Stereoselective synthesis of short benzyl malonyl peptides

Article abstract of DOI:10.1039/c3ra41852a

A Rh-catalyzed addition reaction on non symmetric dehydro alanine retro-peptides is the key step in the reported three-step strategy for the diastereoselective synthesis of differently functionalized benzyl malonyl peptides (74% overall yield).

Full text of DOI:10.1039/c3ra41852a

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Stereoselective synthesis of short benzyl malonyl
peptides3
Cite this: RSC Advances, 2013, 3,
13470
Emanuele Aresu, Stefania Fioravanti,* Simona Gasbarri, Lucio Pellacani
and Fabio Sciubba
Received 17th April 2013,
Accepted 31st May 2013
A Rh-catalyzed addition reaction on non symmetric dehydro alanine retro-peptides is the key step in the
reported three-step strategy for the diastereoselective synthesis of differently functionalized benzyl
malonyl peptides (74% overall yield).
DOI: 10.1039/c3ra41852a
www.rsc.org/advances
Introduction
Peptidomimetics have been designed to overcome the pro-
blems of pharmacokinetics1 of the parent peptides and to be
thus considered in different biological applications. In
particular, retro-peptides are molecules that present the
inversion of one or more peptide bonds of the backbone
chain by the introduction of a malonyl residue and/or a gem-
diaminic unit within the peptide chain.2 These modifications
increase the resistance to enzyme degradation,3 without
decreasing the receptor binding ability and biological
response of native peptides.4 Moreover, the inversion of the
amide bond induces different conformational structures of
peptidomimetics with respect to the native residue, by
obtaining a rearrangement in stable b sheets.5
In the retro-peptide field, we reported the synthesis of non-
symmetric disubstituted malonamides rAA-mGly-AA6 and of
stable N,N9-protected gem-diaminic units P-gAA-P9 (Scheme 1)
that can be successfully used in a tandem deprotection-
coupling reaction to obtain more complex retro-peptide units.7
Continuing our studies, the interest was turned towards the
possibility to modify the mimetic glycine residue of the rAA-
mGly-AA9 structures to obtain different alkyl malonyl units of
kind rAA-mAA99-AA9 and so introduce further modifications in
the peptidomimetic backbone.
Scheme 1 Synthesis of malonamidic and gem-diaminic units.
and/or times) the expected alkylated malonamide is not
formed, probably due to the low electrophilicity of the reactive
species coupled to the instability of the substrate under basic
conditions.
Then, a different synthetic strategy involving a rhodium-
catalyzed conjugate addition to the double bond of malonyl
dehydroalanines10 rAA-mD2Ala-AA9 (obtained from the corre-
sponding rAA-mAla-AA9) was considered (Scheme 2).
As is well known, in the last years rhodium was widely used
as metal catalyst in various addition reactions to alkenes.11
Recently, the synthesis of small peptides containing unnatural
amino acid residues was reported by a rhodium-catalyzed 1,4-
addition of aryl siloxanes or boronates to a,b-dehydroamino
acid moieties.12 Still always by the same group, several useful
alanines were prepared using a rhodium-catalyzed conjugate
addition of aryl boronic acids to dehydro derivatives.13
First of all, we attempted the classical reaction conditions
to alkylate methylene active compounds using opportune
halides (ethyl bromide, methyl iodide) in the presence of
different organic or inorganic bases, such as tertiary (Et3N) or
secondary (piperidine)8 amines or CaO,9 on the symmetric
N,N-Boc protected malonamide rGly-mGly-Gly. However, also
by changing other reaction parameters (solvent, temperature,
`
Dipartimento di Chimica, Universita degli Studi di Roma ‘‘La Sapienza’’, P.le Aldo
Moro 5, I-00185 Roma, Italy. E-mail: stefania.fioravanti@uniroma1.it;
Fax: +39 06490631; Tel: +39 0649913098
Electronic supplementary information (ESI) available: General procedures,
3
analytical and spectroscopic data, 1H and 13C NMR spectra of all new
compounds. See DOI: 10.1039/c3ra41852a
Scheme 2 Synthetic strategy for alkyl malonyl units.
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Scheme 4 Synthesis of different protected aryl boronates.
Scheme 3 Synthesis of symmetric N,N9-Boc protected malonyl dehydroalanines
rAA-mD2Ala-AA9 3a.
the same conditions reported in entry 7 of Table 1. 5a was
obtained after 2 h with similar yields but in lower purity due to
the presence of unidentified by-products.
Thus, different commercial p-substituted phenyl boronic
acids 8–12 were considered to study the influence of a
substituent in para position on the rhodium-catalyzed addi-
tion on 3a. The results are reported in Table 2.
As expected, the presence of an electron donor substituent
(entries 1 and 2) as well as a halogen atom (entries 3 and 4)
favored the addition, while the electron withdrawing nitro
group (entry 5) has the opposite effect and led to the formation
of addition product in lower yields.
Results and discussion
The symmetric N,N9-Boc protected malonamide rGly-mAla-Gly
2a was synthesized in good yields starting from 1 following the
same methodology standardized on the Meldrum’s acid.6
Then, a selenoxide syn elimination was considered to obtain
the corresponding rGly(O-t-Bu)-mD2Ala-Gly(O-t-Bu) 3a, partially
modifying the methodology reported in the literature for
different active methylene compounds14 (Scheme 3).
