Useful synthesis of 2,3,6-tri- and 2,3,5,6-tetrasubstitutedpyridine derivatives from aspartic acid

Article abstract of DOI:10.3987/COM-04-10206

New synthetic methods for 2,3,6-tri- and 2,3,5,6-tetrasubstituted pyridineskeletons, which are the essential main central segments of thiostrepton-type macrocyclic antibiotics, were developed from l-α-aspartic acid (Asp) via the Asp-derived α-dehydroamino

Full text of DOI:10.3987/COM-04-10206

HETEROCYCLES, Vol. 63, No. 12, 2004, pp. 2735 - 2746
Received, 20th August, 2004, Accepted, 7th October, 2004, Published online, 8th October, 2004
USEFUL SYNTHESIS OF 2,3,6-TRI- AND 2,3,5,6-TETRASUB-
STITUTED PYRIDINE DERIVATIVES FROM ASPARTIC ACID
Yasuchika Yonezawa, Akihito Konn, and Chung-gi Shin*
Laboratory of Organic Chemistry, Faculty of Engineering, Kanagawa University,
Rokkakubashi, Kanagawa-ku, Yokohama 221-8686, Japan
Abstract－New synthetic methods for 2,3,6-tri- and 2,3,5,6-tetrasubstituted pyridine
skeletons, which are the essential main central segments of thiostrepton-type
macrocyclic antibiotics, were developed from L-α-aspartic acid (Asp) via the
Asp-derived α-dehydroamino acid ester.
Most thiostrepton-type macrocyclic antibiotics, such as micrococcins P (1)1 and nosiheptide (2)2 (Figure
1), are constituted of a 2,3,6-tri- or 2,3,5,6-tetrasubstituted pyridine skeleton as the main central segment.
So far, two kinds of polysubstituted 6-pyridone derivatives as the promising starting materials for the
above-mentioned pyridines for 1 and 2 have been already synthesized from 1,1-dimethoxypropane via
5-cyano-2- dimethoxymethyl-6-pyridone3 and from 2,3,5-trisubstituted 6-pyridone via the corresponding
pyridine derivative, respectively.4 However, unfortunately, the yield of the former 6-pyridone derivative
was very low (23% yield in two steps) and the latter synthetic route needed many steps. Therefore, it is
necessary to develop newly an alternative efficient method for the preparation of the required
polysubstituted pyridine derivatives.
In particular, concerning the possible formation of the polysubstituted pyridine segment constructing the
thiostrepton-type antibiotics, it was presumed that the pyridine skeletons mentioned above were produced
by an appropriate oxidation and cyclization of a peptide sequence constituted of an L-α-aspartic acid
(Asp) residue through the biosynthetic pathway. Here, to verify and realize chemically the above
presumption, the synthesis of the polysubstituted pyridine derivatives from the Asp via the Asp-derived
α-dehydroamino acid derivative was tried successfully.
First of all, esterification of the authentic benzyloxycarbonyl (Cbz)-L-Asp(OMe)-OH (3), derived from
L-Asp-OH in two steps, with DCC and N-hydroxybenzotriazole (HOBt), followed by selective reduction
of the formed activated α-ester with NaBH4 afforded the corresponding primary alcohol derivative (4).5
The two functional groups of both the (N-Cbz)amino and hydroxyl groups of 45 were protected with an

OH
S
O
N
S
N
HN
S
N
S
N
S
N
N
O
H
N
N
NH2
O
HN
N
N
S
O
HN
OH
O
HO
O
O
S
S
O
NH
HN
HN
S
O
HN
HO
O
O
OH
O N
NH
HN
S
N
O
N
S
O
N
HN
S
HN
O
OH
Nosiheptide (2)
Micrococcin P (1)
Figure 1
isopropylidene (Ip) group by using acetone dimethyl acetal (DMP) in the presence of p-toluenesulfonic
acid hydrate (p-TosOH.H2O) to give methyl [(4S)-3-(N-Cbz)-2,2-dimethyl-1,3-oxazolidin-4-yl]acetate (5).
Subsequently, reduction of the methyl ester (5) with NaBH4 in the presence of CaCl2 gave the
.
corresponding primary hydroxyl derivative (6), which was oxidized with SO3 pyridine in DMSO to form
O
O
O
i) DCC, HOBt
OMe
p-TosOH, DMP
OMe
OH
L-Asp-OH
2 steps
OMe
OH
CbzN
ii) NaBH4
87%
CbzHN
CbzHN
O
O
86% (2 steps)
4
3
5
O
(EtO)2P
O
O
OMe
NHBoc
OH
H
NaBH4, CaCl2
91%
SO3･ Py , DMSO
DBU
CbzN
CbzN
O
91% (2 steps)
O
7
6
Cbz
CbzHN
CbzHN
N
4%TFA / CHCl3
87%
SO3･Py , DMSO
OMe
HO
OMe
OMe
HO
N
O
BocHN
61%
BocHN
Boc
O
O
O
9
10
8
Scheme 1
the corresponding formyl derivative (7) as an intermediate. Without purification, Wittig reaction of the
formyl group of 7 with N-Boc-α-diethoxyphosphorylglycine methyl ester in the presence of DBU was

