Regioselective preparation of functionalized isoxazoline derivatives as key intermediates for the synthesis of selective N-methyl-D-aspartate receptor antagonists

Article abstract of DOI:10.1055/s-0030-1258477

Two highly versatile isoxazoline derivatives, as key intermediates for the synthesis of differently functionalized subtype-selective N-methyl-d-aspartate receptor antagonists, were designed and synthesized. The synthetic strategy is based on an intramolec

Full text of DOI:10.1055/s-0030-1258477

PAPER
1255
Regioselective Preparation of Functionalized Isoxazoline Derivatives as Key
Intermediates for the Synthesis of Selective N-Methyl-D-aspartate Receptor
Antagonists
P
reparationof
F
u
unctionalize
c
dIsoxazoli
i
ne
D
eri
a
vatives Tamborini,*a Andrea Pinto,a Paola Conti,a Maddalena Gallanti,a Maria Celeste Iannuzzi,a Leonardo Lo Presti,b
Carlo De Michelia
a
Dipartimento di Scienze Farmaceutiche ‘Pietro Pratesi’, Università degli Studi di Milano, Via Mangiagalli 25, 20133 Milano, Italy
Fax +39(02)50319326; E-mail: lucia.tamborini@unimi.it
Dipartimento di Chimica Fisica ed Elettrochimica, Università degli Studi di Milano, Via Golgi 19, 20133 Milano, Italy
b
Received 10 December 2010; revised 10 February 2011
tain therapeutic utility without the side effects associated
Abstract: Two highly versatile isoxazoline derivatives, as key in-
with the non-selective examples.
termediates for the synthesis of differently functionalized subtype-
selective N-methyl-D-aspartate receptor antagonists, were designed
and synthesized. The synthetic strategy is based on an intramolecu-
lar nitrile oxide cycloaddition reaction which is a powerful method
capable of controlling both the regio- and stereochemistry of the
pericyclic reaction.
In a previous paper, we disclosed two new competitive
NMDA receptor antagonists, (–)-1 and (+)-2 (Figure 1),
which were characterized by a slight preference for
GluN2A and GluN2B versus GluN2C and GluN2D sub-
types.5 In order to improve their selectivity profile we
planned to increase their molecular complexity, according
to the general concept that new substituents could create
additional interactions with the binding site of one recep-
tor subunit, or determine a steric clash in another. A suit-
able position for further derivatization of our model
compounds is C-4 of the isoxazoline ring.
Key words: N-methyl-D-aspartate receptor antagonist, intramolec-
ular nitrile oxide cycloaddition, regioselective, isoxazoline, amino
acids
N-Methyl-D-aspartate (NMDA) receptors are subtypes of
the family of ligand-gated ion channels activated by en-
dogenous L-glutamate, the major excitatory neurotrans-
mitter of the central nervous system. NMDA receptors
mediate the slow component of glutamatergic excitatory
synaptic currents. The entry of calcium (Ca2+) ions into
neurons, via this receptor channel, triggers a cascade of
biochemical events that leads to physiologically relevant
processes such as synaptic plasticity and synaptogenesis
as well as pathological events, for example, excitotoxici-
ty.1,2
O
O
HO
HO
NH2
COOH
NH2
COOH
4
5
O
4
5
O
N
N
(–)-1
(+)-2
Figure 1 Structures of NMDA receptor antagonists (–)-1 and (+)-2
Thus, we identified compounds ( )-3a,b as useful key in-
termediates that, employing conventional synthetic steps,
could be easily transformed into a series of differently
functionalized amino acid derivatives (R = OH, OR, Nu,
alkyl, aryl, heteroaryl) (Scheme 1). Compounds endowed
with a promising biological profile could eventually be re-
solved into pure enantiomers by chiral high-performance
liquid chromatography (HPLC). As an alternative, enanti-
omerically pure target derivatives could be obtained di-
rectly after enzymatic resolution of racemic intermediates
( )-3a or ( )-3b, a strategy successfully applied to our
model compounds.5
NMDA receptor antagonists have been investigated for
many years as therapeutic agents for the treatment of neu-
rological disorders such as stroke, epilepsy, pain and
Parkinson’s disease. However, it has been discovered that
the majority of drug candidates cause adverse psychoto-
mimetic effects due to the blockade of normal neuronal
function.3
The majority of NMDA receptors are comprised of two
GluN1 and two GluN2 subunits. The GluN1 subunit, the
sole memberof the GluN1 subfamily, is essential for func-
tional NMDA receptor channelsand for their exit from the
endoplasmic reticulum, whereas four members of the
GluN2 subfamily (GluN2A–D) are the major determi-
nants of functional diversity and synaptic localization.4
Current efforts are devoted to the development of sub-
type-selective NMDA receptor antagonists that might re-
The synthesis of 4,5-disubstitued-4,5-dihydroisoxazole
derivatives can be achieved via 1,3-dipolar cycloaddition
of a nitrile oxide to the appropriate cis- or trans-1,2-disub-
stituted alkene.6 The advantage of such a methodology is
the complete translocation of the stereochemical features
of the reagents into the final products. For example, by re-
acting a nitrile oxide with a cis-1,2-disubstituted alkene,
only a cis-4,5-disubstituted 4,5-dihydroisoxazole is ob-
tained.7 However, in many cases this strategy lacks con-
trol of the regiochemistry, producing a mixture of the two
SYNTHESIS 2011, No. 8, pp 1255–1260
x
x
.
