Synthesis of caeliferins, elicitors of plant immune responses: Accessing lipophilic natural products via cross metathesis

Article abstract of DOI:10.1021/ol202541b

A cross metathesis (CM)-based synthesis of the caeliferins, a family of sulfooxy fatty acids that elicit plant immune responses, is reported. Unexpectedly, detailed NMR spectroscopic and mass spectrometric analyses of CM reaction mixtures revealed extensive isomerization and homologation of starting materials and products. It is shown that the degree of isomerization and homologation in CM strongly correlates with substrate chain length and lipophilicity. Side-product suppression requires appropriate catalyst selection and use of 1,4-benzoquinone as a hydride scavenger.

Full text of DOI:10.1021/ol202541b

ORGANIC
LETTERS
2011
Vol. 13, No. 21
5900–5903
Synthesis of Caeliferins, Elicitors of Plant
Immune Responses: Accessing Lipophilic
Natural Products via Cross Metathesis
Inish O’Doherty,† Joshua J. Yim,† Eric A. Schmelz,‡ and Frank C. Schroeder*,†
Boyce Thompson Institute and Department of Chemistry and Chemical Biology, Cornell
University, Ithaca, New York 14853, United States, and Center for Medical,
Agricultural, and Veterinary Entomology, United States Department of Agriculture,
1600/1700 Southwest 23rd Drive, Gainesville, Florida 32608, United States
schroeder@cornell.edu
Received September 19, 2011
ABSTRACT
A cross metathesis (CM)-based synthesis of the caeliferins, a family of sulfooxy fatty acids that elicit plant immune responses, is reported.
Unexpectedly, detailed NMR spectroscopic and mass spectrometric analyses of CM reaction mixtures revealed extensive isomerization and
homologation of starting materials and products. It is shown that the degree of isomerization and homologation in CM strongly correlates with
substrate chain length and lipophilicity. Side-product suppression requires appropriate catalyst selection and use of 1,4-benzoquinone as a
hydride scavenger.
The caeliferins (1ꢀ4) are a family of insect-derived small
molecule signals that play an important role in plant
defenses. Originallyisolatedfrom grasshopper saliva, these
molecules elicit immune responses in corn and Arabidopsis
that attract natural predators of the feeding herbivores.1
Despite their interesting role in insectꢀplant interac-
tions, a lack of synthetic caeliferin samples has prevented
further study of their mode of action, including the identi-
fication of specific receptors in plants that activate immune
responses. We envisioned that a cross metathesis (CM)-
based approach would provide us with the flexibility to
create all known caeliferins 1ꢀ4 from chiral synthon 9 via
variation of its long-chained metathesis partners, 7 and 8
(Scheme 1). Using a large excess of inexpensive 7 or 8 in the
metathesisreaction should allow suppression of competing
dimerization of 9.
The enantiomers of 9 were obtained from THP-pro-
tected glycidol via a Cu(I) catalyzed Grignard reaction
with 4-bromo-1-butene (Scheme 2).2 Alkenes 7 and 8 were
each prepared in one step from commercially available
precursors.3
Next we investigatedconditions for CM of9 with7 or 8.4
Strikingly, CM on substrates 9 and 8 using Grubbs first
generation (G-I), Grubbs second generation (G-II), or
HoveydaꢀGrubbs second generation (HG-II) catalysts
resulted in complex product mixtures, containing only
(2) (a) Cahiez, G.; Chaboche, C.; Jezequel, M. Tetrahedron 2000, 56,
2733–2737. (b) Dıez, E.; Dixon, D. J.; Ley, S. V.; Polara, A.; Rodrıguez,
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F. Helv. Chim. Acta 2003, 86, 3717–3729.
† Cornell University.
‡ United States Department of Agriculture.
(3) Xu, P.; Lin, W.; Zou, X. Synthesis 2002, 8, 1017–1026.
(1) (a) Alborn, H. T.; Hansen, T. V.; Jones, T. H.; Bennett, D. C.;
Tumlinson, J. H.; Schmelz, E. A.; Teal, P. E. Proc. Natl. Acad. Sci. U.S.A.
2007, 104, 12976–12981. (b) Schmelz, E. A.; Engelberth, J.; Alborn,
H. T.; Tumlinson, J. H., III; Teal, P. E. Proc. Natl. Acad. Sci. U.S.A.
2009, 106, 653–657.
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1975, 97, 3265–3267. (b) Connon, S. J.; Blechert, S. Angew. Chem., Int.
Ed. 2003, 42, 1900–1923. (c) Chatterjee, A. K.; Choi, T.-L.; Sanders,
D. P.; Grubbs, R. H. J. Am. Chem. Soc. 2003, 125, 11360–11370. (d)
Hoveyda, A. H.; Zhugralin, A. R. Nature 2007, 450, 243–251.
r
10.1021/ol202541b
Published on Web 10/12/2011
2011 American Chemical Society

