Use of the Kaiser Oxime Resin in the Solid-Phase Synthesis of 3-Aminobenzisoxazoles

Full text of DOI:10.1021/jo9901034

J . Org. Chem. 1999, 64, 4547-4550
4547
Use of th e Ka iser Oxim e Resin in th e
Solid -P h a se Syn th esis of
3-Am in oben zisoxa zoles
Salvatore D. Lepore and Michael R. Wiley*
Eli Lilly and Company, Indianapolis, Indiana
F igu r e 1. Shutske’s Synthesis of 3-Aminobenzisoxazoles.
Received J anuary 20, 1999
Sch em e 1. Rea ction of 2-F lu or oben zon itr iles
w ith Oxim e Resin 1 F ollow ed by Cyclitive
Rem ova l
In connection with our efforts to identify ligands for a
variety of biological targets, we have been interested in
the synthesis of 3-aminobenzisoxazoles.1 To take advan-
tage of parallel synthesis techniques,2 we sought to
develop a method for the solid-phase synthesis of these
interesting heterocycles. Two methods for the preparation
of 3-aminobenzisoxazoles have been reported. Palermo
describes a one-pot procedure involving an SNAr reaction
of an activated ortho-halobenzonitrile with hydroxamate
ion.3 Alternatively, Shutske uses a two-step process
involving acetone oxime addition to 2-fluorobenzonitrile
followed by a subsequent acid-mediated cyclization (Fig-
ure 1).4 We found the two-step approach using polymer-
bound oxime particularly attractive, because in theory,
one could use the aryl oxime intermediate as a linker.
Acid-mediated cyclization would then effect a cyclore-
lease5 from the resin, leaving no apparent residual
functionality from linkage to the solid phase.
To implement this strategy, we obtained the oxime-
containing resin 1 (Scheme 1), which was originally
developed by Kaiser and Degrado for solid-phase peptide
synthesis6 and subsequently used in the synthesis of
cyclic peptides,7 ureas,8 and hydroxamic acids.9 An at-
tractive feature of the Kaiser resin is the incorporation
of the nitro substituent on the benzophenone oxime,
which enhances the stability of acylated intermediates
to anhydrous acid. We hypothesized that aryl oxime
adducts (3) would be at least as stable as acylated oxime
intermediates, providing chemical compatibility of this
linkage for library generation. Therefore, we sought to
demonstrate that the Kaiser resin could serve as a
nucleophile in SNAr reactions and that the resulting aryl
oxime could be used as a linking group in the solid-phase
synthesis of 3-aminobenzisoxazoles.
In preliminary solution-phase studies, the oxime de-
rived from p-nitrobenzophenone was treated with 2-fluo-
robenzonitrile and potassium tert-butoxide in DMF to
give the SNAr product in quantitative yield. Treatment
of this intermediate with anhydrous trifluoroacetic acid
(TFA) for 24 h showed no reaction, whereas the addition
of 20% water gave 50% conversion to 3-aminobenzisox-
azole (4a ) after 24 h.
Although these conditions for acid-promoted cyclization
are not optimized, the stability of the intermediate to
anhydrous acid and the lability under aqueous conditions
suggested that the desired transformations could be
achieved on resin. Therefore, we moved on to optimize
the conditions for these reactions on the solid phase,
where we examined the influence of both steric and elec-
tronic factors on the loading and cyclitive removal steps.
The effect of solvent on the loading reaction was
explored in the case of 4-bromo-2-fluorobenzonitrile (2e,
Table 1), with DMF, N-methylpyrrolidinone, DMSO, and
THF giving 58%, 58%, 52%, and 90% yield, respectively.10
Although DMF is the solvent of choice for the solution
phase reaction, THF is preferred on solid phase, presum-
ably as a result of its superior resin-swelling ability.
Column 3 of Table 1 gives loading yields for a variety of
2-fluorobenzonitriles. The presence of an electron-
withdrawing group facilitates loading at room tempera-
ture. These electron-poor aryl fluorides can also be loaded
equally well at 55 °C, except in the case of 2f, which
(1) Suh has recently incorporated this heterocycle in the design of
potent LTB4 receptor antagonists. Suh, H.; J eong, S.; Han, Y. N.; Lee,
H.; Ryu, J . Bioorg. Med. Chem. Lett. 1997, 7, 389.
