Synthesis and stereochemical analysis of β-nitromethane substituted γ-amino acids and peptides

Article abstract of DOI:10.1039/c2ob27070f

The high diastereoselectivity in the Michael addition of nitromethane to α,β-unsaturated γ-amino esters, crystal conformations of β-nitromethane substituted γ-amino acids and peptides are studied. Results suggest that the N-Boc protected amide NH, conformations of α,β-unsaturated γ-amino esters and alkyl side chains play a crucial role in dictating the high diastereoselectivity of nitromethane addition to E-vinylogous amino esters. Investigation of the crystal conformations of both α,β-unsaturated γ-amino esters and the Michael addition products suggests that an H-C^γ-C^βC ^α eclipsed conformer of the unsaturated amino ester leads to the major (anti) product compared to that of an N-C^γ-C ^βC^α eclipsed conformer. The major diastereomers were separated and subjected to the peptide synthesis. The single crystal analysis of the dipeptide containing β-nitromethane substituted γ-amino acids reveals a helical type of folded conformation with an isolated H-bond involving a nine-atom pseudocycle. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c2ob27070f

Organic &
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Synthesis and stereochemical analysis of
β-nitromethane substituted γ-amino acids and
peptides†
Cite this: Org. Biomol. Chem., 2013, 11,
803
Mothukuri Ganesh Kumar, Sachitanand M. Mali and Hosahudya N. Gopi*
The high diastereoselectivity in the Michael addition of nitromethane to α,β-unsaturated γ-amino
esters, crystal conformations of β-nitromethane substituted γ-amino acids and peptides are studied.
Results suggest that the N-Boc protected amide NH, conformations of α,β-unsaturated γ-amino esters
and alkyl side chains play a crucial role in dictating the high diastereoselectivity of nitromethane addition
to E-vinylogous amino esters. Investigation of the crystal conformations of both α,β-unsaturated γ-amino
esters and the Michael addition products suggests that an H–Cγ–CβvCα eclipsed conformer of the unsatu-
rated amino ester leads to the major (anti) product compared to that of an N–Cγ–CβvCα eclipsed con-
former. The major diastereomers were separated and subjected to the peptide synthesis. The single
crystal analysis of the dipeptide containing β-nitromethane substituted γ-amino acids reveals a helical
type of folded conformation with an isolated H-bond involving a nine-atom pseudocycle.
Received 24th October 2012,
Accepted 23rd November 2012
DOI: 10.1039/c2ob27070f
www.rsc.org/obc
used as effective precursors for the synthesis of amines,
Introduction
carboxylic acids, aldehydes, ketones, substituted alkanes,
Due to their structural diversity and functional versatility, alkenes, complex heterocyclic structures and more.5
backbone homologated α-amino acids and their oligomers,
With these divergent functional group transformations of
such as β-and γ-peptides, emerged as a very important class of nitroalkanes in mind, we asked the question whether these
molecules in medicinal chemistry.1 The β- and γ-peptides have functional properties of nitroalkanes can be transformed to the
been shown to adopt stable secondary structures, such as γ-amino acids and to the peptides, since they are very attractive
helices, β-sheets and reverse turns, similar to the natural entities from the perspective of medicinal chemistry. Hence,
protein secondary structures. Additionally, the backbone we anticipate that α,β-unsaturated γ-amino esters may serve as
functionalized γ4-amino acids, such as β-keto γ4-amino acids, ideal precursors for the synthesis of β-nitromethane functiona-
β-hydroxy-γ4-amino acids and α,β-unsaturated γ4-amino acids, lized γ-amino acids using Michael addition. A variety of nucleo-
are highly versatile non-natural amino acids present in many philes and catalysts have been demonstrated for the C–C and
biologically active peptide natural products.2 Synthetic and C–X (X = N, S, O, etc.) bond formation using the Michael
naturally occurring peptides containing these backbone addition reaction.6 In addition, nitromethane Michael addition
functionalized γ-amino acids have been used as inhibitors of to the N-carbamate protected vinylogous amino esters has also
cysteine, serine and aspartic acid proteases.3 In addition, been reported, however, these reports have offered no insight
simple unprotected β-alkyl or dialkyl substituted γ-amino acids into the diastereoselectivity and the mechanism of the conju-
(pregabalin and gabapentin) have also been marketed as anti- gate addition.7 Recently, we established the synthesis of E-vinyl-
convulsant drugs for the neuropathic pain as well as for the ogous amino acids and their utilization in the construction of
treatment of epilepsy.4
hybrid peptides.8 We sought to utilize these amino acids as
Nitroalkane chemistry is unique with respect to the diverse substrates in the Michael addition reaction with nitromethane
functional group transformations. Nitroalkanes have been to understand the diastereoselectivity as well as to obtain
stereochemically pure β-nitromethane substituted γ-amino
acids to synthesize hybrid peptides. Herein, we are reporting
Department of Chemistry, Indian Institute of Science Education and Research,
Dr. Homi Bhabha Road, Pashan, Pune-411 008, India. E-mail: hn.gopi@iiserpune.ac.in
†Electronic supplementary information (ESI) available: Crystallographic infor-
mation, copies of 1H and 13C NMR spectra as well as HPLC chromatograms are
available. CCDC 802255, 825102, 907818–907820. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c2ob27070f
the highly diastereoselective Michael addition of nitromethane
to various N-Boc-(S,E)-α,β-unsaturated γ-amino esters and the
crystal conformations of β-nitromethane substituted γ-amino
acids and peptides. Results suggest that the high diastereo-
selectivity is mainly depending on the conformations of the
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E-vinylogous amino esters, amino acid side chains and the
amide NH (Boc-NH). In addition, the X-ray analysis demon-
strates that the β-nitromethane substituted γ-dipeptide adopted
a novel C9-helical type of conformation in single crystals.
Results and discussion
Synthesis of (E)-vinylogous amino esters from α-amino acids
The N-protected (S,E)-vinylogous amino esters (1) were syn-
thesized starting from the N-protected amino aldehydes using
the Wittig reaction8a as shown in Scheme 1. The N-Boc amino
aldehydes were synthesized from the oxidation of correspond-
ing amino alcohols.9 A variety of E-vinylogous amino esters
were synthesized using this protocol (Scheme 1, 1a–l). Instruc-
tively, many of these N-protected E-vinylogous amino esters
were prone to crystallize immediately after the column
purification. The crystal conformations of the compounds 1b
and 1c are shown in Fig. 1.8a
Fig. 1 A. The local torsional variables of vinylogous residues. B. X-ray structures
of N-Boc-(S,E)-vinylogous amino esters 1b and 1c showing the N–Cγ–CβvCα
eclipsed conformations.8a
a base. The schematic representation of the conjugate addition
in the presence of a base is shown in Scheme 2. Conditions for
the Michael addition were optimized using the base DBU.7a To
begin with, ethyl ester of N-Boc-(S,E)-α,β-unsaturated γ-valine
(Boc-(S,E)-dgV-OEt, 1a) was subjected to the Michael addition
Conjugate addition of nitromethane to (E)-vinylogous amino
esters
We anticipate that Michael addition of nitromethane to all
E-vinylogous esters (1a–l) can be performed in the presence of
Scheme 1 Synthesis of α,β-unsaturated γ-amino esters using the Wittig reaction.
Scheme 2 Synthesis of β-nitromethane substituted γ-amino esters.
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Table 1 List of β-nitromethane substituted γ-amino acids and their diastereo-
meric ratios
Product
(% yield) dr (anti : syn)
Entry 1
R
X
2 + 3
2 : 3
A
B
C
D
E
F
G
H
I
–CH(CH3)2
–CH(CH3)–CH2–CH3 –C2H5
–C2H5
90
89
86
87
88
84
82
92
91
89 : 11a,c
83 : 17a,c
82 : 18c
90 : 10a
80 : 20c
100 : 0a,c
81 : 19a
80 : 20b
61 : 39a
86 : 14a
81 : 19b
89 : 11a
–CH2–CH(CH3)2
–CH2–NHBoc
–CH2–C6H5
–C2H5
–C2H5
–C2H5
–C2H5
–C2H5
–C2H5
–C2H5
–CH2C6H5 80
–CH2C6H5 79
–CH2C6H5 72
–CH2–OtBu
–CH2–COOtBu
–CH3
Fig. 2 X-ray crystal structures of Boc-NH-γL(β-CH2NO2)-COOEt (2c), Boc-γI-
(β-CH2NO2)-COOH (2b acid) and dipeptide Boc-NH-γV(β-CH2NO2)-γL-
(β-CH2NO2)-COOEt (P3). The backbone and side-chain H-atoms of dipeptide are
not shown for clarity.
