First Total Synthesis, Structure Revision, and Natural History of the Smallest Cytochalasin: (+)-Periconiasin G

Article abstract of DOI:10.1002/chem.201603734

The total synthesis of the smallest cytochalasin isolated so far, periconiasin G, which bears a seven-membered ring in lieu of the usual macrocycle, has been performed from both enantiomers of citronellal, relying on an intramolecular Diels–Alder reaction in favor of the natural endo stereochemistry. We show that, among the four synthesized stereoisomers, including the exo isomers, the one matching the NMR data of the natural product was not that assigned in the original report, imposing structure revision. The natural product, previously isolated from a plant-mutualistic fungus, was biologically investigated taking into account its natural history, showing significant effects against the phytopathogenic fungus Botrytis cinerea and thus opening new opportunities in combating this pest.

Full text of DOI:10.1002/chem.201603734

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Title: First Total Synthesis, Structure Revision and Natural History of the Smallest
Cytochalasin: (+) Periconiasin G
´
Authors: Mehdi Zaghouani; Caroline Kunz; Laura Guedon; Florent Blanchard; Bas-
tien Nay
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COMMUNICATION
First Total Synthesis, Structure Revision and Natural History of
the Smallest Cytochalasin: (+)-Periconiasin G
Mehdi Zaghouani,[a] Caroline Kunz,[a] Laura Guédon,[a] Florent Blanchard[b] and Bastien Nay[a]*
Abstract: The total synthesis of the smallest cytochalasin isolated
so far, periconiasin G which bears a 7-membered ring in lieu of the
usual macrocycle, has been performed from both enantiomers of
citronellal, relying on an intramolecular Diels-Alder reaction in favor
of the natural endo stereochemistry. We show that among the four
synthesized stereoisomers, including the exo isomers, the one
matching the NMR data of the natural product was not that assigned
in the original report, imposing structure revision. The natural
product, previously isolated from a plant-mutualistic fungus, was
biologically investigated taking into account its natural history,
showing significant effects against the phytopathogenic fungus
Botrytis cinerea and thus opening new opportunities in combating
this pest.
lactones) fused to an isoindolone core (Figure 1).1 All of them
biosynthetically result from intramolecular Diels-Alder (IMDA)
reactions occurring on the linear and polyunsaturated products
of polyketide synthase–non ribosomal peptide synthetases
(PKS-NRPS).5 Recently however, several shorter derivatives
were unprecedentedly isolated from the endophytic fungus
Periconia sp. F-31, featuring a 9-membered like periconiasin A
(1), or a 7-membered ring like periconiasin G (2, Figure 1).6
Résumé: La synthèse totale de la plus petite cytochalasine isolée à
ce jour, la périconiasine G qui porte un cycle à 7 chaînons au lieu de
l'habituel macrocycle, a été réalisée à partir des deux énantiomères
du citronellal, reposant sur une réaction de Diels-Alder
intramoléculaire en faveur de la stéréochimie naturelle endo. Nous
montrons que parmi les quatre stéréoisomères endo synthétisés,
incluant les isomères exo, celui dont les données RMN
correspondent à celles du produit naturel n'est pas celui attribué
initialement, imposant une révision de structure. Le produit naturel,
précédemment isolé d'un champignon endophyte, a été évalué
biologiquement en tenant compte de son histoire naturelle, montrant
des effets significatifs contre le champignon phytopathogène Botrytis
cinerea, et ouvrant ainsi la voie vers de nouvelles opportunités de
lutte contre ce fléau.
Figure 1: General structures of cytochalasins with large to medium sized
macrocycles (top); structure of medium-sized natural products (bottom):
periconiasin
A (1) and the reported (2) and revised (3) structures of
periconiasin G.