(2) Backes, B. J .; Ellman, J . A. Curr. Opin. Chem. Biol., 1997, 1,
86. Kaldor, S. W.; Siegel, M. G.; Curr. Opin. Chem. Biol. 1997, 1, 101.
(3) Palermo, M. G. Tetrahedron Lett. 1996, 37, 2885.
(4) (a) Shutske, G. M.; Kapples, K. J . J . Heterocycl. Chem. 1989,
26, 1293. (b) For the addition of other nucleophiles to aryl fluorides,
see: Idoux, J . P.; Gupton, J . T.; McCurry, C. K.; Crews, A. D.; J urss,
C. D.; Colon, C.; Rampi, R. C. J . Org. Chem. 1983, 48, 3771.
(5) For a recent example of cyclorelease to give a heterocycle, see:
Smith, R. A.; Bobko, M. A.; Lee, W. Bioorg. Med. Chem. Lett. 1998, 8,
2369. Dressman, B. A.; Spangle, L. A.; Kaldor, S. W. Tetrahedron Lett.
1996, 37, 937. For other examples of cyclitive removal, see: Nicolaou,
K. C.; Pastor, J .; Winssinger, N.; Murphy, F. J . Am. Chem. Soc. 1998,
120, 5132 and references therein.
(6) Degrado, W. F.; Kaiser, E. T. J . Org. Chem. 1980, 45, 1295.
(7) O¨ sapay, G.; Profit, A.; Taylor, J . W. Tetrahedron Lett. 1990, 31,
6121.
(8) Scialdone, M. A. Tetrahedron Lett., 1996, 37, 8141.
(9) Golebiowski, A.; Klopfenstein, S. Tetrahedron Lett. 1998, 39,
3397.
(10) The % loading determinations of entries a -h in Table 1 are
an average of 3 experiments and were calculated on the basis of weight
difference. IR spectra for these loaded resins showed the requisite
nitrile absorptions from 2225 to 2235 cm-1. In addition, % loading was
also calculated for entries
a and c-f on the basis of nitrogen
combustion analysis (n ) 12) and were found to be in close cor-
respondence with yield determinations based on weight. See ref 8 for
discussion of weight versus elemental analysis for yield determination.
10.1021/jo9901034 CCC: $18.00 © 1999 American Chemical Society
Published on Web 05/25/1999

4548 J . Org. Chem., Vol. 64, No. 12, 1999
Notes
Ta ble 1. Loa d in g Yield s of Oxim e Ad d u cts 3; HP LC
P u r ity a n d % Isola ted Yield s of 3-Am in oben zisoxa zoles 4
Sch em e 2. On -Resin BOC Rem ova l F ollow ed by
Tr ea tm en t w ith Electr op h iles
a
Determined by resin weight difference (av of 3 experiments,
see ref 8). As determined by reverse-phase HPLC. c Compounds
b
4a -h were purified by silica gel chomatography. Yields based on
d
loading yield of 3. Major impurity was 7-carboxy-3-aminobenz-
isoxazole (hydrolysis of the nitrile group).
required room temperature for optimal loading (83% vs
69%). Substrates bearing electronically neutral or donat-
ing groups require heating to 55 °C in order to load in a
reasonable time frame (12 h). As shown by entry b, the
loading reaction is not sensitive to steric hindrance
around the site of nucleophilic substitution.
Ta ble 2. P r od u ct Yield a n d P u r ity of 9
We observed facile cyclitive removal of compounds
4a -c and 4h , with reasonable crude purity and high
isolated yield using 99:1 TFA/H2O at 55 °C for 12 h.
However, application of the same conditions to 3d -f and
3g gave much lower isolated yields (crude purity of 60-
80%) after 12 h and required several days to reach
completion. After we explored a variety of acidic condi-
tions, 4:1 TFA/aqueous 5 N HCl was found to give
markedly better results at much shorter reaction times.
