Investigation of the electrophilic reactivity of the biologically active marine sesquiterpenoid onchidal and model compounds

Article abstract of DOI:10.3762/bjoc.14.197

The structure of the sesquiterpene onchidal (6), a component of the defensive secretion of the shell-less mollusc Onchidella binneyi, contains a masked α,β-unsaturated 1,4-dialdehyde moiety, the presence of which has been proposed to be the cause of the feeding deterrent activity exhibited by the mollusc. We have found onchidal acts as an electrophile, reacting rapidly with the model nucleophile n-pentylamine forming diastereomeric aminated pyrrole adducts. Somewhat surprisingly, no reaction was observed between onchidal and n-pentanethiol. Structurally simplified n-pentyl 11–13 and cyclohexylmethyl 15–17 analogues of onchidal were prepared and demonstrated similar amine-selective reactivity. Onchidal and analogues reacted with the model protein lysozyme, forming covalent adducts and leading to protein cross-linking. These results provide preliminary evidence supporting the molecular mechanism of biological activity exhibited by onchidal.

Full text of DOI:10.3762/bjoc.14.197

Investigation of the electrophilic reactivity of the biologically
active marine sesquiterpenoid onchidal
and model compounds
Melissa M. Cadelis* and Brent R. Copp
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2018, 14, 2229–2235.
School of Chemical Sciences, University of Auckland, Private Bag
doi:10.3762/bjoc.14.197
92019, Auckland 1142, New Zealand
Received: 02 May 2018
Accepted: 02 August 2018
Published: 24 August 2018
Email:
Melissa M. Cadelis* - m.cadelis@auckland.ac.nz
*
Corresponding author
Associate Editor: S. Bräse
Keywords:
© 2018 Cadelis and Copp; licensee Beilstein-Institut.
License and terms: see end of document.
dialdehyde; lysozyme; mollusc; onchidal; pyrrole
Abstract
The structure of the sesquiterpene onchidal (6), a component of the defensive secretion of the shell-less mollusc Onchidella binneyi,
contains a masked α,β-unsaturated 1,4-dialdehyde moiety, the presence of which has been proposed to be the cause of the feeding
deterrent activity exhibited by the mollusc. We have found onchidal acts as an electrophile, reacting rapidly with the model nucleo-
phile n-pentylamine forming diastereomeric aminated pyrrole adducts. Somewhat surprisingly, no reaction was observed between
onchidal and n-pentanethiol. Structurally simplified n-pentyl 11–13 and cyclohexylmethyl 15–17 analogues of onchidal were pre-
pared and demonstrated similar amine-selective reactivity. Onchidal and analogues reacted with the model protein lysozyme,
forming covalent adducts and leading to protein cross-linking. These results provide preliminary evidence supporting the molecular
mechanism of biological activity exhibited by onchidal.
Introduction
More than 80 terpenoid natural products containing the 1,4- The ichthyotoxic masked dialdehyde caulerpenyne (3), a major
dialdehyde moiety have been isolated from sources such as component of extracts of the green alga Caulerpa taxifolia, ex-
fungi, algae, sponges and molluscs [1]. Many of these natural hibits antiproliferative activities as well as wound healing abili-
products exhibit biological activity, ranging from anti-inflam- ties with the latter resulting from rapid transformation to the
matory to antimicrobial and antifeedant activities [1]. The highly reactive 1,4-dialdehyde, oxytoxin 2 (4) [4-6]. Oxytoxin 2
prototypical examples polygodial (1) and scalaradial (2, (4) is itself a natural product, produced by the mollusc Oxynoe
Figure 1) both exhibit antifeedant activity against worms and olivacea from a diet-derived (Caulerpa algae) precursor and is
fish [1,2], with recent studies also showing that 1 is a potential predominantly present in the predator-deterring mucous secre-
lead as a marine antifouling agent [3].
tion of the mollusc [7]. Two structurally-related masked dialde-
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Beilstein J. Org. Chem. 2018, 14, 2229–2235.
