C-H Olefination of Tryptophan Residues in Peptides: Control of Residue Selectivity and Peptide-Amino Acid Cross-linking

Article abstract of DOI:10.1021/acs.orglett.9b02894

There is high demand for new methods to modify peptides, for application in drug discovery and biomedicine. A C-H functionalization protocol for the olefination of tryptophan residues in peptides is described. The modification is successful for Trp residues at any position in the peptide, has broad scope in the styrene coupling partner, and offers opportunities for conjugating peptides with other biomolecules. For peptides containing both Trp and Phe, directing group manipulation enables full control of residue selectivity.

Full text of DOI:10.1021/acs.orglett.9b02894

Letter
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/OrgLett
C−H Olefination of Tryptophan Residues in Peptides: Control of
Residue Selectivity and Peptide−Amino Acid Cross-linking
Myles J. Terrey, Ashley Holmes, Carole C. Perry, and Warren B. Cross*
Department of Chemistry and Forensic Science, School of Science and Technology, Nottingham Trent University, Clifton Lane,
Nottingham, NG11 8NS, U.K.
*
S Supporting Information
ABSTRACT: There is high demand for new methods to modify
peptides, for application in drug discovery and biomedicine. A C−H
functionalization protocol for the olefination of tryptophan residues in
peptides is described. The modification is successful for Trp residues at
any position in the peptide, has broad scope in the styrene coupling
partner, and offers opportunities for conjugating peptides with other
biomolecules. For peptides containing both Trp and Phe, directing group
manipulation enables full control of residue selectivity.
eptide modification has a critical role to play in drug
Scheme 1. C−H Functionalization of Aromatic Amino Acids
in Peptides
P
discovery, the diagnosis of disease, and the understanding
1
,2
of biological mechanism and function. As examples, peptide-
3
−5
fluorophore conjugates,
which enable the imaging of
6
,7
biological systems, have been used to diagnose infection,
8
,9
and to aid surgery.
In drug discovery, where peptide
therapeutics often have higher potency and target specificity
10−12
than small molecule drugs,
synthetic peptide modification
can provide necessary improvements to drug stability, cell
13−16
penetration and oral bioavailability.
Of the large number of methods that have been developed
2
,17
for the modification of peptides,
most rely upon the
reactions of heteroatoms, which may themselves be crucial to
18,19
structure and biological function.
Alternatively, peptides
and proteins have been modified at carbon atoms by metal-
catalyzed cross-coupling reactions, but these methods require
2
0−22
the incorporation of non-natural amino acids.
Con-
sequently, new methods of peptide modification that operate
at carbon atoms and in native peptides are needed.
To achieve this goal, there has been rapid progress in
developing methods for the C−H functionalization of
2
3−38
3
peptides.
For example, C(sp )-H functionalization of
25
alanine residues can give rise to phenylalanine derivates.
2
C(sp )-H functionalization of aromatic amino acids has
focused almost entirely on tryptophan (Trp) residues, that
have been modified using arylation, alkynylation and allylation,
postsynthetic modification of tryptophan-containing peptides,
and demonstrate peptide−amino acid cross-linking; moreover,
for peptides containing both Phe and Trp residues, we describe
methods for the control of residue selectivity.
2
6−35
Scheme 1A.
In addition, the modification of phenyl-
alanine (Phe) residues in peptides has recently been achieved
through C−H olefination, Scheme 1B.
With a view to modifying complex peptides that contain
more than one aromatic amino acid, we sought to determine if
C−H olefination could be applied selectively to Trp residues in
peptides containing potentially competing Phe residues.
Herein, we report a C−H olefination protocol for the
36−38
Our investigation began by employing Fujiwara−Moritani
reaction conditions that we had previously optimized for the
3
7
olefination of phenylalanine-containing peptides. Hence, the
Received: August 15, 2019
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b02894
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
model dipeptide Ac-Gly-Trp-OMe (1a) was treated with 4
Table 1. Optimization of Reaction Solvent
equiv of styrene in the presence of Pd(OAc) (10 mol %) and
2
AgOAc (2.5 equiv) at 130 °C in tert-amyl alcohol, Scheme 2.
