Letter Cite This: Org. Lett. 2018, 20, 2311−2314
pubs.acs.org/OrgLett
Stereochemical Study of Puna’auic Acid, an Allenic Fatty Acid from the Eastern Indo-Pacific Cyanobacterium Pseudanabaena sp Emmanuel Roulland,*,†,∥ Hiren Solanki,‡,∥ Kevin Calabro,‡ Mayalen Zubia,§ Grégory Genta-Jouve,† and Olivier P. Thomas*,‡ †
C-TAC, COMETE UMR 8638 CNRS, Université Paris Descartes, 4 avenue de l’observatoire, 75006 Paris, France Marine Biodiscovery Laboratory, School of Chemistry and Ryan Institute, National University of Ireland Galway (NUI Galway), University Road, H91TK33 Galway, Ireland § UMR Ecosystèmes Insulaires Océaniens, LabEx-CORAIL, University of French Polynesia, Faa’a, BP6570, 98702 Tahiti, French Polynesia ‡
S Supporting Information *
ABSTRACT: The isolation and structure elucidation of puna’auic acid, an allenic fatty acid isolated from a marine cyanobacterium, is described. All configurations were first assessed through molecular modeling of NMR and ECD spectra and then confirmed through a straightforward enantioselective total synthesis of puna’auic acid featuring a key reductive opening of a propargylic epoxide.
D
where frequent blooms negatively impact coral reefs of this fragile marine ecosystem. The chemical investigation of this species started with a fractionation by reversed phase Vacuum Liquid Chromatography (VLC) of the CH2Cl2/MeOH extract obtained from different specimens collected in the lagoon of Puna’auia, Tahiti. The main metabolite 1 of the MeOH fractions showed an intense molecular ion peak at m/z 311.2237 [M − H]−, corresponding to the molecular formula C18H32O4 for the neutral molecule (Figure 1). Together with the 1H NMR
espite outstanding advances in computational chemistry, the determination of the spatial arrangement of atoms in a molecule remains a challenge for natural product chemists. In nature, chirality has attracted much attention due to its importance for applications in the pharmaceutical sector. While central chirality is widespread, the occurrence of axial chirality in natural products is more limited and mostly represented by biaryl atropisomers.1−3 Allene is the simplest cumulene and the most common case of non-atropisomer molecules with an axial chirality, as first suggested by van’t Hoff,4 and then confirmed by Maitland and Mills.5 The first allenic natural product was discovered by Celmer and Solomons,6,7 and since then, about 200 allenic substances were isolated from natural sources, representing only a small part of the natural chemical diversity.8,9 While natural allenes were first described in unsaturated fatty acids, most of them are now identified in terpenoids, such as some carotenoid pigments mainly produced by fungi and plants.9 In marine environments, such a chemical feature is mainly encountered in halogenated allenes from algae. In the context of ongoing projects aimed at the description of the chemical diversity present in marine organisms, and especially, cyanobacteria of the Eastern Indo-Pacific Ocean, we report herein the isolation and structure elucidation of a new allenic fatty acid called puna’auic acid (1) from the cyanobacterium Pseudanabaena sp. collected off Puna’auai, French Polynesia. The full assessment of its configuration was addressed, first, through a complete theoretical study, and further ascertained by an enantioselective total synthesis using a CuI-catalyzed key step to build the chiral allene moiety.10 The discovery of a close enyne analogue in this species finally led us to propose a possible biosynthesis for the allene moiety. The cyanobacterium Pseudanabaena sp. is abundant in the shallow lagoons of Tahiti and Moorea (French Polynesia), © 2018 American Chemical Society
Figure 1. Chemical structure of puna’auic acid (1).
