Structure elucidation via stereoselective synthesis of the acetate center of 1-azabicyclo[2.2.2]oct-3-yl α-hydroxy-α-(1-iodo-1-propen-3-yl)-α-phenylacetate (IQNP). A high affinity muscarinic imaging agent for SPECT

Full text of DOI:10.1021/jo961033t

J . Org. Chem. 1996, 61, 8335-8337
8335
Sch em e 1. Ster eoselective Syn th esis of
(E)-IQNP (1)
Str u ctu r e Elu cid a tion Via Ster eoselective
Syn th esis of th e Aceta te Cen ter of
1-Aza bicyclo[2.2.2]oct-3-yl r-Hyd r oxy-
r-(1-iod o-1-p r op en -3-yl)-r-p h en yla ceta te
(IQNP ). A High Affin ity Mu sca r in ic
Im a gin g Agen t for SP ECT
Daniel W. McPherson* and Furn F. Knapp, J r.
Nuclear Medicine Group, Health Sciences
Research Division, Oak Ridge National Laboratory,
Oak Ridge, Tennessee 37831-6229
Received J une 3, 1996
1-Azabicyclo[2.2.2]oct-3-yl R-hydroxy-R-(1-iodo-1-pro-
pen-3-yl)-R-phenylacetate (IQNP, 1) has been developed
as a new imaging agent for use in single photon emission
computed tomography (SPECT). IQNP has been shown
to be readily radioiodinated, to cross the blood-brain
barrier and to localize in regions of the brain which
contain varying concentrations of the muscarinic acetyl-
choline receptor (mAChR).1 In addition, the various
stereoisomers of IQNP demonstrated modest selectivity
for the various subtypes of mAChR.2-4
The stereochemistry of 3-quinuclidinol is well estab-
lished5 and the (R)-(-) configuration of various 3-quinu-
clidinyl esters has been shown to impart mAChR receptor
binding activity to the ligands.6,7 The acetate moiety of
IQNP (1) has been resolved as the (-)- and (+)-R-
methylbenzylamine salts.2 Biodistribution studies in rats
have demonstrated that (E)- and (Z)-IQNP, which contain
the (-) configuration of the acetate moiety, have signifi-
cant uptake in areas of the brain rich in mAChR and the
heart. However, IQNP is isolated as an oil and the R/ S
orientation of the acetate moiety has yet to identified due
to the absence of a suitable crystal for analysis.
7
A stereoselective R-alkylation of R-heterosubstituted
acids has been reported to afford diastereoselectivity of
>95%.8,9 The condensation of (S)-(+)-mandelic acid with
pivalaldehyde affords cis-(2S,5S)-2-(tert-butyl)-5-phenyl-
1,3-dioxolan-4-one (2). It has been demonstrated that
after deprotonation with base, the electrophilic reaction
with alkyl halides occurs on the less hindered face with
retention of configuration.
Utilizing cis-(2S,5S)-2 and cis-(2R,5R)-2, we have
prepared the various stereoisomers of the acetate moiety
and subsequently (E)-(R,S)- and (E)-(R,R)-IQNP10 as
shown in Scheme 1. A solution of cis-(2S,5S)-2 in
anhydrous tetrahydrofuran was treated with a solution
of lithium 1,1,1,3,3,3-hexamethyldisilazane in anhydrous
tetrahydrofuran at -78 °C. After addition was complete,
the solution was stirred for 30 min at -78 °C and
propargyl bromide was added. The solution was warmed
to room temperature and stirred for 3 h to afford (2S,5S)-
2-(tert-butyl)-5-phenyl-5-(1-propyn-3-yl)-1,3-dioxolan-4-
one ((S,S)-3). (S,S)-3 was then treated with a 2 M
methanolic sodium hydroxide solution to afford (S)-(+)-
R-hydroxy-R-phenyl-R-(1-propyn-3-yl)acetic acid ((S)-4).
An analogous procedure utilizing (2R,5R)-2 afforded
(R,R)-3 and (R)-4. (R)- and (S)-4 were then esterified as
described previously to afford (R)-(-)- and (S)-(+)-ethyl
R-hydroxy-R-phenyl-R-(1-propyn-3-yl)acetate ((R)- and
(S)-5), respectively.
* Address correspondence to: Oak Ridge National Laboratory, P.O.
Box 2008, Building 4501, Mail Stop 6229, Oak Ridge, TN 37831-6229,
FAX (423)-574-6226, E-mail: phm@ornl.gov
By comparison of the NMR analysis and measured
specific rotation of (S)- and (R)-4 and -5 with those
previously reported for 4 and 5 resolved by the use of
the stereoisomers of R-methylbenzylamine (Table 1), the
isomer with the measured (+)-specific rotation can be
assigned the S configuration and the isomer with the
measured (-)-specific rotation can be assigned the R
configuration.
