The first total synthesis of potent human chymase inhibitor SPF32629B via regioselective bromination and O-acylation strategy

Article abstract of DOI:10.1016/j.tetlet.2010.03.091

An efficient total synthesis of (±)-SPF32629B is described. The key features of the synthesis include regioselective bromination followed by carboxylation at C-3 and protecting-group-free regioselective acylation of hydroxyl group present at C7. Structure

Full text of DOI:10.1016/j.tetlet.2010.03.091

Tetrahedron Letters 51 (2010) 2768–2770
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
The first total synthesis of potent human chymase inhibitor SPF32629B via
regioselective bromination and O-acylation strategy
a,
b
Srinivasa Rao Vegi a,b, Shanthaveerappa K. Boovanahalli a, , Balaram Patro , K. Mukkanti
*
*
a Medicinal Chemistry Laboratory, GVK Biosciences Pvt. Ltd, Plot No. 28, IDA, Nacharam, Hyderabad 500 076, AP, India
b Chemistry Division, Institute of Science and Technology, JNT University, Kukatpally, Hyderabad 500 072, AP, India
a r t i c l e i n f o
a b s t r a c t
Article history:
An efficient total synthesis of ( )-SPF32629B is described. The key features of the synthesis include reg-
ioselective bromination followed by carboxylation at C-3 and protecting-group-free regioselective acyl-
ation of hydroxyl group present at C7. Structure was determined by X-ray analysis.
Ó 2010 Elsevier Ltd. All rights reserved.
Received 20 February 2010
Revised 17 March 2010
Accepted 19 March 2010
Available online 25 March 2010
SPF32629A and B (1 and 2, respectively, Fig. 1) make up a family
of biologically interesting pyridone derivatives isolated from the
cultured broth of penicillium sp. by Shimatani and Hosoya in
2004.1 These compounds differ in structure only at C-3 and 2 is
the carboxylated derivative of 1. The preliminary biological evalu-
ation revealed that both 1 and 2 demonstrated significant inhibi-
C7. While in turn 3 could be easily assembled starting from com-
mercially available pyridine by following previously described
protocols.5,6
As illustrated in Scheme 2, the total synthesis of ( )-SPF32629B
commenced with known compound 3, which can be efficiently
prepared in five steps starting from cheaply available pyridine.5,6
tory activities against human chymase (IC50 = 0.25 and 0.42
lg/
mL, respectively) and human elastase (IC50 = 4.9 and 4.5 g/mL,
l
respectively) and therefore expected to be useful for the treatment
of inflammation-related various diseases and procoagulant.1,2 Hu-
man chymase inhibitors are considered to be useful therapeutic
agents in disorders such as congestive heart failure, allergy, vascu-
lar proliferation and chronic inflammation following fibrosis.3 In
this regard several chymase inhibitors have been reported in the
literature.4
OH
R
O
N
H
O
O
Thus, on account of their diverse structural features and their
potential medicinal utility as human chymase inhibitors, 1 and 2
present themselves as important synthetic targets to explore their
therapeutical potential. A systematic exploration of the properties,
mechanism of action and biochemical significance of 1 and 2 re-
quires efficient and versatile syntheses of these molecules. Besides,
synthetic approaches to these molecules will also offer avenues to
the discovery of more analogues with high selectivity and potent
bioactivity than 1 and 2. In this context, we have previously dis-
closed the first total synthesis of racemic 15 and to the best of
our knowledge so far no studies on the synthesis of 2 have been re-
ported. Herein, we describe a concise and an efficient approach, de-
signed to afford the first total synthesis of racemic 2.
1: R = H
SPF32629A
2: R = COOH SPF32629B
Figure 1. Structure of SPF32629A and B.