Figure 1: Masked and unmasked 1,4-dialdehyde natural products 1–6.
hydes, 5 (from Caulerpa ashmeadii) [8] and onchidal (6) [9,10] product, with a protonated molecular ion of m/z 373.3556
from the defensive secretion of the mollusc Onchidella [M + H]+ corresponding to a formula of C25H45N2 (requires
binneyi) also exhibit biological properties including feeding 373.3577). NMR data further supported such a structure, with
deterrence, antibacterial and anticholinesterase activities. pyrrole signals observed at [δH 6.57–6.55, m, H-1" and H-4";
.06, br s, H-3"; δC 120.6 (C-2" and C-4"); 118.5 (C-1"); 106.8
(
6
Chemical reactivity studies using polygodial (1), scalaradial (2) (C-3")] and pentylamine substitution at C-1 [δH 3.50–3.46, m;
and caulerpenyne (3) have demonstrated evidence of pyrrole δC 54.1]. In the case of the more polar products 8 and 9,
formation upon reaction with primary amines, with conclusions (+)-ESIMS derived the same formula as for 7, while differ-
drawn attributing bioactivities such as antifeedant activity to ences observed in 1H NMR shifts for H-1/H-2/H-1' between 7
this chemical reactivity [1,11-13]. In an effort to ascertain and 8 [δ8–7, Δδ +1.29–0.42] suggested 8/9 were purified as
whether the mollusc metabolite onchidal is susceptible to salts.
nucleophilic attack in a similar manner, herein we report on the
reactivity of onchidal and a library of simplified n-pentyl and
cyclohexylmethyl model compounds towards thiol and amine
nucleophiles as well as their reactivity towards a model protein
target, lysozyme.
Results and Discussion
Preliminary studies of the reactivity of onchidal (6) towards
1
-pentanethiol or 1-pentylamine were undertaken in CDCl3 sol-
vent in an NMR tube. Somewhat to our surprise, no reaction
was observed with 1-pentanethiol, even with incubation in the
presence of excess thiol for one week [14]. In contrast, incuba-
tion with excess 1-pentylamine rapidly afforded a mixture of
products, as identified by changes in the 1H NMR spectrum.
Signals attributable to N-alkyl-3-substituted pyrroles 7–9 and
N-pentylacetamide 10 [δH 7.62 t, J = 4.7 Hz; 2.28 m] were ob-
served. Purification by silica gel column chromatography,
eluting with CH2Cl2, afforded pyrrole adduct 7 as the free base.
Elution with CH2Cl2/MeOH afforded two fractions with the
first comprised of a single diastereomer as a salt 8, while a
Scheme 1: Products of the reaction of onchidal (6) with 1-pentylamine.
Reagents and conditions: 1-pentylamine (excess), CDCl3, overnight.
second fraction was obtained as a diastereomeric mixture (8:9, A mechanism that leads to the formation of diaminated pyrrole
:1), again as salts (Scheme 1). Mass spectrometric data ob- adduct 7 starts with amine-induced formation of a 1,4-dialde-
served for 7 supported the formation of a diaminated pyrrole hyde, which then undergoes Paal–Knorr pyrrole formation to
3
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Beilstein J. Org. Chem. 2018, 14, 2229–2235.
give an azafulvinium intermediate (Scheme 2). This intermedi- diesters, purification of which by silica gel column chromato-
ate could then undergo trapping with an additional mole of graphy afforded a fraction of the desired E diester 20 (60%), a
amine nucleophile to give 7 as a mixture of diastereomers. second fraction comprised of a 5:1 E/Z mixture and a third frac-
tion of Z diester 21 (10%, Scheme 3). The reduction of diesters
2
0 (E) and 21 (Z) with LiAlH4 afforded diols 22 and 23 in 63%
and 67% yield, respectively. Subsequent oxidation of 22 with
DMP afforded dialdehyde 11 in 31% yield. Correspondingly,
the reaction of diol 23 with DMP afforded a mixture of dialde-
hyde 11 with dialdehyde 12 (1:1). Attempts at chromatographic
separation of these two isomers resulted in degradation of 12.
Final conversion of 11 to enol acetate 13 was achieved by
overnight reaction with pyridine and acetic anhydride. Purifica-
tion by silica gel column chromatography afforded the desired
E,E enol acetate 13 in 17% yield. A lack of purified dialdehyde
1
2 prevented any attempt at the preparation of enolacetate 14.
Scheme 2: Proposed mechanism for formation of onchidal diaminated
adducts.