intact performed to give the required (Z)-α-dehydroamino acid; methyl (5S,2Z)-2-(N-Boc)amino-
4-[3-(N-Cbz)-2,2-dimethyl-1,3-oxa-zolidin-4-yl]-2-butenoate (8). Deprotection of the isopropylidene
(Ip) group with 4% trifluoroacetic acid (TFA) proceeded smoothly to give the corresponding
.
6-hydroxy-2-hexenoate derivative (9), which was then cyclized with SO3 pyridine in DMSO to give the
expected methyl (5S,6RS)-5-(Cbz)amino-6-hydroxy-1-(N-Boc)-4,5,6-trihydropyridine-2-carboxylate (10),
as shown in Scheme 1.
From the fact that compound (9) could cyclize, the geometric structures of 8 and 9 could be determined to
be (Z)-isomer. Furthermore, in the 1H NMR spectrum, the appearance of β-olefin protons at δ 6.44 and
δ 6.55 ppm for 8 and 9 supported also the above structural confirmation by comparing with the chemical
shifts of the (E)-isomers of appropriate α-dehydroamino acid esters.6 In general, it is well-known that the
olefinic protons of (E)-isomer of α-dehydroamino acids resonate at a lower magnetic field.
Secondly, to synthesize the desired 2,5-disubstituted 6-pyridone derivative for the 2,3,6-trisubstituted
pyridine, the hydroxyl compound (10) was oxidized with 2.67 Jones reagent to give methyl 1-(N-Boc)-
5-(Cbz)amino-6-oxo-4,5,6-trihydropyridine-2-carboxylate (11). After deprotecting the Boc group by
using TFA, the obtained 6-oxo-1,3,4-trihydropyridine derivative (12) was again oxidized with MnO2 to
give the corresponding 6-pyridone derivative (13). Furthermore, to transform the 6-pyridone ring into
the pyridine skeleton, O-methylation of 13 with MeI and Ag2CO3 was carried out to give
5-(N-Cbz)amino- pyridine derivative (14), the Cbz group of which was deprotected with 10% Pd-C/H2 to
give methyl 5-amino-6-methoxypyridine-2-carboxylate (15).
Finally, the amino group of 15 was subjected to cyanization by the method of Anderson and co-workers.7
That is, to replace the 5-amino group of 15 to a cyano group, treatment of 15 with NaNO2 and CuI,
followed by cyanization of the formed 5-iodopyridine derivative as an intermediate with CuI, Pd(PPh3)4,
and KCN gave the corresponding 5-cyanopyridine derivative (16). Furthermore, treatment of 16 with
BBr3 gave the expected methyl 5-cyano-6-pyridone-2-carboxylate (17), as shown in Scheme 2. As a
RHN
MeO
Jones
reagent
MeI,
Ag2CO3
CbzHN
O
CbzHN
O
MnO2
80%
OMe
10
OMe
OMe
OMe
N
N
H
N
90%
87%
R
O
O
O
14 : R=Cbz
15 : R=H
13
10%Pd-C/H2
99%
11 : R=Boc
12 : R=H
TFA
quant
i) NaNO2
ii) CuI
NC
NC
O
BBr3
84%
OMe
MeO
N
N
H
17
iii) CuI,
O
O
Pd(PPh3)4
KCN
40% (3 steps)
16
Scheme 2

result, compound (17) was thought to be the most important substance for the thiostrepton-type
antibiotics, because the reverse conversion of the 6-methoxycarbonyl compound (17) to the
corresponding thioamide derivative proceeds easily.8
On the other hand, with regard to the synthesis of the 2,3,5,6-tetrasubstituted pyridine derivative
constructing 2, firstly, bromination of the 3-position of 12 with NBS and then Et3N gave the
corresponding 3-bromo-6-oxo-1,4,5-trihydropyridine derivative (18), by the method reported earlier.7
Subsequently, oxidation of the ring of 18 to the 6-pyridone ring system was performed under various
conditions, as shown in Table 1. As a result, in the case using DBU, CuBr2, and hexamethylene-
tetramine (HMTA) as an oxidizing agent system, it was found that the reaction of 18 in CH2Cl2 at 0 oC for
4 h proceeded rapidly and the yield of the required 6-pyridone (19) reached 84%.
i) NBS
ii) Et3N
CbzHN
O
Br
O
CbzHN
O
Br
O
Ag2CO3, MeI
87%
oxidation
Table 1.
OMe
12
OMe
N
H
N
H
98%
19
18
CbzHN
MeO
Br
O
RHN
MeO
OMe
OMe
NC
MeO
OMe
i) NaNO2
HCl
NaOMe, CuI
62%
OMe
OMe
N
N
N
ii) CuCN
CuSO4
O
O
20
23
21 : R=Cbz
22 : R=H
10% Pd-C, H2
99%
31% (2 steps)
Scheme 3
Furthermore, similarly to the case of 14, O-methylation of the carbonyl group of 19 with MeI and
Ag2CO3, followed by further substitution at the 3 position of the formed 6-methoxypyridine (20) with CuI
and MeONa gave 3,6-dimethoxypyridine derivative (21). To form the 5-cyanopyridine derivative,
deprotection of the Cbz group of 21 with H2 on 10% Pd/C gave 5-aminopyridine derivative (22), the
amino compound (22) which was cyanized with NaNO2 and HCl and then CuCN and Na2CO3 to give the
Table 1. Oxidation of 18 to 6-Pyridone (19)
Entry
Reagent
DDQ, AcOH
MnO2
Time (h)
Temp. (oC)
Solvent
Yield (%)
1
2
3
96
48
4
50
rt.
0
Dioxane
CHCl3
10
75
84
DBU, CuBr2, HMTA1)
CH2Cl2
1) Hexamethylenetetramine