Finally, a rhodium-catalyzed addition to the CLC double
bond was attempted on 3a (1 mmol), by using commercial
Then, the Rh-catalyzed addition reaction was extended to
non symmetric malonyl dehydro peptides (MDHPs) 3b–f,
choosing commercially available 8 as alkylating agent to
synthesize the different benzyl malonyl peptides 13b–f and
13c9–f9 (Table 3).
In all cases, compounds 13 were obtained in good yields
after fast purification on silica gel, but the reactions on 3c–f
performed under standardized conditions (80 uC) took place
with low (dr = 6 : 4 for 3d–f) or no (for 3c) diastereoselectivity
as determined by 1H NMR spectra of crude mixtures. To
improve the diastereoselectivity, these reactions were repeated
at lower temperatures (50 uC and rt).
While working at room temperature only the reagent
decomposition was observed (24 h), the addition reactions
performed at 50 uC took place with high selectivity (dr ¢ 9 : 1),
as determined by 1H NMR analyses performed on the crude
mixtures, only if the phenylalanine residue was present in the
starting MDPHs (entries 3–5), the stereoselectivity being
controlled by electronic factors (entry 2).
phenyl boronic acid
4 as alkylating agent, chloro(1,5-
cyclooctadiene)rhodium(I) dimer {[Rh(COD)Cl]2} as catalyst,
1,5-cyclooctadiene (COD) as ligand, and potassium hydroxide
(1 mmol) as base.
The reactions were performed under different conditions
(Table 1) and the best appeared using an equimolar ratio
between the reagents, 2.5 mol% of Rh complex and working at
reflux of THF for 2 h (entry 7). After work-up with water and
purification through a short plug of silica gel using CH2Cl2 as
eluent, the product 5a was obtained in 69% yield with very
high purity, as confirmed by NMR spectra.
With the aim of increasing the reaction yields, two different
protected boronate 615 and 716 reagents were synthesized
starting from 4 (Scheme 4) and used as alkylating agents under
Table 1 Rhodium-catalyzed addition reaction on rGly(Ot-Bu)-mD2Ala-
Gly(Ot-Bu) 3a
Table 2 Rh-catalyzed addition with different p-substituted phenyl boronic acids
Catalyst
(mol%)
COD
(mol%)
Time
(h)
Yield
(%)
Entry
4 (eq.)
T/uC
1
2
3
4
5
6
7
16
4
3
3
2
2
1
2.5
2.5
2.5
5.0
2.5
2.5
2.5
10
10
10
20
10
10
10
100a
80
80
80
80
24
24
12
12
12
2
13
20
30
15
43
58
69
Entry
Alkylating agent
X
Addition product
Yield (%)
1
2
3
4
5
8
9
10
11
12
Me
OMe
Br
F
NO2
13a
14a
15a
16a
17a
90
94
89
88
57
80
80
2
a
Performed by using a mixture of dioxane/H2O as solvent.
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Table 3 Synthesis of non symmetric benzyl malonyl peptides
Entry
AA-OR
AA9-OR9
2
Yield (%)
3
Yield (%)
13
Yield (%)
dr 50 uC
1
2
3
4
5
b-AlaOMe
L-LeuOMe
L-PheOt-Bu
L-PheOMe
L-PheOMe
GlyOt-Bu
GlyOMe
GlyOt-Bu
b-AlaOMe
GlyOMe
b
c
d
e
f
96
94
95
93
92
b
c
d
e
f
83
84
78
79
77
b
c,c9
d,d9
e,e9
f,f9a
75
84
79
82
80
—
1 : 1
.9 : 1
.9 : 1
.9 : 1
a
Rh-addition was performed by using 9 as alkylating agent.
catalyzed addition reaction performed on non symmetric
dehydro alanine retro-peptides. The target compounds were
obtained in very high diastereomeric purity probably due to
the formation of a stable coordination complex between the
phenylalanine aromatic residue and the organometallic
reagent. Furthermore, the synthesized short retro-peptide
units are enriched by a mimetic residue of phenylalanine or
p-substituted analogs, in which the aromatic ring substituent
is a potential interesting modification able to modulate the
retro-peptide polarity.
To gain further information, bidimensional NOESY experi-
ments were performed on pure 13d (Fig. 1). 2D NMR spectra
clearly showed that the two aromatic systems of malonyl
peptide are in syn position, due to the presence of dipolar
couplings between the protons of the aromatic moieties. This
permits to attribute the S configuration to the new chiral
center.
Starting from these data, in Scheme 5 a catalytic cycle of the
rhodium-catalyzed addition of aryl boronates to MDHPs 3a–f
is proposed.
The rhodium complex
I was formed in situ and it
coordinates the CLC double bond of 3. If the MDPH contains
the phenylalanine residue, probably the rhodium coordinates
also the aromatic ring leading to the formation of the
intermediate II and determining the observed high diastereos-
electivity. In fact, in II the attack of the nucleophile on the
double bond can occur only on the same side of the benzyl
residue of phenylalanine.
