Regioselective lithiation and functionalization of 3-(benzyloxy)isothiazole: Application to the synthesis of thioibotenic acid

Article abstract of DOI:10.1021/jo0162134

Direct functionalization of the 3-oxygenated isothiazole heteroaromatic parental system has not yet been reported in the literature. Here, we report the first regioselective lithiation of the 5-position of 3-(benzyloxy)isothiazole (4) using LDA in diethyl ether. The versatility of the methodology was explored by quenching with a variety of electrophiles to give the desired products 7a,b,d-g in 54-68% yield. Only benzoylation aiming at the synthesis of 7c was unsuccessful. Furthermore, a highly convergent synthesis of thioibotenic acid (1), the sulfur analogue of the neurotoxic natural product ibotenic acid, was carried out.

Full text of DOI:10.1021/jo0162134

J . Org. Chem. 2002, 67, 2375-2377
2375
Sch em e 1
Regioselective Lith ia tion a n d
F u n ction a liza tion of
3-(Ben zyloxy)isoth ia zole: Ap p lica tion to
th e Syn th esis of Th ioiboten ic Acid
Lennart Bunch, Povl Krogsgaard-Larsen, and
Ulf Madsen*
Department of Medicinal Chemistry, The Royal Danish
School of Pharmacy, Universitetsparken 2,
DK-2100 Copenhagen, Denmark
Sch em e 2
um@dfh.dk
Received October 18, 2001
Abstr a ct: Direct functionalization of the 3-oxygenated
isothiazole heteroaromatic parental system has not yet been
reported in the literature. Here, we report the first regio-
selective lithiation of the 5-position of 3-(benzyloxy)isothia-
zole (4) using LDA in diethyl ether. The versatility of the
methodology was explored by quenching with a variety of
electrophiles to give the desired products 7a ,b,d -g in 54-
68% yield. Only benzoylation aiming at the synthesis of 7c
was unsuccessful. Furthermore, a highly convergent syn-
thesis of thioibotenic acid (1), the sulfur analogue of the
neurotoxic natural product ibotenic acid, was carried out.
Sch em e 3
The synthesis of 3-hydroxyisothiazole can be achieved
in two steps from commercially available 3,3′-dithiane-
dipropionic acid in moderate yield.8 Benzylation provides
an easily separable mixture of the O- and N-benzylated
derivatives 4 and 5, respectively (90% yield, ratio 2:1),
which is in agreement with a previous study9 (Scheme
3). Selective lithiation of the 5-position of isothiazole has
been accomplished using n-BuLi in diethyl ether.10
However, this strategy failed completely in the case of
4, presumably due to competing nucleophilic addition of
n-BuLi to the hetereoaromatic ring. It was therefore
decided to explore the use of more sterically hindered
lithium amide bases. Initially, the influence of solvent
on stability of the 5-lithio anion of 4 generated from LDA
at -78 °C was investigated. In diethyl ether, immediate
precipitation and high stability of this salt was evident,
while in THF no precipitation was observed, and instead,
decomposition proved to be progressing over time. Whereas
only one pathway for decomposition of 5-lithio isothiazole
seems reasonable, two plausible pathways can be pro-
posed for the decomposition of 5-lithio 3-(benzyloxy)-
isothiazole (Scheme 4). Only benzyl alcohol was identified
as a byproduct when the reaction was run in diethyl ether
or in THF. No trace of benzyloxynitrile or the acid-
catalyzed hydrolysis product carbamic acid benzyl es-
ter11,12 was detected in the crude product mixture. Thus,
we suggest that path b is the preferred pathway of
decomposition.
Bioisosteric modification of biological active molecules
is a well-described and often used strategy in medicinal
chemistry research. The 3-hydroxyisoxazole1 moiety is an
extensively used carboxylic acid bioisostere.2,3 It is also
found in the naturally occurring amino acid ibotenic
acid,4,5 a widely used neurotoxin and pharmacological tool
for studies of glutamic acid receptors (Scheme 1). Sub-
stitution of sulfur for the ring oxygen provides the
parental system 3-hydroxyisothiazole, which is less acidic
(pKa ∼7) and more lipophilic compared with the 3-hy-
droxyisoxazole group (pKa ∼5).6 On this basis, we were
interested in synthesizing thioibotenic acid (1), the sulfur
analogue of ibotenic acid (Scheme 1).