spectra, these data were in accordance with a fatty acid derivative containing two unsaturations. However, integration of the olefinic protons only accounted for two protons. While a cycle was first suspected to be responsible for the second hydrogen deficiency, the presence of signals at δC 92.3 (C-11), 93.0 (C-9), and 205.3 (C-10) did not fit with the molecular formula and HMBC correlations (Table 1). The deshielded carbon C-10 could correspond to either a carbonyl group or an allene. The latter hypothesis was in good agreement with the chemical shifts of two methines of an allene Received: February 24, 2018 Published: March 29, 2018 2311
DOI: 10.1021/acs.orglett.8b00654 Org. Lett. 2018, 20, 2311−2314
Letter
Organic Letters
probability.11 Due to the recent developments and efficiencies of DP4, we relied more on the latter configurations. Subsequently, the absolute configuration of 1 was obtained by comparison between experimental and TD-DFT theoretical ECD spectra. As shown in Figure 2, a perfect match was
Table 1. 1H (600 MHz) and 13C (150 MHz) NMR Data for 1 in CD3OD 1 no. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
δH, mult. (J in Hz) − 2.26, 1.60, 1.35, 1.35, 1.36, 1.45, 2.04, 5.20, − 5.18, 3.94, 3.51, 1.55, 1.39, 1.54, 1.33, 1.32, 1.34, 0.92,
δC 178.5 35.6 26.3 30.0 30.3 30.3 30.3 29.7 93.0 205.3 92.3 75.2 75.9 33.6
t (7.0) quint (7.0) m m m quint (7.0) m m m ddd (7.5, 4.5, 2.0) dt (8.5, 4.5) m m m m m m t (7.0)
26.6 33.1 23.7 14.4
Figure 2. Experimental and theoretical ECD spectra for 1.
observed between the experimental spectrum of natural 1 and the TD-DFT predicted spectrum of the Ra,12R,13S enantiomer. A single Cotton Effect was observed at ca. 220 nm, which was assigned to the π→ π* transition of the allene moiety. In the recent literature, many examples of relative and absolute configurations of natural products have been deduced from DFT calculations only.12 However, as this was the first application of this method to the determination of the configurations of a natural allene, we decided to embark into the total enantioselective synthesis of 1. For the asymmetric synthesis of puna’auic acid (1), we started from a building block of the chiral pool. To construct the desired α-hydroxyallene function in a diastereoselective manner, we anticipated that the CuI-catalyzed conjugated hydride addition to propargylic epoxides described by Deutsch et al. would lead to the core moieties of 1.10 Starting from acetal 2 obtained from 2-deoxy-D-ribose,13 a Wittig olefination,10 followed by hydrogenation, yielded the primary alcohol 3 (Scheme 1). In parallel, the terminal alkyne 6 was obtained from the commercially available alcohol 5.14 The corresponding lithium acetylide was then condensed on aldehyde 4 derived from alcohol 3 after Swern oxidation.15 Both 7-syn and 7-anti diasteroisomers were obtained in a 69/31 ratio and an overall yield of 61% over two steps.16 Alcohol 7-anti was then transformed into the corresponding mesylate 8. Under acidic catalysis, the acetonide protective group of 8 was removed, and the addition of K2CO3 after completion of the reaction led to the epoxy-alcohol 9, featuring an efficient one-pot two-step approach. Gratifyingly, Krause’s CuI-catalyzed conjugated hydride addition allowed transformation of the epoxy-alcohol 9 into the desired allene 10 in 84% yield with a high diastereomeric ratio (95/5) and no need for a protective group.10 To our knowledge, this is the first implementation of this method in an asymmetric synthesis of a natural product. The sequence DDQ-promoted MPM removal17/selective TEMPO-catalyzed oxidation18 was found to be incompatible with a nonprotected allenic alcohol function. Therefore, diol 10 was first transformed into the diacetate 11 before the removal
at C-9 and C-11. Location of the allene on the C18 fatty acid chain was expected to be challenging, but fortunately, COSY, HSQC, and HMBC correlations from the methyl terminus through the signals of two oxygenated methines at C-13 and C12 placed the allene between C-9 and C-11. Intense fragment ions observed in HRMS/MS at m/z 129.0935 and 181.1245 [M − H]− were attributed to C-7/C-8 and C-11/C-12 bond breaks respectively and came as a confirmation of the location of the allene. All together puna’auic acid contains two chiral centers and one axis of chirality around the allene, therefore leading to eight possible stereoisomers. Because no clear interpretation of the data could help in the configuration assignment, we first relied on computational studies and comparisons with experimental NMR and chiroptical data. To assign both the relative and absolute configurations of 1, predictions of the theoretical NMR chemical shifts and Electronic Circular Dichroism (ECD) were performed using density functional theory (DFT). For the relative configuration, the B3LYP hybrid functional was used at the 6-31g (d) level on the four possible diastereoisomers. Several metrics were implemented to compare both theoretical and experimental 13 C NMR chemical shifts, and the results are summarized in Table 2. Using the mean average error (MAE), a common metrics, Ra*,12S*,13S*-1 was the most probable relative configuration while the configuration Ra*,12R*,13S*-1 was deduced from both the R 2 values and the recently proposed DP4 Table 2. Comparison of Theoretical and Experimental 13C NMR Chemical Shifts configurations of 1
MAE (ppm)
R2
DP4 (%)
Ra*,12R*,13R* Ra*,12S*,13R* Ra*,12R*,13S* Ra*,12S*,13S*
4.2 4.5 3.7 3.2
0.9987 0.9984 0.9991 0.9986
1.1 0 98.1 0.8 2312
DOI: 10.1021/acs.orglett.8b00654 Org. Lett. 2018, 20, 2311−2314
Letter
Organic Letters
analogous to Krause’s copper-catalyzed conjugated hydride addition used in the enantioselective synthesis (Scheme 2). Consequently, this key reaction might mimic a natural process for the formation of allenes in fatty acids or polyketides.