x
x
.2
0
1
1
Advanced online publication: 16.03.2011
DOI: 10.1055/s-0030-1258477; Art ID: Z52810SS
© Georg Thieme Verlag Stuttgart · New York

1256
L. Tamborini et al.
PAPER
R = OH, OR
R = H
R = Nu
OH
R
EtOOC
HOOC
NHBoc
COOEt
NH2
R = alkyl
COOH
N
N
O
O
R = aryl
( )-3a
( )-3b
R = heteroaryl
Scheme 1 Structures of key intermediates ( )-3a,b and conceivable functionalized amino acids
possible regioisomers.8 Moreover, in the present case, the silyl groups to yield amino alcohol ( )-7 which was im-
additional stereogenic center present in the a-amino acid mediately transformed into the tert-butylcarbamate ( )-8.
moiety would produce a complex mixture of four stereo- The hydroxy group was acylated with 2-bromoacetyl bro-
isomers, which would be difficult to separate by column mide in the presence of pyridine, and the resulting bromo
chromatography. Therefore, we devised a regioselective derivative ( )-9 was treated with sodium iodide in acetone
synthesis of key intermediates ( )-3a and ( )-3b based on to give the corresponding iodo derivative ( )-10
an intramolecular nitrile oxide cycloaddition (INOC) of (Scheme 1).
unsaturated oxime ( )-12. The INOC methodology is a
The iodo moiety was needed in order to accomplish effi-
powerful tool, characterized by excellent control of both
ciently its nucleophilic substitution by a nitrate anion to
regio- and stereochemistry, which has been widely em-
give ( )-11.13 Intermediate ( )-11 was then transformed
ployed to prepare complex medium- and large-sized cy-
into the corresponding oxime by reaction with hydroxyl-
clic compounds.9
amine hydrochloride in dimethyl sulfoxide (Scheme 2).14
The synthetic route to obtain key intermediates ( )-3a and The key step of this synthetic sequence is the one-pot
( )-3b started from commercially available (2Z)-but-2- chlorination and elimination of hydrogen chloride to gen-
ene-1,4-diol, which was transformed into the correspond- erate the nitrile oxide in situ which then underwent an
ing monosilyl ether 4 using tert-butyldimethylsilyl chlo- INOC reaction. This transformation was performed by
ride, following a literature procedure.10 Subsequently, the adding dropwise a 3.5% solution of sodium hypochlorite
second hydroxy group was converted into a mesylate us- (NaOCl) to a vigorously stirred solution of oxime ( )-12
ing standard chemistry, and then submitted to a nucleo- in dichloromethane (Scheme 3). The intramolecular cy-
philic displacement reaction using the preformed anion of cloaddition reaction was very fast and resulted in forma-
ethyl-N-(diphenylmethylene)glycinate,11,12 yielding inter-
mediate ( )-6 (Scheme 2). Treatment of ( )-6 with hydro-
chloric acid (1 M) induced cleavage of both the imino and
NHBoc
NHBoc
EtOOC
EtOOC
Ph
Ph
b
a
( )-10
N
EtOOC
NH2
92%
71%
EtOOC
OH
OMs
O
O
O
O
a
c
b
O2NO
N
79%
94%
79%
( )-11
( )-12
OH
OTBDMS
OTBDMS
OTBDMS
( )-6
OH
( )-7
5
O
4
4
5
6
O
d
NHBoc
COOEt
6a
3a
( )-3a
NHBoc
EtOOC
NHBoc
EtOOC
NHBoc
EtOOC
85%
N
1
3
O
2
c
e
d
f
( )-13a (29%)
90%
86%
86%
O
O
O
O
I
O
O
OH
d
NHBoc
COOEt
( )-3b
84%
N
O
Br
( )-8
( )-9
( )-10
( )-13b (22%)
Scheme 2 Reagents and conditions: (a) MsCl, Et3N, CH2Cl2; (b)
(C6H5)2C=NCH2CO2Et, LDA, THF, –78 °C; (c) 1 M HCl, Et2O; (d)
Boc2O, Et3N, CH2Cl2; (e) BrCH2COBr, py, CH2Cl2, 0 °C; (f) NaI,
acetone.
Scheme 3 Reagents and conditions: (a) AgNO3, MeCN; (b)
NH2OH·HCl, NaOAc, DMSO; (c) NaOCl (3.5%), CH2Cl2; (d)
K2CO3, EtOH.
Synthesis 2011, No. 8, 1255–1260 © Thieme Stuttgart · New York

PAPER
Preparation of Functionalized Isoxazoline Derivatives
1257
1H NMR (300 MHz, CDCl3): d = 0.10 (s, 6 H), 0.88 (s, 9 H), 3.00
(s, 3 H), 4.26–4.31 (m, 2 H), 4.86 (dd, J = 1.1, 6.8 Hz, 2 H), 5.56–
5.66 (m, 1 H), 5.83 (ddt, J = 1.1, 5.5, 5.5 Hz, 1 H).