Just recently reported (February 2016), compound 2 has been
the smallest member so far discovered among cytochalasins,
showing weak anti-HIV activities.6c Due to our chemical and
biological interests for PKS-NRPS derivatives7 as chemical
weapons in the microbial world, we focused on synthetic
approaches toward the dwarf cytochalasins, including 2, in order
to gain sufficient material for biological investigations. In this
context, an elegant total synthesis of periconiasins A-E (e.g. 1),
with the original 9/6/5-fused system, has recently been reported
by Liu and Tang.8 Conversely, we describe herein the total
syntheses of four stereoisomers of the 7-membered
cytochalasin 2, bearing a unique 7/6/5-fused system, resulting
from endo and exo IMDA cycloadditions, as suspected to
happen during biosynthesis. The compounds, among which
periconiasins 2 and 3 (Figure 1), were obtained from linear
IMDA substrates made from both enantiomers of citronellal, and
L-leucine. We show that the reported structure (2) of periconiasin
G has been misassigned due to wrong interpretation of NOESY
NMR data.6c A revision of the natural product structure, as
compound 3, is thus highlighted by this total synthesis, with the
reported NMR of natural periconiasin G matching our synthetic
endo derivative obtained from (R)-citronellal and displaying the
14R configuration. In addition, we provide an ecologically
relevant biological study, showing that 3 is able to delay the
hyphal growth and to decrease sporulation of the economically
Cytochalasins are fungal natural products which have attracted
great interest from chemists and biologists due to their
structures and biological properties.1 Among typical
representatives, cytochalasin B2 has not only proved its utter
ability to destabilize actin filaments, thus disrupting cellular
processes linked to the cytoskeleton,2 but also, for example,
potent inhibition of GLUT transporters involved in glucose
uptake3 and overexpressed by several cancer cell lines.4 In
nature, cytochalasins could interfere with such biological
processes and could be used by fungi to compete with others.
The structural diversity of cytochalasins more frequently involves
macrocyclic cycloalkanones (or oxidation-derived expanded
[a]
[b]
M. Zaghouani, L. Guédon, Dr. C. Kunz, Dr. B. Nay
Muséum National d'Histoire Naturelle and CNRS (UMR 7245)
57 rue Cuvier (CP 54), 75005 Paris, France
E-mail: bnay@mnhn.fr
F. Blanchard
Institut de Chimie des Substances Naturelles (ICSN-CNRS)
1 Avenue de la Terrasse, 91198 Gif-sur-Yvette Cedex, France
Supporting information for this article (experimental synthetic
procedures, copies of NMR spectra, NMR comparison figures and
tables, crystallographic data, biological experimental part) is given
via a link at the end of the document.
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disastrous phytopathogenous fungus Botrytis cinerea, without
affecting spore germination or fungal morphology. This may be
indicative of the advantage brought by this cytochalasin for the
endophytic fungal producer and its plant host, relative to
phytopathogens.9
To synthesize the reported structure 2 of periconiasin G, (S)-(–)-
citronellal (S)-5 was first protected as a 1,3-dioxolane before
cleaving the alkene under modified Lemieux-Johnson oxidizing
conditions, providing the carboxylic acid 6 (Scheme 1). After
esterification of the acid as a benzyl ester, the aldehyde function
(7) was readily and quantitatively unmasked under acidic
conditions (80% AcOH, 110°C). Compound 7 was submitted to
the modified Takai olefination10 in presence of anhydrous CrCl2
and CHI3 at room temperature, to get the iodoalkene 8 in 77%
yield (E/Z = 11:1). A Suzuki-Miyaura cross-coupling of 8 with (Z)-
but-2-en-2-ylboronic acid 9,11 in presence of the commercially
available catalyst PdCl2(dppf)•CH2Cl2 (1.2 mol%), furnished
diene 10 in 75% yield with slight erosion of the E/Z ratio (8:1)
despite optimization. After hydrolysis of the benzyl ester, the
carboxylic acid was activated as an acylimidazole (11) in order
to further install the lactam moiety.12
Scheme 2:. Completion of the synthesis of reported periconiasin G (2, in fact:
14-epi-periconiasin G). Conditions: a) LiHMDS, THF, –78°C (65%); b) LiHMDS,
PhSeBr, THF, –78°C (93%); c) m-CPBA, H2O2, CHCl3, –500°C; d) Sealed
tube, 140°C (29% of 16a and 13% 16b over two steps); e) aq. NaOH, MeOH,
0°Crt (2: 73%; 17: 93%).
