Synthesis and α4β2 nicotinic affinity of unichiral 5-(2-pyrrolidinyl)oxazolidinones and 2-(2-pyrrolidinyl)benzodioxanes

Article abstract of DOI:10.1016/j.bmcl.2006.08.020

The RS and SR enantiomers of 2-oxazolidinone and 1,4-benzodioxane bearing a 2-pyrrolidinyl substituent at the 5- and 2-position, respectively, were synthesized as candidate nicotinoids. One of the two benzodioxane stereoisomers reasonably fits the pharmac

Full text of DOI:10.1016/j.bmcl.2006.08.020

Bioorganic & Medicinal Chemistry Letters 16 (2006) 5610–5615
Synthesis and a4b2 nicotinic affinity of unichiral 5-(2-
pyrrolidinyl)oxazolidinones and 2-(2-pyrrolidinyl)benzodioxanes
Marco Pallavicini,a,* Barbara Moroni,a Cristiano Bolchi,a Antonio Cilia,c
Francesco Clementi,b Laura Fumagalli,a Cecilia Gotti,b Fiorella Meneghetti,a
Loredana Riganti,b Giulio Vistolia and Ermanno Valotia
a
`
Istituto di Chimica Farmaceutica e Tossicologica, Universita di Milano, viale Abruzzi 42, I-20131 Milano, Italy
b
`
CNR, Istituto di Neuroscienze, Dipartimento di Farmacologia Medica, Universita di Milano, via Vanvitelli 32, I-20129 Milano, Italy
cDivisione Ricerca e Sviluppo Farmaceutico, Recordati s.p.a., via Civitali 1, I-20148 Milano, Italy
Received 7 July 2006; revised 1 August 2006; accepted 2 August 2006
Available online 30 August 2006
Abstract—The RS and SR enantiomers of 2-oxazolidinone and 1,4-benzodioxane bearing a 2-pyrrolidinyl substituent at the 5- and
2-position, respectively, were synthesized as candidate nicotinoids. One of the two benzodioxane stereoisomers reasonably fits the
pharmacophore elements of (S)-nicotine and binds at a4b2 nicotinic acetylcholine receptor with submicromolar affinity. Interesting-
ly, both the synthesized pyrrolidinylbenzodioxanes exhibit analogous affinity at a2 adrenergic receptor resembling the behaviour of
some known a2-AR ligands recently proved to possess neuronal nicotinic affinity.
Ó 2006 Elsevier Ltd. All rights reserved.
Neuronal nicotinic acetylcholine receptors (nAChRs)
are ligand-gated ion channels playing an important role
in the regulation of neurotransmission in the CNS. Their
dysfunction has been related to a number of severe brain
pathologies, including Parkinson’s and Alzheimer’s dis-
eases, schizophrenia, anxiety, and some forms of epilep-
sy. As a result, novel ligands for neuronal nAChRs, in
particular for the two major a4b2 and a7 brain sub-
types, may have a great potential as pharmaceuticals
aiming at several neurological disorders.1 Indeed, sub-
type selective nAChR ligands have been developed over
the last decade leading to the formulation of reliable
a4b2 and, more recently, a7 pharmacophores, where
common elements are a cationic centre (N+) and a suit-
ably distanced hydrogen bond acceptor and/or p-elec-
tron-rich group (HBA/p), while a distinctive element
enhancing a7 binding properties seems to be a bulky
hydrophobic moiety directly attached to the cationic
nitrogen containing portion or to HBA/p.2–4 The direc-
tionality of the HBA/p group relative to the
N+ ! HBA/p vector emerges as a critical feature and
there is a continuing interest concerning nicotine and
novel nicotinoids with only one or no rotatable bond be-
tween the cationic centre and HBA/p as useful ligand
templates. Recently, we have reported unichiral E and
Z isomers of pyrrolidinylmethoxyimines and prolinal
oxime ethers as candidate bioisosteres of nicotine and
its isoxazolic analogue ABT 418, demonstrating that
some of them display a submicromolar a4b2 affinity
and a remarkable selectivity over a7 and muscarinic
receptors.5 In a continuation of this investigation, we
present here the synthesis and the nicotinic binding affin-
ity of 1 and 2, where the typical nicotinoid 2-pyrrolidinyl
residue is linked to a HBA/p cyclic moiety, represented
by oxazolidin-2-one and 1,4-benzodioxane, respectively.