A retro-synthetic analysis of thioibotenic acid (1)
suggests the formation of the sp2-sp3 carbon-carbon
bond as the key step (Scheme 2). A possible strategy
would thus be selective generation of the 5-anion of
suitably protected 3, followed by the addition to an imine
2.7 However, in contrast to the extensive research in the
field of direct functionalization of heteroaromatic rings,
no methodology has so far been reported for the func-
tionalization of the 3-oxygenated isothiazole parental
system.
(1) Sørensen, U. S.; Krogsgaard-Larsen, P. Org. Prep. Proced. Int.
2001, 33, 515-564.
We then turned to investigate the influence of the
steric bulk and strength of the lithium amide base and
reaction time on the percentage of deuterium incorpora-
tion. However, only partial incorporation could be achieved
using lithium ethylisopropyl amide (LEIA) or LDA, which
(2) Krogsgaard-Larsen P.; Frølund, B.; Kristiansen, U.; Frydenvang,
K.; Ebert, B. Eur. J . Pharm. Sci. 1997, 5, 355-384.
(3) Madsen, U.; Sløk, F. A.; J ohansen, T. N.; Ebert, B.; Krogsgaard-
Larsen, P. Curr. Top. Med. Chem. 1997, 2, 1-14.
(4) Eugster, C. H. In Fortschritte der Chemie Organischer Natur-
stoffe XXVII; Zechmeister, L., Ed.; Springer-Verlag, New York, 1969;
p 261.
(5) Krogsgaard-Larsen, P.; Honore´, T.; Hansen, J . J .; Curtis, D. R.;
Lodge, D. Nature 1980, 284, 64-66.
(6) Frydenvang, K.; Matzen, L.; Norrby, P.-O.; Sløk, F. A.; Liljefors,
T.; Krogsgaard-Larsen, P.; J aroszewski, W. J . Chem. Soc., Perkin
Trans. 2 1997, 1783-1791.
(8) Beeley, N. R. A.; Harwood: L. M.; Hedger, P. C. J . Chem. Soc.,
Perkin Trans. 1 1994, 2245-2251 and references therein.
(9) Chan, A. W. K.; Crow, W. D.; Gosney, I. Tetrahedron 1970, 26,
2497-2506.
(7) Recently, the synthesis of several R-aryl and R-heteroaryl glycine
derivatives has been reported based on the addition of lithiated aryl
and heteroaryl moieties to imine 6. Cal´ı, P.; Begtrup, M. Synthesis
2002, 63-66.
(10) Iddon, B. Heterocycles 1995, 41, 533-593.
(11) Grigat, E.; Puetter, R. Chem. Ber. 1964, 97, 3018-3021.
(12) Buckley, N.; Oppenheimer, N. J . J . Org. Chem. 1997, 62, 540-
551.
10.1021/jo0162134 CCC: $22.00 © 2002 American Chemical Society
Published on Web 03/03/2002

2376 J . Org. Chem., Vol. 67, No. 7, 2002
Notes
Ta ble 2. Rea ction of 4 w ith Va r iou s Electr op h iles Usin g
th e Gen er a l P r oced u r e
Sch em e 4
Ta ble 1. In vestiga tion of Effects of Lith iu m Am id e Ba se
a n d Tim e on th e P er cen ta ge of Deu ter iu m
In cor p or a tion a a n d Degr ee of Decom p osition b
t ) 15 min
7a (%)
t ) 45 min
7a (%)
base
4 (%)
BnOH
4 (%)
BnOH
LDA
LEIA
LTMP
38
67
86
57
28
9
5
<5
5
38
40
32
57
5
0
55
60c
14
a
The percentage of deuterium incorporation was determined
from 1H NMR. The percentage of decomposition was determined
b
from the amount of BnOH formed. c Extending the reaction time
to 2 h, a further increase in deuterium incorporation was not
observed.
