Enantioselective synthesis of (S)-3-(4-thiazolyl)-2-tert- butoxycarbonylaminopropionic acid: A chiral building block for renin inhibitor

Article abstract of DOI:10.3987/COM-04-10178

(S)-3-(4-Thiazolyl)-2-tert-butoxycarbonylaminopropionic acid (6), an important structural constituent of the renin inhibitor, has been synthesized from (Z)-3-(4-thiazolyl)-2-benzoylaminoprop-2-enoic acid (4b) by enantioselective hydrogenation using the Ru

Full text of DOI:10.3987/COM-04-10178

HETEROCYCLES, Vol. 63, No. 10, 2004, pp. 2385 - 2392
Received, 20th July, 2004, Accepted, 19th August, 2004, Published online, 20th August, 2004
ENANTIOSELECTIVE SYNTHESIS OF (S)-3-(4-THIAZOLYL)-2-tert-
BUTOXYCARBONYLAMINOPROPIONIC ACID: A CHIRAL BUILDING
BLOCK FOR RENIN INHIBITOR
Yoshifumi Yuasa,a Haruki Tsuruta,a and Yoko Yuasab*
aTakasago International Corporation, 1-5-1 Nishiyawata, Hiratsuka, Kanagawa,
254-0073, Japan
bSchool of Pharmacy, Tokyo University of Pharmacy and Life Science, 1432-1
Horinouchi, Hachioji, 192-0392, Tokyo, Japan
E-mail: yuasay@ps.toyaku.ac.jp
Abstract – (S)-3-(4-Thiazolyl)-2-tert-butoxycarbonylaminopropionic acid (6), an
important structural constituent of the renin inhibitor, has been synthesized from
(Z)-3-(4-thiazolyl)-2-benzoylaminoprop-2-enoic acid (4b) by enantioselective
hydrogenation using the Ru-(S)-p-tolyl-BINAP complex as the key step, and then
followed by acid hydrolysis and tert-butoxycarbonylation.
INTRODUCTION
Based on the concept of isosterism in medicinal chemistry, 3-(4-thiazolyl)-L-alanine can be considered as
an annular equivalent of L-histidine. Furthermore, (S)-3-(4-thiazolyl)-2-tert-butoxycarbonylamino-
propionic acid (6) has become an interesting component in the preparation of a potential renin inhibitory
active peptide by its corporation.1 Several preparations of 6 are known:
a) The synthesis from (S)-benzyloxycarbonyl-5-oxo-4-oxazolidinoneacetic acid, which was derived from
the commercially available N-benzyloxycarbonyl-L-aspartic acid.2
b) The enzymatic resolution of the ethyl ester of N-acetyl-3-(4-thiazolyl)-DL-alanine by chymotrypsin2 or
acetylase.3
c) The synthesis from the methyl ester of N-tert-butoxycarbonyl-L-aspartic acid.4
On the other hand, the enantioselective hydrogenation with the Ru-BINAP complex catalyst is one of the
most powerful tools for the synthesis of optically active compounds.5 In paticular, the enantioselective

hydrogenation of 3-substituted 2-propenoic acids and 3-phenyl-2-aminoprop-2-enoic acid are well
known.6-9 The enantioselective hydrogenation of a five-membered heterocycle substituted
2-acylaminoprop-2-enoic acid using chiral diphosphinerhodium catalysts has also been reported.10
However, that of the 3-(4-thiazolyl)-2-acylaminoprop- 2-enoic acids (4a) and (4b) is unknown.
We have studied the enantioselective hydrogenation of 4a and 4b in order to develop the synthesis of 6.
We now report the efficient synthesis of 6 from 3-(4-thiazolyl)-2-benzoylaminopropanoic acid (5b) via
the enantioselective hydrogenation of 4b using Ru-p-tolyl-BINAP complex catalyst.11
RESULTS AND DISCUSSION
The preparation of (Z)-3-(4-thiazolyl)-2-acylaminoprop-2-enoic acids (4a) and (4b) as substrates for the
enantioselective hydrogenation is illustrated in scheme 1. The chlorination of 4-methylthiazole with Cl2
gas in conc. H2SO4 and SO3 in the presence of a catalytic amount of AIBN at 100°C for 20 h afforded a
mixture of 4-dichloromethylthiazole (1) and 4-chloromethylthiazole in 45% and 21% yields,
respectively.12
Scheme 1
CHO
Me
CHCl
2
N
N
N
b
a
2
S
S
S
1
O
CO H
2
N
N
O
c
d
N
NHCOR
R
S
S
4a: R=Me
4b: R=Ph
3a: R=Me
3b: R=Ph
Reagents and conditions: a) Cl2/SO3/PCl3/H2SO4, 100°C, 20 h. b) 10%H2SO4,
100°C, 2 h c) RCONHCH2CO2H, AcONa, rt, 20 min. d) 3a: 25% aqueous acetone,
reflux, 2 h 3b: NaOH, 100°C, 45 min.
The dichloride (1) was then treated with 10% H2SO4 at 100°C for 2 h to give 4-thiazolecarbaldehyde (2) in
85% yield. Subsequently, 2 was condensed with acetylglycine and hippuric acid to give
4-thiazolylazlactone (3a) and (3b) in 52% and 84% yields, respectively. The treatment of 3a with
aqueous acetone solution gave 4a in 68% yield. Alternatively, 3b in aqueous NaOH solution was refluxed
to give 4b in 85% yield.
We examined the hydrogenation of 4a and 4b in the presence of the Ru-(S)- or (R)-p-tolyl-BINAP

