N-Phenyl-N-aceto-vinylsulfonamides as Efficient and Chemoselective Handles for N-Terminal Modification of Peptides and Proteins

Article abstract of DOI:10.1002/ejoc.201701715

A number of vinylsulfonamides were synthesized and screened to identify reagents that can be used to modify octreotide under biological pH and room temperature with improved efficiency. N-Phenyl-N-aceto-vinylsulfonamide exhibits higher reactivity and has emerged as an efficient reagent that has the ability to realize the selective modification of peptides and proteins at the N-terminus via aza-Michael addition. We showed that, after conjugation of peptides and proteins with the reagent containing a bioorthogonal functional group, the derivatives could be further labelled by functionalities, including fluorescent tags, modified drugs and polyethylene glycol (PEG) polymers without the need for prior treatment. Somatostatin, lysozyme, and RNaseA were selectively modified at the N-terminus, which illustrated the application of the method.

Full text of DOI:10.1002/ejoc.201701715

DOI: 10.1002/ejoc.201701715
Full Paper
Protein Modification
N-Phenyl-N-aceto-vinylsulfonamides as Efficient and
Chemoselective Handles for N-Terminal Modification of Peptides
and Proteins
Rong Huang,[a,b,c] Zhihong Li,[a] Peiling Ren,[c] Wenzhang Chen,[a] Yuanyuan Kuang,[a]
Jiakang Chen,[a] Yuexiong Zhan,[a] Hongli Chen,*[a] and Biao Jiang*[a]
Abstract: A number of vinylsulfonamides were synthesized with the reagent containing
a bioorthogonal functional
and screened to identify reagents that can be used to modify group, the derivatives could be further labelled by functionali-
octreotide under biological pH and room temperature with im- ties, including fluorescent tags, modified drugs and polyethyl-
proved efficiency. N-Phenyl-N-aceto-vinylsulfonamide exhibits ene glycol (PEG) polymers without the need for prior treatment.
higher reactivity and has emerged as an efficient reagent that Somatostatin, lysozyme, and RNaseA were selectively modified
has the ability to realize the selective modification of peptides at the N-terminus, which illustrated the application of the
and proteins at the N-terminus via aza-Michael addition. We method.
showed that, after conjugation of peptides and proteins
Introduction
and N-terminal serine and threonine modification through
periodate oxidation to generate aldehydes for subsequent bio-
conjugation.[6] Site-specific N-terminal transamination,[7] imid-
azolidinone formation with 2-pyridinecarboxyaldehydes,[8] N-
terminus targeting oxidative coupling,[9] and reductive alkyl-
ation with aldehydes[10] have also been reported. Alternative
popular strategies in which N-terminal residues are directly
modified using N-hydroxysuccinimide (NHS) esters,[11] imid-
azole-1-sulfonyl azide,[12] and ketenes[13] through control of the
reaction pH have also been effectively achieved. All these meth-
ods provide powerful tools for various applications. However,
additional chemical techniques are required to provide simple
and efficient methods to modify the properties of peptides and
proteins.
Vinylsulfonamides have been successfully used as electro-
philic capture functions to modify proteins through Michael ad-
dition with thiol and amine residues.[14] Recently, we have also
demonstrated that divinylsulfonamides act as efficient linkers
for stapling disulfide bonds in peptides (Figure 1a)[15] and N-
methyl-N-phenylethenesulfonamides are appealing compounds
to selectively modify the lysine residues of native peptides (Fig-
ure 1b).[16] We found that cysteine residues, which have rela-
tively high nucleophilicity, were converted into the desired
product smoothly when reacted through Michael addition with
vinylsulfonamides under mild reaction conditions.[15] However,
targeting amino groups typically requires high pH and we have
demonstrated that the lysine residues of peptides were selec-
tively modified by vinylsulfonamides under alkaline conditions
(pH > 9).[16] In this study, we showed that the reactivity of vinyl-
sulfonamides could be improved efficiently by tuning their
structure and demonstrate selective N-terminal modification of
native peptides and proteins at biological pH or mildly acidic
conditions (Figure 1c).