In an effort to reduce the complexity of the NMR spectra ob-
served for the diastereomeric onchidal–pyrrole adducts, a range
of simpler achiral n-pentyl 11–14 and cyclohexylmethyl 15–18
side-chained model compounds, as either the dialdehyde or
masked dialdehyde variants, were prepared (Figure 2).
Scheme 3: Synthesis of n-pentyl dialdehydes 11 and 12 and enol
acetate 13. Reagents and conditions: a) n-hexanal (0.8 equiv),
LiOH·H2O (1.2 equiv), THF, 4 h, 60% (20), 10% (21);
b) LiAlH4 (2.5 equiv), Et2O, 0 °C, 1 h, 63% (22), 67% (23); c) DMP
(2.5 equiv), CH2Cl2, 4 h, 31% (11); d) Ac2O (2 equiv),
pyridine (4 equiv), overnight, 17% (13).
Having developed a successful synthetic route to n-pentyl side-
chain dialdehyde 11 and enol acetate 13, the synthesis of ana-
logues 15–18 with a side-chain more comparable to onchidal
(
6) were attempted. H.W.E reaction of 2-cyclohexylacetalde-
hyde (24) [16] with phosphonoester 19 afforded a fraction of
the desired E diester 25 in 15% yield, a fraction of Z diester 26
in 1.5% yield and another fraction of a mixture of the two (5:1)
Figure 2: Target onchidal model compounds 11–18.
(
Scheme 4). The reaction of diesters 25 and 26 with LiAlH4
afforded the corresponding diols 27 and 28 in 61% and
Horner–Wadsworth–Emmons (H.W.E.) reaction of n-hexanal 71% yield, respectively, which upon oxidation (DMP) afforded
with phosphonoester 19 [15] afforded an E/Z mixture of olefinic dialdehydes 15 and 16 in 49% and 73% yield, respectively. The
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Beilstein J. Org. Chem. 2018, 14, 2229–2235.
reaction of dialdehyde 15 with Ac2O and pyridine afforded enol
acetate 17 in 43% yield after purification. Interestingly, the
reaction of dialdehyde 16 with Ac2O/pyridine only afforded de-
composition products, failing to give 18.
Figure 3: Pyrrole product 29 and salt 30 obtained from the reaction of
dialdehyde 11 with n-pentylamine.
Paal–Knorr pyrrole formation to form an azafulvenium interme-
diate which is subsequently quenched with another mole of
amine nucleophile to form the observed product (Scheme 5).
Scheme 4: Synthesis of cyclohexylmethyl dialdehydes 15 and 16 and
enol acetate 17. Reagents and conditions: a) phosphonate 19
Scheme 5: Reaction of dialdehyde 11 with excess 1-pentylamine to
form 29. Reagents and conditions: (a) 1-pentylamine (excess), CDCl3,
overnight.
(
1.3 equiv), LiOH·H2O (1.5 equiv), THF, 4 h, 15% (25), 1.5% (26);
b) LiAlH4 (2.5 equiv), Et2O, 0 °C, 1 h, 61% (27), 71% (28); c) DMP
2.5 equiv), CH2Cl2, 4 h, 49% (15), 73% (16); d) Ac2O (2 equiv),
pyridine (4 equiv), overnight, 43% (17).
(
Similar reactivity profiles were observed for each of cyclo-
The electrophilic reactivity of model dialdehydes 11 and 15 and hexylmethyl dialdehyde 15, and enol acetates 13 and 17, with
enol acetates 13 and 17 towards 1-pentanethiol and 1-penty- no reactivity towards 1-pentanethiol being detected, but with
lamine were then studied. As found for onchidal, no reaction rapid reaction with 1-pentyamine to form pyrrole adducts. In the
(
NMR tube) between 11/13/15/17 and 1-pentanethiol was case of dialdehyde 15, the reaction product was determined to
detected, even after one week of incubation. In direct contrast, be 31 (12% plus 18% as the salt, 32, Figure 4), while enol esters
all four model compounds reacted rapidly with 1-pentylamine, 13 and 17 gave 29 and 31 (7% and 5% yields), respectively,
forming pyrrole adducts. The reaction of dialdehyde 11 with upon reaction with the amine nucleophile.
