A general and scalable synthesis of aeruginosin marine natural products based on two strategic C(sp3)-H activation reactions

Article abstract of DOI:10.1002/anie.201500066

An efficient and scalable access to the aeruginosin family of marine natural products, which exhibit potent inhibitory activity against serine proteases, is reported. This synthesis was enabled by the strategic use of two different, recently implemented C(sp³)-H activation reactions. The first method led to the common 2-carboxy-6-hydroxyoctahydroindole (Choi) core of the target molecules on a large scale, whereas the second one provided rapid and divergent access to the various hydroxyphenyllactic (Hpla) subunits. This strategy allowed the synthesis of the aeruginosins 98B and 298A, with the latter being obtained in unprecedentedly large quantities.

Full text of DOI:10.1002/anie.201500066

Angewandte
Chemie
DOI: 10.1002/anie.201500066
Total Synthesis
A General and Scalable Synthesis of Aeruginosin Marine Natural
3
ꢀ
Products Based on Two Strategic C(sp ) H Activation Reactions**
David Dailler, Grꢀgory Danoun, and Olivier Baudoin*
Abstract: An efficient and scalable access to the aeruginosin
Table 1: Structures of target aeruginosins.
family of marine natural products, which exhibit potent
inhibitory activity against serine proteases, is reported. This
Compound
R1
R2
R3
R4
Cl
synthesis was enabled by the strategic use of two different,
3
ꢀ
recently implemented C(sp ) H activation reactions. The first
ꢀ
aeruginosin 98A (1a)
SO3
method led to the common 2-carboxy-6-hydroxyoctahydroin-
dole (Choi) core of the target molecules on a large scale,
whereas the second one provided rapid and divergent access to
the various hydroxyphenyllactic (Hpla) subunits. This strategy
allowed the synthesis of the aeruginosins 98B and 298A, with
the latter being obtained in unprecedentedly large quantities.
(agmatine)
(d-allo-Ile)
d-allo-Ile
d-allo-Ile
ꢀ
aeruginosin 98B (1b)
aeruginosin 98C (1c)
SO3ꢀ agmatine
H
Br
SO3
agmatine
aeruginosin 298A (1d)
H
H
I
solated from marine sponges and cyanobacterial water
blooms, the aeruginosins include more than 20 congeners,
including aeruginosin 98A (1a), 98B (1b), 98C (1c), and
298A (1d; Scheme 1, Table 1), which have shown potent
in vitro inhibition of serine proteases.[1,2] These enzymes are
(l-argol)
(d-Leu)
involved in a number of important physiological
processes, including the blood coagulation cas-
cade. Aeruginosins have been isolated with
yields in the range of 0.01–0.05% from dried
algae, thereby stressing the need for the devel-
opment of an efficient and modular total syn-
thesis of these compounds. Structurally, the
aeruginosins 1a–d display four different ele-
ments linked by amide bonds, namely a 2-
carboxy-6-hydroxyoctahydroindole (Choi) core,
a C-terminus containing a terminal guanidine,
a hydrophobic amino acid, and a d-hydroxyphe-
nyllactic (Hpla) subunit. Retrosynthetic discon-
nections at the amide bonds thus provide four
different fragments (2–5). Different approaches
have been reported to construct the Choi core 2
in previous total syntheses of individual aerugi-
nosins,[3] including a Michael-type addition reported by the
groups of Bonjoch,[3a,c] Wipf,[3b] and Shibasaki,[3e] a ring-
closing metathesis[3d] and an aza-Prins reaction[3f] reported by
Hanessian and co-workers, a nucleophilic opening of an
oxabicyclic system reported by Carreira and co-workers,[3h]
and an intramolecular asymmetric allylic amination by the
Scheme 1. Retrosynthetic analysis. PG=protecting group.