13C NMR (75.5 MHz, CDCl3): d = –5.10, 18.48, 26.08, 38.33,
59.85, 65.82, 122.59, 136.14.
tion of the two diastereoisomers ( )-13a and ( )-13b,
which could be separated easily by column chromatogra-
phy.
Compound ( )-13a was successfully crystallized from
acetonitrile, and its relative stereochemistry secured by
single crystal X-ray analysis (Figure 2).
Anal. Calcd for C11H24O4SSi: C, 47.11; H, 8.63. Found: C, 47.38;
H, 8.89.
Ethyl (4Z)-6-[(tert-Butyldimethylsilyl)oxy]-2-[(diphenylmeth-
ylidene)amino]hex-4-enoate [( )-6]
n-BuLi (15 mL, 24.0 mmol, 1.6 M in hexanes) and DMPU (7 mL,
58.0 mmol) were added dropwise to a soln of diisopropylamine (3.4
mL, 24 mmol) in anhyd THF (37 mL) at –78 °C. A soln of ethyl-N-
(diphenylmethylene)glycinate (6.7 g, 26.4 mmol) in THF (36 mL)
was added dropwise and the resulting mixture stirred at –78 °C for
3 h. A soln of mesylate 5 (6.7 g, 24.0 mmol) in anhyd THF (15 mL)
was added and the mixture was stirred for a further 30 min at –78
°C and then warmed to r.t. After 30 min, a sat. soln of NH4Cl (8 mL)
was added and the mixture was extracted with Et2O (3 × 50 mL).
The combined organic layer was dried over anhyd Na2SO4 and con-
centrated under reduced pressure. The crude residue was purified by
column chromatography (cyclohexane–EtOAc, 98:2) to afford
( )-6.
Figure 2 ORTEP view of the asymmetric unit of compound ( )-
13a; thermal ellipsoids are drawn at the 25% probability level
Colorless oil; yield: 10.2 g (94%); Rf = 0.38 (cyclohexane–EtOAc,
9:1).
Finally, the desired products ( )-3a and ( )-3b were ob-
tained in good yield by ethanolysis of lactones ( )-13a
and ( )-13b.
1H NMR (300 MHz, CDCl3): d = –0.01 (s, 3 H), 0.01 (s, 3 H), 0.84
(s, 9 H), 1.22 (t, J = 7.0 Hz, 3 H), 2.59–2.66 (m, 2 H), 4.02–4.26 (m,
5 H), 5.22–5.33 (m, 1 H), 5.50–5.59 (m, 1 H), 7.10–7.15 (m, 2 H),
7.20–7.45 (m, 6 H), 7.57–7.63 (m, 2 H).
13C NMR (75.5 MHz, CDCl3): d = –4.91, 14.43, 18.57, 26.21,
32.11, 59.85, 61.20, 65.44, 125.75, 128.26, 128.72, 128.85, 129.08,
130.59, 132.83, 136.57, 139.65, 170.84, 171.96.
In conclusion, we have accomplished an efficient synthe-
sis of the key intermediates ( )-3a and ( )-3b by applying
an INOC synthetic strategy on the appropriate unsaturated
oxime ( )-12. Isoxazolines ( )-3a and ( )-3b represent
valuable synthetic tools and will be used in due course for
the preparation of a series of analogues of ( )-1 and ( )-2,
in order to evaluate the influence of the substituent at the
4-position of the isoxazoline nucleus on the potency and
NMDA subtype selectivity.
Anal. Calcd for C27H37NO3Si: C, 71.80; H, 8.26; N, 3.10. Found: C,
71.68; H, 8.08; N, 3.00.
Ethyl (4Z)-2-[(tert-Butoxycarbonyl)amino]-6-hydroxyhex-4-
enoate [( )-8]
An aq soln of HCl (1 M, 83 mL) was added to a stirred soln of ( )-
6 (10.2 g, 22.6 mmol) in Et2O (83 mL). The mixture was stirred at
r.t. for 6 h. The organic layer was separated and the aq layer was
made basic with K2SO4, and extracted with EtOAc (2 × 50 mL). The
combined organic layer was dried over Na2SO4 and concentrated
under reduced pressure to afford the free amine ( )-7 (3.1 g, 18.1
mmol) as a yellow oil; Rf = 0.15 (EtOAc–MeOH, 9:1).
All reagents were purchased from Sigma. TLC analyses were per-
formed on Fluka silica gel 60 F254 aluminum sheets; spots were
made visual by spraying with a dilute alkaline KMnO4 solution or
with ninhydrin. Column chromatography was carried out using
Nova Chimica silica gel LC-60A (60–200 mm). Melting points were
determined using a model B 540 Büchi apparatus and are uncorrect-
1
ed. H and 13C NMR spectra were recorded on a Varian Mercury
Et3N (3.8 mL, 27.1 mmol) and Boc2O (4.3 g, 19.9 mmol) were add-
ed to a soln of the amine ( )-7 (3.1 g, 18.1 mmol) in CH2Cl2 (70 mL)
at 0 °C. The mixture was stirred at r.t. for 12 h and then washed with
aq HCl (1 M, 2 × 20 mL), dried over Na2SO4 and concentrated under
reduced pressure. The residue was purified by column chromatog-
raphy (cyclohexane–EtOAc, 3:2) to afford alcohol ( )-8.