The IMDA reaction was performed at 140°C in a sealed tube to
give a 72:28 mixture of endo (16a) and exo (16b) isomers,
respectively (42% global yields, two steps), supposedly through
endo-15 and exo-15 transition states (Scheme 2). In an attempt
to improve this ratio, in favor of the awaited endo-cycloadduct
16a, the IMDA reaction was tested in the presence of LiBF4,
15
LiClO4 and Schreiner's thiourea.16 It was interesting to note
that both lithium salts inverted the endo/exo ratio at 43:57
(LiBF4) and 37:63 (LiClO4), thus disfavoring endo-isomer 16a,
while the thiourea had no effect on this ratio. Finally,
debenzoylation of the amides 16a and 16b under hydrolytic
conditions afforded the reported structure of periconiasin G (2)
and the exo-isomer 17, respectively (Scheme 2). However, the
proton NMR spectra of compound 2, made from (S)-(–)-
citronellal (S)-5, strikingly mismatched that of the reported
natural product,6c as did the exo isomer 17. Careful analysis of
2D NMR spectra confirmed the synthesized structure 2,
especially its endo stereochemistry, in contrast to the exo-
isomer 17.
Scheme 1:.Synthesis of diene 10 from (S)-citronellal (S)-5. Conditions: a)
(CH2OH)2, (±)-CSA, HC(OEt)3, CH2Cl2, rt (99%); b) OsO4 (1.2 mol%), 2,4,6-
collidine, H2O2, KMnO4, MeOH, 0°Crt; c) NaIO4, H2O/MeOH (10:1), rt (44%
over two steps); d) BnBr, K2CO3, acetone, 130°C (Mw), 8 min (86%); e) AcOH
(80% in H2O), 110°C (Mw), 6 min (quant.); f) CrCl2, CHI3, 1,4-dioxane/THF
(3:1), rt (77%, E/Z = 11:1); g) PdCl2(dppf)·DCM, KOH, THF/H2O (4.7:1) (75%,
E/Z = 8:1); h) aq. NaOH, EtOH, rt (90%); i) CDI, Et3N, DCM, rt (96%).
Addition of the anion of N-benzoyl-5-iso-butyl-pyrrolidin-2-one 12,
prepared from L-leucine,13 to the activated intermediate 11
provided the branched pyrrolidinone 13 in 65% yield (Scheme 2).
After deprotonation by LiHMDS at the acidic methyne position of
13, phenylselenation was achieved in 93% yield, affording an
equimolar diastereomeric mixture of phenylselenide 14.
In
cytochalasins
biosynthesis,
three
stereochemical
determinants are responsible for the structures observed: (i) the
aminoacid is always in the L series and the corresponding
stereocenter makes the late IMDA cycloaddition face-selective
on the pyrrolone dienophile; (ii) the IMDA reaction setting up the
full carbocyclic skeleton is exclusively endo selective, probably
due to enzymatic control, always leading to the same
stereochemistry of the hexahydro-isoindolone moiety; (iii) the
presence of stereocenters in the macrocycle depends on
additional transformations more specific to each natural product.
Consequently, we suspected a misinterpretation of NOESY
spectra regarding the configuration of C-14 in the 7-membered
ring (reported as S),6c and we decided to reproduce this total
synthesis with (R)-(+)-citronellal (R)-5 as the starting material.
The synthetic route proceeded as described above with similar
14
Selenide oxidation in presence of mCPBA and H2O2 released
the dienophile within the lactam ring, affording a highly sensitive
linear IMDA substrate (15) to be kept in a chloroform solution
and promptly engaged in the cycloaddition step.
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yields to reach the linear IMDA substrate 18 (Scheme 3). The
IMDA reaction was performed at lower temperature than
previously (100°C in CHCl3) since 18 was found more fragile
than 15 at higher (140°C) or ambient temperature for longer
reaction times. An inseparable mixture of endo (19a) and exo
(19b) cycloadducts was obtained in a 3:1 ratio, respectively
(44% yield, two steps). The protective benzoyl amides of 19a
and 19b were finally hydrolyzed, giving a chromatographically
separable mixture of 3 and 20.
