Diversification of Peptidomimetics and Oligopeptides through Microwave-Assisted Rhodium(III)-Catalyzed Intramolecular Annulation

Article abstract of DOI:10.1002/adsc.201900550

A chemoselective rhodium(III)-catalyzed cascade annulation for the construction of the indolizinone and quinolizinone scaffolds is developed. Diversification of peptidomimetics and oligopeptides is achieved in a rapid and step-economical manner through the combination of Ugi reaction and microwave-assisted rhodium(III)-catalyzed intramolecular annulation via C(sp²)-H activation without installing a directing group. (Figure presented.).

Full text of DOI:10.1002/adsc.201900550

Title: Diversification of Peptidomimetics and Oligopeptides through
Microwave-Assisted Rhodium(III)-Catalyzed Intramolecular
Annulation
Authors: Liangliang Song, Guilong Tian, Anna Blanpain, Luc Van
Meervelt, and Erik Van der Eycken
This manuscript has been accepted after peer review and appears as an
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To be cited as: Adv. Synth. Catal. 10.1002/adsc.201900550
Link to VoR: http://dx.doi.org/10.1002/adsc.201900550

10.1002/adsc.201900550
Advanced Synthesis & Catalysis
COMMUNICATION
DOI: 10.1002/adsc.201900550
Diversification of Peptidomimetics and Oligopeptides through
Microwave-Assisted Rhodium(III)-Catalyzed Intramolecular
Annulation
Liangliang Song,a Guilong Tian,a* Anna Blanpain,a Luc Van Meerveltb and Erik V.
Van der Eyckenac*
a
Laboratory for Organic & Microwave-Assisted Chemistry (LOMAC), Department of Chemistry, KU Leuven
Celestijnenlaan 200F, 3001, Leuven, Belgium. guilongtian6@gmail.com; erik.vandereycken@kuleuven.be.
Biomolecular Architecture, Department of Chemistry, KU Leuven Celestijnenlaan 200F, 3001, Leuven, Belgium.
Peoples’ Friendship University of Russia (RUDN University), Miklukho-Maklaya Street 6, Moscow, Russia.
b
c
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.((Please
delete if not appropriate))
cells.[8] However, contemporary labelling strategies
Abstract.
A
chemoselective rhodium(III)-catalyzed
cascade annulation for the construction of the indolizinone are mainly limited to classical reactions[9] and
and quinolizinone scaffolds is developed. Diversification
transition metal-catalyzed cross-coupling reactions[10]
,
of peptidomimetics and oligopeptides is achieved in a rapid
and step-economical manner through the combination of
Ugi reaction and microwave-assisted rhodium(III)-
catalyzed intramolecular annulation via C(sp2)-H
activation without installing a directing group.
largely relying on prefunctionalizations, often leading
to the formation of undesired byproducts and lengthy
synthetic procedures.
Keywords: Annulation; C-H Activation; Rhodium;
Peptidomimetic; Peptide
The Ugi four-component reaction (Ugi-4CR) is
regarded as one of the most attractive reactions for
synthesizing multifunctional compounds,[1] especially
for the generation of polymorphous peptidomimetics
and peptides.[2] Moreover it also provides an
opportunity for various post-transformations with
diverse
functional
groups
to
synthesize
pharmacologically important heterocyclic scaffolds,
mostly in two operational steps.[3] Furthermore,
microwave-assisted organic synthesis (MAOS) has
been proven to be a powerful strategy in organic
chemistry. During the past decade, researchers are
investigating
microwave-assisted
C-H
functionalizations to develop green, clean and efficient
access for the construction of diverse heterocycles.[4]
Site specific modification of peptides is necessary
for many biological and therapeutic applications.[5]
Peptides from nonproteinogenic amino acids are
required for medicinal and pharmaceutical chemistry,
since they show improved pharmacokinetics and
bioactivity in comparison with their natural versions.[6]
For instance, peptides labeled with fluorescent groups
could be used for realtime tracking of biomolecules,
Scheme 1. Previous strategies and this approach.
Recently, transition metal-catalyzed C-H activation
has emerged as an increasingly powerful strategy for
direct peptide modification[11-17]. Despite major
advances, late-stage peptide diversification frequently