Scheme 2. Initial Investigation of the C−H Olefination of
a
Model Peptides 1a and 1b
entry
solvent
T/°C
time/h
yield/%
1
2
3
4
5
6
7
8
t-amylOH
1,2-DCE
MeCN
1,4-dioxane
THF
HFIP
PhMe
PhMe
100
100
100
100
100
100
100
100
48
48
48
48
48
48
48
2
60
60
42
40
36
0
85
82
ring; see Supporting Information. ICP analysis of 2b indicated
>
99.9% removal of Pd and Ag (Pd 6 ppm; Ag 50 ppm).
Having determined optimum conditions for the C−H
olefination of dipeptide 1b, we next investigated the scope of
the alkene. In this study, the substituent in the para position of
styrene was varied, Scheme 3. Pleasingly, both electron-
Scheme 3. Scope of the Alkene for the Modification of 1b
a
Reaction conditions: (i) Styrene (4 equiv), Pd(OAc) (10 mol %),
2
Ag(OAc) (2.5 equiv), air, t-amylOH (0.08 M).
However, 1a proved to be unreactive under these conditions.
In contrast, when the indole nitrogen was protected with a Boc
group, the Trp residue was reactive: reaction of the model
peptide Ac-Gly-Trp(Boc)-OMe (1b) gave the olefinated
peptide 2a in 38% yield, in which the Boc group had also
been cleaved. The unmodified and side-chain deprotected
peptide 1a accounted for the mass balance in this reaction.
When the reaction temperature was lowered to 100 °C, the
Boc group was not cleaved and the modified peptide 2b was
isolated in 60% yield. The use of a Boc group is a particularly
attractive feature of this reaction, as Boc is the most commonly
used tryptophan-protecting group in Fmoc-based solid-phase
peptide synthesis, and is readily cleaved to reveal the native
withdrawing (Cl−, F C−, NC−) and electron-donating
3
substituents (H C−, MeO−) were compatible with the C−
3
H olefination reaction. With a view to future applications in
biomolecule cross-linking, we also demonstrated the coupling
of 1b with the unnatural amino acid Ac-Phe(4-vinyl)-OMe
(SI-3) to give the peptide−amino acid conjugate 3f.
In order to establish the general applicability of the C−H
olefination, the reaction was applied to a range of di- and
tripeptides, Scheme 4. For dipeptides with the Trp residue at
the C-terminus (general structure Ac-AA-Trp(Boc)-OMe), the
adjacent aliphatic amino acids alanine and leucine proved
amenable to the reaction (5a−b, 62−68% yields). Methionine,
which contains a thioether group that is susceptible to
oxidation, also proved to be a compatible neighboring amino
acid residue (5c, 74% yield).
3
9
Trp residue.
To further optimize the reaction, the effect of the reaction
solvent was studied, Table 1. Carrying out the reaction in 1,2-
dichloroethane instead of tert-amyl alcohol gave a similar yield
of the modified peptide 2b (60%, entry 2). Other polar aprotic
solvents were also effective, but the yields of 2b were lower
(
entries 3−5). Hexafluoroisopropanol (HFIP) is a commonly
employed solvent in C−H functionalization and in peptide
chemistry, yet it proved ineffective in the reaction (entry
6
2
5,40
).
The best yield for the reaction was obtained using
toluene (85%, entry 7); indeed, with toluene as the solvent, the
reaction time could be significantly reduced to 2 h (82%, entry
8
). Under these high-yielding conditions a second modified
In our previous study of the C−H olefination of phenyl-
alanine residues, we noted that Phe residues were not modified
at the N-terminus of a peptide; to rationalize this finding, we
peptide was also isolated (2b′, 1% yield), in which the Trp
residue had additionally been modified at C-4 of the indole
B
DOI: 10.1021/acs.orglett.9b02894
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 4. C−H Olefination of Trp Residues in Di- and Tripeptides
proposed that, for Phe modification, bidentate coordination of
appears that bidentate coordination of the peptide backbone is
not essential and that the Boc group plays a directing group
role. Further support was obtained from the reaction of the
protected amino acid Ac-Trp(Boc)-OMe (6), which gave the
modified amino acid 7 in 39% yield, eq 1. The importance of
the peptide to the Pd catalyst was required, through two amide
37,41
groups of the peptide backbone.
