Peltate Glandular Trichomes of Colquhounia Coccineavar
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ORGANIC LETTERS 2013 Peltate Glandular Trichomes of Vol. 15, No. 7 Colquhounia coccinea var. mollis Harbor a 1694–1697 New Class of Defensive Sesterterpenoids Chun-Huan Li,†,§ Shu-Xi Jing,†,§ Shi-Hong Luo,† Wei Shi,†,§ Juan Hua,† Yan Liu,† Xiao-Nian Li,† Bernd Schneider,‡ Jonathan Gershenzon,‡ and Sheng-Hong Li*,† State Key Laboratory of Phytochemistry and Plant Resources in West China, Kunming Institute of Botany, Chinese Academy of Sciences, Kunming 650201, P. R. China, Max Planck Institute for Chemical Ecology, Hans-Knoll-Straße€ 8, D-07745 Jena, Germany, and University of Chinese Academy of Sciences, Beijing 100049, P. R. China [email protected] Received February 21, 2013 ABSTRACT A new class of unique sesterterpenoids, colquhounoids AÀC(1À3), were identified from the peltate glandular trichomes of Colquhounia coccinea var. mollis (Lamiaceae) through precise laser-microdissection coupled with UPLC/MS/MS and spectroscopic analyses and X-ray diffraction. Very interestingly, their structural features and defensive function are closely related to leucosceptroid-class sesterterpenoids harbored by the glandular trichomes of another Lamiaceae taxon, Leucosceptrum canum, even though this is morphologically distinct and taxonomically distant. 3 Sesterterpenoids, a special group of C25 terpenoids con- and spiro- and pentanor-skeletons. The core structure of sisting of only 965 known compounds, represent highly leucosceptroids has been synthesized by Horne’s group,4 attractive targets for total synthesis due to their compara- and total synthesis of leucosceptroid B has been completed tively large size, structural complexity, molecular diversity, by Liu’s group.5 and broad biological activities.1 Although sesterterpenoids Colquhounia coccinea var. mollis (Schlechtendal) Prain is have been reported from widespread sources,1 their dis- a large shrub 1À2 m high distributed mainly in the south- tribution and natural functions in plants have not been well west of China, with vivid whorls of orange-scarlet flowers investigated, with merely ca. 80 compounds having been (Figure 1A).6 The plant is never touched by livestock and documented. Recently, we found that the glandular tri- only occasionally attacked by a generalist tussock moth chomes of a woody Himalayan Lamiaceae, Leucosceptrum (Lymantriidae) and some unknown pathogens. Under a canum, with colored nectar, harbor the unique defensive scanning electron microscope, numerous nonglandular sesterterpenoids leucosceptroids A and B (see Figure 2),2 trichomes and peltate and capitate glandular trichomes and its leaves and flowers contain a series of antifeedant leucosceptroids with antipodal cyclopentenones, lactone, (3) (a) Luo, S.-H.; Hua, J.; Li, C.-H.; Jing, S.-X.; Liu, Y.; Li, X.-N.; Zhao, X.; Li, S.-H. Org. Lett. 2012, 14, 5768–71. (b) Luo, S.-H.; Weng, L.-H.; Xie, M.-J.; Li, X.-N.; Hua, J.; Zhao, X.; Li, S.-H. Org. Lett. 2011, † Kunming Institute of Botany. 13, 1864–1867. (c) Luo, S.-H.; Hua, J.; Li, C.-H.; Liu, Y.; Li, X.-N.; ‡ Max Planck Institute for Chemical Ecology. Zhao, X.; Li, S.-H. Tetrahedron Lett. 2013, 54, 235–237. (d) Luo, S.-H.; § University of Chinese Academy of Sciences. Hua, J.; Niu, X.-M.; Liu, Y.; Li, C.-H.; Zhou, Y.-Y.; Jing, S.-X.; Zhao, (1) (a) Hog, D. T.; Webster, R.; Trauner, D. Nat. Prod. Rep. 2012, 29, X.; Li, S.-H. Phytochemistry 2013, 86, 29–35. 752–779. (b) Wang, L.; Yang, B.; Lin, X.-P.; Zhou, X.-F.; Liu, Y. Nat. (4) Xie, J.; Ma, Y.; Horne, D. A. J. Org. Chem. 2011, 76, 6169–6176. Prod. Rep. 2013, 30, 455–473. (5) Huang, X.; Song, L.; Xu, J.; Zhu, G.; Liu, B. Angew. Chem., Int. (2) Luo, S.-H.; Luo, Q.; Niu, X.-M.; Xie, M.-J.; Zhao, X.; Schneider, Ed. 2013, 52, 952–955. B.; Gershenzon, J.; Li, S.-H. Angew. Chem., Int. Ed. 2010, 49, 4471–4475. (6) Li, H.-W.; Hedge, I. C. Flora China 1994, 17, 299. 10.1021/ol4004756 r 2013 American Chemical Society Published on Web 03/21/2013 (Figure 1B,C) were observed to densely cover the leaves, overlapped. The 13C NMR spectrum (Table S1, Support- buds, and stems. The peltate glandular trichomes were pre- ing Information) of 1 demonstrated 25 carbon reso- cisely collected using laser microdissection (Figure 1DÀF) nances, which were further classified by DEPT spectra and analyzed for their secondary metabolites with UPLC/ as five methyls, four methylenes with one of them occur- MS/MS (Figure 1GÀJ). Three major compounds (1À3) ring at relatively highfield (δC 12.4), ten methines includ- were found with retention times of 25.4, 28.9, and 23.7 ing an olefinic methine (δC 117.0), two oxymethines min in the total ion chromatogram (Figure 1G) and with (δC 78.2 and 85.2), two methines appearing at relatively molecular