Scheme 1. Asymmetric Total Synthesis of the Proposed Structure of 1
Scheme 2. Biosynthetic Hypothesis for 1
Chirality in nature is still a fascinating property, and the determination of configurations an exciting challenge for chemists. In this work we demonstrated the complementarity of approaches including molecular modeling and asymmetric synthesis25 to ascertain the chiral properties of the first allenic fatty acid found in a cyanobacterium.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.8b00654. 1D and 2D NMR spectra, as well as HRMS spectra of 1 and 14; 1D NMR spectra of synthetic compounds 2−13; Computational details for NMR and ECD calculations; All reactions and NMR spectra of the intermediates of the synthesis (PDF)
of the MPM protective group, leading to alcohol 12 in good yield. The latter was then oxidized quantitatively into carboxylic acid 13 through a two-step Swern15/Lungren-Pinnick19 sequence. Finally, both acetate protective groups were removed by saponification, delivering fatty-acid 1. The 1H, 13C NMR and mass spectra of synthetic 1 were in perfect agreement with those of the naturally occurring product confirming the relative configuration of 1, while the chiroptical properties confirmed its absolute configuration. The enantioselective synthesis of 1 secured the assignments of both the absolute and relative configurations, and at the same time, it demonstrated the reliability of the molecular modeling method to assess the absolute configuration of chiral allenes. Due to their rare occurrence in natural products, the biosynthesis of allenes has not been clearly demonstrated; feeding experiments have only been performed, and the preliminary results suggest enynes as precursors of the allenes in fungal metabolites.20 Remarkably, some natural allenes have been found, together with their alkyne analogues in two other families of marine fungal metabolites: truncateol from the sponge-associated fungus Truncatella angustata,21 oxirapentyns from marine-sediment-derived fungus Isaria feline, underpinning this hypothesis.22 With the intent of identifying some possible intermediates in the biosynthesis of 1, we inspected the minor metabolites of our fraction. Gratifyingly, we could isolate and identify the related enyne 14. This metabolite is a new natural product already used as an intermediate in the synthesis of (13S)-coriolic acid.23,24 It is worth noting that the S absolute configuration at C-13 for 14 is in agreement with the one found for 1. This observation led us to propose that the biosynthesis of 1 is likely to include a stereoselective reductive addition on the propargylic epoxyde derived from the enyne 14, a reaction
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] (E.R.). *E-mail:
[email protected] (O.P.T.). ORCID
Emmanuel Roulland: 0000-0002-8012-7946 Kevin Calabro: 0000-0001-8962-4810 Grégory Genta-Jouve: 0000-0002-9239-4371 Olivier P. Thomas: 0000-0002-5708-1409 Author Contributions ∥
E.R. and H.S. contributed equally.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This project (Grant-Aid Agreement No. PBA/MB/16/01) is carried out with the support of the Marine Institute and is funded under the Marine Research Programme by the Irish Government. H.S. is grateful to NUI Galway for supporting his PhD scholarship. The computational chemistry and the total synthesis efforts were financially supported by CNRS and Université Paris Descartes. R. Doohan (NUI Galway) is acknowledged for help in recording the NMR data. 2313
DOI: 10.1021/acs.orglett.8b00654 Org. Lett. 2018, 20, 2311−2314
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Organic Letters
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REFERENCES
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DOI: 10.1021/acs.orglett.8b00654 Org. Lett. 2018, 20, 2311−2314