Likewise here, the peptide
Ac-Phe-Trp(Boc)-OMe (4d) was only modified at the Trp
residue and not at the N-terminal Phe residue (5d, 52% yield).
We then explored if N-terminal Trp residues could be
modified, and in contrast to N-terminal Phe, Trp residues at
the N-terminus did react: C−H olefination of the peptides Ac-
Trp(Boc)-Leu-OMe (4e) and Ac-Trp(Boc)-Met-OMe (4f)
gave the modified peptides 5e and 5f with isolated yields of
4
7% and 45% respectively. The C−H olefination was also
applied to three model tripeptides: these reactions proved
successful with the Trp residue at the C-terminus, in the
middle of the peptide and at the N-terminus (5g−j, 55−69%
yield).
There is a clear difference between the lack of reaction of N-
terminal Phe residues and the successful modification of N-
terminal Trp. For the C−H olefination of Trp residues, it
the Boc group is a noteworthy finding, as carbamate directing
42
groups are uncommon in C−H functionalization. Indeed, for
the C−H functionalization of indoles, Boc has been reported
43,44
to be a poor directing group.
It may be that in this case the
C
DOI: 10.1021/acs.orglett.9b02894
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
amide group of the Trp residue acts as a primary directing
group, and Boc is acting as an ancillary directing group.
Scheme 5. Control of Residue Selectivity in the C−H
a
45
Olefination of Trp-Phe Peptides
For broad application, which includes the modification of
peptides containing more than one aromatic amino acid, we
were aware that control of residue selectivity is important.
Having established methods for the C−H olefination of Trp
37
containing peptides and Phe containing peptides, we next
investigated the C−H functionalization of peptides containing
both Trp and Phe, where modification of both aromatic
residues was possible. This investigation began with the
dipeptide Ac-Trp(Boc)-Phe-OMe (8a), which had the
potential to react at the Trp residue via Boc directed C−H
activation, and at the Phe residue via bidentate coordination of
the peptide backbone. Peptide 8a was treated under the
optimized conditions described in Table 1, and the major
product resulted from olefination of just the Trp residue (9a,
2
2% isolated yield, Scheme 5A). However, five modified
peptides were isolated from the reaction, involving all
combinations of Trp and Phe olefination. Modification of
the tripeptide Ac-Gly-Trp(Boc)-Phe-OMe (SI-17) gave a
46
similar product distribution.
We anticipated that manipulation of the directing groups in
a peptide would enable control of the residue selectivity. To
suppress reaction at the Trp residue, alternatives to the Boc
group were studied. First, the peptide Ac-Trp-Phe-OMe (8b),
which lacked a protecting group on the Trp residue, was
exposed to the same reaction conditions as peptide 8a:
reaction of 8b gave peptide 10 in 21% yield, in which the Trp
residue was not modified, but the Phe residue had undergone
mono-olefination, Scheme 5B. In an attempt to increase the
yield of the modified peptide, 8b was treated using conditions
we had previously optimized for Phe modification (heating at a
37
higher temperature of 130 °C for 12 h in t-amylOH); this
harsher reaction, however, gave an intractable mixture of
products. Next we studied the peptide Ac-Trp(TIPS)-Phe-
OMe (8c), which contained a tri-isopropylsilyl protecting
group on the Trp residue. Peptide 8c also underwent exclusive
modification of the Phe residue: the reaction could be
performed at 130 °C, to give the diolefinated peptide 11 as
the only product in 49% yield.
