 (1H, m, H-5), 1.29 (3H, s, H₃-16), 0.76 (6H, s, H₃-19 and H₃-20); ¹³C NMR (CDCl₃, 100 MHz) δ_C: 140.6 (C-14, d), 114.1 (C-15, t), 83.9 (C-8, s), 82.1 (C-12, d), 74.5 (C-13, s), 71.9 (C-18, t), 60.9 (C-17, t), 60.0 (C-9, d), 50.3 (C-5, d), 38.9 (C-1, t), 37.4 (C-10, s), 36.3 (C-4, s), 35.5 (C-3, t), 33.7 (C-7, t), 24.7 (C-11, t), 24.7 (C-16, q), 19.8 (C-6, t), 17.6 (C-2, t), 17.0 (C-19, q), 15.1 (C-20, q); HRESIMS m/z 356.2793 [M + NH₄]⁺ (calcd for C₂₀H₃₈NO₄⁺, 356.2795).

15-Hydroxylabda-7,13(E)-diene-17,12-olide (10): white amorphous powder; [α]²⁶_D −2.08 (c 0.48, in CHCl₃); UV λ_max nm (log ε): 229 (6.87); CD (MeOH) λ_max nm (Δε): 214 (−0.0095), 234 (+0.21), 268 (−0.0084); ¹H NMR (CDCl₃, 400 MHz) δ_H: 7.27 (1H, d, J = 1.6 Hz, H-7), 5.68 (1H, d, J = 6.0 Hz, H-14), 4.58 (1H, d, J = 11.6 Hz, H-12), 4.17 (2H, t, J = 3.0 Hz, H₂-15), 2.33 (1H, m, H₂-6a), 2.05 (1H, m, H₂-6b), 2.23 (1H, d, J = 12.8 Hz, H-9), 1.90 (1H, d, J = 13.2 Hz, H₂-11a), 1.45 (1H, m, H₂-11b), 1.70 (1H, m, H₂-1a), 1.02 (1H, td, J = 12.4, 4.0 Hz, H₂-1b), 1.66 (3H, s, H₃-16), 1.46 (2H, m, H₂-2), 1.42 (1H, m, H₂-3a), 1.15 (1H, td, J = 16.0, 4.0 Hz, H₂-3b), 1.27 (1H, dd, J = 12.0, 4.8 Hz, H-5), 0.88 (3H, s, H₃-19), 0.85 (3H, s, H₃-18), 0.71 (3H, s, H₃-20); ¹³C NMR (CDCl₃, 100 MHz) δ_C: 165.8 (C-17, s), 143.1 (C-7, d), 135.5 (C-13, s), 127.4 (C-14, d), 125.7 (C-8, s), 83.7 (C-12, d), 58.7 (C-15, t), 48.9 (C-9, d), 48.7 (C-5, d), 41.7 (C-3, t), 38.5 (C-1, t), 34.6 (C-10, s), 32.7 (C-18, q), 32.7 (C-4, s), 27.1 (C-11, t), 25.0 (C-6, t), 21.3 (C-19, q), 18.4 (C-2, t), 13.3 (C-20, q), 12.0 (C-16, q); HRESIMS m/z 341.2083 [M + Na]⁺ (calcd for C₂₀H₃₀O₃Na⁺, 341.2087).

10a:¹H NMR (CDCl₃, 400 MHz) δ_H: 7.91 (2H, d, J = 8.4 Hz, H-22, 26), 7.59 (2H, d, J = 8.4 Hz, H-23, 25), 7.36 (1H, ddd, J = 4.8, 2.8, 2.8 Hz, H-7), 5.83 (1H, t, J = 6.8 Hz, H-14), 4.66 (1H, d, J = 11.8 Hz, H-12), 4.90 (2H, t, J = 6.0 Hz, H₂-15), 2.39 (1H, m, H₂-6a), 2.12 (1H, m, H₂-6b), 2.27 (1H, m, H-9), 1.88 (1H, ddd, J = 14.2, 3.6, 1.6 Hz, H₂-11a), 1.58 (1H, m, H₂-11b), 1.78 (1H, m, H₂-1a), 1.02 (1H, td, J = 9.2, 4.0 Hz, H₂-1b), 1.82 (3H, s, H₃-16), 1.53 (2H, m, H₂-2), 1.48 (1H, m, H₂-3a), 1.21 (1H, td, J = 12.8, 4.0 Hz, H₂-3b), 1.34 (1H, dd, J = 12.0, 5.2 Hz, H-5), 0.93 (3H, s, H₃-19), 0.91 (3H, s, H₃-18), 0.77 (3H, s, H₃-20).