N
N
N
O
N
N
O
O
O
O
N
HN
HN
O
O
(S)-nicotine
AR-R17779
1
2
3
Keywords: nAChR; Nicotine; Ligand; Affinity; Oxazolidinone;
Benzodioxane.
Both the designed structures are characterized by the
presence of two vicinal stereocentres connected by the
only bond, whose rotation is relevant to molecule
*
Corresponding author. Tel.: +39 2 50317524; fax: +39 2 50317565;
E-mail: marco.pallavicini@unimi.it
0960-894X/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2006.08.020

M. Pallavicini et al. / Bioorg. Med. Chem. Lett. 16 (2006) 5610–5615
5611
conformation. In view of finding ligands with diversified
affinity profiles, this was judged an intriguing feature in
addition to the existence of four different isomers and to
the fact that such a stereochemical complication takes
place in proximity of the critical cationic head with pre-
dictably important consequences on the mutual disposi-
tion of N+ and HBA/p. The replacement of the HBA/p
moiety of nicotine, namely the pyridine ring, by oxazoli-
din-2-one and 1,4-benzodioxane was undertaken consid-
ering the HBA/p properties of both these systems.
Furthermore, in the case of oxazolidinone, we consid-
ered the reported successful combination of the cyclic
carbamate with quinuclidine to give a conformationally
restricted spiranic analogue of acetylcholine (AR-
R17779) with high and specific a7 nicotinic affinity6,7
and, in the case of benzodioxane, the chance of creating
a partially rigidified analogue of prolinol phenyl ethers
(3), whose nicotinic affinity is well known.8 To our
knowledge, the use of benzodioxane system in nicotinic
ligands was limited to 6-(2-pyrrolidinyl)-1,4-benzodiox-
ane9 and 3-quinuclidinyl 1,4-benzodioxan-5-carboxya-
mide,10 where benzodioxane is linked to the rest of the
molecule through the benzene ring and represents, in
the case of quinuclidine derivative, the additional bulky
hydrophobic moiety typical of many candidate a7
ligands.
isomer of secondary dibenzylaminoalcohol 5 to be iso-
lated, namely that deriving from the SS
pyrrolidinyloxirane and having R configuration at the
exocyclic alcoholic carbon. The Cbz group of this novel
synthetic intermediate was reduced to methyl with
LiAlH4 and the resulting diamine (2R,20S)-614 debenzy-
lated by hydrogenolysis to give (2R,20S)-7,15 which was
finally cyclised to (5R,20S)-1 by reaction with 1,10-car-
1
bonyldiimidazole. Three hundred mega hertz H NMR
spectra confirmed the unitary diastereoisomeric compo-
sition of the two last intermediates 6 and 7 and of the
final pyrrolydinyloxazolidinone (5R,20S)-1.16 On the ba-
sis of the X-ray crystallographic analysis,17 R configura-
tion was assigned to the oxygen-bound asymmetric
carbon of this latter and, retrospectively, of the three
preceding aminoalcohols 5–7 (Fig. 1).
Starting from (R)-N-carbobenzyloxyprolinol, the same
synthetic route illustrated in Scheme 1 led to (5S,20R)-1.19
In order to obtain the pyrrolidinylbenzodioxane 2 with
S configuration at the nitrogen-bound asymmetric
carbon, the diastereomeric mixture of R,S, and SS
(1-carbobenzyloxy-2-pyrrolidinyl)oxirane, epimers at the
epoxide chiral centre, was reacted with 2-benzyloxyphe-
nol and potassium carbonate in 2-propanol (Scheme 2).
