Total synthesis of the elastin crosslinker (+)-desmopyridine

Article abstract of DOI:10.1016/j.tetasy.2012.10.009

Desmopyridine, isolated from the acid hydrolysates of bovine ligamentum nuchae, is an elastin crosslinker and an amino acid that has a 3,4,5-trisubstituted pyridine skeleton. Herein we report the first total synthesis of (+)-desmopyridine in 10% yield over six steps starting from 4-aminopyridine via chemo- and regioselective palladium-catalyzed Sonogashira and Negishi cross-coupling reactions.

Full text of DOI:10.1016/j.tetasy.2012.10.009

Tetrahedron: Asymmetry 23 (2012) 1557–1563
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Tetrahedron: Asymmetry
journal homepage: www.elsevier.com/locate/tetasy
Total synthesis of the elastin crosslinker (+)-desmopyridine
⇑
Yuko Murakami, Hiroto Yanuma, Toyonobu Usuki
Department of Materials and Life Sciences, Faculty of Science and Technology, Sophia University, 7-1 Kioicho, Chiyoda-ku, Tokyo 102-8554, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 6 September 2012
Accepted 1 October 2012
Desmopyridine, isolated from the acid hydrolysates of bovine ligamentum nuchae, is an elastin cross-
linker and an amino acid that has a 3,4,5-trisubstituted pyridine skeleton. Herein we report the first total
synthesis of (+)-desmopyridine in 10% yield over six steps starting from 4-aminopyridine via chemo- and
regioselective palladium-catalyzed Sonogashira and Negishi cross-coupling reactions.
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
model structures of elastin have been proposed,5 elucidation of
the entire three-dimensional structure of elastin utilizing spectro-
Elastic fibers exist in vertebrate tissues such as the lung, skin,
blood vessels, and the heart, and play an important role in provid-
ing their elasticity and stretch properties.1,2 These unique func-
tions are mainly derived from their self-assembling nature and
the crosslinking structure. Elastin, the main component of elastic
fibers, is an extremely insoluble extracellular matrix protein that
consists of soluble precursor tropoelastin monomers crosslinked
by chiral amino acids. Hydrophobic domains in rubber-like pro-
teins make elastin easily stretchable when an external tensile force
is applied, while the stretch can be released without external ac-
tion (Fig. 1). Therefore, the elastin crosslinkers are significant ami-
no acids with regard to their elasticity and stretch properties.
scopic methods, such as NMR (nuclear magnetic resonance), X-ray
diffraction, CD (circular dichroism),6 and so on, has been limited to
date due to its insoluble nature.
However, the chemical structures of the amino acid crosslinkers
have been mostly elucidated. Desmopyridine 1 (Fig. 2), one of the
elastin crosslinkers, was isolated from the acid hydrolysates of bo-
vine ligamentum nuchae and its structure was elucidated by Suy-
ama et al. in 2001.7 The amount of 1 in human aorta elastin was
found to gradually increase with age, suggesting that 1 might be
a useful biomarker for the aortic elastin damaged by ammonia.
On the contrary, the pyridinium amino acids desmosine 2 (Fig. 2)
and isodesmosine 3, known as attractive biomarkers for the diag-
Figure 1. Depiction of elastic fibers.
The crosslinking structures in elastin are spontaneously formed
when the amine moiety of the lysine residue of tropoelastin is spe-
cifically converted into the aldehyde allysine by lysyl oxidase in
the extracellular space.3,4 The allysine generated then reacts with
other allysines and/or lysine to form the crosslinkers. Although
nosis of COPD (chronic obstructive pulmonary disease),8 have also
been identified as elastin crosslinkers.9 Recently the first- and sec-
ond-generation total synthesis of desmosine 2 was achieved in our
laboratory.10 The synthesis involved chemo- and regioselective
palladium-catalyzed cross-coupling reactions between pyridine
cores and the corresponding segments as key steps. In order to
investigate the possible use as a biomarker for damaged elastin in-
duced by ammonia and in order to obtain some clues about the
⇑
Corresponding author. Tel.: +81 3 3238 3446; fax: +81 3 3238 3361.
E-mail address: t-usuki@sophia.ac.jp (T. Usuki).
0957-4166/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetasy.2012.10.009

1558
Y. Murakami et al. / Tetrahedron: Asymmetry 23 (2012) 1557–1563
NH2 NH2
CO2H CO2H
NH2 NH2
CO2H CO2H
CO2H
CO2H
CO2H
H2N
H2N
N
N
desmopyridine 1
H2N
desmosine 2
CO2H
CO2H
NH2
CO2H
NH2
CO2H
H2N
H2N
N
isodesmosine 3
Figure 2. Structures of the elastin crosslinkers desmopyridine 1, desmosine 2, and isodesmosine 3.
three-dimensional structure of elastin, we report the first total syn-
thesis of the elastin crosslinker desmopyridine 1 utilizing stepwise
palladium-catalyzed cross-coupling reactions.
was carried out in the presence of MeOH, alkyne 7 was obtained in
87% yield along with a small amount of the methyl ester.17 Thus
the preparation of alkyne 7 with the L-configuration of the amino
acid was achieved.
In our previous synthetic study,10b we found that the large-scale
2. Results and discussion
preparation of 3,5-dibromo-4-iodopyridine
5 had limitations.