OBn
OH
Regioselective
Bromination
Br
N
O
O
N
H
O
3
O
Carboxylation
Tandem Reduction and
O
O
OH
The retro-synthetic analysis of compound 2 is shown in Scheme
1. We envisioned the construction of 2 starting from 3 via regiose-
lective bromination followed by insertion of carboxylic acid at C-3,
coupled with tandem reduction and regioselective O-acylation at
Regioselective
O
OH
Acylation
HO
HO
N
H
O
N
H
O
O
O
SPF32629B
* Corresponding authors. Tel.: +91 40 6451 3333x1163; fax: +91 40 2715 2999
(S.B.).
E-mail address: sveerappa.boovanahal@gvkbio.com (S.K. Boovanahalli).
Scheme 1. Retrosynthetic analysis of SPF32629B.
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.03.091

S. R. Vegi et al. / Tetrahedron Letters 51 (2010) 2768–2770
2769
OBn
N
OH
OH
OBn
Br
b
a
c
Cl
O
N
H
O
N
H
N
O
O
O
O
O
3
4
5
6
O
O
O
OH
O
O
OH
O
CH3
d
f
e
MeO
HO
Br
N
H
N
H
O
N
H
O
O
O
O
8
9
7
OH
O
OH
HO
HO
g
h
O
N
H
O
N
H
O
OH
10
SPF32629B
O
Scheme 2. Reagents and conditions: (a) POCl3, reflux,12 h, 89%; (b) AcOH, H2O, sealed tube, 180–190 °C, 12 h, 90%; (c) NBS, THF, DCM, À10 °C to 0 °C, 2 h, 97%; (d) Ac2O,
DMAP, rt, 5 h, 98%; (e) bis(benzonitrile)palladium(II) chloride, dppf, TEA, MeOH, CO, 300 psi, 120 °C, 3 h, 70%; (f) LiOHÁH2O, THF, H2O, 50 °C, 36 h, quantitative (crude); (g)
NaBH4, THF, MeOH, 0 °C, 1 h, quantitative (crude); (h) isovaleric acid, EDCÁHCl, DCM, THF, DMF, DMAP, rt, 12 h, 80% (over all yield for three steps).
Reaction of 3 with phosphorous oxychloride under reflux for
12 h yielded 4 in good yield. Compound 5 could be conveniently
prepared by effecting debenzylation and dechlorination in a single
step. Thus, when a solution of 4 in aqueous acetic acid was heated
at 180–190 °C in a sealed tube for 12 h afforded 5 in high yield.
Then, we examined direct conversion of 5 to 9 by insertion of car-
boxylic acid group regioselectively at C-3 under lithiation followed
by carboxylation conditions (BuLi or LDA, then CO2), however all
attempts resulted in complex mixtures and failed to give the de-
sired conversion. We therefore opted to install carboxylic acid
group in a stepwise fashion through sequential bromination and
palladium-catalyzed Heck carbonylation. Gratifyingly, when com-
pound 5 was reacted with N-bromosuccinimide under optimized
conditions afforded a highly regioselective bromination at C-3 to
give 6 in excellent yield. The regioselective bromination at C-3
was confirmed by 1H NMR, HMBC and COSY analysis. At this point,
many attempts to introduce carboxyl group at C-3 in bromo-ana-
logue 6 failed and hence, we thought to introduce protecting
groups for amide and hydroxyl groups that would facilitate car-
bonylation. Accordingly, we examined various protecting groups
including BOC, CBz, benzyl, TBDMS and acetyl protection. Although
we could achieve Boc and CBz protection separately on 6, the
resulting protected compounds did not undergo subsequent car-
bonylation under various experimental conditions (Scheme 3).
On the other hand, all attempts to protect 6 with benzyl and
TBDMS groups failed. Finally, to our delight, we found that the best
results, in terms of protection and carbonylation were achieved
through acetylation of 6. Thus, acetylation of 6 rendered 7 in excel-
lent yield and subsequently, after extensive experimentation,
palladium-catalyzed esterification at C-3 of 7 was accomplished
using bis(benzonitrile)palladium(II) chloride and dppf as palla-
dium source in presence of TEA in methanol under 300 psi CO pres-
sure to afford 8. This procedure allowed a remarkably facile
incorporation of methyl ester moiety at C-3 in conjunction with
regeneration of free hydroxyl group at C-4. Further hydrolysis of
8 with lithium hydroxide monohydrate furnished the requisite
acid derivative 9 in good yield.