[*] D. Dailler,[+] Dr. G. Danoun,[+] Prof. Dr. O. Baudoin
Universitꢀ Claude Bernard Lyon 1, CNRS UMR 5246, Institut de
Chimie et Biochimie Molꢀculaires et Supramolꢀculaires, CPE Lyon
43 Boulevard du 11 Novembre 1918 (France)
E-mail: olivier.baudoin@univ-lyon1.fr
Homepage: http://cosmo.univ-lyon1.fr/Index.html
[+] These authors contributed equally to this work.
group of Trost.[3i] We envisioned that our recently developed
3
ꢀ
[**] Financial support was provided by Rꢀgion Rhꢁne-Alpes (ARC
Santꢀ), Fondation Pierre Potier pour le Dꢀveloppement de la Chimie
des Substances Naturelles et ses Applications, and Institut
Universitaire de France. We thank Dr. G. Pilet, UCBL, for X-ray
diffraction analysis, the group of Prof. M. Lemaire, (in particular Dr.
E. Mꢀtay and Dr. C. Guyon), UCBL, for helpful suggestions, Dr. J.-Y.
Ortholand and Dr. H. Lemoine, Edelris, for assistance with
preparative HPLC.
intramolecular palladium(0)-catalyzed C(sp ) H alkenyla-
tion method[4,5] would allow access to 2 in a straightforward
and scalable manner, thereby enabling the collective synthesis
of the aeruginosins 1a–d. Indeed, the required bromocyclo-
hexene precursor 6 should be accessible from the abundant l-
alaninol in a few steps only. To rapidly and efficiently access
the Hpla fragments 5a–c bearing different substituents on the
benzene ring, a divergent approach from a common precursor
would be the best option. We envisioned that the intermo-
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201500066.
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[6]
ꢀ
lecular palladium(II)-catalyzed directed C H arylation of
attack of 12 onto any of the equivalent terminal carbon atoms
of the allyl cation arising from 11, on the opposite face to the
large OTBDPS group. The desired trans,syn diastereoisomer
was separated, and 6 was isolated after introduction of
a trifluoroacetyl group. Notably, 6 was obtained in 33%
overall yield from 9, and this six-step sequence could be
a suitable derivative of d-lactic acid would fulfill such an
3
ꢀ
objective. Altogether, the above C(sp ) H activation based
retrosynthetic strategy[7] should allow us to prepare 1a–d in
a straightforward and modular manner from the chiral pool.
ꢀ
The synthesis of 2 is depicted in Scheme 2. The C H
activation precursor 6 was prepared through a robust, and
conducted on multigram quantities with only one chromato-
3
ꢀ
graphic purification. The intramolecular C(sp ) H alkenyla-
tion of 6 was then re-optimized, taking previously reported
reaction conditions[4] as a starting point. An optimal yield was
obtained with the well-defined complex [Pd(PCy3)2] as the
catalyst, and K2CO3 and catalytic potassium pivalate as the
basic system in toluene at 1208C. Importantly, this protocol
allowed production of multigram quantities of the hexahy-
droindole 13 with yields in the range 68–71%. Hydrogenation
of the alkene in 13 was best performed using Rh/C as the
catalyst and occurred with high diastereoselectivity, presum-
ably controlled by the angular hydrogen, to give the
octahydroindole 14, the structure and absolute configuration
of which was confirmed by X-ray analysis (Scheme 2).[10]
Then, removal of the TBS group, oxidation of the resulting
primary alcohol to the carboxylic acid, and subsequent
esterification and reductive cleavage of the trifluoroacetyl
group furnished over 1 gram of 2 in 65–67% yield from 13 and
15% overall yield for the entire 11-step sequence from 9.
At the outset of our studies on the synthesis of the Hpla
3
ꢀ
fragments 5a–c through a directed C(sp ) H arylation
strategy, we tested various known directing groups such as
the quinolinyl and methylthiophenyl groups introduced by
Daugulis and co-workers[11] and the 2-pyridinylisopropyl
(PIP) group introduced by Shi and co-workers
(Scheme 3).[12] The latter turned out to be the best choice,
both because it furnished the highest arylation yields and
because it allowed preservation of the integrity of the
sensitive lactate stereogenic center. The PIP group was
Scheme 2. Synthesis of the Choi core 2. Reagents and conditions:
=
a) TBDPSCl, imidazole, CH2Cl2, 08C; b) (PCy3)2Cl2Ru CHPh (3 mol%),
CH2Cl2, 208C; c) CHBr3, NaOH, Et3BnNCl, CH2Cl2, ultrasound, 208C;
d) 1308C (neat), 88% for 4 steps, d.r. 6:1; e) 12, K2CO3, DMF, 20!
