Preparation of N-Substituted N-Arylsulfonylglycines and Their Use in Peptoid Synthesis

Article abstract of DOI:10.1021/acs.orglett.5b02862

To increase the chemical diversity accessible with peptoids and peptide-peptoid hybrids, N-alkylated arylsulfonamides were used to prepare side chain protected N-substituted glycines compatible with solid-phase synthesis. The described procedures give acc

Full text of DOI:10.1021/acs.orglett.5b02862

Letter
pubs.acs.org/OrgLett
Preparation of N‑Substituted N‑Arylsulfonylglycines and Their Use in
Peptoid Synthesis
Steve Jobin,† Simon Vezina-Dawod,† Claire Herby, Antoine Derson, and Eric Biron*
́
Faculty of Pharmacy, Universite
Laboratory of Medicinal Chemistry, Centre de Recherche du Centre Hospitalier Universitaire de Queb
Quebec, Quebec G1V 4G2, Canada
* Supporting Information
́ ́ ́
Laval, Quebec, Quebec G1V 0A6, Canada
́
ec, 2705 Boulevard Laurier,
́
́
S
ABSTRACT: To increase the chemical diversity accessible with peptoids and peptide−peptoid hybrids, N-alkylated
arylsulfonamides were used to prepare side chain protected N-substituted glycines compatible with solid-phase synthesis. The
described procedures give access to peptoid monomers bearing a wide variety of functional groups from commercially available
amines in four straightforward steps. The prepared N-substituted N-arylsulfonylglycines were used as monomers in solid-phase
synthesis to introduce relevant functionalized side chains into peptoid oligomers and peptide−peptoid hybrids.
functionalized peptoid residues need to be incorporated in
the sequence.
eptides are useful tools in chemical biology and drug
Pdiscovery, but their application as therapeutic agents has
been hampered by poor bioavailability and degradation by
proteases. Among the oligomeric peptidomimetics developed
to overcome these drawbacks,1 peptoids showed very attractive
structural and biological properties.2 Peptoids are oligomers of
N-substituted glycines (NSG), and the simple migration of the
side chains to the backbone nitrogens significantly improves
relevant pharmacological properties. Compared to peptides,
peptoids are immune to proteolytic degradation3 and the
removal of highly hydrated N−H bonds improves the cell
permeability and bioavailability of these compounds.4 On the
other hand, the N-alkylation of one or several amino acid
residues or their substitution by NSG residues in bioactive
peptides was also shown to improve pharmacokinetic proper-
ties such as resistance to proteases and bioavailability.5
The synthesis of peptoid oligomers is straightforward, and
two general routes have been developed: the monomer and
submonomer approaches. The submonomer method intro-
duced by Zuckermann et al.6 is the most frequently used and
involves iterative acylation and amination reactions on solid
support to build up peptoid residues and generate oligomers
after multiple rounds. In comparison, the monomer approach
has a direct analogy with solid-phase peptide synthesis where
N-Fmoc NSG monomers are sequentially coupled to the solid
support to create oligomers.2a,5c,d A practical advantage of this
approach is the use of essentially the same synthesis protocol as
peptides, facilitating its implementation on automated peptide
synthesizers to prepare peptoids and peptomers (peptide-
peptoid hydrids) or perform a peptoid scan on a bioactive
peptide. While the submonomer method requires a large excess
of reagents, the monomer approach uses fewer equivalents,
which is more economical when expensive side chain
Typically, NSG derivatives are prepared in solution from
their corresponding amine by N-alkylation with a bromoacetate
alkyl ester followed by ester cleavage and N-Fmoc protec-
tion.5c,d,7 Compared to NSG bearing neutral hydrophobic side
chains, the number of commercially available amines to prepare
side chain protected monomers is very limited. As protection of
functionalized side chains is recommended to avoid side
reactions during NSG synthesis and oligomerization, different
routes have been developed to prepare side chain protected
amines and their corresponding NSG monomers. Depending
on the functional group needing protection, the synthesis of
side chain protected NSG usually requires around 6 steps and
they are often recovered in low yields (15−37%).5c,d,8 In most
cases, their preparation involves a sequence of (1) amine
protection; (2) side chain protection; (3) amine deprotection;
(4) N-alkylation with a bromoacetate ester; (5) ester cleavage;
and (6) amine reprotection with an Fmoc group. In an effort to
increase the chemical diversity accessible with the monomer
approach and introduce interesting reactive or polar function-
alities into peptoids and peptomers, we decided to develop a
simple and affordable approach to prepare functionalized NSG
derivatives bearing protected side chains and to use them in
solid-phase synthesis. Herein we report our results concerning
the use of N-alkylated 2-nitrobenzenesulfonamides to prepare
N-substituted glycine building blocks for peptoid and peptomer
synthesis.