Proline side chain
–CH(CH3)2
–CH2–CH(CH3)2
–CH2–C6H5
J
K
L
a Determined by reverse phase HPLC. b Determined by NMR analysis.
c Determined by isolated products.
almost equal diastereomeric ratio from 1i, enable us to ration-
alize a plausible mechanism of the Michael addition starting
reaction using nitromethane in the presence of DBU. The from the crystal structures of E-vinylogous amino esters. The
Michael addition product was isolated with 90% yield in 2.5 h local conformations of vinylogous residues were determined
at room temperature. Interestingly, the HPLC analysis of the by introducing additional torsional variables θ1 (N–Cγ–CβvCα)
crude product reveals the 89.5 : 9.5 diastereomeric ratio of anti and θ2 (Cγ–CβvCα–C′) along with the ϕ and ψ as shown in
(2) : syn (3) products, respectively. Both major (2) and minor (3) Fig. 1. The crystal conformations of vinylogous amino esters
products were separated using column chromatography and from the literature8,10 and from the present work suggest that
analyzed. Encouraged by the high diastereoselectivity, we they mainly adopt either N–Cγ–CβvCα (θ1 = ∼0°) or H–Cγ–
further subjected other vinylogous amino acids (1b–l) to the CβvCα (θ1 = ∼120°) eclipsed conformations (Fig. 1). This is not
Michael addition reaction and the diastereomeric products surprising because in their seminal work Houk and
were isolated in moderate to good yields (72–92%). The ana- colleagues11 and Wiberg et al.12 proposed that the eclipsed
lyses of all nitromethane conjugate addition products reveal conformations of olefins and the carbonyls are stabilized by
that anti (2b–l) is a major product in all the cases. The dia-
stereomeric major (2a, 2b, 2c and 2e) and minor (3a, 3b, 3c
and 3e) products of 1a, 1b, 1c and 1e were separated using
column chromatography and the diastereomeric ratio was
measured using the isolated yields. Surprisingly, compound 1f
gave a single anti isomer 2f. As the reactions were performed
in small scale, only major anti Michael addition products (2g,
2j, 2k and 2l) were isolated using column chromatography
for the compounds 1g, 1j, 1k and 1l. In addition, we found
difficult to separate the diastereomers for the compounds 1d,
1h and 1i from the column chromatography. However, the dia-
stereomeric ratio (2 : 3) was measured for all the crude
products either using HPLC or by the 1H NMR (Table 1).
Instructively, high diastereoselectivity was observed for all
E-vinylogous esters except the proline side-chain containing 1i.
In addition, both benzyl and ethyl esters gave similar yields
and diastereoselectivity of the nitromethane Michael addition.
Out of all Michael addition products (2a–l and 3a–l) in
Table 1, we were able to obtain the single crystals for 2c and its
X-ray structure is shown in Fig. 2.
In contrast to the nucleophilic addition at the carbonyl
compounds containing α-chiral center, the stereochemical
outcome of 1,4 nucleophilic addition to α,β-unsaturated esters
containing γ-stereogenic center is sometimes bewildering
to rationalize. However, the intriguing results of conjugate
Scheme 3 Schematic representation of the highly diastereoselective Michael
addition of nitromethane to N-Boc-protected vinylogous amino
esters 1a–e, 1h and 1j–k, a single anti isomer of 1f and the
addition of nitromethane to the E-vinylogous amino esters. The crystal structure
of 2c representing the stereochemical outcome from the conformer B.
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σ → π* donations. As the C–C bond is a better σ donor than β-Nitromethane substituted γ-peptides and crystal
the C–N bond, from the analysis of twenty one units of vinylo- conformations
gous residues in both monomers as well as in peptide crystals
In order to synthesize peptides containing β-nitromethane
(see ESI†), we observed that the amino acid side-chains are
substituted γ-amino acids, we subjected ethyl esters of
nearly perpendicular to the double bonds. In addition, the
N-protected nitroamino acids for the saponification and N-Boc
deprotection using 1 N NaOH and 50%TFA in DCM, respecti-
vely. The free amines and carboxylic acids were directly used
for the peptide synthesis without further characterization.
analysis also reveals the preference of an N–Cγ–CβvCα eclipsed
conformation in monomer esters and an H–Cγ–CβvCαeclipsed
conformation in amides/peptides. Based on these analyses, we
speculate that both eclipsed conformers A and B are in equili-
However, we were able to obtain the single crystals for 2b after
brium in solution (Scheme 3).
the ester hydrolysis and its structure is shown in Fig. 2. The
peptide couplings were carried out using standard DCC/HOBt
The preference for A over B is may be due to the reduced
A
1,3 strain. In the conformer B, both Cγ–H and Cα–H are in the
coupling conditions (Scheme 5). The list of β-nitromethane
substituted γ-hybrid peptides (P1–7) synthesized using this
procedure is shown in Scheme 5. All peptides were isolated in
moderate yields (55–78%) after column chromatography. Out
of all the peptides in Scheme 5, P3 gave single crystals after
the slow evaporation from the solution of EtOAc and its X-ray
structure is shown in Fig. 2. Similar to the vinylogous amino
acids (Fig. 1), the backbone torsional angles of β-nitromethane
substituted γ-amino acids were analyzed by introducing
additional torsional variables θ1 and θ2.8 The torsional
variables of β-nitromethane substituted γ-amino residues are
tabulated in Table 2. Inspection of the crystal structure reveals
that the independent amino acids in the dipeptide adopted
g+, g+ backbone conformations similar to the amino acid crystal
structure 2b. In contrast to the anti conformation of 2c (along
the C3–C4 bond), the amino acid 2b and amino acid residues
in the dipeptide P3 follow the general trend of tetraalkyl sub-
stituted ethane by adopting the gauche conformations.13
Instructively, the analysis of the crystal structure of P3 reveals
that it adopts a C9-helical type of structure in single crystals.14
The right handed helical twist is stabilized by a 9-atom
involved intramolecular H-bond between the urethane carbo-
nyl and NH of the C-terminal residue with CvO⋯H–N dis-
tance 1.995 Å [O⋯N dist. 2.845 Å] and O⋯H–N bond angle
170°.
same plane separated by a distance of 2.3 Å, whereas in the
conformer A, N–H is twisted upwards in all N–C–CvC eclipsed
conformations (in single crystals) probably to reduce the steric
strain (N–H⋯H–Cα dist., 2.75 Å and N–H⋯H–Cα angle 67°)
with Cα–H. The crystal structure analysis also suggests that the
approach of the nucleophile from top is restricted due to the
steric clash with an R group (Scheme 3) as the R-group
projected perpendicular to the double bonds. Based on these
analyses, we propose that a nucleophile must approach the
conjugated ester from the opposite face of the R group to avoid
the steric clash. In addition, the excellent diastereoselectivity
observed in all E-vinylogous amino esters containing amide
NH except proline (1i), suggesting the significant role of the
amide NHs in dictating the high diastereoselectivity. In order
to understand the role of NH in the conjugate addition,
we performed the conjugate addition reaction on ethyl ester of
N,N-di-Boc-protected (S,E)-α,β-unsaturated γ-phenylalanine
(4, Scheme 4). Instructively, the conjugate addition of nitro-
methane yields the diastereomeric ratio (5 + 6) 52 : 48 similar
to the compound 1i suggesting the pivotal role of amide NH.
We speculate that amide NH may be involved in the H-bond
formation with the incoming nucleophile and stabilizing the
nucleophilic approach. Based on these overall results we
propose the possible reaction mechanism for the conjugate
addition of the nitromethane as shown in Scheme 3. The con-
former B leads to the major (anti) product due to the H-bond
mediated stabilization of the nucleophile in the conjugate
addition, whereas the conformer A disfavored the formation of
major (syn) as there is no favorable H-bond stabilization for
the nucleophilic approach. Further, the analysis of the crystal
structure 2c reveals that it adopted a similar structure as anti-
cipated from the conformer B.
Conclusions
Overall, the β-nitromethane substituted γ-amino acids were
synthesized with high diastereoselectivity in the absence of
any metal or chiral catalysts. The high diastereoselectivity of
the conjugate addition of nitromethane to the α,β-unsaturated
Scheme 4 Conjugate addition of nitromethane to N,N-di-Boc-protected E-vinylogous amino ester.
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Scheme 5 Synthesis of peptides containing β-nitromethane substituted γ-amino acids.
Table 2 Torsional values of nitro amino acids and nitropeptide
Compound
Residue
ϕ
θ1
θ2
ψ
Back bone conformation
2c
2b acid
P3
γL(β-CH2NO2)
γI(β-CH2NO2)
γV(β-CH2NO2)
γL(β-CH2NO2)
−110
−110
−100
−99
−179
67
68
89
72
72
66
−146
−145/36
−87
t, g+
g+, g+
g+, g+
g+, g+
69
36
γ-amino esters depends on the two relatively stable stereo con- absorbance (νmax) are quoted in wavenumbers (cm−1). 1H NMR
formers of the unsaturated esters as well as the steric effects and 13C NMR spectra were recorded on 400 MHz/500 MHz and
of the amino acid side-chains. In addition amide NH plays on 100 MHz/125 MHz respectively, using residual solvents as
a crucial role in dictating the high diastereoselectivity in an internal standards (CDCl3 and DMSO). Chemical shifts (δ)
H–Cγ–CβvCα eclipsed conformer by stabilizing the nucleo- reported in parts per million (ppm) and coupling constants (J)
philic approach through intermolecular H-bonding. The C9 reported in Hz. Mass spectra were recorded by MALDI TOF/
helical signature of the dipeptide and the ability of nitro- TOF and Electron Spray Ionization (ESI). Diastereomeric ratios
alkane’s diverse functional group transformations offer the were determined by Reverse Phase HPLC using an analytical
glimpse of the potential to generate a new class of peptide fol-
C18 column (5 μm, 4.6 × 250 mm), MeOH–H2O = 75 : 25 (except
damers with diverse functional group transformations from for 2i + 3i mixture, MeOH–H2O = 50 : 50) as a mobile phase
β-nitromethane substituted γ-amino acids.
with flow rate 0.75 mL min−1
.