908C; f) (CF3CO)2O, pyridine, CH2Cl2, 208C, 38% for 2 steps; g) [Pd-
(PCy3)2] (10 mol%), K2CO3, tBuCO2K (30 mol%), toluene, 1208C, 68–
71%; h) 1. Rh/C (20 mol%), H2 (1 atm), ethyl acetate, 208C; 2. HCl
(0.5%), CH3OH, 208C, 95%; i) Jones’ reagent, acetone, 208C, 86%;
j) Me3SiCHN2, MeOH, 08C, 97%; k) NaBH4, MeOH, ꢀ10!208C,
83%. [a] X-ray structure of 14 (shown with 50% probability ellipsoids,
only selected H atoms are displayed for clarity). DMF=N,N-dimethyl-
formamide, TBDPS=tert-butyldiphenylsilyl, TBS=tert-butyldimethyl-
silyl.
easily scalable route starting from the commercially available
dihomoallylalcohol 9.[8] The latter was protected with the
bulky tert-butyldiphenylsilyl (TBDPS) group, and subsequent
ring-closing metathesis employing the Grubbsꢀ first-genera-
tion catalyst afforded the cyclopentenol 10 in quantitative
yield over two steps. Dibromocyclopropanation and thermal
electrocyclic ring opening[9] provided the racemic dibromo-
cyclohexene 11 as an inconsequential 6:1 mixture of diaste-
reoisomers. Nucleophilic substitution with the enantiopure
TBS-protected l-alaninol 12 provided a 1:1 mixture of the
two diastereoisomers possessing the trans relationship
between the nitrogen and oxygen substituents of the cyclo-
hexene ring. This outcome can be explained by the SN1-type
Scheme 3. Synthesis of the Hpla fragments 5a–c. Reagents and
=
conditions: a) BnOC( NH)CCl3, TfOH, hexane/CH2Cl2 (6:1), 208C;
b) LiOH, THF, 208C; c) 2-(pyridin-2-yl)propan-2-amine, HBTU, iPr2NEt,
DMF, 0!208C, 92% for 3 steps; d) 8a or 8b or 8c, Pd(OAc)2
(10 mol%), K2CO3, CH3CN, 1108C, yields: 78% for 16a, 50% for 16b,
61% for 16c; e) NOBF4, pyridine, CH2Cl2, ꢀ30!208C, yields: 92% for
5a, 76% for 5b, 70% for 5c. HBTU=O-(benzotriazol-1-yl)-N,N,N’,N’-
tetramethyluronium hexafluorophosphate, Tf=trifluoromethanesul-
fonyl.
2
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introduced by peptide coupling between 2-(pyridin-2-yl)pro-
pan-2-amine and Bn-protected d-lactic acid, the latter being
accessible in two steps and quantitative yield from commer-
cially available methyl d-lactate (15). After screening various
methyl ester was subsequently hydrolyzed. The coupling of
the resulting carboxylic acid with the Cbz-protected agmatine
3a furnished the intermediate 19 in high yield. The latter
underwent sulfation under previously reported reaction
conditions,[3i] followed by hydrogenolysis of all four benzyl-
containing protecting groups to give aeruginosin 98B (37 mg).
The synthetic product, which showed identical spectroscopic
data to previously reported compounds,[2b,3i] was obtained in
5.1% overall yield (84% average yield per step) for the
longest linear sequence of 17 steps starting from 9. In
principle, the same synthetic sequence employing 5b,c instead
of 5a would lead to aeruginosins 98A and 98C (Table 1). In
these cases, the final hydrogenation step is challenging
because of the presence of sensitive carbon–halogen bonds.