Based on our previous work with N-substituted arylsulfon-
amides as alternative building blocks to prepare peptoids,9 our
Received: October 2, 2015
© XXXX American Chemical Society
A
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Scheme 1. Synthesis of Side Chain Protected N-Substituted N-Arylsulfonylglycines
strategy to obtain side chain protected NSG monomers was to
use commercially available amines as starting material and
protect/activate the amine with a 2-nitrobenzenesulfonyl group
(oNs) followed by side chain protection giving fully protected
submonomers that could be directly N-alkylated with methyl
bromoacetate and saponified (Scheme 1). This approach offers
the opportunity to obtain protected NSG monomers in 4 steps
by avoiding the amine deprotection/reprotection with Fmoc
steps and to increase yields by using larger amounts of
bromoacetate during the N-alkylation step to reach completion
since bis-alkylation of the amine is not possible. The oNs group
was selected based on its compatibility with most standard
protecting groups used in peptide and peptoid solid-phase
synthesis. Initially described by Fukuyama et al.,10 the oNs
protecting group activates the amino function to generate a
nucleophilic anion that can be selectively alkylated under mild
conditions with alkyl halides or a Mitsonobu reaction. The oNs
was found to be very useful to prepare N-methylated
peptides.11 During oligomerization, the oNs group can be
efficiently removed with thiophenol or β-mercaptoethanol in
the presence of a base and the released chromophore is visible
to the naked eye. Moreover, it is compatible with different
reaction conditions used in functional group protection such as
esterification, amide formation, and O-tert-butylation. Finally,
oNs-protected derivatives have a high propensity to form solid
powders, which are significantly more convenient to handle and
use as feedstock.12
The protected NSG monomers 5a−i were prepared by a
straightforward four-step synthesis from different commercially
available amines 1a−i (Scheme 1). First, the oNs protected
amines 2a−i were prepared from oNs-Cl and the corresponding
amine under standard conditions. Due to a lack of selectivity
between the primary amine and the phenolic hydroxyl of
tyramine 1d, a more selective procedure described by Penso et
al.13 with oNs-Cl in a THF/DMF (8:1) mixture in the presence
of lyophilized solid sodium carbonate as base was used to
prepare N-oNs-tyramine 2d in good yields. For the second step,
appropriate protection was performed on the functionalized
side chains. First, O-tert-butylation of compounds 2a−f was
achieved with magnesium perchlorate and di-tert-butylcarbon-
ate in refluxing DCM as described by Bartoli et al.14 and gave
the corresponding tert-butyl ethers 3a−f (Table 1). With yields
Table 1. Isolated Yields for Protected NSG Monomers 5a−j
a
overall yields (%)
amine 1
residue
submonomer 3
monomer 5
a
b
c
d
e
f
Nhpr(tBu)
Nthr(tBu)
N1hipr(tBu)
Ntyr(tBu)
49
48
44
42
55
38
70
94
−
44
37
36
39
38
27
56
86
57
54
N2thcp(tBu)
N4thch(tBu)
Nglu(allyl)
Nlys(Boc)
Ntrp(Boc)
Narg(Boc)2
g
h
i
j
−
a
Isolated yield after purification calculated from starting amines.
between 49% and 64%, the O-tert-butylation reaction is clearly
the limiting step for the synthesis of 3a−f. Improvement of this
reaction would significantly increase the overall yields for
submonomers 3a−f and their respective monomers 5a−f. The
oNs-β-Ala-Oallyl ester 3g was obtained by esterification of 2g
with allyl alcohol and a catalytic amount of p-toluenesulfonic
acid in refluxing toluene. The allyl ester protected side chain is
very useful to prepare a cyclic oligomer on solid support.