General procedure for synthesis of vinylogous amino esters (1)
The N-protected amino aldehyde (10 mmol) was dissolved in
dry THF (40 mL) under the N2 atmosphere. To this solution
Wittig ylide (11.5 mmol) was added. The progress of the reac-
Experimental section
General experimental details
Solvent THF was distilled from sodium/benzophenone. tion was monitored by TLC. After the completion of the reac-
Column chromatography was performed on silica gel tion (8 h), THF was evaporated and the product was purified
(120–200 mesh). N-Protected amino aldehydes and Wittig by column chromatography using a 5 : 95 ethyl acetate–pet
ylides were synthesized using the reported procedures.8a IR ether solvent system.
spectra were recorded on an FT-IR spectrophotometer by using
(S,E)-Ethyl-4-(tert-butoxycarbonylamino)-5-methylhex-2-enoate
KBr pellets and NaCl plates. Wavelengths of maximum (1a).8a White solid (yield 2.43 g, 90%); mp 59 °C; UV (λmax
)
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1
216 nm; [α]2D5 −3.40 (c 1.0, MeOH); H NMR (400 MHz, CDCl3) 14.3; MALDI TOF/TOF m/z value for C18H25NO4 [M + Na+]
δ 6.85–6.81 (dd, 1H, J = 15.8 Hz, J = 5.2 Hz, –CHvCH–COOEt), 342.1681 (calcd), 342.1657 (found).
5.92–5.88 (d, 1H, J = 15.6 Hz, –CHvCH–COOEt), 4.55 (d, 1H,
(R,E)-Ethyl-5-tert-butoxy-4-(tert-butoxycarbonylamino)pent-
J = 6.8 Hz, NH), 4.18–4.16 (q, J = 6.88 Hz, 2H, –OCH2), 2-enoate (1f).8a Colorless oil (2.55 g, 81%); UV (λmax) 211 nm;
1.86–1.84 (m, 1H, –CH–CHvCH), 1.42 (s, 9H, Boc), 1.29–1.25 [α]2D5 +4.4 (c 1.0, MeOH); 1H NMR (400 MHz, CDCl3), δ 6.87
(t, J = 7.32 Hz, 3H, –OCH2–CH3), 0.93–0.88 (q, 6H, J = 6.4 Hz, (dd, J = 15.6 Hz, J = 5.5 Hz, 1H, –CH–CHvCH–), 5.90 (dd, J =
C(CH3)2); 13C NMR (100 MHz, CDCl3); 166.3, 155.4, 147.4, 15.8 Hz, J = 1.6 Hz, 1H, –CH–CHvCH–), 4.99 (br, 1H, NH),
121.5, 79.7, 60.5, 56.7, 32.3, 28.4, 18.9, 17.0, 14.3; MALDI TOF/ 4.33 (br, 1H, –CH–CHvCH–), 4.14 (q, J = 7.3 Hz, 2H, –OCH2–),
TOF m/z calcd for C14H25NO4 [M + Na+] 294.1681, observed 3.42 (d, J = 4.1 Hz, 2H, CH2–OtBu), 1.40 (s, 9H, Boc), 1.23 (t,
294.1686.
3H, J = 7.1 Hz, –OCH2–CH3), 1.11 (s, 9H, OtBu); 13C NMR
(4S,5R,E)-Ethyl-4-(tert-butoxycarbonylamino)-5-methylhept- (100 MHz, CDCl3) δ 166.3, 155.3, 146.9, 121.7, 79.7, 73.4, 63.2,
2-enoate (1b).8a White solid (yield 2.62 g, 92%); mp 62 °C; UV 60.4, 51.7, 28.4, 27.4, 14.2; MALDI TOF/TOF m/z value for
1
(λmax) 216 nm; [α]2D5 −11.20 (c 1.0, MeOH); H NMR (400 MHz, C16H29NO5 [M + Na+] 338.1943 (calcd), 338.1904 (found).
CDCl3) δ 6.87 (d, 1H, J = 15.6 Hz, –CHvCH–COOEt), 5.92 (d,
(S,E)-6-tert-Butyl-1-ethyl 4-((tert-butoxycarbonyl)amino)hex-
1H, J = 15.6 Hz, –CHvCH–COOEt), 4.58 (dd, 1H, J = 17.9 Hz, 2-enedioate (1g). White solid (2.9 g, 80%); mp 68 °C; UV (λmax
)
J = 9.2 Hz, –CH–CHvCH–), 4.32 (br, 1H, NH), 4.2 (q, J = 206 nm; [α]2D5 −10.0 (c 1.0, MeOH); H NMR (CDCl3, 400 MHz)
7.3 Hz, 2H, –OCH2), 1.66 (m, 2H, –CH–CH2–CH3), 1.45 (s, 9H, δ 6.86 (dd, 1H, J = 15.6 Hz, J = 5 Hz, CHvCH–COOEt), 5.92
Boc), 1.29 (t, J = 7.1 Hz, 3H, –OCH2–CH3), 1.14 (m, 1H, –CH– (dd, 1H, J = 15.6 Hz, J = 1.8 Hz, CHvCH–COOEt), 5.3 (d, 1H,
CH2–CH3), 0.91–0.86 (m, 6H, 2 × –CH3); 13C NMR (100 MHz, J = 8.2 Hz, NH), 4.63 (m, 1H, –CH–CHvCH–), 4.17 (q, 2H, J =
CDCl3) δ 166.3, 155.3, 147.1, 121.6, 79.6, 60.4, 55.7, 39.0, 7.2 Hz, –OCH2CH3), 2.53 (m, 2H, –CH2COOtBu), 1.42 (two s,
1
t
28.4, 25.3, 15.3, 14.2, 11.6; MALDI TOF/TOF m/z value for 18H, Boc and Bu), 1.26 (t, 3H, J = 7.1 Hz, –OCH2CH3); 13C
C15H27NO4 [M + Na+] 308.1838 (calcd), 308.1837 (found).
NMR (CDCl3, 100 MHz) δ 169.0, 166.1, 154.9, 146.8, 121.5,
(S,E)-Ethyl-4-(tert-butoxycarbonylamino)-6-methylhept-2-enoate 81.8, 79.9, 60.5, 48.3, 39.8, 28.4, 28.4, 14.3; MALDI TOF/TOF
(1c).8a White solid (2.70 g, 95%); mp 55 °C; UV (λmax) 218 nm; m/z value for C17H29NO6 [M + Na+] 366.1887 (calcd), 366.1813
[α]2D5 −25.50 (c 1.0, MeOH); 1H NMR (500 MHz, CDCl3) (found).
δ 6.85–6.81 (dd, J = 16 Hz, J = 5.5 Hz, 1H, CHvCHCO2Et),
(S,E)-Ethyl-4-(tert-butoxycarbonylamino)pent-2-enoate (1h).8a
5.93–5.90 (d, 1H, J = 16 Hz, CHvCHCO2Et), 4.45 (br, 1H, NH), Colorless oil (yield 2.25 g, 93%); UV (λmax) 216 nm; [α]2D5 −20.8
4.33 (br, 1H, –CH–CHvCH), 4.21–4.17 (q, 2H, J = 7 Hz, (c 1.0, MeOH); 1H NMR (400 MHz, CDCl3) δ 6.87–6.82 (dd, 1H,
–OCH2), 1.72–1.67 (m, 1H, CH–(CH3)2), 1.44 (s, 9H, Boc), J = 15.6 Hz, J = 4.1 Hz, –CHvCH–COOEt), 5.89–5.85 (d, 1H, J =
1.40–1.37 (t, 2H, J = 7 Hz, CH2CH–(CH3)2), 1.30–1.27 (t, J = 15.6 Hz, –CHvCH–COOEt), 4.5 (br, 1H, NH), 4.38 (m, 1H,
7 Hz, 3H, –OCH2CH3), 0.94–0.93 (d, J = 6.5 Hz, 6H, CH– –CH–CHvCH–), 4.16 (q, 2H, J = 7.3 Hz, –OCH2), 1.432 (s, 9H,
(CH3)2); 13C NMR (100 MHz, CDCl3) δ 166.5, 155.1, 148.0, Boc), 1.265–1.247 (m, 6H, –CH(CH3)2); 13C NMR (100 MHz,
120.4, 79.7, 60.5, 49.8, 43.9, 28.4, 24.7, 22.7, 22.2, 14.3; MALDI CDCl3) δ 166.4, 154.9, 154.9, 120.2, 79.8, 60.5, 47.0, 28.4, 20.4,
TOF/TOF m/z value for C15H27NO4 [M + Na+] 308.1838 (calcd), 14.3; MALDI TOF/TOF m/z value for C12H21NO4 [M + Na+]
308.1840 (found).
266.1368 (calcd), 266.1365 (found).