Crotonlaevin J (11): white amorphous powder; [α]²³_D −24.00 (c 0.25, in CHCl₃); IR (KBr) ν_max 3417, 2926, 2867, 1711, and 1639 cm⁻¹; UV λ_max nm (log ε): 229 (7.03); CD (MeOH) λ_max nm (Δε): 215 (−0.079), 235 (+0.078), 268 (−0.025); ¹H NMR (CDCl₃, 400 MHz) δ_H: 7.35 (1H, d, J = 2.0 Hz, H-7), 5.61 (1H, t, J = 6.8 Hz, H-14), 5.16 (1H, dd, J = 11.2, 2.0 Hz, H-12), 4.22 (1H, dd, J = 12.8, 7.6 Hz, H₂-15a), 4.14 (1H, dd, J = 12.8, 7.6 Hz, H₂-15b), 2.39 (1H, m, H₂-6a), 2.10 (1H, m, H₂-6b), 2.30 (1H, m, H-9), 1.80 (3H, s, H₃-16), 1.75 (1H, m, H₂-1a), 1.07 (1H, td, J = 12.8, 4.0 Hz, H₂-1b), 1.71 (1H, m, H₂-11a), 1.65 (1H, m, H₂-11b), 1.59 (1H, m, H₂-2a), 1.52 (1H, m, H₂-2b), 1.48 (1H, m, H₂-3a), 1.20 (1H, td, J = 12.8, 4.0 Hz, H₂-3b), 1.32 (1H, dd, J = 12.0, 4.8 Hz, H-5), 0.93 (3H, s, H₃-19), 0.90 (3H, s, H₃-18), 0.77 (3H, s, H₃-20); ¹³C NMR (CDCl₃, see Table 2); HRESIMS m/z 319.2267 [M + H]⁺ (calcd for C₂₀H₃₁O₃⁺, 319.2268).

Crotonlaevin K (12): white amorphous powder; [α]²³_D +6.25 (c 0.32, in CHCl₃); IR (KBr) ν_max 2923, 2857, 1711, 1674, and 1638 cm⁻¹; UV λ_max nm (log ε): 234 (7.32); CD (MeOH) λ_max nm (Δε): 213 (+0.014), 239 (+0.67), 270 (−0.048); ¹H NMR and ¹³C NMR data (CDCl₃, see Tables 1 and 2); HRESIMS m/z 317.2112 [M + H]⁺ (calcd for C₂₀H₂₉O₃⁺, 317.2111).

Crotonlaevin L (13): white amorphous powder; [α]²³_D +80.00 (c 0.20, in CHCl₃); IR (KBr) ν_max 3385, 2927, 2869, 1705, and 1647 cm⁻¹; UV λ_max nm (log ε): 226 (6.93); CD (MeOH) λ_max nm (Δε): 214 (+0.19), 239 (+0.5), 286 (−0.015); ¹H NMR (acetone-D₆, 400 MHz) δ_H: 6.94 (1H, t, J = 2.8 Hz, H-7), 5.69 (1H, t, J = 6.0 Hz, H-14), 4.72 (1H, d, J = 11.2 Hz, H-12), 4.35 (1H, dt, J = 10.4, 2.8 Hz, H-6), 4.13 (2H, t, J = 5.6 Hz, H₂-15), 2.44 (1H, dq, J = 13.2, 3.2 Hz, H-9), 1.87 (1H, ddd, J = 13.2, 3.2, 2.0 Hz, H₂-11a), 1.49 (1H, m, H₂-11b), 1.72 (1H, m, H₂-1a), 1.17 (1H, m, H₂-1b), 1.59 (1H, dt, J = 13.6, 3.2 Hz, H₂-2a), 1.47 (1H, m, H₂-2b), 1.67 (3H, s, H₃-16), 1.43 (1H, m, H₂-3a), 1.26 (1H, td, J = 13.6, 3.2 Hz, H₂-3b), 1.39 (1H, m, H-5), 1.17 (3H, s, H₃-18), 1.06 (3H, s, H₃-19), 0.84 (3H, s, H₃-20); ¹³C NMR (acetone-D₆, see Table 2); HRESIMS m/z 357.2025 [M + Na]⁺ (calcd for C₂₀H₃₀O₄Na⁺, 357.2034).

