Synthesis and biological evaluation of peptidyl organotrifluoroborates and their corresponding boron analogs

Article abstract of DOI:10.1007/s11164-014-1596-7

Six peptidyl organotrifluoroborates and their corresponding boronate esters and/or boronic acid analogs were designed and synthesized. Their anti-proliferative activity against hepatocellular carcinoma cells (HepG2) and human metastatic breast cancer cell

Full text of DOI:10.1007/s11164-014-1596-7

Res Chem Intermed (2014) 40:2187–2198
DOI 10.1007/s11164-014-1596-7
Synthesis and biological evaluation of peptidyl
organotrifluoroborates and their corresponding boron
analogs
•
Chia-Hua Tsai Chia-Hung Lin Ching-Tien Hsieh
•
•
•
Chih-Cheng Cai Ting-Ju Lin Pin-Yi Liu
•
•
•
Meng-Hsuan Lin Meng-Ju Wu Chia-Chieh Fu
•
•
•
•
Yang-Chang Wu Fang-Rong Chang Po-Shen Pan
Received: 29 December 2013 / Accepted: 21 February 2014 / Published online: 3 April 2014
Ó Springer Science+Business Media Dordrecht 2014
Abstract Six peptidyl organotrifluoroborates and their corresponding boronate
esters and/or boronic acid analogs were designed and synthesized. Their anti-pro-
liferative activity against hepatocellular carcinoma cells (HepG2) and human meta-
static breast cancer cells (MDA-MB231) were evaluated by use of an MTT assay.
Potassium {4-[(3S,6S,9S)-3,6-dibenzyl-9-isopropyl-4,7,10-trioxo-11-oxa-2,5,8-tri-
azadodecyl]phenyl}trifluoroborate (B6) was potent (IC50 = 29.9 lM) against
MDA-MB231, and {4-[(3S,6S,9S)-6-benzyl-3-((benzyloxy)methyl)-9-isopropyl-
4,7,10-trioxo-11-oxa-2,5,8-triazadodecyl]phenyl}boronic acid (B9) and Potassium
{4-[(3S,6S,9S)-6-benzyl-3-((benzyloxy)methyl)-9-isopropyl-4,7,10-trioxo-11-oxa-2,
5,8-triazadodecyl]phenyl}trifluoroborate (B10) had broad anti-proliferative activity
against HepG2 (IC50 = 24.7 and 21.8 lM, respectively) and MDA-MB231
(IC50 = 24.5 and 18.9 lM, respectively).
Keywords Peptide Á Boronic acid Á Organotrifluoroborate Á Potassium
trifluoroborate Á HepG2 Á MDA-MB231
C.-H. Tsai Á C.-H. Lin Á C.-T. Hsieh Á C.-C. Cai Á T.-J. Lin Á P.-Y. Liu Á M.-H. Lin Á
M.-J. Wu Á C.-C. Fu Á P.-S. Pan (&)
Department of Chemistry, Tamkang University, No. 151 Yingzhuan Rd., Tamsui Dist., New Taipei
City 25137, Taiwan, ROC
e-mail: popan@mail.tku.edu.tw
Y.-C. Wu Á F.-R. Chang
Graduate Institute of Natural Products, Kaohsiung Medical University, 100 Shi-Chuan 1st Rd.,
Kaohsiung 80708, Taiwan, ROC
Y.-C. Wu
Graduate Institute of Integrated Medicine, College of Chinese Medicine, China Medical University,
Taichung 404, Taiwan, ROC
F.-R. Chang
Cancer Center, Kaohsiung Medical University Hospital, Kaohsiung 807, Taiwan, ROC
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Introduction
Among boron-containing compounds, organotrifluoroborates are seldom considered
in biological applications because of their lack of an empty p-orbital. Although the
value of organofluoroborates in organic synthesis is well recognized [1], studies of
their biologic applications are limited [2, 3]. Herein, we report the synthesis of two
series of peptidyl organotrifluoroborates and their corresponding boronate esters
and/or boronic acids, and their anti-proliferative activity against hepatocellular
carcinoma cells (HepG2) and human metastatic breast cancer cells (MDA-MB231),
determined by use of MTT assays [4].