Sch em e 5a
could be due to the setup of an equilibrium between the
dialkylamine and the 5-lithiated heterocycle. Surpris-
ingly, the use of the more basic and more sterically
hindered amide base, lithium 2,2,6,6-tetramethylpiperi-
dine (LTMP), resulted in extensive decomposition (Table
1). Further studies with LDA as the base were therefore
carried out. Performing the reaction with inverse or
forward addition of the LDA base did not influence the
reaction outcome significantly. When the reaction was
run at half the concentration (0.05 M), a slight increase
in yield (67%) was observed, whereas the use of 2.2 equiv
and 4.0 equiv of LDA gave 76% and 86% incorporation
of deuterium, respectively. However, a large excess of
LDA may pose a problem on the following reaction with
some electrophiles. Also, addition of a cosolvent, N,N-
dimethylpropyleneurea (DMPU, 30 Vol %), was tried but
resulted only in extensive decomposition.
To investigate the generality of this new methodology
for introduction of substituents in the 5-position of
3-(benzyloxy)isothiazole (4), a series of electrophiles were
chosen. In the presence of 1.1 equiv of LDA in a 0.1 M
diethyl ether solution (Table 2), the desired products
7b,d -f could be isolated in 54-68% yields.13 However,
attempted benzoylation using benzoyl chloride or benzoyl
cyanide to give 7c proved to be unsuccessful and resulted
in a complex reaction mixture.
a
Reagents and reaction conditions: (a) 1.1 equiv of LDA, -78
°C, Et2O, then 1.1 equiv of imine 6 (56%); (b) LiOH, H2O/THF, rt,
4 h (76%); (c) 10% HBr/AcOH, rt, 2 h (60%).
next step was to apply it to the synthesis of thioibotenic
acid (1) (Scheme 5). The 5-lithio anion of 4 was generated
using 1.1 equiv of LDA and subsequently reacted with
imine 6 to give the desired product 7g in 56% yield.14
The following hydrolysis of the ester functionalities also
mediated mono-decarboxylation upon the acidic workup
to give crude 8 in 76% yield. Cleavage of the benzyl ether
and simultaneous removal of the BOC group were
accomplished in HBr/AcOH to give 1 as the zwitterion
in 60% yield after treatment with Et2O and H2O. Pre-
liminary electrophysiological studies show that thio-
ibotenic acid (1) is a moderately potent agonist (EC50
)
With this new methodology for functionalization of the
3-oxygenated isothiazole parental system in hand, our
42 ( 2 µM) at the glutamic acid receptors sensitive to
the N-methyl-D-aspartic acid (NMDA) receptor antago-
nist 4-(3-phosphonopropyl)-2-piperazinylcarboxylic acid
(CPP).
(13) All yields given are isolated yields after chromatography. In
all cases, a maximum of 5% decomposition was observed. To improve
the efficiency of the reaction, recovery of starting material 4 could be
achieved. Also, lowering the concentration of the reaction components
and use of larger excess of LDA may be advantageous, as observed in
the deuterium experiments.
(14) Purification of 7g proved to be very difficult presumably due
to the presence of impurities originating from polymerization of imine
6.

Notes
In conclusion, we have developed a methodology for
J . Org. Chem., Vol. 67, No. 7, 2002 2377
3-Ben zyloxy-5-(m eth oxyca r bon yl)isoth ia zole (7d ). The
reaction was carried out as described in the general procedure.
The crude product was purified using flash chromatography
(heptane/EtOAc 9:1, Rf ) 0.25) to give 7d as a clear oil (73 mg,
56%): 1H NMR δ 7.50-7.30 (m, 5H), 7.16 (s, 1H), 5.41 (s, 2H),
3.91 (s, 3H); 13C NMR δ 168.59, 160.53, 156.36, 136.13, 128.69,
regioselective generation of the 5-lithio anion of the
3-oxygenated isothiazole heterocycle. To our knowledge,
this is the first report on direct functionalization on this
parental system. The 5-lithio anion of 4 showed high
stability in diethyl ether, whereas in THF decomposition
was progressing over time. A mechanism of decomposi-
tion was suggested based on identification of a degrada-
tion product. We were able to demonstrate the capture
of the 5-lithio anion of 4 with a variety of electrophiles,
which gave the desired products 7a ,b,d -g in 54-68%
isolated yield, whereas attempted benzoylation to give
7c was unsuccessful. The intermediate 7g was depro-
tected in two steps to give thioibotenic acid (1), which
will undergo further pharmacological characterization as
a glutamic acid receptor ligand in our laboratory, and
results will be reported elsewhere.