Having efficiently synthesized the appropriately substituted
pyridone core 9, we turned our attention toward construction of fi-
nal target molecule. Based on the poor selectivity of benzylation or
TBDMS protection of OH and CONH groups of 5 and poor stability
of acetyl group in 7; we hypothesized that 2 could be regioselec-
tively assembled in a one-pot procedure by a tandem reduction
and acylation protocol in presence of free OH, COOH and CONH
groups. Accordingly, reduction of 9 under standard sodium boro-
hydride mediated reduction conditions furnished the requisite
crude alcohol 10 in quantitative yield, which upon extractive isola-
tion was acylated without further purification. Gratifyingly, as ex-
pected the resulting crude alcohol 10 underwent coupling with
isovaleric acid in a regioselective manner to produce SPF32629B
in good yield with high purity. The spectral data including IR,
O
O
OR1
OR1
Br
b
X
RO
a
6
O
N
N
R2
O
R2
O
R1 = Boc; R2 = Boc
R1 = H; R2 = CBz
R = H or CH3; R1 = Boc; R2 = Boc
R = H or CH3; R1 = H; R2 = CBz
Scheme 3. Reagents and conditions: (a) Boc anhydride, DCM, TEA, 0 °C to rt, 6 h,
90% or CBzCl, TEA, DCM, 0 °C to rt, 3 h; (b) bis(benzonitrile)palladium(II) chloride,
dppf TEA, MeOH, CO, 300 psi, 120 °C, 12 h or BuLi, THF, À78 °C, 1 h, then CO2.
Figure 2. ORTEP diagram of 2 showing the crystal structure.

2770
S. R. Vegi et al. / Tetrahedron Letters 51 (2010) 2768–2770
MASS, 1H NMR, 13C NMR and LC–MS of our synthetic compound 2
were in consistent with the reported data.1,2,7 The assigned struc-
ture was also firmly established by single-crystal X-ray analysis
as shown in Figure 2.8 With this, the total synthesis of 2 was com-
pleted in an overall yield of 43% and this constitutes the first re-
ported total synthesis of ( )-SPF32629B.
In conclusion, we have described a highly efficient synthesis of
SPF32629B, featuring sequential regioselective carboxylation at C-
3 followed by late-stage in situ C7-keto reduction and regioselec-
tive O-acylation. Studies on the enantioselective synthesis includ-
ing biological activities of SPF32629B and their analogues are
currently in progress and will be reported in due course.
4. (a) Akahoshi, F. Curr. Pharm. Des. 2003, 9, 1191–1199; (b) Akahoshi, F.; Ashimori,
A.; Sakashita, H.; Yoshimura, T.; Imada, T.; Nakajima, M.; Mitsutomi, N.;
Kuwahara, S.; Ohtsuka, T.; Fukaya, C.; Miyazaki, M.; Nakamura, N. J. Med. Chem.
2001, 44, 1286–1296; (c) Akahoshi, F.; Ashimori, A.; Sakashita, H.; Yoshimura,
T.; Eda, M.; Imada, T.; Nakajima, M.; Mitsutomi, N.; Kuwahara, S.; Ohtsuka, T.;
Fukaya, C.; Miyazaki, M.; Nakamura, N. J. Med. Chem. 2001, 44, 1297–1304; (d)
Masaki, H.; Mizuno, Y.; Tatui, A.; Murakami, A.; Koide, Y.; Satoh, S.; Takahashi, A.
Bioorg. Med. Chem. Lett. 2003, 13, 4085–4088; (e) Tanaka, T.; Muto, T.; Maruoka,
H.; Imajo, S.; Fukami, H.; Tomimori, Y.; Fukuda, Y.; Nakatsuka, T. Bioorg. Med.