complex as a catalyst under several different conditions. The results are summarized in Table 1.
The reaction was carried out in a polar solvent such as MeOH or MeOH/H2O. It was found that the
reaction temperature affected the enantioselectivity of the product. Upon the hydrogenation of 4a and 4b
at 20°C, the reaction proceeded at a significantly slower rate, therefore a longer reaction time was
required to complete the reaction, but the optical yield was in a moderate range (56-77% ee) (Entries 1~4).
When the temperature was raised from 20°C to 60°C, the reaction rate accelerated, but the
enantioselectivity was lower than the others (Entry 5). The reaction at 35°C gave a result similar to that at
60°C in yield and enantioselectivity (Entry 6). In addition, it is known that the hydrogenation using the
Ru-p-tolyl-BINAP complex as a catalyst can be carried out in the presence of triethylamine;6 however,
the enantioselective hydrogenation of 4b could not be effected in the presence of a triethylamine (Entry
7).
Scheme 2
CO2H
NHBoc
CO2H
CO2H
*
N
N
N
b, c
a
NHCOR
NHCOR
S
S
S
6
4a: R=Me
4b: R=Ph
5a: R=Me
5b: R=Ph
.
Reagents alnd conditions : a) H , 5MPa, Ru Cl [(S)- or (R)-p-tolyl-BINAP] Et N, MeOH,
2
2
4
2
3
b) HCl, reflux, 20 h. c) ( t-Boc )O, THF rt, 40 h.
2
Table 1. Asymmetric hydrogenation of 4a and 4b
CatalystA
Solvent
Temp. (°C) Time (h)
E.e. (%)C
Entry Substrate
Yield (%)
Config.
(+)-A
(-)-A
(+)-A
(-)-A
(-)-A
(-)-A
(-)-A
MeOH
20
20
20
20
60
35
120
120
140
140
42
75
77
60
56
51
55
—ٛ
1
2
3
4
5
6
7
4a
4a
4b
4b
4b
4b
4b
100
100
100
98
(R)-5a
(S)-5a
(R)-5b
(S)-5b
(S)-5b
(S)-5b
(S)-5b
MeOH
MeOH
MeOH
MeOH
100
100
11
MeOH/H2O (10/1)
MeOH/H2O (10/1)B 35
120
120
Reaction conditions: H2 press. 5 MPa. catalyst; 1 mole%,
A(+)-A; an empirical formula Ru2Cl4[(R)-(+)-p-tolyl-BINAP] 2Et3N, (-)-A; Ru2Cl4[(S)-(-)-p-tolyl-BINAP] 2Et3N.
BEntry 7 was carried out in the presence of 1.1 eq. mole triethylamine.
CThe ee value of 5a was determined by HPLC after being converted to 6. The ee value of 5b was determined by HPLC
after being converted to the corresponding methyl ester.
The absolute configurations of 5a and 5b were determined by comparison with the optical rotations in the
literature2 after conversion to 6. The enantiomeric excess of 3-(4-thiazolyl)-2-benzoylaminopropanoic
acid (5b) was determined by HPLC after converting it to the corresponding methyl ester using
diazomethane (Entries 3~7). However, that of 3-(4-thiazolyl)-2-acetylaminopropanoic acid (5a) could not

