N-substituted glycines with functional side-chains for peptoid synthesis

Article abstract of DOI:10.1002/ejoc.201402864

N-Substituted glycines (NSG) constitute mimics of natural amino acids, and bring stability to peptide analogues. We developed a method to synthesize NSG bearing reactive secondary heterofunctionality. It was shown that persilylation could be used for temp

Full text of DOI:10.1002/ejoc.201402864

Job/Unit: O42864
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FULL PAPER
DOI: 10.1002/ejoc.201402864
N-Substituted Glycines with Functional Side-Chains for Peptoid Synthesis
Adeline René,[a] Jean Martinez,[a] and Florine Cavelier*[a]
Keywords: Amino acids / Peptoids / Peptidomimetics / Protecting groups
N-Substituted glycines (NSG) constitute mimics of natural
obtained by C-terminal deprotection and N-terminal protec-
amino acids, and bring stability to peptide analogues. We de- tion of the resulting amino acid derivatives. This method was
veloped a method to synthesize NSG bearing reactive sec-
ondary heterofunctionality. It was shown that persilylation
could be used for temporary protection of reactive groups
present on the substrate, with the aim of selectively alkylat-
ing the amine groups. NSG ready for peptide coupling were
applied to prepare N-alkylated glycines as analogues of ser-
ine, cysteine, tyrosine and tryptophan. The last two deriva-
tives were obtained in good yields and N-homotyrosine (N-
hTyr) has been introduced into a peptide sequence of interest
using automated solid-phase peptide synthesis (SPPS).
reductive amination between an amine with glyoxylic
acid.[1] In this case, Fmoc-N-alkylglycines have been synthe-
sized with protected amino acid side chains.[14] The synthe-
sis of oligomeric NSG was performed on solid support and
named the submonomer solid-phase synthesis method.[15]
A new approach to the synthesis of peptoids was described
via photolithographic techniques,[16] and has already proved
to be an interesting application for antibody replace-
ments.[17] Oligomers of N-substituted glycines can also be
obtained using microwave-assisted synthesis, especially for
incorporation of electronically deactivated benzylic
amines.[18] Apart from oligomeric NSG, some examples of
bioactive peptide-peptoid hybrids have been reported as li-
gands of the SH2 domain involved in signal transduc-
tion,[19] or as membrane interactive agents.[20]
Peptoids can be designated according to different no-
menclatures. For example, for the amine building block cor-
responding to tyrosine, some use the IUPAC nomenclature,
N-[2-(p-hydroxyphenyl)methyl]glycine, whereas others apply
the three letters codes related to amino acid abbreviations,
for example, N-Tyr.[21]
Here we describe an efficient synthesis of N-substituted
glycine analogues of a selection of functionalized natural
amino acids like tyrosine, tryptophan, cysteine and serine.
NSG that we prepared are the homologues of natural
amino acids. Therefore, the nomenclature of these deriva-
tives is N-hTyr, N-hTrp, N-hCys, N-hSer.[1]
Introduction
Peptides are known for their efficiency as therapeutic
drugs. However, their poor bioavailability and metabolic in-
stability are still significantly problematic for their use as
drugs. N-Substituted glycine oligomers, named peptoids,
constitute an alternative to avoid these drawbacks.[1] In ad-
dition, as it has been recently reviewed,[2] incorporating
various side chains on different amines offers the possibility
of having in hand a large variety of N-substituted glycines
(NSG). The resulting peptoids might maintain affinity
towards targeted receptors while not being recognized by
proteases. The NSG have already proved their biological
interest when incorporated into peptide sequences.[3] They
were successfully combined with glucosaminides to form
glycopeptoids,[4] and were introduced into macrocycles,
forming cyclic functionalized peptoids, which constitute
promising templates for multimeric ligation of biologically
active ligands.[5] Recently, linear oligomeric N-substituted
aminomethyl-benzamides have been cyclized to form novel
macrocycles enabling the determination of the first X-ray
structures of arylopeptoid constructs.[6]
In addition, incorporation of N-substituted glycines en-
ables translocation mutagenesis.[7] Other applications in-
clude drug candidates,[8] peptide mimics for studies of sec-
ondary structure,[9] catalysts,[10] sensors[11] and nanostruc-
tured materials.[12] They can also constitute starting materi-
als for incorporating post-modifications on amino acid side
chains into proteins.[13]
To illustrate their utility in structure–activity relation-
ships, N-hTyr has been introduced into a peptide sequence
of interest, the neurotensin fragment called NT(8–13).