Chemical peptide and protein modifications have been widely
applied in studying biological processes[1] and developing valu-
able therapeutics.[2] To satisfy diverse demands, many strategies
have been developed to expand the field of selective modifica-
tion methodologies, especially for natural residue-specific ap-
proaches.[3] Cysteine is a common labelling site because it func-
tions as a superior nucleophile. Nevertheless, most cysteine res-
idues on proteins tend to form disulfides with others and only
a few proteins offer accessible, unpaired cysteine residues. La-
belling surface lysine residues is also a popular strategy; how-
ever, this approach results in heterogeneous mixtures of prod-
ucts because lysine residues are highly abundant in proteins.
The N-terminus stands out as an appealing target for modifica-
tion because of its unique chemical properties[4] and an increas-
ing number of methods have been reported for site-specific
modification of the N-terminal position. Many studies have
been built upon chemoselective targeting of the N-terminus by
side chain participation, such as N-terminal cysteine functionali-
zation by native chemical ligation,[4] thiazolidine formation,[5]
[a] Shanghai Institute for Advanced Immunochemical Studies,
ShanghaiTech University,
99 Haike Road, Pudong, Shanghai, 201210 P. R. China
E-mail: chenhl@shanghaitech.edu.cn
jiangbiao@shanghaitech.edu.cn
http://siais.shanghaitech.edu.cn/eng/index.asp
[b] Shanghai Institute of Materia Medica, Chinese Academy of Sciences,
555 Zu Chong Zhi Road, Pudong, Shanghai, 201303, P. R. China
[c] University of Chinese Academy of Sciences,
19A Yuquan Road, Shijingshan District, Beijing, 100049, P. R. China
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under https://doi.org/10.1002/ejoc.201701715.
Eur. J. Org. Chem. 2018, 829–836
829
© 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
amides 11 emerged as the most effective reagent for further
studies.
Figure 1. Vinylsulfonamides as efficient handles for natural residue-specific
modification of native peptides and proteins.
Results and Discussion
We investigated a panel of vinylsulfonamides 1–14 with respect
to their Michael addition with octreotide (15). Octreotide carries
Figure 2. Vinylsulfonamide candidate screening for selective N-terminal modi-
fication of octreotide. (a) The reaction scheme. (b) The structures of screened
vinylsulfonamides. (c) The conversion of octreotide using vinylsulfonamides
1–14 as analyzed by RP-HPLC.
an N-terminus and an internal lysine with the sequence H2N-
D-
Phe-Cys-Phe- -Trp-Lys-Thr-Cys-Thr-ol (disulfide bridge Cys2–
D
Cys7) and is a standard peptide used for selective internal lysine
modification in our previous study.[16] The vinylsulfonamide
candidates reacted with octreotide in a mixture of phosphate
buffer (pH 7.4) and CH3CN (3:1) at room temperature (r.t.) for
24 h (Figure 2a, Figure 2b). The conversions of octreotide were
analyzed by RP-HPLC. The results revealed that aryl vinylsulfon-
amides 3 and 4 had slightly higher reactivity than benzyl vinyl-
sulfonamides 1 and 2 (Figure 2c). The installation of F, Cl, or
OMe on the phenyl ring (5–7) improved the reactivity only
slightly (< 20 %). The introduction of an electron-withdrawing
substituent on the phenyl ring (8 and 9) significantly improved
the yield (29 %, 52 %). We also examined the introduction of
different groups on the N-atom directly (4, 10, 11) and, to our
delight, found that compound 11 reacted with high conversion
The products 16a and 16b, generated from the reaction of
octreotide (15) and reagent 11 (Figure 3a), were isolated by RP-
HPLC (Figure 3b) and characterized by MS2. The main product
16a revealed signals at m/z 1244.4933 [M + H]+ and 622.7566
[M + 2H]2+ (Figure 3c) and it was assigned to mono-modified
octreotide. After the treatment of the product 16a with tris(2-
carboxyethyl)phosphine (TCEP) to reduce the disulfide bond,
the peptide fragments obtained from MS2 indicated that the N-
terminus of octreotide was selectively functionalized (Fig-
ure 3d). A trace amount of the product 16b was also confirmed
as di-modified octreotide by MS and MS2 analysis (Figure S1).
