Catalytic Peptide Synthesis: Amidation of N-Hydroxyimino Esters

Article abstract of DOI:10.1021/acscatal.7b04244

A catalytic method for the formation of amide bonds was developed in which the amidation of N-hydroxyimino esters with a broad range of amino acid tert-butyl esters is promoted by a niobium catalyst in the absence of solvent. Contrary to the predominant protocol based on reagent control commonly applied to amidation reactions, this study provides insight into an approach based on substrate control. This system affords the corresponding amides in high yields in addition to being both highly atom efficient and racemization-free. An advantage of this system is shown in that the Lewis acid catalysis proceeds chemoselectively in the presence of nonactivated esters. Furthermore, the resulting amides are easily transformed into their corresponding di- and tripeptides with high diastereoselectivities under simple hydrogenation conditions.

Full text of DOI:10.1021/acscatal.7b04244

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Article
Catalytic Peptide Synthesis: Amidation of N-Hydroxyimino Esters
WATARU MURAMATSU, Hiroaki Tsuji, and Hisashi Yamamoto
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b04244 • Publication Date (Web): 19 Jan 2018
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Catalytic Peptide Synthesis: Amidation of Nꢀ
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Hydroxyimino Esters
Wataru Muramatsu,* Hiroaki Tsuji, and Hisashi Yamamoto*
Molecular Catalyst Research Center, Chubu University, 1200 Matsumotoꢀcho, Kasugai, Aichi
487ꢀ8501, Japan
ABSTRACT: A catalytic method for the formation of amide bonds has been developed, in
which the amidation of Nꢀhydroxyimino esters with a broad range of amino acid tertꢀbutyl esters
is promoted by a niobium catalyst in the absence of solvent. Contrary to the predominant
protocol based on reagent control commonly applied to amidation reactions, this study provides
insight into an approach based on substrate control. This system affords the corresponding
amides in high yields in addition to both high atomꢀefficiency and racemizationꢀfree. An
advantage of this system is shown in that the Lewis acid catalysis proceeds chemoselectively in
the presence of nonꢀactivated esters. Furthermore, the resulting amides are easily transformed
into their corresponding diꢀ and tripeptides with high diastereoselectivities under simple
hydrogenation conditions.
KEYWORDS: amidation, peptides, niobium catalysis, solventꢀfree reactions, hydrogenation
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■ INTRODUCTION
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Peptides have attracted increased interest as pharmaceuticals because of their high specificity
compared to small molecule drugs.1 Cosmetic and health food industries are also interested in
peptidic products. Their industrial rise has prompted industrial and academic chemists to seek
atom efficient manufacturing processes for peptideꢀbased products to reduce the environmental
impact and production cost.
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In organic synthesis of peptides, one of the key routes involves the condensation reaction
between amines and either esters or carboxylic acids.2 Thus, peptides are synthesized stepwise
by coupling the amino functional group of one amino acid to an activated carboxylic acid group,
which is generated in situ from carboxylic acids and stoichiometric amounts of a coupling
reagent,
such
as
N,N'ꢀdicyclohexylcarbodimide
(DCC)
and
(benzotriazolꢀ1ꢀ
yloxy)tripyrrolidinophosphonium hexafluorophosphate (PyBOP) (Scheme 1a).2d In solidꢀphase
synthesis, which is now the most common industrial approach to peptide synthesis, the peptide
backbone is assembled one amino acid at a time while attached to resin beads, which require
washing away residual reagents at each step.3 From these processes, producing 1 kg of peptide
typically requires several metric tons of solvent and thousands of liters of water in addition to the
large quantities of chemicals, creating a significant environmental footprint.4 As such, studies
carried out in the past few decades have focused on the search for a catalytic route to amide bond
formation. Indeed, since our initial report of a highly active catalytic method for the amidation
reaction between carboxylic acids and amines,5 a variety of prominent catalytic approaches have
been reported (Scheme 1b).6 The most recently reported catalysts employed for this
transformation are boronic acid derivatives, with Shibasaki most recently reporting the catalytic
amidation reaction using a 1,3ꢀdioxaꢀ5ꢀazaꢀ2,4,6ꢀtriborinane derivative.7 However, these
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aforementioned methods are aimed at the conversion of the hydroxy group to an active leaving
group based on reagent control, whereas they sometimes lead to nonꢀtrivial issues regarding
racemization.8 As such, considerable room for improvement exists in the context of both waste
production and racemization during amide bond formation. To address this issue, our group
recently developed a powerful catalytic protocol in which Ta(OEt)5 promotes the amidation of a
wide range of βꢀhydroxy esters in good yields using a diverse array of amine nucleophiles
(Scheme 1c).9 This catalytic approach is based on substrate control, with a hydroxy group
located at the βꢀposition of the carbonyl group playing a key role as a directing group in
initiating the siteꢀselective activation of the carbonyl group by the tantalum catalyst, to ultimately
promote amide bond formation. However, this method is limited in its applicability to esters of
specific amino acids such as threonine (Thr) and serine (Ser) for the preparation of peptides.
Herein, we present a novel and highly atomꢀefficient approach for the catalytic amidation of Nꢀ
hydroxyimino esters10 with a broad range of amino acid tertꢀbutyl esters to achieve the desired
C–N bond formation without any racemization (Scheme 1d). Furthermore, we aim to
demonstrate that the resulting amides can be readily transformed into diꢀ and tripeptides in high
diastereoselectivities.
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Scheme 1. Approaches in Peptide Synthesis
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■ RESULTS AND DISCUSSION
Initially we examined the reaction between EꢀNꢀhydroxyꢀαꢀimino ester 111 and amino acid tertꢀ
butyl ester HCl salts using a range of different bases and catalysts (Table 1). 1 was quantitatively
prepared from methyl pyruvate and NH2OH·HCl.10b As the HCl salts of amino acid esters are
generally more stable than the free amines, this allows their longꢀterm storage and use without
polymerization or racemization, and the salts tend to be easy to handle.12 In addition, tertꢀbutyl
esters are advantageous for peptide synthesis, as side reactions such as the selfꢀintermolecular
condensation or intramolecular cyclization of peptides can be avoided due to steric hindrance
from the tertꢀbutyl moiety.13 Based on the route outlined in Scheme 1c, Ta(OEt)5 was first
selected as a catalyst for this transformation. Following optimization of the reaction conditions
using this catalyst, we found that 1 smoothly engaged with LꢀAlaꢀOtꢀBu·HCl to afford the
desired amide 2a with no observation of racemization in the presence of 2 mol% of Ta(OEt)5 and
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1 equiv of Et3N at 50 °C under solventꢀfree conditions (low eꢀvalue, entry 1, 95% yield).
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However, in the absence of Et3N, the reaction was unsuccessful, indicating that a base is required
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to neutralize the HCl salt (entry 2). Although the free amine, LꢀAlaꢀOtꢀBu, can be used instead of
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its HCl salt, the oneꢀpot procedure described herein is technically and practically more
convenient, and the ammonium chloride generated during neutralization has no effect on the
catalyst activity. Although a modicum of the product 2a was produced in the absence of the
catalyst (entry 3), its yield was not improved dramatically at higher temperatures or under
microwave irradiation (entries 4 and 5). We subsequently examined the effect of different types
of bases on the reaction, and found that the obtained yields were significantly lower than those
obtained for Et3N (entries 6–9, <1–71% yields). In addition, upon repeating the reaction using a
series of tantalum catalysts, a range of yields of 2a were observed, including a number of
unsatisfactory results (entries 10–15, 28–98% yields). Furthermore, the investigation of various
metal alkoxide catalysts (entries 16–22) indicated that the Nb(OEt)5ꢀcatalyzed amidation of 1
with LꢀAlaꢀOtꢀBu·HCl produced 2a in the best yield (entry 22, >99% yield). Moreover, these
optimized conditions were applicable to the gramꢀscale synthesis of 2a (entry 22 in the bracket,
>99% yield). We also demonstrated that the reaction time could be significantly shortened (i.e.,
from 24 to 3 h) using microwave irradiation with petroleum ether as the solvent, as only a small
decrease in yield was observed. (entry 23, 95% yield).14 When methyl pyruvate was employed
instead of 1, only imine formation reaction between amino group of LꢀAlaꢀOtꢀBu and carbonyl
group of methyl pyruvate proceeded instead of amidation reaction to give the corresponding
amide compound (entry 24).15
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Table 1. SolventꢀFree Catalytic Amidation of 1 with L
ꢀAlaꢀOtꢀBu·HCl Salta
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Entry
Base
Et3N
Catalyst
Ta(OEt)5
Ta(OEt)5
–
Yield of 2a
(%)b
1
2
3
95
<1
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35
3
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71
<1
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98
93
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–
Et3N
Et3N
Et3N
DABCO
iꢀPr2NH
pyridine
K2CO3
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
4c
5d
6
–
–
Ta(OEt)5
Ta(OEt)5
Ta(OEt)5
Ta(OEt)5
Ta(OMe)5
Ta(OBu)5
Ta(NMe2)5
Ta(acac)(OEt)4
TaCl5
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23d
24f
TaBr5
VO(OEt)3
Cu(OEt)2
Hf(OEt)4
W(OEt)6
Pd(OAc)2
Fe(OTf)3
Nb(OEt)5
Nb(OEt)5
Nb(OEt)5
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39
85
35
>99 (>99)e
95
0g
aConducted on a 1.0 mmol scale of
internal standard. 100 °C. Through the use of microwave irradiation in petroleum ether for 3 h. Isolated yield of the reaction
with 10 mmol of
ꢀAlaꢀOtꢀBu·HCl. fMethyl pyruvate was employed instead of 1. gIsolated yield of the corresponding amide.
LꢀAlaꢀOtꢀBu·HCl. Determined by GC analysis with the aid of dodecane as a calibrated
b
c
d
e
L
To highlight the wide applicability of this method, we applied the most successful catalyst,
Nb(OEt)5, and conditions (entry 22 of Table 1) to the amidation of several Nꢀhydroxyimino
esters10 with a range of amino acid tertꢀbutyl ester HCl salts (Scheme 2). As indicated in the
scheme, the amidation of
proceeded smoothly to afford the corresponding amides 2b, 2c, and 2e in excellent yields.
Unexpectedly, ꢀIleꢀOtꢀBu·HCl showed low reactivity and reacted with 1 to give the amide 2d in
LꢀValꢀOtꢀBu·HCl, LꢀLeuꢀOtꢀBu·HCl, and LꢀPheꢀOtꢀBu·HCl with 1
L
only moderate yield without formation of any side products including a diastereomer of 2d.16
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Although
LꢀPhgꢀOtꢀBu cannot be effectively incorporated using classical amidation methods of
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Scheme 1a due to its high tendency to racemize,17 our substrateꢀcontrolled method dramatically
provided a quantitative yield of 2f with no observation of racemization.18 When amino acids
bearing free hydroxy groups were treated in the presence of Nb(OEt)5, slightly lower yields of
2g, 2i, and 2k (43%, 74%, and 67% yields, respectively) were obtained compared to those of the
corresponding protected amides 2h, 2j, and 2l (90%, 90%, and 98% yields, respectively).
Interestingly, the reaction tolerated amino acids bearing not only hydroxy, carbonyl, and Sꢀalkyl
groups, but also those bearing the indole moiety, providing 2m–2p in 94 to >99% yields. These
results therefore indicate that nucleophilic amines bearing a variety of functional groups can take
part in this amide bond formation reaction to produce high yields of the desired amides.
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Furthermore, even the secondary amine, LꢀProꢀOtꢀBu·HCl, reacted to furnish 2q in 92% yield
with a rotamer ratio of 63:37. Not only βꢀsubstituted EꢀNꢀhydroxyꢀαꢀimino esters 3, 4, and 611
but also ZꢀNꢀhydroxyꢀαꢀimino ester 711 was subject to the amidation to give the corresponding
amides 2r–2u in 82 to 96% yields.19 EꢀNꢀhydroxyimino ester 8, which bears two methyl ester
groups at αꢀ and γꢀpositions, reacted with LꢀAlaꢀOtꢀBu·HCl to produce 2v as a major product.
Moreover, this protocol could be extended to the amidation using the dipeptide 11a as well as Nꢀ
hydroxyꢀγꢀimino methyl ester 9,20 giving the corresponding oligopeptide precursors 2w and 2x in
95% and 85% yields, respectively.
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Scheme 2. SolventꢀFree Nb(OEt)5ꢀCatalyzed Amidationa
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b
aConducted on a 1.0 mmol scale of
Lꢀamino acid tertꢀbutyl ester·HCl. Commercially available free amines,
LꢀPhgꢀOtꢀBu and
Lꢀ
TyrꢀOtꢀBu, and LꢀAlaꢀLꢀAlaꢀOtꢀBu (11a) provided by hydrogenation of 2a in Scheme 4 were used instead of their corresponding
HCl salts with Et3N. cThe ratio of rotational isomers was determined by 1H NMR analysis.
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Unexpectedly, ZꢀNꢀhydroxyꢀαꢀimino ester 511 containing tertꢀbutyl group at βꢀposition was not
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reacted with LꢀAlaꢀOtꢀBu, prepared by neutralization of LꢀAlaꢀOtꢀBu·HCl, in the presence of 2–
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10 mol% of Nb(OEt)5 or Ta(OEt)5 at 50–100 °C. Because steric repulsion between the tertꢀbutyl
group and methoxy group prevents the interconversion of the geometric isomer s-trans-5 to s-
cis-5, the carbonyl moiety of 5 may not be activated by niobium catalyst (Fig 1a). To reveal the
reactivity pattern of Lewis acid (Nb)–imino ester 5 complexes, the most stable structures and
relative free energy levels of TS1 and TS2 were calculated at MP2/(6ꢀ31G(d), LANL2DZ) level.
This confirmed that the complex TS2 is 2.07 kcal/mol higher in relative energy than TS1 (Fig
1b).
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Figure 1. (a) Equilibrium between the geometric isomer s-trans-5 and s-cis-5. (b) The most stable structures and relative free
energy levels of the Nb(OMe)5–imino ester 5 complexes. Red characters indicate relative free energy levels in kcal/mol.
Our success opened new insight to evaluate the application of this niobiumꢀcatalyzed
amidation. When the Nb(OEt)5ꢀcatalyed amidation of 1 was carried out in the presence of 1.05
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equiv of methyl propionate, 2a was chemoselectively obtained in >99% yield (Scheme 3a). In
addition, this catalysis could proceed via a specific geometric conformer. Since the isomerization
between Eꢀ and ZꢀNꢀhydroxyꢀγꢀimino esters such as 10 is much slower than that between Eꢀand
ZꢀNꢀhydroxyꢀαꢀimino esters such as 1, both geometric isomers Eꢀ10 and Zꢀ10 could be prepared
from methyl levulinate and NH2OH·HCl.10b Interestingly, while Zꢀ10 was smoothly converted to
the amide 2y with an E/Z ratio of 76:2421 in the presence of 5 mol% of Nb(OEt)5, the amide 2y
was hardly obtained from Eꢀ10 (Scheme 3b). Thus, these results clearly indicate that the
geometric conformation of Nꢀhydroxyimino esters is related to the progress of this amidation
reaction.
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Scheme 3. Chemoselective Amidation and Effect of the Geometry of 10a
aConducted on a 1.0 mmol scale of
LꢀAlaꢀOt-Bu·HCl.
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We then moved on to demonstrate the diastereoselective reduction of a selection of dipeptide
precursors 2, where we expected that any chirality incorporated into 2 may bias the
stereoselectivity of the reduction (Scheme 4). After testing several known methods for the
transformation of Nꢀhydroxyimines into amines,22 we found that the Pdꢀcatalyzed
hydrogenation22a,b of 2a in acetic acid was particularly effective, preferentially furnishing
dipeptide 11a with (S)ꢀconfiguration at the new chiral carbon center in remarkably high yield and
diastereoselectivity. Similarly, upon hydrogenation of 2b, dipeptide 11b was yielded with (S)ꢀ
configuration at the new chiral carbon center in >99% yield with >99:1 dr. Indeed, the reaction
tolerates amino acids bearing a variety of functional groups, providing dipeptides 11c–11j and
tripeptide 11o in good to excellent yields and diastereoselectivities. Unfortunately, the
hydrogenation of 2m, 2n, and 2x gave incomplete conversion to 11h (40% yield, 94:6 dr), 11i
(79% yield, 87:13 dr), and 11p (89% yield, >99:1 dr). Indeed, Kise previously reported that the
reduction of methyl esters of 2c and 2e with 10 equiv of a reducing agent (i.e.,
Zn/methanesulfonic acid, Zn/trifluoroacetic acid, Zn/TiCl4, or NaBH4/CoCl2·6H2O), afforded the
corresponding dipeptides with 53:47 to 60:40 dr.22e Moreover, although the diastereoselective
reduction of 2q was reported by Mukaiyama, he employed excess quantities of SmI2 as a
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reducing agent to obtain an epiꢀ11j, DꢀAlaꢀL
ꢀProꢀOtꢀBu, in 67% yield with 89:11 dr.22d However,
our method is more practical and provided the other diastereomer 11j in >99% yield and 85:15
dr. This Pdꢀcatalyzed approach for the hydrogenation of Nꢀhydroxyꢀαꢀimino moiety tolerates the
amides 2 bearing different types of sterically hindering R2 groups, giving dipeptides 11k–11n
and 11p in excellent yields with diastereoselectivities ranging from 63:37 dr of 11m for Bn
group to >99:1 dr of 11k for nꢀPr group. In the case of 2u with catalytic amount of Pd(OH)2/C,
only 21% of 11n was isolated with >99:1 dr. Even though the reaction was carried out at 1.0
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MPa pressure of hydrogen, the yield was not improved. Pd/C (K type)23 showed excellent
reactivity (>99% yield), but the diastereoselectivity of 11n was 50:50 dr. When Pt/charcoal was
employed instead of Pd(OH)2/C, 2u was completely consumed. However, reduction of both Ph
and Nꢀhydroxyimino groups to cyclohexyl and methylene groups was observed, respectively.
After optimization studies using several nonꢀchiral hydrogenation catalysts24, we found that the
hydrogenation using Pd(OH)2/C in 0.4N HCl in CPME/AcOH (1:9) proceeded smoothly to
afford 11n in 90% yield with 65:35 dr. When diastereoselectivity is not sufficiently high,
separation of these diastereomers is usually carried out by recrystallization including chiral
resolution, chiral derivatization, or the use of HPLC technology,25 and we accomplished the
separation of these diastereomers to obtain highly pure 11f, 11h, 11i, and 11l–11n by practical
techniques of recrystallization and/or normal phase silicaꢀgel column chromatography without
any chiral sources.26 All spectroscopic data pertaining to the produced diꢀ and tripeptides 11a–
11q were accorded with those of the authentic samples prepared via a common synthetic method
using 1ꢀethylꢀ3ꢀ(3ꢀdimethylaminopropyl)carbodiimide hydrochloride (WSC·HCl) and 1ꢀ
hydroxybenzotriazole (HOBt).2d
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Scheme 4. Diastereoselective Hydrogenationa,b
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aConducted on a 1.0 mmol scale of 2a. bDiastereomeric ratio was determined by 1H NMR analysis (see Supporting Information).
d
cConducted on a 10 mmol scale of 2a. The dipeptides were isolated as their acetic acid salts. eAfter recrystallization with Et2O.
fIn AcOH/MeOH (2:1) at 1.0 MPa pressure. gAfter separation of both diastereomers by normal phase silicaꢀgel column
h
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chromatography. The ratio of rotational isomers was determined by H NMR analysis. 0.4N HCl in CPME/AcOH (1:9) was
used as a solvent.
A plausible pathway of the process is outlined in Fig 2. As 1 can easily convert between two
geometric conformers due to low freeꢀenergy barriers, two possible reaction pathways through
geometric conformers I and II are proposed to account for the amidation of 1 with amino acid
esters via substrateꢀcontrol models. Initially 1 is transformed into either conformer I or II by
overcoming the appropriate freeꢀenergy barrier (2.0 or 3.4 kcal/mol, respectively).27
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Subsequently, the iminoꢀN moiety of conformer I or the hydroxyꢀO moiety of conformer II may
coordinate with the metal center (M) of the catalyst to form 5ꢀmembered intermediate III or 6ꢀ
membered intermediate IV. According to the results of Scheme 3, the second proposal is more
reasonable. Finally, the amine species, supplied by the in situ neutralization of the amino acid
ester HCL salts using Et3N, undergoes nucleophilic substitution at the activated carbonyl group
of intermediate III or IV to form the desired amide products, such as 2a (Fig 2a). In the classical
amidation reaction, two pathways for racemization have been recognized. One is direct
enolization after activation of the carbonyl group by coupling reagents. The other is the
enolization of the oxazolones generated by deprotonation at N–H moieties of activated amino
acid esters (Fig 2b).2d Our catalytic approach, however, can perfectly avoid the racemization
because neither of these two events are possible in our new process. That is because the
enolization in Fig 2b does not proceed due to the absence of an αꢀproton in 1. Additonally, as the
amidation reaction proceeds, coordination of the metal ion to the oxygen atom of tertꢀbutyl ester
is not allowed because of the instability of the nineꢀmember ring intermediate as well as steric
hindrance from the bulky tertꢀbutyl group. Furthermore, the amide oxygen is not able to react
with the bulky and nonꢀactivated tertꢀbutyl ester to generate oxazolone intermediate (Fig 2c).
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Figure 2. Plausible catalytic pathway.
■ CONCLUSION
We have explored a novel and powerful catalytic route for peptide synthesis based on substrate
control. The use of a niobium catalyst to promote amide bond formation between Nꢀ
hydroxyimino esters and amino acid tertꢀbutyl esters bearing a range of functional groups
produced high yields of the corresponding amides. Since the starting Nꢀhydroxyimino esters are
known to be prepared quantitatively in one step from the corresponding keto esters with
NH2OH·HCl,10b from the corresponding esters with nitroso reagents,10e or by other
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methods,10a,c,d,f,g the various artificial amino acids can be easily incorporated by this new
catalytic method. The resulting amides were readily converted into their corresponding diꢀ and
tripeptides in excellent yields and excellent to good diastereoselectivities. Our developed
protocol has several advantages compared to the conventional amidation techniques in liquidꢀ
phase peptide synthesis: 1) solventꢀfree, 2) catalytic, 3) high atomꢀefficiency, 4) no racemization,
5) the use of amino acid ester HCl salts, 6) chemoselective, 7) scalable, 8) applicable to
unnatural amino acids. We expect that this novel substrateꢀcontrolled reaction will express the
clear advantages over current peptide synthetic methods, associated with waste production,
racemization, and limited applicability.
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■ AUTHOR INFORMATION
Corresponding Author
*Eꢀmail for W.M.: muramatsu@isc.chubu.ac.jp
*Eꢀmail for H. Y.: hyamamoto@isc.chubu.ac.jp
ORCID
Wataru Muramatsu: 0000ꢀ0001ꢀ7183ꢀ7981
Hisashi Yamamoto: 0000ꢀ0001ꢀ5384ꢀ9698
Notes
The authors declare no competing financial interests.
■ ASSOCIATED CONTENT
Supporting Information. Experimental procedures, characterization data, and copies of spectra
for all compounds. This material is available free of charge via the Internet at http://pubs.acs.org.
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Experimental procedures (PDF)
1Hꢀ, 13CꢀNMR spectral, and HPLC data (PDF)
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Computational details (PDF)
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■ ACKNOWLEDGMENT
We would like to thank Prof. Matsujiro Akakura from Aichi University of Education for
assistance in computational study. Support has been provided by the Japan Science and
Technology Agency (JST), Adaptable and Seamless Technology Transfer Program through
Targetꢀdriven R&D (AꢀSTEP).
■ REFERENCES
(1) (a) Bode, J. W. Curr. Opin. Drug Disc. Dev. 2006, 9, 765–775. (b) Boas, U. Chem. Rev.
2009, 109, 2092–2118. (c) Li, H.; Aneja, R.; Chaiken, I. Molecules 2013, 18, 9797–9817. (d)
Uhlig, T.; Kyprianou, T.; Martinelli, F. G.; Oppici, C. A.; Heiligers, D.; Hills, D.; Calvo, X. R.;
Verhaert, P. EuPA Open Proteom. 2014, 4, 58–69. (e) Fosgerau, K.; Hoffmann, T. Drug Discov.
Today 2015, 20, 122–128. (f) Recio, C.; Maione, F.; Iqbal, A. J.; Mascole, N.; De Feo, V. Front.
Pharmacol. 2017, 7, 1–11.
(2) (a) Colemann, G. H.; Alvarado, A. M. Org. Synth. 1923, 3, 3. (b) Parreux, L.; Loupy, A.;
Volatron, F. Tetrahedron 2002, 58, 2155–2162. (c) Baxendale I. R.; Ley, S. V.; Smith, C. D.;
Tranmer, G. K. Chem. Commun. 2006, 4835–4837. (d) ElꢀFaham, A.; Albericio, F. Chem. Rev.
2011, 111, 6557–6602. (e) Lundberg, H.; Tinnis, F.; Selander, N.; Adolfsson, H. Chem. Soc. Rev.
2014, 43, 2714–2742. (f) Gabriel, C. M.; Keener, M.; Gallou, F.; Lipshutz, B. H. Org. Lett. 2015,
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2
3
4
5
6
7
8
9
17, 3968–3971. (g) Dunetz, J. R.; Magano, J.; Weisenburger, G. A. Org. Process Res. Dev. 2016,
20, 140–177. (h) Fuse, S.; Mifune, Y.; Nakamura, H.; Tanaka, H. Nat. Commun. 2016, 7, 13491.
(3) (a) Merrifield, R. B. J. Am. Chem. Soc. 1963, 85, 2149–2154. (b) Chang, C.ꢀD.; Meienhofer,
J. Int. J. Pept. Protein Res. 1978, 11, 246–249. (c) Amblard, M.; Fehrentz, J.ꢀA.; Subra, G. Mol.
Biotechnol. 2006, 33, 239–254. (d) Coin, I.; Beyermann, M.; Mienert, M. Nat. Protoc. 2007, 2,
3247–3256. (e) GongoraꢀBenitez, M.; TullaꢀPuche, J.; Albericio, F. ACS Comb. Sci. 2013, 15,
217–228.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(4) Sheldon, R. A. Green Chem. 2007, 9, 1273–1283.
(5) (a) Ishihara, K.; Ohara, S.; Yamamoto, H. J. Org. Chem. 1996, 61, 4196–4197. (b) Maki, T.;
Ishihara, K.; Yamamoto, H. Tetrahedron 2007, 63, 8645–8657.
(6) (a) Rattabiraman, V. R.; Bode, J. W. Nature 2011, 480, 471–479. (b) Ohshima, T.; Hayashi,
Y.; Agura, K.; Fujii, Y.; Yoshiyama, A.; Mashima, K. Chem. Commun. 2012, 48, 5434–5436. (c)
Caldwell, N.; Jamieson, C.; Simpson, I.; Tuttle, T. Org. Lett. 2013, 15, 2506–2509. (d) Liu, S.;
Yang, Y.; Liu, X.; Ferdousi, F. K.; Batsanov, A. S.; Whiting, A. Eur. J. Org. Chem. 2013, 5692–
5700. (e) Dine, T. M. E.; Rouden, J.; Blanchet, J. Chem. Commun. 2015, 51, 16084–16087. (f)
Fatemi, S.; Gernigom, N.; Hall, D. G. Green Chem. 2015, 17, 4016–4028. (g) Krause, T.;
Baader, S.; Erb, B.; Gooßen, L. J. Nat. Commun. 2016, 7, 11732. (h) Figueiredo, R. M.; Suppo,
J.ꢀS.; Campagne, J.ꢀM. Chem. Rev. 2016, 116, 12029–12122. (i) Lundberg, H.; Tinnis, F.;
Zhang, J.; Algarra, A. G.; Himo, F.; Adolfsson, H. J. Am. Chem. Soc. 2017, 139, 2286–2295.
(7) Noda, H.; Furutachi, M.; Asada, Y.; Shibasaki, M.; Kumagai, N. Nat. Chem. 2017, 9, 571–
577.
(8) (a) Bodanszky, M.; Bodanszky, A. Chem. Commun. 1967, 591–593. (b) Antonovics, I.;
Young, G. T. J. Chem. Soc. C 1967, 595–601. (c) Carpino, L.A. J. Org. Chem. 1988, 53, 875–
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878. (d) Bodanszky, M. Pronciples of Peptide Synthesis, 2nd ed.; SpringerꢀVerlag: Berlin, 1993.
(e) LloydꢀWilliams, P.; Albericio, F.; Giralt, E. Chemical Approaches to the Synthesis of Peptide
and Proteins; CRC Press: Boca Raton, FL, 1997.
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10
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(9) Tsuji, H.; Yamamoto, H. J. Am. Chem. Soc. 2016, 138, 14218–14221.
(10) Several methods for preparation of Nꢀhydroxyimino esters were reported, see: (a) Knifton, J.
F. J. Catal. 1974, 33, 289–298. (b) Ritson, D. J.; Cox, R. J.; Berge, J. Org. Biomol. Chem. 2004,
2, 1921–1933. (c) Kitahara, K.; Toma, T.; Shimokawa, J.; Fukuyama, T. Org. Lett. 2008, 10,
2259–2261. (d) Wu, L. Y.; Choi, J. K.; Hatton, K. Y.; Berkman, C. E. Tetrahedron Lett. 2010,
51, 402–403. (e) Baidya, M.; Yamamoto, H. J. Am. Chem. Soc. 2011, 133, 13880–13882. (f)
García, J.; Franci, G.; Pereira, R.; Benedetti, R.; Nebbioso, A.; RodríguezꢀBarrios, F.;
Gronemeyer, H.; Altucci, L.; de Lera, A. R. Bioorg. Med. Chem. 2011, 19, 3637–3649. (g) Kung,
K. K.ꢀY.; Wong, K.ꢀF.; Leung, K.ꢀC.; Wong, M.ꢀK. Chem. Commun. 2013, 49, 6888–6890.
(11) Geometric configurations of Nꢀhydroxyimino esters 1, 5–7 have been reported, see
references in Supporting Information.
(12) (a) Anderson, G. W.; Blodinger, J.; Welcher, A. D. J. Am. Chem. Soc. 1952, 74, 5309–5312.
(b) Anderson, G. W.; Callahan, F. M. J. Am. Chem. Soc. 1960, 82, 3359–3363.
(13) Funasaki, N.; Hada, S.; Neya, S. Anal. Chem. 1993, 65, 1861–1867.
(14) The use of other solvents (nꢀpentane, nꢀhexane, cꢀpentane, cꢀhexane, diethyl ether,
diisopropyl ether, THF, CPME, ethyl acetate, and toluene) also affected the obtained yields (67–
95% yields).
(15) Only a mixture of equilibrium imines was produced.
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(16) If the amidation proceeds with racemization at αꢀposition of amino acids, diastereomers of
2d, 2k, 2l, and 2w should be produced. However, any diastereomers were not isolated in the
formation of 2d, 2k, 2l, and 2w.
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(17) Smith, G. G.; Sivakua, T. J. Org. Chem. 1983, 48, 627–634.
(18) HPLC data of 2f was shown in Supporting Information.
(19) Geometric configurations of Nꢀhydroxyimino esters 3, 4, 8–10, and amides 2a–2y were
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determined by C NMR analysis, see: (a) Levy, G. C.; Nelson, G. L. J. Am. Chem. Soc. 1972,
94, 4897–4901. (b) Afonin, A. V.; Ushakov, I. A.; Tarasova, O. A.; Shmidt, E. Y.; Mikhakeva,
A. I.; Voronov, V. K. Russ. J. Org. Chem. 2000, 36, 1777–1783. (c) EI Idrissi, M.; Faraj, S. Phys.
Chem. News 2003, 14, 124–126 (d) Kline, M.; Pierce, D.; Cheatham, S. Mahn. Reson. Chem.
2017, 55, 154–156.
(20) We previously reported the catalytic amidation of esters containing a hydroxyl group as
directing group at αꢀ, βꢀ, γꢀ, or δꢀposition, see ref. 9.
(21) Eꢀ2y can be isolated by normal phase silicaꢀgel column chromatography. However, a
portion of Eꢀ2y is rapidly isomerized to Zꢀ2y in CDCl3, so that 2y was observed as an
equilibrium mixture of 76:24 (E/Z) by 1H NMR analysis. In contrast, isomerization of Eꢀ2y was
not observed because of the stability of Eꢀ2y in CD3OD.
(22) (a) Weaver, W. E.; Hartung, W. H. J. Org. Chem. 1950, 15, 741–747. (b) Hartung, W. H.;
Kramer, S. N.; Hager, G. P. J. Am. Chem. Soc. 1954, 76, 2261–2262. (c) Kukolja, S.; Draheim,
S. E.; Graves, B. J.; Hunden, D. C.; Pfeil, J. L.; Cooper, R. D. G.; Ott, J. L.; Counter, F. T. J.
Med. Chem. 1985, 28, 1896–1903. (d) Mukaiyama, T.; Yorozo, K.; Kato, K.; Yamada, T. Chem.
Lett. 1992, 21, 181–184. (e) Kise, N.; Takaoka, S.; Yamauchi, M.; Ueda, N. Tetrahedron Lett.
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2002, 43, 7297–7300. (f) Humble, R. W.; Middleton, D. F.; Banoub, J.; Ewing, D. F.; Boa, A.
N.; MacKenzie, G. Tetrahedron Lett. 2011, 52, 6223–6227.
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(23) Pd/C (K type) is commercially available from Wako Pure Chemical Ind., Ltd.
(24) To the best of our knowledge in regards to the methods for catalytic enantioselective
hydrogenation of Nꢀhydroxyimino esters, only three papers and a patent have been reported.
However, these methods require high pressure of H2 gas, and obtained yield and/or
stereoselectivity are not satisfactory. See, (a) Krasik, P.; Alper, H. Tetrahedron Asymm. 1992, 3,
1283–1288. (b) Chan, A. S. C.; Chen, C.ꢀC.; Lin, C.ꢀW.; Lin, Y.ꢀC.; Cheng, M.ꢀC.; Peng, S.ꢀM.
J. Chem. Soc., Chem. Commun. 1995, 1767–1768. (c) Xie, Y.; Mi, A.; Jiang, Y.; Liu, H. Synth.
Commun. 2001, 31, 2767–2771. (d) Ryberg, P.; Berg, R. PCT Int. Appl. 2015178847, November
26, 2015.
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(25) (a) Yamada, T.; Nonomura, S.; Fujiwara, H.; Miyazawa, T.; Kuwata, S. J. Chromatogr. A
1990, 515, 475–482. (b) Schoneich, C.; Huhmer, A. F. R.; Rabel, S. R.; Stobaugh, J. F.; Jois, S.
D. S.; Larive, C. K.; Siahaan, T. J.; Squier, T. C.; Bigelow, D. J.; Williams, T. D. Anal. Chem.
1995, 67, 155–181. (c) Liu, Y.ꢀM.; Schneider, M.; Sticha, C. M.; Toyooka, T.; Sweedler, J. V. J.
Chromatogr. A 1998, 800, 345–354. (d) Wan, H.; Blomberg, K. G. J. Chromatogr. A 2000, 875,
43–88. (e) Winter, D.; Pipkorn, R.; Lehmann, W. D. J. Sep. Sci. 2009, 32, 1111–1119. (f)
Ludewig, R.; Dong, J.; Zou, H.; Scribe, G. K. E. J. Sep. Sci. 2010, 33, 1085–1089. (g) Scriba, G.
K. E. Encyclopedia of Analytical Chemistry: Sepatration of Peptide Diastereomers; John Wiley
& Sons, Ltd. 2014.
(26) We also have confirmed that the dipeptides such as 11i, 11j, 11l, 11m, and 11o could be
easily purified by recycling preparative HPLC chromatography (PU714R from GL Sciences Inc.)
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with a reversedꢀphase silicaꢀgel column (Intertsil® ODSꢀHL from GL Sciences Inc.) under 0.1%
TFA/MeCN system.
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(27) Istomina, N. V.; Shcherbina, N. A.; Krivdin, L. B. Russ. J. Org. Chem. 2009, 45, 481–485.
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