(R,E)-Ethyl-4,5-bis((tert-butoxycarbonyl)amino)pent-2-enoate
(S,E)-tert-Butyl-2-(3-ethoxy-3-oxoprop-1-enyl)pyrrolidine-
(1d). White solid (2.66 g, 70%); mp 120 °C; UV (λmax) 218 nm; 1-carboxylate (1i).8a Colorless oil (2.23 g, 83%); UV (λmax
)
[α]2D5 −3.0 (c 1.0, MeOH); 1H NMR (CDCl3, 400 MHz) δ 6.82 (dd, 214 nm; [α]D25 −72.0 (c 1.0, MeOH); H NMR (400 MHz, CDCl3)
1H, J = 15.8 Hz, J = 5.3 Hz, –CHvCH–COOEt), 5.95 (dd, 1H, J = δ 6.83–6.78 (d, J = 15.6 Hz, 1H, –CH–CHvCH–), 5.83–5.80 (d,
15.8 Hz, J = 1.8 Hz, –CHvCH–COOEt), 5.28 (bs, 1H, NH), 4.86 J = 15.2 Hz, 1H, –CH–CHvCH–), 4.51–4.37 (br, 1H, –CH–
(bs, 1H, NH), 4.36 (m, 1H, –CH–CHvCH–), 4.16 (q, 2H, J = CHvCH–), 4.21–4.17 (q, J = 6.4 Hz, 2H, –OCH2), 3.44–3.43 (t,
7.3 Hz, –OCH2CH3), 3.28 (m, 2H, –CH–CH2–NHBoc), 1.4 (bs, J = 6 Hz, 2H, Boc-N–CH2), 2.14–2.05 and 1.89–1.82 (m, 4H,
18H, 2 Boc), 1.25 (t, 3H, J = 7.1 Hz, –OCH2CH3); 13C NMR –CH2–CH2–), 1.421 (s, 9H, Boc), 1.31–1.27 (t, J = 6.8 Hz, 3H,
δ 166.1, 156.7, 155.4, 145.4, 122.3, 79.9, 60.6, 52.8, 43.6, 28.4, CH3); 13C NMR (100 MHz, CDCl3) δ 166.4, 154.3, 148.5, 120.4,
28.3, 28.0, 14.2; MALDI TOF/TOF m/z value for [M + Na+] 79.6, 60.3, 57.8, 46.2, 31.7, 28.3, 22.9, 14.2; MALDI TOF/TOF
1
C17H30N2O6 381.1996 (calcd), 381.1979 (found).
(S,E)-Ethyl-4-((tert-butoxycarbonyl)amino)-5-phenylpent-2-enoate (found).
m/z value for C14H23NO4 [M + Na+] 292.1525 (calcd), 292.1520
(1e). White powder (4.01 g, 84%); mp 70 °C; UV (λmax) 206 nm,
(S,E)-Benzyl-4-((tert-butoxycarbonyl)amino)-5-methylhex-
1
255 nm; [α]2D5 −2.0 (c 1.0, MeOH); H NMR (400 MHz, CDCl3) 2-enoate (1j). White solid (2.84 g, 82%); mp 83 °C; UV (λmax
)
δ 7.30–7.14 (m, 5H, aromatic), 6.91–6.86 (dd, J = 5.04 Hz, J = 217 nm, 314 nm; [α]2D5 −19.0 (c 1.0, MeOH); H NMR (CDCl3,
11 Hz, 1H, –CHvCHCO2Et), 5.85–5.81 (d, J = 17.4, 1H, 500 MHz) δ 7.34 (m, 5H, aromatic), 6.91 (d, 1H, J = 15.3 Hz,
–CHvCHCO2Et), 4.59 (br, 1H, NH), 4.52 (m, 1H, –CH– –CHvCH–COOBz), 5.97 (d, 1H, J = 15.9 Hz, –CHvCH–
CHvCH), 4.18–4.13 (q, J = 6.8 Hz, 2H, –OCH2), 2.92–2.85 (m, COOBz), 5.17 (bs, 2H, –CH2–C6H5), 4.55 (d, 1H, J = 7.2 Hz,
2H, CH2–Ph), 1.37 (s, 9H, Boc), 1.27–1.23 (t, J = 7.3 Hz, 3H, NH), 4.19 (m, 1H, –CH–CHvCH–), 1.86 (m, 1H, –CH(CH3)2),
–OCH2CH3); 13C NMR (100 MHz, CDCl3) δ 166.2, 155.0, 147.6, 1.43 (s, 9H, Boc), 0.91 (dd, J = 16.2 Hz, J = 5.8 Hz, 6H,
136.4, 129.4, 128.6, 126.9, 121.1, 79.9, 60.5, 52.3, 40.9, 28.3, –CH(CH3)2); 13C NMR (CDCl3, 100 MHz) δ 166.1, 155.4, 148.2,
1
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135.9, 128.6, 128.3, 121.1, 79.8, 66.4, 56.7, 32.3, 28.4, 18.9, –OCH2–CH3); 13C NMR (100 MHz, CDCl3) δ 171.1, 156.3, 79.9,
18.0; MALDI TOF/TOF m/z value for [M + Na+] C19H27NO4 76.3, 61.0, 56.9, 36.6, 34.4, 29.3, 28.3, 20.1, 16.8, 14.2; MALDI
356.1832 (calcd), 356.1856 (found).
(S,E)-Benzyl-4-((tert-butoxycarbonyl)amino)-6-methylhept- 355.1829 (found).
2-enoate (1k). White solid (2.59 g, 70%); mp 60 °C; UV (λmax (3S,4S)-Ethyl-4-((tert-butoxycarbonyl)amino)-5-methyl-
TOF/TOF m/z value for C15H27N2O6 [M + Na+] 355.1840 (calcd),
)
1
217 nm, 314 nm; [α]D25 −33.0 (c 1.0, MeOH); H NMR (CDCl3, 3-(nitromethyl)hexanoate (3a)syn. Colorless oil; yield (0.08 g,
400 MHz) δ 7.3 (m, 5H, aromatic), 6.89 (dd, 1H, J = 15.6 Hz, J = 11%); [α]2D5 −9.0 (c 1.0, MeOH); IR ν cm−1 3370, 2988, 2870,
5 Hz, –CHvCH–COOBz), 5.97 (dd, 1H, J = 15.8 Hz, J = 1.6 Hz, 1720, 1555, 1399, 1234, 1190, 1113, 1020; 1H NMR (CDCl3,
–CHvCH–COOBz), 5.16 (m, 2H, –CH2C6H5), 4.69 (d, 1H, J = 400 MHz) δ 4.53–4.39 (m, 3H, –CH2NO2 and NH), 4.13 (q, J =
8.2 Hz, NH), 4.35 (m, 1H, –CH–CHvCH–), 1.67 (m, 1H, 7.3 Hz, 2H, –OCH2CH3), 3.51–3.45 (m, 1H, γ CH), 2.98–2.9
–CH(CH3)2), 1.43 (s, 9H, Boc), 1.36 (t, J = 7.3 Hz, 2H, –CH2–CH- (m, 1H, β CH), 2.47–2.30 (m, 2H, α CH2), 1.77–1.69 (m, 1H,
(CH3)2), 0.92 (d, 6H, J = 6.9 Hz, –CH(CH3)2); 13C NMR (CDCl3, –CH(CH3)3), 1.4 (s, 9H, Boc), 1.25 (t, J = 7.1 Hz, 3H,
100 MHz) δ 166.2, 155.2, 149.8, 135.9, 128.6, 128.3, 128.3, –OCH2CH3), 0.98–0.91 (m, 6H, –C(CH3)3); 13C NMR (CDCl3,
120.0, 79.6, 66.3, 49.8, 43.7, 28.4, 24.4, 22.8, 22.2; MALDI TOF/ 100 MHz) δ 172.1, 156.1, 79.7, 76.8, 61.1, 56.9, 36.1, 32.3,
TOF value for [M + Na+] C20H29NO4 370.1989 (calcd), 370.1960 30.1, 28.3, 28.1, 19.9, 18.1, 14.1; MALDI TOF/TOF m/z value
(found).
(S,E)-Benzyl-4-((tert-butoxycarbonyl)amino)-5-phenylpent- (observed).
2-enoate (1l). White solid (3.15 g, 78%); mp 80 °C; UV (λmax (3R,4S,5S)-Ethyl-4-(tert-butoxycarbonylamino)-5-methyl-
for C19H28N2O6 [M +
Na+] 355.1840 (calcd), 355.1829
)
207 nm, 314 nm; [α]2D5 −1.0 (c 1.0, MeOH); 1H NMR (CDCl3, 3-(nitromethyl)heptanoate (2b)anti. Yellow oil (0.56 g, 83%, dr
500 MHz) δ 7.25 (m, 10H, aromatic), 6.96 (d, J = 15.6 Hz, 1H, 83 : 17); [α]2D5 +13.0 (c 1.0, MeOH); the dr determined by a
–CHvCH–COOBz), 5.91 (d, J = 15.9 Hz, 1H, –CHvCHOOBz), Reverse Phase HPLC analytical C18 column (5 μm, 4.6 ×
5.16 (s, 2H, –CH2–C6H5), 4.62 (br, 1H, NH), 4.54 (br, 1H, –CH– 250 mm), flow rate 0.75 mL min−1, Rt (major, 2banti) =
CH2–Phe), 2.88 (m, 2H, –CH–CH2–Phe), 1.38 (s, 9H, Boc); 24.9 min, Rt (minor, 3bsyn) = 23.2 min; IR ν (cm−1) 3370, 2970,
13C NMR (CDCl3, 100 MHz) δ 166.0, 155.0, 148.3, 136.4, 135.9, 2930, 1700, 1550, 1520, 1380, 1250, 1170, 1030; 1H NMR
129.4, 128.6, 128.2, 126.9, 120.7, 79.9, 66.3, 52.3, 40.9, 28.3, (400 MHz, CDCl3) δ 4.54–4.41 (m, 2H, –CH2–NO2), 4.28 (d, J =
28.0; MALDI TOF/TOF m/z value for [M + Na+] C23H27NO4 10.5 Hz, 1H, NH), 4.12 (q, J = 8 Hz, 2H, –OCH2–CH3), 3.58 (m,
404.1832 (calcd), 404.1885 (found).
