Novel 5-substituted 3-hydroxyphenyl and 3-nitrophenyl ethers of S-prolinol as α4β2-nicotinic acetylcholine receptor ligands

Article abstract of DOI:10.1016/j.bmcl.2016.10.078

A series of 3-nitrophenyl and 3-hydroxyphenyl ethers of (S)-N-methylprolinol bearing bulky and lipophilic substituents at phenyl C5 were tested for affinity at α4β2 and α3β4 nAChRs. The two phenyl ethers 5-substituted with 6-hydroxy-1-hexynyl showed high α4β2 affinity and significantly increased α4β2/α3β4 selectivity compared to the respective unsubstituted parent compounds. Within the two series of novel phenyl ethers, we observed parallel shifts in affinity and, furthermore, the increase in α4β2/α3β4 selectivity resulting from the hydroxyalkynyl substitution parallels that reported for the same modification at the 3-pyridyl ether of (S)-N-methylprolinol (A-84543), a well-known potent α4β2 agonist. On the basis of these results, our nitrophenyl and hydroxyphenyl prolinol ethers can be considered bioisosteres of the pyridyl ether A-84543 and lead compounds candidable to analogous optimization processes.

Full text of DOI:10.1016/j.bmcl.2016.10.078

Bioorganic & Medicinal Chemistry Letters 26 (2016) 5613–5617
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Bioorganic & Medicinal Chemistry Letters
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Novel 5-substituted 3-hydroxyphenyl and 3-nitrophenyl ethers
of S-prolinol as a4b2-nicotinic acetylcholine receptor ligands
Cristiano Bolchi a, Francesco Bavo a, Laura Fumagalli a, Cecilia Gotti b, Francesca Fasoli b, Milena Moretti b,
Marco Pallavicini a,
⇑
a Dipartimento di Scienze Farmaceutiche, Università degli Studi di Milano, via Mangiagalli 25, I-20133 Milano, Italy
b CNR, Istituto di Neuroscienze, Dipartimento di Biotecnologie Mediche e Medicina Traslazionale, Università degli Studi di Milano, via Vanvitelli 32, I-20129 Milano, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of 3-nitrophenyl and 3-hydroxyphenyl ethers of (S)-N-methylprolinol bearing bulky and lipophi-
Received 7 October 2016
Revised 25 October 2016
Accepted 26 October 2016
Available online 27 October 2016
lic substituents at phenyl C5 were tested for affinity at
substituted with 6-hydroxy-1-hexynyl showed high 4b2 affinity and significantly increased
selectivity compared to the respective unsubstituted parent compounds. Within the two series of novel
phenyl ethers, we observed parallel shifts in affinity and, furthermore, the increase in 4b2/ 3b4 selec-
tivity resulting from the hydroxyalkynyl substitution parallels that reported for the same modification at
the 3-pyridyl ether of (S)-N-methylprolinol (A-84543), a well-known potent 4b2 agonist. On the basis of
these results, our nitrophenyl and hydroxyphenyl prolinol ethers can be considered bioisosteres of the
pyridyl ether A-84543 and lead compounds candidable to analogous optimization processes.
Ó 2016 Elsevier Ltd. All rights reserved.
a
4b2 and
a
3b4 nAChRs. The two phenyl ethers 5-
a
a
4b2/ 3b4
a
a
a
Keywords:
nAChR
A-84543
Ligand
Affinity
a
Bioisosterism
6-Hydroxy-1-hexinyl
Agonists at neuronal nicotinic acetylcholine receptors (nAChRs),
in particular those at the 4b2 subtype, are intensively studied
nowadays because of their potential application in the therapy of
ganglionic a3b4 nAChR subtype, in particular by introducing a ser-
a
ies of sterically bulky substituents at the pyridyl C5 [12–15]. Our
previous researches have been also focused on modifications of
3-pyridyl ether A-84543 aimed at modulating its activity profile.
[16–19] As shown in Fig. 2, the first notable result of these inves-
tigations has been the finding that conformational blocking of
the aryloxymethyl portion of 1 inside a pyridodioxane [20] or ben-
zodioxane [17–19] system or its conformational restriction by
ortho-methoxy substitution [16] results into selective partial
a number of nervous-system disorders, tobacco addiction and alco-
hol dependence [1–3]. Varenicline, a partial
used as an aid to smoking cessation [4–7], while another partial
and highly selective 4b2 agonist, sazetidine-A [8,9], has been
a4b2 agonist, is widely
a
studied as an antidepressant within a wide number of 3-pyridyl
ethers of N-methylprolinol or 2-azetidinemethanol bearing differ-
ent substituents at the pyridine 5-position [1]. Recently, VMY-2-95
a4b2 agonism, but on condition that pyridine nitrogen is correctly
has been proved to have the high and selective
cal of these 5-substituted 3-pyridyl ethers and an interesting char-
acter of low-efficacy agonist and potent desensitizer of 4b2
a
4b2 affinity typi-
positioned in the pyridodioxane system (3) [20] or the benzene
ring is hydroxylated at a certain position (5 and 6). [16,17] Isosteric
replacement of pyridine nitrogen with CH or removal of the OH
a
nAChR. Such a profile and good drug like properties make this
molecule a good candidate for the treatment of nicotine addiction
and other nAChRs related CNS disorders [2,10] (see Fig. 1).