As in the previous oxirane opening with dibenzylamine,
only one diastereomeric secondary alcohol resulted from
the nucleophilic attack by phenoxyphenate, namely the
SS phenoxyalcohol 8,20 where the four substituents at
the oxygen-bound asymmetric carbon have the same re-
ciprocal disposition as in (2R,20S)-5 with phenoxymeth-
yl replacing dibenzylaminomethyl group. Upon this
reaction, the N-protecting group was almost quantita-
tively transformed into isopropoxycarbonyl. From the
above reaction, another main product was isolated and
The synthesis of (5R,20S)-5[10-methylpyrrolidin-20-yl]-
1,3-oxazolidin-2-one [(5R,20S)-1] was accomplished as
outlined in Scheme 1. (S)-N-Carbobenzyloxyprolinol
was oxidized to the corresponding aldehyde (S)-3 with
sulfur trioxide pyridine complex as previously reported
in the literature.11 The aldehyde was then converted into
a nearly equimolar diastereomeric mixture of R,S and
SS (1-carbobenzyloxy-2-pyrrolidinyl)oxirane, epimers
at the newly formed chiral centre, by treatment with
dimethylsulfoxonium methylide.12 As proved by 1H
NMR and HPLC analyses,13 subsequent oxirane ring
opening by dibenzylamine allowed only one diastereo-
1
identified, on the basis of H NMR and mass spectra,
as the tetrahydropyrroloxazolone derivative 9.21 This
latter product, which was not submitted to further syn-
thetic transformations, also showed unitary diastereo-
meric composition and its configuration was
tentatively established to be R and S at the oxygen-
and nitrogen-bound stereogenic carbons, respectively,
presuming that the bicyclic system was originated by
O
H
OH
b
O
a
N
N
N
Cbz
(S)-3
Cbz
(S)-4
OH
Cbz
OH
d
2'
2'
c
Bn
Bn
Bn
2
2
N
Cbz
N
N
N
Bn
(2R,2'S)-5
(2R,2'S)-6
O
OH
O
5
2'
f
2'
e
2
NH
N
N
NH2
(2R,2'S)-7
(5R,2'S)-1
Scheme 1. Synthesis of pyrrolidinyloxazolidinone. Reagents and
conditions: (a) pyridine–SO3 complex, TEA, DMSO 25 °C, 2 h, 89%;
(b) NaH, trimethylsulfoxonium iodide, DMSO, THF, 25 °C, 2 h, 40%;
(c) dibenzylamine, 2-propanol, MW irradiation (150 °C, 100 W),
15 min, 48%; (d) LiAlH4, THF, reflux, 2 h, 54%; (e) H2–Pd/C, MeOH,
96 h, 100%; (f) 1,10-carbonyldiimidazole, THF, reflux, 2 h, 58%.
Figure 1. ORTEP18 of (5R,20S)-1, showing the atom-numbering
scheme (ellipsoids are at the 40% probability).

5612
M. Pallavicini et al. / Bioorg. Med. Chem. Lett. 16 (2006) 5610–5615
(2S,20R)-2 was prepared by the same synthesis as its
BnO
enantiomer but starting from (R)-4, in turn obtained
from (R)-N-carbobenzyloxyprolinol as a mixture of epi-
mers at the epoxide chiral centre.24
8
O
1
(1R,8S)-9
OH
N
O
2'
O
For the reasons explained above, though the common
intermediate 4 was a mixture of two diastereomers, the
syntheses provided only the u stereoisomers of 1 and
2. The l stereoisomers, that is, the SS and RR forms,
were not obtained and the foreseen availability of all
the four stereoisomers of both 1 and 2 was not realized.
This notwithstanding, the biological evaluation was not
deferred considering that the study of the two synthe-
sized enantiomeric pairs could be already highly infor-
mative. In particular, we evaluated the affinity towards
the a4b2 and the a7 subtypes present in rat cortex mem-
branes by binding studies using, as ligands, [3H]-epibati-
dine and [125I]-aBungarotoxin, respectively.25 Nicotine
was included in the series for comparison. As shown
in Table 1, we found that the two pyrrolidinyloxazolid-
inones are virtually devoid of nicotinic affinity, whereas
the two pyrrolidinylbenzodioxanes exhibit a4b2 affinity.
In particular, (2R,20S)-2, which has the same S configu-
ration at the pyrrolidine stereocentre as (ꢀ)-nicotine,
shows a moderate submicromolar affinity for a4b2
nAChR and is fourfold more potent than its enantio-
mer. Both the isomers of 2 display a remarkable similar
selectivity over the a7 nAChR.
1
O
a
N
b
O
O
N
Cbz
O
OBn
(S)-4
(1S,2'S)-8
2'
2'
OSO2CH3
OSO2CH3
1
1
N
N
d
c
O
O
O
O
O
O
OBn
OH
(1S,2'S)-10
(1S,2'S)-11
2'
O
2'
O
2
e
N
2
N
O
O
O
O
(2R,2'S)-12
(2R,2'S)-2
Scheme 2. Synthesis of pyrrolidinylbenzodioxane. Reagents and con-
ditions: (a) 2-benzyloxyphenol, potassium carbonate, 2-propanol,
reflux, 24 h, 42% for
8 and 30% for 9; (b) Mesyl chloride,
triethylamine, dichloromethane, 25 °C, 1 h, 95%; (c) H2–Pd/C, EtOH,
24 h, 79%; (d) potassium carbonate, acetone, reflux, 24 h, 88%; (e)
LiAlH4, THF, reflux, 2 h, 82%.