Column 5 (Table 1) gives the isolated yield of benzisox-
azoles 4a -h using these conditions, based on percent
loading. As shown in column 4 (Table 1), crude cyclization
products (4) were generally quite pure. A noticeable
exception was entry f (Table 1). When 3f was subjected
to cyclitive removal conditions for 2 h, the crude purity
of the corresponding product (4f) was 72% (isolated yield
68%), with the major impurity arising from hydrolysis
of the 4-nitrile group. This purity can be improved to 84%
by reducing the cyclitive removal time to 1 h with a
modest drop in yield.
a
b
As determined by reverse-phase HPLC. Compounds 9a -d
were purified by silica gel chomatography. Yields based on loading
yield of 5.
(as shown in entries a -d in Table 2) to give 8. Table 2
gives the crude purities and isolated yields of 9a -d after
cyclitive removal.
Having identified acceptable conditions for loading and
cyclitive removal of a variety of substrates, we moved on
to confirm the compatibility of the aryl oxime linker with
the anhydrous acidic conditions suggested by the solu-
tion-phase model study. Thus, the BOC-protected amine
5 (Scheme 2) was prepared and loaded onto the Kaiser
resin in 53% yield. The resin was then treated with 25%
TFA/CH2Cl2 for 2 h, rinsed, and vacuum-dried to give 7
(quantitative) as determined by resin weight difference.
The resin washing solutions from this reaction were
concentrated and provided no UV-active material by TLC.
Resin 7 was then treated with a variety of electrophiles
In conclusion, we have developed an efficient method
for the solid-phase synthesis of 3-aminobenzisoxazoles.
Our approach involves the first application of the Kaiser
oxime resin to SNAr reactions and can be used success-
fully with 2-fluorobenzonitriles containing a variety of
electron-withdrawing/-donating groups. The intermediate
aryl oxime linkage is stable to anhydrous acid and can
support BOC-deprotection and amidation reactions, hold-
ing promise for compatibility with a broader variety of
organic transformations. Progress toward the expansion
of this methodology, as well as its application to the syn-
thesis of directed libraries, will be reported in due course.

Notes
J . Org. Chem., Vol. 64, No. 12, 1999 4549
6-Br om o-3-a m in o-1,2-ben zisoxa zole (4e). HPLC tR ) 22.6
Exp er im en ta l Section
1
min. H NMR (400 MHz, DMSO-d6) δ 7.76 (s, 1H), 7.74 (d, J )
8.4 Hz, 1H), 7.41 (d, J ) 8.4 Hz, 1H), 6.44 (bs, 2H). MS (ESI)
m/z 213 [79Br, (M + H)+], 215 [81Br, (M + H)+]. Anal. Calcd for
C7H5BrN2O: C, 39.47; H, 2.37; N, 13.15; Br, 37.51. Found: C,
39.30; H, 2.26; N, 13.09; Br, 37.58.
6-Cya n o-3-a m in o-1,2-ben zisoxa zole (4f). HPLC tR (eluent
gradient 5% A to 40% A over 40 min) 17.2 min. 1H NMR (400
MHz, DMSO-d6) δ 8.09 (s, 1H), 7.99 (d, J ) 8.0 Hz, 1H), 7.66 (d,
J ) 8.0 Hz, 1H), 6.67 (bs, 2H). MS (ESI) m/z 160 (M + H)+. Anal.
Calcd for C8H8N2O2: C, 60.38; H, 3.17; N, 26.40. Found: C,
60.17; H, 3.46; N, 24.82.
Gen er a l. Except for compound 2c and 2f, all reagents used
were obtained from commercial sources. Reagents were used
without further purification. Kaiser oxime resin 1 was purchased
from Novabiochem with a loading capacity of 1.07 mmol/g. All
NMR spectra (400 MHz) were recorded on a Varian Gemini-
400 spectrometer. Mass spectra were obtained with either ESI
or FAB as the ionization method. All purifications were carried
out by radial chromatography (Chromatotron model 8924,
Harrison Research) using 1 mm silica gel plates (Analtech).
