Synthesis of novel benzoxaborole-containing phenylalanine analogues

Article abstract of DOI:10.1016/j.tetlet.2011.07.053

The synthesis of novel benzoxaborole-containing phenylalanine analogues 2 and 3 has been developed. The key steps involve the preparation of appropriate precursors from the readily available amino acids and the formation of benzoxaborole ring directly in

Full text of DOI:10.1016/j.tetlet.2011.07.053

Tetrahedron Letters 52 (2011) 4924–4926
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Synthesis of novel benzoxaborole-containing phenylalanine analogues
Xianfeng Li a, , Jacob J. Plattner a, Vincent Hernandez a, Charles Z. Ding a,b, Wei Wu c, Yang Yang c,
⇑
Musheng Xu c
a Anacor Pharmaceuticals, Inc., 1020 E. Meadow Circle, Palo Alto, CA 94303, USA
b Wuxi AppTec Co. Ltd., 288 Fute Zhong Road, Waigaoqiao Free Trade Zone, Shanghai 200131, PR China
c Wuxi AppTec Co. Ltd., No. 111 HuangHai Road, 4th Avenue, TEDA, Tianjin 300456, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesis of novel benzoxaborole-containing phenylalanine analogues 2 and 3 has been developed.
The key steps involve the preparation of appropriate precursors from the readily available amino acids
and the formation of benzoxaborole ring directly in the corresponding amino acid fragment. The resulting
compounds 2–3 show improved water solubility at physiological pH, suggesting their potential use as
boron delivery agents for boron neutron capture therapy.
Received 8 June 2011
Revised 8 July 2011
Accepted 12 July 2011
Available online 23 July 2011
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
Benzoxaborole
Boronated amino acid
Phenylalanine analogue
Boron delivery agent
p-Boronophenylalanine (BPA, 1 in Fig. 1) is a boronated amino
acid which exhibits a specific affinity for tumor cells. Its 10B-en-
riched form has been employed in Boron Neutron Capture Therapy
(BNCT) for treatments of patients affected by different types of can-
cer.1 Upon irradiation with thermal neutrons, BPA absorbs neu-
(GSK’052).9d These benzoxaborole-containing molecules are meta-
bolically stable and exhibit excellent drug-like properties. The ben-
zoxaborole core has been shown to have a lower pKa value (pKa
7.2)11 compared to that of phenylboronc acid (pKa 8.9),12 suggest-
ing that it exists about 50% in anionic tetrahedral form at physio-
logical pH and thus has an enhanced water solubility. Based on
these considerations, we postulated that the incorporation of ben-
zoxaborole in phenylalanine would result in a new class of poten-
tial boron delivery agents with improved physicochemical
properties including water solubility. This Letter describes the syn-
thesis of benzoxaborole-containing phenylalanine 2 and its close
derivative with an additional oxygen linker 3 (Fig. 1).
trons and self-destructs to release lithium ion (7Li) and
a-
particles (4He) of high energy in tumor cells, leading to radiation-
induced cell death. However, the clinical use of BPA is limited by
its poor water solubility at physiological pH. There has been lack
of new boron delivery agents to achieve effective neutron capture
therapy of cancer.2 To facilitate the delivery of BPA to tissue sites,
the complex formation of BPA with fructose or mannitol has been
proposed to produce a water soluble form of BPA for BNCT.3 Also,
the continued development of new boron delivery agents, includ-
ing boron-containing amino acids4 or BPA analogues with water-
solubilizing groups,5 is the subject of extensive research. In addi-
tion, BPA-containing molecules may be used as potential biosen-
sors6 or as chemical warheads7 for various biological
applications. Furthermore, BPA and its derivatives have been
widely employed as building blocks to construct biologically active
molecules in drug discovery.8
Results and discussion
As shown in Figure 2, two different synthetic routes can be
envisioned to prepare the target benzoxaboroles. The first route
(route a) is based on carbon–carbon bond formation between the
OH
B
OH
B
NH2
Benzoxaboroles are boron-containing heterocycles that have
emerged as a new class of potential therapeutics.9,10 Recently, sev-
eral benzoxaborole-based compounds have entered human clinical
trials for antifungal, anti-inflammatory, and antibacterial indica-
tions, exemplified by AN2690,9a,b AN2728,9c and AN3365
NH2
OH
O
O
O
L
OH
OH
2, L = CH2
3, L = CH2O
1
⇑
Corresponding author. Tel.: +1 650 543 7587; fax: +1 650 543 7660.