1
-pentylamine afforded pyrrole adduct 29 almost instantaneous-
ly as determined by 1H NMR. Purification of the crude reaction
product gave 29 as the free base (15% yield) and as the salt, 30
(
also 15% yield, Figure 3). Spectroscopic and spectrometric
analysis of 29 confirmed the formation of a diamine
adduct, with detection of a protonated molecular ion in the
(
+)-ESI mass spectrum at m/z 307.3097 (C20H39N2 requires
07.3108) and NMR signals appropriate for a 3-substituted
3
Figure 4: Pyrrole product 31 and salt 32 obtained from reaction of
dialdehyde 15 with n-pentylamine.
N-alkylpyrrole [δH 6.57 dd, J = 2.3, 2.3 Hz, H-4"; 6.54 br s,
H-1"; 6.04 dd, J = 2.3, 2.3 Hz, H-3"; δC 120.8 (C-2"), 120.3
(
C-4"), 118.4 (C-1"), 106.2 (C-3")].
We next investigated the reactivity of onchidal (6) and ana-
As proposed for the onchidal–diamine adduct, the formation of logues 11–13 and 15–17 towards the lysine-rich model protein
9 is presumably a consequence of dialdehyde 11 undergoing lysozyme. Previous studies have reported hen egg white
2
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Beilstein J. Org. Chem. 2018, 14, 2229–2235.
lysozyme (HEWL) as a suitable target of electrophiles due to its
commercial availability, a well-characterized amino acid se-
quence and the ability for routine (+)-ESIMS analysis to iden-
tify covalent adduct formation [17].
Reactivity studies were conducted with commercially available
HEWL, in a solution of MeOH/H2O (+ 0.5% formic acid), and
the reaction products were investigated by (+)-ESIMS. Prelimi-
nary reaction of onchidal (6) with lysozyme was conducted in a
solvent mixture of MeOH/H2O (1:15) at 20 °C and examined
regularly by (+)-ESIMS. No adducts were detected at 20 hours,
but by day 3 (72 h), three new peaks representing mass addi-
tions of +198 mu, +216 mu, and +230 mu were detected
Figure 5: Lysine adducts arising from the reaction of onchidal (6) with
lysozyme.
(
Figure 5 and Table 1). These adducts are likely the result of the
reaction of lysine residues present in the enzyme [17]. The latter
two adducts are proposed to be pyrrole adducts with incorpora- be due to formation of insoluble higher order protein adducts.
tion of solvolytic H2O and methanol, respectively. The ESIMS analysis of the supernatant identified only a trace of
+
198 mu adduct could have arisen via elimination of H2O or unreacted lysozyme and detection of ions arising from exten-
methanol from the corresponding adducts, or alternatively, from sive modification of the enzyme. To simplify the analysis of
deprotonation of the anticipated lysozyme-onchidal azafulve- these adducts, the incubation time for 11 was shortened to
nium intermediate. The adduct product distributions were calcu- 4 hours, with resultant ESIMS analysis identifying the presence
lated from the deconvoluted (+)-ESI mass spectrum, identi- of the three expected pyrrole adducts with mass additions of
fying a total lysozyme modification yield of 15% (Table 1). The +132, +150 and +164 (Table 1). Interestingly, Z-dialdehyde 12,
presence of a large amount of unmodified lysozyme (85%), formed the same adducts as 11 but at a much slower rate,
even after 72 h, was attributed to the slow reactivity of the enol requiring overnight incubation. In addition to the expected
acetate functionality of onchidal, as observed in the original mono-adducts [+132, +150, +164], lysozyme di-adducts were
model studies.
also detected at +264 (2 × alkene), +282 (alkene and OH), +296
alkene and OMe), +314 (OH and OMe), and +328 (2 × OMe).
(
Next, the reactivity of dialdehydes 11, 12, 15 and 16 and enol
acetates 13 and 17 with lysozyme were examined in a similar Similar reactivity was observed for cyclohexylmethyl E-dialde-
manner with mass spectrometry identifying varying degrees of hyde 15, leading to the formation of a range of mono- (+158,
modification. Of the dialdehydes, 11 was the most reactive +176, +190) and di-adducts (+316 [2 × alkene], +334 [alkene
leading to rapid formation of a white precipitate, speculated to and OH], +348 [alkene and OMe], +366 [OH and OMe], +380
Table 1: Summary of lysozyme modifications by onchidal (6) and analogues 11–13 and 15–17.a
+
1 (%)c
+2 (%)c
OHd
No.