Crude purities were estimated from integrated peak areas of
HPLC chromatographs with the UV detector monitoring at λ )
215 nm. Analytical HPLC setup: C18 Vydac column with solvent
gradient A ) acetonitrile (0.1% TFA) and B ) water (0.1% TFA)
at 1 mL/min flow rate. Unless otherwise noted, all HPLC tR are
given for an eluent gradient of 10% A to 60% A over 40 min.
The nomenclature for compounds 4a -h and 9a -d is based on
the numbering system given in ref 4a and reproduced here:
5-Nitr o-3-a m in o-1,2-ben zisoxa zole (4g). HPLC tR ) 15.9
1
min. H NMR (400 MHz, DMSO-d6) δ 8.91 (d, J ) 2.8 Hz, 1H),
8.36 (dd, J ) 9.2, 2.8 Hz, 1H), 7.65 (d, J ) 9.2 Hz, 1H), 6.82 (bs,
2H). MS (ESI) m/z 180 (M + H)+. Anal. Calcd for C7H5N3O3: C,
46.94; H, 2.81; N, 23.46. Found: C, 46.85; H, 2.80; N, 23.26.
4-Tr iflu or om eth yl-3-a m in o-1,2-ben zisoxa zole (4h ). HPLC
1
tR ) 21.5 min. H NMR (400 MHz, DMSO-d6) δ 7.86 (d, J ) 8.4
Hz, 1H), 7.71 (t, J ) 8.4 Hz, 1H), 7.64 (d, J ) 7.2 Hz, 1H), 5.81
(bs, 2H). MS (ESI) m/z 203 (M + H)+. Anal. Calcd for
C8H5F3N2O: C, 47.54; H, 2.49; N, 13.86; F, 28.20. Found: C,
47.32; H, 2.62; N, 13.92; F, 27.96.
Gen er a l Syn th esis of 3-Am in oben zisoxa zoles 9a-d . F or -
m a tion of 6-(Acetyla m in om eth yl)-3-a m in o-1,2-ben zisox-
a zole (9a ). To p-nitrobenzophenone oxime polystyrene (Kaiser)
resin (500 mg, 1.07 mmol/g, 0.54 mmol) in a tared 25 mL Kontes
microfilter funnel was added THF (7 mL) and potassium tert-
butoxide (640 µL, 1 M in THF, 0.642 mmol). After being shaken
by hand for several minutes, the resin turned a deep purple color.
To this suspension was added 4-(tert-butoxycarbonylamino-
methyl)-2-fluorobenzonitrile (1.07 mmol, 268 mg). The reaction
vessel was rotated at 55 °C in a Robbins oven for 12 h and
allowed to cool for 1 h. The resin was then rinsed with 2 × 5
mL of CH2Cl2, 2 × 5 mL of 5% TFA/CH2Cl2, 2 × 5 mL of
2-propanol, and 4 × 5 mL of MeOH. The resin was dried in a 35
°C vacuum oven for 12 h to give a ∆wt of 65.0 mg (53% loading
yield). To effect BOC-deprotection, the resin was then suspended
in 25% TFA/CH2Cl2 (7 mL) and rotated for 2 h. The resin was
again rinsed with 2 × 5 mL of CH2Cl2, 2 × 5 mL of 2-propanol,
and 4 × 5 mL of MeOH. The resin was then suspended in DMF
(7 mL), followed by addition of acetic anhydride (505 µL, 6.42
mmol) and N,N-diisopropylethylamine (930 µL, 6.42 mmol).
After the reaction vessel rotated for another 12 h, the resin was
rinsed with 2 × 5 mL of CH2Cl2, 2 × 5 mL of 2-propanol, and 4
× 5 mL of MeOH. TFA (4 mL) and 5 N HClaq (1 mL) were then
added to the resin, and the vessel was rotated for 2 h in a 55 °C
oven. The TFA/HClaq was collected, and the resin was rinsed
with 2 × 5 mL of CH2Cl2. These washings were combined and
concentrated in vacuo to give the crude product 9a (95% purity
by reverse-phase HPLC), which was purified by radial chroma-
tography on a 2 mm plate, eluting with 50% EtOAc/hexanes.
