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Resolving the (19R) Absolute Configuration of Lanciferine, a Monoterpene Indole Alkaloid from Alstonia boulindaensis Mehdi A. Beniddir,*,† Grégory Genta-Jouve,‡ and Guy Lewin† Équipe “Pharmacognosie-Chimie des Substances Naturelles” BioCIS, Université Paris-Sud, CNRS, Université Paris-Saclay, 5 Rue J.-B. Clément, 92290 Châtenay-Malabry, France ‡ C-TAC, UMR 8638 CNRS, Faculté de Pharmacie de Paris, Paris-Descartes University, Sorbonne, Paris Cité, 4, Avenue de l’Observatoire, 75006 Paris, France †
S Supporting Information *
ABSTRACT: Reinvestigation of the structure of lanciferine (1a) through extensive spectroscopic analysis in conjunction with a detailed computational study led to the unambiguous assignment of its (19R) absolute configuration, thus leading to the full (2R, 3S, 7S, 15R, 16R, 19R, 20S) assignment of lanciferine 45 years after its isolation.
L
and (19S)-epilanciferine (1b).12 Targeting the (S)-epimer was reasonable, because it was the preferential structure among the proposed structures for lanciferine.9 Notably, Ang Li et al. used the words, “Proposed Structure of Lanciferine” because the structural reassignment attempt of 1 was hampered by the ambiguous and incomplete 1H NMR data disclosed in the isolation report (Figure S2, Supporting Information).13 In addition, no 13C NMR data were reported.14 Remarkably, when the authors compared the 1H NMR spectrum of (19S)epilanciferine (1b) and that of the natural product (1a), they found that the chemical shift of Me-18 of 1b (δH 1.4) significantly differed from that reported for natural lanciferine (1a) (δH 1.2) (Figure 1). Thus, Ang Li et al. concluded that uncovering the structural mystery of 1 has to rely on reisolation of the sample from nature. As part of our continuing interest in monoterpene indole alkaloid chemistry, we recently implemented an in-house spectroscopic database, constituted of a cumulative collection of alkaloids, for dereplication purposes.15 Satisfyingly, a pure sample of the original lanciferine (1a, ∼3 mg) was retrieved from this historical collection. In order to decipher the “mystery” of lanciferine, comprehensive 1D and 2D NMR data analyses of the original sample were carried out, along with a thorough literature survey regarding all the indolinolidcontaining monoterpene indole alkaloids. With the original sample of 1a in hand, we acquired the 1D and 2D NMR data that permitted the complete assignment of all 1H and 13C NMR signals (Table 1) along with the
anciferine (1a) is a monoterpene indole alkaloid isolated by one of us (G.L.) in 1973 from the aerial parts of Alstonia boulindaensis Boiteau, a New Caledonian Apocynaceae plant.1 This natural product belongs to the Akuammiline family,2 a subset of the monoterpene indole alkaloids comprising numerous complex scaffolds that have captured the interest of synthetic chemists because of their chemical structures and a broad range of biological activities.3 Lanciferine (1a) possesses an oxidized furoindoline motif embedded within a polycyclic framework that was coined “indolinolid” in the original report.1 Even though 1a was the first representative of the akuammiline family to exhibit this molecular architecture, currently, the latter is encountered among nine other congeners, namely, picranitine,4 alstolactines A, B, and C,5 alstoniascholarines L and M,6 and scholarisines L (2), K (3), and M.7 Although detailed biosynthetic studies have not been performed, these indolinolid-containing alkaloids are thought to arise from the oxidation of picraline at C-5 along with the subsequent disconnection of the C-5−N-4 bond. The original assignment of the structure of lanciferine (1a) relied on degradation, derivatization, and the comprehensive analysis of MS, UV, IR, and 1H NMR data.8 The absolute configuration of 1a was unambiguously assigned as (2R, 3S, 7S, 15R, 16R, and 20S), but for C-19.9 However, a structure of lanciferine displaying a (19S) configuration was reported between 2013 and 2016,2,10,11 presumably because the majority of the trisubstituted olefinic bonds of the akuammiline alkaloids possess an E configuration. Following these three reports, a (19S) absolute configuration of lanciferine was favored. Recently, Ang Li et al. reported stereoselective total syntheses of three akuammiline alkaloids, aspidodasycarpine, lonicerine, © XXXX American Chemical Society and American Society of Pharmacognosy
Received: November 12, 2017
A
DOI: 10.1021/acs.jnatprod.7b00957 J. Nat. Prod. XXXX, XXX, XXX−XXX
Journal of Natural Products
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configuration at C-19 was clearly proved to be (19R) (Figure 1).