E-mail address: xli@anacor.com (X. Li).
Figure 1. p-Boronophenylalanine 1 and benzoxaborole-containing phenylalanine
analogues 2–3.
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.07.053

X. Li et al. / Tetrahedron Letters 52 (2011) 4924–4926
4925
OH
OH
OH
OH
B
a
b
NH2
NHP
-
route a
route b
O
B
O
O
H2N
TrtHN
O
O
O
+
O
X
O
OH
9
OP'
10
OH
O
B
X
B
NHP
NH2
Br
OH
O
O
O
O
O
d
c
O
TrtHN
O
TrtHN
O
O
O
OP'
OH
O
O
Figure 2. Two proposed approaches to prepare benzoxaborole-containing phenyl-
alanine 2.
11
12
OH
OH
B
B
synthetic equivalent of an amino acid and benzoxaborole-contain-
ing bromide or aldehyde fragment. The second route (route b) in-
cludes the introduction of benzoxaborole ring directly into the
phenylalanine fragment. We chose the latter since this approach
allows the use of readily available amino acids and the method
developed with DL-amino acids can be directly extended to make
corresponding chiral compounds. Also, this route introduces boron
atom toward the end of synthesis and would involve minimized
transformation of the 10B-enriched product.
O
O
O
H2N
O
e
O
HO
O
NH2
O
3
13
Scheme 2. Reagents and conditions: (a) TrtCl, Et3N, dichloromethane, addition over
1.5 h, then rt, 16 h, 100%; (b) 2-bromo-5-hydroxybenzaldehyde, dry toluene, PPh3,
DIAD, N2, 0 °C, 30 min, then 60 °C, 16 h, 24%; (c) pinacol diboron, Pd(dppf)Cl2, KOAc,
dioxane, N2, 80 °C, 16 h, 56%; (d) NaBH4, MeOH, rt, 2 h, then 6 N HCl, 2 h, 79%; (e)
LiOH, MeOH, H2O, rt 10 min, 27%.
The synthesis of compound 2 is shown in Scheme 1. Hydroxy-
benzaldehyde 4, prepared using Reimer-Tiemann reaction accord-
ing to the reported procedure,13 was converted into its triflate
derivative 5 in 80% yield by reaction with triflate chloride in
dichloromethane. Without further purification, crude 5 was al-
lowed to react with pinacol diboron in the presence of
compounds 7 and 8 was accounted for the 71% yield. Hydrogena-
tion of benzyl ester 7 or basic hydrolysis of methyl ester 8 afforded
the desired compound 2 as a white solid.16
14
The synthesis of compound 3 was carried out using DL-serine
methyl ester 9 according to Scheme 2. A key step in the synthesis
was to convert the hydroxyl group of serine methyl ester into phe-
nyl ether by Mitsunobu reaction. During our initial attempts, we
found that the reaction of Boc or Cbz-protected serine ester with
2-bromo-5-hydroxybenzaldehyde in the presence of DIAD and
PPh3 failed to give any desired ether product. The b-elimination
product was isolated instead, which is attributed to the fact that
Pd(dppf)Cl2 to afford boronate 6 in 55% yield. Subsequently, for-
mation of benzoxaborole ring was accomplished by using a one-
pot procedure as previously described.15 This included the treat-
ment of 6 with sodium borohydride in methanol to convert the
aldehyde into hydroxylmethyl group, which simultaneously cy-
clized to the adjacent boronate and subsequently hydrolyzed to
form the corresponding benzoxaborole ring upon addition of aque-