Experimental
General procedure for conversion of potassium organotrifluoroborate to boronic
acid [5]
Potassium organotrifluoroborate (1.0 equivalent) and silica gel (1.0 equivalent) were
weighed into a 20-ml vial, and H2O–ethyl acetate (1:1) was added. The reaction was
stirred at room temperature for 3–6 h. The reaction mixture was filtered to remove
silica gel, and the filter cake was thoroughly washed with ethyl acetate–acetone
(9:1). The aqueous and organic layers were separated, the aqueous layer was
extracted with ethyl acetate, and the combined organic layers were dried, filtered,
and concentrated in vacuo. The crude product was then purified by reversed-phase
HPLC to afford the pure desired product.
General procedure for conversion of potassium organotrifluoroborate to boronate
ester [5]
Potassium organotrifluoroborate (1.0 equivalent), pinacol (1.1 equivalents), and
silica gel (1.0 equivalent) were weighed into a 20-ml vial and H2O–ethyl acetate
(1:1) was added in one portion. The reaction was stirred at room temperature for
3–5 h. The reaction mixture was then filtered, and the filter cake was thoroughly
rinsed with ethyl acetate–acetone (9:1). The aqueous and organic layers were
separated, the aqueous layer was extracted with ethyl acetate, and the combined
organic layers were dried, filtered, and concentrated in vacuo. The crude ester
analogs were purified by flash column chromatography to afford the desire product.
General procedure for conversion of boronate ester to potassium
organotrifluoroborate
Boronate ester (1.0 equivalent) was added 6.0 equivalents of KHF2 in H2O–
methanol (1:1), and the mixture was stirred at room temperature for 3 h. The solvent
was then removed in vacuo and the crude solid was obtained by Soxhlet extraction
(3 h) with acetone. The collected solution was concentrated and the desired product
was precipitated by addition of 10 mL of acetone–hexane (1:3).
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General procedure for synthesis of series A analogs
Tripeptide (1.1 equivalent) and 4-carboxyphenylboronic acid pinacol ester (3, 1.0
equivalent)wereweighed intoa dryflask withDIPEA(6.0equivalents)and TBTU(1.1
equivalents). Anhydrous dichloromethane was added to generate a 0.1 M solution. On
completion of the reaction the crude material was washed with aqueous HCl (pH 1) and
saturated sodium bicarbonate. The resulting organic solutions were collected, dried by
MgSO4, filtered, and concentrated to yield the crude product. This crude product was
then purified by a short flash column chromatography to afford the desire product.
General procedure for synthesis of series B analogs
To a vial containing potassium 4-formylphenyltrifluoroborate (4, 1.0 equivalent) in
MeOH, tripeptide (1.2 equivalents) was added to generate 0.5 M solution. The
reaction mixture was stirred for 3 h at room temperature. 5-Ethyl-2-methylpyridine
borane (PEMB, 1.0 equivalent) was then added, and stirring was continued for
3–5 h. The solvent was removed in vacuo, and the resulting crude material was
washed with hexane. The crude solid was obtained by Soxhlet extraction (3 h) with
acetone. The collected solvent was concentrated and the desired pure product was
then precipitated by addition of 10 mL of acetone–hexane (1:3).
Compound A1
1H NMR (300 MHz, CDCl3) d 7.83 (s, 4H), 7.74 (br, 1H), 7.51 (br, 1H), 7.45 (br,
1H), 7.07–6.91 (m, 5H), 5.00 (q, J = 8.0 Hz, 1H), 4.70 (t, J = 8.7 Hz, 1H), 4.44
(dd, J = 8.8, 5.9 Hz, 1H), 3.66 (s, 3H), 3.03–2.88 (m, 2H), 2.17–1.99 (m, 2H), 1.34
(s, 12H), 0.90 (dd, J = 13.8, 6.7 Hz, 6H), 0.78 (dd, J = 12.9, 6.8 Hz, 6H). 13C
NMR (75.5 MHz, CDCl3) d 172.1, 171.8, 171.6, 167.6, 136.6, 134.7, 129.5, 128.3,
127.1, 126.6, 84.1, 59.1, 57.4, 54.3, 52.1, 38.9, 31.6, 31.2, 25.0, 19.3, 19.2, 19.0,
18.2. 11B NMR (192.5 MHz, acetone-d6) d 29.4. M.p. 164 °C. HRMS (ESI, positive
ion) m/z calcd for [M ? H]? = 608.3499, m/z found 608.3526.