Sazetidine-A and VMY-2-95 descend from the N-methylprolinol
3-pyridyl ether A-84543 (1) and its azetidinyl analogue A-85380
(2). These were discovered at the end of the nineties [11] as potent
substituent transforms the selective
a4b2 partial agonists 3 and
5 into the unselective 4b2 antagonist 4. [20] On the other hand,
a
the same modification, namely the meta-hydroxylation, made on
the conformationally unrestricted 1 and its phenyl isoster is asso-
ciated to potent a4b2 full agonism (7 and 8), but selective only in
the case of the meta-hydroxylated derivative 7 [16]. These results
suggest different receptor interactions of the aromatic portion
between the forcedly extended bicyclic derivatives 3 and 5 and
the aryl ethers 7 and 8, for which a folded conformation of the
oxymethylene linker is allowed [16].
a4b2 agonists with selectivity over a7 and muscle-type nAChRs.
From 2005, the pyridyl ethers A-84543 and A-85380 were recon-
sidered in order to increase their selectivity also over the
On the basis of these observations, we were induced to postu-
late similar interactions with the receptor counterpart for 1 and
⇑
Corresponding author.
E-mail address: marco.pallavicini@unimi.it (M. Pallavicini).
http://dx.doi.org/10.1016/j.bmcl.2016.10.078
0960-894X/Ó 2016 Elsevier Ltd. All rights reserved.

5614
C. Bolchi et al. / Bioorganic & Medicinal Chemistry Letters 26 (2016) 5613–5617
N
N
OH
NH
O
N
N
N
HN
O
O
O
sazetidine-A
N
varenicline
O
O2N
MeO
(CH2)n
11
10
9
O
OH
X
HN
N
R
N
N
N
N
1 (A-84543) R = Me n = 2
2 (A-85380) R = H n = 1
VMY-2-95
O
O
X
Fig. 1.
a4b2 nAChR agonists: chemical structures of varenicline and of some
reference 3-pyridyl ethers.
HO
O2N
12 X = 6-hydroxy-1-hexynyl
16 X = 6-hydroxy-1-hexynyl
13
14
15
= Br
17
18
19
= Br
= Ph
= p-hydroxyphenyl
= Ph
= p-hydroxyphenyl
α4β2 partial agonists (3, 5, 6) or antagonists (4)
Fig. 3. Chemical structures of the designed phenyl ethers of (S)-N-methylprolinol.
N
N
N
O
O
O
O
OH
O
O
X
2) and our previous prolinol ethers with higher
[16–19,28].
a4b2 affinity
N
The meta-bromo substituted hydroxyphenyl ether 13 [29] was
synthesized from N-Boc protected S-prolinol by Mitsunobu reac-
tion with 3-hydroxy-5-bromophenol, subsequent N-deprotection
and final reductive alkylation with formaldehyde (Scheme 1). To
obtain the hyroxyphenyl ethers 12, [30] 14 [31] and 15, [32] bear-
ing an aryl or alkynyl residue at the meta position, the intermediate
bromophenyl ether 20 was coupled with phenylboronic acid (24)
or 4-hydroxyphenylboronic acid (25), according to previously
reported methods, [33,34] or with 5-hexynol (26), according to
the Sonogashira reaction, and then N-deprotected and N-methy-
lated (Scheme 1). The corresponding meta-substituted nitrophenyl
ethers 16–19 [35–38] were synthesized from N-Boc protected S-
prolinol through the same synthetic sequences followed for 12–
15, but using 3-bromo-5-nitrophenol in the initial Mitsunobu reac-
tion (Scheme 1).
4 X = H
= OH
6
3
5
α4β2 full agonists
N
N
O
O
OH
N
HO
7
8
Fig. 2. Analogues of 1 (A-84543) with different activity and selectivity profiles at
4b2 nAChR.
a
8. The known bioisosteric relationship between pyridine and
phenol further supported such a hypothesis and prompted us to
consider also the isosteric replacement of 3-pyridyl with 3-nitro-
phenyl and to synthesize 9 [21–23]. Indeed, this compound had
The reference compounds 9 [39] and 11 [40] were prepared by
Mitsunobu condensation of N-Boc protected S-prolinol with 3-
nitrophenol (36) and 3-bromophenol (37). The nitrophenyl ether
36 was then deprotected and submitted to reductive alkylation
with formaldehyde to yield 9, while the bromophenyl ether 37
was coupled with 5-hexynol (39) before being N-deprotected and
N-methylated to give 11 (Scheme 2). To prepare the 3-methoxy-
phenyl ether 10, [41] N-Cbz protected S-prolinol was condensed
with 3-methoxyphenol (41) and then reduced with LiAlH4
(Scheme 2).