Figure 1. Structures of natural lanciferine (1a), (19S)-epilanciferine (1b), and scholarisines L (2) and K (3).
configurational assignment of C-19. The NOESY cross-peak observed between Me-18 and H2-21 placed these protons cofacial, while the NOESY correlation of H-19 with H-15 placed them on the opposite face (Figure 2). Thus, the absolute
Figure 2. Key NOESY correlations of 1a.
Table 1. NMR Spectroscopic Data for Natural Lanciferine (1a) and (19S)-Epilanciferine (1b12) in CDCl3 1a position 2 3 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 NH N-Me CO2Me CO2Me C6H5-CHCH-CO2 C6H5-CHCH C6H5-CHCH C6H5-CHCH
δH, mult. (J in Hz)a 3.02, m 3.51, d (18.4) 3.92, d (18.4)
7.60, 6.78, 7.02, 6.67,
d (7.6) t (7.6) t (7.7) d (8.0)
2.08−2.15, m 2.76, t (2.4) 3.77, 4.07, 1.25, 2.99,
d d d q
(11.2) (11.2) (5.6) (5.6)
2.31, 3.11, 4.66, 2.44, 3.66,
d (13.0) d (13.0) s s s
6.04, 7.37, 7.37, 7.44,
d (16.0) d (16.0) m, 3H m, 2H
1b δ Cb
δH, mult. (J in Hz)c,e
106.66 57.71 174.93 41.49
3.01−3.09, m 3.64, d (18.7) 3.77, d (18.7)
53.42** 131.66 128.17 121.19 129.32 111.15 147.28 21.72
7.58, 6.80, 7.03, 6.70,
d (7.5) dd (7.5, 7.5) ddd (7.7, 7.7, 1.0) d (7.8)
2.12, ddd (14.8, 4.0, 4.0) 2.16, ddd (14.8, 2.5, 2.5) 2.66, dd (3.1, 3.1)
36.21 53.45** 67.74 13.48 62.27 61.57 52.22
46.00 51.56 171.72 165.36 117.56 144.96 129.06, 2×CH, 130.53, CH 128.17, 2×CH 134.42, C
3.92, 4.27, 1.41, 2.84,
d d d q
(11.2) (11.2) (5.6) (5.6)
2.22, 3.22, 4.66, 2.41, 3.62,
d (13.5) d (13.5) s s s
6.00, d (16.0) 7,35, d (16.0) 7.37−7.43, m, 3H 7.43−7.48, m, 2H
δCd,e 106.43 57.26**** 174.72 41.94 53.52*** 131.88 128.15* 121.19 129.34 111.29 147.25 22.24 32.65 54.28*** 68.49 10.82 59.29 61.93 56.65****
45.61 51.54 171.44 165.37 117.39 144.86 128.20, 2×CH*, 130.5, CH 129.09, 2×CH 134.34, C
a Data recorded at 400 MHz. bData recorded at 100 MHz. cData recorded at 600 MHz. dData recorded at 151 MHz. *; **; ***; ****: these attributions can be exchanged. eThese chemical shifts were reported from ref 12 (the signals were assigned by us).
B
DOI: 10.1021/acs.jnatprod.7b00957 J. Nat. Prod. XXXX, XXX, XXX−XXX
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Interestingly, the 13C NMR data related to the Nmethylpiperidine and epoxide moieties of the two epimeric alkaloids, namely, scholarisines K (3, 19S, 20S) and L (2, 19R, 20S),7 are in full agreement with those of (19S)-epilanciferine (1b) and natural lanciferine (1a), respectively (Table 2).