Compound A2
1H NMR (300 MHz, CD3OD) d 7.77 (br, 4H), 7.22–7.03 (m, 5H), 4.77 (dd, J = 8.4,
6.3 Hz, 1H), 4.39 (d, J = 5.7 Hz, 1H), 4.30 (d, J = 6.3 Hz, 1H), 3.66 (s, 3H),
3.13–3.08 (m, 1H), 2.94–2.85 (m, 1H), 2.09 (oct, J = 6.6 Hz, 2H), 0.97–0.89 (m,
12H). 13C NMR (75.5 MHz, CD3OD) d 173.6, 173.3, 170.3, 138.2, 135.0, 130.5,
129.5, 127.8, 127.6, 61.0, 59.3, 55.8, 52.6, 39.2, 32.1, 20.0, 19.6, 19.3, 18.7. 11B
NMR (192.5 MHz, CD3OD) d 28.8. M.p. 327 °C. HRMS (ESI, positive ion) m/
z calcd for [M ? H]? = 526.2717, m/z found 526.2763.
Compound A3
1H NMR (300 MHz, CD3OD) d 7.62 (dd, J = 11.6, 8.0 Hz, 4H), 7.23–7.06 (m,
5H), 4.74 (dd, J = 8.6, 6.3 Hz, 1H), 4.31 (dd, J = 13.9, 8.4 Hz, 2H), 3.66 (s, 3H),
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3.15–3.08 (m, 1H), 2.95–2.87 (m, 1H), 2.08 (oct, J = 6.8 Hz, 2H), 0.95 (d,
J = 6.7 Hz, 3H), 0.90 (dd, J = 6.8, 2.6 Hz, 9H). 13C NMR (75.5 MHz, DMSO-d6)
d 171.6, 171.3, 170.9, 167.2, 137.5, 131.0, 129.1, 127.9, 126.2, 125.4, 59.0, 57.4,
53.4, 51.6, 37.5, 30.2, 29.9, 19.2, 18.8, 18.7, 18.2. 11B NMR (192.5 MHz, DMSO-
d6) d 2.6. M.p. 275 °C. HRMS (ESI, negative ion) m/z calcd for [M –
K]- = 548.2546, m/z found 548.2560.
Compound A4
1H NMR (300 MHz, CDCl3) d 7.82 (d, J = 8.1 Hz, 2H), 7.63 (d, J = 8.1 Hz, 2H),
7.24–7.01 (m, 10H), 6.90 (br, 2H), 6.70 (br, 1H), 4.93 (q, J = 6.9 Hz,1H), 4.73 (q,
J = 9.0 Hz, 1H), 4.44–4.39 (m, 1H), 3.68 (s,3H), 3.13 (d, J = 6.9 Hz, 2H), 2.99 (d,
J = 7.2 Hz, 2H), 2.08–2.04 (m, 1H), 1.35 (s, 12H), 0.84 (dd, J = 12.9 Hz, 6H). 13
C
NMR (75.5 MHz, CDCl3) d 172.0, 171.5, 171.0, 167.5, 136.9, 136.3, 134.9, 129.5,
128.7, 128.5, 127.1, 126.9, 126.7, 84.2, 57.6, 54.8, 54.7, 52.2, 38.8, 38.4, 31.3, 25.0,
19.0, 18.3. 11B NMR (192.5 MHz, CDCl3) d 29.3. M.p. 137 °C. HRMS (ESI,
positive ion) m/z calcd for [M ? H]? = 656.3499, m/z found 656.3498.