Peptoids can be synthesized according to different ap-
proaches. One of the first known methods consists of the
[a] IBMM, UMR-CNRS 5247, Universités Montpellier I and II,
Place Eugène Bataillon, 34095 Montpellier, France
E-mail: florine@um2.fr
Results and Discussion
We have developed an alternative route to synthesize N-
substituted glycines with functionalized side chains bearing
heteroatoms. First, we focused on the synthesis of the NSG
http://www.ibmm.fr/
Supporting information for this article is available on the
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A. René, J. Martinez, F. Cavelier
FULL PAPER
resembling a tyrosine homologue, called N-homotyrosine, chloride and triethylamine.[27] Thus, an efficient method of
which synthesis has been described in seven steps with 26% silylation under mild conditions catalyzed by iodine re-
overall yield (Scheme 1).[22] To protect the side-chain func- tained our attention.[28] We chose these persilylation condi-
tionality, this synthetic pathway involved a tert-butylation tions before performing alkylation with ethylbromoacetate
step using isobutene,[23] which often proceeds in low at 0 °C directly on the silylated amine. Then, protection of
yield.[14,24]
the α-amino group with Boc and Fmoc groups and final
ester hydrolysis gave access to building blocks ready for
peptoid synthesis.
The persilylation reaction was performed on tyramine 1,
which was dissolved in dichloromethane in the presence of
iodine, followed by slow addition of hexamethyldisilazane
in dichloromethane.[28] One of the proposed hypotheses
concerning the catalytic mechanism was that iodine could
polarize the Si–N bond in HMDS to produce the reactive
silylating agent (Me3Si)2-NH+-I (I– counter-anion). The re-
action mixture was basic enough for the amine and hydroxy
groups to react. However, addition of triethylamine, which
traps HBr, allowed the intermediate 8a to be observed by
LC-MS monitoring. Usually, alkylation was performed one
pot at 0 °C without additional base. The reaction was
quenched after HPLC monitoring to minimize formation
of the double alkylated product 12a. The optimized mono
9a/dialkylated 12a amines ratio was improved to 95:5. The
temporary silyl-protection on the hydroxy function was re-
moved during the workup, and 9a was isolated in 67%
yield. The usual N-protection followed by saponification af-
forded the desired N-protected N-substituted glycines. Boc
derivatives are generally used both in solution peptide syn-
thesis and solid phase peptide synthesis (SPPS), and Fmoc
derivatives are dedicated to SPPS. In the case of Fmoc de-
rivative saponification, a solution of CaCl2 [0.8 m in iPrOH/
H2O (7:3)] was used to allow both ester deprotection and
preservation of the Fmoc protecting group.[29] Fmoc-N-
homotyrosine 11a was obtained in three steps in 61% over-
all yield (Scheme 2).
Scheme 1. Synthesis of Fmoc-N-hTyr described by Einsiedel.;[22] a:
N-(benzyloxycarbonyloxy)succinimide, THF, 0 °C, for 1 h, then
25 °C for 3 h (96%); b: isobutylene, cat. conc. H2SO4, CH2Cl2/THF
(4:1), –20 to 25 °C for 18 h (53%); c: H2 (49 psi),10% Pd/C (100%);
d: ethylbromoacetate, NEt3, THF, 0 °C to room temp. then r.t. for
2 h (79%); e: NaOH, MeOH, room temp., 30 min; f: Fmoc-OSu,
H2O/dioxane, pH 8.5–9.0, 0 °C to room temp. room temp., 5 h
(66%); g: TFA, room temp., 1 h.
An alternative method using a Lewis acid and di-tert-
butyl dicarbonate was investigated to perform this tricky
step.[25] Indeed, when magnesium perchlorate was used un-
der refluxing dichloromethane, decarboxylation occurred
and generated the tert-butylated phenolic ether.[26] Never-
theless, although yields were improved, the number of steps
remained the same.
We thought that a shorter synthesis could be achieved
using persilylation as a temporary protection, which is usu-
This strategy was extended to other amino acid ana-
ally performed under acidic conditions with hexamethyl- logues, like N-homotryptophan, N-homocysteine and N-
disilazane at high temperature together with trimethylsilyl homoserine (Scheme 3).