1H, γ CH), 2.91 (m, 1H, β CH), 2.45 (m, 2H, α CH), 1.51 (m,
Synthesis of N-protected β-nitromethane substituted 2H, –CH–CH2–CH3), 1.4 (s, 9H, Boc), 1.24 (m, 4H, –OCH2–CH3
γ-amino esters through the Michael addition of nitromethane and –CH–), 0.92 (m, 6H, –CH2–CH3 and CH3); 13C NMR
to N-protected α,β-unsaturated γ-amino esters (2 + 3). The (100 MHz, CDCl3) δ 171.4, 155.9, 79.7, 76.1, 61.0, 50.2, 42.0,
N-Boc α,β-unsaturated γ-amino ester 1 (2.0 mmol) was dis- 39.4, 34.2, 28.3, 25.0, 23.4, 21.5, 14.1; MALDI TOF/TOF m/z
solved in neat nitromethane (10.0 mmol). To this solution value for C16H30N2O6 [M + Na+] 369.2002 (calcd), 369.2072
DBU (2.0 mmol) was added under a N2 atmosphere. The reac- (found).
tion mixture was stirred at room temperature for about 2.5–3 h
(3S,4S,5R)-Ethyl-4-((tert-butoxycarbonyl)amino)-5-methyl-
and the progress of the reaction was monitored by TLC. After 3-(nitromethyl)heptanoate (3b)syn. Light yellow oil; yield
completion of the reaction, EtOAc (100 mL) was added to the (0.12 g, 17%); [α]2D5 −17.0 (c 1.0, MeOH); IR ν cm−1 3380, 2970,
reaction mixture, and washed with H2O (3 × 50 mL), 1 N HCl 2944, 1717, 1563, 1400, 1277, 1190, 1020; 1H NMR (CDCl3,
(2 × 50 mL) and brine (2 × 50 mL) solution. Then the organic 400 MHz) δ ppm 4.56–4.41 (m, 3H, NH and –CH2NO2), 4.16 (q,
layer was dried over anhydrous Na2SO4 and concentrated J = 7.3 Hz, 2H, –OCH2CH3), 3.93 (m, 1H, γ CH), 3.05–2.93 (m,
under reduced pressure. The crude product was purified 1H, β CH), 2.43–2.31 (m, 2H, –CH2NO2), 1.60–1.52 (m, 1H,
by silica gel column chromatography to get the pure diastereo- –CH(CH3)CH2CH3), 1.43 (s, 9H, Boc), 1.27 (t, J = 7.1 Hz,
mers (2 and 3). The ratio of diastereoselectivities was calcu- –OCH2CH3), 0.93–0.89 (m, 6H); 13C NMR (CDCl3, 100 MHz)
lated using Reverse Phase HPLC chromatograms, 1H NMR 172.0, 156.0, 79.7, 76.7, 61.1, 55.8, 36.6, 35.9, 32.1, 28.2,
integrals or from the isolated yields.
(3R,4S)-Ethyl-4-(tert-butoxycarbonylamino)-5-methyl- C19H28N2O6 [M + Na+] 369.2002 (calcd), 369.2072 (observed).
3-(nitromethyl)hexanoate (2a)anti. Light yellow oil (0.57 g,
(3R,4S)-Ethyl-4-(tert-butoxycarbonylamino)-6-methyl-
28.0, 24.8, 16.0, 14.1, 11.1; MALDI TOF/TOF m/z value for
89%, dr 89 : 11); [α]D25 +35.0 (c 1.0, MeOH); the dr determined 3-(nitromethyl)heptanoate (2c)anti. White solid (0.55 g, 82%,
by a Reverse Phase HPLC analytical C18 column (5 μm, 4.6 × dr 82 : 18); mp 67 °C; [α]2D5 −148.0 (c 1.0, MeOH); the dr deter-
250 mm), flow rate 0.75 mL min−1, Rt (major, 2aanti) = mined by a Reverse Phase HPLC analytical C18 column (5 μm,
33.3 min, Rt (minor, 3asyn) = 32.3 min; IR ν (cm−1) 3360, 2970, 4.6 × 250 mm), flow rate 0.75 mL min−1, Rt (major, 2canti) =
1
2930, 1710, 1550, 1510, 1380, 1240, 1170, 1100, 1030; H NMR 33.3 min, Rt (minor, 3csyn) = 32.3 min; IR ν (cm−1) 3370, 2980,
(400 MHz, CDCl3) δ 1.4 (s, 9H, Boc), 4.35 (d, J = 10.1, 1H, NH), 2940, 1710, 1550, 1520, 1380, 1250, 1160, 1010, 904, 852, 764,
3.60–3.54 (m, 1H, γ CH), 1.87–1.81 (m, 1H, –CH(CH3)2), 0.98 629; 1H NMR (400 MHz, CDCl3) δ 4.47 (m, 2H, –CH2–NO2),
(d, J = 6.9 Hz, 6H, CH3), 0.88 (d, J = 6.9 Hz, 6H, CH3), 2.92–2.84 4.27 (d, J = 9.2 Hz, 1H, NH), 4.14 (q, J = 8 Hz, 2H, –OCH2–CH3),
(m, 1H, β CH), 4.55–4.44 (m, 2H, CH2–NO2), 2.53–2.38 (m, 2H, 3.79 (m, 1H, γ CH), 2.75 (m, 1H, β CH), 2.45 (m, 2H, α CH2),
α CH2), 4.16 (q, J = 8 Hz, 2H, –OCH2–), 1.23 (t, J = 8 Hz, 3H, 1.63 (m, 2H, CH2), 1.4 (s, 9H, Boc), 1.33 (m, 1H, CH), 1.24
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(t, J = 7.3 Hz, 3H, –OCH2–CH3), 0.91 (d, J = 6.9 Hz, 3H, CH3), m/z value for C19H28N2O6 [M + Na+] 403.1840 (calcd), 403.1816
0.87 (d, J = 6.9 Hz, 3H, CH3); 13C NMR (100 MHz, CDCl3) δ (observed).
171.2, 156.3, 79.3, 76.2, 60.9, 56.6, 36.4, 36.1, 34.6, 28.3, 23.7,
16.3, 14.1, 11.4; MALDI TOF/TOF m/z value for C16H30N2O6 3-(nitromethyl)pentanoate (2f)anti. Yellow oil (0.63 g, 84%, dr
[M + Na+] 369.2002 (calcd), 369.2062 (found). 100 : 0); [α]2D5 +131.0 (c 1.0, MeOH); the dr determined by a
(3R,4R)-Ethyl-5-tert-butoxy-4-(tert-butoxycarbonylamino)-
(3S,4S)-Ethyl-4-((tert-butoxycarbonyl)amino)-6-methyl- Reverse Phase HPLC analytical C18 column (5 μm, 4.6 ×
3-(nitromethyl)heptanoate (3c)syn. Colorless oil; yield (0.12 g, 250 mm), flow rate 0.75 mL min−1, Rt (single isomer, 2fanti) =
18%); [α]2D5 −30.0 (c 1.0, MeOH); IR ν cm−1 3380, 2990, 1730, 25.3 min; IR ν (cm−1) 3380, 2970, 2930, 2800, 1720, 1550, 1500,
1560, 1523, 1387, 1277, 1182, 1030; 1H NMR (CDCl3, 400 MHz) 1370, 1240, 1170, 1090, 1020, 874, 816; 1H NMR (400 MHz,
δ 4.59–4.42 (m, 3H, NH and –CH2NO2), 4.16 (q, J = 7.3 Hz, 2H, CDCl3) δ 4.95 (d, J = 9.2 Hz, 1H, NH), 4.57 (m, 2H, –CH2–NO2),
–OCH2CH3), 3.81–3.84 (m, 1H, γ CH), 2.86–2.79 (m, 1H, β CH), 4.12 (q, J = 7.3 Hz, 2H, –OCH2–CH3), 3.88 (m, 1H, γ CH), 3.45
2.51–2.33 (m, 2H, α CH2), 1.69–1.62 (m, 1H, –CH(CH3)3), 1.40 (m, 2H, CH2–OtBu), 2.91 (m, 1H, β CH), 2.49 (m, 2H, α CH2),
(s, 9H, Boc), 1.29–1.26 (t, J = 7.1 Hz, 3H, –OCH2CH3), 0.96–0.91 1.41 (s, 9H, Boc), 1.24 (t, J = 7.2 Hz, 3H, –OCH2–CH3), 1.14
(m, 6H, –CH(CH3)3); 13C NMR (CDCl3, 100 MHz) 171.9, 155.7, (s, 9H, tBu); 13C NMR δ 171.4, 155.8, 79.9, 73.5, 62.1,
79.7, 76.6, 61.0, 49.7, 42.1, 38.8, 32.3, 28.2, 28.1, 24.9, 60.9, 50.9, 37.0, 33.9, 28.3, 27.3, 14.2; MALDI TOF/TOF m/z
23.1, 21.7, 14.1; MALDI TOF/TOF m/z value for C19H28N2O6 value for C17H32N2O7 [M + Na+] 399.2102 (calcd), 399.2140
[M + Na+] 369.2002 (calcd), 369.2062 (observed).
Ethyl-4,5-bis((tert-butoxycarbonyl)amino)-3-(nitromethyl)-
(found).