To suppress the reactivity of the Phe residue, bidentate
coordination of the peptide to palladium needs to be
prevented. Our previous study discovered that N-alkylation
of the Phe residue rendered it unreactive in the C−H
37
olefination reaction. Consequently, we prepared the peptide
Ac-Gly-(N-Me)-Phe-Trp(Boc)-OMe (12), containing a Boc-
protected tryptophan residue and an N-methyl phenylalanine
residue. When exposed to the optimized conditions for
tryptophan C−H olefination, peptide 12 was modified
exclusively at the Trp residue, giving the olefinated peptide
a
Reaction conditions: (i) Styrene (4 equiv), Pd(OAc) (10 mol %),
2
4
7
Ag(OAc) (2.5 equiv), air, PhMe (0.08 M), 100 °C, 2 h. (ii) Styrene
(
(
4 equiv), Pd(OAc) (10 mol %), Ag(OAc) (5 equiv), air, t-amylOH
0.12 M), 130 °C, 12 h.
2
1
3 in 55% yield, Scheme 5C.
In summary, the C−H olefination is successful for Trp
residues at the C-terminus, at the N-terminus, or in the middle
of peptides. Crucially, the reaction requires the Trp residue to
be protected with a Boc group, which most likely acts as a
directing group for the C−H activation. The Boc group may
have been overlooked previously for the C−H functionaliza-
tion of peptides, yet it facilitates the desired reactivity, is the
most common Trp protecting group, and is readily cleaved to
reveal the native peptide. Further, we have demonstrated that
manipulation of the directing groups within the peptide
enables full control of residue selectivity in Trp/Phe peptides.
The C−H olefination methodology reported here is
complementary to prior methods for the modification of
tryptophan residues by C−H arylation, alkynylation, and
allylation. This now broad combination of C−H functionaliza-
tion methods offers significant new opportunities in chemical
biology, in peptide therapeutics, and in molecular imaging. By
coupling peptide 1b with the vinyl-phenylalanine compound
SI-3, we have demonstrated how the C−H olefination can be
used for peptide−biomolecule conjugation.
D
DOI: 10.1021/acs.orglett.9b02894
Org. Lett. XXXX, XXX, XXX−XXX

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Letter
Intraoperative Tumor-Specific Fluorescence Imaging in Ovarian
Cancer by Folate Receptor-Α Targeting: First in-Human Results.
Nat. Med. 2011, 17 (10), 1315−1319.
ASSOCIATED CONTENT
Supporting Information
■
*
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b02894.
(10) Lau, J. L.; Dunn, M. K. Therapeutic Peptides: Historical
Perspectives, Current Development Trends, and Future Directions.
Bioorg. Med. Chem. 2018, 26 (10), 2700−2707.
(11) Craik, D. J.; Fairlie, D. P.; Liras, S.; Price, D. The Future of
Detailed experimental procedures, reaction develop-
ment, mechanistic studies, and characterization data
for all compounds (PDF)
Peptide-Based Drugs. Chem. Biol. Drug Des. 2013, 81 (1), 136−147.
12) Fosgerau, K.; Hoffmann, T. Peptide Therapeutics: Current
Status and Future Directions. Drug Discovery Today 2015, 20 (1),
(
1
22−128.
AUTHOR INFORMATION
Corresponding Author
(13) Grison, C. M.; Burslem, G. M.; Miles, J. A.; Pilsl, L. K. A.; Yeo,
D. J.; Imani, Z.; Warriner, S. L.; Webb, M. E.; Wilson, A. J. Double
Quick, Double Click Reversible Peptide “Stapling”. Chem. Sci. 2017, 8
■
*
E-mail: warren.cross@ntu.ac.uk.
(
7), 5166−5171.
ORCID
(14) Lau, Y. H.; de Andrade, P.; Wu, Y.; Spring, D. R. Peptide
Stapling Techniques Based on Different Macrocyclisation Chem-
istries. Chem. Soc. Rev. 2015, 44 (1), 91−102.
