Triconoids A–D, Four Limonoids Possess Two Rearranged Carbon

Letter pubs.acs.org/OrgLett

Triconoids A−D, Four Limonoids Possess Two Rearranged Carbon Skeletons from Trichilia connaroides Guo-Cai Wang,† Yao-Yue Fan,† Sajan L. Shyaula,‡ and Jian-Min Yue*,† †

State Key Laboratory of Drug Research, Shanghai Institute of Materia Medica, Chinese Academy of Sciences, 555 Zu Chong Zhi Road, Zhangjiang Hi-Tech Park, Shanghai 201203, China ‡ Nepal Academy of Science and Technology, Khumaltar, Lalitpur, GPO Box 3323, Kathmandu, Nepal S Supporting Information *

ABSTRACT: Four limonoids, triconoids A−C (1−3) possessing a new rearranged mexicanolide skeleton and triconoid D (4) furnishing a new rearranged 1,2-seco-phragmalin skeleton, were isolated from the Nepalese plant Trichilia connaroides. Two rearranged limonoid skeletons sharing an F ring of methyl 5-oxotetrahydrofuran-2-carboxylate were postulated to be formed biosynthetically via a very unique chemical cascade. Their structures were fully accomplished by spectroscopic data, single-crystal Xray diffraction, and electrostatic circular dichroism analysis.

Trichilia species of the Meliaceae family are well-known for metabolizing structurally diverse limonoids1 with a broad spectrum of biological activities.2 The plants of T. connaroides grow mainly in the southeastern area of Asia3 and have many applications in folk medicine.4 Previous chemical investigations on this plant resulted in the isolation of a number of structurally diverse limonoids.5 As part of our continuing search for structurally interesting limonoids from the Trichilia genus,5a,b four limonoids, triconoids A−C (1−3) possessing a new rearranged mexicanolide skeleton and triconoid D (4) bearing a new rearranged 1,2-seco-phragmalin skeleton, were isolated from the leaves and twigs of T. connaroides that were collected from the capital area of Nepal. Two rearranged limonoid skeletons sharing an F ring of the methyl 5-oxotetrahydrofuran2-carboxylate motif were proposed biosynthetically to be formed via a unique chemical cascade. Herein, we present the isolation, structural elucidation, and biosynthetic consideration of these novel limonoids. Triconoid A (1), colorless crystals, has the molecular formula C33H33NO10, as deduced from the (+)-HRESIMS ions at m/z 604.2181 [M + H]+ (calcd 604.2183) and 13C NMR, requiring 18 double bond equivalents. The IR spectrum revealed the presence of a carbonyl (1727 cm−1) group. Analysis of the 1D NMR (Table 1) with DEPT experiments and HSQC spectrum (Figures S2 and S3) of 1 resolved the carbon resonances for all 33 carbons in the molecule, including four methyls (one oxygenated), three methylenes, 15 methines (eight olefinic and three oxygenated), six quaternary carbons (three olefinic), and five carbonyl carbons. A typical β-substituted furan ring (δH 6.41, 7.42, and 7.58; δC 109.5, 120.2, 141.5, and 143.6), an msubstituted pyridine ring (δH 7.43, 8.04, 8.77, and 9.11; δC 124.0, 124.1, 137.4, 151.3, and 154.6), one trisubstituted double bond (δH 5.42, δC 121.0 and 141.8), one ketocarbonyl (δC 212.9), and © 2017 American Chemical Society

four ester carbonyls (δC 164.1, 168.88, 168.89, and 172.5) were further distinguished by NMR data. The above-mentioned functional groups accounted for 13 out of 18 double bond equivalents and thus required the presence of five additional rings in the structure of 1.

The planar structure of 1 was accomplished by interpretation of the 1H−1H COSY and HMBC spectra (Figure 1A). First, the 1 H−1H COSY correlations revealed five coupling structural fragments as drawn with bold bonds. The coupling fragments, quaternary carbons, and heteroatoms were then connected by the detailed analysis of its HMBC spectrum, in which the correlations of H-2/C-1; H-9/C-8, C-14, and C-30; H-19/C-1, C-5, C-9, and C-10; H-15/C-16; H-17/C-16; and H-18/C-12, C-13, C-14, and C-17 delineated the typical B−D ring system of a mexicanolide-type limonoid for 1. The β-substituted furan ring was located at C-17 via HMBC correlations of H-17/C-20, C-21, Received: March 23, 2017 Published: April 6, 2017 2182