been already characterized for its
of [3H]-cytisine twenty years ago, [24] but we needed new binding
data, fit to be compared with those of our other 4b2 nicotinic
a4b2 affinity by displacement
a
ligands, all tested against epibatidine. Furthermore, for the same
reason, we prepared also the 3-methoxyphenyl analogue 10 [25]
as a comparison term with 8. The aim of the research was to
explore the effect of the introduction of some meta-substituents,
We evaluated the binding affinity of 9–19 towards the
nAChR present on rat cerebral cortex membranes and towards
the human 3b4 nAChR transiently transfected on HEK 243 cells
according to a previously described experimental protocol. [16]
The 4b2 and
3b4 nAChRs were labelled by [3H]-epibatidine
a4b2
which have been described to preserve the high
1 and 2 and, in some cases, also to confer 4b2 vs
a
4b2 affinity of
a
a3b4 selectivity,
a
into the superimposable meta position of the supposedly bioisos-
teric aryl ethers 8 and 9. This was done in order to substantiate
a
a
our hypothesis of bioisosteric relationship and to find new
ligands with good 4b2 vs 3b4 selectivity. We selected, as
meta-substituents, 6-hydroxy-1-hexynyl, conferring high 4b2 vs
3b4 selectivity to 2 (sazetidine-A), and bromine and phenyl,
a4b2
and the binding affinities (Ki) of the compounds were determined
with competition binding experiments. The results are listed on
Table 1 together with the affinities of (S)-(À)-nicotine, determined
as controls, and with those previously reported for the hydrox-
yphenyl ether 8 [16].
a
a
a
a
which are reported to maintain the high
a4b2 affinity of 1 and 2
when linked to the pyridyl C5 [26,27]. Moreover, we thought that
a terminal hydroxyl could reinforce the interaction of the meta-
substituent, as in sazetidine-A and in 5. Therefore, we enclosed into
the series also p-hydroxyphenyl and planned the synthesis of com-
pounds 9–19 (Fig. 3).
As shown in Table 1, both our lead compounds, namely the
meta-hydroxyphenyl ether 8 and the meta-nitro phenyl ether 9
have high a4b2 affinity (1.1 and 31 nM Ki respectively), higher than
that determined for the unsubstituted phenyl ether of N-methy-
lated S-prolinol in 1995 (42 nM Ki) using [3H]-cytisine. [25,42]
On the other hand, compared to 1 (1.9 nM Ki), [12,41] 8 shows
the same affinity, while 9 a lower one. Overall, it can be stated that
The title compounds have the same S configuration as the liter-
ature reference pyridyl ethers (see sazetidine-A, VMY-2-95, 1 and

C. Bolchi et al. / Bioorganic & Medicinal Chemistry Letters 26 (2016) 5613–5617
5615
a
or
b
a
b
N
N
N
HN
N
N
Boc
Boc
Boc
Boc
Boc
O
Y
O
Y
OH
O
OH
d
Br
N
X
O2N
O
f
or
g
Y = OH (20), NO2 (21)
38
Y = NO2 (36), Br (37)
O
Y
c
c
or d
X
9
OH
HN
Y=OH X= Ph
p-OH-C6H4
6-OH-1-hexynyl
24
25
26
O
Y
39 X = Boc
40 X = H
11 X = Me
b
c
N
X
Br
O
27
28
29
Y=NO2 X= Ph
e
p-OH-C6H4
6-OH-1-hexynyl
Y = OH (22), NO2 (23)
N
e
MeO
Cbz
OH
d
13, 17
41 X = Cbz
10 X = Me
f
HN
e
O
12,14, 15, 16, 18, 19
Scheme 2. Reagents, conditions and yields. (a) 3-Y-phenol, PPh3, DIAD, THF, reflux,
12 h, 82% (36) and 48% (37); (b) Methanolic 1.25 N HCl, room temperature, 12 h,
100% (38), 71% (39) and 71% (40); (c) 37% CH2O, CH3COOH, pic-BH3, CH3OH, room
temperature, 4 h, 98% (9); (d) Pd(PPh3)4, CuBr, 5-hexyn-1-ol, TEA, reflux, 12 h, 79%;
(e) 3-Methoxy-phenol, PPh3, DIAD, THF, reflux, 12 h, 63%; (f) LiAlH4, THF, reflux,
90 min, 67%.
X
Y
Y=OH X= Ph
30
31
p-OH-C6H4
6-OH-1-hexynyl 32
Table 1
33
34
35
Y=NO2 X= Ph
p-OH-C6H4
6-OH-1-hexynyl
Nicotine and compounds 9–19: affinity for native
membranes, labelled by [3H]-epibatidine, and for heterologously expressed human
3b4 nAChR, labelled by [3H]-epibatidine and affinity ratios.
