Diastereoselective synthesis of fluorinated piperidine quinazoline spirocycles as iNOS selective inhibitors

Article abstract of DOI:10.1016/j.tetlet.2012.03.050

A diastereoselective synthesis of fluoropiperidine quinazoline spirocycles has been developed through a silyl triflate mediated intermolecular coupling of difluorobenzamidine and racemic N-protected 3-fluoropiperidine dimethyl ketals or piperidones. Combination of the silyl reagents together with Lewis acids (such as BF₃·OEt₂, ZnCl₂, InCl ₃, etc.) accelerated the coupling reaction to afford the desired fluorospirocycles in good yields (40-83%) and high diastereoselectivity. A ratio of the two diastereoisomers of up to 10:1 in favor of the desired isomer can be achieved.

Full text of DOI:10.1016/j.tetlet.2012.03.050

Tetrahedron Letters 53 (2012) 2942–2947
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Diastereoselective synthesis of fluorinated piperidine quinazoline spirocycles
as iNOS selective inhibitors
Chris Walpole a, , Ziping Liu a, Ernest E. Lee b, Hua Yang a, Fei Zhou c, Nicole Mackintosh a,
⇑
Magnus Sjogren d, David Taylor e, Jinyu Shen b, Robert A. Batey b,
⇑
a AstraZeneca R&D Montreal, 7171 Frederick-Banting, Ville Saint-Laurent (Montreal) Quebec, Canada H4S 1Z9
b Department of Chemistry, University of Toronto, 80 St. George St., Toronto, Ontario, Canada M5S 3H6
c AstraZeneca R&D Innovative Medicines Boston, 35 Gatehouse Drive, Waltham, MA 02451, USA
d AstraZeneca Process R&D Södertälje, 151 85 Södertälje, Sweden
e AstraZeneca R&D, Silk Road Business Park, Charter Way, Macclesfield, Cheshire SK10 2NA, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
A diastereoselective synthesis of fluoropiperidine quinazoline spirocycles has been developed through a
silyl triflate mediated intermolecular coupling of difluorobenzamidine and racemic N-protected 3-fluoro-
piperidine dimethyl ketals or piperidones. Combination of the silyl reagents together with Lewis acids
(such as BF3ꢀOEt2, ZnCl2, InCl3, etc.) accelerated the coupling reaction to afford the desired fluorospirocy-
cles in good yields (40–83%) and high diastereoselectivity. A ratio of the two diastereoisomers of up to
10:1 in favor of the desired isomer can be achieved.
Received 24 February 2012
Revised 12 March 2012
Accepted 14 March 2012
Available online 1 April 2012
Keywords:
Spirocycle
Ó 2012 Published by Elsevier Ltd.
Lewis acid
Difluorobenzamidine
Piperidine
Piperidone
The fluorine atom has been routinely utilized in medicinal
chemistry programs in order to fine-tune the physicochemical,
pharmacological, and drug metabolism and pharmacokinetic
(DMPK) properties of drug-like molecules. The unique properties
of the fluorine atom, such as small size and strong electron-
withdrawing ability, frequently make fluorinated compounds more
metabolically stable, yet often with minimum impact on pharma-
cology.1 On the other hand, the C–F bond may change the physico-
chemical features of a molecule markedly (e.g., amine pKa and
hence lipophilicity at physiological pH, LogD) and so may have a
significant impact on the biological properties.2 As a result, the reg-
ioselective and stereoselective introduction of a fluorine atom into
drug candidates is a fairly common strategy in modern medicinal
chemistry.3
We have previously reported the identification of iNOS selective
inhibitors based on fluoropiperidine spirocyclo-3,4-dihydroqui-
nazolines. Inducible nitric oxide synthase (iNOS) is an isoform of
NOS that catalyzes the formation of NO from arginine.4 Inhibitors
of iNOS have been proposed as treatments for a variety of condi-
tions involving the immune response and inflammation, including
rheumatoid arthritis and neuropathic pain. In order to improve the
metabolic stability of the lead compound AR-C 102222 1,5 and to
improve the cell permeability by reducing amidine basicity, we
introduced a fluorine atom at the 3-position of the piperidine ring
of 1 (Fig. 1). Our goal was, (i) to reduce the basicity and increase the
lipophilicity of the amidine moiety at physiological pH (LogD) in
order to improve cellular membrane permeability and CNS
exposure and (ii) to stabilize the aminal moiety. Two promising
O
F
H
N
N
N
CN
N
1 (AR-C102222)
F
NH2
O
O
F
F
F
H
N
H
N
N
N
N
CN
CN
F
F
N
N
⇑
2a
2b
NH2
F
NH2
Corresponding authors. Tel.: +1 514 832 3224; fax: +1 514 832 3230 (C.W.); tel.:
+1 416 978 5059; fax: +1 416 978 5059 (R.A.B.).