Furthermore, the structural analogy between 2 and
a2-adrenergic receptor (a2-AR) ligands such as efaro-
xan, imiloxan, and, especially, idazoxan induced to
evaluate the binding of 2 also to a2-ARs. Interestingly,
we found that both (2R,20S)-2 and (2S,20R)-2 possess
the ability to compete with [3H]RS 79948-197 for rat
cortex a2-ARs displaying submicromolar affinities
(0.77 and 0.47 lM Ki, respectively; Kd 1.01 nM).26,27
the RS diastereomer of 4 through oxirane ring opening
and subsequent intramolecular displacement of Cbz
phenoxy group by oxirane oxygen. The benzyloxyphen-
oxyalcohol (1S,20S)-8 was mesylated and debenzylated.
Successive intramolecular nucleophilic substitution of
mesylate by the ortho-phenoxy moiety gave 1,4-dioxane
ring closure [(2R,20S)-12].22 The isopropoxycarbonyl
group was finally reduced to methyl obtaining
(2R,20S)-2[10-methylpyrrolidin-20-yl]-1,4-benzodioxane
[(2R,20S)-2] as an oily product, which was easily con-
verted into its hydrochloride.23 1H NMR analysis of vic-
inal coupling constants for (2R,20S)-2 showed the
relative anti disposition of the benzodioxane methine
proton not only to one of the two benzodioxane methy-
lene protons, as imposed by chair conformation of diox-
ane ring and by equatorial position of 2-pyrrolidinyl
substituent, but also to pyrrolidine methine proton. As
proved by conformational analysis, such a double anti
disposition of the three vicinal protons is favoured in
the diastereomer with opposite configurations at the
two asymmetric carbons, while the same arrangement
is virtually forbidden, due to the steric hindrance be-
tween N-methyl and benzodioxane methylene, in the
diastereomer with identical configurations at the two
stereocentres. Therefore, R configuration was assigned
to the benzodioxane C2 of the diastereomer of 2 pre-
pared from (S)-N-carbobenzyloxyprolinol. The absolute
configuration of the same stereogenic carbon in the syn-
thetic intermediates 8–12, which are all novel synthetic
intermediates, was consequently established by correla-
tion to (2R,20S)-2 as reported in Scheme 2.
O
N
idazoxan
O
HN
The lack of affinity of (5R,20S)-1 and (5S,20R)-1 for both
a4b2 and a7 nAChRs is likely to be explained consider-
ing the different positions of the pharmacophore ele-
ments in pyrrolidinyloxazolidinone than in nicotine
Table 1. Affinity of nicotine and compounds 1–2 for native receptor
subtypes, present in rat brain membranes, labelled by [3H]-epibatidine
and [125I]-aBungarotoxin
Compound
[3H]-Epi Ki (lM)
[
125I]-aBgtx Ki (lM)
(ꢀ)-Nicotine
0.004 (18)
62 (17)
80 (18)
0.234 (29)
117 (34)
338 (31)
12 (33)
(5R,20S)-1
(5S,20R)-1
(2R,20S)-2ÆHCl
(2S,20R)-2ÆHCl
Kd (nM)
0.62 (18)
2.57 (16)
0.020 (23)
45 (27)
1.06 (32)
The Kd and Ki values were derived from [3H]-epibatidine and [125I]-
aBungarotoxin saturation and two competition binding experiments
on rat brain membranes as described in Ref. 25. The curves were fitted
using a nonlinear least squares analysis program and the F test. The
numbers in brackets represent the %CV.

M. Pallavicini et al. / Bioorg. Med. Chem. Lett. 16 (2006) 5610–5615
5613
˚
˚
Table 2. Interatomic distances (A) for 1 stereoisomers, root mean square (rms) deviations (A) of the coordinates of the four selected matching atoms
of 1 from those of the corresponding atoms of reference compounds ((S)-nicotine and AR-R17779) and distances (A) between N of 1 and N+ of
+
˚
reference compounds
Compounda
N+–N
distance in 1
N+–CO
distance in 1
rms
N+–N+
distance 1-nic.