required 5-cyanopyridine derivative (23), as shown in Scheme 3.
promising 23 for the synthesis of 2 could be synthesized.
Consequently, it was found that the
In conclusion, although the synthetic route needs multiple steps, it is important that the main central
polysubstituted pyridine skeleton in antibiotics (1) and (2) were readily derived from L-α-aspartic acid via
α-dehydroamino acid derivative. Furthermore, this synthetic method is thought to be widely applicable
to the synthesis of various polysubstituted pyridine derivatives.
ACKNOWLEDGEMENT
This work was supported in part by a Grant-in-Aid for Scientific Research No. 14550829 from the
Ministry of Education, Science, Sports and by “High-Tech Research Project” from the Ministry of
Education, Culture, Sports, Science and Technology.
EXPERIMENTAL
The melting points were measured using a Yamato (Model Mp-21) micro-melting point apparatus, and are
uncorrected. The IR spectra were recorded using a Hitachi EPI-G3 spectrophotometer in KBr. The
NMR spectra were measured with JEOL FX 200 and JNE 500 spectrometers in CDCl3 or DMSO-d6
solution with tetramethylsilane used as the internal standard. The specific rotations were measured in a
0.5 dm tube using a JASCO DIP-4 polarimeter in MeOH.
Methyl (3S)-Benzyloxycarbonylamino-4-hydroxybutanoate (4). To a solution of 3 (30.0 g, 106.66
mmol) in THF (150 mL) was added, with stirring, a solution of HOBt (21.62 g, 160.01 mmol) and DCC
o
(26.41 g, 127.99 mmol) in THF (150 mL) at 0 C. After stirring for 30 min, the DCC salt precipitated
was filtered off and the filtrate was stirred with NaBH4 (4.29 g, 117.32 mmol) at 0 oC for 30 min. To the
reaction mixture was added saturated NH4Cl aqueous solution (100 mL) and then THF was evaporated.
The aqueous solution was extracted with CHCl3 (100 mL x 3) and the combined extracts were washed
with saturated NaHCO3 aqueous solution (100 mL x 3) and brine (100 mL x 3) and then dried over
anhydrous Na2SO4. Concentration in vacuo gave a crude syrup, which was purified on a silica gel
column using a mixture of hexane and EtOAc (1 : 3 v/v) to give 4 as a colorless syrup. Yield 86%
25
(23.94 g). [α]D –10.4o (c 1.07, MeOH). IR (KBr) 3568, 3370, 2956, 1728 cm-l. 1H NMR : δ 2.58
(d, 2H, α-H, J=5.9 Hz), 3.62 (s, 3H, COOCH3), 3.70-3.75 (m, 2H, CH2OH), 3.90-4.05 (m, 1H, β-H), 5.11
(s, 2H, Cbz’s CH2), 5.50 (br d, 1H, NH, J=8.6 Hz), 7.32 (s, 5H, Ph). Anal. Calcd for C13H17NO5: C,
58.43; H, 6.41; N, 5.24. Found: C, 58.27; H, 6.59; N, 5.24.
Methyl (4S)-(3-Benzyloxycarbonyl-2,2-dimethyl-1,3-oxazolidin-4-yl)acetate (5). To a solution of 4