E-mail addresses: christopher.walpole@astrazeneca.com (C. Walpole), rbatey@
chem.utoronto.ca (R.A. Batey).
Figure 1. Spirocyclic fluoropiperidine quinazoline selective iNOS inhibitors: 1 (AR-
C102222) and the F-Piperidine Spirocyclic iNOS inhibitors 2a and 2b.
0040-4039/$ - see front matter Ó 2012 Published by Elsevier Ltd.
http://dx.doi.org/10.1016/j.tetlet.2012.03.050

C. Walpole et al. / Tetrahedron Letters 53 (2012) 2942–2947
2943
O
F
H
N
R
N
O
F
N
F
(R,S)-2
H
N
N
R
enantiomer
separation
F
F
NH2
O
N
O
N
F
2
H
R
F
F
NH2
N
F
N
F
F
N
(S,R)-2
4
NH2
NH
NH2
F
NH2
O
R
a R = 4-CN-Ph
b R = cyanopyridyl
c R = OBn
O
i-PrOH
F
F
H
R
N
N
5
O
N
(R,R)-3
F
F
enantiomer
separation
H
N
N
R
F
NH2
O
F
N
F
3
F
H
N
R
N
NH2
N
(S,S)-3
F
NH2
Scheme 1. A synthetic route to fluoropiperidine spirocycles 2 and 3.
compounds 2a and 2b have been identified, which retain iNOS
inhibitory potency and selectivity over other NOS isoforms and
have excellent oral efficacy in rodent models of neuropathic pain.
The potential of 2a and 2b as novel analgesics and the details of
the fluorine atom’s effects on the physicochemical and pharmaco-
logical properties of the iNOS selective inhibitors will be reported
elsewhere.6
As the introduction of a fluorine atom generates two stereogen-
ic centers in the final fluorinated iNOS inhibitors 2a and 2b, a sim-
ple and practical method for the synthesis of these compounds as
single stereoisomers was required. Of the four possible stereoiso-
mers, the desired stereoisomer is the one in which the fluorine
atom resides in an equatorial position on the piperidine ring and
has a cis-relationship to the amidine group of the quinazoline ring.
The formation of 2a and 2b would thus need to be accomplished
with both diastereo- and enantiocontrol. In recent years, various
enantioselective fluorination methods have been achieved utilizing
a variety of conditions, including the use of organic chiral sulfon-
amide-type fluorination reagents, Cinchona alkaloid/SelectfluorÒ
or NFSI combinations, and by using organometallic catalysis, for
example, chiral palladium or chiral titanium complexes.7 There is
however no precedence for achieving a diastereocontrolled synthe-
sis of compounds such as 2a and 2b. In this Letter, we disclose a
synthesis of fluoropiperidine spirocycles
2 as iNOS selective
inhibitors by the diastereocontrolled coupling of a difluoroben-
zeneamidine5 with a racemic fluoropiperidinone8,9 mediated by
Lewis acid agents.10
Initially, we examined the direct fluorination of AR-C102222
using Selectfluor™, but this led to the formation of complex mix-
tures. Therefore an approach in which fluorinated piperidinones
Table 1
TMSOTf mediated coupling of amidine 5 with 3-fluoropiperidinone 4a
O
O
F
F
F
F
F
F
(i) TMSOTf,
Et3N (4.0 equiv)
H
N
H
N
N
N
NH2
NH
NH2
CN
+
CN
N
N
O
N
(ii)
F
F
F
NH2
NH2
5
4a, 24 h
2a
3a
O
C6H4CN
Solvent
Entry
TMSOTf (equiv)
Temp (°C)
2a:3a ratioa
Yieldb (%) (isomer 2a)
1
2
3
4
5
6
7
8
1.0
2.0
3.0
4.0
5.0
5.0
3.0
4.0
THF
THF
THF
THF
THF
THF
Toluene
Toluene
50
50
50
50
50
Reflux
50
1:1.6
1.1:1
2.4:1
2.8:1
3.4:1
3.4:1
1.6:1
2.3:1
19
34
28
22
30
30
39
32
50
a
Determined by HPLC.