be determined using similar HPLC conditions. The enantiomeric excess of 5a was determined by HPLC
after converting it to 6 (Entries 1 and 2).
Furthermore, the recrystallization of 5b (Entry 3, 60% ee) using EtOAc/MeOH gave the pure compound
5b in 40% yield with 100% ee. Attempts to obtain the pure compound (5a) under similar recrystallizations
were unsuccessful. The target compound (6) was derived from 5b in 57% with 99% ee by the usual
method; acid hydrolysis and tert-butoxycarbonylation (Scheme 2 and see EXPERIMENTAL).
CONCLUSION
We have described the efficient synthesis of the chiral building block for the renin inhibitor.
(S)-3-(4-Thiazolyl)-2-tert-butoxycarbonylaminopropionic
acid
(6)
was
synthesized
from
3-(4-thiazolyl)-2-benzoylaminopropanoic acid (5b) via the enantioselective hydrogenation of
(Z)-3-(4-thiazolyl)-2-benzoylaminoprop-2-enoic acid 4b using an optically active Ru-(S)-p-tolyl-BINAP
complex catalyst.
EXPERIMENTAL
All reagents and solvents were obtained from commercial sources and used without further purification.
Melting points were determined using a Yanagimoto micro melting apparatus and are uncorrected. The
IR spectra were run on a JASCO IR-810. The 1H-NMR spectra were recorded in CDCl3 or CD3OD with
TMS as the internal standard using a Bruker AM-400 (400MHz) spectrometer. MS spectra were run on a
Hitachi M-80A spectrometer at 70eV. Optical rotations were recorded using a JASCO DIP-4 digital
polarimeter. The HPLC was done using a Hitachi L-6000 [column, with a L-4000 UV as the detector,
Optical purity; CHIRALCEL OC, 4.6 mm ID x 250 mm; eluent: 2-propanol/n-hexane = 35:65, flow rate,
1 mL/min; detector, UV (254 nm)]; chemical purity; [column: Inertsil ODS-2 (250 mm x 4.6 mm);
eluent: MeCN/H2O, 7:3 (pH 2.3; adjusted by using H3PO4); flow rate: 0.5 mL/min; detection: UV (254
nm)].
4-Thiazolecarbaldehyde (2)
4-Methylthiazole (50 g, 0.5 mol, Aldrich) was dissolved in a mixture of 98% H2SO4 (46 g) and
concentrated H2SO4 containing 25% SO3 (47.5 g). PCl3 (1 mL) was added and AIBN (1.5 g) in 98%
H2SO4 (36 g) was then added to the mixture over a period of 10 h with stirring at 85-90°C. During this
time a stream of Cl2 gas was continually introduced into the reaction mixture. The mixture was then
degassed in vacuo at 60°C, cooled and quenched by pouring over 300 g of crushed ice. The quenched
mixture was then extracted with 3 x 200 mL of benzene. The benzene extracts were combined, washed