a4b2 nAChR, present in rat brain
a
a
4b2-nAChR [3H]Epi Ki
M)
a
3b4-nAChR [3H]Epi
Ki( M)
a
a
3b4 Ki/
4b2 Ki
Scheme 1. Reagents, conditions and yields. (a) 3-Hydroxy-5-bromophenol, PPh3,
DIAD, THF, 130 °C, 30 min,, microwave, 34% (20); (b) 3-Bromo-5-nitrophenol, PPh3,
DIAD, THF, reflux, 12 h, 85% (21); (c) TFA, DCM, room temperature, 2 h, 80% (22); (d)
Methanolic 1.25 N HCl, room temperature, 12 h, 42% (23), 100% (30), 96% (31), 80%
(32), 78% (33), 81% (34), and 95% (35); (e) 37% CH2O, CH3COOH, pic-BH3, CH3OH,
room temperature, 4 h, 87% (12), 75% (13), 80% (14), 51% (15), 80% (16), 98% (17),
86% (18) and 79% (19); (f) Pd(PPh3)4, toluene, 2 M aqueous Na2CO3, XB(OH)2,
ethanol, reflux, 12 h, 75% (24), 97% (25), 93%. (27) and 63% (28); (g) Pd(PPh3)4, CuBr,
5-hexyn-1-ol, TEA, reflux, 12 h, 80% (26) and 61% (29).
(
l
l
8
9
0.0011 (29)
0.0312 (32)
0.600 (32)
4.4(44)
0.0237 (21)
0.186 (25)
0.528 (32)
0.0038 (47)
0.0142 (31)
0.055 (26)
0.330 (45)
0.012 (43)
Nicotine
0.074 (100)
0.946 (33)
4.5 (41)
8.3 (44)
3.1 (34)
0.147 (30)
0.200 (28)
0.030 (53)
1.2 (32)
0.415 (49)
0.947 (38)
0.122 (41)
0.004 (18)
67
30
7
2
131
0.8
0.4
8
85
8
3
10
10
11
12
13
14
15
16
17
18
19
(-)-
65
1
the introduction of NO2 and OH at the meta position favours the
a4b2 nAChR interaction of the phenyl and promotes, in the case
0.261 (30)
of the meta hydroxylation, the phenyl ether to the rank of the cor-
responding pyridyl ether. The importance of the m-OH is further
0.0019
0.0009
0.040 (18)
1.4
63
0.194 (24)
737
74,000
demonstrated by the drop of
a4b2 affinity resulting from its
42
Kd (nM)
methylation (see compound 10) and from its removal (see com-
pound 11 compared to 12). Furthermore, both 8 and 9 show a mod-
erate a4b2 vs a3b4 subtype selectivity.
The Kd and Ki values were derived from[3H]epibatidine saturation and competition
binding experiments using rat cortex membranes ( 4b2) and the membrane of
human 3b4 transfected cells as described in Ref. [16]. The curves were fitted using
a nonlinear least squares analysis program and the F test. The numbers in brackets
represent the% coefficient of variation (CV). The affinities of 8, 1 and 42 are those
previously reported in Refs. [16] and [12].
a
Based on these premises, the same meta substituents (Br, Ph, p-
hydroxyphenyl and 6-hydroxy-1-hexynyl) were introduced into 8
and 9 and, as shown in Table 1, parallel trends of affinity were
found between the series of the m-hydroxyphenyl ethers 12–15
and that of the corresponding m-nitrophenyl analogues 16–19.
Meta-bromination (13 and 17) and, to a greater extent, meta-
a
of 1 (Fig. 4), [12] because the 5-(4-hydroxyphenyl)-substituted
analogue of 1 is not reported and the binding data for the 5-bromo-
phenylation (14 and 18) lower the
4b2/ 3b4 selectivity. On the contrary, both the meta-6-
hydroxy-1-hexynyl substituted derivatives 12 and 16 show high
ten-nanomolar 4b2 affinities and significantly higher 4b2/
3b4 selectivity than the respective parent compounds 8 and 9,
and the meta-4-hydroxyphenyl substituted analogues 15 and 19
have the highest 4b2 affinities (3 and 12 nM Ki respectively),
though with a modest 4b2/ 3b4 selectivity.
a4b2 affinity and cancel the
and 5-phenyl analogues are limited to the
a4b2 affinity and not
a
a
directly comparable to the 1.9 nM Ki of 1. [42] Nevertheless,
though circumscribed to only one substituent, the parallelism with
a
a
1 is revealing: 42, compared to 1, maintains high
a4b2 affinity
a
(0.9 nM Ki), and shows increased 4b2/ 3b4 selectivity with a
a
a
behaviour analogous to that of 12 and 16, compared to 8 and 9
respectively. The meta-6-hydroxy-1-hexynyl substitution similarly
affects the nicotinic affinity profile of 1 and those of 8 and 9, thus
suggesting that the two prolinol phenyl ethers 8 and 9 share a
a
a
a
With regard to the parallelism with 1, this can be drawn only
for 42, namely the 5-(6-hydroxy-1-hexynyl)-substituted analogue

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C. Bolchi et al. / Bioorganic & Medicinal Chemistry Letters 26 (2016) 5613–5617
[16] Bolchi C, Valoti E, Gotti C, Fasoli F, Ruggeri P, Fumagalli L, Binda M, Mucchietto
N
N
N
M, Sciaccaluga M, Budriesi R, Fucile S, Pallavicini M. J. Med. Chem.