Isolated yield by MPLC on silica gel.
b

2944
C. Walpole et al. / Tetrahedron Letters 53 (2012) 2942–2947
Table 2
Trialkylsilyl triflate and BF3ꢀOEt2 mediated coupling of amidine 5 with 3-fluoropiperidinone 4a
O
O
F
F
F
F
F
F
(i) R3SiOTf,
H
N
H
N
N
N
NH2
NH
NH2
Et3N (4.0 equiv), Et2O
+
CN
CN
N
N
O
N
(ii)
F
F
F
NH2
NH2
5
4a
2a
3a
O
C6H4CN
BF3·OEt2 (0.5 equiv),
r.t., 24 h
Entry
R3SiOTf (equiv)
2a:3a ratioa
Yieldb (%) (isomer 2a)
1
2
3
4
TMSOTf (4.0)
TMSOTf (5.0)
TBSOTf (4.0)
TBSOTf (5.0)
3.6:1
3.1:1
4.2:1
3.8:1
33
48
50
38
a
Determined by HPLC.
Isolated by MPLC on silica gel.
b
4 are coupled with the ortho-aminobenzeneamidine 5 was envis-
aged. The precursor fluorinated piperidinones 4 can be readily gen-
erated by electrophilic fluorination of N-acylpiperidinones using
Selectfluor™ under acidic catalysis.9 The coupling reaction of 5
with racemic piperidinones 4 in refluxing ethanol5 afforded the
four stereoisomers (R,S)-2 (desired stereoisomer), (S,R)-2, (R,R)-3,
and (R,R)-3 in a non-stereoselective manner (Scheme 1). Although
the racemic diastereomers 2 and 3 could be simply separated by
flash column chromatography and the enantiomers of both dias-
teroisomers 2 and 3 were separable by OD or AD chiral column
chromatography with a high recovery rate, the overall yield was
limited to about 12% of the desired enantiomer.
An improved and diastereocontrolled formation of the products
using the coupling of 4 and 5 was therefore desired. The use of a
silylating reagent (e.g., TMSOTf) was found to facilitate the
coupling reaction of 4a and 5 at lower reaction temperatures, in
favor of diastereoisomer 2a, which has the amidine functionality
of the quinazoline ring axially oriented and the fluorine substituent
equatorially oriented on the piperidine ring.11–13 The selectivity
achieved was found to be dependent upon the quantity of TMSOTf
used (Table 1). When the amidine 5 was pre-treated with 1.0 equiv
of TMSOTf in THF in the presence of excess triethylamine, followed
by reaction with the 3-fluoropiperidinone 4a at 50 °C for 24 h, the
undesired diastereoisomer 3a was isolated as the major compound
(Table 1, entry 1). By increasing the amount of TMSOTf used, the
diastereoselectivity was reversed in favor of the desired isomer
2a. The ratio of 2a:3a was found to increase with increasing
TMSOTf stoichiometry (Table 1, entries 1–5). When 5.0 equiv of
TMSOTf was used, the ratio of the two diastereomers 2a:3a
reached 3.4:1, with a 30% isolated yield of diastereoisomer 2a
(Table 1, entry 5). The spirocyclization reaction became impractically
slow at temperatures below 50 °C. In addition, the diastereoselec-
tivity and yield remained unchanged on increasing the reaction
temperature from 50 °C to reflux (Table 1, entry 6). Replacing the
THF by toluene as a solvent resulted in slightly lower diastereose-
lectivity but higher isolated product yields were obtained under
otherwise identical conditions (Table 1, entry 3 versus 7, and entry
4 versus 8).