Experimental section
General remarks
All the commercial available reagents and the anhydrous
solvents were purchased from Aldrich and used without further
purification. The reactions were monitored by 1H NMR
spectroscopy. Flash chromatography was performed on
1
Merck silica gel (60, particle size: 0.040–0.063 mm). H NMR
Conclusions
In conclusion, a new three-step synthetic strategy for the
synthesis of differently functionalized benzyl malonyl peptides
(74% overall yield) was developed, the key step being a Rh-
Fig. 1 NOESY spectrum of 13d. Spatial correlations between two aromatic
systems are evidenced. The existence of these cross peaks is possible only if the
two rings face the same plane.
Scheme 5 Proposed catalytic cycle of the rhodium-catalyzed addition of aryl
boronates.
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and 13C NMR spectra were recorded on a VARIAN XL-300
spectrometer at room temperature. CDCl3 was used as the
solvent and CHCl3 as the internal standard. 2D NMR spectra
were recorded by a Bruker Avance III 400 MHz NMR spectro-
meter and used to assist in structure elucidation.17 FT-IR
spectra were recorded on a Perkin-Elmer 1600 spectrophot-
ometer in CHCl3 as the solvent. HRMS analyses were
performed using a Micromass Q-TOF Micro quadrupole-time
of flight (TOF) mass spectrometer equipped with an ESI source
and a syringe pump. The experiments were conducted in the
positive ion mode.
= 7.0 Hz, 3H), 1.42 (s, 18H), 3.02–3.29 (m, 3H), 3.90 (d, J = 5.2
Hz, 2H), 4.72–4.79 (m, 1H), 7.18–7.38 (m, 5H), 7.40 (br, 1H),
7.50 (br, 1H); (diastereomer II) 1.20 (d, J = 7.0 Hz, 3H), 1.48 (s,
18H), 3.02–3.29 (m, 3H), 3.90 (d, J = 5.2 Hz, 2H), 4.72–4.79 (m,
1H), 7.18–7.38 (m, 5H), 7.40 (br, 1H), 7.50 (br, 1H). 13C NMR
(CDCl3): d (diastereomer I) 16.4, 27.7 (6C), 37.6, 41.8, 47.5, 53.7,
81.8, 82.0, 126.6, 128.1 (2C), 129.3 (2C), 136.1, 168.5, 170.1,
170.6, 171.0; (diastereomer II) 16.4, 27.8 (6C), 37.7, 41.9, 47.6,
53.7, 81.9, 82.0, 126.7, 128.1 (2C), 129.3 (2C), 136.1, 168.5,
170.1, 170.8, 171.1. HRMS (ESI Q-TOF) m/z [M + H]+ calcd for
C
23H35N2O6 435.2495, found 435.2493.
Methyl (2S)-2-{3-[(3-methoxy-3-oxopropyl)amino]-2-methyl-3-
General procedure for the synthesis of methyl malonyl
peptides
oxopropanamido}-3-phenylpropanoate (2e). Yield 93%. Yellow
oil. Purified by flash chromatography on silica gel (eluent:
hexane/ethyl acetate = 40 : 60). nmax cm21 3422, 3334, 1733,
1674. 1H NMR (CDCl3): d (diastereomer I) 1.36–1.42 (m, 3H),
2.55 (t, J = 6.1 Hz, 2H), 3.02–3.19 (m, 3H), 3.45–3.59 (m, 2H),
3.71 (s, 3H), 3.75 (s, 3H), 4.79–4.88 (m, 1H), 7.05 (br, 2H), 7.14–
7.34 (m, 5H); (diastereomer II) 1.36–1.42 (m, 3H), 2.55 (t, J = 6.1
Hz, 2H), 3.02–3.19 (m, 3H), 3.45–3.59 (m, 2H), 3.72 (s, 3H), 3.75
Starting from the methyl Meldum’s acid, the same reported
synthetic procedure for the synthesis of malonyl peptides6 was
followed.
Di-tert-butyl 2,29-[(2-methylmalonyl)bis(azanediyl)]diacetate
(2a). Yield 98%. White solid mp 125.5–126.2 uC. Purified by
flash chromatography on silica gel (eluent: hexane/ethyl
1
acetate = 45 : 55). nmax cm21 3424, 3330, 1731, 1672. H NMR
(s, 3H), 4.79–4.88 (m, 1H), 7.14–7.34 (m, 5H), 7.73 (br, 2H). 13
C
(CDCl3): d 1.47 (s, 18H), 1.51 (d, J = 7.3 Hz, 3H), 3.21 (q, J = 7.2
Hz, 1H), 3.93 (d, J = 5.2 Hz, 4H), 7.00 (br, 2H). 13C NMR
(CDCl3): d 16.2, 27.8 (6C), 41.9 (2C), 47.3, 81.7 (2C), 168.5 (2C),
171.4 (2C). HRMS (ESI Q-TOF) m/z [M + H]+ calcd for
C16H29N2O6 345.2026, found 345.2021.