focuses on the synthesis of non-natural peptides or
fluorescence peptide labelling through one-step
coupling, and mostly requires external monodentate
(Scheme 1a) or bidentate auxiliary assistance (Scheme
1b), causing the need to introduce and remove a
directing group. Based on our previous studies on the
metabolites
and
cells
under
physiological
conditions.[7] In addition, peptides labeled with drugs
have been designed for selectively targeting tumor
1
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10.1002/adsc.201900550
Advanced Synthesis & Catalysis
functionalization of amino acids and the construction
of polyheterocyclic frameworks,[4a, 18] we herein report
the chemoselective rhodium(III)-catalyzed cascade
annulation for the synthesis of the indolizinone and
quinolizinone scaffolds, which forms the core of many
natural products with pharmacological relevance, such
100 W for 1 h, yielded 2a in 92% (Table 1, entry 10).
A higher temperature of 120 °C for 1 h (100 W),
delivered 2a in 99% yield (Table 1, entry 11). Only
36% yield of 2a was 1observed when the reaction was
performed under conventional heating for 1h at 120 °C
in t-AmOH (Table 1, entry 12). We also tried other
catalysts, such as [Cp*IrCl2]2 and Cp*Co(CO)I2, but
both of them did not work under the standard
conditions (Table 1, entries 13-14).
as rosettacin, oxypalmatime, camptothecin and
[19-20]
norketoyobyrine
(Scheme 1c and 1d).
Remarkable features of our approach include 1) the
development of a versatile rhodium(III)-catalyzed
intramolecular cascade annulation of peptidomimetics
and peptides, 2) post-translational peptidomimetic and
peptide modification in a rapid and step-economical
manner through the combination of Ugi reaction and
microwave-assisted C-H activation, 3) the utilization
of an internal amide group as directing group to build
up the indolizinone or quinolizinone scaffolds, and 4)
the synthesis of complex fused ring system in a
twofold and threefold manner.
Table
2.
Rhodium(III)-catalyzed
annulation
of
peptidomimetics.a)b)
Table 1. Optimization of the reaction conditions.a)
a)
Conditions: 1a (0.3 mmol), catalyst (0.015 mmol),
Cu(OAc)2 (0.6 mmol), CsOAc (0.6 mmol), solvent (3.0 mL).
b)
1
Determined by H NMR analysis of the crude reaction
c)
d)
e)
mixture.
120 °C.
Isolated yield.
Reaction was
performed conventionally at 120 °C in t-AmOH.
We commenced our exploration with the
optimization studies on the annulation of
peptidomimetic 1a, which was readily constructed
through Ugi-4CR. When the reaction was performed
with [RhCp*Cl2]2 (5 mol%), Cu(OAc)2 (2 equiv) and
CsOAc (2 equiv) in t-AmOH at 110 °C for 1 h with
300 W maximum power, the indolizinone 2a was
obtained in 90% yield (Table 1, entry 1). Then various
solvents were screened, showing that t-AmOH was the
best one (Table 1, entries 2-7). When [Ru(p-
cymene)Cl2]2 was used, a lower conversion was
observed (Table 1, entry 8). Extension of the reaction
time to 2 h resulted in the formation of 2a in 99% yield
(Table 1, entry 9). Decreasing the maximum power to
a)
Conditions: 1 (0.3 mmol), [RhCp*Cl2]2 (0.015 mmol),
Cu(OAc)2 (0.6 mmol), CsOAc (0.6 mmol), t-AmOH (3.0
b)
mL). Isolated yield. Cy = cyclohexyl, TMB = 1,1,3,3-
tetramethylbutyl, ADM
toluenesulfonylmethyl.
= adamantyl, TOSM = p-
With the optimal conditions in hand, we next
evaluated the scope of the protocol (Table 2).
2
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10.1002/adsc.201900550
Advanced Synthesis & Catalysis
Scheme 2. Twofold and threefold rhodium(III)-catalyzed annulation.
yielded the corresponding indolizinones 2b-d in 87-
92%. However, the peptidomimetic 1e’ did not
undergo the reaction. Thereaction was also applicable
to para-Me and -CF3 substrates, giving the
indolizinones 2e (93%) and 2f (79%). The reactions
with meta- or ortho-Me substrates smoothly afforded
the corresponding indolizinones in 86% (2g) and 74%
(2h) yield respectively. The phenyl substituent of the
alkyne could be replaced by a cyclopropyl, cyclohexyl
or TBS group, yielding the indolizinones 2i-k in 58-
82%. However, the terminal alkyne substrate failed to
give the corresponding indolizinone 2l. The substrate