with NaHCO3 solution, dried over anhydrous MgSO4, and the benzene evaporated in vacuo. The residue
was distilled at 53-54°C/0.5 torr to give 4-dichloromethylthiazole (37.5 g, 45%). The benzene extract
water layer was treated with conc. NH4OH until the pH reached 6.0, and the resulting oil extracted into
benzene. The benzene extract was dried over anhydrous MgSO4, and the benzene evaporated in vacuo.
The residue was distilled at 48-49°C/0.7 torr to give 4-chloromethylthiazole (14.0 g, 21%).
4-Dichlorothiazole (24 g, 0.143 mol) in 10% H2SO4 (300 mL) was refluxed under N2 at 125-130°C for 2
h. The mixture was then cooled to rt and a 10% NaOH solution was added until the solution reached pH 6.
The mixture was then extracted with 3 x 200 mL of CHCl3. The organic layer was washed with water,
dried over anhydrous MgSO4 and concentrated in vacuo to give 2 (13.7 g, 85%). mp 65-66°C. lit.,12
65-67°C, lit., 13 59-61°C. 1H NMR (CDCl3): δ 8.32 (d, J = 1.9, 1H), 8.99 (d, J = 1.9, 1H), 10.12 (s, 1H);
IR νmax (CHCl3) 1700, 1585, 1425 cm-1. MS: m/z 113 (62%, M+), 85 (100), 58 (72), 57 (56), 45 (40), 29
(24).
4-Thiazolylazlactone (3a)
A mixture of 2 (3.2 g, 28 mmol), acetylglycine (3.25 g, 28 mmol), sodium acetate (12.35 g, 0.15 mol) and
acetic anhydride (30 mL) were heated for 20 min at 100°C. The reaction products were cooled, filtrated
and washed with water. The crude product was recrystallized from acetone to give 3a (2.8 g, 52%). mp
115-118°C. Anal. Calcd for C8H6N2O2S: C, 49.47; H, 3.11; N, 14.42. Found: C, 49.61; H, 3.09; N, 14.44.
1H NMR (CDCl3): δ 7.53-7.59 (m, 3H), 7.62-7.66 (m, 1H), 8.18-8.21 (m, 2H), 8.83 (d, J = 2.1, 1H), 8.90
(d, J = 2.1, 1H); IR νmax (CHCl3) 3440, 1710, 1650, 1520 cm-1; MS: m/z 194 (95%, M+), 166 (19), 148 (3),
124 (100), 97 (38), 60 (14), 43 (87).
4-Thiazolylazlactone (3b)
A mixture of 2 (10.2 g, 90 mmol), hippuric acid (7.4 g, 90 mmol), sodium acetate (36.3 g, 0.43 mol) and
acetic anhydride (50 mL) was heated for 20 min at 100°C. The reaction products were cooled, filtrated
and washed with water. The crude product was recrystallized from CHCl3 to give 3b (19.4 g, 84%). mp
174-176°C. Anal. Calcd for C13H8N2O2S: C, 60.92; H, 3.15; N, 10.93. Found: C, 61.08; H, 3.09; N, 10.85.
1H NMR (CDCl3): δ 7.53-7.59 (m, 3H), 7.62-7.66 (m, 1H), 8.18-8.21 (m, 2H), 8.83 (d, J = 2.1, 1H), 8.90
(d, J = 2.1, 1H); IR νmax (CHCl3) 3620, 1800, 1660, 1520 cm-1. MS: m/z 256 (13%, M+), 105 (100), 77
(56), 69 (10), 51 (6).
(Z)-3-(4-Thiazolyl)-2-acetylaminoprop-2-enoic acid (4a)
The mixture of 3a (2.5 g, 12.9 mmol) and 25% aqueous acetone (80 mL) was refluxed for 2 h. The
reaction mixture was then concentrated in vacuo. The residue was extracted with CHCl3 and washed with
water. The solvent was removed and the solid residue was recrystallized from MeOH to give 4a (1.86 g,
68%) as a solid. mp 185-187°C. Anal. Calcd for C8H8N2O3S: C, 45.27; H, 3.80; N, 13.20. Found:
C, 45.31; H, 3.78; N, 13.05. 1H NMR (CD3OD): δ 7.09 (s, 1H), 7.53-7.64 (m, 3H), 7.82 (d, J = 1.9, 1H),