N+–N+distanceb
1-AR-R17779
nicotine
AR-R17779
(5R,20R)-1
(5S,20R)-1
(5R,20S)-1
(5S,20S)-1
4.76
4.59
4.68
4.50
5.21
4.89
5.04
5.81
0.45
0.80
0.90
0.99
0.35
0.47
0.33
0.35
0.21
0.29
0.31
0.28
0.30 (1.89)
0.78 (2.50)
0.72 (1.25)
0.57 (1.32)
a S configuration was invariably given to the protonated nitrogen.
b In brackets, the distance between the two N+ when the two oxazolidinone rings are allowed to exactly coincide.
Figure 3. Superposition of (2R,20S)-2 onto (S)-nicotine.
oxazolidinone and the only cationic nitrogen helps in
raising the distance between the N+ atoms, nevertheless
the positions of these latter are to be considered signifi-
cantly different and become quite divergent when the
oxazolidinone rings of 1 and AR-R17779 are allowed
to exactly coincide. Again, it is the RR isomer that gives
the best superposition of ammonium nitrogen atoms
(Fig. 2).
Figure 2. Superposition of (5R,20R)-1 onto (S)-nicotine and
AR-R17779.
and in AR-R17779. All the lowest energy conformers of
the stereoisomers of 1, which have in common a ffi60°
angle between the two ring planes, as indicated by con-
formational analysis28 and by X-ray analysis of
(5R,20S)-1, and N+–HBA (N+–N or N+–CO) distances
compatible with nicotinic pharmacophore models, are
superimposed on nicotine with unfavourably high rms
distance between the four selected matching atoms
(methyl carbon, cationic nitrogen, pyrrolidine stereocen-
tre, and carbonylic oxygen/pyridine nitrogen), with the
only exception of the unfortunately unavailable RR iso-
In the case of pyrrolidinylbenzodioxanes 2, contrary to
pyrrolidinyloxazolidinones 1, the lowest energy con-
formers28 of the two synthesized enantiomers, SR and
RS, are those showing the best superposition with nico-
tine with low rms distances (0.36 and 0.39, for (2R,20S)-
2 and (2S,20R)-2, respectively, versus 0.60 and 0.91 for
the other two streoisomers; matching atoms: methyl car-
bon, cationic nitrogen, pyrrolidine stereocentre, and
benzodioxane O(4)/pyridine nitrogen) (Table 3). This is
consistent with the finding of nicotinic affinity. In partic-
ular, (2R,20S)-2, which displays a submicromolar a4b2
affinity, shows, in addition to the minimum value of
rms distance, the maximum closeness of O(4) to pyridine
´
´
˚
+
+
˚
mer (0.45 A rms distance and 0.21 A N –N distance)
(Table 2 and Fig. 2). The superposition onto AR-
R17779 (fitting of the four heteroatoms) is better, as
shown by the generally lower rms values, but the N+–
N+ distances are sensibly higher (Table 2). Also
accounting that superposition through three atoms of
+
˚
nitrogen of nicotine and a 4.68 A N –O(4) distance.
Both these features candidate O(4) of (2R,20S)-2 to
˚
Table 3. Interatomic distances (A) for 2 stereoisomers, root mean square (rms) deviations (A) of the coordinates of the four selected matching atoms
˚
˚
of 2 from those of the corresponding atoms of the reference compound (S)-nicotine and distances (A) between O(4) of 2 and pyridine N of nicotine
Compounda
N+–O(4) distance in 2
rms
N(pyridine)–O(4) distance
(2R,20R)-2
(2S,20R)-2
(2R,20S)-2
(2S,20S)-2
4.95
3.42
4.68
4.34
0.60
0.39
0.36
0.91
0.71
0.76
0.30
0.78
a S configuration was invariably given to the protonated nitrogen.

5614
M. Pallavicini et al. / Bioorg. Med. Chem. Lett. 16 (2006) 5610–5615
9. Elliott, R. L.; Ryther, K. B.; David, J. A.; Raszkiewicz, J.
L.; Campbell, J. E.; Sullivan, J. P.; Garvey, D. S. Bioorg.
Med. Chem. Lett. 1995, 5, 991.
10. Walker, D. P.; Jacobsen, J. E.; Acker, B. A.; Groppi, V.
E.; Piotrowski, D. W. PCT WO 03/042210, 2003.
11. Dei, S.; Bellucci, C.; Buccioni, M.; Ferrarono, M.;
Gualtieri, F.; Guandalini, L.; Manetti, D.; Matucci, R.;
Romanelli, M. N.; Scapecchi, S.; Deodori, E. Bioorg. Med.