Concentration of the product-containing fractions gave pure 9a
(45 mg; 4 step yield, 41% based on 1.07 mmol/g given by the
manufacturer or 79% yield based on loading yield).
Gen er a l Syn th esis of 3-Am in o-1,2-ben zisoxa zoles 4a -
h . F or m a tion of 3-Am in o-1,2-ben zisoxa zole (4a ). To p-
nitrobenzophenone oxime polystyrene (Kaiser) resin (500 mg,
1.07 mmol/g, 0.54 mmol) in a tared 25 mL Kontes microfilter
funnel was added THF (7 mL) and potassium tert-butoxide (640
µL, 1 M in THF, 0.642 mmol). After being shaken by hand for
several minutes, the resin turned a deep purple color. To this
suspension was added 2-fluorobenzonitrile (1.07 mmol, 214 mg),
and the reaction vessel was rotated at 55 °C in a Robbins oven
for 12 h, removed from the oven, and allowed to cool for 1 h.
The resin was then rinsed with 2 × 5 mL of CH2Cl2, 2 × 5 mL
of 5% TFA/CH2Cl2, 2 × 5 mL of 2-propanol, and 4 × 5 mL of
MeOH and then dried in a 35 °C vacuum oven for 12 h to give
a ∆wt of 36.5 mg (66% loading yield). TFA (4 mL) and 5 N HClaq
(1 mL) were then added to the resin, followed by rotation for 2
h in a 55 °C oven. The TFA/HClaq was collected, and the resin
was rinsed with 2 × 5 mL of CH2Cl2. These washings were
combined and concentrated in vacuo to give the crude product
4a (>96% purity by reverse-phase HPLC), which was purified
by radial chromatography using a 2 mm plate and eluting with
25% EtOAc/hexanes. Concentration of the product-containing
fractions gave pure 4a (35 mg; 2 step yield, 49% based on
theoretical equivalents of oxime in the starting resin or 74%
based on loading yield). 3-Am in o-1,2-ben zisoxa zole (4a ).
HPLC tR ) 12.3 min. 1H NMR (400 MHz, DMSO-d6) δ 7.79 (d,
J ) 7.7 Hz, 1H), 7.48 (t, J ) 8.4 Hz, 1H), 7.40 (d, J ) 8.4 Hz,
1H), 7.21 (t, J ) 7.0 Hz, 1H), 6.38 (bs, 2H). MS (ESI) m/z 135
(M + H)+. Anal. Calcd for C7H6N2O: C, 62.68; H, 4.51; N, 20.88.
Found: C, 62.61; H, 4.46; N, 20.83.
6-(Acetyla m in om eth yl)-3-a m in o-1,2-ben zisoxa zole (9a ).
1
HPLC tR ) 10.5 min. H NMR (400 MHz, DMSO-d6) δ 8.39 (bs,
1H), 7.69 (d, J ) 8.0 Hz, 1H), 7.24 (s, 1H), 7.10 (d, J ) 8.0 Hz,
1H), 6.32 (bs, 1H), 4.32 (d, J ) 6.4 Hz, 2H), 1.86 (s, 3H). MS
(ESI) m/z 206 (M + H)+. Anal. Calcd for C10H11N3O2: C, 58.53;
H, 5.40; N,20.48. Found: C, 57.77; H, 5.45; N, 19.92.
7-Tr iflu or m eth yl-3-a m in o-1,2-ben zisoxa zole (4b). HPLC
1
tR ) 21.9 min. H NMR (400 MHz, DMSO-d6) δ 7.86 (d, J ) 8.4
Hz, 1H), 7.71 (t, J ) 8.4 Hz, 1H), 7.64 (d, J ) 7.2 Hz, 1H), 5.82
(bs, 2H). MS (ESI) m/z 203 (M + H)+. Anal. Calcd for
C8H5F3N2O: C, 47.54; H, 2.49; N, 13.86; F, 28.20. Found: C,
47.71; H, 2.60; N, 13.85; F, 28.37.