Table 3
tert-Butyldimethylsilyltriflate and boron trifluoride etherate Lewis acid mediated coupling of amidine 5 and 3-fluoropiperidinone 4c
(i) TBSOTf (6.0 equiv),
F
F
F
F
Cbz
F
F
Cbz
H
N
H
N
N
N
Et3N (4.0 equiv),
MeCN
NH2
NH
NH2
+
N
N
O
(ii)
F
F
F
NH2
NH2
5
4c
2c
3c
N
Cbz
BF3·OEt2, 24 h
Entry
TBSOTf (equiv)
BF3ꢀOEt2 (equiv)
Temp (°C)
Solvent
2c:3c ratioa
Yieldb (%) (2c + 3c)
1
2
3
4
5
6
7
5.0
4.0
4.0
5.0
4.0
6.0
6.0
0.1
0.5
0.5
0.1
0.5
1.6
1.6
4
rt
4
THFc
Et2O
Et2O
Et2O
CH2Cl2
CH2Cl2
MeCN
4.5:1
3.0:1
4.6:1
4.6:1
1:1.1
4.0:1
4.3:1
70
80
71
57
48
—
4
40
rt
rt
65
a
b
c
Determined by 19F NMR.
Isolated by MPLC on silica gel.
THF polymerization occurred at 40 °C or room temperature.

C. Walpole et al. / Tetrahedron Letters 53 (2012) 2942–2947
2945
Table 4
tert-Butyldimethylsilyltriflate mediated coupling of amidine 5 and 3-fluoropiperidinone 4c using various Lewis acids
(i) TBSOTf (6.0 equiv),
F
F
F
F
Cbz
F
F
Cbz
Et3N (4.0 equiv),
MeCN
H
N
H
N
N
N
NH2
NH
NH2
+
N
N
O
(ii)
F
F
F
NH2
NH2
4c,
5
2c
3c
Lewis Acid
(1.5 equiv)
N
Cbz
Entry
Lewis acid
Time (h)
2c:3c ratioa
Yieldb,c (%),(2c + 3c)
1
2
3
4
5
6
7
8
9
BF3ꢀOEt2
LiClO4
MgBr2
96
24
48
48
48
36
72
24
48
4.3:1
5.4:1
4.4:1
4.8:1
5.0:1
5.3:1
6.4:1
6.2:1
10:1
65
46
19
69
60
33
16
46
40
ZnCl2
Mg(ClO4)2
Yb(OTf)3
Dy(OTf)3
Sc(OTf)3
InCl3
a
b
c
Determined by 19F NMR.
Isolated yield by MPLC on silica gel.
The reactions were performed at room temperature.
The use of 4 equiv of TMSOTf together with BF3ꢀOEt2 in diethyl
ether improved both the diastereomeric ratio of 2a:3a and the
product yield, and could be achieved at room temperature (Table
2, entry 1). The isolated yield of diastereomer 2a could be increased
further to 48% using 5.0 equiv of TMSOTf. The best diastereoselec-
tivity was achieved using 4.0 equiv of TBSOTf and 0.5 equiv of
BF3ꢀOEt2, resulting in a ratio of 4.2:1 in favor of the desired diaste-
reomer 2a and a 50% isolated yield of pure 2a (Table 2, entry 3).