unmod (%)b
alkened
OHd
OCH3d
alkened
OCH3d
6
1
1
1
1
1
1
a,e
1f
2
85
37
13
82
10
30
93
5
24
10
0
4
18
8
6
21
21
0
0
0
0
0
0
0
0
0
0
0
0
5
14
0
3
18
7
0
5
14
18
2
15
22
2
11
0
11
0
6
30
3
7e
0
0
aStandard reaction conditions: 50 µM substrate, 10 µM lysozyme, in MeOH/H2O at 20 °C for 20 hours (unless otherwise noted). Product distribution
determined from deconvoluted (+)-HRESIMS data. bPercentage of unmodified lysozyme. cPercentage of mono-adduct (+1) and di-adduct (+2) prod-
ucts detected by (+)-ESIMS. dAlkene-, hydroxy and methoxy group containing adducts detected. In the case of di-adducts, ions observed consistent
for mixed nucleophilic quenching products, i.e., one hydroxy and one methoxy group are not reported in the Table. eIncubation time of 3 days.
fIncubation time of 4 hours.
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Beilstein J. Org. Chem. 2018, 14, 2229–2235.
[
2 × OMe]) (Table 1), while Z-dialdehyde 16 was comparative-
Supporting Information
ly less reactive, forming only mono-adducts. As expected, enol
acetates 13 and 17 were only slowly reactive, giving 18% and
Supporting Information File 1
Experimental procedures and characterization data of new
compounds.
7
% yield of adducts, respectively, with 17 requiring 72 hour
incubation.
[https://www.beilstein-journals.org/bjoc/content/
Sodium dodecyl sulfate polyacrylamide gel electrophoresis
supplementary/1860-5397-14-197-S1.pdf]
(
SDS-PAGE) was used to look for the presence of protein
crosslinking arising from the incubation of dialdehydes 11 and
5 and enol acetate 13 with lysozyme. Bands corresponding to
Supporting Information File 2
1
H and 13C NMR spectra of synthesized compounds.
https://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-14-197-S2.pdf]
1
[
dimers (28 kDa) were evident for both the n-pentyl and cyclo-
hexylmethyl dialdehydes, with a faint band at 50 kDa also
evident in the n-pentyl dialdehyde incubation reaction, indicat-
ing the presence of lysozyme trimers (Figure 6). No cross-
linking was detected for enol acetate 13, likely due to its low re- Acknowledgements
activity as determined from the n-pentylamine incubation We thank Dr. Michael Schmitz and Tony Chen for their assis-
studies.
tance with acquisition of NMR and mass spectrometric data.
ORCID® iDs
Melissa M. Cadelis - https://orcid.org/0000-0003-3883-0109
Brent R. Copp - https://orcid.org/0000-0001-5492-5269
References
1.
Dasari, R.; De Carvalho, A.; Medellin, D. C.; Middleton, K. N.;
Hague, F.; Volmar, M. N. M.; Frolova, L. V.; Rossato, M. F.;
De La Chapa, J. J.; Dybdal-Hargreaves, N. F.; Pillai, A.; Mathieu, V.;
Rogelj, S.; Gonzales, C. B.; Calixto, J. B.; Evidente, A.; Gautier, M.;
Munirathinam, G.; Glass, R.; Burth, P.; Pelly, S. C.;
van Otterlo, W. A. L.; Kiss, R.; Kornienko, A. ChemMedChem 2015, 10,
2
014–2026. doi:10.1002/cmdc.201500360
2. Cimino, G.; Sodano, G.; Spinella, A. Tetrahedron 1987, 43,
401–5410. doi:10.1016/S0040-4020(01)87721-8
Figure 6: SDS-PAGE separation of lysozyme after modification with
11 (left), 13 (middle), 15 (right).
5
3.