In addition, we have previously shown that the various
E- and Z-stereoisomers of the acetate center of IQNP can
be separated utilizing a normal phase column (mobile
phase: methylene chloride:ethanol:triethylamine [97:3:
0.03]). Also, the preparation of 1 from 5 as shown in
Scheme 1 has been shown not to cause racemization of
chiral centers.2 Therefore, utilizing (S)- and (R)-5,
(1) McPherson, D. W.; DeHaven, D. L.; Callahan, A. P.; Knapp. F.
F., J r. J . Med. Chem. 1993, 36, 848.
(2) McPherson, D. W.; Lambert, C. R.; J ahn, K.; Sood, V.; McRee,
R. C.; Zeeberg, B.; Reba, R. C.; Knapp, F. F., J r. J . Med. Chem. 1995,
38, 3908.
(3) McPherson, D. W.; Lambert, C. R.; Knapp, F. F., J r. Eur. J . Nucl.
Med. 1994, 21, 1293.
(4) Gilter, M. S.; Boulay, S. F.; Sood, V. S.; McPherson, D. W.; Knapp,
F. F., J r.; Zeeberg, B. R.; Reba, R. C. Brain Res. 1995, 687, 71.
(5) Meyerhoffer, A. J . Med. Chem. 1972, 15, 994.
(6) Inch, T. D.; Green, D. M.; Thompson, P. B. J . J . Pharm.
Pharmacol. 1973, 25, 359.
(7) Frater, G.; Muller, U.; Gunther, W. Tetrahedron Lett. 1981, 22,
4221.
(8) Seebach, D.; Naef, R.; Calderari, G. Tetrahedron 1984, 40, 1313.
(9) Ringdahl, B.; Resul, B.; Dahlbom, R. Acta. Pharm. Succ. 1979,
16, 281.
(10) The assignment of stereochemistry refers to (R,S)-IQNP, for
example, the first stereocenter designation as the quinuclidinyl moiety
and the second as the acetate moiety.
S0022-3263(96)01033-X
This article not subject to U.S. Copyright. Published 1996 by the American Chemical Society

8336 J . Org. Chem., Vol. 61, No. 23, 1996
Notes
Ta ble 1. Com p a r ison of th e Sp ecific Rota tion a of th e E
Isom er s of IQNP (1) a n d In ter m ed ia tes
orientation of the acetate center as being either R or S.
In conclusion, a facile stereoselective synthesis of
R-hydroxy-R-phenyl-R-(1-propyn-3-yl)acetic acid (4) in
high enantiomeric excess has been developed and allows
determination of the (R/S) conformation at this center.
In addition, comparison of the specific rotation, HPLC,
and NMR data of (E)-(R,R)- and (E)-(R,S)-IQNP to those
prepared by a classical resolution of the acetate moiety
allows the assignment of (E)-(R,R)-IQNP as the isomer
demonstrating binding to the M1 mAChR subtype and
(Z)-(R,R)-IQNP as the isomer binding to both the M1 and
M2 mAChR subtypes.
stereoselective
synthesis
classical
compound
resolutionb
(R,R)-2
(S,S)-2
(R,R)-3
(S,S)-3
(R)-4
(S)-4
(R)-5
(S)-5
(R,R)-6
(S,S)-6
(E)-(R,R)-7
(E)-(R,S)-7
(E)-(R,R)-1
(E)-(R,S)-1
-82.4°
+87.3° c
+27.2°
-27.8°
-20.6°
+20.8°
-24.9°
+25.2°
-13.4°
+40.6°
-17.5°
+30.7°
-17.2°
+42.4°
-10.7°
+12.7°
-13.6°
+18.4°
-4.5°
+41.8°
-12.5°
+29.0°
-20.2°
+39.5°
Exp er im en ta l Section
Gen er a l. Anhydrous THF, (S)-(+)-mandelic acid, (R)-(-)-
mandelic acid, butyllithium (2.5 M in hexanes), 1,1,1,3,3,3-
hexamethyldisilazane, propargyl bromide, and pivalaldehyde
(trimethylacetaldehyde) were purchased from Aldrich Chemical
Co. All other chemicals and solvents were analytical grade and
were used without further purification. Thin layer chromato-
graphic analyses (TLC) were performed using 250 µm layers of
silica gel coated on glass (Alltech). High performance liquid
chromatographic (HPLC) analysis was performed with a ISCO
pump and ISCO Model A4 variable wavelength detector using
a Waters Nova Pak (3.9 mm × 30 cm) column. Proton spectra
are reported using tetramethylsilane as the internal standard,
and carbon spectra are reported using CHCl3 as the reference
signal (77.0 ppm). Melting points are reported uncorrected.