300 (300 MHz) spectrometer. Chemical shifts (d) are expressed in
ppm and coupling constants (J) in Hz. Elemental analyses were ob-
tained using a Thermo Finnigan Elemental Analyzer Flash EA
1112.
(2Z)-4-[(tert-Butyldimethylsilyl)oxy]but-2-en-1-yl Methane-
sulfonate (5)
Yellow oil; yield: 4.4 g (71% over two steps); Rf = 0.19 (cyclohex-
ane–EtOAc, 7:3).
1H NMR (300 MHz, CDCl3): d = 1.24 (t, J = 7.1 Hz, 3 H), 1.39 (s,
9 H), 2.40 (br s, 1 H), 2.44–2.53 (m, 1 H), 2.53–2.65 (m, 1 H), 4.09–
4.12 (m, 2 H), 4.16 (q, J = 7.1 Hz, 2 H), 4.26–4.34 (m, 1 H), 5.30
(br d, J = 7.5 Hz, 1 H), 5.36–5.48 (m, 1 H), 5.71–5.82 (m, 1 H).
A soln of alcohol 4 (7.1 g, 35.0 mmol) in CH2Cl2 (150 mL) was
cooled to 0 °C, and Et3N (5.3 mL, 38.5 mmol) and MsCl (3 mL, 38.5
mmol) were added dropwise. The mixture was stirred at r.t. for 3 h.
H2O was added (100 mL), and the aq layer was extracted with
CHCl3 (2 × 100 mL). The combined organic layer was dried over
anhyd Na2SO4 and the solvent was evaporated under reduced pres-
sure. The residue was purified by column chromatography (cyclo-
hexane–EtOAc, 4:1) to give the corresponding mesylate 5.
13C NMR (75.5 MHz, CDCl3): d = 14.37, 26.47, 30.48, 53.31,
58.18, 61.70, 80.19, 126.17, 132.88, 155.52, 172.25.
Anal. Calcd for C13H23NO5: C, 57.13; H, 8.48; N, 5.12. Found: C,
57.46; H, 8.59; N, 5.21.
Yellow oil; yield: 7.7 g (79%); Rf = 0.28 (cyclohexane–EtOAc,
9:1).
Synthesis 2011, No. 8, 1255–1260 © Thieme Stuttgart · New York

1258
L. Tamborini et al.
PAPER
Ethyl (4Z)-6-[(Bromoacetyl)oxy]-2-[(tert-butoxycarbonyl)ami-
no]hex-4-enoate [( )-9]
Ethyl (4Z)-2-[(tert-Butoxycarbonyl)amino]-6-[(2E)-2-(hydroxy-
imino)acetoxy]hex-4-enoate [( )-12]
Pyridine (2.2 mL, 27.0 mmol) and 2-bromoacetyl bromide (1.6 mL,
18.9 mmol) were added dropwise to an ice-cold soln of ( )-8 (3.7 g,
13.5 mmol) in CH2Cl2 (60 mL). The mixture was stirred at 0 °C for
30 min. Cold H2O (30 mL) was added and the organic layer was
separated. The aq layer was extracted with CH2Cl2 (2 × 20 mL) and
the combined organic layers dried over anhyd Na2SO4 and concen-
trated under reduced pressure. The residue was purified by column
chromatography (cyclohexane–EtOAc, 7:3) to afford bromide ( )-
9.
NH2OH·HCl (0.88 g, 12.7 mmol) and NaOAc (2.1 g, 25.5 mmol)
were added to a soln of compound ( )-11 (3.2 g, 8.5 mmol) in
DMSO (50 mL). The mixture was stirred for 2 h at r.t., and then
brine (25 mL) was added. The mixture was extracted with Et2O (3
× 15 mL), and the combined organic layer dried over anhyd Na2SO4
and concentrated under reduced pressure. The residue was purified
by column chromatography (cyclohexane–EtOAc, 1:1) to give
oxime ( )-12.
Yellow oil; yield: 2.1 g (71%); Rf = 0.23 (cyclohexane–EtOAc,
Yellow oil; yield: 4.6 g (86%); Rf = 0.27 (cyclohexane–EtOAc,
75:25).
85:15).
1H NMR (300 MHz, CDCl3): d = 1.25 (t, J = 7.1 Hz, 3 H), 1.40 (s,
9 H), 2.49–2.68 (m, 2 H), 4.15 (q, J = 7.1 Hz, 2 H), 4.34–4.40 (m, 1
H), 4.65–4.80 (m, 2 H), 5.30 (br d, J = 7.3 Hz, 1 H), 5.58–5.70 (m,
2 H), 7.48 (br s, 1 H), 7.50 (s, 1 H).
13C NMR (75.5 MHz, CDCl3): d = 14.34, 28.43, 30.76, 53.12,
60.89, 61.85, 80.38, 126.40, 129.84, 141.43, 155.70, 162.31,
172.23.
