Determination of the relative and absolute stereochemistry of a potent and α(1A)-selective adrenoceptor antagonist

Article abstract of DOI:10.1016/S0960-894X(00)00524-2

The binding affinities and selectivities of antagonists 1-4 for the α(1A)-adrenoceptor are dependent on the stereochemical orientation of the groups at the C-4 and C-5 positions of the oxazolidinone ring. The unambiguous assignment of the relative and abs

Full text of DOI:10.1016/S0960-894X(00)00524-2

Bioorganic & Medicinal Chemistry Letters 10 (2000) 2705±2707
Determination of the Relative and Absolute Stereochemistry of a
Potent and ꢀ1A-Selective Adrenoceptor Antagonist
Bharat Lagu,a,* John M. Wetzel,a Carlos Forray,b Michael A. Patanec
and Mark G. Bockc
aDepartment of Chemistry, Synaptic Pharmaceutical Corporation, 215 College Road, Paramus, NJ 07652, USA
bDepartment of Pharmacology, Synaptic Pharmaceutical Corporation, 215 College Road, Paramus, NJ 07652, USA
cDepartment of Medicinal Chemistry, Merck Research Laboratories, West Point, PA 19486, USA
Received 8 June 2000; accepted 5 September 2000
AbstractÐThe binding anities and selectivities of antagonists 1±4 for the a1A-adrenoceptor are dependent on the stereochemical
orientation of the groups at the C-4 and C-5 positions of the oxazolidinone ring. The unambiguous assignment of the relative and
absolute con®gurations of the diastereomers of SNAP 7915 (1) is reported. # 2000 Elsevier Science Ltd. All rights reserved.
We have recently reported the discovery of the struc-
turally novel, orally bioavailable a1A-adrenoceptor
antagonist 1 (SNAP 7915) as a potential treatment for
benign prostatic hyperplasia (BPH).1 Of the four possi-
ble diastereomers with this structure (viz. 1±4, Fig. 1),
one of them (1) binds the a1A-adrenoceptor with the
highest anity and selectivity in radioligand binding
assays2 as summarized in Table 1. We therefore sought
to unambiguously assign the relative and absolute
con®gurations of compounds 1±4.
(®rst isomer, 8b: Rf 0.6, second isomer, 8a: Rf 0.5; hex-
ane:EtOAc 3:1). The separation of the enantiomers was
accomplished by preparative chiral HPLC.5
The relative con®gurations (cis/trans) of 8a and 8b were
assigned on the basis of 1H NMR analysis of the
respective p-nitrophenyloxycarbonyl derivatives 11a and
11b (Scheme 1, eq 2), as shown in Figure 2. For com-
pound 11b, an NOE was observed between the protons
of the C-5 methyl group and the proton at C-4. No
NOE was observed between the protons at the C-4 and
C-5 positions of this isomer, which was thus assigned
trans stereochemistry. For oxazolidinone 11a, no NOE
was observed between the protons of the C-5 methyl
group and the proton at C-4. However, an NOE was
observed between the protons at the C-4 and C-5 posi-
tions, leading us to assign this isomer cis stereo-
chemistry. The vicinal coupling constants of the C-4
protons of 11a (J=7.8 Hz) and 11b (J=5.1 Hz) are also
consistent with the values reported for similar oxa-
zolidinones, and were thus helpful in making the
stereochemical assignments.6
Two workable synthetic routes were devised to access
the oxazolidinone ring system. The ®rst approach
(Scheme 1, eq 1) involved the addition of imine 5 to
acetaldehyde, a€ording intermediate 6, which after
deprotection provided amino alcohol 7.3 Compound 7
was then converted to oxazolidinone 8 via a two-step
sequence of N-acylation and cyclization with base.1
In the alternative route (Scheme 1, eq 2), the dianion of
commercially available 3,4-di¯uorophenylacetic acid 9
was treated with acetaldehyde to obtain the b-hydroxy
acid 10 as a 1:1 mixture of cis (erythro) and trans (threo)
isomers. The crude product was then transformed into
the corresponding oxazolidinones 8a±b in excellent yield
via Curtius rearrangement.4 The cis and trans diaster-
eomers (8a and 8b) were separated by ¯ash column chro-
matography or preparative thin-layer chromatography
In order to assign the absolute con®gurations at the
stereogenic centers of the oxazolidinone rings, a new
synthetic route (Scheme 2) was designed which
employed an enantiomerically pure substrate derived
from the chiral pool. Commercially available (S)-(+)-
methyl lactate was converted into amide 13 according to
the method of Martin et al.7 Treatment of compound 13
with 3,4-di¯uorophenyllithium yielded ketone 14 as the
sole product, which was then converted to oxime 15.
*Corresponding author at present address: R. W. Johnson Pharma-
ceutical Research Institute, Raritan, NJ, USA. Tel.: +1-908-704-4291;
fax: +1-908-526-6469; e-mail: blagu@prius.jnj.com
0960-894X/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0960-894X(00)00524-2