Studies towards these targets are currently on-going in our
laboratory and will be reported in due course.
Following this first success, we decided to perform the
synthesis of our second target, aeruginosin 298A (1d), on
a larger scale. The compound 1d possesses the same Choi core
and Hpla fragment as aeruginosin 98B, but a different hydro-
phobic amino acid and C-terminus (Scheme 5). First, 5a was
reacted with d-leucine methyl ester on gram scale, and the
resulting product 20 was hydrolyzed and coupled with 2,
thereby providing the intermediate 21. Then, similarly as
above with 1b, cleavage of the TBDPS group, followed by
ester hydrolysis and coupling with the protected l-argol
fragment 3b, which was synthesized through an improved
protocol starting from Boc-protected l-ornithine in four steps
(55% overall yield; Scheme 5, bottom), furnished 1.9 g of the
intermediate 22 in excellent yield from 21. Finally, acid-
mediated cleavage of the TBS group on the
ꢀ
reported reaction conditions for the C H arylation of 7 with
the benzyl-protected iodoarene 8a under palladium(II)
acetate catalysis, we found that K2CO3 as the base and
acetonitrile as the solvent allowed isolation of the aryllactate
16a in an optimal 78% yield. Notably, this procedure could be
conducted on gram scale without any racemization despite
the use of basic conditions, as verified by HPLC analysis on
a chiral stationary phase employing independently prepared
racemic samples of 7 and 16a as references. To remove the
PIP directing group in 16a without racemization of the base-
sensitive stereocenter, we designed a new, very mild protocol
employing NOBF4 as a nitrosation agent[13] and pyridine at
low temperature. In this manner, the enantiopure carboxylic
ꢀ
acid 5a was obtained in excellent yield. The same C H
arylation/PIP cleavage sequence was then applied to the
halogenated iodoarenes 8b,c, which are relevant to the
synthesis of halogenated aeruginosins 98A and 98C, thereby
furnishing the halogenated Hpla fragments 5b,c chemoselec-
tively and in good overall yields.
With the various requisite fragments in hand, we turned to
the synthesis of our first target, aeruginosin 98B (1b;
Scheme 4). First, 5a was reacted with d-allo-isoleucine
methyl ester[3i,14] under classical peptide coupling conditions
to give the intermediate 17, which upon hydrolysis and
peptide coupling with 2 gave the methyl ester 18. Then, the
TBDPS group of 2 was removed using Olahꢀs reagent, and the
argol fragment and hydrogenolysis provided
700 mg of 1d, which was isolated as the salt
formed with trifluoroacetic acid, as described
previously.[2a,3b,e,g] Remarkably, the synthetic
product was obtained in 8.2% overall yield
(86% average yield per step) for the longest
linear sequence of 17 steps starting from 9.
Both the overall yield and scale of this total
synthesis are unprecedented.[15]
In conclusion, we have provided a general
and scalable access to the aeruginosin family
of marine natural products possessing inter-
esting pharmacological properties, albeit low
availability from natural sources. For this
3
ꢀ
purpose, two recently discovered C(sp ) H
activation reactions were employed in a stra-
tegic manner, with the first one enabling
a large-scale synthesis of the common (Choi)
heterocyclic core of the molecules and the
second one a rapid and divergent access to
diversely decorated Hpla fragments. This
powerful strategy was successfully imple-
mented in the synthesis of aeruginosins 98B
and 298A, with the latter being performed on
unprecedentedly large scale, and should
streamline the access to other members of
this family of marine products, including the
halogenated congeners. Importantly, this
Scheme 4. Synthesis of aeruginosin 98B (1b). Reagents and conditions: a) d-allo-Ile-
OMe, PyBOP, iPr2NEt, CH2Cl2, 208C, 77%; b) LiOH, THF/MeOH 3:1, 0!208C; c) 2
(1.2 equiv), PyBOP, iPr2NEt, CH2Cl2, 208C, 60% for 2 steps; d) HF·pyridine, CH3CN,
08C, 95%; e) LiOH, THF/MeOH (2:1), 0!208C; f) 3a (1.2 equiv), HBTU, iPr2NEt,
DMF, 0!208C, 100% for 2 steps; g) SO3·pyridine, pyridine, 508C; h) H2, Pd(OH)2/C,
MeOH/AcOEt (1:1), 208C, 60% for 2 steps. PyBOP=(benzotriazol-1-yloxy)tripyrrolidino-
phosphonium hexafluorophosphate, THF=tetrahydrofuran.