ous hydrochloric acid. Boc protecting group was also removed in
this step resulting in the desired benzyl ester 7 in 32% yield. Also
methyl ester 8 was isolated in 39% yield due to transterification
in methanol under these reaction conditions. Combination of two
the relative acidity of the a-proton of the carbamate-protected ser-
ine ester causes its facile elimination under Mitsunobu condi-
tions.17 Subsequently, compound 9 was protected with trityl
group, a non-carbamate type protecting group, by reacting with
trityl chloride in the presence of triethylamine to convert into ester
10. Mitsunobu reaction of 10 and 2-bromo-5-hydroxybenzalde-
hyde in the presence of DIAD and PPh3 afforded phenyl ether com-
pound 11. Full conversion of 10 to 11 was observed, and low
isolated yield (24%) was due to compound loss during column puri-
fication and was not optimized. Compound 11 was reacted with
pinacol diboron in the presence of Pd(dppf)Cl2 to afford boronate
12 in 56% yield. Reduction of 12 with sodium borohydride in meth-
anol converted the aldehyde into its hydroxylmethyl group which
simultaneously cyclized to the adjacent boronate and hydrolyzed
to give benzoxaborole, upon addition of 6 N HCl. Under these con-
ditions, trityl protecting group was also removed resulting in ben-
zoxaborole 13 in 79% yield. Hydrolysis of methyl ester 13 gave the
desired compound 3 as off white solid.18
The water solubility of 1–3 was determined in 0.1 M, pH 7.4
phosphate buffer.19 BPA 1 has a solubility of 1.7 mg/mL, which is
consistent with the reported value (1.6 mg/mL).3a,5a Compared to
1, compounds 2 and 3 exhibit improved water solubility of 5.2
and 3.6 mg/mL, respectively. Their better solubility could be attrib-
utable to the lower pKa of benzoxaborole since the benzoxaborole
would exist in anionic tetrahedral form to a more significant extent
at physiological pH. Compound 2 is more soluble than 3. One po-
tential explanation is that the presence of the electron-donating
oxygen atom para to the benzoxaborole in 3 would increase its
pKa value and thus decrease its water solubility.
OTf
OH
O
O
b
a
BocHN
COOBn
BocHN
COOBn
5
4
O
B
OH
O
O
B
NH2
c
O
RO2C
BocHN
CO2Bn
7, R = Bn
8, R = Me
6
OH
B
NH2
d for 7
e for 8
O
HO
O
2
Scheme 1. Reagents and conditions: (a) Triflate chloride, DMAP, dichloromethane,
0 °C, 1 h, 80%; (b) pinacol diboron, Pd(dppf)Cl2, KOAc, dioxane, N2, 80 °C, 16 h, 55%;
(c) NaBH4, MeOH, rt, 2 h, then 6 N HCl, 12 h, 32% for 7, 39% for 8; (d) Pd/C, EtOH, H2,
10 h, 89%; (e) LiOH, MeOH, H2O, 30 min, rt, 64%.

4926
X. Li et al. / Tetrahedron Letters 52 (2011) 4924–4926
10. (a) Berube, M.; Dowlut, M.; Hall, D. G. J. Org. Chem. 2008, 73, 6471–6479; (b)
In summary, we have developed the first synthesis of novel
Pal, A.; Berube, M.; Hall, D. G. Angew. Chem., Int. Ed. 2010, 49, 1492; (c) Jay, J. I.;
Lai, B. E.; Myszka, D. G.; Mahalingam, A.; Langheinrich, K.; Katz, D. F.; Kiser, P. F.
