Determination of the absolute configuration of fasidotril, a potent dual ACE/NEP inhibitor

Article abstract of DOI:10.1016/S0957-4166(03)00458-0

The absolute configuration of fasidotril has been established by a chemical correlation leading to the stereochemical assignment of the intermediate (S)-3-acetylsulfanyl-2-(benzo[1,3]dioxol-5-ylmethyl)propanoic acid.

Full text of DOI:10.1016/S0957-4166(03)00458-0

TETRAHEDRON:
ASYMMETRY
Pergamon
Tetrahedron: Asymmetry 14 (2003) 2335–2337
Determination of the absolute configuration of fasidotril, a
potent dual ACE/NEP inhibitor
V. Grosset, D. Danvy and M. Capet*
Bioprojet-Biotech, 4 rue du Chesnay Beauregard, BP95206, 35762 Saint Gre´goire Cedex, France
Received 28 May 2003; accepted 12 June 2003
Abstract—The absolute configuration of fasidotril has been established by a chemical correlation leading to the stereochemical
assignment of the intermediate (S)-3-acetylsulfanyl-2-(benzo[1,3]dioxol-5-ylmethyl)propanoic acid.
© 2003 Elsevier Ltd. All rights reserved.
Angiotensin I converting enzyme (ACE, EC 3.4.25.1)
and neprilysin (NEP, EC 3.4.24.11, also named neutral
endopeptidase, enkephalinase or atriopeptidase) are
two ectoenzymes that play a major role in regulating
cardiovascular and renal functions such as angiotensin
II, bradykinin and natriuretic peptides. Whereas ACE
inhibitors are largely used drugs in the treatment of
hypertension, congestive heart failure or renal insuffi-
ciency, the potential interest of achieving NEP inhibi-
tion in such diseases was only recently proposed.1–3
Fasidotril 1 was the first drug proposed to combine in
a single molecule equal inhibitory potency towards
NEP and ACE, hence resulting in protection of natri-
uretic peptides and bradykinin to prevent angiotensin II
formation.4,5 Fasidotril is in phase II clinical trials,
whereas similar compounds (e.g. omapatrilat) are
undergoing larger scale clinical trials and are often
designated ‘vasopeptidase inhibitors’ (Fig. 1).6
given the (S)-configuration based on structure activity
relationships.
Figure 1.
Among the routes leading to fasidotril,7 one involves
the separation of the enantiomers of the acid 2 by
successive recrystallizations of its a-phenethylamine or
ephedrine salt.8 The levorotatory acid can be converted
to biologically active fasidotril† by condensation with
(S)-alanine benzyl ester8 as depicted in Scheme 1.
Fasidotril contains two stereogenic centers, one of
which stems from natural alanine. The other one was
Scheme 1. Reagents and conditions: (i) Et3N (1.0 equiv.),
(S)-AlaOBn, CH3SO3H (1.0 equiv.), DCCI (1 equiv.), HOBT
(1 equiv.), THF, 0°C“rt, 6 h.
* Corresponding author. Tel.: 33 2 99 28 04 53; fax: 33 2 99 28 04 44;
e-mail: m.capet@bioprojet.com
1
† Mp=104°C; H NMR (250 MHz, CDCl3): 7.25 (s, 5H), 6.6 (s, 3H),
6.0 (d, 1H, J=7.5 Hz), 5.85 (s, 2H), 5.0 (s, 2H), 4.5 (quint., 1H,
J=7.5 Hz), 3.05 (d, 2H, J=6.1 Hz), 3.0–2.4 (m, 3H), 2.25 (s, 3H),
1.3 (d, 3H, J=7.5 Hz). Anal. calcd for C23H25NO6S: C, 62.30; H,
5.64; N, 3.16. Found: C, 62.2; H, 5.30; N, 3.20. [h]2D0=−50.6 (c 1.3,
CH3OH).
Analogous condensation of the dextrorotatory acid
with alanine gives a compound far less active as an
ACE inhibitor. The NMR signal of the alanine methyl
0957-4166/$ - see front matter © 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/S0957-4166(03)00458-0