2015;58:6665.
[17] Bolchi C, Gotti C, Binda M, Fumagalli L, Pucci L, Pistillo F, Vistoli G, Valoti E,
Pallavicini M. J. Med. Chem. 2011;54:7588.
O
O
O
[18] Pallavicini M, Moroni B, Bolchi C, Cilia A, Clementi F, Fumagalli L, Gotti C,
Meneghetti F, Riganti L, Vistoli G, Valoti E. Bioorg. Med. Chem. Lett.
2006;16:5610.
N
O2N
HO
[19] Pallavicini M, Bolchi C, Binda M, Cilia A, Clementi F, Ferrara R, Fumagalli L,
Gotti C, Moretti M, Pedretti A, Vistoli G, Valoti E. Bioorg. Med. Chem. Lett.
2009;19:854.
[20] C. Bolchi, F. Bavom, C. Gotti, L. Fumagalli, F. Fasoli, M. Binda, V. Mucchietto, M.
Sciaccaluga, S. Plutino, S. Fucile, M. Pallavicini, Eur. J. Med. Chem. DOI:
10.1016/j.ejmech.2016.10.048 (in press).
12
16
42
HO
HO
HO
Fig. 4. Hydroxyhexynyl-aryl ethers of S-prolinol.
[21] Sun W, Blanton MP, Gabriel JL, Canney DJ. Med. Chem. Res. 2005;15:241.
[22] Poupaert J, Carato P, Colacino E. Curr. Med. Chem. 2005;12:877.
[23] Meanwell NA. J. Med. Chem. 2011;54:2529–2591.
[24] D.E. Gunn, R.L. Elliott, N.H. Lin, H.A. Kopecka, M.W. Holladay, US5472958,
1995.
similar binding mode at
which they are veritable bioisosteres.
a4b2 nAChR with the pyridyl ether 1, of
As previously discussed, [43] the long and flexible alkynyl sub-
stituent at C5 of prolinol pyridyl ethers would be able to reach b-
side areas relatively remote from the binding site of the charged
pyrrolidine nitrogen and there interact, respectively, with those
non-conserved b2- and b4-residues which are mainly responsible
[25] Elliott RL, Kopecka HA, Gunn DE, Lin NH, Garvey DS, Ryther KB, Holladay MW,
Anderson DJ, Campbell JE, Sullivan JP, Buckley MJ, Gunther KL, O’Neill AB,
Decker MW, Arneric SP. Bioorg. Med. Chem. Lett. 1996;6:2283.
[26] Lin NH, Gunn DE, Li Y, He Y, Bai H, Ryther KB, Kuntzweiler T, Donnelly-Roberts
DJ, Anderson DJ, Campbell JE, Sullivan JP, Arneric SP, Holladay MW. Bioorg.
Med. Chem. Lett. 1998;8:249.
[27] Lin NH, Li Y, He Y, Holladay MW, Kuntzweiler T, Anderson DJ, Campbell JE,
Arneric SP. Bioorg. Med. Chem. Lett. 2001;11:631.
[28] Pallavicini M, Moroni B, Bolchi C, Clementi F, Fumagalli L, Gotti C, Vailati S,
Valoti E, Villa L. Bioorg. Med. Chem. Lett. 2004;14:5827.
for the difference between
remote interactions of the alkynyl appendage would result in a cor-
rect positioning of pyrrolidine N+ relative to an
-conserved Trp
residue in the 4b2 [43,44], but not in the 3b4 binding site and
this would justify the high 4b2/ 3b4 selectivity. On the basis of
a4b2 and a3b4 nAChRs. Such different
a
[29] (S)-13: obtained as
a
waxy solid:
[
a
]
25 = À18.36 (c 1, CHCl3); 1H NMR
D
a
a
(300 MHz, CDCl3) d 6.56 (t, J = 1.9 Hz 1H); 6.47 (t, J = 1.9 Hz, 1H); 6.28 (t,
J = 1.9 Hz, 1H); 4.80 (bs, 1H, exchange with D2O); 4.00–3.81 (m, 2H); 3.21–
3.14 (m, 1H); 2.86–2.79 (m, 1H); 2.60 (s, 3H); 2.42–2.31 (m, 1H); 2.12–2.00
(m, 1H); 1.93–1.89 (m, 2H); 1.82–1.61 (m, 1H); 13C NMR (75 MHz, CDCl3) d
159.9, 158.7, 122.8, 113.1, 108.4, 102.6, 70.7, 64.6, 58.0, 42.8, 28.2, 23.1.