Mol. Pharm. 2010, 7, 116–129; (d) Wozniack, A. A.; Cyranski, M. K.; Zubrowska,
A.; Sporzynski, A. J. Organomet. Chem. 2009, 694, 3533–3541. and references
cited therein; (e) Ye, L.; Ding, D.; Feng, Y.; Xie, D.; Wu, P.; Guo, H.; Meng, Q.;
Zhou, H. Tetrahedron 2009, 65, 8738–8744; (f) Kumar, J. S.; Bashian, C. M.;
Corsello, M. A.; Jonnalagadda, S. C.; Mereddy, V. R. Tetrahedron Lett. 2010, 51,
4482–4485.
benzoxaborole-containing phenylalanine analogues 2 and 3. The
key steps include the preparation of appropriate precursors from
the readily available amino acids and the formation of benzoxabo-
role ring directly in the corresponding amino acid fragment. The
resulting compounds 2–3 show improved water solubility at phys-
iological pH, suggesting their potential use as boron delivery
agents for boron neutron capture therapy.
11. Dowlut, M.; Hall, D. G. J. Am. Chem. Soc. 2006, 128, 4226–4227.
12. Westmark, P. R.; Gardiner, S. J.; Smith, B. D. J. Am. Chem. Soc. 1996, 118, 11093–
11100.
Acknowledgments
13. Kalesh, K. A.; Tan, L. P.; Lu, K.; Gao, L.; Wang, J.; Yao, S. Q. Chem. Commun. 2010,
46, 589–591.
14. Nakamura, H.; Fujiwara, M.; Yamamoto, Y. J. Org. Chem. 1998, 63, 7529–7530.
15. Zhang, Y.-K.; Plattner, J. J.; Akama, T.; Baker, S. J.; Hernandez, V. S.; Sanders, V.;
Freund, Y.; Kimura, R.; Bu, W.; Hold, K. M.; Lu, X.-S. Bioorg. Med. Chem. Lett.
2010, 20, 2270–2274.
16. 2-Amino-3-(1-hydroxy-1,3-dihydrobenzo[c][1,2] oxaborol-5-yl)propanoic acid
2: To a solution of compound 7 (550 mg, 1.77 mmol) in ethanol (20 ml) was
added Pd/C (50 mg) portionwise. The solution was stirred at rt under H2
atmosphere for 10 h. After the reaction was completed as indicated by HPLC,
ethanol was evaporated in vacuo. The residue was purified by prep-HPLC
We would like to thank Dr. Tsutomu Akama for helpful discus-
sions, Dr. Kurt Jarnagin for his support, and Dr. Lan Lin at WuXi
AppTec Co. for high-resolution mass analysis.
References and notes
1. For reviews, see: (a) Yang, W.; Gao, X.; Wang, B. Biological and Medicinal
Applications of Boronic Acids. In Boronic Acids; Hall, D. G., Ed.; Wiley-VCH:
Weinheim, Germany, 2005; pp 481–512; (b) Kabalka, G. W.; Yao, M.-L. Anti-
Cancer Agents Med. Chem. 2006, 6, 111–125; (c) Sivaev, I. B.; Bregadze, V. I.
ARKIVOC 2008, 4, 47–61.
(column: Luna 300 Â 50.0 mm, 10
l; mobile phase: [A-H2O; B-MeCN + 0.1%
TFA] B%: 0–10%, 20 min) to give compound 2 as white solid (350 mg, 89%).
Alternatively, LiOH (252 mg, 6 mmol) in 5 mL water was added to a solution of
compound 8 (500 mg, 2.12 mmol) in MeOH (20 mL) portionwise. The solution
was stirred at rt for 30 min. After the reaction was completed as indicated by
2. Barth, R. F. Appl. Radiat. Isot. 2009, 67, S3–S6.
HPLC, the mixture was acidified to pH
evaporated in vacuo. The residue was purified by prep-HPLC (column: Luna
; mobile phase: [A-H2O; B-MeCN + 0.1% TFA] B%: 0–10%,
5 with 2 N HCl, and MeOH was
3. (a) Mori, Y.; Suzuki, A.; Yoshino, K.; Kakihana, H. Pigment Cell Res. 1989, 2, 273–
277; (b) Shull, B. K.; Spielvogel, D. E.; Head, G.; Gopalaswamy, R.; Sankar, S.;
Devito, K. J. Pharm. Sci. 2000, 89, 215–222.