To provide a privileged site for further bioorthogonal conju-
(> 95 %). Pyridine vinylsulfonamides 12–14 exhibited moderate gation, a bifunctional N-phenyl-N-aceto-vinylsulfonamide 17,
reactivity (34–52 %). Therefore, N-phenyl-N-aceto-vinylsulfon- equipped with an alkynyl group as a reactive handle, was syn-
Eur. J. Org. Chem. 2018, 829–836
www.eurjoc.org
830
© 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
Figure 3. Selective modification of the N-terminus of octreotide using 11. (a) The reaction scheme. (b) RP-HPLC of the reaction. (c) The ESI-HRMS spectrum of
product 16a. (d) The MS2 spectrum of product 16a.
Eur. J. Org. Chem. 2018, 829–836
www.eurjoc.org
831
© 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
thesized (Scheme 1). The synthesis started from commercially butyl dicarbonate (Boc2O) to form compound 19. The latter was
available 4-aminophenol (18), which was protected by di-tert- propargylated to give 20 and the Boc protecting group of was
Scheme 1. Synthesis of 17 bearing an alkyne moiety for further modification.
Figure 4. Stepwise modification of the N-terminus of octreotide using 17. (a) The reaction scheme. (b) RP-HPLC of the reaction. (c) The ESI-HRMS spectrum of
product 23. (d) The MS2 spectrum of product 23. (e) The ESI-HRMS spectrum of product 29–33.
Eur. J. Org. Chem. 2018, 829–836
www.eurjoc.org
832
© 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
Figure 5. Site-specific attachment of reagent 17 to somatostatin, lysozyme, and RNaseA.
removed to give the product 21. Subsequently, compound 21 for 18 h. The reaction mixture was analyzed by RP-HPLC (Fig-
was reacted with 2-chloroethanesulfonyl chloride and the re- ure 6a). A single major product 34 was obtained (44 % yield
sulting product 22 was functionalized with an acetyl group to and 90 % yield based on recovered somatostatin) and assigned
afford the desired bifunctional vinylsulfonamide 17.
as a mono-modified somatostatin (Figure 5) based on the mass
peak of m/z 959.3980 [M + 2H]2+ and 639.5986 [M + 3H]3+
(Figure 6b). After treating the isolated compound 34 with TCEP
to reduce the disulfide bond, LC-MS2 analysis of the peptide
fragments revealed that the modification of somatostatin oc-
curred at the N-terminus (Figure 6c).
Compound 17 reacted with octreotide to afford the ex-
pected single N-terminal modified product 23 in excellent yield
(92 %, Figure 4a), which was determined by RP-HPLC (Fig-
ure 4b), MS and MS2 (Figure 4c–d). Using the alkyne handle,
the modified peptide 23 was further conjugated with different
functionalities, such as fluorescent tags (24, 25), modified drugs
(26, 27), and polyethylene glycol (PEG) polymers (28), via click
chemistry without the need for prior treatment. The corre-
sponding selective N-terminal modified bioconjugates 29–33
were obtained conveniently in good yields (Figure 4e).
To demonstrate the applicability of vinylsulfonamide 17 to
modify peptides and proteins, we examined the ability of 17 to
label commercially available peptides and proteins including
somatostatin (with N-terminal alanine and two internal lysines),
lysozyme (with N-terminal lysine and five internal lysines), and
RNaseA (with N-terminal lysine and nine internal lysines). The
results showed that selective modification of all these peptides
and proteins proceeded smoothly with 17 to afford the prod-
ucts mono-modified on the N-terminus (Figure 5).
We further tested vinylsulfonamide 17 for site-specific modi-
fication of lysozyme and RNaseA. Protein was treated with com-
pound 17 (3 equiv.) in phosphate buffer (pH 6.03) at 37 °C
for 5 h. The ESI-HRMS spectrum confirmed that mono-modified
lysozyme (35, unmodified lysozyme/mono-modified lyso-
zyme = 2:1) and RNaseA (36, unmodified RNaseA/mono-modi-
fied RNaseA = 5:2) were produced with signals at m/z 14584 Da
(Figure 7a) and 13963 Da (Figure 7b), respectively. A trace
amount of di-modified products were also detected. After
trypsin digestion, the modification sites of products 35 and 36
were determined as the N-terminus based on the detection of
the expected fragments in LC-MS2 (Figure 7c, Figure 7d). In ad-
dition, the conversion of the modified proteins was improved
significantly by adjusting the pH, temperature, amount of rea-
In the case of somatostatin, modification was performed in gent, and reaction time. RNaseA was completely modified at
phosphate buffer (pH 7.4) with 3 equiv. 17 at room temperature higher pH, but at the cost of decreased selectivity (Figure S2).