Chem. 2003, 11, 3153.
replace pyridine nitrogen as HBA in the interaction with
the binding site. As shown in Figure 3 for (2R,20S)-2, the
benzene ring, whose plane makes a ffi55° angle with the
pyrrolidine plane, cannot overlay the pyridine ring of
nicotine because condensed with dioxane; however,
though entirely exceeding the overlap volume, it does
not preclude the interaction.
Finally, we would like to draw attention to the a2-AR
affinity of pyrrolidinylbenzodioxanes. Recently, Abelson
and Ho¨glund29 have demonstrated that a2-AR ligands,
such as clonidine, rilmenidine, and efaroxan, possess
micromolar or submicromolar affinity for rat spinal
cord receptors labelled by [3H]epibatidine and affect
spinal ACh release. Their observation would provide
strong evidence to that nicotinic receptors are involved
in the increase or decrease of intraspinal ACh produced
by the a2-AR ligands and in the antinociception medi-
ated by these later. Proceeding in the opposite direction,
we evidenced an analogous double-faced profile of bind-
ing affinity for a novel compound, namely (2R,20S)-2,
whose structure resembles that of a2-AR ligand idazo-
xan. Our findings are consistent with Abelson’s results,
indicating the same interesting coexistence of affinities
for two quite different receptor systems, an unexpected
feature, which could be not undesirable.
12. Ashton, W. T.; Cantone, C. L.; Meurer, L. C.; Tolman, R.
L.; Greenlee, W. J.; Patchett, A. A.; Lynch, R. J.; Schorn,
T. W.; Strouse, J. F.; Siegl, P. K. S. J. Med. Chem. 1992,
35, 2103.
13. Compound (2R,20S)-5: isolated as an oil after repeated
chromatographic separations on silica gel (eluent 99:1
dichloromethane/methanol); 1H NMR (CDCl3, 300 MHz)
d 1.50–1.90 (m, 4H), 2.35–2.60 (m, 2H), 3.20–4.20 (m, 8H),
5.00–5.20 (m, 2H), 7.15–7.45 (m, 15H). Analytical HPLC:
LiChrospher 5 lm or lPorasil 10 lm column, 98:2 dichlo-
romethane/methanol, 2 ml/min.
14. Compound (2R,20S)-6: isolated as an oil by chromatog-
raphy on silica gel (eluent: from 95:5 to 92:8 dichloro-
1
methane/methanol); H NMR (CDCl3, 300 MHz) d 1.15–
1.30 (m, 1H), 1.55–1.75 (m, 3H), 2.38 (s, 3H), 2.25–2.41
(m, 1H), 2.46–2.61 (m, 3H), 3.17–3.23 (m, 1H), 3.51 (d,
J = 13.4 Hz, 2H), 3.72 (d, J = 13.4 Hz, 2H), 3.98 (t, 1H),
7.18–7.39 (m, 10H).
15. Compound (2R,20S)-7: isolated as an oil by concentration
of the reaction solution in methanol after removal of the
catalyst; 1H NMR (CDCl3) d 1.67–1.86 (m, 4H), 2.30–2.51
(m, 2H), 2.44 (s, 3H), 2.79 (d, J = 5.9 Hz, 2H), 3.15–3.18
(m, 1H), 3.62 (br s, 3H, disappears on exchange with
D2O), 3.82–3.91 (m, 1H).
Acknowledgments
16. Compound (5R,20S)-1: mp 143–144 °C (isolated by chro-
This research was financially supported by funds of the
Italian Ministry of University and Scientific Research
(FIRB Grant RBNE03FH5Y 002). We are grateful to
Prof. Riccardo Stradi (University of Milan) for the help-
matography on silica gel; eluent 80:20 toluene/methanol);
20
D
½aŠ ꢀ66.5 (c 0.4, MeOH); 1H NMR (CDCl3, 300 MHz) d
1.56–1.67 (m, 1H), 1.69–1.80 (m, 2H), 1.87–2.00 (m, 1H),
2.33 (pseudo q, J = 8.4 Hz, 1H), 2.42 (s, 3H), 2.62–2.68
(m, 1H), 3.05–3.11 (m, 1H), 3.51 (pseudo t, J = 8.4 Hz,
1H), 3.64 (pseudo t, J = 8.4 Hz, 1H), 4.66–4.73 (m, 1H),
5.80 (br s, 1H). Anal. Calcd for C8H14N2O2: C, 56.45; H,
8.29; N, 16.46. Found: C, 56.22; H, 8.36; N, 16.35.