(3R,4S)-1-tert-Butyl-6-ethyl 3-(tert-butyloxycarbonylamino)-
pentanoate (crude mixture, 2d + 3d, inseparable diastereo- 4-(nitromethyl)hexanedioate (2g)anti. White solid (0.66 g, 82%,
mers). White solid (0.69 g, 87%, dr 90 : 10); the dr determined dr 81 : 19); mp 86 °C; [α]2D5 −3.0 (c 1.0, MeOH); the dr deter-
by a Reverse Phase HPLC analytical C18 column (5 μm, 4.6 × mined by a Reverse Phase HPLC analytical C18 column (5 μm,
250 mm), flow rate 0.75 mL min−1, Rt (major, 2danti) = 4.6 × 250 mm), flow rate 0.75 mL min−1, Rt (major, 2ganti) =
1
25.96 min, Rt (minor, 3dsyn) = 25.26 min; H NMR (500 MHz, 30.0 min, Rt (minor, 3gsyn) = 26.7 min; IR ν (cm−1) 3350, 2980,
CDCl3) 5.19–5.06 (br, 2NH), 4.61–4.48 (m, 2H, –CH2NO2), 2930, 1700, 1550, 1520, 1380, 1240, 1170, 1030, 860, 775; 1H
4.17–4.13 (m, 2H, –OCH2CH3), 3.83–3.78 (m, 1H, γ CH), 3.29 NMR (500 MHz, CDCl3) δ 5.19 (d, J = 9.2 Hz, 1H, NH), 4.55 (m,
(m, 2H, –CH2NHBoc), 2.85–2.83 (m, 1H, β CH), 2.62–2.44 (br 2H, –CH2–NO2), 4.14 (m, 2H, –OCH2–), 4.04 (m, 1H, γ CH), 2.9
m, 2H, α CH2), 1.41 (br m, 18H, 2Boc), 1.27–1.24 (br m, 3H, (m, 1H, β CH), 2.48 (m, 4H, α and δ CH2’s), 1.45 (s, 9H, Boc),
t
–OCH2CH3); 13C NMR (100 MHz, CDCl3) δ 171.9, 171.4, 156.9, 1.4 (s, 9H, Bu),1.26 (t, J = 5.6 Hz, 3H, –OCH2–CH3); 13C NMR
156.4, 80.0, 76.1, 61.1, 60.5, 53.1, 52.8, 42.3, 42.1, 36.4, 34.1, (100 MHz, CDCl3) δ 170.1, 170.2, 155.5, 81.9, 80.1, 78.1, 61.1,
31.9, 31.0, 29.7, 29.4, 28.3, 28.3, 22.7, 14.7. MALDI TOF/TOF 49.0, 38.2, 34.1, 28.3, 28.0, 14.1; HRMS m/z value for
m/z value for C13H24N2O6 [M + Na+] 419.2160 (calcd), 419.2174 C18H32N2O8 [M + Na+] 427.2051 (calcd), 427.2063 (found).
(found).
Ethyl-4-((tert-butoxycarbonyl)amino)-3-(nitromethyl)pentanoate
(3R,4S)-Ethyl-4-(tert-butoxycarbonylamino)-3-(nitromethyl)- (crude mixture, 2h + 3h, inseparable diastereomers). Light
5-phenyl pentanoate (2e)anti. White solid (0.6 g, 80%, dr yellow oil (0.560 g, 92%, dr 80 : 20); the dr determined by NMR
80 : 20); mp 117 °C; [α]2D5 −114.2 (c 1.0, MeOH); the dr deter- analysis; 1H NMR (500 MHz, CDCl3) 4.58–4.44 (m, 2H,
mined by isolated products; IR ν (cm−1) 3370, 2970, 2930, –CH2NO2), 4.18–4.14 (m, 2H, –OCH2–CH3), 3.92–3.82 (m, 1H, γ
1700, 1550, 1510, 1380, 1250, 1170, 1030, 860, 766; 1H NMR CH), 2.84–2.77 (m, 1H, β CH), 2.56–2.38 (m, 2H, α CH2), 1.44
(400 MHz, CDCl3) δ 7.23 (m, 5H, aromatic), 4.54 (m, 2H, –CH2– (br, 9H, Boc), 1.28–1.26 (m, 3H, –OCH2CH3), 1.22–1.21 (br, 3H,
NO2), 4.34 (d, J = 8.24 Hz, 1H, NH), 4.15 (q, J = 7.2 Hz, 2H, side chain –CH3); 13C NMR (100 MHz, CDCl3) δ 171.9, 171.4,
–OCH2–CH3), 3.99 (m, 1H, γ CH), 2.91 (m, 2H, CH2–Phe), 2.74 155.4, 79.8, 79.5, 76.3, 76.2, 60.9, 60.3, 47.5, 47.3, 39.8, 39.4,
(m, 1H, β CH), 2.55 (m, 2H, α CH2), 1.29 (s, 9H, Boc), 1.26 33.8, 32.7, 28.6, 18.5, 17.7, 14.0; MALDI TOF/TOF m/z value for
(t, J = 7.32 Hz, 3H, –OCH2–CH3); 13C NMR (100 MHz, CDCl3) C13H24N2O6 [M + Na+] 327.1527 (calcd), 327.1554 (found).
δ 171.2, 155.6, 137.0, 129.1, 128.7, 126.9, 79.9, 76.0, 61.1, 53.1,
38.8, 38.3, 34.3, 28.2, 14.2; MALDI TOF/TOF m/z value for 1-carboxylate (crude mixture, 2i + 3i, inseparable diastereo-
C19H28N2O6 [M + Na+] 403.1840 (calcd), 403.1816 (found).
mers). Yellow oil (0.6 g, 91%, dr 61 : 49); the dr determined by
tert-Butyl-2-(4-ethoxy-1-nitro-4-oxobutan-2-yl)pyrrolidine-
(3S,4S)-Ethyl-4-((tert-butoxycarbonyl)amino)-3-(nitromethyl)- a Reverse Phase HPLC analytical C18 column (5 μm, 4.6 ×
5-phenyl pentanoate (3e)syn. White solid (0.15 g, 20%); mp 250 mm), flow rate 0.75 mL min−1, Rt (major, 2ianti) =
120 °C; [α]2D5 −15.0 (c 1.0, MeOH); the dr determined by iso- 52.3 min, Rt (minor, 3isyn) = 47.9 min; 1H NMR (400 MHz,
lated products; IR ν (cm−1) 3374, 2980, 2930, 1718, 1691, 1553, DMSO)
δ 4.53 (br, 2H, CH2NO2), 4.07–3.93 (m, 2H,
1520, 1378, 1249, 1175, 1022, 701, 605; 1H NMR (CDCl3, –OCH2CH3), 3.73 (br, 1H, γ CH), 3.32–3.19 (m, 2H, –CH2NBoc),
400 MHz) δ 7.33–7.18 (m, 5H, aromatic), 4.59–4.51 (m, 3H, NH 3.02–2.95 (m, 1H, β CH), 2.31–2.19 (m, 2H, α CH2), 1.69 (br,
and –CH2NO2), 4.18 (q, J = 7.2 Hz, 2H, –OCH2CH3), 4.09 (m, 4H, –CH2–CH2–), 1.36 (br, 9H, Boc), 1.15–1.52 (m, 3H,
1H, γ CH), 2.94–2.88 (m, 1H, β CH), 2.58–2.53 (m, 2H, α CH2), –OCH2CH3); 13C NMR (100 MHz, DMSO) δ 170.8, 154.9, 154.8,
1.33–1.27 (m, 12H, Boc and –OCH2CH3); 13C NMR (CDCl3, 154.1, 79.4, 79.2, 77.6, 77.39, 60.7, 58.6, 58.2, 47.2, 37.4, 33.2,
100 MHz) 171.9, 155.6, 136.9, 129.0, 128.7, 126.9, 79.9, 74.7, 32.2, 28.5, 27.2, 24.0, 23.5, 14.5. MALDI TOF/TOF m/z value for
61.2, 52.7, 39.2, 37.7, 32.4, 29.7, 28.2, 14.2; MALDI TOF/TOF C15H26N2O6 [M + Na+] 353.1683 (calcd), 353.1686 (found).
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(3R,4S)-Benzyl-4-((tert-butoxycarbonyl)amino)-5-methyl- by column chromatography using 2 : 98 ethyl acetate–pet ether
3-(nitromethyl)hexanoate (2j)anti. Colorless oil (dr 86 : 14); [α]2D5 solvent systems.
−42.0 (c 1.0, MeOH); the dr determined by a Reverse Phase
Boc2N-dgF-OEt (4). Colorless liquid; yield 20%; [α]2D5 −29.0
HPLC analytical C18 column (5 μm, 4.6 × 250 mm), flow rate (c 1.0, MeOH); UV (λmax) 218 nm, 256 nm; 1H NMR δ (ppm)
0.75 mL min−1, Rt (major, 2janti) = 28.9 min, Rt (minor, 3jsyn) = 7.29–7.18 (m, 5H, aromatic), 7.10 (dd, J = 16 Hz, J = 5 Hz, 1H,
26.4 min; IR ν (cm−1) 3380, 2970, 2390, 1710, 1550, 1510, 1380, α CH), 5.91 (dd, J = 15.8 Hz, J = 2.1 Hz, 1H, β CH), 5.21–5.15
1240, 1170, 1150, 966, 753, 689; 1H NMR (400 MHz, CDCl3) (m, 1H, γ CH), 4.19 (q, J = 7.3 Hz, 2H, –OCH2CH3), 3.19 (m,
δ 7.36–7.32 (m, 5H, aromatic), 5.13 (s, 2H, –CH2–Phe), 4.53–4.41 2H, –CH2C6H5), 1.41 (s, 18H, (Boc)2), 1.28 (t, J = 7.1 Hz, 3H,
(m, 2H, α CH2), 4.34 (d, J = 10.1 Hz, NH), 3.60–3.54 (m, 1H, –OCH2CH3); 13C NMR δ (ppm) 166.0, 152.1, 147.0, 137.3,
γ CH), 2.93–2.85 (m, 1H, β CH), 2.62–2.43 (m, 2H, –CH2–NO2), 129.3, 128.3, 126.5, 121.5, 82.5, 60.3, 57.9, 38.4, 27.8, 14.1;
1.84–1.77 (m, 1H, δ CH), 1.42 (s, 9H, Boc), 0.95 (d, J = 6.9 Hz, HR-MS m/z value for C23H33NO6 [M + Na+] 442.2200 (calcd),
3H, CH3), 0.88 (d, J = 6.4 Hz, 3H, CH3); 13C NMR (100 MHz, 442.2212 (observed).