Therefore, as an alternative route, commercially available 4-ami-
nopyridine was selected as the starting material to prepare 5 (Ta-
ble 1). The regioselective introduction of a bromide into the 3,5-
positions in the pyridine ring was achieved by treatment with N-
bromosuccinimide (NBS) to afford 4-amino-3,5-dibromopyridine
12 in 92% yield.18 Diazonium salt formation of the amine group
of 12 utilizing sodium nitrite under acidic conditions and the fol-
lowing Sandmeyer reaction using KI were then optimized (Table 1,
entries 1–5). When hydrochloric acid was used as an acid, only 40%
of the starting material was recovered (entry 1).19 The use of sulfu-
ric acid and tetrafluoroboric acid (HBF4) gave the desired product 5
in 10% and 12% yields, respectively (entries 2 and 3).20 When the
reaction, in the presence of HBF4, was carried out in dimethylform-
amide (DMF) in order to dissolve the substrates, no product was
obtained (entry 4). Eventually, the reaction with 10 equiv of so-
dium nitrite in the presence of HBF4 afforded 5 in 55% yield (entry
5). The synthetic route now established is capable for the gram-
scale synthesis of 3,5-dibromo-4-iodopyridine 5.
With the coupling substrates 5, 6, and 7 in hand, we explored C–
C bond formation between the pyridine core and the correspond-
ing alkyne and iodo amino acid utilizing palladium-catalyzed
cross-coupling reactions in order to achieve the total synthesis of
1 (Scheme 3). Based on the established synthetic strategy,10b the
Sonogashira cross-coupling reaction between the 4-position in 5
and alkyne 7 was carried out with 10 mol % Pd2(dba)3, 40 mol %
P(2-furyl)3, and 40 mol % CuI at 40 °C to give monocoupled pyri-
The target molecule 1 consists of a pyridine core and two types
of amino acids. As presented in Scheme 1, it was thus envisioned
that 1 would be retrosynthetically derived from 3,4,5-trisubsti-
tuted pyridine 4, which can be accessed by chemo- and regioselec-
tive palladium-catalyzed Sonogashira and Negishi cross-coupling
reactions between trihalogenated pyridine 5 and the correspond-
ing iodoalkylated
mo-4-iodopyridine
L
-glycine 6 and terminal alkyne 7. 3,5-Dibro-
would then be formed from 4-
5
aminopyridine. Benzyl 2-(S)-[(tert-butoxycarbonyl)amino]-4-iodo-
butanoate 6 can be prepared from commercially available 4-benz-
oxyl-(S)-3-[(tert-butoxycarbonyl)amino]-4-oxobutanoic
and tert-butyl 2-(S)-((tert-butoxycarbonyl)amino)pent-4-ynoate 7
can be formed from -serine, which is a naturally derived -amino
acid with the -configuration.
Our synthesis started with the stereoselective preparation of
terminal alkyne 7 starting from -serine (Scheme 2). The protection
of -serine using di-tert-butyl dicarbonate ((Boc)2O) in a mixed sol-
acid,11
L
a
L
L
L
vent of MeOH/Et3N/1 M NaOH (9:1:1 v/v/v) gave N-tert-butoxycar-
bonyl amino acid 8 in 90% yield.12 The carbonyl group of 8 was
then selectively protected by t-butyl group in the presence of a free
hydroxyl group by utilizing O-tert-butyl-N,N0-diisopropylisourea,
which was freshly prepared from N,N0-diisopropylcarbodiimide,13
to afford N-tert-butoxycarbonyl-O-tert-butyl amino acid 9 in 74%
yield.14 The use of the impure isourea in the reaction gave an
inseparable by-product. Replacement of the hydroxyl group in 9
with iodine gave iodoalkanoate 10 in 86% yield via an Appel reac-
tion. The coupling reaction of the organocopper compound and (2-
bromoethynyl)trimethylsilane was achieved as follows; the
organozinc reagent was prepared from 10 by treatment with Zn
and trimethylsilyl chloride (TMSCl), and then the coupling reaction
between organocopper substrate (RCu(CN)ZnI),15 prepared from
CuCN and LiCl, and freshly distilled (2-bromoethynyl)trimethylsi-
lane was carried out to afford product 11 in 51% yield.16 Treatment
with tetrabutylammonium fluoride (TBAF) in tetrahydrofuran
(THF) in the presence of EtOH then led to the desired terminal al-
dine 13 in 58% yield. The introduction of iodoalkylated L-glycine
6 into the 3,5-positions of the obtained 13 was then attempted
by a Negishi cross-coupling reaction using the catalyst Pd-PEP-
PSI-IPr.21 The reaction was run with an improved procedure,22
which can be used to remove the extra Zn from the organozinc re-
agent via centrifugation, to afford tricoupled pyridine 4 as the ma-
jor product in 52% yield, along with dicoupled pyridine 14 in 17%
yield as a by-product. After the reduction of the alkyne and re-
moval of the benzyl groups with H2 and Pd/C, the t-butyl and t-
butoxycarbonyl protecting groups were successfully removed
kyne 7 {½aꢀD ¼ þ44:7 (c 0.1, CHCl3)} in 93% yield. When the reaction

Y. Murakami et al. / Tetrahedron: Asymmetry 23 (2012) 1557–1563
1559
NH2
CO2H
CO2H
NH2
CO2H
H2N
N
1
NHBoc
CO2tBu
CO2Bn
NHBoc
CO2Bn
BocHN
N
4
NH2
I
NHBoc
CO2Bn
Br
Br
I
N
N
6
+
5
4-aminopyridine
NHBoc
NH2
CO2H
L-serine
HO
CO2tBu
7
Scheme 1. Retrosynthesis of 1.
using trifluoroacetic acid (TFA) to give the crude product contain-
ing 1. Purification by reverse-phase silica gel column chromatogra-
phy afforded pure (+)-desmopyridine 1 in 66% yield over two steps.