Compound A5
1H NMR (300 MHz, D2O) d 7.91 (d, J = 8.1 Hz, 2H), 7.78 (d, J = 7.8 Hz, 2H),
7.38–7.24 (m, 10H), 4.86–4.78 (m, 2H), 4.34 (d, J = 6.3 Hz, 1H), 3,75 (s, 3H),
3.20–3.17 (m, 2H), 3.09–2.96 (m, 2H), 2.21–2.14 (m, 1H), 1.02–0.98 (m, 6H). 13C
NMR (75.5 MHz, DMSO-d6) d 171.7, 171.4, 171.2, 166.3, 138.3, 137.5, 135.2,
134.1, 133.7, 129.2, 127.9, 126.7, 57.4, 54.7, 53.5, 51.6, 37.5, 36.9, 30.0, 18.9, 18.2.
11B NMR (192.5 MHz, CD3OD) d 28.2. M.p. 267 °C. HRMS (ESI, positive ion) m/
z calcd for [M ? H]? = 574.2717, m/z found 574.2744.
Compound A6
1H NMR (300 MHz, DMSO-d6) d 8.29 (d, 1H), 8.21 (d, 1H), 7.54–7.51 (m, 2H),
7.36–7.33 (m, 4H), 7.26 (t, J = 7.3 Hz, 2H), 7.18–7.13 (m, 1H), 4.81–4.73 (m, 1H),
4.23 (dd, J = 8.1, 6.2 Hz, 1H), 3.64 (s, 3H), 3.11–2.96 (m, 2H), 2.07 (sext,
J = 6.7 Hz, 1H), 0.90 (dd, J = 8.6, 6.9 Hz, 6H). 13C NMR (75.5 MHz, DMSO-d6) d
172.0, 171.8, 167.3, 138.3, 130.9, 130.8, 129.2, 128.0, 126.1, 125.3, 57.4, 54.4, 51.7,
36.8, 30.6, 30.0, 18.9, 18.1. 11B NMR (192.5 MHz, CD3OD) d 3.8. M.p. 268 °C.
HRMS (ESI, negative ion) m/z calcd for [M – K]- = 596.2546, m/z found 596.2546.
Compound A8
1H NMR (300 MHz, CDCl3) d 7.77 (dd, J = 48.8, 8.1 Hz, 4H), 7.36–7.28 (m, 5H),
7.18–7.10 (m, 5H), 7.03–7.0 (m, 2H), 6.51 (d, 1H), 4.72–4.67 (m, 2H), 4.56 (d,
J = 4.0 Hz, 2H), 4.44 (dd, J = 8.5, 5.3 Hz, 1H), 4.02–3.97 (m, 1H), 3.71 (s, 3H),
3.66–3.60 (m, 1H), 3.11–3.06 (m, 2H), 2.06 (oct, J = 5.4 Hz, 1H), 1.36 (s, 12H),
0.81 (dd, J = 13.3, 6.9 Hz, 6H). 13C NMR (75.5 MHz, CDCl3) d 172.0, 170.5,
170.2, 167.5, 137.4, 136.3, 135.7, 135.2, 129.5, 128.9,128.8, 128.3, 128.2, 127.3,
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126.4, 84.4, 73.8, 69.1, 57.7, 54.8, 53.2, 52.3, 37.7, 31.3, 25.1, 19.0, 18.1. 11B NMR
(192.5 MHz, Acetone-d6) d 31.0. M.p. 107 °C. HRMS (ESI, positive ion) m/z calcd
for [M ? H]? = 686.3605, m/z found 686.3576.
Compound A9
1H NMR (300 MHz, CD3OD) d 8.21 (d, J = 8.6 Hz, 1H), 7.73 (br, 4H), 7.32–7.25
(m, 5H), 7.20–7.11 (m, 5H), 4.82–4.73 (m, 2H), 4.54 (s, 2H), 4.31–4.26 (m, 1H),
3.77 (d, J = 5.7 Hz, 1H), 3.67 (s, 3H), 3.13 (dd, J = 13.9, 6.0 Hz, 1H), 2.95 (dd,
J = 13.9, 8.1 Hz, 1H), 2.07 (sext, J = 6.7 Hz, 1H), 0.88 (d, J = 6.8 Hz, 6H). 13C
NMR (75.5 MHz, CD3OD) d 173.4, 173.3, 172.0, 170.3, 139.3, 138.1, 136.4, 135.0,
130.8, 130.5, 129.5, 129.0, 128.9, 127.8, 127.7, 74.3, 70.7, 59.5, 55.9, 55.3, 55.2,
52.6, 39.0, 32.0, 19.5, 18.8. 11B NMR (192.5 MHz, CD3OD) d 28.9. M.p. 254 °C.