Carole C. Perry: 0000-0003-1517-468X
Warren B. Cross: 0000-0001-6277-400X
Notes
(15) Chang, Y. S.; Graves, B.; Guerlavais, V.; Tovar, C.; Packman,
K.; To, K.-H.; Olson, K. A.; Kesavan, K.; Gangurde, P.; Mukherjee,
A.; Baker, T.; Darlak, K.; Elkin, C.; Filipovic, Z.; Qureshi, F. Z.; Cai,
H.; Berry, P.; Feyfant, E.; Shi, X. E.; Horstick, J.; Annis, D. A.;
Manning, A. M.; Fotouhi, N.; Nash, H.; Vassilev, L. T.; Sawyer, T. K.
Stapled α-Helical Peptide Drug Development: a Potent Dual Inhibitor
of MDM2 and MDMX for P53-Dependent Cancer Therapy. Proc.
Natl. Acad. Sci. U. S. A. 2013, 110 (36), E3445−E3454.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by Nottingham Trent University
(NTU) through studentships for M.J.T. and A.H. We thank
Oliver Bolland and Mahrukh Mukhtar (both NTU) for
carrying out preliminary investigations, Nigel Mould (NTU)
for carrying out the HPLC experiments, Dr. Graham Hickman
(16) White, C. J.; Yudin, A. K. Contemporary Strategies for Peptide
Macrocyclization. Nat. Chem. 2011, 3 (7), 509−524.
(17) Stephanopoulos, N.; Francis, M. B. Choosing an Effective
Protein Bioconjugation Strategy. Nat. Chem. Biol. 2011, 7 (12), 876−
84.
18) Lee, H. G.; Lautrette, G.; Pentelute, B. L.; Buchwald, S. L.
(
NTU) for assisting with the ICP analysis, and the National
8
Mass Spectrometry Facility at Swansea University for high
resolution mass spectrometry analysis.
(
Palladium-Mediated Arylation of Lysine in Unprotected Peptides.
Angew. Chem., Int. Ed. 2017, 56 (12), 3177−3181.
(19) Cheng, W.-M.; Lu, X.; Shi, J.; Liu, L. Selective Modification of
Natural Nucleophilic Residues in Peptides and Proteins Using
Arylpalladium Complexes. Org. Chem. Front. 2018, 5 (21), 3186−
REFERENCES
■
(
1) Schumacher, D.; Hackenberger, C. P. R. More Than Add-on:
Chemoselective Reactions for the Synthesis of Functional Peptides
and Proteins. Curr. Opin. Chem. Biol. 2014, 22, 62−69.
(
3
193.
20) Lang, K.; Chin, J. W. Cellular Incorporation of Unnatural
Amino Acids and Bioorthogonal Labeling of Proteins. Chem. Rev.
014, 114 (9), 4764−4806.
21) Chalker, J. M.; Wood, C. S. C.; Davis, B. G. A Convenient
2) deGruyter, J. N.; Malins, L. R.; Baran, P. S. Residue-Specific
(
Peptide Modification: a Chemist’s Guide. Biochemistry 2017, 56 (30),
863−3873.
3) Sinkeldam, R. W.; Greco, N. J.; Tor, Y. Fluorescent Analogs of
3
(
2
(
Biomolecular Building Blocks: Design, Properties, and Applications.
Chem. Rev. 2010, 110 (5), 2579−2619.
(
Albericio, F.; Lee, M.; Serrels, A.; Kielland, N.; Read, N. D.; Lavilla,
R.; Vendrell, M. Spacer-Free BODIPY Fluorogens in Antimicrobial
Peptides for Direct Imaging of Fungal Infection in Human Tissue.
Nat. Commun. 2016, 7, 10940.
Catalyst for Aqueous and Protein Suzuki−Miyaura Cross-Coupling. J.
Am. Chem. Soc. 2009, 131 (45), 16346−16347.
4) Mendive-Tapia, L.; Zhao, C.; Akram, A. R.; Preciado, S.;
(22) Spicer, C. D.; Triemer, T.; Davis, B. G. Palladium-Mediated
Cell-Surface Labeling. J. Am. Chem. Soc. 2012, 134 (2), 800−803.