Scheme 2. Synthesis of Boc-N-hTyr 10a and Fmoc-N-hTyr.11a.
2
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Peptoid Synthesis
their corresponding dialkylated products 12c–d by
chromatography. N-Protection of the crude mixtures al-
lowed a better separation of N-protected amino esters (13c–
d and 14c–d) from dialkylated derivatives 12c–d. Finally,
compounds 10c–d and 11c–d were isolated in 7 to 26%
overall yield (Table 1). These results could be explained by
the low steric hindrance of the side chains that did not limit
formation of di-alkylated product in addition to a high
nucleophilic character of the side chain functional group.
Table 1. Overall yields per side chain.
Entry
Side-chain
10
11
1
2
3
4
a
b
c
67
52
26
7
61
44
18
8
d
We were then interested in the synthesis of modified
peptides containing these NSG building blocks to exemplify
their utility in structure–activity relationships. We chose the
neurotensin fragment NT(8–13) that we are studying in an-
other program.[30] It has been previously shown that re-
placement of tyrosine in a closely related sequence of
NT(8–13) (JMV 438) by N-homotyrosine led to a highly
NTS2-selective ligand.[22] After obtaining the N-substituted
Scheme 3. Extension of the synthetic strategy.
The one pot persilylation/alkylation step was studied to glycine 9a as presented in Scheme 2, we used the Fmoc de-
evaluate the reaction yield for each side chain. N-homo- rivative 11a in a SPPS strategy using a Liberty CEMTM
tyrosine 9a and N-homotryptophan 9b were isolated in 67% microwave peptide synthesizer without any special condi-
and 53% yield respectively. However, N-homocysteine 9c tions for peptoid building block coupling (Scheme 4). We
and N-homoserine 9d could not be clearly separated from obtained the desired pseudo-hexapeptide JMV 4961.
Scheme 4. Solid phase peptide synthesis of JMV 4961 using Fmoc-N-hTyr; (a) HBTU, DIEA, DMF; (b) piperidine 20%/DMF; (c) TFA,
TIS, CH2Cl2.
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A. René, J. Martinez, F. Cavelier
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I2 (0.01 equiv.) was added, followed by HMDS (2 equiv.) in CH2Cl2
(0.8 m) dropwise within 5 min. The mixture became gradually
colourless. After 2 h stirring at room temp., the pH was checked
Conclusions
We developed a rapid route to access N-substituted
glycines in three steps in satisfactory yields depending on and verified to be basic. The reaction mixture was cooled to 0 °C
and ethyl bromoacetate (1 equiv.) was added dropwise. After
30 min stirring at room temp., the reaction mixture was washed
with water. The organic layer, which contained the dialkylated
product 13c–d and the desired product 9c–d, was dried with MgSO4
and then concentrated. This crude product was directly engaged in
the amine protection reaction.
the nature of aliphatic or aromatic side chains. These N-
protected building blocks in can be directly used in SPPS,
in particular in the synthesis of peptide–peptoid hybrids.
Experimental Section
Amine Group Protection for Homoalkylamines
General: All reactions involving air-sensitive reagents were per-
formed under nitrogen or argon. Purifications were performed with
column chromatography using silica gel (Merck 60, 230–400 mesh)
or with a Biotage instrument Isolera 4 using SNAP KP-SIL flash
cartridges. Proton nuclear magnetic resonance (1H NMR) and
carbon nuclear magnetic resonance (13C-NMR) spectra were re-
corded with a Bruker Avance 200 spectrometer at 200 and 50 MHz,
a Bruker Avance 300 spectrometer at 300 and 75 MHz or a Bruker
600 spectrometer at 600 and 150 MHz, respectively. All chemical
shifts were recorded relative to internal tetramethylsilane when
CDCl3 was used as solvent. Low-resolution electrospray ionization
(ESI) mass spectra were recorded with a Micromass platform elec-
trospray mass spectrometer. Spectra were recorded in the positive
mode (ESI+). HPLC chromatography was performed using a
Waters 2796 HPLC instrument equipped with a Phenomenex®
Onyx Monolithic HD-C18 column (50 μmϫ4.6 mm).
N-Boc-Protection Procedure: To a solution of monoalkylated amine
9 (1 equiv.) in THF (0.2 m) was added di-tert-butyl dicarbonate
(1.2 equiv.) and triethylamine (1 equiv.). The reaction mixture was
stirred overnight at room temp. After removing THF in vacuo, the
crude product was dissolved in ethyl acetate. This organic layer was
washed with a solution of potassium hydrogen sulfate (ϫ3) and a
saturated solution of sodium hydrogen carbonate (ϫ3). Then, the
organic layer was dried with MgSO4 and concentrated.