CDCl3) δ 171.2, 156.5, 135.7, 128.8, 128.6, 127.2, 80.1, 76.0,
(Boc)2N-γF(β-CH2NO2)-OEt (5 + 6, diastereomeric mixture).
1
67.0, 57.1, 36.8, 34.6, 29.5, 28.5, 20.2, 17.1; MALDI TOF/TOF Light yellow liquid; yield 88%; the dr determined by H NMR
m/z value for C20H30N2O6 [M + Na+] 417.1996 (calcd), 417.1915 analysis; 1H NMR δ (ppm) 7.31–7.26 (m, 5H, Ar), 7.24–7.16 (m,
(observed).
5H, Ar′), 5.41–5.24 (m, 2H, –CH2NO2) and 4.75–4.39 (m, 2H,
(3R,4S)-Benzyl-4-((tert-butoxycarbonyl)amino)-6-methyl- –CH2NO2), 4.19–4.09 (m, 6H, 2 –OCH2CH3, 2 γ CH), 3.08–2.47
3-(nitromethyl)heptanoate (2k)anti. White solid (dr 81 : 19); mp (m, 10H, 2 β CH’s, 2 –CH2C6H5 and 2 –CH2COOEt), 1.51–1.49
85 °C; [α]2D5 −7.0 (c 1.0, MeOH); the dr determined by 1H NMR; (m, 18H, 2Boc), 1.30–1.25 (m, 28H, 2 Boc and 2 –OCH2CH3);
IR ν (cm−1) 3382, 2969, 2361, 1727, 1685, 1545, 1510, 1451, HR-MS m/z value for C24H36N2O8 [M + Na+] 503.2364 (calcd),
1381, 1255, 1156, 962, 746, 696; 1H NMR (500 MHz, CDCl3) 503.2346 (observed).
δ 7.36–7.31 (m, 5H, aromatic), 5.13 (s, 2H, –OCH2–Phe),
4.53–4.41 (m, 2H, –CH2NO2), 4.33 (br, 1H, NH), 3.82 (m, 1H,
General procedure for peptide synthesis
γ CH), 2.81–2.78 (m, 1H, β CH), 2.62–2.57 (m, 2H, α CH2), 1.63
Saponification of the ethyl ester of N-Boc-protected β-nitro-
(m, 1H, ω CH), 1.42 (s, 9H, Boc), 1.37–1.29 (m, 2H, δ CH2), methane substituted γ-amino acids. The Boc-γX(β-CH2NO2)-
0.91 (d, J = 6.4 Hz, 3H, CH3), 0.87 (d, J = 6.4 Hz, 3H, CH3); COOEt (3 mmol) was dissolved in 4 mL methanol followed by
13C NMR (100 MHz, CDCl3) δ 171.2, 155.9, 135.5, 128.6, 128.4, 8 mL 1 N NaOH was added slowly to the solution. The reaction
127.0, 79.8, 76.0, 66.8, 65.2, 60.4, 50.2, 42.1, 39.4, 28.3, 25.0, mixture was stirred for about 24 h. The progress of the reaction
23.4, 21.5, 21.1, 14.2; MALDI TOF/TOF m/z value for was monitored by TLC. After completion of the reaction,
C21H32N2O6 [M + Na+] 431.2153 (calcd), 431.2160 (observed).
methanol was evaporated. The aqueous layer was diluted with
(3R,4S)-Benzyl-4-((tert-butoxycarbonyl)amino)-3-(nitromethyl)- water (50 mL), acidified (pH ∼ 4) with 5% HCl and extracted
5-phenyl pentanoate (2l)anti. White solid (dr 89 : 11); mp with EtOAc (3 × 30 mL). The combined organic layer was
106 °C; [α]2D5 −112.0 (c 1.0, MeOH); the dr determined by a washed with brine solution (2 × 10 mL), dried over anhydrous
Reverse Phase HPLC analytical C18 column (5 μm, 4.6 × Na2SO4 and concentrated under reduced pressure to get free
250 mm), flow rate 0.75 mL min−1, Rt (major, 2lanti) = carboxylic acid. The carboxylic acids were directly used for the
37.6 min, Rt (minor, 3lsyn) = 35.7 min; IR ν (cm−1) 3370, 2978, peptide synthesis without further purification.
2929, 2363, 2342, 1726, 1693, 1542, 1509, 1364, 1248, 1147,
Deprotection of an N-Boc-group from ethyl ester of β-nitro-
1
1108, 982, 845, 754, 734, 702, 638; H NMR (400 MHz, CDCl3) methane substituted γ-amino acids. The solution of Boc-NH-
δ 7.37–7.12 (m, 10H, aromatic), 5.15 (s, 2H, –OCH2–Phe), γX(β-CH2NO2)-COOEt (3.6 mmol) in 3 mL of DCM was cooled
4.60–4.48 (m, 2H, –CH2–NO2), 4.36 (d, J = 9.2 Hz, 1H, NH), at 0 °C followed by addition of neat TFA. After completion of
4.05–3.98 (m, 1H, γ CH), 2.95–2.86 (m, 2H, –CH–CH2–Phe), the reaction (1 h), the solvent was evaporated under reduced
2.76–2.53 (m, 3H, β CH and α CH2), 1.30 (s, 9H, Boc); 13C NMR pressure. The residue was then treated with saturated Na2CO3
(100 MHz, CDCl3) δ 171.0, 155.6, 136.9, 135.4, 129.0, 128.7, solution in cold condition. The aqueous layer was extracted
128.5, 126.9, 80.0, 75.8, 66.9, 61.8, 38.9, 38.3, 34.3, 28.2; with EtOAc (3 × 40 mL). The combined organic layer was
HR-MS m/z value for C24H30N2O6 [M + Na+] 465.1996 (calcd), washed with brine solution, dried over anhydrous Na2SO4 and
465.1989 (observed).
concentrated under reduced pressure to 3 mL.
Coupling strategy. The solution of NH2-γX(β-CH2NO2)-
COOEt in EtOAc (3.6 mmol) was added to the solution of Boc-
γX(β-CH2NO2)-COOH in DMF (3.0 mmol) under ice cold con-
General procedure for the synthesis of di-Boc protected
vinylogous amino ester (4)
Di-tert-butyl dicarbonate (0.436 mL, 2 mmol), DMAP (0.122 g, ditions. The mixture was treated with DCC (3.0 mmol) and
1 mmol) and compound 1e were dissolved in dry acetonitrile HOBt (3.0 mmol). The reaction mixture was stirred for about
(2 mL). The resulting solution was stirred for about 6 h. After 12 h at room temperature and the completion of the reaction
that the reaction mixture was dissolved in 50 mL of EtOAc was monitored by TLC. After completion of the reaction, the
and washed with 1 M HCl (3 × 50 mL) and brine solution reaction mixture was diluted with EtOAc (100 mL) and DCU
(3 × 50 mL). Then the organic layer was dried over Na2SO4 and was filtered through the sintered funnel. The EtOAc layer
evaporated under reduced pressure. The product was purified was washed with brine (3 × 50 mL) followed by 5% aq. HCl
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(3 × 50 mL), 10% aq. Na2CO3 (1 × 50 mL), brine (3 × 50 mL) (m, 1H), 2.59–2.38 (m, 2H), 2.3 (d, J = 6.4 Hz, 2H), 1.66–1.43
and dried over anhydrous Na2SO4. The crude product was puri- (m, 4H), 1.40 (s, 9H), 1.23 (t, J = 7.1 Hz, 3H), 0.97–0.84
fied by column chromatography using EtOAc/pet ether to get (m, 12H); 13C NMR (100 MHz, CDCl3) δ 171.2, 156.8, 80.4,
the pure product.
76.0, 75.7, 60.9, 54.6, 49.9, 42.3, 40.1, 37.0, 36.2, 36.1, 34.8,
Boc-Ala-γPhe(β-CH2NO2)-COOEt (P1). White solid (0.94 g, 28.3, 25.3, 24.5, 23.0, 21.7, 16.2, 14.2, 11.2; MALDI TOF/TOF m/
78%); mp 108 °C; [α]2D0 −29.0 (c 1.0, MeOH); IR ν (cm−1) 3360, z value for C25H46N4O9 [M + Na]+ 569.3157 (calcd), 569.3146
3318, 2980, 1727, 1680, 1544, 1449, 1378, 1318, 1255, 1164, (found).
1068, 1027, 704, 602; 1H NMR (400 MHz, CDCl3) δ 7.30–7.15
(m, 5H, aromatic), 6.39 (d, J = 9.2 Hz, 1H, NH), 4.74 (d, J =
6 Hz, 1H, NH), 4.60–4.48 (m, 2H, –CH2NO2), 4.41–4.35 (m, 1H,
α Ala CH), 4.17 (q, J = 7 Hz, 2H, –OCH2–CH3), 3.99–3.93 (m,
1H, γ CH), 3.02–2.95 (m, 1H, β CH), 2.93–2.74 (m, 2H, α CH2),
2.65–2.46 (m, 2H, –CH2–Phe), 1.43 (s, 9H, Boc), 1.28–1.25 (t, J =
7.1 Hz, 3H, –OCH2CH3), 1.1 (d, J = 6 Hz, 3H, –CH3); 13C NMR
(100 MHz, CDCl3) δ 173.0, 171.2, 155.6, 136.9, 129.0, 128.7,
126.9, 79.2, 75.9, 61.1, 51.3, 50.7, 38.6, 38.3, 34.4, 28.3, 17.6,
14.2; MALDI TOF/TOF m/z value for C22H33N3O7 [M + Na+]
474.2211 (calcd); 474.2294 (observed).