(2.32 g, 8.75 mmol) in acetone (50 mL) were added, with stirring, acetone dimethyl acetal (2.15 mL,
17.49 mmol) and p-toluenesulfonic acid hydrate (0.33 g, 1.75 mmol) at rt. After stirrng overnight, the
reaction mixture was neutralized with saturated NaHCO3 aqueous solution and then evaporated. The
aqueous solution was extracted with EtOAc (100 mL x 3), the combined extracts were washed with brine
(100 mL x 3) and dried over anhydrous Na2SO4. Concentration in vacuo gave a crude syrup, which was
purified on a silica gel column using a mixture of hexane and EtOAc (1 : 1 v/v) to give 5 as a colorless
25
syrup. Yield 87% (1.72 g). [α]D +21.7o (c 1.04, MeOH). IR (KBr) 3449, 2988, 1717 cm-l. 1H
NMR: δ 1.52, 1.62 (each s, 3H x 2, Ip’s CH3), 2.48-2.98 (m, 2H, α-H), 3.65 (s, 3H, COOCH3), 3.86-3.90
(m, 2H, CH2), 4.02-4.31 (m, 1H, NCH), 5.44 (s, 2H, Cbz’s CH2), 7.36 (s, 5H, Ph). Anal. Calcd for
C16H21NO5: C, 62.53; H, 6.89; N, 4.56. Found: C, 62.37; H, 6.84; N, 4.32.
2-[(4S)-(3-Benzyloxycarbonyl-2,2-dimethyl-1,3-oxazolidin-4-yl)]ethanol (6). To a solution of NaBH4
(3.30 g, 87.12 mmol) and CaCl2 (3.30g, 29.73 mmol) in EtOH (100 mL) was added, with stirring, a
o
solution of 5 (22.24 g, 72.60 mmol) in EtOH (150 mL) at 0 C. The reaction mixture was treated with
saturated NH4Cl aqueous solution (200 mL) and EtOH was evaporated. The aqueous solution was
extracted with EtOAc (100 mL x 3), the combined extracts were washed with brine (100 mL x 3) and then
dried over anhydrous Na2SO4. Concentration in vacuo gave a crude syrup, which was purified on a
silica gel column using a mixture of hexane and EtOAc (1 : 1 v/v) to give 6 as a colorless syrup. Yield
25
91% (18.45 g). [α]D +9.4o (c 0.66, MeOH). IR (KBr) 3448, 2980, 2944, 2878, 1698 cm-l. 1H
NMR: δ 1.49, 1.55 (each s, 3H x 2, Ip’s CH3), 1.68-l.96 (m, 2H, β-H), 3.45-4.39 (m, 6H, NCH, CH2 x 2,
OH), 5.16 (s, 2H, Cbz’s CH2), 7.35 (s, 5H, Ph). Anal. Calcd for C15H21NO4: C, 64.50; H, 7.58; N, 5.01.
Found: C, 64.42; H, 7.52; N, 4.96.
Methyl
(5S,2Z)-4-(3-Benzyloxycarbonyl-2,2-dimethyl-1,3-oxazolidin-4-yl)-2-t-butoxycarbonyl-
amino-2-butenoate (8). To a solution of 6 (5.50 g, 19.70 mmol) in CHCl3 (20 mL) was added, with
.
stirring, Et3N (21.87 mL, 157.50 mmol) at 0 oC and then a solution of SO3 pyridine (15.67 g, 98.50 mmol)
in DMSO (40 mL). After stirring for 1 h at 0 oC, ether (20 mL) and EtOAc (20 mL) were further added.
The reaction mixture was washed with 10% citric acid (50 mL x 3) and brine (50 mL x 3) and then dried
over anhydrous Na2SO4. Concentration in vacuo gave a cude syrup, to which was added, with stirring, a
solution of N-Boc-α-ethoxyphosphorylglycine methyl ester (6.41 mg, 19.7 mmol) in CHCl3 (20 mL) and
then DBU (4.40 mL, 29.53 mmol). The whole was stirred for 30 min at 0 oC and then overnight at rt.
The reaction mixture was dissolved in ether (50 mL) and the resulting solution was washed successively
with 10% citric acid (50 mL x 3), brine (50 mL x 3), and saturated NaHCO3 aqueous solution (50 mL x 3)
and then dried over anhydrous Na2SO4. Concentration in vacuo gave a crude syrup, which was purified