Moodie, L. W. K.; Trepos, R.; Cervin, G.; Larsen, L.; Larsen, D. S.;
Pavia, H.; Hellio, C.; Cahill, P.; Svenson, J. J. Nat. Prod. 2017, 80,
Conclusion
515–525. doi:10.1021/acs.jnatprod.6b01056
A chemical reactivity study of the opisthobranch mollusc
metabolite onchidal (6) has identified that it can react with
amines to form pyrrole products. The reaction was presumed to
proceed via amine-mediated conversion of the enolester con-
taining natural product to a 1,4-dialdehyde, which then under-
goes Paal–Knorr pyrrole formation. Structurally simplified
n-pentyl- and cyclohexylmethyl-dialdehydes were synthesized
and found to undergo similar pyrrole forming reactions with
pentylamine. These reactions were also apparent with the
lysine-rich enzyme hen egg white lysozyme, with onchidal (6)
and model compounds 11–13 and 15–17 affording pyrrole
4. Barbier, P.; Guise, S.; Huitorel, P.; Amade, P.; Pesando, D.; Briand, C.;
Peyrot, V. Life Sci. 2001, 70, 415–429.
doi:10.1016/S0024-3205(01)01396-0
5
.
.
Adolph, S.; Jung, V.; Rattke, J.; Pohnert, G. Angew. Chem., Int. Ed.
005, 44, 2806–2808. doi:10.1002/anie.200462276
2
6
Jung, V.; Pohnert, G. Tetrahedron 2001, 57, 7169–7172.
doi:10.1016/S0040-4020(01)00692-5
7. Cimino, G.; Crispino, A.; Di Marzo, V.; Gavagnin, M.; Ros, J. D.
Experientia 1990, 46, 767–770. doi:10.1007/BF01939963
8
.
.
Paul, V. J.; Littler, M. M.; Littler, D. S.; Fenical, W. J. Chem. Ecol. 1987,
3, 1171–1185. doi:10.1007/BF01020547
1
9
Ireland, C.; Faulkner, D. J. Bioorg. Chem. 1978, 7, 125–131.
doi:10.1016/0045-2068(78)90043-3
adducts of the enzyme that were detected by (+)-ESIMS. 10.Abramson, S. N.; Radic, Z.; Manker, D.; Faulkner, D. J.; Taylor, P.
The more reactive dialdehydes were also found to lead to
protein crosslinking with formation of lysozyme dimers
and trimers. Taken together, these results support the hypoth-
esis that onchidal (6) could be used in chemical defense in a
similar manner to related sesquiterpenoid dialdehydes and enol
esters.
Mol. Pharmacol. 1989, 36, 349–354.
1
1
1
1.Guerriero, A.; Depentori, D.; D’Ambrosio, M.; Pietra, F.
Helv. Chim. Acta 1995, 78, 1755–1762. doi:10.1002/hlca.19950780709
2.Potts, B. C. M.; Faulkner, D. J.; De Carvalho, M. S.; Jacobs, R. S.
J. Am. Chem. Soc. 1992, 114, 5093–5100. doi:10.1021/ja00039a021
3.Cimino, G.; Spinella, A.; Sodano, G. Tetrahedron Lett. 1984, 25,
4151–4152. doi:10.1016/S0040-4039(01)90207-2
2234

Beilstein J. Org. Chem. 2018, 14, 2229–2235.
14.Avonto, C.; Taglialatela-Scafati, O.; Pollastro, F.; Minassi, A.;
Di Marzo, V.; De Petrocellis, L.; Appendino, G. Angew. Chem., Int. Ed.
2011, 50, 467–471. doi:10.1002/anie.201005959
1
5.Li, S.; Wang, X.; Liu, L.; Kang, J.; Wang, L.; Liu, H.; Ruan, C.; Nie, A.;
Zheng, Z.; Xie, Y.; Zhao, G.; Xiao, J.; Hu, Y.; Zhong, W.; Cui, H.;
Zhou, X. 5-Membered-S-Heterocyclic Compounds and Their Use in
Ppreparing of Medicines for Treating or Preventing the
Obesity-Relating Diseases. CN Pat. PCT/CN2004/001118, April 6,
2006.
1
6.Peterlin, Z.; Li, Y.; Sun, G.; Shah, R.; Firestein, S.; Ryan, K.
Chem. Biol. 2008, 15, 1317–1327. doi:10.1016/j.chembiol.2008.10.014
7.Schnermann, M. J.; Beaudry, C. M.; Genung, N. E.; Canham, S. M.;
Untiedt, N. L.; Karanikolas, B. D. W.; Sütterlin, C.; Overman, L. E.
J. Am. Chem. Soc. 2011, 133, 17494–17503. doi:10.1021/ja207727h
1
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