weights of 430, 432, and 448, respectively, in high field (δC 8.7 and 18.6), and six quaternary carbons their positive ESI mass spectra (Figure 1HÀJ). From the consisting of a keto carbon (δC 212.5), an ester carbonyl methanolic extract of the whole leaves of C. coccinea var. carbon (δC 173.1), an olefinic quaternary carbon (δC mollis, these trichome metabolites were traced and iso- 170.3), a ketal carbon (δC 109.4), and two oxygenated lated, and their structures were successfully identified. quaternary carbons (δC 82.4 and 85.5). These data were Compound 1, obtained as colorless blocks, was deter- consistent with the signals observed in the 1H NMR mined to have a molecular formula of C25H34O6 on the spectrum, suggesting a highly oxygenated cyclopropyl- basis of EI mass spectrometry as well as high-resolution containing sesterterpenoid for 1. À1 EI-MS. Its IR spectrum showed absorptions at 3406 cm All proton signals, except for the singlet at δH 3.62, could for hydroxyl group and at 1730 and 1682 cmÀ1 for car- be assigned to their respective carbons unambiguously bonyl groups. The 1H NMR spectrum (Table S1, Supporting through analysis of the HSQC spectrum (Figure S4, Information) of 1 clearly displayed two tertiary methyls at Supporting Information), suggesting that the signal at δH δH 1.33 and 2.08 and three secondary methyls at δH 1.03, 3.62 was ascribable to a free hydroxyl group. In addition, 1 1 1 13 1.09, and 1.13. An olefinic proton at δH 5.77 indicated the detailed analysis of the HÀ Hand HÀ C long-range presence of a trisubstituted double bond. Three one- correlations in the COSY and HMBC spectra (Figures S3 proton singlets at δH 3.62, 4.22, and 4.90 were ascribable and S5, Supporting Information) of 1 led to the establish- to either methines or free hydroxyl groups. At a relatively ment of a 5/6/5 ring system for its corestructure, which high field, two one-proton multiplets at δH 0.29 and 1.05 appeared to be similar to that found in leucosceptroids 2,3 together with a two-proton multiplet at δH 0.89 were (Figure 2), a class of sesterterpenoids discovered in the indicative of a disubstituted cyclopropyl residue. Other glandular trichomes and whole leaves and flowers of L. signals were centered between 1.05 and 2.50 and mostly canum. Further analysis of the NMR spectra of 1 and comparison of its data with those of leucosceptroids revealed that the 6/5 carbon framework of 1 was closer to that of leucosceptroids B and J2,3 because of the absence of hydroxylation at C-11, and its C-14 side chain contained amethylatedR,β-unsaturated lactone, as has been found in Figure 1. (A) C. coccinea var. mollis plant in bloom, (B and C) peltate and capitate glandular trichomes on the abaxial leaf surface, (DÀF) collection of peltate glandular trichomes with laser microdissection, (GÀJ) microchemical analysis of second- ary metabolites in the microdissected peltate glandular tri- Figure 2. Chemical structures of colquhounoids AÀC(1À3) and chomes with UPLC/MS/MS. leucosceptroids A and B. Org. Lett., Vol. 15, No. 7, 2013 1695 leucosceptroids E-J,3 judging from the HMBC correla- determined the absolute stereochemistry of 1 to be tions from H-17 to C-15, C-16, C-18, C-19, and C-20, and 2S,3S,4R,5R,6R,7R,8S,10S,11R,13R,14R,17S.Consequently, from Me-25 to C-17, C-18, and C-19 in 1. However, the C-4 the core structure of compound 1 was found to possess an side chain of 1 was obviously different from that of interesting complex cagelike all-cis-fused 5/6/5/6 frame- leucosceptroids. The C-4 isobutenyl side chain of leucos- work due to the occurrence of the 4,8-oxygen bridge. ceptroids was found to be replaced by a methylated Thus, compound 1 was identified as shown in Figure 2 cyclopropyl group in 1, which could be deduced from the and was named colquhounoid A. 1 1 HÀ H coupling relationship of Me-1/H-2/H-3/H2-21/H- Compound 2 was also obtained as colorless blocks. 2in1. In addition, an oxygenation was found to have High-resolution EI-MS and 1Hand13C NMR (including occurred at C-8 in 1 due to the 1HÀ1HcorrelationsofH-8 DEPT) spectra (Table S1, Supporting Information) (δH 4.22) with H-7 and H2-9, and the aforementioned ketal showed a molecular formula of C25H36O6, which was only carbon (δC 109.4) in 1 wasassignabletoC-4owingtothe two hydrogens more than that of 1. Comparison of the simultaneous 1HÀ13C correlations from 5-OH, H-13, and NMR spectra of 2 with those of 1 clearly disclosed that 2 the cyclopropyl protons to the ketal carbon. Thus, more was another sesterterpenoid structurally similar to 1. clear differences existing between 1 and leucosceptroids However, the appearance of an isobutyl moiety in 2,as resulted from the oxygenation at C-8 and further oxygena- indicated by the 1HÀ1H coupling relationship of Me-1/H- tion at C-4 in 1.