Scheme 1. Caeliferin Structures and Retrosynthesis
Scheme 2. Synthesis of Caeliferins 1, 3, and 4
20ꢀ50% of desired6 inaddition tosignificant quantities of
side products. ESIþ-MS analysis of CM reactions revealed
CH2 insertions and deletions for substrate 9 as well as CH2
deletions for product 6 (Figures 1, S3). Similar results were
obtained in the CM of 9 with 7 (Figure S4). Using the more
active G-II or HG-II catalysts, significant amounts of
chain shortened starting materials and products were
already observed after 2 h, at which time less than 40%
of starting material 9 had been consumed. Previous studies
have shown that isomerization and CH2 insertion and
deletion during metathesis reactions are likely the result
of ruthenium hydride formation,5,6 which can be sup-
pressed by the addition of hydride scavengers, e.g., 1,4-
benzoquinone (BQ).7 We found that, even with the addi-
tion of BQ, the use of either G-II or HG-II resulted in the
formation of significant quantities of side products
(Figures S3, S4).
Our ESIþ-MS-based analyses revealed significant
homologation; however, the extent of CM-induced pro-
duct and starting material isomerization remained unclear.
Building on recent experience using 2D NMR spectro-
scopy fortheanalysisofmixtures,8 we used high-resolution
dqfCOSY spectra to further characterize CM reaction
outcomes. To simplify NMR spectroscopic analysis, we
isolated the mixture of residual 9 as well as its isomers and
homologues from G-II-catalyzed CM of 8 and 9 and subse-
quently removed the stereogenic THP group, producing a
corresponding mixture of 17 and derivatives (Figure 2).
dqfCOSY spectra8 of this mixture enabled identification
Figure 1. ESIþ-MS analysis of CM reaction mixtures showing
[MþNaþ] for starting material 9 and product 6. Ion signals
corresponding to chain shortened and chain extended homo-
logues are shown in red and blue, respectively. Conditions: (a)
G-II (0.05 equiv), BQ (0.1 equiv), 40 °C, 20 h; (b) HG-II (0.05
equiv), BQ (0.1 equiv), 40 °C, 20 h; (c) G-I (0.05 equiv), BQ (0.1
equiv), 40 °C, 20 h.
(5) (a) Schmidt, B. Eur. J. Org. Chem. 2004, 9, 1865–1880. (b)
Courchay, F. C.; Sworen, J. C.; Ghiviriga, I.; Abboud, K. A.; Wagener,
K. B. Organometallics 2006, 25, 6074–6086.
(6) (a) Fokou, P. A.; Meier, M. A. R. J. Am. Chem. Soc. 2009, 131,
1664–1665. (b) Mutlu, H.; de Espinoza, L. M.; Meier, M. A. R. Chem.
Soc. Rev. 2011, 40, 1404–1445.
(7) Hong, S. H.; Sanders, D. P.; Lee, C. W.; Grubbs, R. H. J. Am.
Chem. Soc. 2005, 127, 17160–17161.
(8) (a) Taggi, A. E.; Meinwald, J.; Schroeder, F. C. J. Am. Chem. Soc.
2004, 126, 10364–10369. (b) Pungaliya, C.; Srinivasan, J.; Fox, B. W.;
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Natl. Acad. Sci. U.S.A. 2009, 106, 7708–7713.
ofall significant isomerizationand homologation products
(17aꢀi), revealing extensive double-bond isomerization
and confirming CH2 deletions in 9 during CM, which
Org. Lett., Vol. 13, No. 21, 2011
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explains the formation of nor-homologues of metathesis
product 6. Analysis of the dqfCOSY spectra further
revealed large quantities of the chain-extended compounds
17d and 17e. However, only trace amounts of the chain-
Figure 3. Amounts of detected homologation and isomerization
products correlate with chain lengths of CM reactants. Percen-
tages refer to amounts of homologated products formed in G-II-
catalyzed CM without BQ, as determined by ESIþ-MS (see
Supporting Information for conditions). Additional side pro-
duct formation occurs due to product isomerization.
Given that both G-II and HG-II have been employed
successfully in many CM reactions,9 we investigated whether
the degree of homologation and isomerization depends
on specific properties of the reactants 7ꢀ9 (Figure 3).
Using the G-II catalyst without BQ, we found that CM
of 9 with allyl benzyl ether proceeded cleanly without
formation of isomerized products or substrates, whereas
CM of 9 and homoallyl benzyl ether and homodimeriza-
tion of 9 consistently yielded significant amounts of iso-
merized and homologated products (Figure S6). Even
larger quantities of isomerized and homologized products
were obtained in G-II-catalyzed homodimerization reac-
tions of 7 and 8 (Figures S7, S8), similar to amounts of side
products produced in G-II-catalyzed CM reactions of 9