128.40, 128.22, 115.98, 70.61, 52.66. Anal. Calcd for C12H11
-
NO3S: C, 57.82; H, 4.45; N, 5.62. Found: C, 58.20; H, 4.49; N,
5.64.
3-Ben zyloxy-5-for m ylisot h ia zole (7e). The reaction was
carried out as described in the general procedure. The crude
product was purified on flash chromatography (heptane/EtOAc
9:1, Rf ) 0.2) to give 7e as a light yellow solid (62 mg, 54%): 1H
NMR δ 10.00 (s, 1H), 7.50-7.35 (m, 5H), 7.14 (s, 1H), 5.43 (s,
2H); 13C NMR δ 181.84, 169.06, 163.71, 135.93, 128.70, 128.53,
128.33, 117.09, 70.92. Anal. Calcd for C11H9NO2S: C, 60.26; H,
4.14; N, 6.39. Found: C, 60.03; H, 3.97; N, 6.32.
3-Ben zyloxy-5-(1-h ydr oxycycloh exyl)isoth iazole (7f). The
reaction was carried out as described in the general procedure.
The crude product was purified using flash chromatography
(heptane/EtOAc 4:1, Rf ) 0.25) to give 7f as a clear oil (102 mg,
68%): 1H NMR δ 7.47-7.30 (m, 5H), 6.48 (s, 1H), 5.36 (s, 2H),
2.19 (s, 1H), 1.98-1.58 (m, 10H); 13C NMR δ 179.62, 168.59,
136.65, 128.58, 128.18, 107.44, 72.59, 69.94, 39.34, 24.94, 21.63.
Anal. Calcd for C16H19NO2S: C, 66.40; H, 6.62; N, 4.84. Found:
C, 66.28; H, 6.75; N, 4.65.
Exp er im en ta l Section
All reagents were obtained from commercial suppliers and
used without further purification. THF was distilled from
sodium/benzophenone and Et2O was dried over sodium. NMR
(300 MHz) spectra were recorded in CDCl3 using CHCl3 as
reference. Merck Kieselgel (35-70 mesh) was used for flash
chromatography.
Eth yl 2-ter t-Bu tyloxyca r bon yla m in o-2-eth oxyca r bon yl-
2-(3-ben zyloxy-5-isoth ia zolyl)a ceta te (7g). The reaction was
carried out as described in the general procedure using imine 6
as the electrophile.7 Isolation by flash chromatography (heptane/
EtOAc 4:1, Rf ) 0.25) gave crude 7g as a clear oil (135 mg, 56%),
which was not purified further: 1H NMR δ 7.47-7.30 (m, 5H),
3-(Ben zyloxy)isoth ia zole (4). To a solution of 3-hydroxy-
isothiazole8 (2.02 g, 20 mmol) in DMF (20 mL) at 0 °C was added
K2CO3 (5.53 g, 40 mmol) followed by benzyl bromide (2.74 mL,
23 mmol). The reaction mixture was allowed to stir for 48 h at
rt, diluted with H2O (200 mL), and extracted with Et2O. The
organic layer was washed with brine, dried (Na2SO4), and
evaporated to give a crude product (2:1 O/N mixture based on
1H NMR) that was purified by short column flash chromatog-
raphy (heptane/EtOAc 4:1) followed by submission to high
vacuum for 72 h to give 4 as a clear oil (2.29 g, 60%): 1H NMR
δ 8.46 (d, 1H, J ) 5 Hz), 7.49-7.30 (m, 5H), 6.65 (d, 1H, J ) 5
Hz), 5.42 (s, 2H); 13C NMR δ 169.61, 149.00, 136.66, 128.65,
128.29, 128.25, 112.05, 70.49. Anal. Calcd for C10H9NOS: C,
62.80; H, 4.74; N, 7.32. Found: C, 63.05; H, 4.45; N, 7.17.
