Azaspirovesamicols - Regioselective synthesis and crystal structure analysis of a novel class of vesamicol analogues as potential ligands for the vesicular acetylcholine transporter

Article abstract of DOI:10.1246/cl.2007.276

This report describes the high regioselectivity of nucleophilic epoxide ring-opening reactions which resulted in two of four possible regioisomers of N-benzoyl- (5a and 5b) and W-fluorobenzoylazaspirovesamicol derivatives (6a and 6b), respectively. Based

Full text of DOI:10.1246/cl.2007.276

276
Chemistry Letters Vol.36, No.2 (2007)
Azaspirovesamicols—Regioselective Synthesis and Crystal Structure Analysis
of a Novel Class of Vesamicol Analogues as Potential Ligands
for the Vesicular Acetylcholine Transporter
Barbara Wenzel,ꢀ1 Jan W. Bats,2 Matthias Scheunemann,1 and Jorg Steinbach1
¨
1Institut fur Interdisziplinare Isotopenforschung, Permoserstr. 15, 04318 Leipzig, Germany
¨
¨
2Institut fur Organische Chemie und Chemische Biologie, Johann Wolfgang Goethe-Universitat,
Max-von-Laue-Str. 7, 60438 Frankfurt am Main, Germany
¨
¨
(Received November 8, 2006; CL-061315; E-mail: wenzel@iif-leipzig.de)
This report describes the high regioselectivity of nucleophil-
O
O
8
ic epoxide ring-opening reactions which resulted in two of four
possible regioisomers of N-benzoyl- (5a and 5b) and N-fluoro-
benzoylazaspirovesamicol derivatives (6a and 6b), respectively.
Based on structural information obtained from X-ray crystal
structure analyses of 5a and 5b the mode of epoxide ring-open-
ing is discussed.
8
9
9
+
N
N
O
O
R
R
( )-3b (R = H)
( )-4b (R = F)
( )-3a (R = H)
( )-4a (R = F)
a
NH
The drug vesamicol [2-(4-phenylpiperidin-1-yl)cyclohexa-
nol] binds with high affinity to an allosteric binding site of the
vesicular acetylcholine transporter (VAChT).1 Suitable radioli-
gands for the VAChT provide an opportunity to visualize cho-
linergic deficits in brain using PET (positron emission tomogra-
phy), which is of importance for the diagnosis of neurodegener-
ative disorders. Since the VAChT only tolerates vesamicol-like
structures,2 all known ligands are based on the vesamicol skele-
ton. However, for many of them a low selectivity or other causes
prevented their clinical application. Therefore, there is a need for
further evaluation of VAChT ligands with improved properties.
In this study, we report the synthesis and structures of azaspiro-
vesamicols, a novel class of vesamicol analogues. Radiolabeled
with 18F, these compounds could be potential PET radioligands
for the VAChT.
N
N
OH
OH
+
N
O
N
O
R
( )-5a (R = H)
( )-6a (R = F)
( )-5b (R = H)
( )-6b (R = F)
R
Scheme 2. Synthesis of azaspirovesamicols 5a–6b. (a) Ethanol
at 75 ꢁC for 5 days.
The four new vesamicol analogues 5a–6b and their required
epoxide precursors 3a–4b were synthesized as outlined in
Schemes 1 and 2.11
Because of their similar Rf values, they could not be separated
via flash chromatography on silica gel. The synthesis of 5a
and 5b was accomplished by nucleophilic ring-opening reaction
of the epoxide mixture 3a/3b with 4-phenylpiperidine in ethanol
at 75 ꢁC.
2-Azaspiro[5,5]undec-8-ene was prepared according to the
synthetic route described by Liebowitz et al.3 Subsequent ben-
zoylation and fluorobenzoylation, respectively, yielded in 1
and 2. These amides were epoxidized with ethyl chloroformate
and hydrogen peroxide to give a mixture of syn/anti epoxides
3a and 3b4 and 4a and 4b. Using this epoxidation method, the
isomers were formed at an averaged ratio of 65:35 (anti-
3a:syn-3b and anti-4a:syn-4b, resp.) as determined by HPLC.11
In theory, the nucleophilic attack of an amine can proceed
on the two positions, C8 and C9, of syn/anti epoxide and finally
should result in the formation of four isomers. However, only
two compounds, 5a and 5b, were obtained in an averaged ratio
of 66:34 (5a:5b). These two isomers could be separated in a
moderate yield via fractionated crystallization from an ethanolic
solution. Therefore, preparative HPLC separation of 5a and 5b
as well as of the epoxide precursors 3a and 3b was unnecessary.
Precise determination of molecular structures of 5a and 5b
was accomplished by X-ray structure analysis (Figure 2).5 On
the basis of the structural information it is possible to disclose
the mode of epoxide ring-opening. Nucleophilic attack of 4-
phenylpiperidine on C8 of the anti epoxide 3a leads to the re-
gioisomer 5a. In contrast, regioisomer 5b was formed by nucle-
ophilic attack on C9 of the syn epoxide 3b, exclusively. There-
fore, we can conclude that the formation of these two isomers
is strongly favored. Furthermore, we observed a correlation of
the ratios of the regioisomers 5a:5b (66:34) and the precursors
R
R
R
+
O
O
O
O
N
N
NH
N
a
b
O
2-Azaspiro[5.5]-
undec-8-ene
( )-1 (R = H)
( )-2 (R = F)
( )-3a (R = H)
( )-4a (R = F)
( )-3b (R = H)
( )-4b (R = F)
Scheme 1. Synthesis of syn/anti epoxide precursors 3a–4b.
(a) Benzoyl and 4-fluorobenzoyl chloride, resp./NaHCO3, (b)
H2O2/ethyl chloroformate/Na2HPO4.
Copyright Ó 2007 The Chemical Society of Japan