1
Compound 13a: Yield 100%. ESI-MS: m/z = 324.4 [M + H]+. H
NMR (CDCl3, 300 MHz, 25 °C): δ = 1.30 (t, J = 6.9 Hz, 3 H,
CH2CH3), 1.50 (s, 9 H, tBu), 2.70–2.90 (m, 2 H, CH2CH2N), 3.40–
3.50 (m, 2 H, CH2CH2N), 3.75 and 3.77 (2s, 2 H, CH2CO2), 4.20–
4.30 (q, J = 6.9 Hz, 2 H, CH2CH3), 5.1 (br., 1 H, OH), 6.60–6.75
(m, 2 H, CH meta), 7.00–7.15 (m, 2 H, CH ortho) ppm.
1
Compound 13b: Yield 100%. ESI-MS: m/z = 347.4 [M + H]+. H
Alkylation Procedure for Homotyramine and Homotryptamine: To
a stirred suspension of the amine (1 equiv.) and I2 (0.01 equiv.) in
CH2Cl2 (0.25 m) was added diluted HMDS (2 equiv.) in CH2Cl2
(0.8 m) dropwise within 5 min. The mixture became gradually
homogeneous. After stirring at room temp. for 2 h and basic pH
was verified , the reaction mixture was cooled to 0 °C and ethyl-
bromoacetate (1 equiv.) was added dropwise. After stirring for 1.5 h
at room temp., the reaction mixture was washed with a 10% citric
acid solution. The pH of the aqueous layer was increased with po-
tassium carbonate to a value of 8–9. Then, the monoalkylated
amine was extracted with ethyl acetate. The organic layer was dried
with MgSO4 and then concentrated.
NMR (CDCl3, 300 MHz, 25 °C): δ = 1.24 (t, J = 6 Hz, 3 H,
CH2CH3), 1.51 (s, 9 H, Boc), 2.95–3.03 (m, 2 H, CH2CH2N), 3.50–
3.56 (m, 2 H, CH2CH2N), 3.76 and 3.89 (2s, rotamers, CH2CO2),
4.09–4.18 (q, J = 12 Hz, 2 H, CH2CH3), 6.98–7.02 (m, 1 H, CH
para), 7.07–7.20 (m, 2 H, CH ortho, metaЈ), 7.33–7.36 (m, 1 H, CH
meta), 7.57–7.62 (m, 1 H, CHNH indole), 7.99 (br. s, 1 H, indole)
ppm.
Compound 13c: Purification by silica flash chromatography using
cyclohexane/EtOAc 8:2 as the eluent. Yield 27% (2 steps) ESI-MS:
m/z = 264.3 [M + H]+. 1H NMR (CDCl3, 300 MHz, 25 °C): δ =
1.26 (t, J = 6 Hz, 3 H, CH2CH3), 1.41 (s, 9 H, Boc), 2.74 (t, J
= 7.5 Hz, 2 H, CH2CH2N), 3.2 (s, 1 H, SH), 3.29–3.38 (m, 2 H,
CH2CH2N), 3.20 (s, 2 H, CH2CO2), 4.16 (q, J = 6 Hz, 2 H,
CH2CH3) ppm.
1
Compound 9a:.[31] Yield 67% ESI-MS: m/z = 224.1 [M + H]+. H
NMR (CDCl3, 300 MHz, 25 °C): δ = 1.24 (t, J = 9 Hz, 3 H,
CH2CH3), 2.74 (d, J = 6 Hz, 2 H, CH2Ph), 2.84 (d, J = 6 Hz, 2 H,
CH2NH), 3.39 (s, 3 H, CH2CO2), 4.15 (q, J = 9 Hz, 2 H, CH2CH3),
6.72 (d, J = 9 Hz, 2 H, CH ortho), 7.02 (d, J = 9 Hz, 2 H, CH meta)
ppm. 13C NMR (CDCl3, 75 MHz, 25 °C): δ = 14.2 (CH2CH3), 35.4
(CH2CH2NH), 50.8 (CH2CH2NH), 50.9 (CH2CO2), 60.9
(CH2CH3), 115.4 (Cm), 129.8 (Co), 131.3 (NHCH2CH2C), 154.3
(Cp), 172.3 (CO2) ppm.