on a silica gel column using a mixture of hexane and EtOAc (2.5 : 1 v/v) to give 8 as colorless syrup.
Yield 91% (8.0 g). [α]D28 +66.8o (c 4.04, MeOH). IR(KBr) 3334, 2980, 1710, 1500, 1455 cm-l. 1H
NMR: δ 1.38 (s, 9H, Boc’s t-Bu), 1.44, 1.56 (each s, 3H x 2, Ip’s CH3), 2.47-2.52 (m, 2H, CH2), 3.68 (s,
3H, COOCH3), 3.87-4.08 (m, 3H, NCH, CH2), 5.08 (s, 2H, Cbz’s CH2), 6.00 (br s, 1H, NH), 6.44 (t, 1H,
olefin-H, J=7.3 Hz), 7.28 (s, 5H, Ph). Anal. Calcd for C23H32N2O7: 61.59; H, 7.19; N, 6.25. Found: C,
61.68; H, 7.25; N, 6.40.
Methyl (5S,2Z)-5-Benzyloxycarbonylamino-2-t-butoxycarbonylamino-6-hydroxy-2-hexenoate (9).
A solution of 8 (318 mg, 0.71 mmol) in a mixture of CHCl3 and TFA (96 : 4 v/v, 6 mL) was stirred
overnight at rt. The resultant solution was made to weak alkali with Et3N and then concentrated in
vacuo to give a residual syrup, which was dissolved in EtOAc (10 mL). The reaction mixture was
washed with 10% citric acid (20 mL x 3) and brine (20 mL x 3) and then dried over anhydrous Na2SO4.
Concentration in vacuo gave a crude syrup, which was purified on a silica gel column using a mixture of
26
hexane and EtOAc (1 : 1 v/v) to give 9 as a colorless syrup. Yield 87% (252 mg). [α]D +23.7o (c
3.38, MeOH). IR (KBr) 3406, 2974, 1713, 1524 cm-l. 1H NMR: δ 1.45 (s, 9H, Boc’s t-Bu), 2.50-2.55
(m, 2H, CH2), 3.53-3.91 (m, 4H, NCH, CH2, OH), 3.77 (s, 3H, COOCH3), 5.10 (s, 2H, Cbz’s CH2), 5.81
(br d, 1H, NH, J=7.7 Hz), 6.38 (br s, 1H, BocNH), 6.55 (t, 1H, olefin-H, J=7.7 Hz), 7.34 (s, 5H, Ph).
Anal. Calcd for C20H28N2O7: C, 58.81; H, 6.91; N, 6.86. Found: C, 58.40; H, 7.33; N, 6.83.
Methyl (6RS,5S)-5-Benzyloxycarbonylamino-6-hydroxy-1-t-butoxycarbonyl-4,5,6-trihydro-
pyridine-2-carboxylate (10). To a solution of 9 (278 mg, 0.682 mmol) in CHCl3 (10 mL) was added,
.
with stirring, Et3N (0.764 mL, 5.456 mmol) and then a solution of SO3 pyridine (543 mg, 3.41 mmol) in
o
DMSO (20 mL) at 0 C. After stirring for 30 min and for 1 h at rt, the reaction mixture was mixed to
ether (10 mL) and the resulting solution was washed with 10% citric acid (30 mL x 3) and brine (30 mL x
3) and then dried over anhydrous Na2SO4. Concentration in vacuo gave crude crystals, which were
recrystallized from CHCl3-hexane to give 10 as colroelss needles. Yield 61% (168 mg). mp 169-170
28
oC. [α]D –67.6o (c 0.82, MeOH). IR (KBr) 3484, 3316, 1722, 1689, 1647, 1536 cm-l. 1H NMR: δ
1.40 (s, 9H, Boc’s t-Bu), 1.96-2.24 (m, 1H, H-4a), 2.58-2.91 (m, 1H, H-4b), 3.19 (d, 1H, OH, J=3.7 Hz),
3.61 (s, 3H, COOCH3), 3.95-4.l8 (m, 1H, H-5), 5.08 (s, 2H, Cbz’s CH2), 5.87 (m, 2H, H-3, H-6), 7.32 (s,
5H, Ph). Anal. Calcd for C20H26N2O7: C, 59.10; H, 6.45; N, 6.89. Found: C, 59.19; H, 6.29; N, 6.84.
Methyl (5S)-5-Benzyloxycarbonylamino-6-oxo-1-t-butoxycarbonyl-4,5,6-trihydropyridine-2-
carboxylate (11). To a solution of 10 (70 mg, 0.17 mmol) in acetone (10 mL) was added, with stirring,
o
2.67 M Jones reagent (0.13 mL) at 0 C. After stirring for 30 min and overnight at rt, to the resulting
o
solution was added 2-propanol (10 mL) at 0 C for 10 min. The precipitates were filtered off and the

filtrate was neutralized with saturated NaHCO3 aqueous solution and then acetone was evaporated in
vacuo. The aqueous solution was extracted with EtOAc (5 mL x 3) and the combined extracts were
washed with saturated NaHCO3 aqueous solution (5 mL x 3) and brine (5 mL x 3) and then dried over
anhydrous Na2SO4. Concentration in vacuo gave crude crystals, which were recrystallized from
o
25
hexane-EtOAc to give 11 as colorless needles. Yield 90% (62 mg). mp 158-160 C. [α]D –9.3o (c
0.67, MeOH). IR (KBr) 3448, 2980, 1785, 1728, 1707, 1656, 1521 cm-l. 1H NMR: δ 1.53 (s, 9H,
Boc’s t-Bu), 2.32-2.44 (m, 1H, H-4a), 2.96-3.08 (m, 1H, H-4b), 3.81 (s, 3H, COOCH3), 4.23-4.33 (m, 1H,
H-5), 5.10 (s, 2H, Cbz’s CH2), 5.83 (br d, 1H, NH, J=4.5 Hz), 6.51 (dd, 1H, H-3, J=2.6, 7.6 Hz), 7.37 (s,
5H, Ph). Anal. Calcd for C20H24N2O7: C, 59.40; H, 5.98; N, 6.93. Found: C, 59.34; H, 5.99; N, 7.32.
Methyl (5S)-Benzyloxycarbonylamino-6-oxo-1,4,5-trihydropyridine-2-carboxylate (12). A solution
of 11 (2.255 g, 5.58 mmol) in a mixture of CHCl3 and TFA (1 : 1 v/v, 30 mL) was stirred for 1 h at rt.
The reaction mixture was made to weak alkali with Et3N and then the solvent was evaporated in vacuo.
The residual substance was dissolved in EtOAc (50 mL) and the resulting solution was washed
successively with 10% citric acid (25 mL x 2), brine (25 mL x 3), and saturated NaHCO3 aqueous
solution (25 mL x 3) and then dried over anhydrous Na2SO4. Concentration in vacuo gave crude crystals,
which were recrystallized from a EtOAc-hexane to give 12 as colorless needles. Yield 99 % (1.697 g).
o
25
mp 111-112 C. [α]D –40.2o (c 0.86, MeOH). IR (KBr) 3448, 3334, 1716, 1683, 1536 cm-l. 1H
NMR: δ 2.33 (ddd, 1H, H-4a, J=2.9, J=14.7, 17.6 Hz), 3.05 (ddd, 1H, H-4b, J=7.2, 7.3 Hz, 17.6 Hz), 3.81
(s, 3H, COOCH3), 4.28 (ddd, 1H, H-5, J=5.5, 7.3, 14.7 Hz), 5.11 (s, 2H, Cbz’s CH2), 5.96 (br d, 1H,
CbzNH, J=5.5 Hz), 6.27 (dd, 1H, H-3, J=2.9, 7.2 Hz), 7.33 (s, 5H, Ph), 7.92 (br s, 1H, H-1). Anal.
Calcd for C15H16N2O5: C, 59.21; H, 5.30; N, 9.21. Found: C, 58.94; H, 5.13; N, 8.97.
Methyl 5-Benzyloxycarbonylamino-6-oxo-1-hydropyridine-2-carboxylate (13). A suspension of 12
(0.50 g, 1.64 mmol) and MnO2 (4.28 g, 49.29 mmol) in CHCl3 (20 mL) was stirred under sonication for 6
h at rt. The MnO2 was filtered off and the residue was concentrated in vacuo to give crude crystals.
Recrystallization from a EtOAc-hexane gave 13 as colorless needles. Yield 80% (397 mg). mp
o
185-186 C. IR 3430, 3404, 1737, 1716, 1650, 1632, 1569, 1524 cm-l. 1H NMR: δ 3.39 (s, 3H,
COOCH3), 5.22 (s, 2H, Cbz’s CH2), 7.10 (d, 1H, H-3, J=7.6 Hz), 7.37 (s, 5H, Ph), 8.10 (d, 1H, H-4, J=7.6
Hz), 8.13 (br s, 1H, CbzNH), 10.18 (br s, 1H, H-1). Anal. Calcd for C15H14N2O5: C, 59.60; H, 4.67; N,
9.27. Found: C, 59.55; H, 4.29; N, 9.15.
Methyl 5-Benzyloxycarbonylamino-6-methoxypyridine-2-carboxylate (14). A suspension of 13 (320
mg, 0.84 mmol), Ag2CO3 (116 mg, 0.42 mmol), and MeI (0.1 mL, 1.68 mmol) in CHCl3 (15 mL) was