1H NMR (300 MHz, CDCl3): d = 1.25 (t, J = 7.1 Hz, 3 H), 1.40 (s,
9 H), 2.48–2.70 (m, 2 H), 3.82 (s, 2 H), 4.17 (q, J = 7.1 Hz, 2 H),
4.35 (dd, J = 7.1, 12.6 Hz, 1 H), 4.56–4.73 (m, 2 H), 5.18 (br d,
J = 7.1 Hz, 1 H), 5.55–5.75 (m, 2 H).
13C NMR (75.5 MHz, CDCl3): d = 14.40, 25.96, 28.50, 30.76,
53.06, 61.76, 61.77, 80.17, 126.50, 129.96, 155.40, 167.26, 171.91.
Anal. Calcd for C15H24BrNO6: C, 45.70; H, 6.14; N, 3.55. Found: C,
45.42; H, 5.89; N, 3.38.
Anal. Calcd for C15H24N2O7: C, 52.32; H, 7.02; N, 8.13. Found: C,
51.99; H, 6.89; N, 8.00.
Ethyl (4Z)-2-[(tert-Butoxycarbonyl)amino]-6-[(iodo-
acetyl)oxy]hex-4-enoate [( )-10]
Ethyl (2R*)-2-[(tert-Butoxycarbonyl)amino]-3-[(3S*,3aR*)-6-
oxo-3a,4-dihydro-3H,6H-furo[3,4-c]isoxazol-3-yl]propanoate
[( )-13a] and Ethyl (2S*)-2-[(tert-Butoxycarbonyl)amino]-3-
[(3S*,3aR*)-6-oxo-3a,4-dihydro-3H,6H-furo[3,4-c]isoxazol-3-
yl]propanoate [( )-13b]
A soln of NaOCl (3.5%, 7 mL) was added dropwise to a soln of the
oxime ( )-12 (2.1 g, 6.1 mmol) in CH2Cl2 (100 mL). The mixture
was stirred for 30 min at r.t., and the organic layer was separated,
washed with H2O (3 × 10 mL) and dried over anhyd Na2SO4. The
solvent was evaporated under vacuum and the crude material puri-
fied by flash chromatography (cyclohexane–EtOAc, 7:3) to give
compounds ( )-13a and ( )-13b as white solids in 51% combined
yield.
NaI (4.0 g, 27.0 mmol) was added to a stirred soln of compound ( )-
9 (4.3 g, 10.8 mmol) in acetone (60 mL). The mixture was stirred at
r.t. for 1 h. The resulting solid was removed by filtration and the fil-
trate was concentrated under vacuum, redissolved in EtOAc (120
mL) and washed with 10% aq Na2S2O3 soln (60 mL), dried over an-
hyd Na2SO4 and concentrated under reduced pressure. The residue
was purified by column chromatography (cyclohexane–EtOAc,
85:15) to afford iodide ( )-10.
Yellow oil; yield: 4.1 g (86%); Rf = 0.27 (cyclohexane–EtOAc,
85:15).
1H NMR (300 MHz, CDCl3): d = 1.26 (t, J = 7.1 Hz, 3 H), 1.40 (s,
9 H), 2.50–2.70 (m, 2 H), 3.67 (s, 2 H), 4.18 (q, J = 7.1 Hz, 2 H),
4.30–4.40 (m, 1 H), 4.56–4.70 (m, 2 H), 5.16 (br d, J = 7.0 Hz, 1 H),
5.55–5.74 (m, 2 H).
Compound ( )-13a
Yield: 0.60 g (29%); crystallized as colorless prisms from EtOAc–
hexane (1:1); mp 144–146 °C; Rf = 0.39 (cyclohexane–EtOAc,
1:1).
1H NMR (300 MHz, CDCl3): d = 1.30 (t, J = 7.1 Hz, 3 H), 1.44 (s,
9 H), 2.17 (dd, J = 6.9, 6.9 Hz, 2 H), 4.20 (dd, J = 5.8, 12.2 Hz, 1
H), 4.26 (q, J = 7.1 Hz, 2 H), 4.42–4.60 (m, 2 H), 4.65–4.75 (m, 1
H), 5.20 (ddd, J = 7.7, 7.7, 10.2 Hz, 1 H), 5.42 (br s, 1 H).
13C NMR (75.5 MHz, CDCl3): d = –5.44, 14.42, 28.53, 30.78,
53.10, 61.63, 61.76, 80.17, 126.57, 129.77, 155.42, 168.79, 171.93.
Anal. Calcd for C15H24INO6: C, 40.83; H, 5.48; N, 3.17. Found: C,
41.08; H, 5.62; N, 3.24.
13C NMR (75.5 MHz, CDCl3): d = 14.33, 28.50, 32.70, 50.59,
51.51, 62.56, 66.35, 80.79, 84.78, 155.50, 155.73, 158.40, 171.10.
Ethyl (4Z)-2-[(tert-Butoxycarbonyl)amino]-6-[(nitrooxy)acet-
oxy]hex-4-enoate [( )-11]
AgNO3 (3.1 g, 18.5 mmol) was added to a soln of iodide ( )-10 (4.1
g, 9.3 mmol) in MeCN (50 mL) in the dark under an N2 atm. The
mixture was stirred for 12 h at r.t. Et2O (80 mL) was added, and the
solid removed by filtration. The soln was concentrated under re-
duced pressure and the residue purified by column chromatography
(cyclohexane–EtOAc, 4:1) to afford compound ( )-11.