Compound 13d: Purification by silica flash chromatography, using
cyclohexane/EtOAc 8:2 as the eluent. Yield 14% (2 steps). ESI-MS:
m/z = 248.3 [M + H]+, 270.3 [M + Na]. 1H NMR (CDCl3,
300 MHz, 25 °C): δ = 1.23–1.27 (m, 3 H, CH2CH3), 1.42 (d, J =
15 Hz, 9 H, Boc), 3.40–3.44 (m, 2 H, CH2CH2N), 3.62–3.78 (m, 2
H, CH2CH2N), 3.83 and 3.99 (2s, rotamers, 2 H, CH2CO2), 4.19
(q, J = 6 Hz, 2 H, CH2CH3) ppm.
Compound 9b: Yield 53% ESI-MS: m/z = [M + H]+ = 247.3 1H
NMR (CDCl3, 600 MHz, 0 °C): δ = 1.27 (t, J = 7.1 Hz, 3 H,
CH2CH3), 2.98–3.05 (m, 4 H, CH2CH2NH), 3.47 (s, 2 H,
CH2CO2), 4.19 (q, J = 7.1 Hz, 2 H, CH2CH3), 7.12 (d, J = 1.9 Hz,
1 H, CH para), 7.15–7.17 (m, 1 H, CH meta), 7.23–7.25 (m, 1 H,
CH ortho), 7.40–7.42 (m, 1 H, CH metaЈ), 7.65–7.67 (m, 1 H,
CHNH indole), 8.23 (br. s, 1 H, indole) ppm. 13C NMR (CDCl3,
150 MHz, 25 °C): δ = 13.9 (CH2CH3), 25.6 (CH2CH2NH), 49.3
(CH2CH2NH), 50.8 (CH2CO2), 60.5 (CH2CH3), 110.91 (CmЈ),
113.5 (CCHNH), 118.6 (Co), 118.8 (Cm), 121.7 (Cp), 121.8 (Co),
127.2 (NHCH2CH2CC), 136.1 (NHC indole), 172.2 (CO2) ppm.
N-Fmoc Protection Procedure: To a solution of monoalkylated
amine 9 (1 equiv.) in THF (0.2 m) was added N-(9-fluorenyl-
methoxycarbonyloxy)succinimide (1.3 equiv.) and triethylamine
(1 equiv.). The reaction mixture was stirred for 5 h at room temp.
After removing THF in vacuo, the residue was dissolved in ethyl
acetate. This organic layer was washed with a solution of potassium
hydrogen sulfate (ϫ3) and a saturated solution of sodium hydrogen
carbonate (ϫ3). Then, the organic layer was dried with MgSO4
and concentrated. The crude product was purified by silica flash
chromatography using cyclohexane/ethyl acetate 8:2 as the eluent.
Alkylation Procedure for Homocysteamine and Homoethanolamine: Compound 14a: Yield 91%. ESI-MS: m/z = 446.5 [ M + H]+. 1H
To a stirred suspension of the amine hydrochloride (1 equiv.) in
CH2Cl2 (0.25 m) was added triethylamine (1 equiv.). After 30 min,
NMR (CDCl3, 300 MHz, 25 °C): δ = 1.22 (t, J = 9 Hz, 3 H,
CH2CH3), 2.63 (m, 2 H, CH2CH2N), 3.39 (m, 2 H, CH2CH2N),
4
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Peptoid Synthesis
3.76 and 3.80 (2s, rotamers, 2 H, CH2CO2), 4.08–4.15 (m, 2 H,
CH2CH2N), 3.94 and 3.99 (2s, rotamers, 2 H, CH2CO2) ppm. 13C
CH2CH3), 4.42 (d, J = 6 Hz, 1 H, CH2, Fmoc), 4.55 (d, J = 6 Hz, NMR (CDCl3, 75 MHz, 25 °C): δ = 28.4 (CH3, tBu), 53.1
1 H, CH, Fmoc), 5.35 (d, J = 12 Hz, 1 H, CH, Fmoc), 6.7–7.75
(m, 12 H, CH, aromatic) ppm.
(CH2CH2N), 54.2 (CH2CO2), 58.5 (CH2OH), 81.2 (C tBu), 152.4
(CO2, Boc), 178.4 (CO2H) ppm.