between 1989 and 2014, three combinations of the four possible configurations at C-19 and C-20 were reported: (i) (19S, 20R) in scholarisines B, E,22 and M,7 scholarisins IV and V,23 alistonitrine A,24 and 4-methylraucubaininium chloride;25,26 (ii) (19R, 20S) in scholarisine L (2);7 and (iii) (19S, 20S) in scholarisine K (3).7 In conclusion, this paper sheds light on the stereochemical aspect of this intriguing alkaloid through a comprehensive NMR analysis along with a detailed computational study, resulting in (i) the complete absolute configuration assignment of lanciferine 45 years after its isolation and (ii) amelioration of a literature report that casted doubt on the true identity of lanciferine.12
Table 2. 13C NMR Chemical Shifts of 1a, 1b, 2, and 3 in CDCl3 position 3 14 15 18 19 20 21 N-Me a
3
2
δ C,
δ C,
7a
57.5 25.8 27.1 11.2 58.1 62.2 56.3 45.6
7a
57.7 25.0 31.4 13.4 62.0 61.9 52.1 46.0
Δ 2/3 δC +0.2 −0.8 +4.3 +2.2 +3.9 −0.3 −4.2 +0.4
1b δC,
12b
57.26 22.24 32.65 10.82 59.29 61.93 56.65 45.61
1a δC
a
57.71 21.72 36.21 13.48 62.27 61.57 52.22 46.00
Δ 1a/1b δC +0.45 −0.52 +3.56 +2.66 +2.98 −0.36 −4.43 +0.39
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EXPERIMENTAL SECTION
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ASSOCIATED CONTENT
General Experimental Procedures. Optical rotation at 578 nm was measured at 20 °C on an electronic Zeiss polarimeter. The UV spectrum was recorded on a Unicam SP 1800 spectrophotometer, and the IR data on a PerkinElmer type 457 spectrometer. HRMS data were recorded on a Thomson mass spectrometer (Institut de Chimie de Strasbourg). The NMR spectra were recorded on a Bruker AM-400 NMR spectrometer using CDCl3 as solvent. The solvent signal was used as reference. Chemicals and solvents were purchased from SigmaAldrich. Lanciferine (1a): 1 white, microcrystalline powder; mp 259−262 °C (from EtOH); [α]20578 −60 (c 1, CHCl3); UV (EtOH) λmax (log ε) 236 (3.87), 280 (4.20) nm; IR (KBr) νmax 3400 (NH), 1765 (lactone), 1740 (ester), 1715 (conjugated ester), 1640 (conjugated olefin), 1610 (dihydroindole) cm−1; 1H and 13C NMR data, see Table 1; HRMS m/ z [M]+• 544.2209 (calcd for C31H32N2O7, 544.2204). Computational Methods. NMR calculations have been realized on the most stable conformers obtained after geometry optimization using the m062x method at the 6-31G(d) level27 as implemented in the Gaussian 09 software package.28 The absence of an imaginary frequency was confirmed by a frequency calculation at the same level of theory. NMR prediction was performed using the hybrid Becke3LYP functional29,30 at the 6-311+G(d,p) level in CHCl3 with the SCRF method. The polarizable continuum model (PCM) using the integral equation formalism variant (IEFPCM) was used to perform the calculation in CHCl3. Assignments of the relative configurations of compounds 1a and 1b were performed by calculating the CP3 probability proposed by Goodman et al.16
Data recorded at 100 MHz. bData recorded at 151 MHz.
Next, the aforementioned assignments were subjected to Smith and Goodman’s CP3 parameter.16 The latter is based on comparing the differences in the experimental and calculated chemical shifts of diastereomers. This method proved to be a powerful tool in natural products structure elucidation.17 The CP3 parameter generated values of 0.21 and −0.03 for respectively correct and incorrect assignments between calculated and experimental data, which, in conjunction with Bayes’ theorem, has led, with 94.3% certainty, to the assignment of the experimental data of lanciferine (1a) and (19S)epilanciferine (1b) (Figure 1). As is evident in Figure 3, a correct assignment is indicated by green and red pairs of bars pointing in the same direction.
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jnatprod.7b00957. Experimental procedures, original 1H NMR spectrum, and 1D and 2D NMR spectra for 1a (PDF)
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AUTHOR INFORMATION
Corresponding Author
*Tel: + 33 1 46 83 55 87. Fax: + 33 1 46 83 53 99. E-mail:
[email protected].
Figure 3. Differences in calculated and experimental NMR chemical shifts for 1a and 1b.
ORCID
Mehdi A. Beniddir: 0000-0003-2153-4290
Besides lanciferine (1a) and its 10-hydroxy and 10-methoxy analogues isolated from A. boulindaensis,1,8 the epoxy ring at C19 and C-20 is encountered in 12 representatives among the akuammiline family, mainly described within the Alstonia genus. For three of these, isolated between 1974 and 1982, the configuration at the epoxy ring was either not assigned (quaternoxine18,19 and caberine20) or tentatively assigned (raucubainine21). For the nine remaining alkaloids, isolated
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank J.-C. Jullian for his assistance with the NMR spectroscopy and one of the reviewers for the insightful and constructive comments. C
DOI: 10.1021/acs.jnatprod.7b00957 J. Nat. Prod. XXXX, XXX, XXX−XXX
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Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, O.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09 Revision D.01; Gaussian, Inc., Wallingford, CT, 2013. (29) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B: Condens. Matter Mater. Phys. 1988, 37, 785−789. (30) Becke, A. D. J. Chem. Phys. 1993, 98, 5648−5652.