The optimization of the coupling of benzamidine 5 with N-Cbz-
3-fluoropiperidinone 4c was also investigated in order to make
the two desired products 2a and 2b by parallel synthesis. Both
solvent and temperature played important roles in the trialkylsilyl
triflate mediated coupling reaction in combination with a Lewis
acid. Using 5.0 equiv of TBSOTf and 0.1 equiv of BF3ꢀOEt2 at 4 °C in
THF, led to an improvement of the ratio of the two diastereoisomers
2c/3c to 4.5:1 and the combined yield to 70% (Table 3, entry 1).14,15
When the solvent was changed to diethyl ether, using 4.0 equiv of
TBSOTf and 0.5 equiv of BF3ꢀOEt2 at room temperature, the ratio
dropped slightly to 3.0:1 with a combined yield of 80% (Table 3,
entry 2). At lower temperature, the selectivity could be improved
(Table 3, Entry 3). The use of additional TBSOTf did not improve
the diastereoselectivity (Table 3, entry 4). When the reaction was
performed in dichloromethane at an elevated temperature (40 °C)
in the presence of 4.0 equiv of TBSOTf and 0.5 equiv of BF3ꢀOEt2,
the reaction was not diastereoselective (Table 3, entry 5). Although
the diastereoselectivity in favor of 2a was restored by employing
6.0 equiv of TBDMSOTf and 1.6 equiv of BF3ꢀOEt2 at room tempera-
ture, the yield was very poor (Table 3, entry 6). The use of acetoni-
trile as a solvent led to comparable diastereoselectivity to that
observed for the reaction run in THF (Table 3, entry 7).
A range of other Lewis acids were also tested for the coupling of
4c and 5 to evaluate their effects on diastereoselectivity and chem-
ical yield (Table 4). Lithium perchlorate, magnesium bromide, zinc
chloride, ytterbium triflate, dysprosium triflate, or scandium triflate
mediated cyclization gave modestly improved diastereoselectivity
compared to BF3ꢀOEt2. The diastereoselectivity ranged from 4.4:1
Table 5
Trimethylsilyltriflate and boron trifluoride etherate mediated coupling of amidine 5 and fluoroketal 6
(i) TMSOTf,
F
F
F
F
Cbz
F
F
Cbz
Et3N (4.0 equiv),
MeCN
H
N
H
N
N
N
NH2
NH
NH2
·HCl
+
N
N
(ii)
MeO OMe
F
F
F
NH2
NH2
5
6
, 24 h
2c
3c
N
Cbz
Entry
TMSOTf (equiv)
Lewis acid (equiv)
Temp (°C)
2c:3c ratioa
Yieldb (%) (2c + 3c)
1
2
3
4
5
6
7
8
4.0
8.0
8.0
16.0
16.0
8.0
—
—
—
—
rt
rt
0
—
0
6.3:1
6.3:1
6.1:1
4.9:1
6.3:1
5.9:1
—
55
83
71
53
50
43
Trace
0
—
ꢁ25
InCl3 (1.5)
ZnCl2 (1.5)
Sc(OTf)3 (1.5)
0
8.0
8.0
0
0
a
Determined by 19F NMR.
Isolated by MPLC on silica gel.
b

2946
C. Walpole et al. / Tetrahedron Letters 53 (2012) 2942–2947
COR
N
F
F
F
F
F
O
R3SiOTf /
Et3N
NH2
NH
NH2
N(SiR3)2
NSiR3
N(SiR3)2
N
8
F
Lewis Acid
- (R3Si)2O
N(SiR3)2
F
N
F
NCOR
R3Si
5
7
F
F
COR
Lewis
Acid (M+)
H
N
N(SiR3)2
+
N
M
N
NCOR
3
2
+
N
F
F
F
9
R3SiN
F
NH2
Scheme 2. Possible mechanism for formation of fluoropiperidine spirocycles 2 and 3.
Res. Dev. 2008, 12, 305; (d) Mueller, K.; Faeh, C.; Diederich, F. Science 1881,
2007, 317; (e) Kirk, K. L. J. Fluorine Chem. 2006, 127, 1013; (f) Kirch, P. Modern
Fluoroorganic Chemistry: Synthesis, Reactivity, Applications; Wiley-VCH:
Weinheim, 2004; (g) Bohm, H. J.; Banmer, D.; Bendels, S.; Kansy, M.; Kuhn,
B.; Muller, K.; Obst-Sander, U.; Stahl, M. ChemBioChem 2004, 5, 637; (h) Ismail,
F. M. D. J. Fluorine Chem. 2002, 118, 27; (i) Smart, B. E. J. Fluorine Chem. 2001,
109, 3.
to 6.4:1 in favor of isomer 2c, with the combined yield of 2c and 3c
varying from 16% to 69% (Table 4, entries 2–8). Interestingly, the use
of indium chloride together with TBSOTf, led to the highest diaste-
reoisomeric ratio of 10:1, although in only 40% overall yield.