stirred 48 h at rt. The Ag salt was filtered off and the filtrate concentrated in vacuo to give crude crystals,
which were recrystallized from a EtOAc-hexane to give 14 as colorless needles. Yield 87% (288 mg).
o
mp 90-92 C. IR (KBr) 3431, 2941, 1734, 1714, 1587, 1521, 1467 cm-l. 1H NMR: δ 3.86 (s, 3H,
COOCH3), 4.00 (s, 3H, OCH3), 5.12 (s, 2H, Cbz’s CH2), 7.26-7.33 (m, 6H, Ph, H-3), 7.69 (d, 1H, H-4,
J=8.3 Hz), 8.34 (d, 1H, NH). Anal. Calcd for C16H16N2O5: C, 60.75; H, 5.10; N, 8.86. Found: C,
60.87; H, 5.59; N, 8.60.
Methyl 5-Amino-6-methoxypyridine-2-carboxylate (15). A suspension of 14 (88 mg, 0.278 mmol)
and 10% Pd/C (l0 mg) in MeOH (10 mL) was stirred under H2 gas atmosphere for 2 h at rt. The Pd/C
was filtered off and the filtrate was concentrated in vacuo to give crude crystals, which were
recrystallized from a EtOAc-hexane to give 15 as colorless needles. Yield 99% (50 mg). mp 161-163
oC. IR (KBr) 3487, 3346, 2360, 2341, 1716, 1618, 1581, 1566 cm-l. 1H NMR: δ 3.91 (s, 3H,
COOCH3), 4.07 (s, 3H, OCH3), 4.84 (br s, 2H, NH2), 6.85 (d, 1H, H-3, J=7.7 Hz), 7.65 (d, 1H, H-4, J=7.7
Hz). Anal. Calcd for C8H10 N2O3: C, 52.74; H, 5.53; N, 15.38. Found: C, 52.68; H, 5.20; N, 14.94.
Methyl 5-Cyano-6-methoxypyridine-2-carboxylate (16). To a solution of 14 (24 mg, 0.131 mmol)
in THF (1 mL) and 3M HCl (5 mL) was added, with stirring, a solution of NaNO2 (11 mg, 0.158 mmol)
in water (2 mL) at 0 oC for 5 min. After stirring for 15 min, further for 15 min at rt, and then 2 h at 60
oC, the reaction mixture was made to weak alkali with saturated NaHCO3 aqueous solution and extracted
with EtOAc (10 mL x 2). The combined extracts were washed with saturated Na2S2O3 aqueous solution
(5 mL x 3), brine (5 mL x 3), and then dried over anhydrous Na2SO4. Concentration in vacuo gave a
residual substance, to which were added CuI (2 mg, 1.317 x 10-2 mmol), Pd(0) (PPh3)4 (7 mg, 6.587 x
10-2 mmol), and KCN (17 mg, 0.263 mmol) and then CH3CN (10 mL). After refluxing for 12 h, the
precipitates were filtered off and the filtrate was washed with water (10 mL x 2) and brine (10 mL x 2)
and then dried over anhydrous Na2SO4. Concentration in vacuo gave crude crystals, which were
purified on a silica gel column using a mixture of hexane and EtOAc (2 : 1 v/v) to give colorless crystals.
Recrystallization from EtOAc-hexane gave 16 as colorless needles. Yield 40% (10 mg). mp 125-126
oC. IR (KBr) 3097, 3072, 3039, 3007, 2229, 1730, 1712 cm-l. 1H NMR: δ 3.99 (s, 3H, COOCH3), 4.15
(s, 3H, OCH3), 7.76 (d, 1H, H-3, J=7.6 Hz), 8.02 (d, 1H, H-4, J=7.6 Hz). Anal. Calcd for C9H8N2O3: C,
56.25; H, 4.20; N, 14.58. Found: C, 56.33; H, 4.00; N, 14.62.
Methyl 5-Cyano-6-oxohydropyridine-2-carboxylate (17). To a solution of 16 (100 mg, 0.52 mmol)
in CH2Cl2 (10 mL) and was added, with stirring, a solution of BBr3 (1 M solution in CH2Cl2, 0.78 mL,
0.78 mmol) under Ar gas atmosphere at -20 oC for 30 min. After stirring for 1 h, further for and then 1 h