Chem. Lett. 2007, 17, 3431–3434; (f) Iijima, K.; Katada, J.; Hayashi, Y. Bioorg. Med.
Chem. Lett. 1999, 9, 413–418; (g) Koide, Y.; Tatsui, A.; Hasegawa, T.; Murakami,
A.; Satoh, S.; Yamada, H.; Kazayama, S.-I.; Takahashi, A. Bioorg. Med. Chem. Lett.
2003, 13, 25–29; (h) Groutas, W. C.; Schechter, N. M.; He, S.; Yu, H.; Huang, P.; Tu
Bioorg. Med. Chem. Lett. 1999, 9, 2199–2204.
5. Vegi, S. R.; Boovanahalli, S. K.; Sharma, A. P.; Mukkanti, K. Tetrahedron Lett. 2008,
49, 6297–6299.
6. Ochiai, E. J. Org. Chem. 1953, 18, 534–551.
7. Spectral data of ( )-SPF32629B: Rf = 0.3 (5% methanol/dichloromethane); mp
Acknowledgments
129–132 °C; IR (KBr pellet): mmax 3461, 1742, 1688, 1639 cmÀ1 1H NMR
;
(400 MHz CDCl3)) d 14.24 (1H, br s, D2O exchangeable COOH), 13.51 (1H, br s,
D2O exchangeable OH), 10.1 (1H, br s, D2O exchangeable CONH), 7.42–7.34 (5H,
m), 6.63 (1H, s), 6.22 (1H, s), 2.34 (2H, d, J = 7.2 Hz), 2.15–2.09 (1H, m), 0.94 (6H,
d, J = 6.8 Hz); 13C NMR (100 MHz, CDCl3) d 174.94 (C), 171.83 (C), 171.35 (C),
165.93 (C), 151.07 (C), 134.75 (C), 129.82 (CH), 129.34 (2CH), 127.21 (2CH)
101.35 (CH), 96.84 (C), 72.67 (CH); 42.95 (CH2); 25.69 (CH); 22.28 (CH3); 22.26
(CH3); MS (ESI) m/z 344.20 (MÀH)À; LC–MS (ES) m/z calcd for C18H18NO6
[MÀH]À: 344.36, found: 344.2; Develosil ODS MG-3 (4.6 Â 33 mm), mobile
phase: A: 0.1% aq HCOOH, B: 0.1% HCOOH (acetonitrile/methanol (50:50)), T/%B:
0/30, 4/90, 10/90, 10.1/30; flow rate: 1 mL/min, diluent: acetonitrile; UV:
301 nm, tR = 4.61, purity = 98.72%; HPLC—column used: Eclipse XDB-C18
We thank Management, GVK Biosciences Private Limited for the
financial support and encouragement. Help from the analytical
department for the analytical data is appreciated. We thank Dr.
Raghavaiah Pallepogu for the single-crystal X-ray analysis data.
References and notes
1. Shimatani, T.; Hosoya, Y. Japanese Patent JP 2004067584, 2004; Chem. Abstr.
2004, 140, 216278.
2. Shimatani, T.; Hosotani, N.; Ohnishi, M.; Kumagai, K.; Saji, I. J. Antibiot. 2006, 59,
29–34.
(4.6 Â 150 mm) 5
l, mobile phase: A: 0.1 M formic acid; B: methanol; T/%B: 0/
50, 8/90, 15/90, 15.1/50; flow rate: 1.0 mL/min, diluent: (A/ACN, 1:1); UV:
303 nm; tR = 9.13; purity = 99.12%. Chiral HPLC—column used: Chiral PAK-AD-H
(4.6 Â 250 mm), 5
l, mobile phase: A: 0.1% TFA in n-hexanes, B: ethanol (70:30),
isocratic; flow rate: 0.7 mL/min, diluent: ethanol, run time: 25 min, UV: 304 nm,
tR = 8.69 and 12.57.