Boc-Phe-γVal(β-CH2NO2)-COOEt, (P2). White solid (0.67 g,
70%); mp 110 °C; [α]2D0 −14.0 (c 1.0, MeOH); IR ν (cm−1) 3281,
2971, 2925, 1713, 1684, 1651, 1553, 1461, 1380, 1250, 1172,
1097, 960, 857, 754, 706; 1H NMR (400 MHz, CDCl3)
δ 7.32–7.21 (m, 5H, aromatic), 6.02 (d, J = 10.1 Hz, 1H, NH),
5.05 (d, J = 7.8 Hz, 1H, NH), 4.28 (q, J = 7.8 Hz, 1H, α Phe), 4.15
(q, J = 7.3 Hz, 2H, –OCH2CH3), 4.02 (m, 2H, –CH2NO2), 3.85
(m, 1H, γ CH), 3.06 (m, 2H, –CH2–Phe), 2.75 (m, 1H, β CH),
2.26 (m, 2H, α CH2), 1.79 (m, 1H, –CH(CH3)2), 1.41 (s, 9H,
Boc), 1.27 (t, J = 7.1 Hz, 3H, –OCH2–CH3), 0.88 (m, 6H, –
(CH3)2); 13C NMR (100 MHz, CDCl3) δ 171.9, 171.0, 136.5,
129.3, 128.9, 127.2, 80.6, 75.7, 61.0, 56.5, 55.0, 37.4, 36.2, 34.2,
29.2, 28.3, 20.0, 17.1, 14.2; HR-MS m/z value for C24H37N3O7
[M + Na+] 502.2524 (calcd), 502.2513 (found).
Boc-γSer(OBut) (β-CH2NO2)-γVal(β-CH2NO2)-COOEt (P5). Yellow
oil (0.56 g, 55%); [α]D20 −8.0 (c 1.0, MeOH); IR ν (cm−1) 3323,
2973, 1721, 1552, 1376, 1242, 1173, 1085, 1028, 873, 701;
1H NMR (400 MHz, CDCl3) δ 6.32 (d, J = 10.1 Hz, 1H), 5.04
(d, J = 9.2 Hz, 1H), 4.88 (dd, J = 13.1 Hz, J = 4.4 Hz, 1H), 4.58
(m, 2H), 4.29 (m, 1H), 4.16–4.06 (m, 3H), 3.96 (m, 1H), 3.87
(m, 1H), 3.53 (m, 2H), 2.91 (m, 2H), 2.58–2.24 (m, 4H), 1.41
(s, 9H), 1.24 (m, 6H), 1.15 (s, 9H), 0.92 (m, 6H); 13C NMR
(100 MHz, CDCl3) δ 171.1, 156.7, 80.5, 76.2, 73.9, 63.8,
61.0, 60.5, 55.5, 54.9, 50.3, 49.3, 39.5, 37.2, 36.8, 33.9,
29.3, 28.3, 27.3, 24.9, 21.1, 20.2, 17.0, 14.2; HR-MS m/z
value for C25H46N4O10 [M + Na+] 585.3106 (calcd), 585.3110
(found).
Boc-γVal(β-CH2NO2)-γVal(β-CH2NO2)-COOEt (P6). Yellow solid
(0.55 g, 58%); mp 120 °C; [α]2D0 −5.0 (c 1.0, MeOH); IR ν (cm−1
)
3427, 3254, 3062, 2964, 2950, 2564, 1742, 1642, 1554, 1461,
1
1295, 1253, 1166, 1033, 885, 735; H NMR (400 MHz, CDCl3)
δ 6.31 (d, J = 10.1 Hz, 1H, NH), 4.57 (m, 2H, –CH2NO2), 4.38
(m, 3H, NH and –CH2NO2), 4.14 (q, J = 7.3 Hz, 2H, –CH2CH3),
3.96 (dt, J = 10.1 Hz, J = 6.6 Hz, 1H), 3.38 (td, J = 9.4 Hz, J = 3.7
Hz, 1H), 2.97 (m, 2H), 2.49 (m, 2H), 2.3 (d, J = 6.9 Hz, 2H),
1.42 (m, 11H, Boc and 2 CH(CH3)2), 1.24 (t, J = 7.1 Hz,
3H, –OCH2–CH3), 0.95 (m, 12H, 2 –CH(CH3)2); 13C NMR
(100 MHz, CDCl3)
δ 171.2, 170.8, 157.2, 80.5, 76.0,
61.0, 58.0, 55.1, 37.2, 37.0, 36.5, 34.6, 34.0, 30.3, 29.4, 28.3,
25.6, 25.0, 20.1, 19.9, 19.5, 17.5, 14.2; MALDI TOF/TOF m/z
value for C23H42N4O9 [M + Na+] 541.2844 (calcd), 541.2827
(observed).
Boc-γVal(β-CH2NO2)-γLeu(β-CH2NO2)-COOEt (P3). Light yellow
solid (0.9 g, 70%); mp 93 °C; [α]2D0 −12.0 (c 1.0, MeOH);
IR ν (cm−1) 3258, 3067, 2967, 1744, 1651, 1555, 1460, 1379,
1
1293, 1253, 1167, 1107, 865, 742; H NMR (400 MHz, CDCl3)
Boc-γVal(β-CH2NO2)-γPhe(β-CH2NO2)-COOEt (P7). White solid
(0.85 g, 65%); mp 147 °C; [α]2D0 −15.0 (c 1.0, MeOH); IR ν
(cm−1) 3360, 2973, 2929, 1729, 1687, 1552, 1437, 1377, 1294,
1246, 1168, 1086, 1021, 697; 1H NMR (400 MHz, CDCl3)
δ 7.27–7.14 (m, 5H, aromatic), 6.36 (d, J = 8.7 Hz, 1H),
4.67–4.54 (m, 2H), 4.51–4.42 (m, 1H), 4.3 (d, J = 9.6 Hz, 1H),
4.17–4.01 (m, 4H), 3.36–3.30 (m, 1H), 2.99–2.91 (m, 2H),
2.87–2.80 (m, 1H), 2.72–2.47 (m, 4H), 2.21–2.02 (m, 2H),
1.66–1.58 (m, 1H), 1.42 (m, 9H), 1.24 (t, J = 7.1 Hz, 3H), 0.92
(dd, J = 16.3 Hz, J = 6.6 Hz, 6H); 13C NMR (100 MHz, CDCl3)
δ 171.1, 170.4, 157.1, 136.9, 128.9, 128.7, 127.0, 80.4, 76.0,
75.6, 61.1, 57.8, 51.5, 38.6, 38.3, 37.0, 34.4, 30.0, 28.3,
20.0, 18.8, 14.2; MALDI TOF/TOF m/z value for C31H42N4O9
[M + Na+] 589.2844 (calcd), 589.2828 (found).
δ 6.25 (d, J = 9.2 Hz, 1H, NH), 4.62–4.49 (m, 2H, –CH2NO2),
4.43–4.30 (m, 3H,–CH2NO2 and NH), 4.26–4.19 (m, 1H, γ CH),
4.14 (q, J = 7.3 Hz, 2H, –OCH2–), 3.40–3.34 (m, 1H, γ CH),
3.02–2.95 (m, 1H, β CH), 2.89–2.82 (m, 1H, β CH), 2.60–2.37
(m, 2H, α CH2), 2.27–2.25 (m, 2H, α CH2), 1.70–1.65 (m, 2H,
Leu δ CH2), 1.60–1.55 (m, 1H, Val δ CH), 1.41 (s, 9H, Boc),
1.38–1.34 (m, 1H, Leu ω CH), 1.24 (t, J = 7.1 Hz, 3H, –OCH2–
CH3), 1.00–0.86 (m, 12H, Leu and Val side chain (CH3)2);
13C NMR (100 MHz, CDCl3) δ 171.3, 170.5, 157.2, 80.4, 75.9,
61.0, 60.5, 57.9, 48.5, 41.7, 39.0, 37.1, 37.0, 34.4, 30.2, 28.3,
25.0, 23.3, 21.6, 21.1, 19.9, 19.4, 14.2; MALDI TOF/TOF m/z
value for C24H44N4O9 [M + Na+] 555.3001 (calcd), 555.3064
(observed).
Boc-γLeu(β-CH2NO2)-γIle(β-CH2NO2)-COOEt (P4). White solid
(0.655 g, 64%); mp 103 °C; [α]2D0 −6.0 (c 1.0, MeOH); IR ν
(cm−1) 3394, 2967, 2362, 1700, 1654, 1553, 1457, 1378, 1254,
Acknowledgements
1
1169, 1035; H NMR (400 MHz, CDCl3) δ 6.08 (d, J = 10.1 Hz,
1H, NH), 4.62–4.51 (m, 2H, –CH2NO2), 4.46–4.32 (m, 3H, This work is supported by Department of Science and Techno-
–CH2NO2 and NH), 4.12 (q, J = 6.9 Hz, 2H, –OCH2–CH3), logy, Govt of India. M. G. K. and S. M. M are thankful to CSIR,
4.03–3.9 (m, 1H, γ CH), 3.8 (m, 1H, γ CH), 3.03 (m, 1H), 2.79 Govt of India for a research fellowship.
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