Chemistry Letters Vol.36, No.2 (2007)
277
computational investigations as well as additional synthetic
work are in process.
anti epoxide
syn epoxide
Ph
Ph
Ph
The X-ray crystal structures5 of 5a and 5b are shown in
Figure 2. All saturated six-membered rings have chair conforma-
tions. Both the hydroxy and the 4-phenylpiperidine substituents
are in equatorial positions with respect to the central cyclohex-
ane ring. Compound 5a shows an intramolecular O-HꢂꢂꢂN hydro-
gen bond between the hydroxy group and the amine nitrogen
atom. Both the hydroxy group and the amine N–H group of 5b
are hydrogen bonded to a chloride anion. The crystal structure
of 5b has two independent molecules. These molecules only
differ in the relative orientation of the phenyl group attached
to the piperidine ring.
O
O
N
N
NH
a
a
O
e
C9
a
C8
C8
C9
e
a
O
HN
Ph
Figure 1.
In conclusion, we have started the development of a novel
class of vesamicol analogues as ligands for the vesicular acetyl-
choline transporter. Four azaspirovesamicol derivatives (5a–6b)
were synthesized by reaction of syn/anti epoxides (3a–4b) with
4-phenylpiperidine. These nucleophilic ring-opening reactions
were found to proceed in a highly regioselective manner. The
molecular structures of the regioisomers 5a and 5b were deter-
mined by X-ray structure analysis. The next step in this project
will be to synthesize further azaspirovesamicol derivatives bear-
ing different fluoro-substituted groups, in respect of future 18F
labeling. Furthermore, the binding affinity and selectivity to
the VAChT of the described compounds will be determined.
Cl1
N2
N2
O2
O2
N1
N1
O1
O1
Figure 2. ORTEP drawings of the molecular structures of 5a
(left) and 5b (right). Hydrogen atoms other than O–H and N–
H have been omitted for clarity.
Financial support for this project was provided by the
Deutsche Forschungsgemeinschaft (WE 2927/1-1).
References and Notes
1
3a:3b (65:35). Hence, under reaction conditions described
above, the amine does not attack preferred the anti or syn epox-
ide. As expected, reaction of 4-fluorobenzoyl substituted syn/
anti epoxides 4a and 4b (anti-4a:syn-4b = 65:35) with 4-phen-
ylpiperidine also resulted in the formation of two isomers in an
averaged ratio of 63:37 (6a:6b).
The high regiocontrol of epoxide ring-opening is remarkable
and was also observed for the comparable spiro[1,3]-dioxalane-
2,30-[7]oxabicyclo[4,1,0]-heptane reported by Cheng et al.6 and
Matzanke et al.7 However, the authors did not discuss possible
reasons for this selectivity. On closer examination of the three-
dimensional structure of one of the anti epoxide conformers
and supposing a trans diaxial transition state,8 the following
explanation seems to be plausible: By formation of hydrogen
bonds between the benzoyl oxygen and the proton of the amine,
the nitrogen is sterically closer to C8 than to C9 (Figure 1). This
could also apply for the dioxalane compounds mentioned above,
but not for the syn epoxide.
Due to the syn orientation, the benzoyl oxygen and the ep-