at 0 oC, the reaction mixture was poured into H2O (30 mL) and the resulting solution was extracted with
CH2Cl2 (10 mL x 3). The combined extracts were washed with saturated brine (10 mL x 3), and then
dried over anhydrous Na2SO4. Concentration in vacuo gave crude crystals, which were purified on a silica
gel column using a EtOAc to give colorless solids. Recrystallization from EtOAc gave 17 as colorless
crystals. Yield 84% (78 mg). mp 261-263 oC (from EtOAc). IR (KBr) 3095, 2225, 1737, 1658, 1610
cm-l. 1H NMR: δ in DMSO-d6 3.31 (br s,1H, NH or OH), 3.88 (s, 3H, COOCH3), 7.10 (br d, 1H, H-3,
J=7.5 Hz), 8.27 (d, 1H, H-4, J=7.5 Hz). Anal. Calcd for C8H6N2O3: C, 53.94; H, 3.39; N, 15.73.
Found: C, 54.12; H, 3.41; N, 15.68.
Methyl (5S)-5-Benzyloxycarbonylamino-3-bromo-6-oxo-1,4,5-trihydropyridine-2-carboxylate (18).
To a solution of 12 (200 mg, 0.66 mmol) in CHCl3 (10 mL) was added, with stirring, NBS (129 mg, 0.72
mmol) at rt for 1 h. After adding Et3N (0.09 mL, 0.72 mmol), the resulting solution was stirred
overnight at rt. The precipitated material was filtered off and the filtrate was concentrated in vacuo to
give crude crystals. Recrystallization from CHCl3-hexane gave 18 as pale yellow crystals. Yield 98%
o
26
(227 mg). mp 141-143 C. [α]D –36.1o (c 1.44, MeOH). IR 3316, 3244, 1743, 1674, 1536 cm-l.
1H NMR: δ 2.88 (dd, 1H, H-4a, J=17.5, 14.5 Hz), 3.48 (dd, 1H, H-4b, J=17.5, 6.8 Hz), 3.90 (s, 3H,
COOCH3), 4.41 (ddd, 1H, H-5, J=6.8, 14.5, 5.3 Hz), 5.13 (s, 2H, Cbz’s CH2), 5.67 (br d, 1H, CbzNH,
J=5.3 Hz), 7.36 (s, 5H, Ph), 7.75 (br s, 1H, H-1). Anal. Calcd for C15H15N2O5Br: C, 47.02; H, 3.95; N,
7.31. Found: 47.32; H, 3.91; N, 7.13.
Methyl (5S)-5-Benzyloxycarbonylamino-3-bromo-6-oxo-1-hydropyridine-2-carboxylate (19). To a
solution of CuBr2 (473 mg, 2.12 mmol) and hexamethylentetramine (297 mg, 2.12 mmol) in CH2Cl2 (10
o
mL) was added, with stirring, DBU (0.30 mL, 2.12 mmol) at 0 C under Ar gas atmosphere. After
stirring for 10 min, to the resulting solution was further added a solution of 18 (203 mg, 0.53 mmol) in
CH2Cl2 (2 mL). After stirring for 4 h at rt, the reaction mixture was poured into EtOAc (50 mL) and the
resulting solution was washed with saturated NaHCO3 aqueous solution (30 mL) and brine (30 mL x 3)
and then dried over anhydrous Na2SO4. Concentration in vacuo gave crude crystals, which were
recrystallized from EtOAc-hexane to give 19 as colorless needles. Yield 84% (170 mg). mp 201-203
oC. IR 3388, 1722, 1656, 1608, 1551, 1527 cm-1.
1H NMR: δ 3.98 (s, 3H, COOCH3), 5.23 (s, 2H,
Cbz’s CH2), 7.39 (s, 5H, Ph), 8.00 (br s, 1H, CbzNH), 8.29 (s, 1H, H-4), 9.79 (br s, 1H, H-1). Anal.
Calcd for C15H13N2O5Br: C, 47.27; H, 3.44; N, 7.35. Found: C, 47.12; H, 3.43; N, 7.21.
Methyl 5-Benzyloxycarbonylamino-3-bromo-6-methoxypyridine-2-carboxylate (20). To a solution
of 19 (320 mg, 0.84 mmol) in toluene (15 mL) were added, with stirring, Ag2CO3 (116 mg, 0.42 mmol)