2706
B. Lagu et al. / Bioorg. Med. Chem. Lett. 10 (2000) 2705±2707
Reduction of the oxime with LiAlH4, N-acylation, and
base induced cyclization provided oxazolidinone iso-
mers 17 and 18, which were separated by ¯ash column
chromatography. The enantiomeric purity of these iso-
mers was con®rmed by chiral HPLC analysis and their
relative con®gurations were assigned by comparison of
their 1H NMR spectra with those of the racemic isomers
8a and 8b. To complete the analysis, oxazolidinones 17
and 18 were converted into products 1 and 4, respec-
tively. As the absolute con®guration at C-5 is (S), the C-
4 center in trans compounds 1 and 17 also has the (S)
con®guration. Accordingly, the absolute con®gurations
for the stereogenic centers in the cis compounds 4 and
18 are assigned as (4R,5S). Compounds 2 and 3, the
enantiomers of 1 and 4, thus have the (4R,5R) and
(4S,5R) stereochemistry, respectively.
in a number of in vitro and in vivo functional assays,
and exhibited a long plasma half-life (6 h in rat and
>12 h in dog) and good oral bioavailability (25% in rat
and 74% in dog).1 The present study has provided an
unambiguous stereochemical assignment of 1 and its
isomers, and three methods for its synthesis. These
results will facilitate the further study of 1 and its ana-
logues as potential therapeutics for BPH.
1H NMR data for selected compounds. Spectra were
recorded at 300 MHz in CDCl3. Chemical shifts are
referenced to TMS.
8a (cis): d 0.96 (d, J=6.6 Hz, 3H), 4.91 (d, J=8.1 Hz,
1H), 4.99 (dq, J=6.6, 8.1 Hz, 1H), 6.63 (br s, 1H), 7.08±
7.28 (m, 3H).
Among the four diastereomers in Figure 1, compound 1
clearly stood out on the basis of its high binding anity
and selectivity for the cloned human a1A-adrenoceptor.
This compound was also found to be a potent antagonist
8b (trans): d 1.49 (d, J=6.0 Hz, 3H), 4.37 (dq, J=6.0,
7.2 Hz, 1H), 4.45 (d, J=7.2 Hz, 1H), 6.63 (br s, 1H),
7.08±7.28 (m, 3H).
11a (cis): d 1.10 (d, J=6.6 Hz, 3H, C-5 CH3), 5.09 (dq,
J=6.6, 7.8 Hz, 1H, C-5H), 5.39 (d, J=7.8 Hz, 1H, C-
4H), 7.00±7.30 (m, 3H), 7.22 (d, J=8.7 Hz, 2H), 8.21 (d,
J=8.7 Hz, 2H).
11b (trans): d 1.61 (d, J=6.3, 3H, C-5 CH3), 4.56 (dq,
J=5.1, 6.3 Hz, 1H, C-5H), 4.89 (d, J=5.1 Hz, 1H, C-
4H), 7.10±7.30 (m, 3H), 7.24 (d, J=9.3, 2H), 8.23 (d,
J=9.3, 2H).
4 (cis): d 1.03 (d, J=6.6 Hz, 3H), 1.70±1.85 (m, 6H),
1.97±2.09 (m, 2H), 2.42 (t, J=6.9 Hz, 2H), 2.39±2.51
Figure 1.
Table 1. Binding anities of compounds 1±4 for the recombinant human a1-adrenoceptors2
Ki
(nM)a
Compound
Relative stereochemistry
a1A
a1B
a1D
Selectivity
Ki a1B,1D/Ki a1A
1
2
3
4
(+)-trans
(
0.17Æ0.03
13Æ0.6
0.9Æ0.3
30Æ5
119Æ24
74Æ16
48Æ11
76Æ9
122Æ6
193Æ15
92Æ5
>500
<10
<60
<10
)-trans
(+)-cis
(
)-cis
253Æ49
aKi values were obtained using [125I]-HEAT in competition binding assays with recombinant human receptors.
.
Scheme 1. (a) tert-BuLi, THF, then CH3CHO; (b) MeONH2 HCl, MeOH, 68% for two steps; (c) Boc2O, CHCl3, 91%; (d) NaH, THF, 80%; (e)
LDA, then CH3CHO; (f) NaHCO3, diphenylphosphorylazide, DMF, 60 ꢀC, 96% over three steps. (g) NaH, THF followed by inverse addition to 4-
nitrophenyl chloroformate in THF, 79±84%.