oxide oxygen are located on the same side of the molecule,
hence the amine has to converge from the other side. In this case,
we assume a conformer in which the nitrogen atom of the aza
ring is arranged in the unexpected axial position again, because
the nucleophilic attack has to proceed on C9 in order to accom-
plish a trans diaxial transition state. However, we could not
find an explanation for the preferred existence of this conformer.
If we consider a diequatorial cleavage, the three-dimensional
structure of the correspondingly conformer shows clearly, that
sterical factors do not exist, which would force the unfavored
diequatorial attack on C9 (often described in literature).8,9 To
gain deeper insight into these reaction mechanisms, detailed
a) C. A. Altar, M. R. Marien, Synapse 1988, 2, 486. b) G. Marshall, S. M.
Parsons, Trends Neurosci. 1987, 10, 174.
2
a) D. C. Andersons, S. C. King, S. M. Parsons, Mol. Pharmacol. 1983,
24, 48. b) G. A. Rogers, S. M. Parsons, D. C. Andersons, L. M. Nilsson,
B. A. Bahr, W. D. Kornreich, R. Kaufmann, R. S. Jacobs, B. Kirtman,
J. Med. Chem. 1989, 32, 1217.
3
4
5
S. M. Liebowitz, E. J. Belair, D. T. Witiak, D. Lednicer, Eur. J. Med.
Chem. 1986, 21, 439.
W. Carruthers, J. D. Prail, S. M. Roberts, J. Chem. Soc., Perkin Trans. 1
1990, 2854.
The structures were determined by direct methods and refined by least-
squares against all measured F2 values.10 5a: C28H36N2O2, Mr ¼
ꢀ
432:59, triclinic, P1 (no. 2), a ¼ 6:1634ð9Þ, b ¼ 9:970ð3Þ, c ¼
ꢁ
˚
19:717ð3Þ A, ꢀ ¼ 86:432ð12Þ, ꢁ ¼ 84:113ð11Þ, ꢂ ¼ 76:328ð19Þ , V ¼
3
1170:2ð4Þ A , Z ¼ 2, T ¼ 149 K, Dcalcd ¼ 1:228 g cmꢃ3, ꢃ ¼ 0:077
˚
mmꢃ1, 15658 refections measured, 6911 unique reflections, Rint
¼
0:024, 294 refined parameters, R1ðFÞ ½I > 2ꢄðIÞꢄ ¼ 0:043, GOF ¼
þ
1:05; 5b: C28H37N2O2 Clꢃ, Mr ¼ 469:05, orthorhombic, Pca21 (no.
˚
29), a ¼ 14:439ð4Þ, b ¼ 8:8908ð18Þ, c ¼ 39:195ð8Þ A, V ¼ 5032ð2Þ
3
A , Z ¼ 8, T ¼ 157 K, Dcalcd ¼ 1:238 g cmꢃ3
,
ꢃ ¼ 0:179 mmꢃ1
,
˚
43010 measured reflections, 11251 unique reflections, Rint ¼ 0:140,
525 refined parameters, R1ðFÞ ½I > 2ꢄðIÞꢄ ¼ 0:082, GOF ¼ 1:05.
Crystallographic data reported in this manuscript have been deposited
with Cambridge Crystallographic Data Centre as supplementary publi-
cation no. CCDC 624382 (5a) and CCDC 624383 (5b). Copies of the
data can be obtained free of charge via www.ccdc.cam.ac.uk./conts/
retrieving.html.
6
7
C. Y. Cheng, S. C. Wu, L. W. Hsin, S. W. Tam, J. Med. Chem. 1992, 35,
2243.
N. Matzanke, W. Lowe, S. Perachon, P. Sokoloff, J. C. Schwartz, H.
Stark, Eur. J. Med. Chem. 1999, 34, 791.
8
9
R. E. Parker, N. S. Isaacs, Chem. Rev. 1959, 59, 737.
L. I. Kas’yan, S. I. Okovityi, A. O. Kas’yan, Russ. J. Org. Chem. 2004,
40, 11.
10 G. M. Sheldrick, Programs for the Solution and Refinement of Crystal
Structures, University of Gottingen, Germany, 1997.
¨
11 Supporting information is available electronically on the CSJ-Journal
Web site http://www.csj.jp/journals/chem-lett/index.html.
Published on the web (Advance View) January 18, 2007; doi:10.1246/cl.2007.276