Finally, a similar approach could be employed for coupling ketal
6 with the hydrochloride salt of amidine 5 (Table 5). Formation of
2c over 3c was favored, with similar levels of diastereoselectivity
to that obtained from the ketone 4c. Optimal conditions were
found to use TMSOTf (8.0 equiv) and triethylamine (4.0 equiv) at
0 °C for 24 h, giving 2c and 3c in combined 83% yield, in a 6.3:1 ra-
tio (Table 5, entry 3). Reaction at higher or lower temperatures led
to lower product ratios or yields.
The origins of the diastereoselectivity differences obtained un-
der the various conditions are not clear. The major product 2 is
likely the thermodynamically more stable diastereomer,11 but it
is unlikely that the reaction is occurring under the conditions of
thermodynamic control, since a control reaction showed that
isomerization of either 2a or 3a did not occur under the reaction
conditions. A plausible mechanism for the formation of 2 and 3 in-
volves initial formation of the persilylated amidine 7, followed by
reaction with the fluoropiperidone 4 (or corresponding ketal 6)
to give 8. Subsequent Lewis acid promoted cyclization via 9 or a
similar intermediate and subsequent hydrolysis would then afford
2 and 3 (Scheme 2).
In summary, a practical and efficient route for the diastereose-
lective synthesis of novel spirocyclic fluoropiperidine quinazoline
selective iNOS inhibitors has been established. Initial attempts at
coupling the amidine and ketone partners by analogy with non-
fluorinated substituted piperidones led to unselective reaction
and poor reaction yields. However, unexpectedly we demonstrate
that the presence of silylation reagents and Lewis acids can signif-
icantly alter product selectivities and yields. Thus, the use of
TMSOTf or TBSOTf in excess, along with added triethylamine and
Lewis acids, was found to facilitate the coupling reaction of fluori-
nated piperidinones with ortho-aminobenzeneamidine. The spiro-
cyclic core generated in this manner was formed with improved
yields and diastereoselectivity favoring the desired diastereoiso-
mer 2. Similar conditions could also be employed using ketals
rather than the fluorinated piperidinone.
2. (a) O’Hagan, D. Chem. Soc. Rev. 2008, 37, 308; (b) Hunter, L. Beilstein J. Org. Chem.
2010, 6, 38; (c) Smart, B. E. Characteristics of C-F systems. In Organofluorine
Chemistry, Principles and Commercial Applications; Banks, R. E., Smart, B. E.,
Tatlow, J. C., Eds.; Plenum Press: New York, 1994; pp 57–88.
3. (a) Shibata, N.; Ishimaru, T.; Nakamura, S.; Toru, T. J. Fluorine Chem. 2007, 128,
469; (b) Nakagawa, K.; Okano, T.; Ozona, K.; Kato, S.; Kubodera, N.; Ohba, S.;
Itoh, Y.; Mikami, K. J. Fluorine Chem. 2007, 128, 654; (c) Kirk, K. L. Curr. Top. Med.
Chem. 2006, 6, 1447; (d) Sun, S.; Adejare, A. Curr. Top. Med. Chem. 2006, 6, 1457;
(e) Hamashima, Y. Chem. Pharm. Bull. 2006, 54, 1351; (f) Giannini, G. Curr. Med.
Chem. 2002, 9, 687.
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Annedi, S. C.; Ramnauth, J.; Rakhit, S. Annu. Rep. Med. Chem. 2009, 44,
27.
5. Tinker, A. C.; Beaton, H. G.; Boughton–Smith, N.; Cook, T. R.; Cooper, S. L.;
Fraser–Rae, L.; Hasllam, K.; Hamley, P.; McInally, T.; Nicholls, D. J.; Pimm, A. D.;
Wallace, A. V. J. Med. Chem. 2003, 46, 913.