DOI: 10.1021/acs.orglett.7b00873 Org. Lett. 2017, 19, 2182−2185

Letter

Organic Letters Table 1. 1H and 13C NMR Data for Compounds 1−4 in CDCl3 1 no. 1 2 3 4 5 6 7 8 9 10 11α 11β 12α 12β 13 14 15α 15β 16 17 18 19 20 21 22 23 28 29 30 1′ 2′ 3′ 4′ 5′ 6′ 7′ OMe

δH (mult, J, Hz) 3.53 dd (8.6, 6.5) 5.54 d (8.6) 4.57 s

2.41 dd (12.2, 6.1) 1.99 m 1.81 m 1.49 td (14.1, 4.9) 1.68 brd (14.1) 2.28 d (5.7) 2.85 dd (18.8, 5.7) 2.69 d (18.8) 5.46 s 1.10 s 1.38 s 7.58 s 6.41 brs 7.42 s 4.53 s 1.27 s 5.42 d (6.5)

2 δC 212.9 48.8 71.3 47.5 44.9 172.5 168.9 141.8 55.4 48.7 21.6 33.8 37.0 45.2 29.8 168.9 77.1 21.5 14.7 120.2 141.5 109.5 143.6 81.2 22.5

δH (mult, J, Hz) 3.56 ddd (8.5, 6.4, 1.5) 5.59 d (8.5) 4.60 s

2.92 m 2.00 m 1.84 m 2.08 td (14.3, 5.2) 1.40 m

2.95 d (18.4) 2.89 d (18.4) 5.49 s 1.14 s 1.44 s 7.64 s 6.48 d (1.8) 7.46 m 4.56 s 1.32 s 5.70 dd (6.4, 2.0)

3 δC 212.8 49.0 71.5 47.6 44.9 172.5 169.0 144.5 52.2 49.0 21.2 28.0 41.4 73.4 39.1 168.2 77.4 15.7 14.8 119.6 141.9 109.7 143.7 81.3 22.8

9.11 brs

121.0 164.1 124.1 151.3

9.14 s

122.2 164.2 124.1 151.4

8.77 d (4.6) 7.43 dd (8.0, 4.6) 8.04 d (8.0) 3.11 s

154.6 124.0 137.4 53.1

8.81 d (4.8) 7.46 m 8.07 d (8.0) 3.15 s

154.7 124.1 137.6 53.3

δH (mult, J, Hz) 2.88 d (6.7) 3.87 s 3.60 s

2.37 dd (11.5, 6.0) 1.96 m 1.69 m 1.46 td (14.9, 5.2) 1.63 m 2.27 d (6.0) 2.92 dd (18.5, 6.0) 2.82 d (18.5)

4 δC 213.9 56.0 74.1 47.5 44.8 172.7 168.8 138.8 55.3 47.8 21.5 33.8 36.8 44.6 30.2

7.50 s 6.38 s 7.43 brs 4.72 s 1.19 s

170.3 77.5 21.7 14.8 120.1 141.2 109.4 143.7 83.9 15.6

5.80 d (6.7)

126.2

5.26 s 1.09 s 1.39 s

δH (mult, J, Hz) 5.94 m 5.39 dd (5.5, 2.2) 3.90 s

2.73 m 1.72 m 2.05 ddt (14.8, 4.8, 2.5) 1.28 m 1.48 td (13.9, 2.9)

3.08 d (22.0) 2.86 d (22.0) 4.79 s 1.02 s 1.28 m 7.50 s 6.41 dd (1.9, 0.9) 7.43 t (1.7) 4.78 s a, 2.66 d (17.7) b, 2.55 d (17.7) 5.93 m

6.70 m 1.69 d (7.1) 1.75 s

3.69 s

53.5

3.60 s

δC 216.0 127.9 68.2 51.0 46.8 174.2 168.4 130.5 43.7 53.4 19.2 28.5 38.2 133.7 33.2 168.1 81.8 16.7 22.5 120.2 141.1 109.8 142.4 80.0 46.1 136.3 165.4 127.8 141.1 14.8 12.2