and MeI (0.1 mL, 1.68 mmol). The resulting solution was further stirred at 60 oC overnight. After Ag
salt was filtered off, the filtrate was concentrated in vacuo to give crude crystals, which were
recrystallized from EtOAc-hexane to give 20 as colorless needles. Yield 87% (288 mg). mp 155-156
oC. IR 3448, 2984, 1722, 1698 cm-l. 1H NMR: δ 3.95 (s, 3H, COOCH3), 4.02 (s, 3H, OCH3), 5.21 (s,
2H, Cbz’s CH2), 7.31 (s, 5H, Ph), 8.12 (br s, 1H, CbzNH), 8.17 (s, 1H, H-4). Anal. Calcd for
C16H15N2O5Br: C, 48.63; H, 3.83; N, 7.09. Found: C, 48.59; H, 3.73; N, 7.12.
Methyl 5-Benzyloxycarbonylamino-3, 6-dimethoxypyridine-2-carboxylate (21). To a solution of 20
(174 mg, 0.44 mmol) in DMF (5 mL) was added, with stirring, a solution of MeONa (29 mg, 0.53 mmol)
o
in MeOH (5 mL) under 0 C . After adding CuI (16.8 mg, 0.09 mmol), the resulting solution was
refluxed for 3 h. Precipitated Cu salt was filtered off and the residue was extracted with EtOAc (5 mL x
3). The combined extracts were washed with brine (5 mL x 3) and dried over anhydrous Na2SO4.
Concentration in vacuo gave a crude substance, which was purified on a silica gel column using a mixture
of EtOAc and hexane (1 : 3 v/v) to give 21 as colorless crystals. Yield 62% (95 mg). mp 134-135 oC.
IR 3448, 2980, 2236, 1731 cm-l. 1H NMR: δ 3.83 (s, 3H, 3-OCH3), 3.98 (s, 3H, COOCH3), 4.07 (s, 3H,
6-OCH3), 5.19 (s, 2H, PhCH2), 7.32 (s, 5H, Ph), 7.81 (s, 1H, H-4), 8.08 (br s, 1H, CbzNH). Anal. Calcd
for C17H18N2O6: C, 58.96; H, 5.24; N, 8.09. Found: C, 59.21; H, 5.17; N, 7.86.
Methyl 5-Amino-3,6-dimethoxypyridine-2-carboxylate (22). A suspension of 21 (217 mg, 0.63
mmol) and 10% Pd-C (21 mg) in EtOH (15 mL) was stirred under H2 gas atmosphere at rt for 2 h. The
Pd-C was filtered off and the filtrate was concentrated in vacuo to give crude crystals, which were
recrystallized from EtOAc-hexane to give 22 as colorless prisms. Yield 99% (133 mg). mp 162-164
oC. IR 3298, 2932, 2860, 1734, 1659 cm-l. 1H NMR: δ 3.84 (s, 3H, 3-OCH3), 3.97 (s, 3H, COOCH3),
4.10 (s, 3H, 6-OCH3), 4.31 (br s, 2H, NH2), 7.31 (s, 1H, H-4). Anal. Calcd for C9H12N2O4: C, 50.94; H,
5.70; N, 13.20. Found: C, 50.88; H, 5.67; N, 13.45.
Methyl 5-Cyano-3,6-dimethoxypyridine-2-carboxylate (23). A buffer solution was prepared from a
.
mixture of CuSO4 5H2O (146 mg, 0.58 mmol) and CuCN (209 mg, 2.34 mmol) in H2O (10 mL), and a
small amount of Na2CO3. On the other hand, to a solution of a concentrated HCl (0.1 mL, 1.64 mmol)
and H2O (1 mL) was added a solution of 22 (133 mg, 0.63 mmol) in THF (3 mL) at 0 oC and then added
slowly a solution of NaNO2 (40 mg, 584 mmol) in H2O (3 mL). To the latter solution was added, with
stirring, the former buffer at 0 oC, the resulting solution was stirred at 0 oC for 1 h and then 50 oC for 4 h.
The reaction mixture was mixed with EtOAc (5 mL) and then the resulting solution was washed with
saturated aquous NaHCO3 solution (3 mL x 5) and brine (5 mL x 3) and then dried over anhydrous

Na2SO4. Concentration in vacuo gave a crude residue, which was purified on a silica gel column using a
mixture of EtOAc and hexane (1 : 2 v/v) to give colorless crystals. Recrystallization from EtOAc-
o
hexane gave 23 as colorless needles. Yield 31% (43 mg). mp 159-160 C. IR 3448, 2944, 2224,
1731 cm-l. 1H NMR: δ 3.87 (s, 3H, 3-OCH3), 3.97 (s, 3H, COOCH3), 4.04 (s, 3H, 6-OCH3), 7.58 (s, 1H,
H-4). Anal. Calcd for C19H10N2O4: C, 54.05; H, 4.53; N, 12.61. Found: C, 53.78; H, 4.44; N, 12.51.
ACKNOWLEDGEMENT
This work was supported in part by Grant-in Aid for Scientific Research No. 14550829 from the Ministry
of Education, Culture, Sports, Science in Japan and Technology and by “High-Tech Research Project”
from the Ministry of Education, Culture, Sports, Science and Technology in Japan.
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