4. Experimental
4.1. General
The specific rotation of synthetic 1 was observed as ½a D20
¼ þ10:2 (c
ꢀ
0.1, H2O), suggesting that natural desmopyridine 1 is a chiral mol-
All non-aqueous reactions were conducted under an atmo-
sphere of nitrogen with magnetic stirring using dry solvents unless
otherwise indicated. Diisopropylethylamine (iPr2NEt) was dried by
distillation and stored over activated molecular sieves. DMF was
distilled by MgSO4 and stored over activated molecular sieves.
TMSCl and Et3N were dried by distillation. THF, MeOH, and EtOH
were stored over activated molecular sieves. All reagents were ob-
tained from commercial suppliers and used without further purifi-
cation unless otherwise stated. Analytical thin layer
chromatography (TLC) was performed on Silica gel 60 F254 plates
produced by Merck. Column chromatography was performed with
ecule derived from L
-amino acid as expected.23 Other spectroscopic
data for 1 including NMR spectra were also in good agreement
with those for natural 1.7
3. Conclusion
The first total synthesis of the elastin crosslinker (+)-desmo-
pyridine 1 has been achieved with chemo- and regioselective
Sonogashira and Negishi cross-coupling reactions as the key
steps in 10% yield over six steps starting from 4-aminopyri-
dine. This achievement could be used to contribute to the
studies on the three-dimensional structure of elastin fibers
and to develop potential biomarkers for aortic elastin damaged
by ammonia.
acidic Silica gel 60 (spherical, 40–50
lm) or neutral Silica gel 60 N
(spherical, 40–50 m) produced by Kanto Chemicals. Reversed-
l
phase silica gel column chromatography was performed with
Wako gel 100 C18 produced by Wako Chemicals. The enatiomeri-

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Y. Murakami et al. / Tetrahedron: Asymmetry 23 (2012) 1557–1563
N
OtBu
C
HN
(Boc)2O
Et3N/NaOHaq
NH2
NHBoc
CO2H
HO
HO
MeOH, rt
90%
CH2Cl2, reflux
74%
CO2H
L-serine
8
Zn, TMSCl
DMF, rt
I2, PPh3
imidazole
then CuCN
LiCl, -20 °C
NHBoc
CO2tBu
NHBoc
HO
I
CH2Cl2, 0 °C
86%
TMS
Br
CO2tBu
10
-20 °C to rt
51%
9
NHBoc
NHBoc
TBAF, EtOH
THF, 0 °C
93%
TMS
CO2tBu
CO2tBu
11
7
[α]D = +44.7
Scheme 2. Synthesis of terminal alkyne 7.
Table 1
wavenumbers (cmꢁ1). 1H and 13C NMR spectra were recorded on
a JEOL JNM-EXC 300 spectrometer (300 MHz) or on a JEOL JNM-
ECA 500 spectrometer (500 MHz). 1H NMR data are reported as fol-
low: chemical shift (d, ppm), integration, multiplicity (s, singlet; d,
doublet; t, triplet; q, quartet; m, multiplet), coupling constants (J)
in Hz, assignments. 13C NMR data are reported in terms of chemical
shift (d, ppm). EI-MS spectra were recorded on a Shimadzu GCMS
QP-5050 instrument or on a JEOL JMS-700 instrument. FAB-MS
spectra were also observed on a JEOL JMS-700. ESI-MS spectra
were recorded on a JEOL JMS-T100LC instrument. Elemental anal-
ysis was performed by a Perkin Elmer PE2400 II apparatus.
The carbon numbering in the NMR spectra of all compounds
corresponds with natural desmopyridine 1, as reported in the
literature.7
Synthesis of 12 and 5.a
NH2
NH2
N
Br
Br
NBS, CH2Cl2
reflux
92%
N
4-aminopyridine
12
I
reagents and
solvent
Br
Br
(see Table)
then KI aq
acetone, rt
N
5
4.2. 2-(S)-((tert-Butoxycarbonyl)amino)-3-hydroxypropionic
acid 8
Entry
NaNO2 (equiv)
Acid
HCl
H2SO4
HBF4
HBF4
HBF4
Solvent
Yieldb (%)
1
2
3
4
5
1.1
1.1
1.1
1.1
10
H2O
H2O
H2O
DMF
H2O
0 (s.m. 40%)
10
12
0
To a solution of starting material L-serine (5.01 g, 47.6 mmol,
1.0 equiv) in MeOH (135 mL) was added a solution of 1 M NaOH
(15 mL), Et3N (15 mL) and (Boc)2O (20.8 g, 95.1 mmol, 2.0 equiv)
in one portion. The mixture was stirred overnight at room temper-
ature. The organic solvent was removed in vacuo, and then diluted
with ethyl acetate (EtOAc, 75 mL). The solution was acidified with
3 M HCl solution to pH 2. The aqueous layer was then extracted
with EtOAc. The combined organic layers were washed with brine,
dried over Na2SO4, and concentrated in vacuo. The product 8 was a
colorless solid (8.89 g, 43.3 mmol, 90%) without requiring further
55
a
The reaction time was 30 min.