with 7 or 8 (Figures S3, S4). Thus it appears that the
severity of isomerization in our metathesis reactions gen-
erally correlates with the chain lengths of the reaction
partners and products (Figures 3, S3, S4, S6ꢀS8). That
long-chain, lipophilic substrates are particularly suscepti-
ble to homologation and isomerization during CM is also
suggested by recent acyclic diene metathesis (ADMET)
studies.6,10 Avoiding homologation and isomerization is of
importance for both ADMET and natural product synthesis,
as physical or biological properties may be affected greatly
by small amounts of structurally similar impurities. Our
results show that among tested catalysts, G-I with added
BQ is most suitable for CM involving long-chain substrates.
To complete the synthesis of the caeliferins, CM pro-
ducts 5 and 6 were deprotected11 and converted into acids
15 and 16 via sequential Swern and Pinnick oxidation.12
Figure 2. Section of dqfCOSY spectrum of the mixture of 17 and its
isomers and homologues, as derived from the corresponding
mixture of isomers and homologues of 9 isolated from CM of 8
with 9 using G-II and BQ (600 MHz, CDCl3, see Figure S1 for full
spectrum). Intensity of parts of the spectrum was scaled (1.5x, 2.5x).
extended terminal alkene 17c were found. This is con-
sistent with the hypothesis that CH2-extended variants
of 9 form mostly via ethylidene transfer during CM of
isomerized starting materials, without significant contribu-
tion from isomerized product 6 (isomerization and homo-
logation pathways are shown in Figure S2). The low
abundance of the CH2-extended terminal alkene 17c also
explainsthatonlytrace amountsof CH2 insertion products
of 6 were observed (m/z 745.5 in Figure 1), as their
formation in the absence of chain-extended terminal al-
kenes would require multiple isomerization and metathesis
steps (see Figure S2). In contrast to G-II and HG-II, use of
the G-I catalyst in the presence of BQ did not result in
homologation (Figure 1c) or isomerization (as confirmed
by dqfCOSY), even when using as much as 0.1 equiv of
catalyst and reaction times of up to 48 h. Using the G-I
catalyst, 6 was obtained in 80% yield (E/Z = 4:1), based
on consumed 9.
(10) Qin,H.;Chakulski, B. J.;Rousseau, I.A.;Chen,J.;Xie, X.-Q.;Mather,
P. T.; Constable, G. S.; Coughlin, E. B. Macromolecules 2004, 37, 5239–5249.
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at concentrations corresponding to those found in grass-
hopper saliva,1 the tested caeliferins strongly induced
ethylene production (Figure 4), revealing differences in
the relative potency of different compounds.1 The ethylene-
inducing activity of synthetic caeliferin A16:0 (3) is similar
to that previously reported.1
In summary, we report the first enantioselective synth-
eses of (R)-1, (R)-3, (R)-4, and (S)-4, using CM conditions
suitable for long-chained substrates. Our CM-based caeli-
ferin synthesis is flexible and provides access to related
compounds, which will enable studies aimed at identifying
molecular targets of these elicitors of plant immune
responses. A better understanding of the caeliferins’ biolog-
ical mode of action may facilitate the development of
sustainable pesticides that take advantage of natural plant
defense responses.
Figure 4. Ethylene emission of Arabidopsis seedlings after treat-
ment with synthetic caeliferin solutions; errors bars, s.d.; two-
tailed Student’s t test; *P < 0.05, **P < 0.005.
Acknowledgment. This work was supported by the
National Institutes of Health (GM079571 to F.C.S.) and
the O’Reilly Foundation (to I.O.D.). We thank Prof.
Geoffrey Coates for helpful discussions.
Desilylation followed by hydrogenation and sulfation
produced 1 and 3. For the preparation of pure (E)-4,
desilylated 16 was methylated and freed from the contam-
inating (Z)-isomer using AgNO3-impregnated silica gel.13
Samples of synthetic caeliferins were tested in Arabidopsis1
for their activity to elicit immune responses and thus mimic
the effects of a feeding herbivore’s saliva. When delivered
Supporting Information Available. Supporting figures,
experimental procedures, and spectroscopic data. This
material is available free of charge via the Internet at
http://pubs.acs.org.
(13) Williams, C. M.; Mander, L. N. Tetrahedron 2001, 57, 425–447.
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