HRMS (ESI, positive ion) m/z calcd for [M ? H]? = 604.2822, m/z found
604.2802.
Compound A10
1H NMR (300 MHz, CD3OD) d 8.16 (br, 1H), 7.61 (dd, J = 15.3, 8.3 Hz, 4H),
7.33–7.25 (m, 5H), 7.17–7.11 (m, 5H), 4.76–4.70 (m, 2H), 4.54 (s, 2H), 4.26 (dd,
J = 11.3, 4.9 Hz, 1H), 3.77 (d, J = 5.7 Hz, 2H), 3.68 (s, 3H), 3.16–3.10 (m, 1H),
3.0–2.92 (m, 1H), 2.08 (oct, J = 6.8 Hz, 1H), 0.89 (dd, J = 6.8, 2.4 Hz, 6H). 13C
NMR (75.5 MHz, DMSO-d6) d 171.7, 171.0, 169.5, 167.3, 138.2, 137.4, 131.0,
130.7, 129.2, 128.1, 127.9, 127.5, 127.4, 126.2, 125.4, 72.0, 69.4, 57.5, 53.4, 51.6,
37.4, 29.8, 18.8, 18.2. 11B NMR (192.5 MHz, DMSO-d6) d 3.1. M.p. 221 °C.
HRMS (ESI, negative ion) m/z calcd for [M – K]- = 626.2652, m/z found
626.2657.
Compound B1
1H NMR (300 MHz, CDCl3) d 7.81 (br, 1H), 7.73 (d, J = 6 Hz, 2H), 7.26–7.08 (m,
8H), 4.85 (q, J = 6.3 Hz, 1H), 4.44 (dd, J = 8.4, 5.1 Hz, 1H), 3.68–3.64 (m, 4H),
3.47 (d, J = 12.9 Hz, 1H), 3.12–3.05 (m, 2H), 2.91 (d, J = 4.8 Hz, 1H), 2.09–1.70
(m, 2H), 1.27 (s, 12H), 0.82 (dd, J = 6.9, 5.1 Hz, 6H), 0.72 (dd, J = 28.5, 6.9 Hz,
6H). 13C NMR (75.5 MHz, CDCl3) d 174.2, 172.0, 171.3, 142.3, 137.0, 136.9,
135.2, 129.5, 128.9, 128.7, 127.8, 127.1, 126.9, 84.0, 67.6, 57.6, 54.5, 53.4, 52.3,
37.7, 31.4, 31.3, 31.1, 30.9, 25.0, 19.5, 19.1, 17.9, 17.7. 11B NMR (192.5 MHz,
CDCl3) d 31.1. M.p. 96 °C. HRMS (ESI, positive ion) m/z calcd for
[M ? H]? = 594.3706, m/z found 594.3726.
Compound B2
1H NMR (300 MHz, DMSO-d6) d 8.48 (br, 1H), 7.74 (br, 3H), 7.39–7.05 (m, 8H),
5.20–5.00 (m, 1H), 4.33 (dd, J = 8.4, 6.0 Hz, 1H), 3.69–3.61 (m, 4H), 3.54–3.49
(m, 2H), 3.22–3.15 (m, 1H), 2.82 (dd, J = 14.1, 10.5 Hz, 1H), 2.13 (oct,
J = 6.6 Hz, 2H), 1.05–0.88 (m, 12H). 13C NMR (75.5 MHz, DMSO-d6) d 173.7,
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173.2, 167.5, 138.3, 135.6, 134.7, 133.1, 129.7, 128.1, 127.0, 104.7, 66.3, 59.4,
55.6, 52.7, 51.7, 39.3, 32.0, 31.4, 19.6, 19.2, 18.6, 18.4. 11B NMR (192.5 MHz,
CD3OD) d 28.2. M.p. 223 °C. HRMS (ESI, positive ion) m/z calcd for
[M ? H]? = 512.2924, m/z found 512.2948.
Compound B3
1H NMR (300 MHz, CD3OD) d 7.48–7.46 (m, 2H), 7.34–7.30 (m, 4H), 7.18–7.16
(m, 1H), 6.99–6.97 (m, 2H), 4.95 (m, 2H), 4.35 (d, J = 6 Hz, 1H), 3.69 (s, 3H), 3.58
(d, J = 12.9 Hz, 1H), 3.37–3.30 (m, 1H), 3.16–3.09 (m, 2H), 2.93–2.85 (m, 1H),
2.18–2.11 (m, 1H), 1.93–1.91 (m, 1H), 0.96–0.82 (m, 12H). 13C NMR (75.5 MHz,
CD3OD) d 173.6, 173.4, 138.4, 133.6, 133.1, 130.5, 129.7, 129.0, 128.1, 66.7, 59.4,
55.5, 52.7, 39.2, 32.1, 19.6, 19.1, 18.6. 11B NMR (192.5 MHz, CD3OD) d 4.2. M.p.
215 °C. HRMS (ESI, negative ion) m/z calcd for [M – K]- = 534.2753, m/z found
534.2777.
Compound B4
1H NMR (300 MHz, CDCl3) d 7.96 (br, 1H), 7.66 (d, J = 7.5 Hz, 2H), 7.39–7.18
(m, 9H), 7.05–6.97 (m, 4H), 6.72 (br, 1H), 4.71 (q, J = 7.5 Hz, 1H), 4.47 (dd,
J = 8.4, 4.8 Hz, 1H), 3.70 (s, 3H), 3.66–3.48 (m, 3H), 3.16–3.02 (m, 3H), 2.56 (br,
1H), 2.11 (oct, J = 5.4 Hz, 1H), 1.33 (s, 12H), 0.86 (t, J = 7.5 Hz, 6H). 13C NMR
(75.5 MHz, CDCl3) d 172.1, 171.1, 136.9, 136.5, 135.3, 129.6, 129.3, 129.0, 128.9,
127.8, 127.4, 127.2, 84.0, 62.9, 57.6, 54.7, 52.3, 38.7, 37.8, 31.4, 25.1, 19.1, 18.0.
11B NMR (192.5 MHz, CDCl3) d 31.0. M.p. 105 °C. HRMS (ESI, positive ion) m/
z calcd for [M ? H]? = 642.3706, m/z found 642.3733.
Compound B5
1H NMR (300 MHz, CD3OD) d 8.49 (br, 1H), 7.74 (br, 2H), 7.34–7.05 (m, 13H),
4.99–4.90 (m, 1H), 4.27 (dd, J = 8.1, 6.0 Hz, 1H), 3.95 (t, J = 6.6 Hz, 1H),
3.71–3.61 (m, 3H), 3.28 (s, 1H), 3.19–3.07 (m, 3H), 2.77 (dd, J = 13.8, 10.5 Hz,
1H), 2.16 (oct, J = 6.6 Hz, 1H), 0.99 (dd, J = 6.6, 4.8 Hz, 6H). 13C NMR
(75.5 MHz, CD3OD) d 173.4, 172.9, 172.8, 168.0, 138.3, 135.6, 135.3, 130.7,
130.6130.4, 130.1, 129.7, 128.9, 128.2, 62.6, 59.5, 55.5, 55.1, 52.7, 51.3, 39.7, 37.9,
31.9, 19.7, 18.9. 11B NMR (192.5 MHz, CD3OD) d 28.2. M.p. 265 °C. HRMS (ESI,
positive ion) m/z calcd for [M ? H]? = 560.2942, m/z found 560.2926.
Compound B6
1H NMR (300 MHz, CD3OD) d 7.51 (d, J = 7.8 Hz, 2H),7.34–7.23 (m, 8H),
7.14–7.11 (m, 3H), 6.95 (d, J = 7.8 Hz, 2H), 5.00–4.95 (m, 1H), 4.29–4.27 (m,
1H), 3.87 (t, J = 6.9 Hz, 1H), 3.70 (s, 3H), 3.43 (q, J = 17.1 Hz, 2H), 3.17 (dd,
J = 5.1, 4.8 Hz, 1H), 3.06–3.02 (m, 2H), 2.78 (dd, J = 10.8,10.5 Hz,1H),
2.18–2.15 (m, 1H) 1.01–0.97 (m, 6H). 13C NMR (75.5 MHz, DMSO-d6) d 171.8,
170.8, 166.4, 137.3, 134.6, 131.7, 129.4, 128.4, 128.1, 127.6, 127.1, 126.6, 59.9,
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57.5, 53.2, 51.8, 49.7, 36.2, 29.9, 19.0, 18.2. 11B NMR (192.5 MHz, DMSO-d6) d
3.2. M.p. 210 °C. HRMS (ESI, negative ion) m/z calcd for [M – K]- = 582.2753,
m/z found 582.2779.
Compound B8
1H NMR (300 MHz, CD3OD) d 7.67 (d, J = 8.0 Hz, 2H), 7.35–7.14 (m, 12H), 4.82
(dd, J = 8.9, 5.5 Hz, 1H), 4.44 (s, 2H), 4.33 (d, J = 6.1 Hz, 1H), 3.70 (s, 3H), 3.57
(d, J = 14.6 Hz, 1H), 3.53–3.47 (m, 4H), 3.16 (dd, J = 13.9, 5.5 Hz, 1H), 2.90 (dd,
J = 13.9, 8.8 Hz, 1H), 2.10 (sext, J = 6.5 Hz, 1H), 1.34 (s, 12H), 0.92 (dd,
J = 6.8, 1.7 Hz, 6H). 13C NMR (75.5 MHz, CD3OD) d 173.8, 173.5, 173.4, 143.4,
139.4, 138.2, 136.1, 130.7, 129.6, 129.5, 129.1, 129.0, 128.9, 128.0, 85.2, 76.0,
74.3, 71.3, 62.5, 59.4, 55.3, 52.8, 39.2, 32.1, 25.4, 19.6, 18.7. 11B NMR
(192.5 MHz, CD3OD) d 30.9. M.p. 95 °C. HRMS (ESI, positive ion) m/z calcd
for [M ? H]? = 672.3812, m/z found 672.3818.
Compound B9
1H NMR (300 MHz, Acetone-d6) d 8.44 (br, 1H), 7.85 (d, J = 7.8 Hz, 2H), 7.73
(br, 1H), 7.33–7.15 (m, 12H), 7.16 (br, 1H), 5.04 (m, 1H), 4.50 (s, 2H), 4.41 (dd,
J = 8.5, 5.9 Hz, 1H), 4.18 (m, 1H), 4.09 (d, J = 12.9 Hz, 1H), 3.99–3.86 (m, 3H),
3.69 (s, 3H), 3.26 (dd, J = 14.0, 5.0 Hz, 1H), 2.94 (dd, J = 13.9, 9.3 Hz, 1H), 0.91
(d, J = 6.8 Hz, 6H). 13C NMR (75.5 MHz, Acetone-d6) d 172.5, 171.8, 167.1,
138.9, 138.5, 136.0, 135.0, 130.8, 130.3, 129.6, 129.5, 129.0, 128.9, 128.0, 74.4,
69.4,60.7, 58.9, 55.7, 52.6, 51.7, 39.5, 32.0, 19.8, 19.0. 11B NMR (192.5 MHz,
CDCl3) d 29.6. M.p. 121 °C. HRMS (ESI, positive ion) m/z calcd for
[M ? H]? = 590.3030, m/z found 590.3029.
Compound B10
1H NMR (300 MHz, CD3OD) d 7.53 (d, J = 7.7 Hz, 2H), 7.34–7.13 (m, 10H), 7.04
(d, J = 7.6 Hz, 2H), 4.96–4.92 (m, 1H), 4.47 (s, 2H), 4.33 (d, J = 3.7 Hz, 1H),
3.79–3.62 (m, 7H), 3.24–3.17 (m, 1H), 2.89 (dd, J = 13.9, 9.9 Hz, 1H), 2.08 (sext,
J = 6.8 Hz, 1H), 0.91 (dd, J = 6.8, 3.7 Hz, 6H). 13C NMR (75.5 MHz, CD3OD) d
173.3, 173.2, 167.4, 138.5, 138.1, 133.9, 133.5, 130.5, 129.7, 129.5, 129.0, 128.1,
74.5, 69.0, 59.9, 59.4, 55.8, 52.7, 51.6, 39.2, 31.9, 19.6, 18.7. 11B NMR
(192.5 MHz, CD3OD) d 4.6. M.p. 139 °C. HRMS (ESI, negative ion) m/z calcd
for [M – K]- = 612.2859, m/z found 612.2850.
Results and discussion
Building blocks used for constructing the desired products are shown in Fig. 1. Six
potassium peptidyl organotriboroborates and their corresponding boron-containing
analogs were successfully synthesized (Fig. 2). The general synthetic strategies are
outlined in Schemes 1 and 2.
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Compounds B3, B6, and B10 were successfully obtained by formation of imines
from the free amine building blocks, by use of potassium 4-formylphenyltrifluor-
oborate (4), followed by reductive amination [6]. The resulting organotrifluorob-
orates were then converted into boronate esters (B1, B4, and B8) or boronic acids
(B2, B5, and B9). Compounds B7 and B11 were also synthesized and used as
positive controls. The structures of the synthesized compounds are summarized in
Fig. 2.
The anti-proliferative activity of the synthesized compounds against HepG2 and
MDA-MB231 were evaluated by use of an MTT assay; the IC50 values are listed in
Table 1. Doxorubicin was used as positive control because it is currently used to
treat different types of breast cancer and other cancers [7, 8].
Fig. 1 Amino acid and boron
building blocks
O
O
NH2
NH2
O
O
1a
1b
BnO
HO
HO
HO
NHBoc
NHBoc
NHBoc
O
O
O
2a
2c
2b
O
O
HO
H
Bpin
BF3K
3
4
BnO
O
O
O
H
H
H
MeO
N
X
MeO
N
X
MeO
N
X
N
N
N
N
N
H
N
H
H
H
H
H
O
O
O
O
O
O
Y
Y
Y
A1: X = CO, Y = Bpin
A2: X = CO, Y = B(OH)2
A3: X = CO, Y = BF3K
A4: X = CO, Y = Bpin
A8: X = CO, Y = Bpin
A9: X = CO, Y = B(OH)2
A10: X = CO, Y= BF3K
A11: X = CO, Y = H
A5: X = CO, Y = B(OH)2
A6: X = CO, Y = BF3K
A7: X = CO, Y = H
B1: X = CH2, Y = Bpin
B2: X = CH2, Y = B(OH)2
B3: X = CH2, Y = BF3K
B4: X = CH2, Y = Bpin
B5: X = CH2, Y = B(OH)2
B6: X = CH2, Y = BF3K
B7: X = CH2, Y = H
B8: X = CH2, Y = Bpin
B9: X = CH2, Y = B(OH)2
B10: X = CH2, Y = BF3K
B11: X = CH2, Y = H
Fig. 2 Summary of synthesized boron-containing peptidyl analogs
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2197
Table 1 SAR study of series
A and series B analogs
Entry
Compound
IC50 (µM)a
HepG2 MDA-MB231
1
2
3
4
5
6
7
> 100
> 100
24.7
29.9
B6
> 100
24.5
B7 (control)
B9
21.8
18.9
B10
> 100
> 100
> 100
> 100
> 100
> 100
B11 (control)
KF (control)
KBF4 (control)
O
O
OH
OH
O
OH
OH
8
0.4
2.3
OH
O
H
O
O
Doxorubicin (control)
NH2
Conclusions
Preliminary structure activity relationships (SAR) analysis revealed that the
presence of potassium trifluoroborate is vital for a compound’s biological activity;
replacing it with boronic acid or boronate ester greatly diminished or eliminated this
activity. The presence of a large hydrophobic and/or aromatic residue, for example a
benzyl or benzyloxymethyl group, on the tripeptidyl compounds significantly
enhanced the compounds’ activity. Compared with the full peptide backbones, an
aminomethyl linking group was preferred for the tri-peptidyl compounds. These
results provide insight into the structural optimization necessary for future
syntheses. Further work is in progress and the results will be presented in due
course.
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