Isolated yield.
b
cally pure (>99.5%) amino acid was purchased from Watanabe
Chemical Industries, Ltd (Hiroshima, Japan).
Melting points were measured by an AS one ATM-01 apparatus.
Optical rotations were measured on a JASCO P-2200 digital polar-
imeter at the sodium lamp (k = 589 nm) D line and are reported
purification; ½a D20
ꢀ
¼ þ1:2 (c 1.0, MeOH) {lit. value: ½a D20
¼ þ1:95
ꢀ
(c 1.0, MeOH)24}; IR (KBr, cmꢁ1) 3531, 3436, 2985, 2879, 1736,
1674, 1524, 1369, 1236, 1167, 1074, 1027, 849, 781, 752, 463; 1H
NMR (300 MHz, CDCl3) d 5.54 (1H, br s, NH), 4.32–4.35 (1H, br s,
H14), 4.11–4.08 (1H, m, H13), 3.89–3.84 (1H, m, H13), 1.47 (9H,
as follows: ½a TD
ꢀ
(c g/100 mL, solvent). Infrared (IR) spectra were re-
corded on a JASCO FT-IR 4100 spectrometer and are reported in

Y. Murakami et al. / Tetrahedron: Asymmetry 23 (2012) 1557–1563
1561
NHBoc
CO2tBu
NHBoc
CO2tBu
NHBoc
CO2Bn
I
I
Br
Br
7
6
Br
Br
Zn, TMSCl, DMF, rt
Pd2(dba)3 (10 mol%)
P(2-furyl)3 (40 mol%)
CuI (40 mol%)
N
then
Pd-PEPPSI-IPr (20 mol%)
50 °C
5
DMF, iPr2NEt
N
40 °C, 58%
13
NHBoc
NHBoc
CO2tBu
CO2tBu
CO2Bn
NHBoc
CO2Bn
NHBoc
CO2Bn
+
Br
BocHN
N
N
4 (52%)
14 (17%)
NH2
CO2H
NH2
CO2H
1) H2, Pd/C
MeOH, rt
CO2H
2) TFA/H2O = 95/5
rt, 66% (2 steps)
H2N
N
(+)-1
[α]D = +10.2
Scheme 3. Total synthesis of 1.
s, tBu); Anal. Calcd for C8H15NO5: C, 46.8; H, 7.4; N, 6.8. Found: C,
(9H, s, tBu); Anal. Calcd for C12H23NO5: C, 55.2; H, 8.9; N, 5.4.
47.0; H, 7.4; N, 6.9.
Found: C, 55.5; H, 8.4; N, 5.4.
4.3. tert-Butyl 2-(S)-((tert-butoxycarbonyl)amino)-3-hydroxy-
4.4. tert-Butyl 2-(S)-((tert-butoxycarbonyl)amino)-3-iodoprop-
propanoate 9
anoate 10
To a solution of 2-(S)-((tert-butoxycarbonyl)amino)-3-hydroxy-
propionic acid 8 (498 mg, 2.44 mmol, 1.0 equiv) in CH2Cl2 (5 mL)
Triphenylphosphine (2.50 g, 9.54 mmol, 1.5 equiv) and imidaz-
ole (0.65 g, 9.54 mmol, 1.5 equiv) were dissolved in CH2Cl2
(30 mL) with stirring under nitrogen at 0 °C. After powdered iodine
(2.42 g, 9.54 mmol, 1.5 equiv) was added to the solution, the mix-
ture was warmed to room temperature and then cooled to 0 °C.
Next, a solution of tert-butyl 2-(S)-((tert-butoxycarbonyl)amino)-
3-hydroxypropanoate 9 (1.66 g, 6.36 mmol, 1.0 equiv) in CH2Cl2
(12 mL) was added to the cooled solution. After stirring for 1.5 h
at 0 °C, the reaction mixture was concentrated in vacuo. The resi-
due was filtered through a short column of neutral silica gel eluting
with hexane/Et2O (1/1), and the filtrate was concentrated. Purifica-
tion by silica gel column chromatography (hexane/EtOAc = 60/1 ?
30/1) afforded 10 as a yellow solid. Recrystallization from pentane
gave as a colorless solid (1.99 g, 5.36 mmol, 86%); Rf 0.49 (hexane/
was
added
2-tert-butyl-1,3-diisopropyl-isourea
(0.9 mL,
3.66 mmol, 1.5 equiv). The solution was stirred and refluxed for
3 h, and then 2-tert-butyl-1,3-diisopropyl-isourea (0.9 mL,
3.66 mmol, 1.5 equiv) was added to the reaction mixture again.