NMR (CDCl3): d (diastereomer I) 16.6, 33.6, 35.0, 37.7, 48.1,
51.8, 53.2 (2C), 127.1, 128.5 (2C), 129.1 (2C), 135.8, 170.9,
171.0, 171.6 172.5; (diastereomer II) 16.5, 33.6, 35.0, 37.6, 48.2,
52.3, 53.3 (2 C), 127.1, 128.5 (2C), 129.2 (2C), 135.8, 170.8,
170.9, 171.6, 172.5. HRMS (ESI Q-TOF) m/z [M + H]+ calcd for
Methyl 3-{3-[(2-tert-butoxy-2-oxoethyl)amino]-2-methyl-3-
oxopropanamido}propanoate (2b). Yield 96%. White solid,
mp 123.2–124.7 uC. Purified by flash chromatography on silica
gel (eluent: hexane/ethyl acetate = 5 : 95). nmax cm21 3425,
C
18H25N2O6 365.1713, found 365.1710.
Methyl (2S)-2-(3-(2-methoxy-2-oxoethylamino)-2-methyl-3-
oxopropanamido)-3-phenylpropanoate (2f). Yield 92%. Yellow
oil. Purified by flash chromatography on silica gel (eluent:
hexane/ethyl acetate = 25 : 75). nmax cm21 3420, 3335, 1733,
1
3336, 1732, 1673. H NMR (CDCl3): d 1.47 (s, 9H), 1.63 (d, J =
1
7.2 Hz, 3H), 2.55 (t, J = 6.5 Hz, 2H), 3.12 (q, J = 7.2 Hz, 1H),
3.49–3.55 (m, 2H), 3.70 (s, 3H), 3.91 (d, J = 6.5 Hz, 2H), 7.02 (br,
2H). 13C NMR (CDCl3) d 75.5 MHz): 16.6, 27.9 (3C), 33.6, 35.1,
42.0, 48.0, 51.7, 82.1, 168.5, 171.3, 171.4, 172.5. HRMS (ESI
Q-TOF) m/z [M + H]+ calcd for C14H25N2O6 317.1713, found
317.1720.
1675. H NMR (CDCl3): d (diastereomer I) 1.25 (d, J = 7.1 Hz,
3H), 3.02–3.17 (m, 2H), 3.34 (q, J = 7.1 Hz, 1H), 3.74 (s, 3H),
3.76 (s, 3H), 4.01 (d, J = 6.2 Hz, 2H), 4.79–4.88 (m, 1H), 6.83 (br,
2H), 7.08–7.31 (m, 5H); (diastereomer II) 1.27 (d, J = 7.1 Hz, 3H),
3.02–3.17 (m, 2H), 3.34 (q, J = 7.1 Hz, 1H), 3.74 (s, 3H), 3.76 (s,
3H), 4.01 (d, J = 6.2 Hz, 2H), 4.79–4.88 (m, 1H), 6.96 (br, 2H),
7.08–7.31 (m, 5H). 13C NMR (CDCl3): d (diastereomer I) 16.4,
37.6, 41.2, 47.9, 51.3, 51.4, 52.4, 127.1, 128.5 (2C), 129.2 (2C),
135.7, 169.3, 169.9, 170.9, 171.3; (diastereomer II) 16.5, 37.7,
41.2, 48.0, 51.4, 51.5, 53.3, 127.7, 128.5 (2C), 130.2 (2C), 136.3,
169.4, 170.7, 171.1, 171.6. HRMS (ESI Q-TOF) m/z [M + H]+
calcd for C17H23N2O6 351.1556, found 351.1559.
Methyl (2S)-2-[3-(2-methoxy-2-oxoethylamino)-2-methyl-3-
oxopropanamido]-4-methylpentanoate (2c). Yield 94%. White
solid, mp 64.0–65.6 uC. Purified by flash chromatography on
silica gel (eluent: hexane/ethyl acetate = 30 : 70). nmax cm21
3424, 3342, 1742, 1672. 1H NMR (CDCl3): d (diastereomer I) 0.92
(d, J = 6.1, 6H), 1.49 (d, J = 1.5 Hz, 3H), 1.54–1.68 (m, 3H), 3.21
(q, J = 7.3 Hz, 1H), 3.73 (s, 3H), 3.76 (s, 3H), 4.03 (d, J = 5.3 Hz,
2H), 4.54–4.60 (m, 1H), 6.89 (br, 2H); (diastereomer II) 0.94 (d, J
= 6.1, 6H), 1.51 (d, J = 1.5 Hz, 3H), 1.54–1.68 (m, 3H), 3.22 (q, J
= 7.3 Hz, 1H), 3.73 (s, 3H), 3.77 (s, 3H), 4.05 (d, J = 5.5 Hz, 2H),
4.54–4.60 (m, 1H), 7.10 (br, 2H). 13C NMR (CDCl3): d
(diastereomer I) 16.2, 21.3, 22.5 (2C), 40.3, 40.9, 47.2, 50.7
(2C), 51.9, 169.8, 171.2, 171.8, 172.9; (diastereomer II) 16.3,
21.4, 24.5 (2C), 40.4, 41.0, 47.2, 50.7 (2C), 51.9, 169.9, 171.3,
171.8, 173.0. HRMS (ESI Q-TOF) m/z [M + H]+ calcd for
C14H25N2O6 317.1713, found 317.1720.