23) Noisier, A. F. M.; Brimble, M. A. C-H Functionalization in the
Synthesis of Amino Acids and Peptides. Chem. Rev. 2014, 114 (18),
775−8806.
24) Wang, W.; Lorion, M. M.; Shah, J.; Kapdi, A. R.; Ackermann, L.
(
8
(
5) Jadhav, P. D.; Shen, J.; Sammynaiken, R.; Reaney, M. J. T. Site
(
Covalent Modification of Methionyl Peptides for the Production of
FRET Complexes. Chem. - Eur. J. 2015, 21 (47), 17023−17034.
(
Late-Stage Peptide Diversification by Position-Selective C-H
Activation. Angew. Chem., Int. Ed. 2018, 57 (45), 14700−14717.
6) Akram, A. R.; Avlonitis, N.; Lilienkampf, A.; Perez-Lopez, A. M.;
(
25) Gong, W.; Zhang, G.; Liu, T.; Giri, R.; Yu, J.-Q. Site-Selective
Mcdonald, N.; Chankeshwara, S. V.; Scholefield, E.; Haslett, C.;
Bradley, M.; Dhaliwal, K. A Labelled-Ubiquicidin Antimicrobial
Peptide for Immediate in Situ Optical Detection of Live Bacteria in
Human Alveolar Lung Tissue. Chem. Sci. 2015, 6 (12), 6971−6979.
3
C(sp )−H Functionalization of Di-, Tri-, and Tetrapeptides at the N-
Terminus. J. Am. Chem. Soc. 2014, 136 (48), 16940−16946.
(26) (a) Williams, T. J.; Reay, A. J.; Whitwood, A. C.; Fairlamb, I. J.
S. A Mild and Selective Pd-Mediated Methodology for the Synthesis
of Highly Fluorescent 2-Arylated Tryptophans and Tryptophan-
Containing Peptides: a Catalytic Role for Pd Nanoparticles? Chem.
Commun. 2014, 50 (23), 3052−3054. (b) Reay, A. J.; Williams, T. J.;
Fairlamb, I. J. S. Unified Mild Reaction Conditions for C2-Selective
Pd-Catalysed Tryptophan Arylation, Including Tryptophan-Contain-
ing Peptides. Org. Biomol. Chem. 2015, 13 (30), 8298−8309.
(27) Reay, A. J.; Hammarback, L. A.; Bray, J. T. W.; Sheridan, T.;
Turnbull, D.; Whitwood, A. C.; Fairlamb, I. J. S. Mild and
(
7) van Oosten, M.; Schafer, T.; Gazendam, J. A. C.; Ohlsen, K.;
̈
Tsompanidou, E.; de Goffau, M. C.; Harmsen, H. J. M.; Crane, L. M.
A.; Lim, E.; Francis, K. P.; Cheung, L.; Olive, M.; Ntziachristos, V.;
van Dijl, J. M.; van Dam, G. M. Real-Time in Vivo Imaging of
Invasive- and Biomaterial-Associated Bacterial Infections Using
Fluorescently Labelled Vancomycin. Nat. Commun. 2013, 4 (1), 2584.
0
(
8) Staderini, M.; Megia-Fernandez, A.; Dhaliwal, K.; Bradley, M.
Peptides for Optical Medical Imaging and Steps Towards Therapy.
Bioorg. Med. Chem. 2018, 26 (10), 2816−2826.
(
9) van Dam, G. M.; Themelis, G.; Crane, L. M. A.; Harlaar, N. J.;
Regioselective Pd(OAc) -Catalyzed C-H Arylation of Tryptophans
2
Pleijhuis, R. G.; Kelder, W.; Sarantopoulos, A.; de Jong, J. S.; Arts, H.
J. G.; van der Zee, A. G. J.; Bart, J.; Low, P. S.; Ntziachristos, V.
by [ArN
5179.
]X, Promoted by Tosic Acid. ACS Catal. 2017, 7 (8), 5174−
2
E
DOI: 10.1021/acs.orglett.9b02894
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
(
28) Noisier, A. F. M.; García, J.; Ionut,
̧
I. A.; Albericio, F. Stapled
Peptides by Late-Stage C(sp )-H Activation. Angew. Chem., Int. Ed.