7.99-8.02 (m, 2H), 9.11 (d, J = 1.5, 1H); IR νmax (CHCl3) 3440, 1710, 1650, 1520 cm-1; MS: m/z 212
(43%, M+), 194 (3), 184 (7), 170 (68), 168 (67), 126 (100), 99 (80), 93 (95), 82 (5), 72 (13).
(Z)-3-(4-Thiazolyl)-2-benzoylaminoprop-2-enoic acid (4b)
The mixture of 3b (19 g, 74 mmol) and 0.3% NaOH (780 mL) was stirred at 100°C for 45 min. The
reaction mixture was then cooled and filtered. The filtrate was carefully acidified with 10% HCl to give a
solid. The solid was washed with acetone and ether. The crude product was recrystallized from a mixture
of EtOAc and MeOH to give 4b (21.7 g, 85%). mp 206-208°C. Anal. Calcd for C13H10N2O3S: C, 56.92;
1
H, 3.67; N, 10.21. Found: C, 59.88; H, 3.61; N, 10.16. H NMR (CD3OD): δ 7.09 (s, 1H), 7.53-7.64 (m,
3H), 7.82 (d, J = 1.9, 1H), 7.99-8.02 (m, 2H), 9.11(d, J = 1.5, 1H); IR νmax (CHCl3): 3440, 1710, 1650,
1520 cm-1; MS: m/z 275 (80%, M++1), 257 (27), 231 (23), 230 (27), 153 (14), 122 (6), 105 (100), 77
(17).
(S)-3-(4-Thiazolyl)-2-acetylaminopropanoic acid (5a)
Into a 100 mL autoclave were charged 4a (0.5 g, 2.35 mmol) in MeOH (20 mL) and
Ru2Cl4[(S)-(-)-p-tolyl-BINAP]2Et3N (20 mg, 0.022 mmol) under an atmosphere of N2. The atmosphere
was then replaced with H2 at a pressure of 5 MPa. The reaction mixture was stirred for the appropriate
time and at the temperature (see Table 1). The reaction solution was concentrated in vacuo and the solid
residue was recrystallized from CHCl3 to give 5a (0.45 g, 90%) as a solid. For the measurement of the
enantiomeric purity by HPLC, 5a was converted into 6. mp 166-170°C, The enantiomeric purity was 77%
ee. [α]D23 -8.0º(c 1.0, MeOH). 1H NMR (CD3OD) δ 3.40 (dd, J = 8.5, 14.5, 1H), 3.52 (dd, J = 4.5, 14.5,
1H), 7.35 (s, 1H), 7.41-7.46 (m, 2H), 7.50-7.55 (m, 1H), 7.76-7.78 (m, 1H), 8.95(d, J = 1.8, 1H); IR νmax
(CHCl3): 3410, 3310, 1705, 1630, 1520 cm-1; MS: m/z 214 (11%, M+), 169 (16), 155 (15), 127 (100), 111
(60), 99 (95), 74 (23), 43 (78). HRMS calcd for C8H10N2O3S: 214.2386, Found: 214.2394.
(S)-3-(4-Thiazolyl)-2-benzoylaminopropanoic acid (5b)
Into a 100 mL autoclave were charged 4b (1.0 g, 3.6 mmol) in MeOH (20 mL) and
Ru2Cl4[(S)-(-)-p-tolyl-BINAP]2Et3N (20 mg, 0.022 mmol) under an atmosphere of N2. The atmosphere
was then replaced with H2 at a pressure of 5 MPa. The reaction mixture was stirred for the appropriate
time and at the temperature (see Table 1). The reaction solution was concentrated in vacuo and the solid
residue was recrystallized from a mixture of EtOAc and MeOH to give 5b (0.41 g, 40%). For the
measurement of the enantiomeric purity by HPLC, 5b were converted into the methyl ester of 5b using
diazomethane. The enantiomeric purity was 100% ee. [(S)-tr 21.7 min, (R)-tr 27.4 min]. mp 198-200°C,
23
1
[α]D -22.0º (c 1.0, MeOH). H NMR (CD3OD): δ 3.40 (dd, J = 8.5, 14.5, 1H), 3.52 (dd, J = 4.5, 14.5,
1H), 7.35 (s, 1H), 7.41-7.46 (m, 2H) 7.50-7.55 (m, 1H), 7.76-7.78 (m, 1H), 8.95 (d, J = 1.8, 1H); IR νmax
(CHCl3): 3410, 3310, 1705, 1630, 1520 cm-1. MS: m/z 276 (6%, M+), 232 (6), 171 (5), 155 (6), 127 (36),
111 (24), 105 (100), 99 (36), 77 (30). HRMS calcd for C13H12N2O3S: 276.3094, Found: 276.3098.

(S)-3-(4-Thiazolyl)-2-tert-butoxycarbonylaminopropanoic acid (6)
5b (0.30 g, 1.4 mmol) and 6 N HCl (25 mL) were refluxed for 20 h and then cooled to rt. The reaction
mixture was concentrated in vacuo. The residue was washed with toluene to remove the benzoic acid. The
crude product (0.24 g) was dissolved in THF (25 mL) and adjusted to pH 9 with 3N NaOH solution.
Di-tert-butyl dicarbonate (0.23 g, 1.05 mmol) in THF (5 mL) was added to the mixture and stirred for 40
h at rt. The solvent was removed in vacuo and the residue was adjusted to pH 2 with 3 N HCl, and then
extracted with EtOAc (50 mL), washed with brine and dried using anhydrous MgSO4. The solvent was
concentrated in vacuo and the solid residue was recrystallized from acetone to give 6 (0.18 g, 57%) as a
solid. The enantiomeric purity was 99% ee. [(S)- tr 7.8 min., (R)- tr 9.6 min]. mp 122-123°C, lit.,2 mp
24
24
122-123°C, [α]D +11.0º (c 1.0, MeOH). [α]D +125.9º (c 1.0, CHCl3) lit.,2 [α]D +124.9 (c 1.04,
CHCl3), 1H NMR (CDCl3): δ 1.46 (s, 9H), 3.31-3.39 (m, 1H), 3.49-3.55 (m , 1H), 4.5 (m, 1H), 5.61-5.65
(m, 1H), 7.18 (s, 1H), 8.89-8.91 (m, 1H); IR νmax (CHCl3): 3430, 1710, 1505 cm-1; MS: m/z 273 (9%,
M++1), 259 (4), 257 (4), 217 (90), 199 (100), 173 (28), 153 (6), 127 (50), 111 (3), 99 (23), 71 (18).
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