Protection of the free amine of 2h with a Boc group was
conducted under standard conditions to give submonomer 3h
in good yields. Interestingly, compounds 3a−h can be directly
used as building blocks in peptoid synthesis by the
submonomer approach using the appropriate bromine displace-
ment conditions. Finally, because protection of 2i with the Boc
group occurred on both sulfonamide and indole nitrogens, a
selective N-alkylation on the sulfonamide of 2i was first
performed with methyl bromoacetate to generate 3i followed
by protection of the indole with a Boc group to afford 4i in 80%
yields. N-Alkylation of submonomers 3a−f and 3h was
achieved with methyl bromoacetate in the presence of K2CO3
to yield compounds 4a−f and 4h. Submonomer 3g was
alkylated with tert-butyl bromoacetate to avoid allyl ester
hydrolysis with LiOH, and treatment of the fully protected
NSG 4g with a TFA solution afforded monomer 5g. On the
other hand, the methyl ester of compounds 4a−f, 4h, and 4i
was cleaved with LiOH to give side chain protected NSG 5a−f,
B
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Letter
5h, and 5i in good yields (70−91% for 2 steps). Finally, NSG 5j
bearing a guanyl moiety to mimic arginine was prepared from
fully protected NSG 4h by Boc-deprotection with a TFA
solution and guanylation with N,N′-bis-Boc-1-guanylpyrazole
followed by treatment with LiOH (Scheme 1). Overall, NSG
monomers 5a−j were obtained in moderate to good yields, and
besides 5j, their synthesis required 4 straightforward steps. The
oNs protection and N-alkylation reactions were the most
efficient steps with 78−99% and 80−99% yields, respectively.
Besides the O-tert-butylation reaction, side chain protection on
amino and carboxyl groups was very efficient. Compared to the
reported peptoid monomers synthesis of homothreonine
(Fmoc-Nthr(tBu)-OH, 6 steps, 15% yield),5c lysine (Fmoc-
Nlys(Boc)-OH, 4 steps, 20−38% yield),8c,d and homotyrosine
Fmoc-Ntyr(tBu)−OH, 6 steps, 33−37%),5d,8a,b our approach
allows a faster access to side chain protected NSG and in many
cases with 2−3-fold increased yields.
5a−j and 7a−c were incorporated using HATU and HOAt
with NMM as a base for 3 h and the oNs group was cleaved
with a solution of p-methoxybenzenethiol and DBU in DMF
for 2 × 10 min (Scheme 3). In this case, β-mercaptoethanol
was replaced by p-methoxybenzenethiol to avoid incomplete
oNs removal as described by different groups for poly-N-
alkylated peptides and peptoids.9,16 Cyclic peptoid 9 was
obtained after allyl ester removal on peptoid 8g with a catalytic
amount of Pd(PPh3)4 in the presence of phenylsilane and
macrocyclization on solid support with PyAOP. Peptoid 10 was
prepared exclusively by the monomer method with N-
substituted N-oNs-glycines. Following side chain deprotection
and cleavage from the resin with a TFA solution, the crude
purity of the oligomers was evaluated by HPLC (Table 2).
Excellent purities were obtained for most oligomers showing
that N-substituted N-oNs-glycines can be efficiently used as
monomers in peptoid synthesis.
Peptoid residues bearing amides can be useful to increase the
molecular diversity of a library and explore a binding site in a
compound optimization process. Amidated side chains have
also been used to induce a particular folding and promote
secondary or tertiary structures via hydrogen bonding.15 To
expend our approach to monomers with amidated side chains,
NSG 7a−c were prepared from commercially available β-
alanine tert-butyl ester by a five-step synthesis (Scheme 2).