[30] (S)-12: obtained as a yellow waxy solid after chromatography on silica gel
a
a
our binding data, we think that these structure–activity and struc-
ture-selectivity relationships, formulated for the alkynyl substi-
tuted pyridyl ethers of prolinol, [12,43] can be applied also to 12
and 16, alkynyl substituted hydroxyphenyl and nitrophenyl ethers
(eluent 90:10:1 DCM/methanol/conc NH3); Rf = 0.42; [
a]
D
25 = À19.02 (c 0.5,
methanol); 1H NMR (300 MHz, CD3OD) d 6.39 (m, 2H); 6.31 (m, 1H); 3.90 (m,
2H); 3.60 (t, J = 6.4 Hz, 2H); 3.10–3.02 (m, 1H); 2.73–2.64 (m, 1H); 2.48 (s,
3H); 2.38–2.29 (m, 3H); 2.09–1.99 (m, 1H); 1.84–1.75 (m, 2H); 1.73–1.61 (m,
5H); 13C NMR (75 MHz, CD3OD) d 159.8, 158.1, 125.3, 110.9, 108.4, 101.7, 88.5,
80.5, 69.7, 64.4, 61.1, 57.3, 40.6, 31.4, 27.7, 24.9, 22.1, 18.3.
of prolinol, which are in fact new potent and selective
ligands.
a4b2 nAChR
[31] (S)-14: obtained as a waxy yellow solid after chromatography on silica gel
Acknowledgements
(eluent 90:10:1 DCM/methanol/conc NH3); Rf = 0.38; [
a
]
D
25 = À27.80 (c 0.5,
methanol); 1H NMR (300 MHz, CD3OD) d 7.54 (d, J = 7.5 Hz, 2H); 7.39 (t,
J = 7.5 Hz, 2H); 7.29 (t, J = 7.5 Hz, 1H); 6.63 (m, 2H); 6.38 (m, 1H); 4.01 (m,
2H); 3.12–3.07 (m, 1H); 2.76–2.73 (m, 1H); 2.42 (s, 3H); 2.40–2.33 (m, 1H);
2.13–2.07 (m, 1H); 1.88–1.70 (m, 3H); 13C NMR (75 MHz, CD3OD) d 160.4,
158.6, 143.3, 141.1, 128.3, 127.0, 126.5, 106.6, 104.4, 100.4, 69.8, 64.6, 57.3,
40.6, 27.7, 22.1.
This research was financially supported by the CNR Research
Project on Aging, and the fondazione Giancarla Vollaro and the
Fondazione Monzino.
[32] (S)-15: obtained as a yellow oil after chromatography on silica gel (eluent
90:10:2 DCM/methanol/conc NH3); Rf = 0.30; [
a
]
25 = À20.93 (c 1, methanol);
D
References and notes
1H NMR (300 MHz, CD3OD) d 7.39 (m, 2H); 6.81 (m, 2H); 6.58 (m, 2H); 6.30 (t,
J = 1.8 Hz, 1H); 3.99 (m, 2H); 3.10 (m, 1H); 2.75 (m, 1H); 2.50 (s, 3H); 2.36 (m,
1H); 2.09 (m, 1H); 1.81 (m, 3H); 13C NMR (75 MHz, CD3OD) d 160.3, 158.4,
156.9, 143.2, 132.4, 130.0, 127.5, 115.0, 105.9, 103.8, 99.5, 69.8, 64.5, 57.3,
40.6, 27.8, 22.1.
[1] Yu LF, Zhang HK, Caldarone BJ, Eaton JB, Lukas RJ, Kozikowski AP. J. Med. Chem.
2014;57:8204.
[2] Kong H, Song J, Yenugonda VM, Zhang L, Shuo T, Cheema AK, Kong Y, Du G,
Brown ML. Mol. Pharm. 2015;12:393.
[3] Rahman S, Engleman EA, Bell RL. Front. Neurosci. 2015;9 426.
[4] Philip NS, Carpenter LL, Tyrka AR, Whiteley LB, Price LH. J. Clin. Psychiatry.
2009;70:1026.
[5] Caldarone BJ, Wang D, Paterson NE, Manzano M, Fedolak A, Cavino K, Kwan M,
Hanania T, Chellappan SK, Kozikowski AP, Olivier B, Picciotto MR, Ghavami A.
Psychopharmacology. 2011;217:199.
[6] Mihalak KB, Carroll FI, Luetje CW. Mol. Pharmacol. 2006;70:801.
[7] Rollema H, Chambers LK, Coe JW, Glowa J, Hurst RS, Lebel LA, Lu Y, Mansbach
RJ, Mather RJ, Rovetti CC, Sands SB, Schaeffer E, Schulz DW, Tingley 3rd FD,
Williams KE. Neuropharmacology. 2007;52:985.
[8] Xiao Y, Fan H, Musachio JL, Wei ZL, Chellappan SK, Kozikowski AP, Kellar KJ.
Mol. Pharmacol. 2006;70:1454.
[9] Levin ED, Rezvani AH, Xiao Y, Slade S, Cauley M, Wells C, Hampton D, Petro A,
Rose JE, Brown ML, Paige MA, McDowell BE, Kellar KJJ. Pharmacol. Exp. Ther.
2010;332:933.
[10] Yenugonda VM, Xiao Y, Levin ED, Rezvani AH, Tran T, Al-Muhtasib N,
Sahibzada N, Xie T, Wells C, Slade S, Johnson JE, Dakshanamurthy S, Kong HS,
Tomita Y, Liu Y, paige M, Kellar KJ, Brown ML. J. Med. Chem. 2013;56:8404.