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Hanessian, R. Margarita, A. Hall, S. Johnstone,
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V. Gnanadesikan, T. Shibuguchi, Y. Fukuta, T.
Nemoto, M. Shibasaki, J. Am. Chem. Soc. 2003,
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Scheme 5. Synthesis of aeruginosin 298A (1d). Reagents and conditions: a) d-Leu-
OMe, PyBOP, iPr2NEt, CH2Cl2, 208C, 82%; b) LiOH, THF/MeOH (3:1), 0!208C; c) 2
(1.2 equiv), PyBOP, iPr2NEt, CH2Cl2, 208C, 76% for 2 steps; d) HF·pyridine, CH3CN,
08C, 99%; e) LiOH, THF/MeOH (2:1), 0!208C; f) 3b (1.2 equiv), HBTU, iPr2NEt,
DMF, 0!208C, quant. for 2 steps; g) HCl (0.5%), MeOH, 08C, 78%; h) H2, Pd(OH)2/
=
=
C, MeOH/AcOEt (1:1), 208C, then CF3CO2H, 93%; i) TfN C(NHCbz)2, CF3C( O)-
N(Me)SiMe3, Et3N, CH2Cl2, 208C, 76%; j) isobutyl chloroformate, N-methylmorpholine,
THF, ꢀ108C, then NaBH4, THF/H2O (4:1), 0!208C, 74%; k) TBSCl, imidazole,
CH2Cl2, 0!208C, 99%; l) TMSOTf, 2,6-lutidine, CH2Cl2, ꢀ78!208C, 100%. TMS=tri-
methylsilyl.
ꢀ
[7] For reviews on C H functionalization strategies
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study allows access to significant quantities of pure com-
pound, and therefore should enable more advanced pharma-
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[9] For a review: B. Halton, J. Harvey, Synlett 2006, 1975.
[10] CCDC 1029844 (14) contains the supplementary crystallo-
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Received: January 5, 2015
Published online: && &&, &&&&
ꢀ
Keywords: C H activation · natural products · palladium ·
synthetic methods · total synthesis
.
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Yamaguchi, Tetrahedron Lett. 1994, 35, 3129 – 3132; b) M.
Murakami, K. Ishida, T. Okino, Y. Okita, H. Matsuda, K.
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13, 2649 – 2652.
[15] The previous largest-scale synthesis allowed production of 59 mg
of 1d in 3.1% overall yield.[3a,c] The previous most efficient
synthesis of 1d displayed an overall yield of 5.3%.[3g]
[3] a) N. Valls, M. Lꢁpez-Canet, M. Vallribera, J. Bonjoch, J. Am.
Chem. Soc. 2000, 122, 11248 – 11249 (aeruginosin 298A); b) P.
4
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Angew. Chem. Int. Ed. 2015, 54, 1 – 5
These are not the final page numbers!

Angewandte
Chemie
Communications
Total Synthesis
Family ties: An efficient access to the
aeruginosin family of marine natural
products, which exhibit potent inhibitory
activity against serine proteases, was
D. Dailler, G. Danoun,
O. Baudoin*
&&& — &&&
achieved. The strategic use of two differ-
3
ꢀ
A General and Scalable Synthesis of
ent C(sp ) H activation reactions led to
Aeruginosin Marine Natural Products
the synthesis of aeruginosins 98B and
298 A, with the latter being obtained on
an unprecedentedly large scale.
3
ꢀ
Based on Two Strategic C(sp ) H
Activation Reactions
Angew. Chem. Int. Ed. 2015, 54, 1 – 5
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!