Crotonlaevin M (14): white amorphous powder; [α]²⁶_D +10.00 (c 0.10, in CHCl₃); IR (KBr) ν_max 3381, 3296, 2944, 2920, 2849, 1683, and 1625 cm⁻¹; UV λ_max nm (log ε): 221 (6.88); CD (MeOH) λ_max nm (Δε): 217 (−0.79), 256 (−0.28); ¹H NMR (C₅D₅N, 400 MHz) δ_H: 6.11 (1H, t, J = 6.0 Hz, H-14), 5.25 (1H, d, J = 4.0 Hz, H-7), 4.74 (1H, dd, J = 12.0, 3.6 Hz, H-12), 4.53 (2H, d, J = 6.0 Hz, H₂-15), 2.48 (1H, ddd, J = 17.2, 12.0, 1.2 Hz, H₂-11a), 2.36 (1H, dd, J = 17.2, 3.6 Hz, H₂-11b), 2.11 (1H, d, J = 13.6 Hz, H₂-6a), 1.68 (1H, dd, J = 13.6, 4.4 Hz, H₂-6b), 1.96 (1H, dd, J = 12.8, 1.2 Hz, H-5), 1.78 (3H, s, H₃-16), 1.68 (1H, m, H₂-1a), 1.07 (1H, td, J = 12.8, 3.6 Hz, H₂-1b), 1.58 (1H, m, H₂-2a), 1.47 (1H, m, H₂-2b), 1.40 (1H, d, J = 13.6 Hz, H₂-3a), 1.17 (1H, td, J = 12.8, 3.6 Hz, H₂-3b), 1.03 (3H, s, H₃-18), 0.92 (3H, s, H₃-20), 0.85 (3H, s, H₃-19); ¹³C NMR data (C₅D₅N, see Table 2); HRESIMS m/z 357.2034 [M + Na]⁺ (calcd for C₂₀H₃₀O₄Na⁺, 357.2036).

Crotonlaevin N (15): colorless crystal (mixed solvents: petroleum ether/EtOAc/acetone); mp 125–127 °C; [α]²³_D −18.75 (c 0.32, in CHCl₃); IR (KBr) ν_max 3427, 2926, 2869, 1707, and 1641 cm⁻¹; UV λ_max nm (log ε): 223 (6.85); CD (MeOH) λ_max nm (Δε): 219 (−0.44), 254 (+0.057); ¹H NMR and ¹³C NMR data (CDCl₃, see Tables 1 and 2); HRESIMS m/z 319.2257 [M + H]⁺ (calcd for C₂₀H₃₁O₃⁺, 319.2268).

Crotonlaevin O (16): colorless crystal (mixed solvents: petroleum ether/EtOAc); mp 167–168 °C; [α]²⁰_D +30.00 (c 0.3, in CHCl₃); IR (KBr) ν_max 3428, 2927, 2850, 1703, and 1640 cm⁻¹; UV λ_max nm (log ε): 228 (6.66); CD (MeOH) λ_max nm (Δε): 230 (+0.24), 264 (−0.019); ¹H NMR (CDCl₃, 400 MHz) δ_H: 7.33 (1H, dd, J = 2.4, 4.4 Hz, H-7), 5.96 (1H, dd, J = 17.2, 17.2 Hz, H-14), 5.40 (1H, d, J = 17.2 Hz, H₂-15a), 5.22 (1H, d, J = 10.8 Hz, H₂-15b), 4.12 (1H, dd, J = 10.8, 1.6 Hz, H-12), 2.38 (1H, m, H₂-6a), 2.11 (1H, m, H₂-6b), 2.16 (1H, m, H-9), 1.93 (1H, m, H₂-11a), 1.34 (1H, m, H₂-11b), 1.79 (1H, m, H₂-1a), 1.06 (1H, td, J = 12.8, 4.0 Hz, H₂-1b), 1.58 (1H, m, H₂-2a), 1.53 (1H, m, H₂-2b), 1.48 (1H, m, H₂-3a), 1.20 (1H, td, J = 12.8, 4.0 Hz, H₂-3b), 1.35 (3H, s, H₃-16), 1.32 (1H, m, H-5), 0.92 (3H, s, H₃-19), 0.90 (3H, s, H₃-18), 0.75 (3H, s, H₃-20); ¹³C NMR data (CDCl₃, see Table 2); HRESIMS m/z 319.2261 [M + H]⁺ (calcd for C₂₀H₃₁O₃⁺, 319.2268).