Table 2. Crude Purities and Isolated Yields for Oligomers
Containing Monomers 5a−j and 7a−c
a
b
oligomer
sequence
purity (%) yield (%)
8a
8b
8c
8d
8e
8f
Npr-N1a-Npr-Npm-Nlys
Npr-N1b-Npr-Npm-Nlys
Npr-N1c-Npr-Npm-Nlys
Npr-N1d-Npr-Npm-Nlys
Npr-N1e-Npr-Npm-Nlys
Npr-N1f-Npr-Npm-Nlys
N1h-Nme-Npm-Npr-N1g
Npr-N1h-Npr-Npm-Nlys
Npr-N1i-Npr-Npm-Nlys
Npr-N1j-Npr-Npm-Nlys
Npr-7a-Npr-Npm-Nlys
Npr-7b-Npr-Npm-Nlys
Npr-7c-Npr-Npm-Nlys
c[N1h-Nme-Npm-Npr-N1g]-NH2
N1i-7b-N1h-N1d-7a
91
84
93
89
67
75
91
92
95
87
93
95
98
96
70
89
67
73
67
71
55
74
78
95
75
70
82
75
55
69
41
58
Scheme 2. Synthesis of Monomers with Amide Side Chains
8g
8h
8i
8j
8k
8l
8m
9
c
c
10
11
7b-Ala-N1i-Lys-N1d
a
b
Crude purities were determined by HPLC. Isolated yield after
First, the fully protected NSG 6 was obtained after amine
protection with an oNs group and N-alkylation as described
above. After tert-butyl ester removal on 6, different amines were
coupled to the side chain followed by methyl ester cleavage to
give monomers 7a−c in 47−64% overall yields.
purification by preparative HPLC. Based on initial loading of Rink
Amide AM resin (0.56 mmol/g). Three peaks containing only the
expected cyclic compound were observed on the HPLC chromato-
gram. The reported crude purity and yield for 9 include all the peaks
containing exclusively the expected compound.
c
To demonstrate the compatibility of NSG monomers 5a−j
and 7a−c with solid-phase synthesis, a series of peptoids were
prepared on solid support (Scheme 3). The synthesis was
performed on 50 mg of Rink amide resin using both
submonomer and monomer approaches. For the submonomer
method, peptoid residues were built up by an acylation step
with bromoacetic acid and DIC (40 equiv) in DMF for 40 min
followed by amination with a primary amine (30 equiv) in
DMF for 40 min. For the monomer method, protected NSG
As expected, oligomers 8e and 8f bearing hindered side
chains were obtained in lower purities of 67% and 75%,
respectively. Nevertheless the approach with monomers 5e and
5f showed an important improvement compared to purities of
39% for 8e and 50% for 8f obtained by the submonomer
approach at room temperature.9 These results suggest that the
monomer approach could be more appropriate for hindered α-
disubstituted amines. A previous study with submonomers 3e
and 3f in solid-phase peptoid synthesis showed that the
bromine displacement on solid support was the limiting step. In
the monomer approach, this step is performed in solution and
could be more efficient since a greater quantity of alkylating
reagent can be used. This strategy offers, for more hindered
residues, significant building block economy and the oppor-
tunity to increase oligomer purities and isolated yields.
Moderate purities were observed for cyclic peptoid 9, but
LC-MS analysis confirmed the absence of the linear precursor
and showed the presence of only one molecular ion
Scheme 3. Synthesis of Peptoid Oligomers
C
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Org. Chem. 2009, 2009, 5099−5111. (c) Adessi, C.; Soto, C. Curr.
Med. Chem. 2002, 9, 963−978.
corresponding to the expected mass for the cyclic product in
each peak (Supporting Information, Figure S3). Such behavior
has also been observed for cyclic peptoids by Kwon et al.17 and,
due to the absence of chirality, could be explained by the
presence of different conformations. A peptomer containing
three NSG and two amino acids was prepared by standard
solid-phase peptide synthesis with HATU as a coupling reagent
and a piperidine solution to remove the Fmoc group from the
amino acid residues while the oNs group from the NSG was
cleaved with the solution described above. Peptomer 11 was
obtained in good purities and yields showing that N-substituted
N-oNs-glycines can also be used as monomers in the synthesis
of peptide−peptoid hybrids.