3. (a) Muto, T.; Fukami, H. IDrugs 2002, 5, 1141–1150; (b) Akahoshi, F. Drugs Future
2002, 27, 765–770; (c) Doggrell, S. A.; Wanstall, J. C. Expert Opin. Investig. Drugs
2003, 12, 1429–1432; (d) Fukami, H.; Okunishi, H.; Miyazaki, M. Curr. Pharm.
Des. 1998, 4, 439–453; (e) Tomimori, Y.; Muto, T.; Fukami, H.; Saito, K.;
Horikawa, C.; Tsuruoka, N.; Saito, M.; Sugiura, N.; Yamashiro, K.; Sumida, M.;
Kakutani, S.; Fukuda, Y. Lab. Invest. 2002, 82, 789–794; (f) Watanabe, N.;
Tomimori, Y.; Saito, K.; Miura, K.; Wada, A.; Tsudzuki, M.; Fukuda, Y. Int. Arch.
Allergy Immunol. 2002, 128, 229–234; (g) Kokkonen, J. O.; Lindstedt, K. A.;
Kovanen, P. T. Circulation 2003, 107, 2522–2524; (h) Matsumoto, T.; Wada, A.;
Tsutamoto, T.; Ohnishi, M.; Isono, T.; Kinoshita, M. Circulation 2003, 107, 2555–
2558; (i) Kanemitsu, H.; Takai, S.; Tsuneyoshi, H.; Nishina, T.; Yoshikawa, K.;
Miyazaki, M.; Ikeda, T.; Komeda, M. Hypertens. Res. 2006, 29, 57–64; (j)
Tomimori, Y.; Muto, T.; Saito, K.; Tanaka, T.; Maruoka, H.; Sumida, M.; Fukami,
H.; Fukuda, Y. Eur. J. Pharmacol. 2003, 478, 179–185; (k) Sakaguchi, M.; Takai, S.;
Jin, D.; Okamoto, Y.; Muramatsu, M.; Kim, S.; Miyazaki, M. Eur. J. Pharmacol.
2004, 493, 173–176; (l) Takai, S.; Jin, D.; Muramatsu, M.; Okamoto, Y.; Miyazaki,
M. Eur. J. Pharmacol. 2004, 501, 1–8; (m) Aoyama, Y. Expert Opin. Ther. Patents
2001, 11, 1423–1428.
8. Crystal data for ( )-SPF32629B: C18H19NO6; M = 345.34, temperature = 293(2) K,
ꢀ
wavelength = 0.71073 Å, crystal system, space group = triclinic, P, unit cell
dimensions a = 5.4767(8) A
c = 14.733(3) A
= 88.399(13) °. Volume = 881.8(2) Å3, Z, calculated density = 2,
1.301 Mg/m3, absorption coefficient = 0.098 mmÀ1
F(000) = 364, crystal
range for data collection = 3.51–24.71 °,
a = 70.750(15) °. b = 11.6443(18) A b = 83.818(13) °,
c
,
size = 0.42 Â 0.20 Â 0.18 mm,
h
limiting indices = À6 ꢀ h ꢀ 5, À13 ꢀ k ꢀ 10, À16 ꢀ l ꢀ 17, reflections collected/
unique = 4399/2902 [R(int) = 0.0270], completeness to h = 24.71 96.3%, max. and
min. transmission = 0.9825 and 0.9599, refinement method = full-matrix least-
squares on F2, data/restraints/parameters = 2902/0/281, goodness-of-fit on
F
2 = 0.908, final R indices [I > 2
r(I)] R1 = 0.0466, wR2 = 0.1092, R indices (all
data) R1 = 0.0908, wR2 = 0.1228, extinction coefficient = 0.004(4), largest diff.
peak and hole = 0.169 and À0.176 e Å3. Further details of the crystal structure
investigation can be obtained from the Cambridge Crystallographic Data Centre,
12 Union Road, Cambridge, CB2 1EZ, UK (CCDC Deposition No. 759752).