Compound 14b: Yield 83%. ESI-MS: m/z = 469.5 [M + H]+. 1H
NMR (CDCl3, 300 MHz, 25 °C): δ = 1.17 (t, J = 6 Hz, 3 H,
CH2CH3), 2.89 (m, 2 H, CH2CH2N), 3.56 (m, 2 H, CH2CH2N),
3.79 and 3.84 (2s, rotamers, 2 H, CH2CO2), 4.03–4.13 (m, 3 H,
CH2CH3, CH Fmoc), 4.42 (t, J = 9 Hz, 2 H, CH2, Fmoc), 6.79–
7.52 (m, 12 H, CH aromatic), 8.06 (br. s, 1 H, CHNH indole) ppm.
Ester Deprotection Procedure of Fmoc Derivatives 14: Fmoc ester
derivative 14 (1 equiv.) was dissolved in a CaCl2 solution (0.8 M)
prepared in iPrOH/H2O (7:3). Solid sodium hydroxide (1.5 equiv.)
was added, and the mixture was stirred at room temp. overnight.
After HPLC monitoring, the solvent was removed in vacuo. The
crude product was diluted in water, and the pH of the aqueous
layer pH was decreased to 2–3 using solid citric acid. This acidic
aqueous layer was extracted with ethyl acetate. The combined or-
ganic phases were washed with saturated a solution of sodium
chloride , dried with MgSO4 and concentrated.
Compound 11a: Yield 100%. ESI-MS: m/z = 418.5 = [M + H]+.
1H NMR ([D6]DMSO, 300 MHz, 25 °C): δ = 2.29–2.33 (m, 1 H,
CH2CH2N), 2.63–2.68 (m, 1 H, CH2CH2N), 3.04–3.09 (m, 1 H,
CH2CH2N), 3.35–3.40 (m, 1 H, CH2CH2N), 3.76 and 3.88 (2s, rot-
amers, 2 H, CH2CO2), 4.16–4.23 (m, 2 H, CH2 Fmoc), 4.50 (d, J
= 6 Hz, 1 H, CHCH2 Fmoc), 6.62–6.74 (m, 4 H, CH tyramine),
6.98–7.90 (m, 8 H, CH Fmoc) ppm. 13C NMR ([D6]DMSO,
75 MHz, 25 °C): δ = 33.5 (CH2CH2N), 47.1 (CH Fmoc), 49.2
(CH2CH2N), 50.1 (CH2CO2), 66.4 (CH2 Fmoc), 115.3 (C m tyr-
amine), 120.41 (C Fmoc), 124.9 (C Fmoc), 127.4 (C Fmoc), 129.3
(C o tyramine), 140.9 (C Fmoc), 141.2 (C Fmoc), 144.2 (C Fmoc),
155.5 (NCH2CH2C), 155.9 (CO2 Fmoc), 171.3 (CO2H) ppm.
Compound 14c: Purification by silica flash chromatography, using
cyclohexane/EtOAc 8:2 as the eluent. Yield 18%. ESI-MS: m/z =
1
386.5 [M + H]+. H NMR (CDCl3, 300 MHz, 25 °C): δ = 1.27 (t,
J = 9 Hz, 3 H, CH2CH3), 2.71–2.83 (m, 2 H, CH2CH2N), 3.22 (s,
2 H, CH2CO2), 3.36–3.5 (m, 2 H, CH2CH2N), 4.14–4.25 (m, 3 H,
CH2CH3 and CH Fmoc), 4.30–4.33 (d, J = 9 Hz, 1 H, CH2 Fmoc),
5.29 (br. s, 1 H, NH), 7.27–7.76 (CHaro) ppm.
Compound 14d: Yield 22%. ESI-MS: m/z = 370.4 [M + H]+. 1H
NMR (CDCl3, 300 MHz, 25 °C): δ = 1.17–1.29 (m, 3 H, CH2CH3),
3.20 (m, 2 H, CH2CH2N), 3.48–3.60 (m, 2 H, CH2CH2N, SH),
3.88–3.92 (m, 2 H, CH2CH2N), 3.92 and 3.98 (2s, rotamers, 2 H,
CH2CO2), 4.10–4.27 (m, 3 H, CH2CH3 and CH Fmoc), 4.43–4.49
(m, J = 18 Hz, 2 H, CH2 Fmoc), 7.29–7.75 (CH aromatic) ppm.