In conclusion, a convenient four-step synthesis was
developed to prepare N-substituted N-oNs-glycines from
readily available amines. The described approach eliminates
the amine deprotection/Fmoc reprotection steps commonly
observed in the synthesis of side chain protected NSG and
allows the synthesis of peptoid monomers bearing a wide
variety of functional groups. The results obtained in this work
demonstrate that these N-substituted N-oNs-glycines can be
used as buildings blocks in peptoid and peptomer synthesis to
introduce interesting reactive or polar side chains. Moreover,
the conditions used in this work allow a substantial building
block economy compared to standard submonomer method
conditions. Simple and affordable, the described procedures
represent a complementary and interesting alternative approach
to introduce relevant functionalities into peptoids and peptide−
peptoid hybrids, increasing the chemical diversity accessible
with the monomer method.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.5b02862.
■
(b) Shreder, K.; Zhang, L.; Gleeson, J.-P.; Ericsson, J. A.; Yalamoori, V.
S
V.; Goodman, M. J. Comb. Chem. 1999, 1, 383−387. (c) Schroder, T.;
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Niemeier, N.; Afonin, S.; Ulrich, A. S.; Krug, H. F.; Brase, S. J. Med.
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Chem. 2008, 51, 376−379. (d) Unciti-Broceta, A.; Diezmann, F.; Ou-
Yang, C. Y.; Fara, M. A.; Bradley, M. Bioorg. Med. Chem. 2009, 17,
959−966.
Detailed experimental procedures, analytical and charac-
terization data for all compounds (PDF)
́
(9) Vezina-Dawod, S.; Derson, A.; Biron, E. Tetrahedron Lett. 2015,
56, 382−385.
(10) Fukuyama, T.; Jow, C.-K.; Cheung, M. Tetrahedron Lett. 1995,
36, 6373−6374.
AUTHOR INFORMATION
Corresponding Author
■
(11) (a) Biron, E.; Chatterjee, J.; Kessler, H. J. Pept. Sci. 2006, 12,
213−219. (b) Miller, S. C.; Scanlan, T. S. J. Am. Chem. Soc. 1997, 119,
2301−2302.
*E-mail: eric.biron@pha.ulaval.ca.
Author Contributions
†S.J. and S.V.-D. contributed equally.
Notes
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2690−2691. (b) Biron, E.; Kessler, H. J. Org. Chem. 2005, 70, 5183−
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J. Org. Chem. 2003, 2003, 4513−4517.
The authors declare no competing financial interest.
(14) Bartoli, G.; Bosco, M.; Carlone, A.; Dalpozzo, R.; Locatelli, M.;
Melchiorre, P.; Sambri, L. J. Org. Chem. 2006, 71, 9580−9588.
(15) (a) Burkoth, T. S.; Beausoleil, E.; Kaur, S.; Tang, D.; Cohen, F.
E.; Zuckermann, R. N. Chem. Biol. 2002, 9, 647−654. (b) Lee, B.-C.;
Zuckermann, R. N.; Dill, K. A. J. Am. Chem. Soc. 2005, 127, 10999−
11009.
ACKNOWLEDGMENTS
■
This work was supported by the Natural Sciences and
Engineering Research Council of Canada (NSERC) (371503-
2010 and RGPIN-2015-06364). S.J. and S.V.-D. thank the
(16) (a) Turner, R. A.; Hauksson, N. E.; Gipe, J. H.; Lokey, R. S. Org.
Lett. 2013, 15, 5012−5015. (b) Narayan, R. S.; VanNieuwenhze, M. S.
Org. Lett. 2005, 7, 2655−2658.
Fonds d’enseignement et de Recherche de la Faculte
pharmacie de l’Universite Laval for scholarships. The authors
are grateful to Pierre-Luc Plante of Centre de recherche du
CHU de Quebec for HRMS and MS/MS analyses.
́
de
́
(17) Park, S.; Kwon, Y.-U. ACS Comb. Sci. 2015, 17, 196−201.
́
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