Crotonlaevin P (17): colorless crystal (mixed solvents: petroleum ether/EtOAc/MeOH); mp 122–124 °C; [α]²³_D +32.00 (c 0.25, in CHCl₃); IR (KBr) ν_max 3432, 2927, 2851, 1706, and 1640 cm⁻¹; UV λ_max nm (log ε): 230 (6.97); CD (MeOH) λ_max nm (Δε): 208 (+0.032), 232 (+0.41), 268 (−0.024); ¹H NMR (CDCl₃, 400 MHz) δ_H: 7.33 (1H, m, H-7), 5.89 (1H, dd, J = 17.3, 17.3 Hz, H-14), 5.38 (1H, dd, J = 17.3, 0.8 Hz, H₂-15a), 5.21 (1H, d, J = 10.8, 0.8 Hz, H₂-15b), 4.10 (1H, dd, J = 10.8, 1.6 Hz, H-12), 2.37 (1H, m, H₂-6a), 2.09 (1H, m, H₂-6b), 2.16 (1H, m, H-9), 1.87 (1H, m, H₂-11a), 1.41 (1H, m, H₂-11b), 1.77 (1H, m, H₂-1a), 1.04 (1H, td, J = 12.0, 4.0 Hz, H₂-1b), 1.57 (1H, m, H₂-2a), 1.51 (1H, m, H₂-2b), 1.47 (1H, m, H₂-3a), 1.18 (1H, td, J = 12.0, 4.0 Hz, H₂-3b), 1.38 (3H, s, H₃-16), 1.29 (1H, dd, J = 12.0, 4.0 Hz, H-5), 0.91 (3H, s, H₃-19), 0.89 (3H, s, H₃-18), 0.74 (3H, s, H₃-20); ¹³C NMR data (CDCl₃, see Table 2); HRESIMS m/z 341.2085 [M + Na]⁺ (calcd for C₂₀H₃₀NaO₃⁺, 341.2087).

X-ray single-crystal diffraction analysis of 7, 8, 10a, 14, 15, 16, and 17

All data were collected using a Bruker Smart Apex CCD diffractometer using graphite-monochromated Mo Kα and Cu Kα radiation. The structure was solved by the direct method SHELXS-97 (Sheldrick, G.M., University of Gottingen, Gottingen, Germany, 1997) and refined by a full-matrix least-square method on F² by means of SHELXL-97 (Sheldrick, G.M., University of Gottingen, Gottingen, Germany, 1997). The hydrogen atoms were not included in the refinement, and all of the non-hydrogen atoms were refined anisotropically. In the final step of structural refinement, the positional parameters of the hydrogen atoms were calculated under a fixed C–H bond length of 1.00 Å with sp³ configuration of the bonding carbon atoms.†

Crystal data of 7: Cu Kα radiation (294 K):C₂₂H₃₆O₅; F.W. 380.51; monoclinic space group I2; unit cell dimensions a = 20.3298(3) Å, b = 9.17075(12) Å, c = 24.6190(4) Å, V = 4233.70(12) ų; α = 90°, β = 112.7227(19)°, γ = 90°, Z = 8, ρ_calc = 1.194 Mg m⁻³, crystal dimensions 0.2 × 0.08 × 0.06 mm, μ = 0.665 mm⁻¹, F(000) = 1664. A total of 20139 reflections were measured, of which 7487 were unique (R_int = 0.0274) and used in all calculations. The final refinement gave R₁ = 0.0411 and wR₂ = 0.1160 [I > 2σ(I)].

Crystal data of 8: Mo Kα radiation (296 K):C₂₂H₃₆O₅; F.W. 380.51; monoclinic space group C2; unit cell dimensions a = 26.328(15) Å, b = 11.042(7) Å, c = 19.217(19) Å, V = 4336(6) ų; α = 90°, β = 129.100(4)°, γ = 90°, Z = 8, ρ_calc = 1.166 Mg m⁻³, crystal dimensions 0.23 × 0.22 × 0.19 mm, μ = 0.081 mm⁻¹, F(000) = 1664. A total of 14628 reflections were measured, of which 7876 were unique (R_int = 0.0507) and used in all calculations. The final refinement gave R₁ = 0.0785 and wR₂ = 0.1415 [I > 2σ(I)].