Eur. J. Org. Chem. 2018, 829–836
www.eurjoc.org
833
© 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
Figure 6. Modification of somatostatin using 17. (a) RP-HPLC of the reaction
mixture. (b) The ESI-HRMS spectrum of product 34. (c) The MS2 spectrum of
product 34.
Modified proteins 35 and 36, containing an alkyne moiety,
were further conjugated with a fluorescent tag (25) via click
chemistry (Figure 8a). Successful conjugation was confirmed by
SDS PAGE analysis (Figure 8b).
Figure 7. Selective N-terminal modification of lysozyme and RNaseA using
17. (a) The ESI-HRMS spectrum of 35. (b) The ESI-HRMS spectrum of 36.
(c) The MS2 spectrum of 35. (d) The MS2 spectrum of 36.
Eur. J. Org. Chem. 2018, 829–836
www.eurjoc.org
834
© 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
Figure 8. Labelling of modified proteins 35 and 36 with FITC-N3 25.
Conclusions
Experimental Section
Synthetic Procedures
Taking advantage of the pKa difference between N-terminal
amine (pKa ca. 8) and lysine amine (pKa ca. 10), N-terminal se-
lective modification has been achieved under acidic to neutral
pH conditions. However, a lower pH often significantly de-
creased conjugation efficiency.[4] To achieve selective N-termi-
nal modification by useful Michael acceptors, vinylsulfonamides
were synthesized as activated electrophilic reagents and their
bioconjugation capabilities were tested against a pool of com-
mercial peptides and proteins (octreotide, somatostatin, lyso-
zyme and RNaseA) lacking free cysteines to avoid interference
with them considering their superior nucleophilicity. Our results
demonstrated that new N-phenyl-N-aceto-vinylsulfonamides
with higher reactivity can be used to label the N-terminus of
native peptides and proteins site-specifically at physiological
pH. We also demonstrated the feasibility of using the modified
peptides and proteins bearing a bioorthogonal group (alkyne)
for further conjugation with functionalities, including fluoro-
phores, modified drugs, and PEG polymers via click chemistry.
On the other hand, the extent to which peptides and proteins
are modified can also be manipulated by adjusting the amount
of reagent, temperature, and pH. These highly reactive Michael
acceptors offer opportunities for effective labelling of proteins
to achieve desirable conversion under appropriate conditions.
Vinylsulfonamides are attractive reagents because they are
easily prepared and modify biomolecules through Michael addi-
tion without any by-products to form stable products. Consider-
ing the good reactivity of the N-phenyl-N-aceto-vinylsulfon-
amides, the simple and clean bioconjugation reaction, the easy
installation of bioorthogonal handles for further protein modifi-
cation, and the wide application of Michael addition, we antici-
pate that this method will expand the reported methodologies
and become a useful tool in scientific research. Efforts to apply
the method to generate bioactive proteins are under way.
General Procedure for the Synthesis of Vinylsulfonamides 1–14:
To a stirred solution of amine (5 mmol, 1 equiv.) and trimethylamine
(15 mmol, 3 equiv.) in DCM (30 mL) at 0 °C, 2-chloroethanesulfonyl
chloride (6 mmol, 1.1 equiv.) was added slowly. The resulting mix-
ture was stirred at 0 °C until the amine was consumed as deter-
mined by TLC. The reaction was quenched with water (10 mL) and
the mixture was extracted with DCM (3 × 30 mL). The combined
organic extracts were washed with brine (10 mL) and dried with
anhydrous Na2SO4. The solvent was removed under vacuum and
the residue was purified by column chromatography.
Modification of Octreotide Using a Panel of Vinylsulfonamides
1–14 and 17: In a 330 μL 96 well plate, a solution of octreotide
(5 mM in H2O, 50 μL) and vinylsulfonamide (5 mM in CH3CN, 150 μL)
in a mixture of phosphate buffer (pH 7.4, 100 μL) and CH3CN
(35 μL.) was shaken at room temp. for 24 h. The resulting reaction
mixture was analyzed by RP-HPLC.