cis-(2S,5S)-2-(ter t-Bu t yl)-5-p h e n yl-1,3-d ioxola n -4-on e
((S,S)-2). (S)-(+)-Mandelic acid (5.0 g, 33.2 mmol), pivalalde-
a
b
Specific rotation measured in chloroform. Reference 2. c Lit-
erature value +88.5°, reference 8.
hyde (11.6 g, 134.7 mmol), and
a catalytic amount of p-
toluenesulfonic acid (73.6 mg) were added to anhydrous pentane
(100 mL) under argon. A drop of sulfuric acid was added, and
the solution was refluxed utilizing a Dean-Stark trap for 7 h.
The solution was cooled, diluted with ether (200 mL), and
washed twice with water (100 mL). The ether solution was dried
over MgSO4 and evaporated to dryness to afford a white solid.
The solid was recrystallized from ether/hexane to afford cis-
(S,S)-2 (5.6 g, 76%). Mp 139.5-140 °C; 1H NMR (CDCl3) δ 7.44-
7.38 (m, 5H), 5.32 (d, J ) 1.4 Hz, 1H), 5.23 (d, J ) 1.4 Hz, 1H),
1.08 (s, 9H); 13C NMR (CDCl3) δ 171.7, 131.4, 129.1, 128.6, 127.0,
109.3, 77.0, 34.5, 23.7; [R]D ) +87.3° (c ) 0.15 g/mL, CHCl3).
cis-(2R,5R)-2-(ter t-Bu t yl)-5-p h en yl-1,3-d ioxola n -4-on e
(R,R)-2. (R,R)-2 was prepared as above using (R)-mandelic acid
(5.0 g, 32.9 mmol), pivalaldehyde (11.4 g, 134.9 mmol), and
p-toluenesulfonic acid (79.0 mg) to afford cis-(R,R)-2 as white
needles (6.2 g, 85%). Mp 140 °C; 1H NMR (CDCl3) δ 7.43-7.38
(m, 5H), 5.31 (d, J ) 1.3 Hz, 1H), 5.23 (d, J ) 1.1 Hz, 1H), 1.07
(s, 9H); 13C NMR (CDCl3) δ 172.0, 141.8, 129.1, 128.8, 127.0,
109.2, 77.0, 34.5, 23.7; [R]D ) -82.4° (c ) 0.15 g/mL, CHCl3).
(2R,5R)-2-(ter t-Bu tyl)-5-p h en yl-5-(1-p r op yn -3-yl)-1,3-d i-
oxola n -4-on e ((R,R)-3). A solution of 1,1,1,3,3,3-hexamethyl-
disilazane (0.8 g, 5.2 mmol) in anhydrous THF (25 mL) was
cooled to -78 °C (dry ice/acetone) under argon. The solution
was stirred and n-butyllithium (2.2 mL, 5.5 mmol) was slowly
added. After addition was complete, the solution was stirred
at -78 °C for 15 min followed by the dropwise addition of a
solution of cis-(R,R)-2 (1.1 g, 5.0 mmol) in anhydrous THF (40
mL). The solution was stirred at -78 °C for 30 min, and
propargyl bromide (80% in toluene, 1.0 mL, 11.2 mmol) was
added. The solution was then allowed to warm to rt over 3 h.
The solution was poured into a cold 30% NH4Cl solution (100
mL). The aqueous solution was washed with ether (200 mL),
and the ether solution was washed with water (100 mL), dried
over MgSO4, and evaporated to dryness to afford an orange oil.
The product was purified by Kugelrohr distillation under high
vacuum (100-110 °C) to afford (R,R)-3 as a pale yellow oil (0.9
g, 68%). 1H NMR (CDCl3) δ 7.71-7.66 (m, 2H), 7.41-7.26 (m,
3H), 5.68 (s, 1H), 3.03-2.75 (qd, J ) 2.6 Hz, 2H), 2.10 (t, J )
2.6 Hz, 1H), 0.96 (s, 9H); 13C NMR (CDCl3) δ 172.0, 137.4, 128.3,
124.6, 110.4, 81.6, 78.1, 71.7, 35.2, 31.1, 23.7; [R ]D ) +27.2° (c
) 0.088, CHCl3); TLC (silica gel, hexane:ethyl acetate [8:2]) Rf
) 0.72.
F igu r e 1. HPLC analysis of (E)-(R,R)-IQNP (A) and (E)-(R,S)-
IQNP (B).