After stirring for 2 h at reflux, the mixture was evaporated in va-
cuo. The residue was filtered through a short column of silica gel
eluting EtOAc, and the filtrate was concentrated. Purification by sil-
ica gel column chromatography (hexane/EtOAc = 5/1) afforded 9 as
a colorless solid (470 mg, 1.80 mmol, 74%); Rf 0.61 (hexane/
EtOAc = 1/1); ½a 2D5
ꢀ
¼ ꢁ20:8 (c 1.6, EtOH) {lit. value: ½a D22
¼ ꢁ22:5
ꢀ
(c 1.8, EtOH)25}; IR (KBr, cmꢁ1) 3322, 3171, 2976, 2890, 1700,
1478, 1365, 1294, 1186, 1060, 930, 854, 791, 755, 694, 585, 437;
1H NMR (300 MHz, CDCl3) d 5.40 (1H, br s, NH), 4.26 (1H, br s,
H14), 3.90–3.89 (2H, d, J = 3.81 Hz, H13), 1.48 (9H, s, tBu), 1.45
EtOAc = 10/1); ½a D25
ꢀ
¼ ꢁ11:5 (c 1.0, MeOH); IR (KBr, cmꢁ1) 3398,
3023, 2984, 2935, 2868, 2293, 1729, 1482, 1410, 1352, 1300,

1562
Y. Murakami et al. / Tetrahedron: Asymmetry 23 (2012) 1557–1563
1248, 1150, 1075, 977, 920, 842, 799, 778, 650, 537, 469, 442; 1H
NMR (300 MHz, CDCl3) d 5.34 (1H, d, J = 6.79 Hz, NH), 4.36–4.33
(1H, m, H14), 3.57 (2H, d, J = 3.53 Hz, H13), 1.48 (9H, s, tBu), 1.45
(9H, s, tBu); 13C NMR (75 MHz, CDCl3) d 168.6, 155.0, 83.3, 80.2,
53.8, 28.4, 28.1, 8.9; ESI-HRMS (m/z) Calcd for C14H21INNaO4
[M+Na]+ 394.0515. Found 394.0489.
4.7. 4-Amino-3,5-dibromopyridine 12
To suspension of 4-aminopyridine (188.2 mg, 2.0 mmol,
a
1.0 equiv) in CH2Cl2 (10 mL), N-bromosuccinimide (818.7 mg,
4.6 mmol, 2.3 equiv) in CH2Cl2 (30 mL) was added over 1 h with
intensive stirring. The reaction mixture was stirred for 24 h, and
then the solvent was concentrated in vacuo to obtain a mixture
of succinimide and 12. Purification by silica gel column chromatog-
raphy (hexane/EtOAc = 1/1) afforded 12 (464.8 mg, 1.84 mmol,
92%) as a colorless solid; Rf 0.62 (hexane/EtOAc = 1/1); mp 163–
167 °C; IR (KBr, cmꢁ1); 3535, 3429, 3266, 3100, 2667, 2536,
2329, 1872, 1825, 1785, 1626, 1566, 1522, 1485, 1408, 1323,
1262, 1154, 1138, 1081, 1051, 892, 855, 747, 726, 658, 589, 573,
517, 468, 431; 1H NMR (300 MHz, CDCl3) d 8.31 (2H, s, H2/6),
5.06 (2H, s, NH); 13C NMR (75 MHz, CDCl3) d 149.9, 147.8, 106.2;
EI-MS (m/z) Calcd for C5H4Br2N2 [M]+ 249.87. Found 249.90.
4.5. tert-Butyl 2-(S)-((tert-butoxycarbonyl)amino)-5-(trimethyl-
silyl)pent-4-ynoate 11
At first, CuCN (174 mg, 1.94 mmol, 1.0 equiv) and LiCl (164 mg,
3.88 mmol, 2.0 equiv) were dried together by heating under vac-
uum at 150 °C for 2 h in a flask. While the CuCN and LiCl were dry-
ing, zinc powder (785 mg, 12.0 mmol, 6.0 equiv) was dried under
vacuum with heating using a heat gun over 5 min. To the flask con-
taining the zinc powder were added DMF (0.9 mL) and TMSCl
(76 lL, 0.6 mmol) at room temperature. After stirring for 30 min
4.8. 3,5-Dibromo-4-iodopyridine 5
at room temperature, to this suspension was slowly added a solu-
tion of tert-butyl 2-(S)-((tert-butoxycarbonyl)amino)-3-iodopro-
panoate 10 (719 mg, 1.94 mmol, 1.0 equiv) in DMF (0.9 mL,
washed with 0.1 mL ꢂ 2). After confirming the generation of 11-
Zn as judged by TLC, to another flask containing the CuCN and LiCl
in DMF (3.2 mL) cooling to ꢁ20 °C was added a solution of 11-Zn
over 30 min.
After stirring for 15 min at ꢁ20 °C, freshly prepared (2-bromoe-
thynyl)trimethylsilane (0.6 mL, 3.88 mmol, 2.0 equiv) in DMF
(0.6 mL) was added dropwise to the reaction mixture over 5 min.
Next, the mixture was allowed to slowly warm to room tempera-
ture and then stirred for 17 h. The mixture was diluted with EtOAc,
and quenched with a saturated NH4Cl solution. The aqueous layer
was then extracted with EtOAc. The combined organic layers were
washed with brine, dried over Na2SO4, and concentrated in vacuo.