6-[(p-Br om op h en ylca r bon yl)a m in om eth yl]-3-a m in o-1,2-
1
ben zisoxa zole (9b). HPLC tR ) 21.5 min. H NMR (400 MHz,
DMSO-d6) δ 9.17 (bs, 1H), 7.82 (d, J ) 8.4 Hz, 2H), 7.72 (d, J )
8.1 Hz, 1H), 7.67 (d, J ) 8.4 Hz, 2H), 7.31 (s, 1H), 7.18 (d, J )
8.1 Hz, 1H), 6.34 (bs, 2H), 4.55 (d, J ) 5.9 Hz, 2H). MS (ESI)
m/z 346 [79Br, (M + H)+], 348 [81Br, (M + H)+]. Anal. Calcd for
C15H12BrN3O2: C, 52.05; H, 3.49; N, 12.14. Found: C, 51.80; H,
3.54; N, 11.67.
6-Meth oxy-3-a m in o-1,2-ben zisoxa zole (4c). HPLC tR
)
1
14.2 min. H NMR (400 MHz, DMSO-d6) δ 7.61 (d, J ) 8.4 Hz,
1H), 6.95 (d, J ) 2.4 Hz, 1H), 6.80 (dd, J ) 8.4, 2.4 Hz, 1H),
6.23 (bs, 2H). MS (ESI) m/z 165 (M + H)+. Anal. Calcd for
C8H8N2O2: C, 58.53; H, 4.91; N, 17.06. Found: C, 58.78; H, 4.99;
N, 17.10.
6-[(p -Br om op h e n yla m in oca r b on yl)a m in om e t h yl]-3-
a m in o-1,2-ben zisoxa zole (9c). HPLC tR ) 28.2 min. 1H NMR
(400 MHz, DMSO-d6) δ 8.77 (bs, 1H), 7.71 (d, J ) 8.1 Hz, 1H),
7.35 (s, 4H), 7.29 (s, 1H), 7.15 (d, J ) 8.4 Hz, 1H), 6.76 (t, J )
6.2 Hz, 1H), 6.33 (bs, 2H), 4.38 (d, J ) 6.2 Hz, 2H). MS (ESI)
m/z 361 [79Br, (M + H)+], 363 [81Br, (M + H)+]. Anal. Calcd for
C15H13BrN4O2: C, 49.88; H, 3.63; N, 15.51. Found: C, 49.78; H,
3.93; N, 15.10.
6-Tr iflu or m eth yl-3-a m in o-1,2-ben zisoxa zole (4d ). HPLC
1
tR ) 24.6 min. H NMR (400 MHz, DMSO-d6) δ 8.03 (d, J ) 8.4
Hz, 1H), 7.90 (s, 1H), 7.58 (d, J ) 8.4 Hz, 1H), 6.64 (bs, 2H). MS
(ESI) m/z 203 (M + H)+. Anal. Calcd for C8H5F3N2O: C, 47.54;
H, 2.49; N, 13.86; F, 28.20. Found: C, 47.47; H, 2.50; N, 13.59;
F, 27.96.

4550 J . Org. Chem., Vol. 64, No. 12, 1999
Notes
6-[(p-Br om op h en yla m in oth ioca r bon yl)a m in om eth yl]-3-
a m in o-1,2-ben zisoxa zole (9d ). HPLC tR ) 30.5 min. 1H NMR
(400 MHz, DMSO-d6) δ 9.72 (bs, 1H), 8.32 (bs, 1H), 7.72 (d, J )
8.1 Hz, 1H), 7.47 (d, J ) 8.8 Hz, 2H), 7.39 (d, J ) 8.8 Hz, 2H),
7.32 (s, 1H), 7.19 (d, J ) 8.1 Hz, 1H), 6.34 (bs, 2H), 4.81 (s, J )
5.5 Hz, 2H). MS (FAB) m/z 377 [79Br, (M + H)+], 379 [81Br, (M
+ H)+]. Anal. Calcd for C15H13BrN4OS: C, 47.76; H, 3.47; N,
14.85. Found: C, 47.79; H, 3.29; N, 14.68.
Ack n ow led gm en t. The authors would like to thank
Dr. Steven W. Kaldor for many helpful discussions.
J O9901034