General procedure for the synthesis of dehydro malonyl
peptides
Methyl malonyl peptide (1.0 mmol) was added slowly to a
stirred suspension of NaH (0.036 g, 1.5 mmol) in 40 mL
anhydrous THF at 0 uC. After 1 h, a solution of PhSeCl (0.766 g,
4.0 mmol) in 5 mL anhydrous THF was added dropwise. After
24 h, the solutions were diluted with diethyl ether, washed
with saturated NaHCO3 and saturated NaCl, and dried over
Na2SO4. The solvents were evaporated under reduced pressure.
The crude mixtures were dissolved in 20 mL CH2Cl2 and
stirred with 40% H2O2 (0.3 mL, 4 mmol) for 3 h at 0 uC. Then,
the mixtures were washed with saturated NaHCO3 and
saturated NaCl and dried over Na2SO4. After solvent evapora-
tert-Butyl (2S)-2-{3-[(2-tert-butoxy-2-oxoethyl)amino]-2-
methyl-3-oxopropanamido}-3-phenylpropanoate (2d). Yield
95%. Yellow oil. Purified by flash chromatography on silica
gel (eluent: hexane/ethyl acetate = 20 : 80). nmax cm21 3423,
3335, 1731, 1675. 1H NMR (CDCl3): d (diastereomer I) 1.20 (d, J
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tion, the expected dehydro malonyl peptides were obtained as
pure compounds and used without further purification.
Di-tert-butyl 2,29-[(2-methylenemalonyl)bis(azanediyl)] dia-
cetate (3a). Yield 84%. Yellow oil. nmax cm21 3423, 3334, 1731,
1679, 1632. 1H NMR (CDCl3): d 1.47 (s, 18H), 4.00 (d, J = 5.1 Hz,
4H), 6.44 (s, 2 H), 7.77 (br, 2H). 13C NMR (CDCl3): d 27.9 (6C),
42.1 (2C), 82.2 (2C), 128.6, 129.1, 168.5 (2C), 170.0 (2C). HRMS
(ESI Q-TOF) m/z [M + H]+ calcd for C16H27N2O6 343.1869,
found 343.1865.
Methyl 3-[2-(2-tert-butoxy-2-oxoethylcarbamoyl)acrylamido]
propanoate (3b). Yield 83%. Yellow oil. nmax cm21 3425,
3336, 1735, 1678, 1633. 1H NMR (CDCl3): d 1.41 (s, 9H), 2.54 (t,
J = 6.2 Hz, 2H), 3.53 (q, J = 6.2 Hz, 2H), 3.63 (s, 3H), 3.92 (d, J =
5.5 Hz, 2H), 6.29 (s, 1H), 6.35 (s, 1H), 7.90 (br, 1H), 8.06 (br,
1H). 13C NMR (CDCl3): d 27.8 (3C), 33.4, 35.1, 42.1, 51.6, 82.1,
136.1, 137.0, 165.1, 165.2, 168.4, 172.4. HRMS (ESI Q-TOF) m/z
[M + H]+ calcd for C14H23N2O6 315.1556, found 315.1560.
Methyl (2S)-2-[(2-{[(2-methoxy-2-oxoethyl)amino]carbonyl}
acryloyl)amino]-4-methylpentanoate (3c). Yield 84%. Yellow
oil. nmax cm21 3422, 3330, 1742, 1674,1631. 1H NMR (CDCl3). d
0.75–0.98 (m, 6H), 1.50–1.71 (m, 3H), 3.70 (s, 3H), 3.72 (s, 3H),
4.06 (d, J = 5.0 Hz, 2H), 4.50–4.63 (m, 1H), 6.41 (s, 1H), 6.43 (s,
1H), 7.94–7.97 (m, 1H), 8.05 (br, 1H). 13C NMR (CDCl3): d 21.7,
22.7, 24.8, 40.9, 41.3, 51.1, 52.2, 52.3, 135.7, 136.5, 164.8,
165.5, 169.8, 172.9. HRMS (ESI Q-TOF) m/z [M + H]+ calcd for
C14H23N2O6 315.1556, found 315.1550.
tert-Butyl (2S)-2-[2-(2-tert-butoxy-2-oxoethylcarbamoyl)acry-
lamido]-3-phenylpropanoate (3d). Yield 78%. Orange oil. nmax
cm21 3427, 3336, 1739, 1678, 1630. 1H NMR (CDCl3): d 1.41 (s,
9H), 1.48 (s, 9H), 3.10–3.21 (m, 2H), 4.00 (d, J = 5.4 Hz, 2H),
4.76–4.81 (m, 1H), 6.26 (s, 1H), 6.45 (s, 1H), 7.16–7.29 (m, 5H),
7.59 (br, 1H), 8.01 (br, 1H). 13C NMR (CDCl3): d 27.8 (3C), 27.9
(3C), 37.7, 42.2, 54.0, 82.2, 82.4, 126.9, 128.3 (2C), 129.4 (2C),
129.8, 136.0, 137.0, 164.5, 164.8, 168.4, 170.0. HRMS (ESI
Q-TOF) m/z [M + H]+ calcd for C23H33N2O6 433.2339, found
433.2333.