Purification by silica gel column chromatography (hexane/
EtOAc = 60/1) afforded 11 as a yellow solid (339 mg, 0.99 mmol,
To a cooled (0 °C) solution of 4-amino-3,5-dibromopyridine 12
(2.0 g, 7.94 mmol, 1.0 equiv) in 48% HBF4 aq (30 mL) was added
dropwise NaNO2 (5.4 g, 79.4 mmol, 10 equiv) in water (16 mL)
ensuring that no gas evolution could be detected. The resultant
slurry was stirred at 0 °C for 30 min. The reaction mixture was fil-
tered to afford a white solid. The solid was quickly transferred por-
tionwise to a stirred solution of KI (2.1 g, 12.7 mmol, 1.6 equiv) in
25 mL of acetone/H2O (2/3). The resultant brown slurry was decol-
orized with saturated Na2S2O3 and carefully neutralized with NaH-
CO3. The solution was then extracted with EtOAc. The combined
organic layers were washed with brine, dried over Na2SO4, and
concentrated in vacuo. Purification by silica gel column chroma-
tography (hexane/EtOAc = 10/1) yielded
5 (1.59 g, 4.38 mmol,
55%) as a colorless solid; Rf 0.61 (hexane/EtOAc = 5/1); mp 186–
190 °C; IR (KBr, cmꢁ1) 1858, 1808, 1634, 1547, 1522, 1496, 1404,
1389, 1205, 1175, 1096, 1020, 883, 756, 687, 517, 422; 1H NMR
(300 MHz, CDCl3) d 8.56 (2H, s, H2/6); 1H NMR (300 MHz, DMSO-
d6) d 8.65 (2H, s, H2/6); 13C NMR (CDCl3, 75 MHz) d 148.4, 129.9,
120.3; 13C NMR (75 MHz, DMSO-d6) d 148.3, 129.4, 122.1; EI-MS
(m/z) Calcd for C5H2Br2IN [M]+ 360.76. Found 360.75; EI-HRMS
(m/z) Calcd for C5H2Br2IN [M]+ 360.7599. Found 360.7589.
51%);
Rf
0.56
(hexane/EtOAc = 10/1);
mp = 42–44 °C;
½
a 2D5
ꢀ
¼ þ54:0 (c 0.1, CHCl3); IR (KBr, cmꢁ1) 3444, 2979, 2179,
1719, 1498, 1392, 1368, 1251, 1156, 1051, 998, 845, 761, 699,
641; 1H NMR (300 MHz, CDCl3) d 5.29 (1H, d, J = 8.1 Hz, NH),
4.36–4.31 (1H, m, H14), 2.80–2.63 (2H, m, H13), 1.48 (9H, s, tBu),
1.46 (9H, s, tBu), 0.14 (9H, s, TMS); 13C NMR (75 MHz, CDCl3) d
170.1, 155.7, 83.0, 80.5, 79.4, 71.8, 52.8, 28.5, 28.1, 24.6, 1.90,
1.25, 1.15, 0.08; FAB-MS (m/z) Calcd for C17H32NO4Si [M+H]+
342.2. Found 342.3.
4.9. (S)-tert-Butyl 2-(tert-butoxycarbonylamino)-5-(3,5-dibro-
mopyridin-4-yl)pent-4-ynoate 13
A
43.0
solution of 3,5-dibromo-4-iodopyridine
mol, 1.0 equiv), tert-butyl 2-(S)-((tert-butoxycar-
(18.7 mg, 69.5 mol, 1.5 equiv),
mol, 10 mol %), P(2-furyl)3 (4.2 mg,
mol, 40 mol %), and CuI (3.6 mg, 18.9 mol, 40 mol %) in
5
(15.6 mg,
4.6. tert-Butyl 2-(S)-((tert-butoxycarbonyl)amino)pent-4-yno-
ate 7
l
bonyl)amino)pent-4-ynoate
Pd2(dba)3 (4.2 mg, 4.6
18.0
DMF (2.3 mL) was degassed by freeze/pump/thaw techniques.