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D.; Srivastava, S.; Zhou, F.; Mackintosh, N.; Carpenter, K.; Perkins, M. N.;
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Walpole, C.; Yang, H. Preparation of spiro[piperidine-4,20(10H)-quinazoline]-
40-amines as nitric oxide synthase inhibitors.PCT Int. Appl.2001, 64 pp.WO
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Hamashima, Y.; Sodeoka, M. Adv. Syn. Catal. 2010, 352, 2708; (b) Ma, J.-A.;
Cahard, D. Chem. Rev. 2008, 108, 1; (c) Prakash, G. K. S.; Beirer, P. Angew. Chem.,
Int. Ed. 2006, 45, 2172.
8. van Niel, M. B.; Collins, I.; Beer, M. S.; Broughton, H. B.; Cheng, S. K. F.;
Goodacre, S. C.; Heald, A.; Locker, K. L.; MacLeod, A. M.; Morrison, D.; Moyes, C.
R.; O’Connor, D.; Pike, A.; Rowley, M.; Russel, M. G. N.; Sohal, B.; Stanton, J. A.;
Thomas, S.; Verrier, H.; Watt, A. P.; Castro, L. J. J. Med. Chem. 1999, 42,
2087.
9. Liu, J.; Chan, J.; Bryant, C. M.; Duspara, P. A.; Lee, E. E.; Powell, D.; Yang, H.; Liu,
Z.; Walpole, C.; Roberts, E.; Batey, R. A. Tetrahedron Lett. 2012, 53, 2971.
10. The coupling of the benzamidine 5 and the optically pure 3-fluoropiperidine
ketones 4a and 4c in the presence of TMSOTf and triethylamine retained the
chirality of the fluorine-attached stereogenic centre. However, the fluorine-
attached stereogenic centre racemized during the cyclization reaction in the
presence of protic acid or Lewis acid.
11. DFT calculations (B3LYP/6-31G⁄, THF/ZPVE corrected) on the diastereoisomers
2a and 3a, reveal that the lowest energy conformations are A (2a) and B (3a)
respectively. Diastereoisomer A (2a) was found to be 3.3 kcal/mol lower in
energy than B (3a). In both cases the lowest energy conformations A and B
have the amidine group of the quinazoline ring adopting an axial orientation
on the piperidine ring. The conformation A (2a) is also the biologically relevant
conformation found in co-crystal structures of the inhibitor with the iNOS
enzyme (Ref.6a). The conformations (not shown) corresponding to the amidine
group of the quinazoline ring in an equatorial orientation were both found to
be 4.6 kcal/mol higher in energy than A (2a). This compares to a 1.3 kcal/mol
energy difference (B3LYP/6-31G⁄, THF/ZPVE corrected) between the
corresponding conformations of the unfluorinated analog of 2a:3a, the
lowest energy conformation also favoring an axially oriented amidine. The
lower energy of A (2a) compared to B (3a) may be due to electronic effects,
since there is a known preference of fluorine to adopt an axial orientation in 3-
fluoropiperidinium ring systems (5.4 kcal/mol), see: (a) Sum, A. M.; Lankin, D.
C.; Hardcastle, K.; Snyder, J. P. Chem. Eur. J. 2005, 11, 1579.; (b) Lankin, D. C.;
Chandrakumar, N. S.; Shashidhar, N. R.; Spangler, D. P.; Snyder, J. P. J. Am. Chem.
Soc. 1993, 115, 3356.
Acknowledgments
We thank Dr. John Wei at Pharmaron and Prof. Stephen Hanes-
sian at the University of Montreal for helpful discussions.
References and notes
1. (a) Purser, S.; Moore, P. R.; Swallow, S.; Gouverneur, V. Chem. Soc. Rev. 2008, 37,
320; (b) Hagmann, W. K. J. Med. Chem. 2008, 51, 4359; (c) Kirk, K. L. Org. Process

C. Walpole et al. / Tetrahedron Letters 53 (2012) 2942–2947
2947
nitrogen lone pairs that occurs when these groups adopt
orientation.