Anal. Calcd for C15H22N2O7: C, 52.63; H, 6.48; N, 8.18. Found: C,
52.68; H, 6.56; N, 7.93.
Compound ( )-13b
Yield: 0.46 g (22%); crystallized as colorless prisms from EtOAc–
hexane (1:1); mp 46–48 °C; Rf = 0.29 (cyclohexane–EtOAc, 1:1).
Yellow oil; yield: 3.2 g (92%); Rf = 0.20 (cyclohexane–EtOAc,
4:1).
1H NMR (300 MHz, CDCl3): d = 1.28 (t, J = 7.1 Hz, 3 H), 1.44 (s,
9 H), 2.50–2.74 (m, 2 H), 4.20 (q, J = 7.1 Hz, 2 H), 4.37 (dd, J = 6.6,
13.2 Hz, 1 H), 4.64–4.81 (m, 2 H), 4.92 (s, 2 H), 5.20 (br d, J = 7.4
Hz, 1 H), 5.60–5.76 (m, 2 H).
1H NMR (300 MHz, CDCl3): d = 1.26 (t, J = 7.1 Hz, 3 H), 1.40 (s,
9 H), 1.70–1.90 (m, 1 H), 2.26 (ddd, J = 4.1, 10.2, 14.3 Hz, 1 H),
4.12–4.28 (m, 2 H), 4.34 (ddd, J = 4.1, 9.1, 9.1 Hz, 1 H), 4.40–4.46
(m, 1 H), 4.48–4.64 (m, 2 H), 5.22 (ddd, J = 3.8, 10.2, 10.2 Hz, 1
H), 5.34 (br d, J = 9.1 Hz, 1H).
13C NMR (75.5 MHz, CDCl3): d = 14.36, 28.49, 33.14, 51.08,
51.21, 62.33, 66.20, 80.85, 85.21, 155.50, 155.70, 158.27, 171.56.
13C NMR (75.5 MHz, CDCl3): d = 14.34, 28.45, 30.73, 53.07,
61.52, 61.77, 67.25, 80.16, 126.12, 130.31, 155.38, 165.93, 171.85.
Anal. Calcd for C15H22N2O7: C, 52.63; H, 6.48; N, 8.18. Found: C,
52.89; H, 6.57; N, 7.89.
Anal. Calcd for C15H24N2O9: C, 47.87; H, 6.43; N, 7.44. Found: C,
48.00; H, 6.56; N, 7.60.
Synthesis 2011, No. 8, 1255–1260 © Thieme Stuttgart · New York

PAPER
Preparation of Functionalized Isoxazoline Derivatives
1259
Ethyl (4R*,5S*)-5-{(2R*)-3-Ethoxy-2-[(tert-butoxycarbon-
yl)amino]-3-oxopropyl}-4-(hydroxymethyl)-4,5-dihydro-
isoxazole-3-carboxylate [( )-3a]
Compound ( )-13a (600 mg, 1.8 mmol) was dissolved in EtOH (30
mL) and K2CO3 (248 mg, 1.8 mmol) was added. The mixture was
stirred at r.t. for 1 h. The solvent was evaporated, Et2O (30 mL) was
added and the solid was removed by filtration. The solvent was re-
moved under vacuum and the crude material was purified by col-
umn chromatography (cyclohexane–EtOAc, 1:1) to give isoxazole
( )-3a.
(l = 0.71073 Å) on a Bruker AXS Smart Apex diffractometer,
equipped with an Apex II CCD detector. Integration of the diffrac-
tion frames was performed using the APEX2 program suite [Bruker
(2010); APEX2, AXScale and SAINT; Bruker AXS Inc., Madison,
Wisconsin, USA], while the SADABS and XPREP programs were
employed to analyze, scale and merge multiple measurements with-
in the raw dataset.
Structure Solution and Refinement
The structure was solved by dual-space direct methods available
within the APEX2 program package. Some attempts were also
made to solve and refine the structure as P1, as we judged the E2 sta-
tistics as not conclusive to assess the presence of the inversion cen-
ter because of the large amount of individual very weak
measurements. These attempts, however, were all unsuccessful.
Careful analysis of the heavy atom positions of the symmetry-inde-
pendent molecules in P1 symmetry revealed that, in fact, inversion
centers existed in the unit cell. Hence the P1 space group was even-
tually chosen as being the most reasonable. Due to the small dimen-
sions of the crystals and limited scattering power of the substance,
the final precision of the refined thermal and geometrical parame-
ters was quite low. Such problems however, did not prevent us from
reliably assessing the relative orientation of the two stereogenic
centers. Final least-squares agreement factors (SHELX)15:
R(F) = 0.0927 (for 433 independent reflections with I>2s(I),
wR(F2) = 0.2034 (all independent data), goodness-of-fit = 1.062, re-
sidual Fourier density in the unit cell: Drmax = +0.24/–0.21 e/Å3. Se-
lected bond lengths (Å) and angles (°): N1–O3 1.39(2), N1–C3
1.29(3), O3–C5 1.49(2), N2–C7 1.47(2), N2–C11 1.38(3), C3–N1–
O3 106(2), N1–O3–C5 110(2), C7–N2–C11 117(2).