F igu r e 2. NMR spectra of (E)-(R,R)-IQNP (A) and (E)-(R,S)-
IQNP (B).
respectively, (E)-(R,S)- and (E)-(R,R)-1 were prepared to
determine the diastereoselectivity of the electrophilic
reaction of 2 with propargyl bromide. HPLC analysis of
(E)-(R,R)- and (E)-(R,S)-1 demonstrated the alkylation
did indeed occur on the less hindered side of 2 with 94%
and 98% enantiomeric excess, respectively (Figure 1).
A significant difference in the splitting pattern corre-
sponding to hydrogen atoms R to the quinuclidinyl
1
nitrogen was observed in the H NMR analysis of (E)-
(R,R)- and (E)-(R,S)-1 synthesized from 5 by either the
classical resolution of the acetate moiety or as described
above (Figure 2). Therefore this apparent difference in
the splitting pattern was also utilized to confirm the
(2S,5S)-2-(ter t-Bu t yl)-5-p h en yl-5-(1-p r op yn -3-yl)-1,3-d i-
oxola n -4-on e ((S,S)-3). (S,S)-3 was prepared as above using

Notes
J . Org. Chem., Vol. 61, No. 23, 1996 8337
1,1,1,3,3,3-hexamethyldisilazane (1.7 g, 10.4 mmol), anhydrous
THF (50 mL), n-butyllithium (4.8 mL, 12.0 mmol), (S,S)-2 (2.7
g, 22.6 mmol) in anhydrous THF (40 mL), and propargyl bromide
(80% in toluene, 2.0 mL, 22.6 mmol) to afford (S,S)-3 as a pale
yellow oil (2.1 g, 82%). 1H NMR (CDCl3) δ 7.71-7.66 (m, 2H),
7.42-7.31 (m, 3H), 5.68 (s, 1H), 3.03-2.75 (qd, J ) 2.6 Hz, 2H),
2.10 (t, J ) 2.7 Hz, 1H), 0.96 (s, 9H); 13C NMR (CDCl3) δ 172.0,
137.4, 128.2, 124.6, 110.4, 81.6, 78.0, 71.7, 35.1, 31.1, 23.5; [R]D
) -27.8° (c ) 0.140 g/mL, CHCl3); TLC (silica gel, hexane:ethyl
acetate [8:2]) Rf ) 0.77.
(R)-(-)-r-Hyd r oxy-r-p h en yl-r-(1-p r op yn -3-yl)a cetic a cid
((R)-4). (R,R)-3 (0.9 g, 3.4 mmol) was dissolved in methanol
(25 mL), and a 1.8 M KOH solution (25 mL) was added. The
solution was stirred at 60 °C for 20 min, cooled to rt, and diluted
with water (100 mL). The aqueous solution was washed with
ether (100 mL) and then made acidic with the slow addition of
1 N HCl (100 mL) and washed with ether (100 mL). The ether
solution washed with water (100 mL), dried over MgSO4, and
evaporated to dryness to afford (R)-4 as a pale orange oil (0.6 g,
98%). 1H NMR (CDCl3) δ 7.63-7.58 (m, 2H), 7.41-7.32 (m, 3H),
3.27-3.17 (dd, J ) 2.6 Hz, 1H), 2.93-2.83 (dd, J ) 2.6 Hz, 1H),
2.06 (t, J ) 2.7 Hz, 1H); [R]D ) -20.6° (c ) 0.063 g/mL, CHCl3).
(S)-(+)-r-Hyd r oxy-r-p h en yl-r-(1-p r op yn -3-yl)a cetic a cid
((S)-4). (S)-4 (1.1 g, 100%) was prepared in the same manner
as above from (S,S)-3 (1.5 g, 5.8 mmol). 1H NMR (CDCl3) δ
7.61-7.56 (m, 2H), 7.38-7.28 (m, 3H), 3.22-3.13 (dd, J ) 2.6
Hz, 1H), 2.92-2.82 (dd, J ) 2.6 Hz, 1H), 2.01 (t, J ) 2.6 Hz,
1H); [R]D ) +20.8° (c ) 0.156 g/mL, CHCl3).
Ack n ow led gm en t. Research was supported at Oak
Ridge National Laboratory by the Office of Health and
Environmental Research, U. S. Department of Energy,
under contract DE-AC05-96OR22464 with Lockheed
Martin Energy Research Corporation.
Su p p or t in g In for m a t ion Ava ila b le: 1H and 13C NMR
spectra of cis-(2S,5S)-2, (2S,5S)-3, (+)-5, cis-(2R,5R)-2, (2R,5R)-
3, (-)-5 and 1H NMR spectra of (+)-4 and (-)-4 (14 pages).
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