13. Cis-1-(4-cyanobenzoyl)-3,50,80-trifluorospiro[piperidine-4,20(10H)-quinazoline]
-40-amine trifluoroacetate (2a). IR (free amine, NaCl disc):
3400, 2229,
1624 cmꢁ1 1H NMR (amine, CDCl3/TMS): d 1.70–2.03 (m, 2H), 3.27–3.46 (m,
a
trans-diaxial
t
;
2H), 3.51–3.80 (m, 2H), 4.30–5.00 (m, 1H), 4.71 (s, 1H), 5.18 (s, 2H), 6.34 (m,
1H), 7.01 (m, 1H), 7.54 (d, J = 7.2 Hz, 2H), 7.71 (d, J = 7.2 Hz, 2H); MS (m/z):
399.89 (M + H); Analysis for C20H16F3N5OꢀCF3COOHꢀ0.3H2O: calcd C 50.93, H
3.42, N 13.5: obtained: C 50.90, H 3.37, N 13.26.
Trans-1-(4-cyanobenzoyl)-3,50,80-trifluorospiro[piperidine-4,20(10H)-quinazoline]-
40-amine trifluoroacetate (3a). IR (amine, NaCl disc):
t
3322 (br), 2229,
1623 cmꢁ1
;
1H NMR (amine, CDCl3/TMS): d 1.50–2.23 (m, 2H), 3.35–3.80 (m,
4H), 4.20–4.62 (m, 1H), 4.28 (s, 1H), 5.30 (br s, 2H), 6.30 (m, 1H), 6.97 (m, 1H),
7.52 (d, J = 8.0 Hz, 2H), 7.73 (d, J = 8.0 Hz, 2H); MS (m/z): 400.14 (M + H);
Analysis for
obtained C 50.90, H 3.37, N 13.26.
C
20H16F3N5Oꢀ1.10CF3CO2H: calcd
C 50.81, H 3.28, N 13.34;
14. Representative coupling protocol: To a stirred solution of amidine 4c (1.0 equiv),
in dry, freshly distilled THF under an atmosphere of N2 at 0 °C, TBSOTf
(5.0 equiv) and NEt3 (6.0 equiv) were added. A 5% by volume solution of
BF3ꢀOEt2 (0.1 equiv) in THF was then added. After stirring the solution at 0 °C
for 45 min, a solution of CBz protected piperidone 4c (1.0 equiv) in THF (2 mL)
was added. The reaction mixture was then warmed to 4 °C, sealed, and stirred
for 24 h. The reaction was then quenched with sat. NaHCO3, extracted with
Et2O (3ꢂ), and the combined organic phases washed with brine and dried over
MgSO4. After filtration and concentration, the ratio of the diastereoisomers was
determined by 19F NMR (or HPLC). Flash chromatography (5% MeOH in CH2Cl2)
yielded a mixture of diastereoisomers 2c and 3c. Purification by MPLC (CH2Cl2/
MeOH 20:1–10:1) gave the spirocycles as two separated diastereoisomers.
15. Benzyl cis-40-amino- 3,50,80-trifluorospiro[piperidine-4,20(10H)-quinazoline]-1-
carboxylate (Diastereoisomer 2c): IR (amine, NaCl disc):
t 3322 (br), 2229,
1623 cmꢁ1 1H NMR (amine, CDCl3/TMS): d 1.61 (br, 3H), 2.01 (br, 1H), 3.50 (br,
;
2H), 3.91 (br, 1H), 4.01–4.40 (m, 2H), 5.14 (s, 2H), 5.21 (br, 1H), 6.31 (m, 1H),
6.90 (m, 1H), 7.40 (m, 5H); MS (m/z): 405 (M + H); 19F NMR (CDCl3) d ꢁ119.4
(m, 1F, aromatic F), –143.3 (m, 1F, aromatic F), –196.9 (dm, 1F, J = 53.0 Hz,
piperidine F). Benzyl trans-40-amino-3,50,80-trifluorospiro[piperidine-4,2’(1’H)-
quinazoline]-1-carboxylate (Diastereoisomer 3c): MS (m/z): 405 (M + H); 19F
NMR (CDCl3) d ꢁ119.5 (m, 1F, aromatic F), ꢁ142.1 (m, 1F, aromatic F), ꢁ194.4
(m, 1F, piperidine F).
12. A referee suggests that the preferred conformation A (2a) is due to the greater
opposition of dipoles from the fluorine lone pairs and the amidine imino-type