Next, iPr2NEt (0.45 mL) was added to the resulting solution. After
stirring at 40 °C for 4 h, the reaction mixture was diluted with
EtOAc and quenched with a saturated NH4Cl solution. The aqueous
layer was then extracted with EtOAc. The combined organic layers
were washed with brine, dried over Na2SO4, and concentrated in
vacuo. Purification by silica gel column chromatography (hexane/
7
l
l
To a solution of 11 (280.0 mg, 0.82 mmol, 1.0 equiv) in THF
l
l
(9.5 mL) and EtOH (240
was added TBAF (1.0 M solution in THF; 410
l
L, 4.10 mmol, 5.0 equiv) cooled to 0 °C
L, 0.41 mmol,
l
0.5 equiv). After stirring at 0 °C for 1.5 h, the reaction mixture
was diluted with EtOAc, and quenched with saturated NH4Cl solu-
tion. The aqueous layer was then extracted with EtOAc. The com-
bined organic layers were washed with brine, dried over Na2SO4,
and concentrated in vacuo. Purification by silica gel column chro-
matography (hexane/EtOAc = 20/1) afforded 7 as a yellow solid
(204.1 mg, 0.76 mmol, 93%); Rf 0.37 (hexane/EtOAc = 15/1);
EtOAc = 10/1) yielded 13 as a colorless solid (12.5 mg, 24.8 lmol,
58%);
Rf
0.18
(hexane/EtOAc = 5/1);
mp = 120–122 °C;
mp = 63–66 °C; ½a 2D5
ꢀ
¼ þ44:7 (c 0.1, CHCl3); IR (KBr, cmꢁ1) 3397,
½
a 2D0
ꢀ
¼ þ25:6 (c 0.1, CHCl3); IR (KBr, cmꢁ1) 3220, 2972, 2241,
3278, 2985, 1699, 1496, 1423, 1362, 1154, 1063, 1018, 979, 874,
844, 781, 668, 543; 1H NMR (300 MHz, CDCl3) d 5.33 (1H, d,
J = 7.3 Hz, NH), 4.35–4.32 (1H, m, H14), 2.71–2.69 (2H, m, H13),
2.01 (1H, t, J = 2.45 Hz, H11), 1.48 (9H, s, tBu), 1.45 (9H, s, tBu);
13C NMR (75 MHz, CDCl3) d 169.8, 155.2, 101.3, 88.0, 82.3, 79.9,
77.6, 52.5, 28.9, 28.5, 23.6; EI-HRMS (m/z) Calcd for C14H23NO4
[M]+ 269.1628. Found 269.1603.
1707, 1534, 1450, 1356, 1160, 1055, 896, 851, 761, 721, 654,
437; 1H NMR (300 MHz, CDCl3) d 8.63 (2H, s, H2/6), 5.52 (1H, d,
J = 7.65 Hz, NH), 4.48–4.44 (1H, m, H14), 3.20–3.04 (2H, m, H13),
1.48 (9H, s, tBu), 1.46 (9H, s, tBu); 13C NMR (75 MHz, CDCl3) d
169.0, 154.7, 149.3, 100.0, 82.5, 79.7, 51.9, 29.4, 28.0, 27.7, 24.0;
ESI-HRMS (m/z) Calcd for C19H24Br2N2NaO4 [M+Na]+ 526.9980.
Found 526.9974.

Y. Murakami et al. / Tetrahedron: Asymmetry 23 (2012) 1557–1563
1563
4.10. (2S,20S)-Benzyl 4,40-(4-((S)-5-(tert-butoxy)-4-(tert-butoxy-
carbonylamino)-5-oxopent-1-ynyl)pyridine-3,5-diyl)bis(2-(tert-
butoxycarbonylamino)butanoate)) 4 and 3-bromo-(2S,20S)-ben-
zyl 4,40-(4-((S)-5-(tert-butoxy)-4-(tert-butoxycarbonylamino)-5-
oxopent-1-ynyl)pyridine-5-yl)-2-(tert-butoxycarbonylamino)-
butanoate)) 14
yielded a crude product. The product was used for the next reac-
tion without further purification.
A mixture of TFA and distilled water (6.8 mL, TFA/H2O = 95/5)
was added to the crude product (36.9 mg) at room temperature
and stirred for 2 h. The solvent was removed in vacuo. Purification
by C18 silica gel column chromatography (0.1% TFA in distilled
water) yielded desmopyridine 1 as a colorless solid (16.8 mg,
Zinc dust (200 mg, 3.0 mmol) was placed in a nitrogen-purged
37.0 lmol, 66%); Rf 0.30 [MeOH (0.1% TFA)/H2O (0.1% TFA) = 1/9];
1.5 mL Eppendorf microtube. Dry DMF (150
lL) and TMSCl
½
a 2D0
ꢀ
¼ þ10:2 (c 0.1, H2O); 1H NMR (500 MHz, D2O) d 8.49 (2H, s,
(60.0 L, 0.47 mmol) were then added to the microtube, and the
l
H2/6), 4.13 (2H, t, J = 6.3 Hz, H9/18), 4.05 (1H, m, H14), 3.11–2.91
(6H, m, H7/11/16), 2.25, (4H, d, J = 5.1 Hz, H8/17), 2.14 (2H, m,
H13), 1.71–1.64 (2H, m, H12); 13C NMR (125 MHz, D2O) d 172.0
(C10/15/19), 160.6 (C4), 139.4 (C2/3/5/6), 56.1 (C9/14/18), 44.2
(C11), 30.8 (C8/17), 30.6 (C13), 26.1 (C7/16), 25.2 (C12); ESI-HRMS
(m/z) Calcd for C18H29N4O6 [M+H]+ 397.2087. Found 397.2067.