with a fluoro or dimethoxy group in the phenyl
substituent derived from the benzaldehyde, turned out
to be compatible, offering the indolizinones 2m (95%)
and 2n (93%). The 2-ethynylquinoline substrate
smoothly reacted to yield the corresponding
indolizinone 2o (78%), which has the same skeleton as
the natural product rosettacin. The annulation worked
well with substrates bearing a longer carbon chain
between the benzamide and the phenylalkyne, leading
to the corresponding quinolizinones 2p (89%) and 2q
(73%), having the same scaffold as the natural product
oxypalmatime.
Table
3.
Rhodium(III)-catalyzed
annulation
of
oligopeptides.a)b)
Considering the fact that the strong coordination
activity of medicinally important heterocycles
continues to constitute major challenges to transition
metal catalysts, often resulting in poisoning of the
[21]
catalyst or undesired C-H bond activation,
we
extended the reaction scope to heteroaryl
peptidomimetics, containing a furan, a thiophene, a
pyrrole, a benzofuran, a benzothiophene or an indole
moiety. They all smoothly afforded the corresponding
indolizinones 2r-w in 52-96% yield. Even
isonicotinamide
and
thiazole-5-carboxamide
substrates delivered the corresponding indolizinones
2x (78%) and 2y (46%). It was also observed that the
benzamide substrates could be replaced by α-
substituted acrylamide substrates, resulting in the
corresponding indolizinones 2z-b’ in 59-89% yield.
However, β-substituted and α, β-disubstituted
acrylamide substrates 1c’ and 1d’ did not undergo the
a)
Conditions: 5 (0.3 mmol), [RhCp*Cl2]2 (0.015 mmol),
Cu(OAc)2 (0.6 mmol), CsOAc (0.6 mmol), t-AmOH (3.0
mL). b) Isolated yield.
Peptidomimetics bearing various secondary amide
groups derived from the corresponding isocyanides,
3
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10.1002/adsc.201900550
Advanced Synthesis & Catalysis
reaction. Maybe the β-substitution or α, β-
disubstitution of acrylamides sterically prevented the
cyclometalation. Interestingly, the robustness of the
RhIII-catalyzed C-H activation approach enabled
twofold and threefold annulation to deliver the di- and
trimers 4 in high yields (Scheme 2).
Subsequently, we evaluated our procedure for
oligopeptides (Table 3). Dipeptides 5a-c were well
tolerated, affording the corresponding indolizinones
6a-c in 72-86%. Compound 6c was unambiguously
characterized by X-ray crystallography.[22] Diverse tri-,
tetra- and pentapeptides were compatible with this
approach, delivering the corresponding indolizinones
6d-k with excellent levels of chemoselectivity. LC-
MS analysis showed that there was no detectable
racemization of indolizinone 6h (see SI).
and threefold cascade annulation is carried out to
produce complex fused ring system.
Experimental Section
To a 10 mL glass tube equipped with a stir bar were added
1, 3 or 5 (0.3 mmol), [RhCp*Cl2]2 (0.015 mmol), CsOAc
(0.6 mmol), Cu(OAc)2 (0.6 mmpl) and t-AmOH (3 mL)
without any particular precautions to extrude oxygen or
moisture. The reaction mixture was irradiated under MW at
120 °C with maximum power of 100 W for 1 h, then cooled
to room temperature. The solvent was removed in vacuo and
the remaining residue was purified by a silica gel column
chromatography (n-heptane/ethyl acetate) to afford the
product 2, 4 or 6.
Acknowledgements
Liangliang Song and Guilong Tian appreciate the China
Scholarship Council (CSC) for providing a doctoral scholarship.
We acknowledge the FWO [Fund for Scientific Research-Flanders
(Belgium)] for financial support. We acknowledge the support of
RUDN University Program 5-100.
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Advanced Synthesis & Catalysis
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5
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10.1002/adsc.201900550
Advanced Synthesis & Catalysis
COMMUNICATION
Diversification of Peptidomimetics and
Oligopeptides through Microwave-Assisted
Rhodium(III)-Catalyzed Intramolecular Annulation
Adv. Synth. Catal. Year, Volume, Page – Page
Liangliang Song,a Guilong Tian,a* Anna Blanpain,a
Luc Van Meerveltb and Erik V. Van der Eyckenac*
6
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