300 Â 50.0 mm, 10
l
20 min) to give compound 2 as white solid (300 mg, 64%). 1H NMR (400 MHz,
MeOD) d 7.65–7.67 (d, 1H, J = 7.6 Hz), 7.32 (s, 1H), 7.27–7.29 (d, 1H, J = 7.6 Hz),
5.07 (s, 2H), 4.20–4.23 (m, 1H), 3.35–3.40 (m, 1H), 3.16–3.21 (m, 1H); 13C NMR
(400 MHz, MeOD) d 170.27 (C@O), 154.91 (C), 137.56 (C), 130.51 (C), 128.03
(C), 121.84 (C), 70.75 (CH2), 54.19 (CH2), 36.35 (CH), carbon adjacent to boron
was not observed20; HRMS calcd for C10H13BNO4 (M+H)+, 222.0938; found,
222.0938; HPLC purity: 99.4% (MaxPlot 190–370 nm), 99.4% (220 nm).
17. Panda, G.; Rao, N. V. Synlett 2004, 714–716.
4. (a) Kabalka, G. W.; Wu, Z.; Yao, M.-L. Appl. Organomet. Chem. 2008, 22, 516–
522; (b) Hattori, Y.; Yamamoto, H.; Ando, H.; Kondoh, H.; Asano, T.; Kirihata,
M.; Yamaguchi, Y.; Wakamiya, T. Bioorg. Med. Chem. 2007, 15, 2198–2205.
5. (a) Nemoto, H.; Cai, J.; Asao, N.; Iwamoto, S.; Yamamoto, Y. J. Med. Chem. 1995,
38, 1673–1678; (b) Takagaki, M.; Ono, K.; Oda, Y.; Kikuchi, H.; Nemoto, H.;
Iwamoto, S.; Cai, J.; Yamamoto, Y. Cancer Res. 1996, 56, 2017–2020.
6. Duggan, P. J.; Offermann, D. A. Aust. J. Chem. 2007, 60, 829–834.
7. (a) Brustad, E.; Bushey, M. L.; Lee, J. W.; Groff, D.; Liu, W.; Schultz, P. G. Angew.
Chem., Int. Ed. 2008, 47, 8220–8223; (b) Liu, C. C.; Mack, A. V.; Brustad, E. M.;
Mills, J. H.; Groff, D.; Smider, V. V.; Schultz, P. G. J. Am. Chem. Soc. 2009, 131,
9616–9617.
8. For examples, see: (a) Gong, Y.; Barbay, J. K.; Dyatkin, A. B.; Miskowski, T. A.;
Kimball, E. S.; Prouty, S. M.; Fisher, M. C.; Santulli, R. J.; Schneider, C. R.;
Wallace, N. H.; Ballentine, S. A.; Hageman, W. E.; Masucci, J. A.; Maryanoff, B. E.;
Damiano, B. P.; Andrade-Gordon, P.; Hlasta, D. J.; Hornby, P. J.; He, W. J. Med.
Chem. 2006, 49, 3402–3411; (b) Capek, P.; Pohl, R.; Hocek, M. Org. Biomol. Chem.
2006, 4, 2278–2284.