Methyl (2S)-2-[2-(3-methoxy-3-oxopropylcarbamoyl)acryla-
mido]-3-phenylpropanoate (3e). Yield 79%. Orange oil. IR
(CHCl3) nmax cm21 3426, 3334, 1736, 1677, 1631. 1H NMR
(CDCl3) d: 2.57 (t, J = 6.3 Hz, 2H), 3.07–3.23 (m, 2H), 3.57 (q, J =
6.3 Hz, 2H), 3.69 (s, 3H), 3.72 (s, 3H), 4.80–4.88 (m, 1H), 6.24
(s, 1H), 6.29 (s, 1H), 7.10–7.29 (m, 5H), 7.87 (br, 1H), 7.95 (br,
1H). 13C NMR (CDCl3): d 33.4, 35.0, 37.5, 51.7, 52.2, 53.6, 127.0,
128.4 (2C), 129.0 (2C), 135.8, 136.4, 137.0, 164.9, 165.2, 171.4,
172.3. HRMS (ESI Q-TOF) m/z [M + H]+ calcd for C18H23N2O6
363.1556, found 363.1550.
General procedure for the synthesis of alkyl malonyl peptides
An oven dried flask was charged with malonyl dehydro peptide
(1 mmol), [Rh(COD)Cl]2 (2.5 mol%), cyclooctadiene (10 mol%),
KOH (1 mmol), an appropriate p-substituted phenyl boronic
acid (1 mmol) or (6) or (7), and tetrahydrofuran (4 mL),
evacuated and backfilled with argon and stirred at 80 uC or 50
uC for 2 h. The reaction mixture was diluted with ethyl acetate,
washed with a saturated NaHCO3 solution and brine, dried
over MgSO4 and the solvent was evaporated. The residue was
purified by column chromatography to afford the desired
compound.
Di-tert-butyl 2,29-{[2-(benzyl)malonyl]bis(azanediyl)}diace-
tate (5a). Yield 89%. Dark yellow oil. nmax cm21 3424, 3337,
1735, 1678. 1H NMR (CDCl3): d 1.45 (s, 18H), 3.55–3.70 (m,
3H), 3.84–3.97 (m, 4H), 6.81–6.86 (m, 5H), 7.15–7.20 (br, 1H),
8.08 (br, 1H). 13C NMR (CDCl3): d 27.9 (6C), 33.7, 41.9 (2C),
53.5, 82.6 (2C), 115.3, 119.8 (2C), 129.4 (2C), 156.4, 166.2,
167.9, 169.4, 170.0. HRMS (ESI Q-TOF) m/z [M + H]+ calcd for
C
22H33N2O6 421.2339, found 421.2330.
Di-tert-butyl 2,29-{[2-(4-methylbenzyl)malonyl]bis(azane-
diyl)}diacetate (13a). Yield 90%. Dark yellow oil. nmax cm21
1
3422, 3333, 1734, 1679. H NMR (CDCl3): d 1.44 (s, 18H), 2.28
(s, 3H), 3.16–3.27 (m, 3H), 3.73–3.97 (m, 4H), 6.92 (br, 2H),
7.05–7.10 (m, 4H). 13C NMR (CDCl3): d 20.9, 27.9 (6C), 37.7,
42.1 (2C), 56.5, 82.2 (2C), 128.6, 129.1 (2C), 134.8 (2C), 136.2,
168.4 (2C), 170.0 (2C). HRMS (ESI Q-TOF) m/z [M + H]+ calcd
for C23H35N2O6 435.2495, found 435.2498.
Di-tert-butyl 2,29-{[2-(4-methoxybenzyl)malonyl]bis(azane-
diyl)}diacetate (14a). Yield 94%. Dark yellow oil. nmax cm21
3423, 3336, 1736, 1677. 1H NMR (CDCl3): d 1.45 (s, 18H), 3.52–
3.56 (m, 3H), 3.73 (s, 3H), 3.86–4.03 (m, 4H), 6.74–6.75 (m, 4H),
8.03 (br, 2H). 13C NMR (CDCl3): d 27.9 (6C), 33.8, 41.9 (2C),
53.6, 55.7, 82.6 (2C), 114.5, 114.7, 116.0 (2C), 120.6 (2C), 166.1,
168.0, 169.5, 170.0. HRMS (ESI Q-TOF) m/z [M + H]+ calcd for
C23H35N2O7 451.2444, found 451.2450.
Di-tert-butyl 2,29-{[2-(4-bromobenzyl)malonyl]bis(azane-
diyl)}diacetate (15a). Yield 89%. Dark yellow oil. nmax cm21
3425, 3334, 1737, 1679. 1H NMR (CDCl3): d 1.44 (s, 18H), 3.52–
3.69 (m, 3H), 3.85–3.96 (m, 4H), 6.68–6.72 (m, 2H), 7.22–7.25
(m, 2H), 8.07 (br, 2H). 13C NMR (CDCl3): d 27.9 (6C), 33.7, 41.9
(2C), 53.6, 82.7 (2C), 111.5, 117.3 (2C), 132.1 (2C), 155.8, 166.2,
167.9, 169.5, 170.0. HRMS (ESI Q-TOF) m/z [M + H]+ calcd for
C22H32BrN2O6 499.1444, found 499.1446.