Gratifyingly, the proton NMR spectra of 3 (Figure S42) and other
data – e.g. [α]20D +62.0 (c 0.2, MeOH), litt.: +61.4 (c 0.07, MeOH)
– were in full accordance with that of the reported natural
product.6c Comforted by X-ray crystallography (Scheme 3),17 we
propose a structural revision of periconiasin G as the (14R)-
antifungal tests on the phytopathogenic fungal model Botrytis
cinerea, responsible for grey mould disease and crop losses in
cultures of temperate and subtropical regions.19 Indeed, three
assays were performed to assess the plant-protective role of
periconiasin G (3) as an inhibitor of (i) spore germination, (ii)
hyphal growth, and (iii) sporulation of B. cinerea. We found that
the percentage of germinated spores in the presence or
absence of periconiasin G was the same at 10 µg·mL-1 (30 µM)
of 3. The hyphal length, however, was significantly shorter at
early time points of culture (Figure 2) but the growth rate was
identical. Based on preliminary results, we show that
periconiasin slows down the hyphal growth just after spore
germination, and that over time, the fungus is capable to
overcome the inhibitory activity of 3 and to grow at the same
growth rate as in absence of the natural product. Another
interesting effect of periconiasin G was found when calculating
the number of spores produced by B. cinerea mycelium after 4
days of culture, significantly falling from 22.02·105 spores·mL–1
to 5.79·105 spores·mL–1 (p<0.001). Morphological analysis of
fungal structures involved in the production of asexual spores
and the morphology of hyphae did not reveal anomalies. Further
study will be needed to determine the mechanism of action of 3
but, as a cytochalasin, it could interact with fungal actin, which is
crucial in the hyphal growth.20
stereoisomer
3 instead of 2 (i.e. 14-epi-periconiasin G).
Incidentally, NMR experiments revealed the difficult analysis of
spatial correlations on NOESY spectra when dealing with
protons 8-H, 13-H and 13-H, 14-H and 21-H3 due to signal
overlapping.
Figure 2: Plot of the hyphal length of B. cinerea germlings in function of
incubation time, showing a 3h delay between control germlings and germlings
in presence of 3 (dissolved in ethanol) at 30 µM final concentration. Ethanol
had no effect on germling growth (data not shown). Linear regression curves
and coefficients are indicated. * p<0.05; ** p<0.01; ***p<0.001 (n=2).
Scheme 3:.Total synthesis of the revised structure of periconiasin G (3) from
(R)-citronellal (R)-5 and X-ray crystallographic structure of 3 (bottom left).
Conditions: a) Sealed tube, 100°C (44% over two steps from the selenide
intermediate, endo/exo: 3:1 based on NMR); b) aq. NaOH, MeOH, 0°Crt
(64% of 3 and 32% of 20).
In conclusion, the total synthesis of four stereoisomers of
periconiasin G was performed, allowing its structure revision as
compound 3 bearing the 14R configuration, instead of 2
originally proposed with the 14S configuration.6c It provided
enough material of 3 for biological investigations in order to
assess its role in Nature, showing a significant reduction of
sporulation, and a delay in the hyphal growth at 30 µM of 3.
Interestingly, neither antibacterial activity, nor inhibition of A.
thaliana seed germination was observed, showing an antifungal
specificity of action. In the ecological context, this observation
Periconiasin G (3) was isolated from an endophytic fungus
(Periconia sp. F-31) living inside an Asian medicinal plant
(Annona muricata).6c At the Muséum National d'Histoire
Naturelle, we seek to understand the role of such compounds in
the plant-fungus mutualism,9 either as signaling molecules or as
chemical weapons to fight against competitors. In the microbial
world, the observation of such phenomena has led to the
discovery of nowadays essential, or promising natural
products.18 As for periconiasin G, we investigated its biological
function in a chemical ecology context. In vitro antibacterial
(Micrococcus luteus, Staphylococcus aureus) and plant seed
germination (Arabidopsis thaliana) essays were found without
impact. Yet, interesting results were obtained when performing
indicates that periconiasin
G could provide a significant
advantage for the endophytic fungal producer and its host-plant,
relative to phytopathogenous competitors. It supports the
importance of chemicals in the plant-fungus mutualism,9a giving
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agriculture.
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Keywords: cytochalasin • periconiasin • total synthesis •
structure revision • antifungal activities
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Entry for the Table of Contents (Please choose one layout)
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The total synthesis of the smallest
cytochalasin isolated so far,
Mehdi Zaghouani, Caroline Kunz, Laura
Guédon, Florent Blanchard, Bastien
Nay*
periconiasin G, is presented. Thanks
to the synthesis of four stereoisomers,
comparison of data with the isolation
report revealed a misassignment,
imposing structure revision. Biological
investigations showed significant
effects against the phytopathogenic
fungus Botrytis cinerea, and suggest a
potential role of this compound in the
mutualistic relationship between the
fungal producer and its host plant.
Page No. – Page No.
First Total Synthesis, Structure
Revision and Natural History of the
Smallest Cytochalasin:
(+)-Periconiasin G
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