1
ful discussion of the H NMR spectrum of (2R,20SR)-2
and to Dr. Marica Orioli (University of Milan) for mass
spectra of (1S,20S)-8 and (1R,8S)-9.
17. Crystallographic data of 1 were obtained using an Enraf
Nonius CAD-4 diffractometer (MoKa radiation) at room
temperature. The structures were solved by direct methods
[Altomare, A.; C. Burla, M.; Camalli, M.; Cascarano, G.;
Giacovazzo, C.; Gagliardi, A.; Polidori, G. J. Appl.
Crystallogr. 1994, 27, 435.] and the refinements were
carried out by full-matrix least-squares using SHELX-97
[Sheldrick, G. M.; SHELX-97, University of Go¨ttingen,
Germany]. The absolute configuration of oxazolidinone
asymmetric carbon was assigned on the basis of the known
chirality of the pyrrolidine asymmetric centre. CCDC-
616375 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/conts/retrieving.html (or from
the Cambridge Crystallographic Data Centre, 12, Union
Road, Cambridge CB21EZ, UK; fax: +44 1223 336 033;
or deposit@ccdc.cam.ac.uk).
References and notes
1. Jensen, A. A.; Frølund, B.; Liljefors, T.; Krogsgaard-
Larsen, P. J. Med. Chem. 2005, 48, 4705.
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651.
3. Nicolotti, O.; Pellegrini-Calace, M.; Altomare, C.; Carotti,
A.; Carrieri, A.; Sanz, F. Curr. Med. Chem. 2002, 9, 1.
4. Mazurov, A.; Hauser, T.; Miller, C. H. Curr. Med. Chem.
2006, 13, 1567.
5. 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.
6. Mullen, G.; Napier, J.; Balestra, M.; DeCory, T.; Hale,
G.; Macor, J.; Mack, R.; Loch, J., III; Wu, E.; Kover, A.;
Verhoest, P.; Sampognaro, A.; Phillips, E.; Zhu, Y.;
Murray, R.; Griffith, R.; Blosser, J.; Gurley, D.; Mac-
hulskis, A.; Zongrone, J.; Rosen, A.; Gordon, J. J. Med.
Chem. 2000, 43, 4045.
7. Tatsumi, R.; Fujio, M.; Satoh, H.; Katayama, J.; Takan-
ashi, S.; Hashimoto, K.; Tanaka, H. J. Med. Chem. 2005,
48, 2678.
8. Elliott, R. L.; Kopecka, H.; Gunn, D. E.; Lin, N. H.;
Garvey, D. S.; Ryther, K. B.; Holladay, M. W.; Anderson,
D. J.; Campbell, J. E. Bioorg. Med. Chem. Lett. 1996, 6,
2283.
18. Johnson, C. K.; ORTEP 11, Report ORNL-5138, Oak
Ridge National Laboratory, TN, 1976.
20
D
19. Compound (5S,20R)-1: ½aŠ +56.5 (c 0.4, MeOH); mp and
1H NMR identical to the enantiomer.
20. Compound (1S,20S)-8: isolated as an oil by chromatogra-
phy on silica gel (eluent: from 80:20 to 50:50 cyclohexane/
ethyl acetate); 1H NMR (CDCl3, 300 MHz) d 1.23 (d,
J = 6.2 Hz, 6H), 1.68–2.04 (m, 3H), 2.12–2.22 (m, 1H),
3.20–3.30 (m, 1H), 3.40–3.55 (m, 2H), 3.98–4.03 (m, 2H),
4.07–4.15 (m, 1H), 4.90 (sp, J = 6.2 Hz, 1H), 5.08 (s, 2H),

M. Pallavicini et al. / Bioorg. Med. Chem. Lett. 16 (2006) 5610–5615
5615
20
D
6.88–6.97 (m, 4H), 7.29–7.43 (m, 5H). MS m/z 422.3
(M+Na+); Analytical HPLC: lPorasil 10 lm column,
70:30 n-hexane/ethyl acetate, 1 ml/min.
193 °C (ethanol); ½aŠ +20.3 (c 2.0, EtOH); 1H NMR
(DMSO-d6, 300 MHz) d 1.82–2.19 (m, 4H), 2.88 (s, 3H),
3.05–3.16 (m, 1H), 3.50–3.65 (m, 2H), 4.11 (dd, J = 12.1,
5.5 Hz, 1H), 4.31 (d, J = 12.1 Hz, 1H), 4.70–4.79 (m, 1H),
6.85–6.96 (m, 4H), 11.12 (br s, 1H). Anal. Calcd for
C13H18ClNO2: C, 61.06; H, 7.09; Cl, 13.86; N, 5.48.