017, 56 (1), 314−318.
29) Mendive-Tapia, L.; Preciado, S.; García, J.; Ramon
(b) Lanke, V.; Prabhu, K. R. Highly regioselective C2-alkenylation
of indoles using the N-benzoyl directing group: an efficient Ru-
catalyzed coupling reaction. Org. Lett. 2013, 15 (11), 2818−2821.
(c) Gong, B.; Shi, J.; Wang, X.; Yan, Y.; Li, Q.; Meng, Y.; Xu, H. E.;
Yi, W. Rhodium(III)-Catalyzed Regioselective Direct C-2 Alkenyla-
tion of Indoles Assisted by the Removable N-(2-Pyrimidyl) Group.
Adv. Synth. Catal. 2014, 356 (1), 137−143. (d) Yan, Z.; Chen, W.;
Gao, Y.; Mao, S.; Zhang, Y.; Wang, Y. Palladium-Catalyzed
Intermolecular C-2 Alkenylation of Indoles Using Oxygen as the
Oxidant. Adv. Synth. Catal. 2014, 356 (5), 1085−1092.
(44) For C−H borylation of N-Boc indole that is selective for C3
rather than C2, see: Kallepalli, V. A.; Shi, F.; Paul, S.; Onyeozili, E. N.;
Maleczka, R. E., Jr.; Smith, R. E., III Boc Groups as Protectors and
Directors for Ir-Catalyzed C−H Borylation of Heterocycles. J. Org.
Chem. 2009, 74 (23), 9199−9201.
3
2
(
́
, R.;
Kielland, N.; Albericio, F.; Lavilla, R. New Peptide Architectures
Through C-H Activation Stapling Between Tryptophan-Phenyl-
alanine/Tyrosine Residues. Nat. Commun. 2015, 6, 7160.
(30) Liu, T.; Qiao, J. X.; Poss, M. A.; Yu, J.-Q. Palladium(II)-
3
Catalyzed Site-Selective C(sp )−H Alkynylation of Oligopeptides: a
Linchpin Approach for Oligopeptide−Drug Conjugation. Angew.
Chem., Int. Ed. 2017, 56 (36), 10924−10927.
(31) Wang, W.; Lorion, M. M.; Martinazzoli, O.; Ackermann, L.
3
BODIPY Peptide Labeling by Late-Stage C(sp )−H Activation.
Angew. Chem., Int. Ed. 2018, 57 (33), 10554−10558.
(32) Zhu, Y.; Bauer, M.; Ackermann, L. Late-Stage Peptide
Diversification by Bioorthogonal Catalytic C-H Arylation at 23 °C
(45) Leitch, J. A.; McMullin, C. L.; Mahon, M. F.; Bhonoah, Y.;
Frost, C. G. Remote C-6 Selective Ruthenium-Catalyzed C-H
Alkylation of Indole Derivatives via σ-Activation. ACS Catal. 2017,
7 (4), 2616−2623.
in H O. Chem. - Eur. J. 2015, 21 (28), 9980−9983.
2
(33) Schischko, A.; Ren, H.; Kaplaneris, N.; Ackermann, L.
Bioorthogonal Diversification of Peptides Through Selective
Ruthenium(II)-Catalyzed C-H Activation. Angew. Chem., Int. Ed.
(
46) See Supporting Information for full details of the C−H
olefination of 8a and SI-17.
47) We first attempted to prepare the peptide Ac-Trp(Boc)-(N-
2
(
017, 56 (6), 1576−1580.
(
34) Lorion, M. M.; Kaplaneris, N.; Son, J.; Kuniyil, R.; Ackermann,
Me)-Phe-OMe, as a direct analogue of 8a−c. However, the coupling
of H-(N-Me)-Phe-OMe with Ac-Trp-OH proved difficult. Instead,
peptide 12 was readily prepared and enabled us to investigate the
protecting group strategy.