53.0

nicotinate and the methoxy groups were attached to C-3 and C-7, respectively. The planar structure of 1 was constructed as an unprecedented rearranged 7(6→28)-abeo-mexicanolide-type limonoid. The relative configuration of 1 was assigned on the basis of a NOESY spectrum (Figure 1B). The NOESY correlations of H29/H-28, H-29/H-19, H-29/H-3, H-3/H-2, H-19/H-11α, H19/H-9, H-9/H-14, and H-14/H-18 showed that they were cofacial and were arbitrarily assigned in an α-orientation. Consequently, H-5 and H-17 were assigned to be β-oriented by the NOESY correlations of H-5/H-11β and H-5/H-17. A single-crystal X-ray diffraction study of 1 (Figure 2) was successfully carried out, which confirmed the above structural deduction and determined the absolute configuration of 1 as drawn (2S, 3R, 4S, 5S, 9S, 10R, 13R, 14S, 17R, and 28S) with excellent absolute structure parameter [0.09(6)].6 Triconoid B (2) was assigned a molecular formula of C33H33NO11 by the (+)-HRESIMS ion peak at m/z 620.2126 [M + H]+ (calcd 620.2132) and 13C NMR, which showed 16 mass units more than that of 1, suggestive of an oxygenated

Figure 1. 1H−1H COSY and selected HMBC (A) and key NOESY (B) correlations of 1.

and C-22. Furthermore, a fused A/F ring system was furnished by the HMBC cross-peaks of H-5/C-6; H-29/C-3, C-4, C-5, and C-28; and H-28/C-6 and C-7. HMBC correlations from H-3, H3′, and H-7′ to C-1′ and from OCH3 to C-7 indicated that the 2183

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Organic Letters

the assignment of the absolute configurations of 2 and 3. In the ECD spectra of 1−3, positive Cotton effects were observed at λmax = 213 nm (Δε + 6.8) for 1, 213 nm (Δε + 13.8) for 2, 216 nm (Δε + 10.7) for 3 and negative Cotton effects detected at λmax = 194 nm (Δε − 14.9) for 1, 196 nm (Δε − 9.8) for 2, 193 nm (Δε − 12.4) for 3.7 Triconoid D (4) possessed a molecular formula of C32H34O10 based on the (+)-HRESIMS ion at m/z 601.2309 [M + Na]+ (calcd 601.2050) and 13C NMR data, requiring 16 double bond equivalents. The NMR data (Table 1) of 4 revealed functional groups of one ketocarbonyl (δC 216.0), four ester carbonyls (δC 165.4, 168.1, 168.4, and 174.2), three double bonds, and a typical β-substituted furan ring. The 13C NMR and DEPT data also showed the presence of five methyls (one oxygenated, δC 53.0), four sp3 methylenes, five sp3 methines (three oxygenated, δC 68.2, 80.0, and 81.8), and three sp3 quaternary carbons. The aforementioned functionalities accounted for 11 out of 16 double bond equivalents, indicative of the existence of five additional rings in the structure. Comprehensive analysis of 2D NMR data (Figure 4A) allowed the construction of 4. Four structural fragments (shown as bold

Figure 2. ORTEP drawing of compound 1.

analogue of 1. Extensive comparison of its NMR data (Table 1) with those of 1 revealed that an oxygenated tertiary carbon C-14 (δC 73.4) in 2 replaced the C-14 methine of 1, indicating the presence of 14-OH in 2, which was confirmed by the HMBC correlations of H-18/C-12, C-13, C-14, and C-17 (Figure S13). The relative stereochemistry of the stereogenic centers of 2 except for C-14 was assigned to be the same as 1 by the NOESY data (Figure S15). Compared to compound 1, the 14α-OH was assigned by the deshielded H-9 (ΔδH 0.51) and H-12α (ΔδH 0.59) and the shielded C-9 (ΔδC 3.2) and C-12 (ΔδC 5.8) due to the γ-gauche effects from the 14-OH, which formed pseudo-1,3diaxial relationships with H-9 and H-12α. Triconoid C (3) possessed a molecular formula of C27H30O9 as determined by the (−)-HRESIMS ion at m/z 497.1814 [M − H]− (calcd 497.1812) and 13C NMR data. Comparison of its NMR data (Table 1) with those of 1 showed that they are structural congeners, with major differences occurring in the A ring. The absence of the NMR signals for a nicotinate group and the shielded H-3 (ΔδH 1.72) of 3 suggested that a hydroxy group was located at C-3. This was supported by the molecular formula and further confirmed by the HMBC correlations from H-3 (δH 3.87) to C-1, C-2, C-4, C-5, C-28, and C-29 (Figure S22). The relative configurations of stereocenters in 3, except for the C-3, were assigned the same as those of 1 by the NOESY experiment (Figure S24). The 3-OH was fixed as an α-orientation by the key NOESY correlation between H-3 and H-5β. Thus, the structure of 3 was elucidated. ECD curves at 190 to 225 nm for compounds 2 and 3 showed good consistency with that of 1 (Figure 3), which resulted from the exciton coupling of two different chromophores of a βsubstituted furan ring and the Δ8(30) double bond and allowed