9. (a) Rock, F. L.; Mao, W.; Yaremchuk, A.; Tukalo, M.; Crepin, T.; Zhou, H.; Zhang,
Y.-K.; Hernandez, V.; Akama, T.; Baker, S. J.; Plattner, J. J.; Shapiro, L.; Martinis,
S. A.; Benkovic, S. J.; Cusack, S.; Alley, M. R. K. Science 2007, 316, 1759–1761; (b)
Baker, S. J.; Zhang, Y.-K.; Akama, T.; Lau, A.; Zhou, H.; Hernandez, V.; Mao, W.;
Alley, M. R. K.; Sanders, V.; Plattner, J. J. J. Med. Chem. 2006, 49, 4447–4450; (c)
Akama, T.; Baker, S. J.; Zhang, Y.-K.; Hernandez, V.; Zhou, H.; Sanders, V.;
Freund, Y.; Kimura, R.; Maples, K. R.; Plattner, J. J. Bioorg. Med. Chem. Lett. 2009,
19, 2129–2132; (d) Hernandez, V.; Akama, T.; Alley, M. R. K.; Baker, S.; Mao, W.;
Rock, F.; Zhang, Y. K.; Zhang, Y.; Zhou, Y.; Crepin, T.; Cusack, S.; Palencia, A.;
Nieman, J.; Anugula, M.; Baek, M.; Diaper, C.; Ha, C.; Keramane, M.; Lu, X.;
Mohammad, R.; Savariraj, K.; Sharma, R.; Singh, R.; Subedi, R.; Plattner, J. 50th
Interscience Conference on Antimicrobial Agents and Chemotherapy, Boston,
September 12–15, 2010; F1-1637.; (e) Xia, Y.; Cao, K.; Zhou, Y.; Alley, M. R.;
Rock, F.; Mohan, M.; Meewan, M.; Baker, S. J.; Lux, S.; Ding, C. Z.; Jia, G.; Kully,
M.; Plattner, J. J. Bioorg. Med. Chem. Lett. 2011, 21, 2533–2536; (f) Li, X.; Zhang,
S.; Zhang, Y. K.; Liu, Y.; Ding, C. Z.; Zhou, Y.; Plattner, J. J.; Baker, S. J.; Bu, W.; Liu,
L.; Kazmierski, W. M.; Duan, M.; Grimes, R. M.; Wright, L. L.; Smith, G. K.;
Jarvest, R. L.; Ji, J. J.; Cooper, J. P.; Tallant, M. D.; Crosby, R. M.; Creech, K.; Ni, Z.
J.; Zou, W.; Wright, J. Bioorg. Med. Chem. Lett. 2011, 21, 2048–2054.
18. 2-Amino-3-(1-hydroxy-1,3-dihydrobenzo[c][1,2] oxaborol-5-yloxy)propanoic
acid 3: To a solution of compound 13 (2 g, 8 mmol) in MeOH (60 mL) was
added LiOH (1.3 g, 32 mmol) in 5 mL water portionwise. The solution was
stirred at rt for 40 min. After the reaction was completed as indicated by HPLC,
the mixture was acidified to pH 6–6.5 with 2 N HCl, to precipitate the product.
The solid was collected by filtration and washed with methanol to give
compound 3 as off white solid (515 mg, 27%). 1H NMR: (400 MHz, MeOD): d
7.59–7.61 (d, 1H, J = 8.0 Hz), 7.02 (s, 1H), 6.99–7.01 (d, 1H, J = 8.0 Hz), 5.03 (s,
2H), 4.43–4.53 (m, 2H), 4.36–4.38 (m, 1H); 13C NMR (400 MHz, MeOD) 169.50
(C@O), 161.91 (C), 157.98 (C), 132.79 (C), 116.08 (C), 107.69 (C), 72.10 (CH2),
67.09 (CH2), 54.07 (CH), carbon adjacent to boron was not observed;20 HRMS
calcd for C10H13BNO5 (M+H)+, 238.0887; found, 238.0889; HPLC purity: 98.3%
(MaxPlot 190–370 nm), 98.5% (220 nm).
19. Assay for solubility: Various amounts of compounds 1–3 were added to 0.1 M,
pH 7.4 phosphate buffer. After vortex-mixing, the mixture was sonicated and
incubated at room temperature for 24 h. If material is precipitated after
incubation then the solubility is lower than the concentration assayed. If the
solution becomes transparent, then solubility is equal to or higher than the
concentration assayed. The solubility was calculated by dividing the amount of
material by total volume of buffer in final transparent solution. Multiple
replications of each experiment were carried out and the results were
averaged.
20. It has been reported that the carbon adjacent to boron is not observed in 13C
NMR spectrum. For examples, see: (a) Adamczyk-Wozniak, A.; Madura, I.;
Velders, A. H.; Sporzynski, A. Tetrahedron Lett. 2010, 51, 6181–6185; (b) Korner,
C.; Starkov, P.; Sheppard, T. D. J. Am. Chem. Soc. 2010, 132, 5968–5969.