Azide–Alkyne Cycloaddition of 17-Modified Octreotide 23 and
24–28 via Click Chemistry: To the crude product 23 (100 μL) with-
out further purification, azide reagent 24–28 in DMSO (10 μmol/
mL, 1 μL), a solution of
1 μL), a solution of tris(benzyltriazolymethyl)amine (TBTA) in 1:4
DMSO-tert-butanol (10 m , 1 μL) and a CuSO4 solution in H2O
(100 m , 1 μL) were added. The mixture was reacted on a Micro
L-ascorbic acid sodium salt in H2O (200 mM,
M
M
plate fast oscillator at room temperature for 1 h. The modified
octreotides were characterized by HPLC and LC–MS analysis.
Modification of Somatostatin Using 17: In a 330 μL 96 well plate,
a solution of somatostatin in phosphate buffer (pH 7.4, 5 m
M, 50 μL)
and 17 (50 m , 15 μL in CH3CN) in a mixture of 100 μL PBS buffer
M
(pH 7.4) and 35 μL CH3CN was vibrated at room temp. for 18 h.
The reaction mixture was analyzed by RP-HPLC. The product was
determined by LC-MS/MS.
Modification of Lysozyme Using 17: In a 1.0 mL Eppendorf tube,
lysozyme solution in phosphate buffer (pH 6.03) (2 mg/mL, 1 mL)
and 17 (50 m
M, 8.40 μL in CH3CN) was vibrated at 37 °C for 5 h.
The product was analyzed by LC-MS/MS.
Eur. J. Org. Chem. 2018, 829–836
www.eurjoc.org
835
© 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
[3] a) C. D. Spicer, B. G. Davis, Nat. Commun. 2014, 5, 4740; b) O. Boutureira,
G. J. Bernardes, Chem. Rev. 2015, 115, 2174–2195; c) O. Koniev, A. Wag-
ner, Chem. Soc. Rev. 2015, 44, 5495–5551.
[4] C. B. Rosen, M. B. Francis, Nat. Chem. Biol. 2017, 13, 697–705.
[5] a) H. Faustino, M. J. S. A. Silva, L. F. Veiros, G. J. L. Bernardes, P. M. P. Gois,
Chem. Sci. 2016, 7, 5052–5058; b) A. Bandyopadhyay, S. Cambray, J. Gao,
Chem. Sci. 2016, 7, 4589–4593; c) H. Ren, F. Xiao, K. Zhan, Y. P. Kim, H.
Xie, Z. Xia, J. Rao, Angew. Chem. Int. Ed. 2009, 48, 9658–9662; Angew.
Chem. 2009, 121, 9838.
[6] K. F. Geoghegan, J. G. Stroh, Bioconjugate Chem. 1992, 3, 138–146.
[7] a) L. S. Witus, C. Netirojjanakul, K. S. Palla, E. M. Muehl, C. H. Weng, A. T.
Iavarone, M. B. Francis, J. Am. Chem. Soc. 2013, 135, 17223–17229; b)
J. M. Gilmore, R. A. Scheck, A. P. Esser-Kahn, N. S. Joshi, M. B. Francis,
Angew. Chem. Int. Ed. 2006, 45, 5307–5311; Angew. Chem. 2006, 118,
5433.
Trypsin Digestion of 17-Modified Lysozyme: In a 1.0 mL Eppen-
dorf tube, 17-modified lysozyme mixture (100 μL) was mixed with
DTT (50 μL, 10 m
60 min at 56 °C. The tube was cooled to room temperature, fresh
IAA solution (50 m ) was added, and the solution was incubated
M) and the resulting solution was incubated for
M
for 30 min on a shaker in the dark. Trypsin solution (1 mg/mL,
2.74 μL) was added to the Eppendorf tube at 0 °C, the reaction
mixture was incubated at 37 °C overnight and the trypsin digested
mixture was analyzed by LC-MS/MS.
Modification of RNaseA Using 17: In a 1.0 mL Eppendorf tube,
RNaseA solution in phosphate buffer (pH 6.03, 2 mg/mL, 1 mL) and
17 (50 mM, 8.40 μL in CH3CN) were vibrated at 37 °C for 5 h. The
product was analyzed by LC-MS/MS.