resulting mixture was stirred vigorously for 15 min at room tem-
perature. After stirring, the solution was removed by a microsy-
ringe. The remaining solid was dried using a hot air gun at
reduced pressure. The activated zinc was then cooled to room tem-
perature. A solution of benzyl 2-(S)-((tert-butoxycarbonyl)amino)-
3-iodobutanoate 6 (211 mg, 0.5 mmol, 5.0 equiv) in dry DMF
(150 lL and washed with 100 lL DMF) was then added to the acti-
Acknowledgments
vated zinc. The reaction mixture was stirred at room temperature
for 1 h until completion of the insertion of zinc, as monitored by
TLC analysis (hexane/EtOAc = 3/1). The zinc duct was then allowed
to settle using a centrifuge for 1 min at room temperature. The
organozinc solution was removed from the activated zinc via
We thank Dr. Yong Y. Lin (Columbia University) and Professor
Kyozo Suyama (Tohoku University) for their suggestions, Professor
Masayuki Satake (University of Tokyo) for NMR measurements,
and Dr. Hidehiko Watanabe (Watanabe Chemical Industries, Ltd)
for providing data. This work was supported by a Grant-in-Aid
for Young Scientists (B) from the Ministry of Education, Culture,
Sports, Science and Technology (MEXT) of Japan (KAKENHI Grant
Number 22710224), the SEI Group CSR Foundation, and the Naito
Science Foundation.
microsyringe with 200
lL DMF and added to a 10 mL flask contain-
ing Pd-PEPPSI-IPr (13.6 mg, 0.02
l
mol, 20 mol %) and 13 (49.5 mg,
0.1 mmol, 1.0 equiv). After stirring for 6 h at 40 °C and 18 h at
50 °C, the reaction mixture was diluted with EtOAc and quenched
with saturated NH4Cl solution. The aqueous layer was then ex-
tracted with EtOAc. The combined organic layers were washed
with brine, dried over Na2SO4, and concentrated in vacuo to give
a crude product as a yellow oil. Purification by silica gel column
chromatography (hexane/EtOAc = 3/1 ? 1/1) afforded 4 (47.3 mg,
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50.9 lmol, 17%) as a yellow oil and 14 (12.9 mg, 18.0 lmol, 17%)
as a yellow oil, respectively. 4: Rf 0.34 (hexane/EtOAc = 1/1);
½
a 2D0
ꢀ
¼ þ32:1 (c 0.1, CHCl3); IR (neat, cmꢁ1) 3382, 2976, 2230,
1712, 1506, 1362, 1251, 1164, 1056, 906, 851, 756, 699, 592,
467; 1H NMR (300 MHz, CDCl3) d 8.20 (2H, s, H2/6), 7.34 (10H,
m, Bn), 6.01 (1H, s, 14NH), 5.52 (2H, d, J = 7.76 Hz, 9NH/18NH),
5.23–5.13 (4H, m, Bn), 4.46 (3H, m, H9/14/18), 3.10–2.87 (2H, m,
H13), 2.72–2.67 (4H, m, H7/16), 2.13–2.07 and 2.01–1.96 (4H, m,
H8/17), 1.44 (36H, m, tBu); 13C NMR (75 MHz, CDCl3) d 172.3,
169.8, 155.7, 135.4, 128.9, 128.8, 128.7, 83.1, 80.5, 80.3, 67.5,
53.5, 53.0, 33.0, 31.2, 29.5, 28.6, 28.5, 28.2, 14.4; ESI-HRMS (m/z)
Calcd for C51H68N4NaO12 [M+Na]+ 951.4731. Found 951.4737. 14:
Rf 0.58 (hexane/EtOAc = 1/1); ½a D20
¼ þ7:9 (c 0.1, CHCl3); IR (neat,
ꢀ
cmꢁ1) 3384, 2974, 2233, 1714, 1505, 1456, 1362, 1252, 1162,
1057, 851, 755, 698, 589, 468; 1H NMR (300 MHz, CDCl3) d 8.58
(1H, s, H6), 8.25 (1H, s, H2), 7.35 (5H, m, Bn), 5.80 (1H, d,
J = 5.95 Hz, 14NH), 5.52 (1H, d, J = 6.23 Hz, 9NH), 5.23–5.13 (2H,
m, Bn), 4.46 (2H, m, H9/14), 3.15–3.00 (2H, m, H13), 2.77–2.72
(2H, m, H7), 2.17–2.07 and 1.98–1.96 (2H, m, H8), 1.44 (27H, m,
tBu); 13C NMR (75 MHz, CDCl3) d 172.3, 169.6, 135.4, 128.9,
128.8, 128.7, 128.6, 82.9, 80.3, 67.4, 55.9, 53.5, 52.8, 52.7, 32.9,
29.8, 28.5, 28.2, 24.4; ESI-HRMS (m/z) Calcd for C35H46BrN3NaO8
[M+Na]+ 738.2366. Found 738.2355.
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4.11. Desmopyridine 1
22. Usuki, T.; Yanuma, H.; Hayashi, T.; Yamada, H.; Suzuki, N.; Masuyama, Y. J.
Heterocycl. Chem. http://dx.doi.org/10.1002/jhet.1807.
23. Currently natural desmopyridine 1 is not available. Therefore, the specific
rotation for natural 1 is unknown.
24. Private communication by Dr. H. Watanabe (Watanabe Chemical Industries,
Ltd, Hiroshima, Japan).
A
solution of 4 (52.1 mg, 56.0 lmol, 1.0 equiv) in MeOH
(2.1 mL) was treated with 10% Pd/C (357.6 mg, 0.34 mmol,
6.0 equiv) and hydrogenated at room temperature. After stirring
for 6 days, the insoluble material was separated by filtration
through a Celite pad on neutral silica gel eluting with MeOH and
the filtrate was concentrated in vacuo. Concentration of the filtrate
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