B. Lagu et al. / Bioorg. Med. Chem. Lett. 10 (2000) 2705±2707
2707
Scheme 2. (a) Pyrrolidine (neat), rt; (b) TBDMSCl, imidazole, DMAP, DMF, 86% for two steps; (c) n-BuLi, 1-bromo-3,4-di¯uorobenzene, THF,
78% (96% based on recovered 13); (d) H2NOH.HCl, NaOAc, MeOH, 94%; (e) LiAlH4, Et2O, re¯ux, 92%; (f) Boc2O, CHCl3, 91%; (g) NaH, THF,
80%; (h) NaH, p-nitrophenylchloroformate; (i) 3-[4-(4-¯uorophenyl)piperidin-1-yl]propylamine, THF, 80%.
and Mr. Jim Peng for technical assistance. We also
thank Dr. Charles Gluchowski for his encouragement.
References and Notes
1. Lagu, B.; Tian, D.; Jeon, Y.; Li, C.; Nagarathnam, D.;
Shen, Q.; Wetzel, J.; Gluchowski, C.; Forray, C.; Chang, R. S.
L.; Broten, T. P.; Ransom, R. W.; Rodrigues, D.; Kassahun,
K.; Pettibone, D. J.; Freidinger, R. M. J. Med. Chem. 2000,
43, 2775.
2. Patane, M. A.; Scott, A. L.; Broten, T. P.; Chang, R. S. L.;
Figure 2.
Ransom, R. W.; DiSalvo, J.; Forray, C.; Bock, M. G. J. Med.
Chem. 1998, 41, 1205.
3. Ding, C. Z. Tetrahedron Lett. 1996, 37, 945.
(m, 1H), 3.06 (br d, J=11.4 Hz, 2H), 3.26±3.41 (m, 2H),
4.99 (dq, J=6.6, 7.8 Hz, 1H), 5.35 (d, J=7.8 Hz, 1H),
6.90±7.03 (m, 4H), 7.15±7.27 (m, 3H), 8.14 (t,
4. A similar procedure has recently been reported. Takacs, J.
J=5.4 Hz, 1H).
M.; Jaber, M. R.; Vellekoop, A. S. J. Org. Chem. 1998, 63,
2742.
1 (trans): d 1.54 (d, J=6.3 Hz, 3H), 1.68±1.82 (m, 6H),
1.95±2.03 (m, 2H), 2.40 (t, J=6.9 Hz, 2H), 2.41±2.52 (m,
1H), 3.01 (br d, J=11.4 Hz, 2H), 3.33 (q, J=6.3 Hz, 2H),
4.47 (dq, J=4.5, 6.3 Hz, 1H), 4.91 (d, J=4.5 Hz, 1H),
6.97±7.21 (m, 7H), 8.12 (t, J=5.4 Hz, 1H).
5. A Chiralcel OD column (20Â250 mm) was used with iso-
i
cratic 80% hexane/20% PrOH/0.1% Et2NH as the eluting
system (12 mL/min, UV 254 nm). The retention times for the
enantiomers of the trans oxazolidinone were 12.1 min {[a]D
+36ꢀ (c 0.25, acetone)} and 15.6 min {[a]D 31ꢀ (c 0.20, ace-
tone)}, respectively. The retention times for the enantiomers of
the cis oxazolidinone were 13.7 min {[a]D +65.8ꢀ (c 0.92, ace-
tone)} and 19.9 min {[a]D 65.0ꢀ (c 0.74, acetone)}, respec-
tively.
Acknowledgements
6. Dondoni, A.; Perrone, D.; Semola, T. Synthesis 1995, 181.
7. Martin, R.; Pascual, O.; Romea, P.; Rovira, R.; Urpi, F.;
Vilarrasa, J. Tetrahedron Lett. 1997, 38, 1633.
We thank Ms. Yong Zheng, Mr. Boshan Li, and Mr.
Vincent Jorgensen for conducting the biological assays,