L. Late-Stage Peptide Diversification Through Cobalt-Catalyzed C−
H Activation: Sequential Multicatalysis for Stapled Peptides. Angew.
Chem., Int. Ed. 2019, 58, 1684−1688.
(
35) Kaplaneris, N.; Rogge, T.; Yin, R.; Wang, H.; Sirvinskaite, G.;
Ackermann, L. Late-Stage Diversification by Manganese-Catalyzed C-
H Activation: Access to Acyclic, Hybrid and Stapled Peptides. Angew.
Chem., Int. Ed. 2019, 58, 3476−3480.
(
36) Bai, Z.; Cai, C.; Yu, Z.; Wang, H. Backbone-Enabled
Directional Peptide Macrocyclization Through Late-Stage Palladi-
2
um-Catalyzed δ-C(sp )−H Olefination. Angew. Chem., Int. Ed. 2018,
5
(
7 (42), 13912−13916.
37) Terrey, M. J.; Perry, C. C.; Cross, W. B. Postsynthetic
Modification of Phenylalanine Containing Peptides by C−H
Functionalization. Org. Lett. 2019, 21 (1), 104−108.
2
(
38) Zheng, Y.; Song, W. Pd-Catalyzed Site-Selective C(sp )−H
Olefination and Alkynylation of Phenylalanine Residues in Peptides.
Org. Lett. 2019, 21 (9), 3257−3260.
́
(39) Isidro-Llobet, A.; Alvarez, M.; Albericio, F. Amino Acid-
Protecting Groups. Chem. Rev. 2009, 109 (6), 2455−2504.
(
40) Dherbassy, Q.; Schwertz, G.; Chesse, M.; Hazra, C. K.; Wencel-
́
Delord, J.; Colobert, F. 1,1,1,3,3,3-Hexafluoroisopropanol as a
Remarkable Medium for Atroposelective Sulfoxide-Directed Fuji-
wara-Moritani Reaction with Acrylates and Styrenes. Chem. - Eur. J.
2
(
016, 22 (5), 1735−1743.
41) For prior calculations on bidentate directing groups in C−H
activation, see: (a) Cross, W. B.; Hope, E. G.; Lin, Y.-H.; Macgregor,
S. A.; Singh, K.; Solan, G. A.; Yahya, N. N,N-Chelate-Control on the
Regioselectivity in Acetate-Assisted C-H Activation. Chem. Commun.
2013, 49 (19), 1918−1920. (b) Tang, H.; Huang, X.-R.; Yao, J.;
Chen, H. Understanding the Effects of Bidentate Directing Groups: a
Unified Rationale for sp(2) and sp(3) C-H Bond Activations. J. Org.
Chem. 2015, 80 (9), 4672−4682.
(
42) For examples of Boc as a directing group in C−H
functionalization, see: (a) Beck, E. M.; Grimster, N. P.; Hatley, R.;
Gaunt, M. J. Mild Aerobic Oxidative Palladium (II) Catalyzed C−H
Bond Functionalization: Regioselective and Switchable C−H
Alkenylation and Annulation of Pyrroles. J. Am. Chem. Soc. 2006,
1
28 (8), 2528−2529. (b) Morita, T.; Satoh, T.; Miura, M.
Rhodium(III)-Catalyzed Ortho-Alkenylation of Anilines Directed by
a Removable Boc-Protecting Group. Org. Lett. 2017, 19 (7), 1800−
1
(
803.
43) For C−H functionalization of N-Boc indole that is completely
unproductive or low yielding, see: (a) García-Rubia, A.; Urones, B.;
́ ́
Gomez Arrayas, R.; Carretero, J. Pd -Catalysed C-H Functionalisa-
II
tion of Indoles and Pyrroles Assisted by the Removable N-(2-
Pyridyl)sulfonyl Group: C2-Alkenylation and Dehydrogenative
Homocoupling. Chem. - Eur. J. 2010, 16 (31), 9676−9685.
F
DOI: 10.1021/acs.orglett.9b02894
Org. Lett. XXXX, XXX, XXX−XXX