[11] Abreo MA, Lin NH, Garvey DS, Gunn DE, Hettinger AM, Wasicak JT, Pavlik PA,
Martin YC, Donnelly-Roberts DL, Anderson DJ, Sullivan JP, Williams M, Arneric
MW, Holladay MW. J. Med. Chem. 1996;39:817.
[33] Bolchi C, Pallavicini M, Bernini SK, Chiodini G, Corsini A, Ferri N, Fumagalli L,
Straniero V, Valoti E. Bioorg. Med. Chem. Lett. 2011;21:5408.
[34] Bolchi C, Pallavicini M, Rusconi C, Diomede L, Ferri N, Corsini A, Fumagalli L,
Pedretti A, Vistoli G, Valoti E. Bioorg. Med. Chem. Lett. 2007;17:6192.
[35] (S)-16: obtained as a light yellow oil after chromatography on silica gel (eluent
90:10:2 DCM/methanol/conc NH3); Rf = 0.57; [
a
]
25 = À30.23 (c 1, methanol);
D
1H NMR (300 MHz, CD3OD) d 7.76 (s, 1H); 7.70 (s, 1H); 7.31 (s, 1H); 4.07 (d,
J = 5.3 Hz, 2H); 3.61 (t, J = 6.4 Hz, 2H); 3.01 (m, 1H); 2.75 (m, 1H); 2.49 (m,
5H); 2.38 (m, 1H); 2.05 (m, 1H); 1.86–1.67 (m, 7H); 13C NMR (75 MHz, CD3OD)
d 159.3, 149.1, 126.3, 123.0, 118.0, 108.4, 92.3, 78.3, 70.6, 64.2, 61.0, 57.3, 40.6,
31.4, 27.6, 24.6, 22.2, 18.3.
[36] (S)-17: obtained as a waxy yellow solid after chromatography on silica gel
(eluent 90:10:2 DCM/methanol/conc NH3); Rf = 0.71;
[
a
]
D
25 = À6.67 (c 1,
methanol); 1H NMR (300 MHz, CD3OD) d 7.96 (t, J = 1.8 Hz, 1H); 7.76 (t,
J = 1.8 Hz, 1H); 7.55 (t, J = 1.8 Hz, 1H); 4.08 (d, J = 5.3 Hz, 2H); 3.12–3.06 (m,
1H); 2.80–2.71 (m, 1H); 2.49 (s, 3H); 2.42–2.33 (q, J = 9.4 Hz, 1H); 2.13–2.03
(m, 1H); 1.88–1.70 (m, 3H); 13C NMR (75 MHz, CD3OD) d 160.1, 149.6, 123.7,
122.6, 118.4, 108.3, 70.9, 64.1, 57.2, 40.6, 27.6, 22.2.
[37] (S)-18: obtained as a yellow oil after chromatography on silica gel (eluent 95:5
DCM/methanol); Rf = 0.24;
[a]
D
25 = –38.42 (c 0.5, methanol); 1H NMR
(300 MHz, CHCl3) d 8.08 (s, 1H); 7.71 (s, 1H); 7.62 (m, 2H); 7.50 (m, 4H);
4.12 (m, 2H); 3.18 (m, 1H); 2.68 (m, 1H); 2.49 (s, 3H); 2.36 (m, 1H); 2.15–2.02
(m, 1H); 1.92–1.75 (m, 3H); 13C NMR (75 MHz, CDCl3) d 159.8, 149.5, 143.6,
138.7, 129.0, 128.6, 127.1, 120.2, 114.5, 107.3, 71.5, 64.0, 57.7, 41.7, 28.5, 23.1.
[38] (S)-19: obtained as a yellow solid after chromatography on silica gel (eluent
[12] Wei ZL, Xiao Y, Yuan H, Baydyuk M, Petukhov PA, Musachio JL, Kellar KJ,
Kozikowski AP. J. Med. Chem. 2005;48:1721.
[13] Liu J, Eaton JB, Caldarone B, Lukas RJ, Kozikowski AP. J. Med. Chem.
2010;53:6973.
[14] Yu LF, Eaton JB, Fedolak A, Zhang HK, Hanania T, Brunner D, Lukas RJ,
Kozikowski AP. J. Med. Chem. 2012;55:9998.
[15] Zhang HK, Yu LF, Eaton JB, Whiteaker P, Onajole OK, Hanania T, Brunner D,
Lukas RJ, Kozikowwski AP. J. Med. Chem. 2013;56:5495.