Ester Saponification Procedure of Boc Derivatives 13
To a stirred solution of Boc ester derivative 13 (1 equiv.) in EtOH
(0.2 m) was added a solution of KOH (4 n , 3 equiv.). The mixture
was stirred at room temp. for 1.5 h. After HPLC monitoring, EtOH
was removed in vacuo. The crude product was diluted in water, and
the aqueous layer pH was decreased to 2–3 using solid citric acid.
Then, this acidic aqueous layer was extracted with ethyl acetate.
The combined organic layers were washed with a saturated solution
of sodium chloride , dried with MgSO4 and concentrated.
1
Compound 11b: Yield 100%. ESI-MS: m/z = 441.5 [M + H]+. H
NMR (CDCl3, 300 MHz, 25 °C):
δ = 2.78–2.82 (m, 1 H,
CH2CH2N), 2.96–3.01 (m, 1 H, CH2CH2N), 3.48–3.56 (m, 2 H,
CH2CH2N), 3.75 and 3.85 (2s, rotamers, 2 H, CH2CO2), 4.1–4.25
(m, 1 H, CHCH2 Fmoc), 4.46–4.48 (m, 2 H, CH2 Fmoc), 7.02–
7.69 (m, 14 H, CH Fmoc), 8.01 (br. d, 1 H, NH indole) ppm. 13C
NMR (CDCl3, 75 MHz, 25 °C): δ = 25.0 (CH2CH2N), 47.9 (CH
Fmoc), 49.9 (CH2CH2N), 68.3 (CH2CO2), 111.9 (CH2 Fmoc),
113.2 (C Fmoc), 119.0 (C Fmoc), 120.2 (C Fmoc), 122.7 (C Fmoc),
127.8 (C Fmoc), 136.9 (C Fmoc), 141.9 (C Fmoc), 144.5 (C Fmoc),
157.5 (CO2 Fmoc), 175.0 (CO2H) ppm.
1
Compound 10a: Yield 100%. ESI-MS: m/z = 296.3 [M + H]+. H
NMR ([D6]DMSO, 600 MHz, 25 °C): δ = 1.34 (s, 9 H, Boc), 2.61–
2.65 (m, 2 H, CH2CH2N), 3.29–3.31 (m, 2 H, CH2CH2N), 3.75
and 3.77 (2s, rotamers, J = 13.3 Hz, 2 H, CH2CO2), 6.66–6.68 (m,
2 H, CH meta), 6.95–6.99 (m, 2 H, CH ortho) ppm. 13C NMR
Compound 11c: Yield 100%. ESI-MS: m/z = 358.4 [M + H]+, 380.4
[M + Na]. H NMR ([D6]DMSO, 600 MHz, 25 °C): δ = 2.64–2.66
1
([D6]DMSO, 150 MHz, 25 °C):
δ = 27.9 (CH3, tBu), 33.4
(m, 1 H, NCH2CH2SH), 3.19–3.21 (m, 2 H, NCH2CH2SH), 3.35
(s, 2 H, CH2CO2), 4.2–4.22 (m, 1 H, CHCH2 Fmoc), 4.30–4.31 (m,
2 H, CHCH2 Fmoc), 6.91–7.94 (m, 9 H, CH Fmoc) ppm. 13C
NMR ([D6]DMSO, 150 MHz, 25 °C): δ = 31.4 (NCH2CH2SH),
32.9 (CH Fmoc), 46.7 (NCH2CH2SH), 65.3 (CH2 Fmoc), 120.1 (C
Fmoc), 125.1 (C Fmoc), 127.1 (C Fmoc), 127.6 (C Fmoc), 140.7
(C Fmoc), 143.9 (C Fmoc), 156.1 (CO2 Fmoc), 171.5 (CO2 H) ppm.
(CH2CH2NH), 48.4 (CH2CH2N), 49.7 (CH2CO2), 78.7 [C(CH3)3],
115.1 (Cm), 129.2 (NCH2CH2C), 129.6 (Co), 154.8 (CO2, Boc),
155.6 (COH), 171.4 (CO2H) ppm.