Di-tert-butyl 2,29-{[2-(4-fluorobenzyl)malonyl]bis(azane-
diyl)}diacetate (16a). Yield 88%. Dark yellow oil. nmax cm21
3421, 3335, 1734, 1676. 1H NMR (CDCl3): d 1.44 (s, 18H), 3.65–
3.77 (m, 3H), 3.87–4.02 (m, 4H), 6.86–6.90 (m, 2H), 8.06–8.10
(m, 2H), 8.40 (br, 2H). 13C NMR (CDCl3). d 27.9 (6C), 33.8, 41.9
(2C), 53.6, 82.6 (2C), 115.5, 115.8 (2C), 116.1 (2C), 152.5, 166.1,
168.0, 170.0, 170.3. HRMS (ESI Q-TOF) m/z [M + H]+ calcd for
C22H32FN2O6 439.2244, found 439.2239.
Methyl (2S)-2-[2-(2-methoxy-2-oxoethylcarbamoyl)acryla-
mido]-3-phenylpropanoate (3f). Yield 77%. Yellow oil. nmax
1
cm21 3428, 3332, 1738, 1678, 1631. H NMR (CDCl3): d 3.02–
3.25 (m, 2H), 3.74 (s, 3H), 3.77 (s, 3H), 4.10 (d, J = 4.6 Hz, 2H),
4.83–4.92 (m, 1H), 6.25 (s, 1H), 6.48 (s, 1H), 7.08–7.29 (m, 5H),
7.97 (br, 2H). 13C NMR (CDCl3): d 33.7, 37.5, 52.2, 53.6, 63.6,
127.0, 128.4 (2C), 129.0 (2C), 130.2, 135.8, 136.5, 164.5, 165.9,
171.3, 171.8. HRMS (ESI Q-TOF) m/z [M + H]+ calcd for
C17H21N2O6 349.1400, found 349.1398.
Di-tert-butyl 2,29-{[2-(4-nitrobenzyl)malonyl]bis(azanediyl)}
diacetate (17a). Yield 57%. Orange oil. nmax cm21 3423, 3335,
1
1735, 1680, 1540. H NMR (CDCl3): d 1.44 (s, 18H), 3.65–3.77
(m, 3H), 3.87–4.02 (m, 4H), 6.86–6.90 (m, 2H), 8.06–8.11 (m,
2H), 8.77 (br, 2H). 13C NMR (CDCl3): d 27.9 (6C), 33.6, 41.9
(2C), 53.6, 82.8 (2C), 115.0, 115.6 (2C), 126.0 (2C), 140.6, 166.3,
13474 | RSC Adv., 2013, 3, 13470–13476
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167.9, 169.5, 170.0. HRMS (ESI Q-TOF) m/z [M + H]+ calcd for
C22H32N3O8 466.2189, found 466.2187.
129.2 (2C), 129.9 (2C), 133.8, 135.8, 165.4 (2C), 172.8 (2C) [(S,S)
major]; 21.6, 33.2, 35.0, 37.7, 38.5, 51.7 (2C), 53.7, 68.1, 114.3,
115.1 (2C), 127.0, 128.5 (2C), 129.1 (2C), 129.9 (2C), 133.8,
135.8, 165.8 (2C), 172.6 (2C) [(S,R) minor]. HRMS (ESI Q-TOF)
m/z [M + H]+ calcd for C25H31N2O6 455.2182, found 455.2187.
Methyl (2S)-2-(3-(2-methoxy-2-oxoethylamino)-2-(4-methoxy-
benzyl)-3-oxopropanamido)-3-phenylpropanoate (13f,f9). Yield
80%. Yellow oil. nmax cm21 3425, 3335, 1734, 1676. 1H NMR
(CDCl3): d 2.86–3.28 (m, 4H), 3.72 (s, 3H), 3.74 (s, 6H), 3.85–
3.91 (m, 1H), 3.97–3.99 (m, 2H), 4.72–4.83 (m, 1H), 6.74–7.30
(m, 9H), 8.01 (br, 2H) [(S,S) major]; 2.86–3.28 (m, 4H), 3.71 (s,
3H), 3.74 (s, 6H), 3.85–3.91 (m, 1H), 4.01–4.03 (m, 2H), 4.72–
Methyl 3-{3-[(2-tert-butoxy-2-oxoethyl)amino]-2-(4-methyl-
benzyl)-3-oxopropanamido}propanoate (13b). Yield 75%.
Dark yellow oil. nmax cm21 3424, 3336, 1736, 1679. 1H NMR
(CDCl3): d 1.44 (s, 9H), 2.22 (s, 3H), 2.50–2.58 (m, 2H), 3.06–
3.09 (m, 2H), 3.45–3.47 (m, 2H), 3.54 (s, 3H), 3.71–3.74 (m, 1H),
3.89–3.95 (m, 2H), 6.69–6.72 (m, 2H), 6.93–6.99 (m, 2H), 8.01
(br, 1H), 8.10 (br, 1H). 13C NMR (CDCl3): d 20.3, 27.9 (3C), 33.4,
33.7, 35.0, 40.7, 51.8, 53.5, 82.5, 115.1, 128.4 (2C), 129.7 (2C),
133.8, 165.9, 166.2, 167.9, 172.2. HRMS (ESI Q-TOF) m/z [M +
H]+ calcd for C21H31N2O6 407.2182, found 407.2183.