95:5:2 DCM/methanol/conc NH3); Rf = 0.33; m.p. = 163.77 °C, [
a]
25 = À23.45
D
(c 1, methanol); 1H NMR (300 MHz, DMSO-d6) d 9.75 (bs, 1H, exchange with
D2O); 7.92 (s, 1H); 7.58 (m, 4H); 6.85 (m, 2H); 4.18 (m, 1H); 4.02 (m, 1H); 2.96
(m, 1H); 2.61 (m, 1H); 2.39 (s, 3H); 2.31–2.15 (m, 1H); 2.01–1.85 (m, 1H);

C. Bolchi et al. / Bioorganic & Medicinal Chemistry Letters 26 (2016) 5613–5617
5617
1.78–1.62 (m, 3H); 13C NMR (75 MHz, CDCl3) d 159.2, 157.7, 149.3, 143.1,
129.5, 128.0, 119.6, 116.5, 113.9, 105.7, 69.5, 64.8, 57.7, 42.0, 27.6, 22.8.
[39] (S)-9: obtained as a light yellow oil after chromatography on silica gel (eluent
5.1 Hz, 1H); 3.87 (dd, J = 9.3, 6.0, 1H); 3.78 (s, 3H); 3.12 (m, 1H); 2.66 (m, 1H);
2.49 (s, 3H); 2.32 (m, 1H); 2.07–1.98 (m, 1H); 1.89–1.71 (m, 3H); 13C NMR
(75 MHz, CDCl3) d 161.0, 160.4, 130.1, 106.9, 106.7, 101.2, 70.7, 64.7, 57.9,
55.5, 41.7, 28.9, 23.1.
90:10:2 DCM/methanol/conc NH3); Rf = 0.64; [
a]
D
25 = À33.90 (c 1, methanol);
1H NMR (300 MHz, CDCl3) d 7.83 (dd, J = 8.2, 2.4 Hz, 1H); 7.77 (m, 1H); 7.52 (t,
J = 8.2 Hz, 1H); 7.37 (m, 1H); 4.08 (d, J = 4.7 Hz, 2H); 3.08 (m, 1H); 2.75 (m,
1H); 2.42 (s, 3H); 2.37 (q, J = 9.4 Hz, 1H); 2.04–2.13 (m, 1H); 1.88–1.70 (m,
3H); 13C NMR (75 MHz, CD3OD) d 159.5, 149.2, 130.0, 120.9, 115.3, 108.6, 70.5,
64.2, 57.3, 40.6, 27.7, 22.2.
[42] The a
4b2 binding affinity (Ki) determined by using [3H]-cytisine is 42 nM for
the unsubstituted phenyl ether of N-methylated S-prolinol (Ref. [25]), 0.15 nM
for 1 (Ref. [11]), 0.27 nM for the 5-bromo substituted derivative of 1 (Ref.
[26]), 0.11 nM for the 5-phenyl substituted derivative of 1 (Ref. [27]), 9 nM for
9 (Ref. [24]), and 175 nM for 10 (Ref. [25]). Instead, by using [3H]-epibatidine,
significantly lower affinities were found: 1.9 nM for 1 (Ref. [12]), 31.2 nM for 9
(Table 1), and 600 nM for 10 (Table 1). Considering such unidirectional
discrepancies, we think that the only literature affinity data with which we
can compare our results are the 1.9 nM Ki of 1 and the 0.9 nM Ki of 42 and that
the 1.1 and 31 nM affinities of 8 and 9, determined by using [3H]-epibatidine,
can be rightly considered sensibly higher than the ‘overvalued’ 42 nM affinity
of the unsubstituted phenyl ether, determined against [3H]-cytisine .
[43] Yuan H, Petukhov PA. Bioorg. Med. Chem. Lett. 2006;14:7936.
[40] (S)-11: obtained as a yellow oil after chromatography on silica gel (eluent
90:10:2 DCM/methanol/conc NH3); Rf = 0.24; [
a
]
25 = À5.07 (c 1, CHCl3); 1H
D
NMR (300 MHz, DMSO-d6) d 7.21 (m, 1H); 6.90 (m, 3H); 4.40 (bs, 1H, exchange
with D2O); 3.94 (dd, J = 5.3, 9.6 Hz, 1H); 3.80 (dd, J = 5.9, 9.6 Hz, 1H); 3.30 (m,
2H); 2.94 (m, 1H); 2.51 (m, 1H); 2.40 (m, 2H), 2.38 (m, 3H); 2.17 (m, 1H);
1.96–1.86 (m, 1H); 1.71–1.50 (m, 7H); 13C NMR (75 MHz, DMSO-d6) d 158.9,
130.1, 124.8, 124.0, 117.1, 115.2, 91.0, 81.0, 71.3, 64.0, 60.6, 57.5, 41.8, 32.1,
28.9, 25.3, 23.0, 18.9.
[41] (S)-10: obtained as a light yellow oil after chromatography on silica gel (eluent
[44] Pedretti A, Marconi C, Bolchi C, Fumagalli L, Ferrara R, Pallavicini M, Valoti E,
Vistoli G. Biochem. Biophys. Res. Commun. 2008;369:648.
Ethyl Acetate/TEA 3%); Rf = 0.40;
[
a]
D
25 = À52.67 (c 1, CHCl3); 1H NMR
(300 MHz, CDCl3) d 7.17 (t, J = 7.7 Hz, 1H); 6.50 (m, 3H); 4.00 (dd, J = 9.3,