Colorless oil; yield: 594 mg (85%); Rf = 0.35 (cyclohexane–EtOAc,
1:1).
1H NMR (300 MHz, CDCl3): d = 1.26 (t, J = 7.2 Hz, 3 H), 1.35 (t,
J = 7.2 Hz, 3 H), 1.40 (s, 9 H), 2.27 (ddd, J = 6.9, 9.2, 15.0 Hz, 1 H),
2.38–2.50 (m, 1 H), 2.84 (br s, 1 H), 3.58 (ddd, J = 4.7, 4.7, 9.2 Hz,
1 H), 3.80–3.96 (m, 2 H), 4.20 (q, J = 7.2 Hz, 2 H), 4.32 (q, J = 7.2
Hz, 2 H), 4.39–4.47 (m, 1 H), 4.82 (ddd, J = 4.7, 9.2, 9.2 Hz, 1 H),
5.42 (br d, J = 6.9 Hz, 1 H).
13C NMR (75.5 MHz, CDCl3): d = 14.28, 28.50, 29.88, 31.44,
51.19, 51.77, 58.33, 62.07, 62.55, 80.39, 83.04, 153.15, 155.70,
161.58, 171.94.
Anal. Calcd for C17H28N2O8: C, 52.57; H, 7.27; N, 7.21. Found: C,
52.62; H, 7.32; N, 7.01.
Ethyl (4R*,5S*)-5-{(2S*)-3-Ethoxy-2-[(tert-butoxycarbon-
yl)amino]-3-oxopropyl}-4-(hydroxymethyl)-4,5-dihydro-
isoxazole-3-carboxylate [( )-3b]
Compound ( )-13b (460 mg, 1.3 mmol) was dissolved in EtOH (15
mL) and K2CO3 (180 mg, 1.3 mmol) was added. The mixture was
stirred at r.t. for 1 h. The solvent was evaporated, Et2O (15 mL) was
added and the solid was removed by filtration. The solvent was re-
moved under vacuum and the crude material was purified by col-
umn chromatography (cyclohexane–EtOAc, 1:1) to afford
isoxazole ( )-3b.
CCDC 802274 contains the supplementary crystallographic data for
the crystal structure in this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
Colorless oil; yield: 424 mg (84%); Rf = 0.35 (cyclohexane–EtOAc,
1:1).
Acknowledgment
Financial support from the Italian Ministry of Education (MIUR)
and University of Milan are gratefully acknowledged.
1H NMR (300 MHz, CDCl3): d = 1.26 (t, J = 7.0 Hz, 3 H), 1.36 (t,
J = 7.0 Hz, 3 H), 1.41 (s, 9 H), 2.22–2.40 (m, 2 H), 3.06 (br s, 1 H),
3.58 (ddd, J = 5.3, 5.3, 9.9 Hz, 1 H), 3.80–3.95 (m, 2 H), 4.18 (q,
J = 7.0 Hz, 2 H), 4.34 (q, J = 7.0 Hz, 2 H), 4.42–4.50 (m, 1 H), 4.82
(ddd, J = 5.0, 9.9, 9.9 Hz, 1 H), 5.50 (br d, J = 8.5 Hz, 1 H).
References
(1) Dingledine, R.; Borges, K.; Bowie, D.; Traynelis, S. F.
Pharmacol. Rev. 1999, 51, 7.
13C NMR (75.5 MHz, CDCl3): d = 14.27, 28.50, 30.53, 31.22,
51.21, 51.88, 58.10, 61.95, 62.62, 80.44, 83.45, 153.18, 155.82,
161.48, 172.17.
(2) Carroll, R. C.; Zukin, R. S. Trends Neurosci. 2002, 25, 571.
(3) Chen, H.-S. V.; Lipton, S. A. J. Neurochem. 2006, 97, 1611.
(4) Paoletti, P.; Neyton, J. Curr. Opin. Pharmacol. 2007, 7, 39.
(5) Conti, P.; De Amici, M.; Grazioso, G.; Roda, G.; Pinto, A.;
Hansen, K. B.; Nielsen, B.; Madsen, U.; Bräuner-Osborne,
H.; Egebjerg, J.; Vestri, V.; Pellegrini-Giampietro, D. E.;
Sibille, P.; Acher, F. C.; De Micheli, C. J. Med. Chem. 2005,
48, 6315.
Anal. Calcd for C17H28N2O8: C, 52.57; H, 7.27; N, 7.21. Found: C,
52.63; H, 7.33; N, 6.99.
Ethyl (2R*)-2-[(tert-Butoxycarbonyl)amino]-3-[(3S*,3aR*)-6-
oxo-3a,4-dihydro-3H,6H-furo[3,4-c]isoxazol-3-yl]propanoate
[( )-13a]; X-ray Analysis
Crystals of the title compound were grown overnight by slow evap-
oration at 4 °C from a drop of MeCN covered by an inert oil. After
testing several specimens, a very thin needle (0.225 × 0.025 × 0.025
mm3) was eventually judged of suitable quality for single-crystal X-
ray diffraction analysis.
(6) (a) Karlsson, S.; Högberg, H.-E. Org. Prep. Proced. Int.