Gen er a l P r oced u r e for th e P r ep a r a tion of 5-Su bstitu ted
3-(Ben zyloxy)isoth ia zoles (7a ,b,d -g). To freshly prepared
LDA (0.58 mmol) in Et2O (4.5 mL) at -78 °C under N2 was added
dropwise 3-(benzyloxy)isothiazole (4) (100 mg, 0.52 mmol) dis-
solved in Et2O (0.3 mL). After 15 min, the appropriate electro-
phile (0.58 mmol) dissolved in Et2O (0.2 mL) was added and
stirring continued at -78 °C for 15 min. The reaction mixture
was quenched with saturated NH4Cl and extracted with EtOAc.
The organic layer was washed with brine, dried (Na2SO4), and
evaporated to give the crude products 7a ,b,d -g.
3
1
6.75 (s, 1H), 6.50 (br s,
/ H), 6.30 (br s, / H), 5.35 (s, 2H),
4
4
4.40-4.20 (m, 4H), 1.43 (br s, 9H), 1.26 (t, 6H, J ) 7 Hz).
2-Am in o-2-(3-h yd r oxy-5-isoth ia zolyl)a cetic Acid (1). To
a solution of 7g (1.87 g, 4.02 mmol) in THF (50 mL) at rt was
added LiOH (aq) (50 mL, 2.5 M) and the mixture stirred for 4
h. The reaction mixture was then cooled to 0 °C and the pH
adjusted to 2 with 1 M HCl (aq). The aqueous phase was
extracted with EtOAc, and the collective organic layers were
washed with brine, dried (Na2SO4), and evaporated to give the
crude product. Isolation by flash chromatography (CH2Cl2,
MeOH, AcOH 100:5:2, Rf ) 0.28) gave crude 8 as a clear oil (1.25
g, 76%) that was used without further purification. To a solution
of 8 (1.25 g, 3.06 mmol) in glacial AcOH (15 mL) was added 10%
HBr/AcOH (15 mL), and the reaction mixture was stirred at rt
for 2 h. After evaporation, the crude product was trituated with
Et2O, H2O (20 mL) was added, and the crystals that formed were
filtered off. The crystals were washed with H2O and then dried
to give 1 as a yellow semicrystalline solid (360 mg, 60%):. 1H
NMR (DMSO) δ 9.05 (br s, 3H), 6.84 (s, 1H), 5.62 (br s, 1H); 13
C
NMR (DMSO/-OH) δ 178.84, 176.08, 167.05, 127.45, 113.19.
Recrystallization of a sample from H2O: mp ) 178-180 °C.
Anal. Calcd for C5H6N2O3S: C, 34.48; H, 3.47; N, 16.08. Found:
C, 34.45; H, 3.32; N, 15.80.
3-Ben zyloxy-5-(r-h yd r oxyben zyl)isoth ia zole (7b). The
reaction was carried out as described in the general procedure.
The crude product was purified using flash chromatography
(heptane/EtOAc 4:1, Rf ) 0.25) to give 7b as a white solid (102
mg, 65%): 1H NMR δ 7.45-7.30 (m, 10H), 6.37 (s, 1H), 6.02 (s,
1H), 5.34 (s, 2H); 13C NMR δ 172.70, 168.60, 141.90, 128.97,
128.79, 128.60, 128.25, 128.18, 126.52, 109.49, 71.88, 70,11. Anal.
Calcd for C17H15NO2S: C, 68.66; H, 5.08; N, 4.71. Found: C,
68.46; H, 4.83; N, 4.68.
Ack n ow led gm en t. We would like to thank Dr. Tine
Bryan Steensbøl at the Royal Danish School of Phar-
macy for the preliminary biological testing.
J O0162134