1
Compound 10b: Yield 100%. ESI-MS: m/z = 319.4 [M + H]+. H
NMR (CDCl3, 300 MHz, 25 °C): δ = 1.35 (s, 9 H, Boc), 2.94–2.99
(m, 2 H, CH2CH2N), 3.54–3.59 (m, 2 H, CH2CH2N), 3.79 and 4.08
(2s, rotamers, 2 H, CH2CO2), 7–7.6 (m, 5 H, aromatic), 8.26 (br.
s, 1 H, NH, indole) ppm. 13C NMR (CDCl3, 75 MHz): δ = 21.4
(CH2CH2N), 24.5 (CH2CH2N), 28.6 (CH3, tBu), 49.8 (CH2CO2),
81.2 [C(CH3)3], 111.7 (C Ar), 112.9 (C Ar), 118.9 (C Ar), 120.1 (C
Ar), 122.3 (C Ar), 123.5 (C Ar), 127.7 (C cycle junction), 136.7 (C
cycle junction), 156.8 (CO2tBu), 175.2 (CO2 H) ppm.
Compound 11d: Yield 100%. ESI-MS: m/z = 342.4 [M + H]+, 364.4
[M + Na]. H NMR (CDCl3, 300 MHz, 25 °C): δ = 3.24–3.50 (m,
1
2 H, CH2CO2), 3.36–3.39 (m, 2 H, NCH2CH2OH), 3.50–3.55 (m,
2 H, NCH2CH2OH), 4.12–4.20 (m, 1 H, CHCH2 Fmoc), 4.43–4.49
(m, 2 H, CHCH2 Fmoc), 7.29–7.76 (m, 9 H, CH Fmoc) ppm. 13C
NMR ([D6]DMSO, 150 MHz, 25 °C): δ = 48.7 (CH Fmoc), 51.2
(NCH2CH2OH), 56.4 (NCH2CH2OH), 66.7 (CH2 Fmoc), 120.1 (C
Fmoc), 126.3 (C Fmoc), 127.6 (C Fmoc), 128.3 (C Fmoc), 141.7
(C Fmoc), 143.6 (C Fmoc), 158.6 (CO2 Fmoc), 173.5 (CO2 H) ppm.
1
Compound 10c: Yield 100%. ESI-MS: m/z = 236.3 [M + H]+. H
NMR ([D6]DMSO, 200 MHz, 25 °C): δ = 1.38 (s, 9 H, Boc), 2.57–
2.64 (m, 2 H, CH2CH2N), 3.06–3.16 (m, 2 H, CH2CH2N), 3.23–
3.25 (m, 2 H, CH2CO2), 12.5 (br. s, 1 H, CO2H) ppm. 13C NMR Supporting Information (see footnote on the first page of this arti-
(CDCl3, 50 MHz, 25 °C): δ = 28.2 (CH3, tBu), 31.5 (CH2CH2N), cle): 1H NMR and 13C NMR spectra of compounds, HPLC and
32.9 (CH2CH2N), 77.7 (CH2CO2), 99.5 [C(CH3)3], 155.5 (CO2 HR-MS of peptides.
Boc), 171.5 (CO2H) ppm.
1
Compound 10d: Yield 100%. ESI-MS: m/z = 220.2 [M + H]+. H
Acknowledgments
NMR (CDCl3, 300 MHz, 25 °C): δ = 1.44–1.47 (2s, rotamers, 9 H,
Boc), 3.40–3.45 (m,
2
H, CH2CH2N), 3.70–3.75 (m,
2
H,
The authors thank Medincell SA for a grant to A. R.
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Job/Unit: O42864
/KAP1
Date: 17-11-14 15:13:04
Pages: 7
A. René, J. Martinez, F. Cavelier
FULL PAPER
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Received: July 4, 2014
Published Online: ᭿
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Eur. J. Org. Chem. 0000, 0–0

Job/Unit: O42864
/KAP1
Date: 17-11-14 15:13:04
Pages: 7
Peptoid Synthesis
Peptoid Monomer Synthesis
A. René, J. Martinez, F. Cavelier* .... 1–7
N-Substituted Glycines with Functional
Side-Chains for Peptoid Synthesis
N-Substituted glycines (NSG) constitute
mimics of natural amino acids that are
used in peptoid synthesis. We developed a
method to synthesize NSG bearing reactive
secondary heterofunctionality. Persilylation
was used as temporary protection, and
analogues of serine, cysteine, tyrosine and
tryptophan were prepared. N-Homotyro-
sine (N-hTyr) has been introduced into a
peptide sequence of interest.
Keywords: Amino acids / Peptoids / Pept-
idomimetics / Protecting groups
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