4.83 (m, 1H), 6.74–7.30 (m, 9H), 8.17 (br, 2H) [(S,R) minor]. 13
C
Methyl (2S)-2-[3-(2-methoxy-2-oxoethylamino)-2-(4-methyl-
benzyl)-3-oxopropanamido]-4-methylpentanoate (13c,c9).
Yield 84%. Dark yellow oil. nmax cm21 3425; 3337; 1739;
1679. 1H NMR (CDCl3): d (diastereomer I) 0.84–1.00 (m, 6H),
1.19–1.32 (m, 1H), 1.54–1.75 (m, 2H), 2.26 (s, 3H), 3.09–3.18
(m, 2H), 3.73 (s, 6H), 3.95–4.01 (d, J = 10.7 Hz, 2H), 4.06–4.13
(m, 1H), 4.43–4.66 (m, 1H), 6.69–6.76 (m, 2H), 6.97–7.06 (m,
2H), 8.12 (br, 2H); (diastereomer II) 0.84–1.00 (m, 6H), 1.19–
1.32 (m, 1H), 1.54–1.75 (m, 2H), 2.35 (s, 3H), 3.09–3.18 (m, 2H),
3.76 (s, 6H), 3.95–4.01 (d, J = 10.7 Hz, 2H), 4.06–4.13 (m, 1H),
4.43–4.66 (m, 1H), 6.69–6.76 (m, 2H), 6.97–7.06 (m, 2H), 8.22
(br, 2H). 13C NMR (CDCl3): d (diastereomer I) 20.4, 21.6 (2C),
24.8, 36.9, 41.0, 41.3, 51.0, 52.2 (2C), 53.6, 128.3 (2C), 133.0
(2C), 140.6, 142.9, 169.0 (2C), 172.5 (2C); (diastereomer II) 20.7,
22.7 (2C), 24.9, 36.9, 41.1, 41.3, 51.3, 52.4 (2C), 53.8, 129.9 (2C),
133.9 (2C), 140.6, 142.9, 169.7 (2C), 173.2 (2C). HRMS (ESI
Q-TOF) m/z [M + H]+ calcd for C21H31N2O6 407.2182, found
407.2191.
NMR (CDCl3): d 37.2, 37.8, 41.2, 52.3 (2C), 53.4, 55.1, 56.6,
113.9 (2C), 120.6, 127.0, 128.4 (2C), 129.1 (2C), 129.7 (2C),
135.4, 135.8, 169.4, 169.7, 169.9, 171.3 [(S,S) major]; 37.3, 37.7,
41.2, 52.2 (2C), 53.2, 55.1, 56.9, 114.0 (2C), 120.7, 127.0, 128.5
(2C), 129.2 (2C), 129.9 (2C), 135.4, 135.8, 169.5, 169.7, 169.9,
171.3 [(S,R) minor]. HRMS (ESI Q-TOF) m/z [M + H]+ calcd for
C24H29N2O7 457.1975, found 457.1981.
Acknowledgements
`
Universita degli Studi di Roma ‘‘La Sapienza’’ is gratefully
acknowledged for financial support. We thank Professor C. G.
Frost of the University of Bath for preliminary helpful advice.
References
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1676. H NMR (CDCl3): d 1.46 (s, 18H), 2.25 (s, 3H), 3.01–3.15
(m, 4H), 3.44–3.51 (m, 1H), 3.89–3.96 (m, 2H), 4.67–4.76 (m,
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+
H]+ calcd for
C30H41N2O6 525.2965, found 525.2958.
Methyl (2S)-2-{3-[(3-methoxy-3-oxopropyl)amino]-2-(4-
methylbenzyl)-3-oxopropanamido}-3-phenylpropanoate
(13e,e9). Yield 82%. Brown oil. nmax cm21 3422, 3334, 1735,
1678. 1H NMR (CDCl3): d 2.25 (s, 3H), 2.43–2.60 (m, 2H), 3.02–
3.19 (m, 4H), 3.63–3.77 (m, 3H), 3.68 (s, 6H), 4.69–4.92 (m, 1H),
6.71–6.75 (m, 2H), 6.98–7.03 (m, 2H), 7.08–7.25 (m, 5H), 8.08
(br, 2H) [(S,S) major]; 2.35 (s, 3H), 2.43–2.60 (m, 2H), 3.02–3.19
(m, 4H), 3.63–3.77 (m, 3H), 3.69 (s, 6H), 4.69–4.92 (m, 1H),
6.71–6.75 (m, 2H), 6.98–7.03 (m, 2H), 7.08–7.25 (m, 5H), 8.16
(br, 2H) [(S,R) minor]. 13C NMR (CDCl3): d 20.4, 33.5, 35.0, 37.6,
38.7, 51.9 (2C), 53.5, 68.0, 114.5, 115.1 (2C), 127.2, 128.6 (2C),
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13476 | RSC Adv., 2013, 3, 13470–13476
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