2001, 33, 103. (b) Easton, C. J.; Hughes, C. M. M.; Savage,
G. P.; Simpson, G. W. Adv. Heterocycl. Chem. 1994, 60,
261. (c) Gallos, J. K.; Koumbis, A. E. Curr. Org. Chem.
2003, 7, 397.
(7) (a) Bianchi, G.; Gandolfi, R. In 1,3-Dipolar Cycloaddition
Chemistry; Padwa, A., Ed.; John Wiley & Sons: New York,
1984, 477. (b) Yoshida, Y.; Ukaji, Y.; Fujinami, S.;
Inomata, K. Chem. Lett. 1998, 10, 1023. (c) Askri, M.;
Jgham, N.; Rammah, M.; Ciamala, K.; Monnier-Jobe, K.;
Vebrel, J. Heterocycles 2007, 71, 289.
Crystal data: C15H22N2O7, formula weight = 342.3, space group: P1
(No 2), unit cell (Å, deg, Å3): a = 6.033 (4), b = 12.131 (8),
c = 13.610 (8), a = 114.76 (2), b = 99.73 (2), g = 94.09 (2),
V = 880.2 (10), Z = 2, Dc = 1.292 g cm–3, F000 = 364, m = 0.103 mm–1.
Data Collection and Reduction
(8) (a) Caramella, P.; Grunanger, P. Nitrile Oxides and Imines,
In 1,3-Dipolar Cycloaddition Chemistry, Vol. 1; Padwa, A.,
A total of 5692 reflections within an entire sphere [(sinq/
l)max = 0.36 Å–1], corresponding to 674 independent data, were col-
lected using a graphite-monochromated MoKa radiation source
Synthesis 2011, No. 8, 1255–1260 © Thieme Stuttgart · New York

1260
L. Tamborini et al.
PAPER
Synthesis 2006, 3815. (j) Shing, T. K. M.; Zhong, Y.-L.
Synlett 2006, 1205. (k) Park, K.-H.; Marshall, W. J.
Tetrahedron Lett. 2004, 45, 4931. (l) Akritopoulou-Zanze,
I.; Gracias, V.; Moore, J. D.; Djuric, S. W. Tetrahedron Lett.
2004, 45, 3421. (m) Yip, C.; Handerson, S.; Tranmer, G. K.;
Tam, W. J. Org. Chem. 2001, 66, 276. (n) Yip, C.;
Handerson, S.; Jordan, R.; Tam, W. Org. Lett. 1999, 1, 791.
(10) Mauleon, P.; Krinsky, J. L.; Toste, F. D. J. Am. Chem. Soc.
2009, 131, 4513.
Ed.; John Wiley & Sons: New York, 1984, Chap. 3, 291–
392. (b) Conti, P.; De Amici, M.; Pinto, A.; Tamborini, L.;
Grazioso, G.; Frølund, B.; Nielsen, B.; Thomsen, C.; Ebert,
B.; De Micheli, C. Eur. J. Org. Chem. 2006, 5533.
(c) Faita, G.; Mella, M.; Mortoni, A.; Paio, A.; Quadrelli, P.;
Seneci, P. Eur. J. Org. Chem. 2002, 1175. (d) Trogu, E.;
De Sarlo, F.; Machetti, F. Chem. Eur. J. 2009, 15, 7940.
(9) (a) Ramachandiran, K.; Karthikeyan, K.; Muralidharan, D.;
Perumal, P. T. Tetrahedron Lett. 2010, 51, 3006.
(b) Chapelon, A.-S.; Giorgi, M.; Ollivier, C.; Santelli, M.
Synth. Commun. 2010, 40, 87. (c) Shing, T. K. M.; So, K.
H.; Kwok, W. S. Org. Lett. 2009, 11, 5070. (d) Kamimura,
A.; Yoshida, T.; Uno, H. Tetrahedron 2008, 64, 11081.
(e) Vaidya, V. V.; Wankhede, K. S.; Salunkhe, M. M.;
Trivedi, G. K. Synth. Commun. 2008, 38, 2392. (f) Nair, V.;
Suja, T. D. Tetrahedron 2007, 63, 12247. (g) Pellissier, H.
Tetrahedron 2007, 63, 3235. (h) Shing, T. K. M.; Wong, W.
F.; Cheng, H. M.; Kwok, W. S.; So, K. H. Org. Lett. 2007,
9, 753. (i) Xu, J.; Wang, J.; Ellis, E. D.; Hamme, A. T. II
(11) O’Donnell, M. J.; Polt, R. L. J. Org. Chem. 1982, 47, 2663.
(12) Bachi, M. D.; Balanov, A.; Bar-Ner, N. J. Org. Chem. 1994,
59, 7752.
(13) Tschaen, D.; Whittle, R.; Weinreb, S. J. Org. Chem. 1986,
51, 2604.
(14) Hassner, A.; Murthy, K. S. K.; Padwa, A.; Chiacchio, U.;
Dean, D. C.; Schoffstall, A. M. J. Org. Chem. 1989, 54,
5277.
(15) Sheldrick, G. M. Acta Crystallogr. A 2008, 64, 112.
Synthesis 2011, No. 8, 1255–1260 © Thieme Stuttgart · New York