Figure 4. 1H−1H COSY and selected HMBC (A) and key NOESY (B) correlations of 4.

bonds) were indicated by the 1H−1H COSY spectrum. HMBC correlations of H-19/C-1, C-5, C-9, and C-10; H-29/C-1, C-3, C-4, and C-5; H-9/C-8, C-14, and C-30; H-15/C-14 and C-16; H-17/C-16; and H-18/C-12, C-13, C-14, and C-17 then constructed the typical A−D ring system of a 1,2-secophragmalin-type limonoid. Moreover, HMBC correlations of H-5/C-6; H-29/C-28; H-28/C-6 and C-7; and OCH3/C-7 revealed that the F ring of a methyl 5-oxotetrahydrofuran-2carboxylate was fused with the A ring. The furan group was attached to the C-17 by HMBC correlations from H-17 to C-20, C-21, and C-22. Finally, HMBC correlations of H-5′/C-1′, C-2′, and C-3′; H-3/C-1′ showed the presence of a tigloyloxy group at C-3. Thus, the planar structure of 4 was accomplished as a rearranged 7(6→28)-abeo-1,2-seco-phragmalin-type limonoid that was unprecedented. In the NOESY spectrum (Figure 4B), the correlations of H-5 with H-12β, H-17, and H-3′ indicated that H-5, H-17, and the tigloyloxy moiety were cofacial and were assigned randomly as βoriented. As a result, the NOESY cross-peaks of H-3/H-2, H-3/ H-28, H-3/H-29b, H-28/H-29a, H-9/H-30, and H-30/H-15 then indicated that the H-3, H-28, and the C-1−C-29 bond were in an α-orientation. In the ECD spectrum of 4 (Figure 5), the positive Cotton effect at λmax = 230 nm (Δε + 17.5) and the negative Cotton

Figure 3. ECD spectra of compounds 1−3. 2184

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formed to 4 by the same biosynthetic procedures used in the cases of 1−3.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.7b00873. Experimental section; X-ray crystallographic data for 1; and raw spectroscopic data including IR, MS, and NMR spectra for compounds 1−4 (PDF) X-ray data for compound 1 (CIF)

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Figure 5. ECD spectrum of 4; arrow denotes the electric transition dipole of two chromophores.

AUTHOR INFORMATION

Corresponding Author

*Tel: 86-21-50806718. E-mail: [email protected].

effect at λmax = 201 nm (Δε − 63.5) indicated a positive chirality for 4, which resulted from the exciton coupling of two different chromophores of a β-substituted furan ring and the conjugated Δ2(30) and Δ8(14) double bonds. The positive chirality and clockwise arrangement of the electric transition dipole of two chromophores allowed the assignment of the absolute configuration of triconoid D as 3R, 4S, 5S, 9S, 10S, 13R, 17R, and 28S. The biosynthetic pathways for 1−4 are shown in Scheme 1. The precursors for 1−3 were considered to be mexicanolide-type

ORCID

Jian-Min Yue: 0000-0002-4053-4870 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support of the National Natural Science Foundation (Nos. 21532007, U1302222) of P.R. China and the “Personalized MedicinesMolecular Signature-based Drug Discovery and Development”, Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDA12020321), is gratefully acknowledged.

Scheme 1. Hypothetical Biosynthetic Pathways of 1−3

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REFERENCES

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limonoids, which were transformed to intermediate i by an oxidation process involving likely a free radical reaction. Intermediate i would undergo further free-radical-mediated reactions and elimination procedures to return the key intermediate iv, which would readily produce the new skeletal intermediate vi by Baeyer−Villiger oxidation8 followed by hydrolysis. Intermediate vi would undergo a series of enzyme−catalytic structural modifications to produce 1−3. Similarly, the biosynthetic precursor for 4 was suggested to be a 1,2-seco-phragmalin-type limonoid, which would be trans2185

DOI: 10.1021/acs.orglett.7b00873 Org. Lett. 2017, 19, 2182−2185