Found: C, 60.84; H, 7.01; Cl, 13.69; N, 5.43.
21. Compound (1R,8S)-9: isolated as a white solid by chro-
matography on silica gel (eluent: from 80:20 to 50:50
cyclohexane/ethyl acetate); mp 117–119 °C;1H NMR
(CDCl3, 300 MHz) d 1.49–1.65 (m, 1H), 1.70–1.90 (m,
2H), 1.95–2.10 (m, 1H), 3.11–3.19 (m, 1H), 3.52–3.61 (m,
1H), 3.91–3.99 (m, 1H), 4.15 (dd, 1H), 4.31 (dd, 1H), 5.00
(q, 1H), 5.08 (s, 2H), 6.87–6.96 (m, 4H), 7.27–7.45 (m,
5H). MS m/z 362.3 (M+Na+). Anal. Calcd for
C20H21NO4: C, 70.78; H, 6.24; N, 4.13. Found: C, 70.61;
H, 6.31; N, 4.06.
24. Compound (2S,20R)-2: 1H NMR spectrum identical to
20
D
(2R,20S)-2. (2S,20R)-2 Hydrochloride: ½aŠ ꢀ17.8 (c 2.0,
1
EtOH); mp and H NMR identical to the enantiomer.
25. Carbonelle, E.; Sparatore, F.; Canu-Boido, C.; Salvano,
C.; Baldani-Guerra, B.; Terstappen, G.; Zwart, R.;
Vijverberg, H.; Clementi, F.; Gotti, C. Eur. J. Pharmacol.
2003, 471, 85.
22. Compound (2R,20S)-12: obtained as an oil after removal
of acetone, treatment of the resultant residue with water
and ethyl acetate and concentration of the dried organic
phase; 1H NMR (CDCl3, 300 MHz): d 1.24 (d, J = 6.2 Hz,
6H), 1.83–1.96 (m, 1H), 2.02–2.20 (m, 3H), 3.33–3.65 (m,
2H), 3.92 (dd, J = 9.2, 12.2 Hz, 1H), 4.20–4.44 (m, 3H),
4.90 (br s, 1H), 6.76–6.90 (m, 4H). Analytical HPLC:
lPorasil 10 lm column, 70:30 n-hexane/ethyl acetate, 1 ml/
min.
26. Diop, L.; Dausse, J-P.; Meyer, P. J. Neurochem. 1983, 41,
710.
27. Uhlen, S.; Dambrova, M.; Nasman, J.; Schioth, H. B.; Gu,
Y.; Wikberg-Matsson, A.; Wikberg, J. E. Eur. J. Phar-
macol. 1998, 343, 93.
28. The stereoisomers of 1 and 2 were built in their protonated
form using the VEGA program (freely downloadable at
www.ddl.unimi.it). Their conformational analyses were
performed systematically rotating the interannular rotat-
able bond (yielding 360 conformers) and minimizing the
obtained conformers to avoid high-energy geometries
(conjugate gradients until rms = 0.01). The calculations
were carried out using Quanta/CHARMm package (MSI,
Burlington, MA) with the force-field CHARMm v22 and
the atomic charges calculated by Gasteiger’s method.
29. Abelson, K. S. P.; Ho¨glund, A. U. Basic Clin. Pharmacol.
Toxicol. 2004, 94, 153.
23. Compound (2R,20S)-2: isolated as an oil by chromatog-
raphy on silica gel (eluent: 92:8 dichloromethane/metha-
1
nol); H NMR (CDCl3, 300 MHz): d 1.66–1.86 (m, 3H),
1.87–2.03 (m, 1H), 2.28–2.49 (m, 1H), 2.52 (s, 3H), 2.65–
2.72 (m, 1H), 3.13–3.19 (m, 1H), 4.00 (dd, J= 11.0, 7.0 Hz,
1H), 4.22 (dt, J = 7.0, 2.2 Hz, 1H), 4.29 (dd, J = 11.0,
2.2 Hz, 1H), 6.78–6.91 (m, 4H). Analytical HPLC: lPora-
sil 10 lm column, 95:5:0.6 dichloromethane/methanol/
NH3, 1 ml/min. (2R,20S)-2 Hydrochloride: mp 190–