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V. Grosset et al. / Tetrahedron: Asymmetry 14 (2003) 2335–2337
group in this compound appears as a doublet at 1.1
ppm instead of 1.3 ppm for fasidotril. These chemical
shifts are constant within a whole series of aromatic
derivatives, including the unsubstituted one,9 which is
of known configuration. These results indicate that the
absolute configuration of fasidotril is (S,S). This abso-
lute configuration is now demonstrated by a chemical
correlation leading to 3. This product can be obtained
from (−)-2 by a modification of the Curtius rearrange-
ment using diphenylphosphorylazide10 (DPPA) in 67%
yield (Fig. 2 and Scheme 2).‡
piperonal was prepared using the reported procedure.11
Esterification with ethanol and thionyl chloride leads to
aminoester 6, which is reduced with lithium aluminum
hydride to aminoalcohol 7. Selective protection of the
amino group is achieved under conventional two phase
system conditions. Conversion of the alcohol into the
thioacetate by a Mitsunobu12 reaction gives (R)-3.§
The measurement of the rotatory power at five different
wavelengths (Na and Hg) is within the confidence mar-
gin for the two samples (+)-3 and (R)-3. As the Curtius
rearrangement is known to occur with retention of
configuration,¶ the absolute stereochemistry of levo-
rotatory acid 2 is (S). Conversion of this latter with
alanine benzyl ester gives the biologically active
diastereomer. The absolute configuration of fasidotril is
thus to be assigned as (S,S) (Fig. 3).
Figure 2.
Figure 3.
Scheme 2. Reagents and conditions: (i) 1. DPPA (1.05 equiv.),
Et3N (1.05 equiv.), toluene, 80°C, 15 min, 2. C6H5CH2OH
(1.2 equiv.), 80°C, 17 h.
Acknowledgements
The protected aminothiol 3 can also be prepared start-
ing from a material of known absolute configuration as
depicted in Scheme 3. Aminoacid (R)-5 derived from
We thank Centre Re´gional de Mesures Physiques de
l’Ouest (CRMPO) for microanalyses and UMR 6052
for polarimetric facilities.
Scheme 3. Reagents and conditions: (i) SOCl2, EtOH, reflux, 3 h (ii) LiAlH4 (1.15 equiv.), Et2O, rt, 24 h; (iii) C6H5CH2OCOCl
(1 equiv.), K2CO3 (2 equiv.), AcOEt/H2O 60/40, rt, 24 h; (iv) 1. PPh3 (2 equiv.), DEAD (2 equiv.), THF, 0°C, 40 min, 2. AcSH
(2 equiv.), THF, 0°C, 2 h then rt, 20 h.
‡ To a solution of 0.44 g (1.56 mmol) of (−)-2 in toluene (2 mL) was added dropwise 353 mL (1.64 mmol) of DPPA and 228 mL (1.64 mmol) of
triethylamine. The reaction mixture was heated at 80°C until gas evolution ceased (20 min) then 194 mL (1.87 mmol) of benzyl alcohol was
added. The mixture was further heated for 17 h at 80°C, then solvent was evaporated and replaced with 15 mL of ethyl acetate. Washing
successively with water, saturated NaHCO3 solution and water, drying over MgSO4 and concentrating afforded an off-white solid. Purification
by column chromatography (heptane/AcOEt 7:3; ¥ 3 cm; 50 g SiO2), trituration in heptane and drying over P2O5 gave 0.4 g (67%) of 3 as a
white solid melting at 142.8°C. 1H NMR (250 MHz, CDCl3): 7.45–7.20 (m, 5H), 6.80–6.50 (m, 3H), 5.95 (s, 2H), 5.10 (s, 2H), 4.85 (d, 1H, J=7.7
Hz), 4.00 (m, 1H), 3.15–2.60 (m, 4H), 2.35 (s, 3H). Anal. calcd for C20H21NO5S: C 62.00; H, 5.46; N, 3.62; S, 8.28. Found: C, 62.30; H, 5.45;
N, 3.43; S, 7.98. [h]D20=+7.6 (c 1.0, CHCl3), [h]32065=+19.7 [h]42036=+14.0 [h]25046=+8.8 [h]52708=+7.7.
§ 1H NMR (250 MHz, CDCl3): 7.45–7.20 (m, 5H), 6.80–6.50 (m, 3H), 5.95 (s, 2H), 5.10 (s, 2H), 4.85 (d, 1H, J=7.7 Hz), 4.00 (m, 1H), 3.20–2.60
(m, 4H), 2.35 (s,3H). Anal. calcd for C20H21NO5S: C, 62.00; H, 5.46; N, 3.62; S, 8.28. Found: C, 61.53; H, 5.38; N, 3.52; S, 8.26. [h]2D0=+7.4
(c 1.0, CHCl3), [h]23605=+18.3 [h]42036=+12.9 [h]25046=+8.2 [h]25708=+7.0.
¶ (S)-2 gives (R)-3 as the result of the priority order change when applying the Cahn–Ingold–Prelog rules.

V. Grosset et al. / Tetrahedron: Asymmetry 14 (2003) 2335–2337
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