[8] J. I. MacDonald, H. K. Munch, T. Moore, M. B. Francis, Nat. Chem. Biol.
2015, 11, 326–331.
Trypsin Digestion of 17-Modified RNaseA: In a 1.0 mL Eppendorf
tube, 17-modified RNaseA mixture (100 μL) was mixed with DTT
[9] a) A. M. ElSohly, J. I. MacDonald, N. B. Hentzen, I. L. Aanei, K. M. El Musle-
many, M. B. Francis, J. Am. Chem. Soc. 2017, 139, 3767–3773; b) A. M.
ElSohly, M. B. Francis, Acc. Chem. Res. 2015, 48, 1971–1978.
[10] D. Chen, M. M. Disotuar, X. Xiong, Y. Wang, D. H. Chou, Chem. Sci. 2017,
8, 2717–2722.
(50 μL, 10 m
at 56 °C. The tube was cooled to room temperature, fresh IAA solu-
tion (50 m ) was added, and the solution was incubated for 30 min
M) and the resulting solution was incubated for 60 min
M
on a shaker in the dark. Trypsin solution (1 mg/mL, 2.74 μL) was
added to the Eppendorf tube at 0 °C. The reaction mixture was
incubated at 37 °C overnight and the trypsin digested mixture was
analyzed using LC-MS/MS.
[11] a) I. Selo, L. Negroni, C. Creminon, J. Grassi, J. M. Wal, J. Immunol. Methods
1996, 199, 127–138; b) G. Gaudriault, J. P. Vincent, Peptides 1992, 13,
1187–1192.
[12] S. Schoffelen, M. B. van Eldijk, B. Rooijakkers, R. Raijmakers, A. J. R. Heck,
J. C. M. van Hest, Chem. Sci. 2011, 2, 701.
[13] a) A. O. Chan, C. M. Ho, H. C. Chong, Y. C. Leung, J. S. Huang, M. K. Wong,
C. M. Che, J. Am. Chem. Soc. 2012, 134, 2589–2598; b) W. Chan, C. Ho,
M. Wong, C. Che, J. Am. Chem. Soc. 2006, 128, 14796–14797.
[14] a) J. Dadová, M. Vrabel, M. Adamik, M. Brazdova, R. Pohl, M. Fojta, M.
Hocek, Chem. Eur. J. 2015, 21, 16091–16102; b) J. L. Furman, M. Kang, S.
Choi, Y. Cao, E. D. Wold, S. B. Sun, V. V. Smider, P. G. Schultz, C. H. Kim, J.
Am. Chem. Soc. 2014, 136, 8411–8417; c) E. H. Suh, Y. Liu, S. Connelly,
J. C. Genereux, I. A. Wilson, J. W. Kelly, J. Am. Chem. Soc. 2013, 135,
17869–17880; d) J. Dadová, P. Orsag, R. Pohl, M. Brazdova, M. Fojta, M.
Hocek, Angew. Chem. Int. Ed. 2013, 52, 10515–10518; Angew. Chem.
2013, 125, 10709.
Acknowledgments
This work was supported by ShanghaiTech University.
Keywords: Protein modifications · N-terminal modification ·
Regioselectivity · Aza-Micheal addition · Proteins
[15] Z. Li, R. Huang, H. Xu, J. Chen, Y. Zhan, X. Zhou, H. Chen, B. Jiang, Org.
Lett. 2017, 19, 4972–4975.
[1] L. Xue, I. A. Karpenko, J. Hiblot, K. Johnsson, Nat. Chem. Biol. 2015, 11,
[16] H. Chen, R. Huang, Z. Li, W. Zhu, J. Chen, Y. Zhan, B. Jiang, Org. Biomol.
Chem. 2017, 15, 7339–7345.
917–923.
[2] a) N. Krall, F. P. da Cruz, O. Boutureira, G. J. Bernardes, Nat. Chem. 2016,
8, 103–113; b) B. E. de Goeij, J. M. Lambert, Curr. Opin. Immunol. 2016,
40, 14–23.
Received: December 11, 2017
Eur. J. Org. Chem. 2018, 829–836
www.eurjoc.org
836
© 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim