Aziridine based electrophilic handle for aspartic acid ligation

Article abstract of DOI:10.1039/c8ob00676h

A one-pot ligation strategy at aspartic acid junctions has been developed by successfully incorporating aziridin-2,3-dicarboxylate to the N-side of a peptide fragment, affording N-aziridine appended peptides, which were ligated in solution phase with a va

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Aziridine Based Electrophilic Handle for Asparatic Acid Ligation
Kiran Bajaj,*a Devesh S. Agarwal,a Rajeev Sakhujaa and Girinath G. Pillaib
A one-pot ligation strategy at aspartic acid junction has been developed by successfully incorporating aziridin-2,3-
Received 00th January 20xx,
Accepted 00th January 20xx
dicarboxylate to the N-side of a peptide fragment, affording N-aziridine appended peptide, which was ligated in solution
phase with
a variety of small peptide thio acids to afford native peptides, following ring-opening/peptidyl
DOI: 10.1039/x0xx00000x
migration/desulfurization strategy. The reaction proceeds in highly regiospecific manner, and provides short native
peptides in good isolable yields. A variety of aspartame based peptides were synthesized to showcase the generality of
aziridine based ligation. Computational studies have also been performed to get insight about the reaction pathway.
www.rsc.org/
towards the construction of peptides at specific amino acid
junctions such as leucine,4a-b valine,5a-b phenylalanine,6a-b
glutamic,7 lysine,8 proline,9a-c tryptophan10 and aspartic
Introduction
Peptide ligation is an ever-emerging field that targets to
discover novel synthetic strategies for peptide synthesis.
Several advancements have been witnessed in recent years
that marked a notable impact to the current literature on
peptide synthesis. The key strategy in peptide ligation involves
the coupling of two peptide fragments in a chemo-selective
manner. In this context, Kent’s group reported pioneer work in
exploring native chemical ligation (NCL) towards chemo-
selective coupling of two unprotected peptide segments at
cysteine junction.1a-b NCL methodology has revolutionized the
area of protein synthesis, and served as powerful tool for
constructing various protein targets such as glycoproteins,
phosphoproteins, oncogenic KRas(G12V), ubiquitinated
peptide conjugates, membrane protein, human chemokine
hCCL21, erythropoietin, enzyme barnase, and many more.2a-i
However, limitations associated with NCL strategy such as
requirement of a cysteine residue at N-side of one of the
peptide fragment, limits its usage. Thus, several modifications
have been proposed to broaden the scope of NCL based
peptide synthesis in the past decade. One such breakthrough
was reported by Dawson’s group, where desulfurization was
introduced after ligation step for synthesizing non-cysteine
containing proteins, exemplifying the ligation at alanine
junction.3 This strategy emerges an idea of generating cysteine
surrogates by suitably placing the thiol auxiliary at natural
amino acid templates that could be employed in peptide
synthesis via ligation-desulfurization strategy. This ligation-
desulfurization strategy has extended the scope of NCL
acid.11a-c Along with NCL, other ligation strategies have also
been emerged time-to-time to advance the peptide syntheses,
namely Staudinger ligation12a-d that utilizes the scope of
Staudinger reaction, KAHA ligation13a-b that ligates two peptide
fragments, one having α-ketoacid at C-side and another having
O-unsubstituted hydroxylamine at N-side of the peptide
fragment and others. These emerged peptide synthetic
strategies
undoubtedly
have
provided
significant
advancements in this field, however they are associated with
inherent challenges, and thus provide the scope of further
advancements especially with vast array of complex amide
functionalities.
In striking contrast, site-selective aziridine ring-opening has
been explored reasonably in the area of peptide synthesis.14a-d
In this context, Tam’s group showcased the peptide ligation
protocol by orthogonal coupling of two unprotected peptide
fragments, one having thio acid at the C-side of peptide
fragment, and another having β-bromo amino acid at N-side of
peptide fragment. During this ligation process, an aziridine ring
was formed that undergoes subsequent ring-opening by
thiocarboxylate to afford α- and β- peptides.15 However, no
further investigations were reported based on this protocol. In
2011, Garner’s group utilized the aziridine ring to ligate two
peptide fragments via Cu(I)-mediated acylation, followed by
hydrolysis to produce serine/threonine amino acid at ligation
junction,16a-b however this moiety has not much explored as an
electrophilic handle in peptide ligation. In order to expand the
utilization of aziridine ring in peptide ligation at particular
amino acid junction, we have recently exploited suitably
derivatized aziridine moiety as cysteine surrogate in NCL, and
exemplified ligation at phenylalanine and tryptophan
junctions.17 While extending the scope of this ligation
methodology, in present work, we aimed to utilize inexpensive
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aziridin-2,3-dicarboxylate towards studying the scope of intermediate 4a in 4 h. Formation of 4a was confirmed by
peptide ligation at aspartic acid junction. HRMS. Intermediate 4a when further exposed to reduction
Previously reported peptide ligation methods at aspartic acid conditions yielded the corresponding aspartic acid appended
junction involves: (i) the use of -thioaspartic acid unit at the dipeptide (5a) in 88% yield (Scheme 2). The scope of this
N-side of peptide fragment, or (ii) the use of peptide fragment strategy was now further extended to a variety of other amino
having aspartic acid at the C-side on NCL based ligation.11 In thioacids
and peptide thioacids to afford their
context to the first method, Payne and Tan’s group’s corresponding aspartic acid containing di- and tripeptides (5a-
independently showcased the use of differently protected β- ) (Scheme 2). All the products were obtained as single
β
(
2)
(3)
g
thioaspartic acid unit as cysteine surrogate in producing a regioisomer in good-to-excellent yields (70-88%). In this overall
variety of aspartic acid based native peptides (Scheme 1a).11b-c process of ring-opening-migration and thiol reduction, the
However, there are numerous challenges involved in preparing configuration at the other carbon of (S,S)-aziridine-2,3-
β-thioaspartic acid such as the use of sophisticated dicarboxylate and the chiral carbon of amino acyl moieties are
sulfenylating agents, tedious and multi-step low-yielding always retained. This was quite evident by the high
1
protocols. While in the second method, competitive formation diastereomeric excess as observed from the H NMR spectra
of isopeptides are observed during ligation process (Scheme (>97% de) of the resultant peptides (5a-g). (Supporting
1b).11a Acknowledging the importance of aspartic acid unit in Information)
various naturally occurring peptides (RGD etc), and artificial
sweetener aspartame (aspartyl-phenylalanine methyl), we
have demonstrated the use of dialkyl (S,S)-aziridine-2,3-
dicarboxylate as an electrophilic handle to obtain a series of
aspartic acid based native peptides in a one-pot fashion via
ligation-desulfurization (Scheme 1c).
Aspartic Ligation
Previous work
COOH
O
HS COOH
O
OBz
O
O
Ligation-desulfurization
1a)
SPh
H2N
O
N
O
O
H
H
H
H
Cbz
N
Cbz
N
N
N
OEt
O
Boc
MeO
OMe
N
OEt
O
N
H
OEt
O
H
H
O
O
TrtS CO2t-Bu
Boc OMe
O
OEt
15
OEt
5a, 88%15
OMeS CO2t-Bu
Boc OMe
OEt
N
5c, 78%
H
5b
, 83%
O
N
H
O
Cbz
Tan's work
HN
Payne's work
Org. lett. 2013,15: 6128-6131.
Angew. Chem. Int. Ed. 2013, 52: 9723–9727.
O
OBz
O
O
H
N
O
O
O
O
H
N
O
Cbz
H
N
H
Cbz
N
OEt
O
HS
1b)
Boc
N
N
H
OEt
O
H
N
OEt
O
O
S
O
Ligation
O
H
O
HS
O
H2N
R
OEt
O
COOH
OEt
O
O
O
OEt
5e, 82%
5f, 75%
5d, 72%
N
H
O
O
O
OMe
Eur. J. Org. Chem. 2003: 3267-3272
O
O
H
N
H
Present work
1c)
N
Cbz
N
OEt
O
H
COOR
O
COOR
O
O
Ligation-desulfurization
HN
OEt
SH
N
H
O
O
5g, 70%
Aspartic Ligation
Scheme 2. Aziridine based ligation to produce aspartic acid appended di- and tri-
peptides.
Scheme 1. Previous (1a-b) and present work (1c) on aspartic acid ligation
To further extend the scope of this strategy towards
synthesizing aspartame appended peptides in solution phase,
we plan to affix aziridine-2,3-dicarboxylate (1a-b) to the N-
terminal of a phenylalanine moiety. Thus, alkaline hydrolysis of
aziridine-2,3-dicarboxylates (1a-b) under controlled conditions
in methanol was carried, which on coupling with L-Phe-OMe
using DPPA as coupling reagent yielded aziridine tagged
Results and Discussion
To investigate our studies, we first plan to perform the
reaction between (S,S)-diethyl aziridine-2,3-dicarboxylate (1a
)
and Cbz-protected glycyl thioacid (2a) via recently explored
one-pot ligation-desulfurization strategy.17 Gratifyingly, the
aziridine ring-opening followed by S to N acyl-migration
proceeded smoothly at 25 oC using DMF without using any
external additive, affording thiol substituted peptide
phenylalanine derivatives (6a-b) in 62-64% yields (Scheme 3).
2 | J. Name., 2012, 00, 1-3
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peptide 7f but not isopeptide 7f'. (For detailed study on other
O
O
O
O
O
O
LiOH.2H2O (0.9 equiv.)
MeOH, 25 oC, 12 h
L-Phe-OMe (1.0 equiv.)
OMe
RO
O-
RO
OR
long range correlations, see supporting information)
RO
N
DPPA (1.2 equiv.)
Et3N (2.5 equiv.)
DMF, 25 oC, 16 h
N
H
N
N
O
H
H
H
6a-b
R = Et, Me
62-64%
1a-b
O
O
H
H
N
N
O
O
Cbz
OMe
N
H
H
O
H
Scheme 3. Tagging of aziridine to N-side of phenylalanine methyl ester
H
N
N
OMe
O
Cbz
N
N
H
O
N
OMe
H
O
O
O
These two aziridine tagged phenylalanine derivatives (6a-b
were then explored as electrophilic handles for different
amino thioacids and peptide thioacids to afford aspartame
containing tri- and tetrapeptides (7a ) in good-to-excellent
)
OMe
7f
α−peptide,
7f'
isopeptide,
2
3
-g
yields via our optimized one-pot ligation-desulfurization
strategy (Scheme 4). All the isolated native peptides were
completely characterized by detailed spectroscopic analysis.
Interestingly, here again the products were obtained as single
regioisomers with retention of configuration at stereocentres.
O
R'
O
O
H
O
R'
O
(i) DMF, 25 oC, 6-12 h
H
H
N
SH
OMe
N
N
OMe
Pg
N
RO
N
Pg
N
N
H
H
H
O
n
N
H
O
(ii) NiCl2.6H2O (3 equiv.),
NaBH4 (9 equiv)
MeOH, -10 oC, 15 min.
R
R
O
n
O
H
O
OR
2/3
n = 0,1
6a-b
R = Et, Me
7a-g
Cbz
NH
O
O
H
N
H
O
Cbz
OMe
H
Boc
N
OMe
N
N
H
O
Cbz
N
OMe
N
N
H
O
H
N
N
H
O
H
O
O
O
O
H
O
O
OMe
OMe
OEt
Figure 1. HMBC spectrum of tetrapeptide, 7f (only selective correlations are
indicated)
7a
, 75%
7b
, 80%
7c, 68%
OMe
O
Finally, we applied the designed aziridine-mediated ligation
protocol for preparing a pentapeptide in solution phase. For
O
O
H
H
O
O
H
H
N
N
N
OMe
N
OMe
Cbz
N
N
Cbz
N
H
N
H
H
O
O
O
H
this purpose, aziridine appended dipeptide
synthesized in a one-pot two step procedure by hydrolyzing
aziridine-2,3-dicarboxylate 1b followed by coupling with
(
6c
)
was
O
O
O
OMe
OEt
7d, 78%
7e, 84%
,
dipeptidyl ester (L-Phe-L-Ala-OMe). This aziridine handled
dipeptide 6c was subjected to one-pot ligation-desulfurization
strategy, using dipeptidyl thioacid (Cbz-L-Ala-L-Phe-SH) to yield
O
O
H
H
N
O
O
N
OMe
H
N
H
N
Cbz
N
N
H
OMe
H
Cbz
N
H
N
O
O
O
H
O
O
O
OMe
aspartame based peptapeptide (8) in 60% yield. (Scheme 5)
OMe
7g, 74%
7f, 82%
This extended experiment further confirms the scope of
presented aziridine-mediated one-pot ligation-desulfurization
method for peptide synthesis.
Scheme 4. Synthesis of aspartame containing native peptides.
To ascertain the formation of α-isomer (7f) (formed by
selectively ring opening at C-2 position) over isopeptide (7f'
O
O
)
O
O
O
LiOH.2H2O (0.9 equiv.)
MeOH, 25 oC, 12 h
L-Phe-L-Ala-OMe (1.0 equiv.)
H
MeO
OMe
N
DPPA (1.2 equiv.), Et3N (2.5 equiv.)
DMF, 25 oC, 16 h
MeO
N
OMe
N
H
(that would have been formed by selectively ring opening at C-
3 position), COSY, HSQC and HMBC studies (Figure 1) were
performed on one of the representative peptide, Cbz-L-Ala-L-
Phe-L-Asp(OMe)-L-Phe-OMe (7f). All the proton and carbon
peaks of tetrapeptide, Cbz-L-Ala-L-Phe-L-Asp(OMe)-L-Phe-OMe
H
N
H
O
1b
6c
, 68%
(i) Cbz-L-Ala-L-Phe-SH (1.1 equiv.)
DMF, 25 oC, 8 h
(ii) NiCl2.6H2O (3 equiv.), NaBH4 (9 equiv)
MeOH, -10 oC, 15 min.
(
7f) were assigned via COSY and HSQC spectrum respectively
O
O
O
H
H
H
(Supporting Information for COSY and HSQC). Long range 1H-
13C correlation (HMBC) spectra implied the correlation of Asp-
OCH3 (δ 3.59 ppm) protons with Asp-CO ester peaks (δ 171.7
ppm), which in turn correlates with CH2 protons of aspartic
acid (δ 2.58 and 2.73 ppm). These specific correlations along
with other spectroscopic data confers the compound to be α-
O
N
N
N
N
N
H
O
OMe
H
O
O
O
OMe
8
, 60%
Scheme 5. Aziridine based ligation to produce aspartic acid appended
pentapeptide
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Computational Analysis
To get an insight into reaction mechanism, we performed
computational experiments to understand the relative
energies in the reaction pathway. The systematic
representation of attack of thioacid to aziridine 6a is
represented in Scheme 6.
H
N
O
H
O
N
O
O
2
3
O
O
O
O
O
N
NH2
O
O
H
H
R1
(
)
N
O
S
NH2
H
S
O
O
N
O
O
O
O
O
O
O
NH
SH
O
N
H
O
O
Transition State 1 (TS 1)
R2
(
)
Transition State 2 (TS2)
Figure 2. Energy profile diagram showing the optimized geometries, relative
energies, dipole moment and HOMO-LUMO gap of possible transition
states/intermediates of the reaction pathways
O
O
O
O
N
O
H
NH
O
HS NH
H
O
NH
O
N
O
O
Figure 3 shows the optimized geometry of transition state. As
the reaction progress from the reactant to the transition state,
the distance between the C and N atoms changes substantially
from 4.606 Å to 3.651 Å in TS1, and the bond length of C-N
changes from 4.397 Å to 3.684 Å with respect to the energy
profile calculations. The frontier molecular orbitals of a
compound are at the “frontier” of electron occupation—the
highest-energy occupied and lowest-energy unoccupied
molecular orbitals (the HOMO and LUMO).19b The HOMO is
logically viewed as nucleophilic or electron donating, while the
LUMO is electrophilic and electron accepting. Furthermore,
both chemical reactions and resonance can be explained by
interactions (overlap) between a filled HOMO and an empty
LUMO on one or more molecules.
O
O
H
O
O
N
HS
O
Intermediate 1
O
Intermediate 2
Reduction
Reduction
O
O
NH
O
O
O
N
H
O
O
O
H
N
NH
O
HN
O
H
O
O
N
O
O
O
O
α- peptide (P1)
β- peptide (P2)
Scheme 6. Schematic representation of peptide formation via aziridine-mediated
ligation
The techniques employed include a full conformation search
followed by scoring of the conformers based on the spatial
distances between the reactive atoms. The full conformation
search was performed considering rotatable bonds, and the
calculations were executed by using the MMX force field, in
PCMODEL v.9.3 software.18a The best pre-organized conformer
for each compound is obtained. Figure 3 presents the
optimized geometry obtained at the BP86/def-TZVP19 level of
theory for the reactants (R), transition states (TS), and product
(P) for reactions shown in Scheme 6. As the reaction goes from
the transition state to the product, the lone pair of nitrogen
TS1
TS2
d(N-C) = 3.651 Å
HOMO-LUMO Gap = 3.462eV
Dipole Moment = 3.213 Debye
d(N-C) = 3.684 Å
HOMO-LUMO Gap = 3.561eV
Dipole Moment = 7.363 Debye
attacks the carbonyl group of O=C−S followed by acyl
migration and the C-N bond becomes completely formed. The
reaction process can also be monitored by observation of the
changes in the energy levels. In the transition state, the
density map mainly delocalizes to the N and C atoms. In the
product, the density map mainly focuses on the C atom. Some
spin density also delocalizes into the nearby connected methyl
and carbonyl group. The optimized geometry, relative
energies, dipole moment and HOMO-LUMO gap were also
calculated for intramolecular rearrangement as indicated in
Figure 2.
Figure 3. Preorganized transition states (TS1 and TS2)
Energy of FMOs (Frontier Molecular Orbitals - HOMO and
LUMO) of
a chemical species plays the key role in
nucleophilic/electrophilic behaviour of a chemical species.
Higher HOMO energy corresponds to the more reactive
molecules towards electrophiles while lower LUMO energy is
essential for nucleophilic attacks. Larger the HOMO-LUMO,
energy gap the harder is the molecule. Based on the minimum
HOMO-LUMO gap and lower bond distance of N-C including
the lower dipole moment proves that TS1 is more favourable
We further investigated the minimum-energy conformations
of the radicals, in which the substitution group and the thiol
group are always in trans configuration. The optimized
geometry and energies for all of the structures are given in the
Figure 2 & 3.
to yield
α-Peptide (P1). Due to polar nature of sulfur atom,
both nucleophilic and electrophilic attacks are possible at S
center in organosulfur compounds. It is expected that
replacement of H-atom of thiol group (-SH) enhances its
nucleophilic nature that undergoes ring opening of aziridine
ring at C-2 position linked with ester group as it would
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enhance the electrophilic nature at C-2 carbon more than C-3 basis set in Turbomole software.19a,21 All points were
position linked with amide group. We can also conclude that confirmed as minima or transition states by calculation of the
steric hindrance also plays a crucial role in this reaction. harmonic vibrational frequencies, by use of analytical second
Stability of a species has been correlated with global hardness derivatives. Vibrational frequency analysis was performed in
by maximum hardness principle; proposed by Parr and all cases to ensure that the molecules possess- no imaginary
Pearson.20 It states that maximum hardness leads to maximum frequency. To obtain corrections for the zero-point energies
stability including favourable chemical changes.
(ZPEs) and to ascertain that the computed transition states
were first-order saddle points. In order to probe the quality of
these B-P86/def-TZVP calculations, we also carried out
optimizations at the MP2 level with the same basis set.22 The
pathway for this reaction was obtained by employment of the
reaction modelling and energy profile. Single-point energy
calculations were also carried out at the B-P86/def-TZVP level
of theory to get more accurate results. The ZPE corrections
were scaled by a factor of 0.98. Transition state reaction path
sampling computations were done to confirm that the
transition states connected the appropriate reactants and
products for the radical cyclization reactions examined in our
study.23 Solvent effects on the reactions have also been
considered by employing B-P86/def-TZVP optimizations of the
gas-phase stationary points and using a relatively simple
COSMO-RS method based on the continuum model of Andreas
Conclusions
In conclusion, an amide-forming aziridine based ligation
approach has been introduced that extended the scope of
proposed ligation to aspartic acid junction. The strategy
requires an aziridine based electrophilc handle to the N- side
of an peptide fragment that ligate with various peptide thio
acids to produce native peptides in good yields. This
methodology adopts one pot ligation-desulfurization approach
and could be extended to the other amino acid linkage sites by
introducing properly designed aziridine handle to N- side of
one of the peptide fragment. The proposed ligation reaction
works smoothly and provides a new route to synthesize
aspartic acid based peptides with highly regiospecfic manner.
The present strategy has opened new doors for extending the
scope of chemical ligation at asparatic acid junction for
preparing long chain peptides of choice.
Klamt.24 dialkyl (S,S)-aziridine-2,3-dicarboxylates25
(
1a-b) and
amino and peptide thioacids26
following literature procedure.
(2/3) were prepared by
General procedure for aziridine-2,3-dicarboxylate hydrolysis
followed by amino acid/peptide coupling. To the solution of
o
aziridine-2,3-dicarboxylate (1.0 equiv.) (1a-b) in MeOH at 0 C
Experimental Section
LiOH.2H2O (0.9 equiv.) was added, and the reaction mixture
was stirred for 12 h at room temperature. The progress of the
reaction was monitored by TLC. After the complete
General Experimental Methods. All chemicals were obtained
from commercial suppliers and used without further
purification. All reactions were carried out in oven-dried
glassware with magnetic stirring. Purification of reaction
products was carried out by flash column chromatography
using silica gel 60 (100-120 mesh) and TLC visualization was
accompanied by UV light, KMnO4 and Phosphomolybidic acid
stains. Nuclear Magnetic Resonance spectra were recorded on
consumption of
1, the solvent was evaporated to dryness
under reduced pressure and the crude mixture was washed by
diethyl ether. The crude mass was then used as such for the
next coupling step. To this stirred crude mass in dry DMF,
DPPA (1.2 equiv) and L-Phe-OMe/ L-Phe-L-Ala-OMe (1.0 equiv)
were added at 0 oC under nitrogen atmosphere. Thereafter,
triethyl amine (2.5 equiv) was added drop-wise and the stirring
continued for 16-18 h at room temperature. After desired
time, the reaction was quenched with ice-cold water, and
extracted with ethyl acetate. The organic layer was first
washed with saturated solution of NaHCO3 and then with citric
acid solution. Thereafter, the organic layer was dried using
anhydrous Na2SO4 and the solvent was dried under reduced
pressure to yield crude product. Flash Column
chromatography of this crude product yielded the pure
aziridine based amino acid/peptide (6a-c) using ethyl acetate–
hexanes eluents.
1
Bruker 400 spectrometer. All H NMR (400MHz) and 13C NMR
(101 MHZ) spectra were recorded either in CDCl3 or DMSO-d6
with TMS as standard in δ units, parts per million (ppm). All
coupling constants J were reported in Hz. The following
abbreviations were used to describe peak splitting patterns
when appropriate: s = singlet, d = doublet, t = triplet, dd =
doublet of doublet, m = multiplet and brs = broad singlet. High
resolution mass spectrometry (HRMS) was performed with an
Agilent 6210 instrument using time-of-flight (TOF-MS) with
electro spray ionization (ESI). Melting points were determined
on
a capillary point apparatus equipped with a digital
thermometer and are uncorrected.
Ethyl
(2S,3S)-3-(((S)-1-methoxy-1-oxo-3-phenylpropan-2-
Computational Analysis. The full conformation search was
performed by using the MMX force field, in PCMODEL v.9.3
software.18a The molecular structure rearrangement is an
important factor to understand the best global minimum
energy conformer.18b All of the reactant, product, and
transition state geometry involved in the reactions were
optimized using density functional theory (DFT) calculations
(specifically, BP86 functional) by employment of the def-TZVP
yl)carbamoyl)aziridine-2-carboxylate. (Azy(OEt)-L-Phe-OMe)
(6a). (2S,3S)-diethyl-aziridine-2,3-dicarboxylate (0.50 g, 2.67
mmol), LiOH.2H2O (0.10 g, 2.40 mmol), L-Phe-OMe (0.478 g,
2.67 mmol), DPPA (0.88 g, 3.204 mmol), Et3N (0.67 g, 6.67
mmol); Rf = 0.3 (silica gel, EtOAc/Hexanes, 2:3); Yield 62% (0.53
1
g); Pale yellow oil; H NMR (400 MHz, Chloroform-d) δ 7.31 –
7.25 (m, 3H), 7.08 (d, J = 7.2 Hz, 2H), 6.87 – 6.78 (m, 1H), 4.83 –
4.73 (m, 1H), 4.19 (q, J = 7.2 Hz, 2H), 3.76 (s, 3H), 3.22 – 3.13
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 5
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DOI: 10.1039/C8OB00676H
ARTICLE
(m, 1H), 3.00 (dd, J = 14.0, 7.0 Hz, 1H), 2.79 (d, J = 9.0 Hz, 1H), Diethyl
Journal Name
((S)-3-(4-(benzyloxy)phenyl)-2-((tert-
2.29 (d, J = 6.1 Hz, 1H)), 1.75 (t, J = 8.4 Hz, 1H), 1.25 (t, J = 7.2 butoxycarbonyl)amino)propanoyl)-L-aspartate
Hz, 3H). 13C NMR (101 MHz, CDCl3) δ 171.2, 170.9, 167.5, Tyr(OBz)-L-Asp(OEt)-OEt) (5c). Compound
(Boc-L-
1
(0.10 g, 0.53
135.9, 129.5, 129.3, 128.7, 127.3, 61.8, 53.1, 52.5, 37.9, 37.7, mmol), Boc-L-Tyr(OBz)-SH (0.24 g, 0.64 mmol); Rf = 0.2 (silica
1
35.5, 14.2. HRMS (ESI) m/z, calcd for C16H20N2O5 [M+H]+ gel, EtOAc/Hexanes, 3:7); Yield 78% (0.21 g); Gel. H NMR (400
321.1445, found 321.1451.
MHz, Chloroform-d) δ 7.44 – 7.31 (m, 5H), 7.14 – 7.09 (m, 2H),
Methyl (2S,3S)-3-(((S)-1-methoxy-1-oxo-3-phenylpropan-2- 6.90 (dd, J = 8.7, 2.3 Hz, 2H), 6.85 (d, J = 7.9 Hz, 1H), 5.03 (s,
yl)carbamoyl)aziridine-2-carboxylate. (Azy(OMe)-L-Phe-OMe) 2H), 4.98-4.89 (m, 1H), 4.83-4.74 (m, 1H), 4.39-4.31 (m, 1H),
(6b). (2S,3S)-dimethyl-aziridine-2,3-dicarboxylate (0.50 g, 3.14 4.19 (q, J = 7.2 Hz, 2H), 4.14 – 4.06 (m, 2H), 3.06-2.98 (m, 2H),
mmol), LiOH.2H2O (0.12 g, 2.83 mmol), L-Phe-OMe (0.562 g, 2.95 (dd, J = 16.3, 4.3 Hz, 1H), 2.80 (dd, J = 17.2, 4.7 Hz,
3.14 mmol), DPPA (1.03 g, 3.768 mmol), Et3N (0.79 g, 7.85 1H),1.41 (s, 9H), 1.25 – 1.19 (m, 6H). 13C NMR (101 MHz, CDCl3)
mmol); Rf = 0.2 (silica gel, EtOAc/Hexanes, 1:1); Yield 64% (0.61 δ 171.3, 170.9, 170.4, 158.1, 155.5, 137.2, 130.6, 130.5, 128.8,
1
g); Pale yellow gel. H NMR (400 MHz, Chloroform-d) δ 7.35 – 128.2, 127.7, 115.2, 80.4, 70.2, 62.1, 61.2, 48.9, 18.5, 36.5,
7.24 (m, 3H), 7.11 – 7.03 (m, 2H), 6.81 (d, J = 8.7 Hz, 1H), 4.85 – 36.4, 29.9, 28.5, 14.3. HRMS (ESI) m/z, calcd for C29H39N2O8
4.76 (m, 1H), 3.77 (s, 3H), 3.75 (s, 3H), 3.18 (dd, J = 13.9, 5.6 [M+H]+ 543.2701, found 543.2702.
Hz, 1H), 3.00 (dd, J = 13.9, 7.1 Hz, 1H), 2.80 (d, J = 9.1 Hz, 1H),
2.28 (d, J = 7.6 Hz, 1H), 1.75 (t, J = 8.5 Hz, 1H). 13C NMR (101 Diethyl ((S)-5-(benzyloxy)-2-((tert-butoxycarbonyl)amino)-5-
MHz, CDCl3) δ 171.7, 170.9, 167.5, 135.8, 129.4, 129.3, 128.8, oxopentanoyl)-L-aspartate (Boc-L-Glu(OBz)-L-Asp(OEt)-OEt)
128.7, 127.4, 53.1, 52.6, 52.5, 37.9, 37.3, 35.6. HRMS (ESI) m/z, (5d). Compound
calcd for C15H19N2O5 [M+H]+ 307.1288, found 307.1284.
(0.22 g, 0.64 mmol); Rf = 0.2 (silica gel, EtOAc/Hexanes, 3:7);
Methyl
(2S,3S)-3-(((S)-1-(((S)-1-methoxy-1-oxopropan-2- Yield 72% (0.19 g); Oil. 1H NMR (400 MHz, Chloroform-d) δ
yl)amino)-1-oxo-3-phenylpropan-2-yl)carbamoyl)aziridine-2- 7.40 – 7.30 (m, 5H), 7.12 (dd, J = 31.7, 8.4 Hz, 1H), 5.30 (dd, J =
1 (0.10 g, 0.53 mmol), Boc-L-Glu(OBz)-SH
carboxylate. (Azy(OMe)-L-Phe-L-Ala-OMe (6c). (2S,3S)- 18.7, 7.4 Hz, 1H), 5.14 (s, 2H), 4.86 – 4.77 (m, 1H), 4.26 – 4.07
dimethyl-aziridine-2,3-dicarboxylate (0.20 g, 1.23 mmol), (m, 5H), 3.05-2.96 (m, 1H), 2.80 (dt, J = 17.1, 4.7 Hz, 1H), 2.58 –
LiOH.2H2O (0.05 g, 1.13 mmol), L-Phe-L-Ala-OMe (0.3 g, 1.23 2.40 (m, 2H), 2.22 – 2.11 (m, 1H), 1.97 (dd, J = 14.5, 7.3 Hz,
mmol), DPPA (0.41 g, 1.51 mmol), Et3N (0.32 g, 3.14 mmol); Rf 1H), 1.43 (s, 9H), 1.33 – 1.19 (m, 6H). 13C NMR (101 MHz,
= 0.2 (silica gel, EtOAc/Hexanes, 7:3); Yield 68% (0.31 g); CDCl3) δ 173.2, 171.5, 171.0, 170.5, 155.7, 135.9, 128.8, 28.5,
White solid, mp: 68 °C. 1H NMR (400 MHz, Chloroform-d) δ 80.3, 66.8, 62.1, 61.3, 54.1, 53.8, 48.8, 36.4, 30.5, 29.9, 28.5,
7.34 – 7.27 (m, 3H), 7.20 – 7.15 (m, 2H), 6.87 (d, J = 8.2 Hz, 1H), 14.3. HRMS (ESI) m/z, calcd for C25H35N2O10 [M+H]+, found.
6.42 (d, J = 7.2 Hz, 1H), 4.60 (q, J = 8.0, 1H), 4.50 (p, J = 7.2 Hz,
1H), 3.76 (s, 3H), 3.74 (s, 3H), 3.12 (dd, J = 14.0, 6.7 Hz, 1H), Diethyl
((benzyloxy)carbonyl)-L-phenylalanyl-L-aspartate
(0.10 g, 0.53
3.03 – 2.94 (m, 1H), 2.78 (d, J = 2.3 Hz, 1H), 2.24 (d, J = 2.2 Hz, (Cbz-L-Phe-L-Asp(OEt)-OEt) (5e). Compound
1
1H), 1.90 (brs, 1H), 1.37 (d, J = 7.2 Hz, 3H). 13C NMR (101 MHz, mmol), Cbz-L-Phe-SH (0.20 g, 0.64 mmol); Rf = 0.3 (silica gel,
CDCl3) δ 173.0, 170.2, 168.1, 136.4, 129.4, 128.8, 127.3, 53.8, EtOAc/Hexanes, 3:7); Yield 82% (0.20 g); White solid, mp: 123-
53.1, 52.7, 48.4, 38.2, 37.5, 35.5, 18.3. HRMS (ESI) m/z, calcd 124 °C. 1H NMR (400 MHz, Chloroform-d) δ 7.38 – 7.27 (m,
for C18H24N3O6 [M+H]+ 378.166, found 378.1742.
6H), 7.25 – 7.15 (m, 4H), 6.79 (d, J = 8.0 Hz, 1H), 5.25 (d, J = 8.2
Hz, 1H), 5.08 (s, 2H), 4.77 (dt, J = 8.3, 4.4 Hz, 1H), 4.50 – 4.44
General procedure for Ligation strategy. To a solution of (m, 1H), 4.19 (q, J = 7.1 Hz, 2H), 4.15 – 4.06 (m, 2H), 3.11 (d, J =
aziridine-2,3-dicarboxylate (or aziridine based phenylalanine 6.1 Hz, 2H), 2.99 (dd, J = 17.2, 4.4 Hz, 1H), 2.79 (dd, J = 17.3,
derivative) (
) (1.1 equiv) was added and the mixture was stirred for 4-6 170.9, 170.8, 170.4, 156.0, 136.2, 129.5, 128.7, 128.4, 128.2,
1/6
) (1.0 equiv) in DMF (0.1 mmol/mL), thioacid 4.6 Hz, 1H), 1.28-1.19 (m, 6H). 13C NMR (101 MHz, CDCl3) δ
(2/3
o
h at 25 C until TLC shows the complete consumption of
1
/
6
.
127.2, 67.2, 62.1, 61.3, 56.1, 48.9, 38.6, 36.4, 14.3, 14.3. HRMS
After completion of reaction, the reaction mixture was (ESI) m/z, calcd for C25H30N2O7 [M+H]+ 471.2126, found
quenched with ice-cold water, and extracted with ethyl 471.2120.
acetate (20 mL x 2) to give the crude intermediate that was
used as such for next step without any purification.
Diethyl
N2,N6-bis(((benzyloxy)carbonyl)-L-lysyl-L-aspartate
(0.10 g, 0.53
General procedure for thiol group reduction. The crude (Cbz-L-Lys(Cbz)-L-Asp(OEt)-OEt) (5f). Compound
1
ligated reaction mass was dissolved in MeOH (0.03 M) and mmol), Cbz-L-Lys(Cbz)-SH (0.27 g, 0.64 mmol); Rf = 0.2 (silica
temperature of reaction mass was lowered to -10 oC (using ice- gel, EtOAc/Hexanes, 3:7); Yield 75% (0.23 g); White solid, mp:
salt bath). At this temperature, NiCl2.6H2O (3 equiv.) and 105-106 °C.1H NMR (400 MHz, Chloroform-d) δ 7.29 – 7.20 (m,
NaBH4 (9 equiv) was added and reaction mixture was stirred at 10H), 6.93 – 6.85 (m, 1H), 5.53 (d, J = 8.0 Hz, 1H), 5.05 – 4.93
the same temperature for 15 min. After 15 min stirring, the (m, 5H), 4.79 – 4.69 (m, 1H), 4.20 – 3.98 (m, 5H), 3.16 – 3.06
reaction mixture was filtered through celite pad and filtrate (m, 2H), 2.93 (dd, J = 17.1, 4.6 Hz, 1H), 2.71 (dd, J = 17.1, 4.6
was concentrated to give crude peptide product that was Hz, 1H), 1.88 – 1.71 (m, 2H), 1.67 – 1.54 (m, 1H), 1.53 – 1.39
further purified by flash chromatography using gradients of (m, 1H), 1.35 – 1.30 (m, 2H), 1.21 – 1.10 (m, 6H). 13C NMR (101
ethyl acetate-hexane as eluent.
MHz, CDCl3) δ 171.7, 171.1, 170.6, 156.8, 156.3, 136.8, 136.4,
128.7, 128.7, 128.2, 67.2, 66.8, 62.2, 61.4, 54.8, 48.7, 40.5,
6 | J. Name., 2012, 00, 1-3
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ARTICLE
36.3, 29.9, 29.5, 22.2, 14.3, 14.2. HRMS (ESI) m/z, calcd for 6a (0.10 g, 0.31 mmol), Cbz-L-Lys(Cbz)-SH (0.16 g, 0.38 mmol);
C30H39N3O9 [M+H]+ 586.2759, found 586.2764.
Rf = 0.2 (silica gel, EtOAc/Hexanes, 2:3); Yield 68% (0.14 g);
White solid, mp: 120–121°C. 1H NMR (400 MHz, Chloroform-d)
Diethyl
((S)-2-((S)-2-(((benzyloxy)carbonyl)amino) δ 7.40 – 7.27 (m, 13H), 7.25 – 7.19 (m, 2H), 7.16 – 7.12 (m, 2H),
propanamido)-5-methoxy-5-oxopentanaoyl)-L-aspartate
(Cbz-L-Ala-L-Glu(OMe)-L-Asp(OEt)-OEt) (5g). Compound
6.97 (d, J = 7.7 Hz, 1H), 5.55 (d, J = 7.1 Hz, 1H), 5.12 (d, J = 5.4
Hz, 1H), 5.08 (s, 2H), 5.05 (d, J = 4.1 Hz, 3H), 4.80– 4.69 (m,
1
(0.10 g, 0.53 mmol), Cbz-L-Ala-L-Glu(OMe)-SH (0.25 g, 0.64 2H), 4.11 (q, J = 6.5, 5.5 Hz, 2H), 3.69 (s, 3H), 3.20 – 2.97 (m,
mmol); Rf = 0.2 (silica gel, EtOAc/Hexanes, 2:3); Yield 70% (0.19 5H), 2.62 (dd, J = 17.2, 6.5 Hz, 1H), 1.85 – 1.58 (m, 4H), 1.57 –
g); Light brown solid, mp: 123-124 °C.1H NMR (400 MHz, 1.48 (m, 1H), 1.47 – 1.36 (m, 1H), 1.31 – 1.22 (m, 1H), 1.18 (t, J
Chloroform-d) δ 7.43 (d, J = 8.5 Hz, 1H), 7.38 – 7.28 (m, 5H), = 7.1 Hz, 3H). 13C NMR (101 MHz, CDCl3) δ 172.5, 171.9,171.3,
7.26 (d, J = 6.2 Hz, 1H), 5.72 (d, J = 7.4 Hz, 1H), 5.12 (d, AB 169.8, 156.9, 136.7, 136.6, 136.3, 132.2, 129.6, 128.7, 128.7,
pattern, J = 11.9 Hz, 1H), 5.08 (d, AB pattern, J = 12.2 Hz, 1H), 128.4, 128.3, 128.3, 128.2, 127.2, 67.3, 66.9, 65.2, 61.8, 53.8,
4.87 – 4.78 (m, 1H), 4.64 – 4.54 (m, 1H), 4.33 (t, J = 7.1 Hz, 1H), 53.1, 52.3, 49.3, 40.4, 37.8, 35.5, 29.9, 29.5, 22.8, 14.2. HRMS
4.19 (qd, J = 8.7, 7.9, 2.5 Hz, 2H), 4.16 – 4.07 (m, 2H), 3.66 (s, (ESI) m/z, calcd for C38H47N4O10 [M+H]+ 719.3287, found
3H), 2.98 (dd, J = 17.1, 4.9 Hz, 1H), 2.79 (dd, J = 17.8, 4.2 Hz, 719.3288.
1H), 2.53 – 2.42 (m, 2H), 2.16 (dd, J = 14.2, 6.9 Hz, 1H), 2.04 –
1.96 (m, 1H), 1.37 (d, J = 7.0 Hz, 3H), 1.27 – 1.17 (m, 6H). 13C Methyl (5S,8S,11S,14S)-14-benzyl-11-(2-ethoxy-2-oxoethyl)-8-
NMR (101 MHz, CDCl3) δ 174.0, 172.7, 170.9, 170.5, 156.1, (3-methoxy-3-oxopropyl)-5-methyl-3,6,9,12-tetraoxo-1-
136.4, 128.6, 128.3, 128.2, 67.1, 62.0, 61.3, 52.4, 52.0, 50.7, phenyl-2-oxa-4,7,10,13-tetraazapentadecan-15-oate. (Cbz-L-
48.8, 36.3, 30.2, 28.0, 19.0, 14.2, 14.2. HRMS (ESI) m/z, calcd Ala-L-Glu(OMe)-L-Asp(OEt)-L-Phe-OMe) (7d). Compound 6a
for C25H35N3O8 [M+H]+ 538.2395, found 538.2396.
(0.10 g, 0.31 mmol), Cbz-L-Ala-L-Glu(OMe)-SH (0.14 g, 0.38
Methyl (8S,11S)-11-benzyl-8-(2-methoxy-2-oxoethyl)-3,6,9- mmol); Rf = 0.1 (silica gel, EtOAc/Hexanes, 1:1); Yield 78% (0.16
trioxo-1-phenyl-2-oxa-4,7,10-triazadodecan-12-oate. (Cbz-L- g); White solid, mp: 138–139°C. 1H NMR (400 MHz,
Gly-L-Asp(OMe)-L-Phe-OMe) (7a). Compound 6b (0.10 g, 0.33 Chloroform-d) δ 7.58 (d, J = 7.3 Hz, 1H), 7.50 (d, J = 8.4 Hz, 1H),
mmol), Cbz-L-Gly–SH (0.10 g, 0.39 mmol); Rf = 0.6 (silica gel, 7.41 – 7.29 (m, 5H), 7.25 – 7.14 (m, 6H), 5.42 (d, J = 5.7 Hz, 1H),
EtOAc/Hexanes, 7:3); Yield 75% (0.12 g); White solid, mp: 128- 5.16 (d, AB pattern, J = 12.2 Hz, 1H), 5.07 (d, AB pattern, J =
1
130 °C. H NMR (400 MHz, Chloroform-d) δ 7.30 (d, J = 4.5 Hz, 12.3 Hz, 1H), 4.84 (q, J = 6.7 Hz, 1H), 4.75 (q, J = 7.2 Hz, 1H),
5H), 7.24 – 7.20 (m, 2H), 7.17 – 7.12 (m, 1H), 7.06 (d, J = 7.6 Hz, 4.42 (q, J = 6.4 Hz, 1H), 4.20 – 4.06 (m, 3H), 3.65 (s, 3H), 3.64
2H), 6.93 (d, J = 6.9 Hz, 1H), 5.27 – 5.14 (m, 1H), 5.06 (s, 2H), (s, 3H), 3.15 (dd, J = 14.0, 5.9 Hz, 1H), 3.07 (dd, J = 13.8, 7.4 Hz,
4.78 – 4.64 (m, 2H), 4.45 – 4.31 (m, 1H), 3.74 (d, J = 5.6 Hz, 2H), 1H), 2.85 (dd, J = 16.9, 6.5 Hz, 1H), 2.78 (dd, J = 15.1, 7.3 Hz,
3.64 (s, 3H), 3.62 (s, 3H), 3.09 (dd, J = 13.9, 5.6 Hz, 1H), 2.97 1H), 2.44 (q, J = 7.0, 6.4 Hz, 2H), 2.06 (p, J = 7.9, 7.1 Hz, 2H),
(dd, J = 14.0, 7.0 Hz, 1H), 2.93 – 2.85 (m, 1H), 2.53 (dd, J = 17.1, 1.39 (d, J = 7.2 Hz, 3H), 1.19 (t, J = 7.2 Hz, 3H). 13C NMR (101
6.8 Hz, 1H). 13C NMR (101 MHz, CDCl3) δ 172.7, 171.7, 169.9, MHz, CDCl3) δ 174.7, 173.1, 171.8, 171.1, 170.6, 169.7, 156.4,
169.2, 156.7, 136.2, 136.0, 129.4, 128.8, 128.5, 128.4, 127.3, 136.3, 136.0, 129.3, 128.6, 128.5, 128.4, 128.3, 128.0, 126.8,
67.6, 53.7, 52.6, 52.4, 49.1, 44.8, 37.7, 35.3. HRMS (ESI) m/z, 67.2, 61.4, 53.7, 53.4, 52.1, 52.0, 51.4, 49.5, 37.7, 35.6, 30.1,
calcd for C25H30N3O8 [M+H]+ 500.2027, found 500.2000.
26.2, 18.2, 14.0. HRMS (ESI) m/z, calcd for C33H43N4O11 [M+H]+
671.2923, found 671.2927.
Methyl
(6S,9S,12S)-12-benzyl-6-isopropyl-9-(2-methoxy-2-
oxoethyl)-2,2-dimethyl-4,7,10-trioxo-3-oxa-5,8,11-
Methyl (8S,11S,14S)-14-benzyl-8-isopropyl-11-(2-methoxy-2-
triazatridecan-13-oate. (Boc-L-Val-L-Asp(OMe)-L-Phe-OMe) oxoethyl)-
3,6,9,12-tetraoxo-1-phenyl-2-oxa-4,7,10,13-
(7b). Compound 6b (0.10 g, 0.33 mmol), Boc-L-Val-SH (0.09 g, tetraazapentadecan-15-oate. (Cbz-Gly-L-Val-L-Asp(OMe)-L-
0.39 mmol); Rf = 0.2 (silica gel, EtOAc/Hexanes, 2:3); Yield 80% Phe-OMe) (7e). Compound 6b (0.10 g, 0.33 mmol), Cbz-Gly-L-
(0.13 g); White solid, mp: °C.1H NMR (400 MHz, Chloroform-d) Val-SH (0.13 g, 0.39 mmol); Rf = 0.3 (silica gel, EtOAc); Yield
1
δ 7.28 – 7.20 (m, 2H), 7.19 – 7.15 (m, 1H), 7.12 – 7.02 (m, 3H), 84% (0.16 g); White solid, mp: 135–136°C. H NMR (400 MHz,
6.93 (d, J = 7.7 Hz, 1H), 4.92 (d, J = 8.2 Hz, 1H), 4.76 – 4.65 (m, Chloroform-d) δ 7.30 – 7.25 (m, 5H), 7.24 – 7.20 (m, 2H), 7.20
2H), 3.88 (t, J = 7.1 Hz, 1H), 3.62 (s, 3H), 3.61 (s, 3H), 3.03 (d, J – 7.10 (m, 2H), 7.09 – 6.98 (m, 3H), 6.71 (d, J = 8.2 Hz, 1H), 5.54
= 6.1 Hz, 2H), 2.96 (dd, J = 17.3, 3.8 Hz, 1H), 2.54 (dd, J = 17.3, – 5.44 (m, 1H), 5.05 (s, 2H), 4.81 – 4.67 (m, 2H), 4.21 (t, J = 7.2
6.7 Hz, 1H), 1.80 – 1.70 (m, 1H), 1.37 (s, 9H), 0.81 – 0.74 (m, Hz, 1H), 3.84 (dd, J = 17.0, 5.6 Hz, 1H), 3.76 (dd, J = 17.8, 6.6
6H). 13C NMR (101 MHz, CDCl3) δ 172.6, 171.8, 171.5, 169.9, Hz, 1H), 3.61 (s, 3H), 3.59 (s, 3H), 3.10 – 2.95 (m, 2H), 2.88 (dd,
156.0, 135.9, 129.5, 128.8, 127.4, 80.4, 59.6, 53.9, 52.5, 52.4, J = 17.0, 5.0 Hz, 1H), 2.58 (dd, J = 17.1, 5.1 Hz, 1H), 2.06 – 1.88
49.1, 38.0, 37.4, 35.7, 28.5, 24.8, 15.8, 11.6. HRMS (ESI) m/z, (m, 1H), 0.80 (d, J = 6.8 Hz, 3H), 0.75 (d, J = 6.9 Hz, 3H). 13
calcd for C25H37N3O8Na [M+Na]+ 530.2473, found 530.2593.
NMR (101 MHz, CDCl3) δ 172.4, 171.6, 171.0, 169.9, 156.9,
136.3, 136.0, 129.5, 128.8, 128.5, 128.3, 127.3, 67.5, 58.8,
C
Methyl
(9S,12S,15S)-15-benzyl-9- 53.8, 52.5, 52.4, 49.4, 44.9, 37.8, 35.9, 31.0, 19.4, 17.9. HRMS
(((benzyloxy)carbony)amino)-12-(2-ethoxy-2-oxoethyl)-
3,10,13-trioxo-1-phenyl-2-oxa-4,11,14-triazahexadecan-16-
oate. (Cbz-L-Lys(Cbz)-L-Asp(OEt)-L-Phe-OMe) (7c). Compound
(ESI) m/z, calcd for C30H39N4O9 [M+H]+ 599.2712, found
599.2718.
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DOI: 10.1039/C8OB00676H
ARTICLE
Methyl
Journal Name
(5S,8S,11S,14S)-8,14-dibenzyl-11-(2-methoxy-2- 51.6, 50.2, 49.4, 47.7, 37.6, 37.3, 36.0, 18.1, 16.9. HRMS (ESI)
m/z, calcd for C38H46N5O10 [M+H]+ 732.3239, found 732.3282.
oxoethyl)-5-methyl-3,6,9,12-tetraoxo-1-phenyl-2-oxa-
4,7,10,13-tetraazapentadecan-15-oate.
(Cbz-L-Ala-L-Phe-L-
Asp(OMe)-L-Phe-OMe) (7f). Compound 6b (0.10 g, 0.33
mmol), Cbz-L-Ala-L-Phe-SH (0.15 g, 0.39 mmol); Rf = 0.2 (silica
gel, EtOAc/Hexanes, 7:3); Yield 82% (0.18 g); White solid, mp:
168–170°C.1H NMR (400 MHz, Chloroform-d) δ 7.33 – 7.21 (m,
6H), 7.18 – 7.10 (m, 6H), 7.08 – 7.01 (m, 5H), 6.67 (d, J = 7.3 Hz,
1H), 5.25 (d, J = 6.4 Hz, 1H), 5.01 (d, J = 12.0 Hz, 1H), 4.91 (d, J
= 12.3 Hz, 1H), 4.78 – 4.70 (m, 1H), 4.69 – 4.63 (m, 1H), 4.56 (q,
J = 6.9 Hz, 1H), 4.05 (q, J = 6.8 Hz, 1H), 3.60 (s, 3H), 3.55 (s, 3H),
3.05 (dd, J = 14.0, 5.9 Hz, 1H), 3.00 – 2.91 (m, 3H), 2.74 (dd, J =
17.0, 5.6 Hz, 1H), 2.59 (dd, J = 16.7, 6.3 Hz, 1H), 1.20 (d, J = 7.1
Hz, 3H). 13C NMR (101 MHz, CDCl3) δ 172.8, 172.0, 171.7,
170.8, 169.9, 156.4, 136.3, 136.2, 136.1, 129.4, 129.4, 129.3,
128.9, 128.8, 128.7, 128.5, 128.3, 127.3, 127.2, 67.5, 54.5,
53.9, 52.5, 52.2, 51.2, 49.5, 37.8, 37.5, 35.8, 18.2. HRMS (ESI)
m/z, calcd for C35H41N4O9 [M+H]+ 661.2868, found 661.2828.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This project was funded by Department of Science and
Technology India. Thus authors acknowledge with thanks DST
for financial support. Dr G. G. Pillai thanks COSMOlogic GmbH
for permitting to use the Turbomole software.
Notes and references
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DOI: 10.1039/C8OB00676H
Aziridine Based Electrophilic Handle for Asparatic Acid Ligation
Kiran Bajaj,*a Devesh S. Agarwal,a Rajeev Sakhujaa and Girinath G. Pillaib
aDepartment of Chemistry, Birla Institute of Technology and Science, Pilani 333031, Rajasthan, India
bComputational Chemistry Division, Zastra Innovations Pvt. Ltd., Kalyan Nagar, Bengaluru 560043, Karnataka, India