Crystal data of 10a: Mo Kα radiation (296 K):C₂₂H₃₃BrO₄; F.W. 501.44; monoclinic space group P2(1); unit cell dimensions a = 11.718(16) Å, b = 8.241(11) Å, c = 13.602(18) Å, V = 1251(3) ų; α = 90°, β = 107.699(13)°, γ = 90°, Z = 2, ρ_calc = 1.331 Mg m⁻³, crystal dimensions 0.23 × 0.21 × 0.19 mm, μ = 1.672 mm⁻¹, F(000) = 524. A total of 6336 reflections were measured, of which 4296 were unique (R_int = 0.0387) and used in all calculations. The final refinement gave R₁ = 0.0498 and wR₂ = 0.0982 [I > 2σ(I)].

Crystal data of 14: Cu Kα radiation (290 K):C₂₀H₃₀O₄; F.W. 334.44; monoclinic space group C2; unit cell dimensions a = 9.1613(4) Å, b = 11.8102(7) Å, c = 16.9960(9) Å, V = 1829.40(17) ų; α = 90°, β = 95.829(5)°, γ = 90°, Z = 4, ρ_calc = 1.214 Mg m⁻³, crystal dimensions 0.29 × 0.25 × 0.24 mm, μ = 0.663 mm⁻¹, F(000) = 728. A total of 17362 reflections measured, of which 3503 were unique (R_int = 0.0334) and used in all calculations. The final refinement gave R₁ = 0.0341 and wR₂ = 0.0917 [I > 2σ(I)].

Crystal data of 15: Cu Kα radiation (291 K):C₂₀H₃₀O₃; F.W. 318.44; orthorhombic space group P2(1)2(1)2(1); unit cell dimensions a = 7.1319(6) Å, b = 14.3359(11) Å, c = 17.4859(15) Å, V = 1787.8(3) ų; α = 90°, β = 90°, γ = 90°, Z = 4, ρ_calc = 1.183 Mg m⁻³, crystal dimensions 0.25 × 0.14 × 0.1 mm, μ = 0.611 mm⁻¹, F(000) = 696. A total of 4991 reflections were measured, of which 2976 were unique (R_int = 0.0183) and used in all calculations. The final refinement gave R₁ = 0.0374 and wR₂ = 0.0977 [I > 2σ(I)].

Crystal data of 16: Mo Kα radiation (296 K):C₂₀H₃₀O₃; F.W. 318.44; orthorhombic space group P2(1)2(1)2(1); unit cell dimensions a = 15.646(17) Å, b = 10.703(12) Å, c = 10.876(12) Å, V = 1821(3) ų; α = 90°, β = 90°, γ = 90°, Z = 4, ρ_calc = 1.161 Mg m⁻³, crystal dimensions 0.23 × 0.21 × 0.19 mm, μ = 0.076 mm⁻¹, F(000) = 696. A total of 12994 reflections were measured, of which 3376 were unique (R_int = 0.0499) and used in all calculations. The final refinement gave R₁ = 0.0391 and wR₂ = 0.0839 [I > 2σ(I)].

Crystal data of 17: Mo Kα radiation (296 K):C₂₀H₃₀O₃; F.W. 318.44; orthorhombic space group P2(1)2(1)2(1); unit cell dimensions a = 6.519(2) Å, b = 17.905(6) Å, c = 31.128(10) Å, V = 3633.2(19) ų; α = 90°, β = 90°, γ = 90°, Z = 8, ρ_calc = 1.164 Mg m⁻³, crystal dimensions 0.28 × 0.26 × 0.21 mm, μ = 0.076 mm⁻¹, F(000) = 1392. A total of 26369 reflections were measured, of which 6748 were unique (R_int = 0.1056) and used in all calculations. The final refinement gave R₁ = 0.0809 and wR₂ = 0.1654 [I > 2σ(I)].

Acknowledgements

Financial support from the National Natural Science Foundation of China (no. 21202075), the National Science Foundation for Fostering Talents in Basic Research of the National Natural Science Foundation of China (J1103307), and the Fundamental Research Funds for the Central Universities (lzujbky-2014-64) is gratefully acknowledged.

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Footnote

† Electronic supplementary information (ESI) available: HRESIMS, IR, UV, CD, and NMR spectra of 1–17, and X-ray crystallographic in CIF data for 7, 8, 10a, and 14–17. Selected key HMBC correlations for new compounds 1–9 and 11–17. CCDC 1002242–1002248. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4ra04863f

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