REFERENCES
(1) Lewin, G.; Kunesch, N.; Poisson, J.; Sévenet, T. J. Indian Chem. Soc. 1978, 55, 1096−1098. (2) Adams, G.; Smith, A. B., III. The Chemistry of Akuammiline Alkaloids. In The Alkaloids; Knolker, H.-J., Ed.; Elsevier: New York, 2016; Vol. 76, pp 171−257. (3) Smith, J. M.; Moreno, J.; Boal, B. W.; Garg, N. K. Angew. Chem., Int. Ed. 2015, 54, 400−412. (4) Tane, P.; Tene, M.; Sterner, O. Bull. Chem. Soc. Ethiop. 2002, 16, 165−168. (5) Yang, X.-W.; Qin, X.-J.; Zhao, Y.-L.; Lunga, P. K.; Li, X.-N.; Jiang, S.-Z.; Cheng, G.-G.; Liu, Y.-P.; Luo, X.-D. Tetrahedron Lett. 2014, 55, 4593−4596. (6) Qin, X.-J.; Zhao, Y.-L.; Song, C.-W.; Wang, B.; Chen, Y.-Y.; Liu, L.; Li, Q.; Li, D.; Liu, Y.-P.; Luo, X.-D. Nat. Prod. Bioprospect. 2015, 5, 185−193. (7) Yang, X.-W.; Luo, X.-D.; Lunga, P. K.; Zhao, Y.-L.; Qin, X.-J.; Chen, Y.-Y.; Liu, L.; Li, X.-N.; Liu, Y.-P. Tetrahedron 2015, 71, 3694− 3698. (8) Lewin, G.; Kunesch, N.; Poisson, J. J. C. R. Acad. Sc. Paris Série C 1975, 280, 987−990. (9) Lewin, G.; Poisson, J. Bull. Soc. Chim. Fr. 1980, II-400−II-404. (10) Adams, G. L.; Carroll, P. J.; Smith, A. B. J. Am. Chem. Soc. 2013, 135, 519−528. (11) Eckermann, R.; Gaich, T. Synthesis 2013, 45, 2813−2823. (12) Li, Y.; Zhu, S.; Li, J.; Li, A. J. Am. Chem. Soc. 2016, 138, 3982− 3985. (13) Lewin, G. Thèse de Doctorat es-Sciences Pharmaceutiques, ̈ d’Alstonia boulindaensis Boiteau (Apocynacées); Université Alcaloides Paris-Sud, 1978. (14) Indeed, in the mid 1970s, 13C NMR analysis was still very much a specialist’s technique, requiring a large amount of compound, and not readily accessible for several natural products research groups. (15) Fox Ramos, A. E.; Alcover, C.; Evanno, L.; Maciuk, A.; Litaudon, M.; Duplais, C.; Bernadat, G.; Gallard, J.-F.; Jullian, J.-C.; Mouray, E.; Grellier, P.; Loiseau, P. M.; Pomel, S.; Poupon, E.; Champy, P.; Beniddir, M. A. J. Nat. Prod. 2017, 80, 1007−1014. (16) Smith, S. G.; Goodman, J. M. J. Org. Chem. 2009, 74, 4597− 4607. (17) Komlaga, G.; Genta-Jouve, G.; Cojean, S.; Dickson, R. A.; Mensah, M. L. K.; Loiseau, P. M.; Champy, P.; Beniddir, M. A. Tetrahedron Lett. 2017, 58, 3754−3756. (18) Mamatas-Kalamaras, S.; Sévenet, T.; Thal, C.; Potier, P. Phytochemistry 1975, 14, 1849−1854. (19) Mamatas-Kalamaras, S.; Sévenet, T.; Thal, C.; Potier, P. Phytochemistry 1975, 14, 1637−1639. (20) Jacquier, M. J.; Vercauteren, J.; Massiot, G.; Le Men-Olivier, L.; Pusset, J.; Sévenet, T. Phytochemistry 1982, 21, 2973−2977. (21) Sierra, P.; Novotný, L.; Samek, Z.; Buděsí̌ nský, M.; Dolejš, L.; Bláha, K. Collect. Czech. Chem. Commun. 1982, 47, 2912−2921. (22) Feng, T.; Cai, X.-H.; Zhao, P.-J.; Du, Z.-Z.; Li, W.-Q.; Luo, X.-D. Planta Med. 2009, 75, 1537−1541. (23) Wang, W.; Cheng, M.-H.; Wang, X.-H. Molecules 2013, 18, 7309−7322. (24) Zhu, G.-Y.; Yao, X.-J.; Liu, L.; Bai, L.-P.; Jiang, Z.-H. Org. Lett. 2014, 16, 1080−1083. (25) Proksa, B.; Uhrín, D.; Grossmann, E.; Votický, Z. Planta Med. 1989, 55, 188−190. (26) Uhrín, D.; Proksa, B. J. Nat. Prod. 1989, 52, 637−639. (27) Hehre, W. J.; Schleyer, P. V. R.; Pople, J. A. Ab Initio Molecular Orbital Theory; Wiley: New York, 1986. (28) Frisch, M. J.; Trucks, H. B.; Schlegel, G. W.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Keith, T.; Kobayashi, R.; Normand, J.; D
DOI: 10.1021/acs.jnatprod.7b00957 J. Nat. Prod. XXXX, XXX, XXX−XXX
