Synthesis of the CDK-inhibitor paullone by cyclization of a deprotonated α-aminonitrile

Article abstract of DOI:10.1055/s-0028-1083250

Cyclization of a deprotonated N-monosubstituted α-aminonitrile obtained by Strecker reaction of a protected 2-amino-benzaldehyde with ethyl 2-aminocinnamate serves as the key step in a short synthesis of the tetracyclic ε-lactam paullone. Georg Thieme Ver

Full text of DOI:10.1055/s-0028-1083250

PAPER
3941
Synthesis of the CDK-Inhibitor Paullone by Cyclization of a Deprotonated
a-Aminonitrile
S
ynthesis of the
i
C
DK
-
l
Inhibito
l
r
Paullone Opatz,*,a Dorota Ferencb
a
Institut für Organische Chemie, Universität Hamburg, Martin-Luther-King-Platz 6, 20146 Hamburg, Germany
Institut für Organische Chemie, Universität Mainz, Duesbergweg 10-14, 55128 Mainz, Germany
E-mail: opatz@chemie.uni-hamburg.de
b
Received 15 July 2008
O
R
Abstract: Cyclization of a deprotonated N-monosubstituted a-ami-
nonitrile obtained by Strecker reaction of a protected 2-amino-
benzaldehyde with ethyl 2-aminocinnamate serves as the key step
in a short synthesis of the tetracyclic e-lactam paullone.
Y
+ CN–
N
2
O
N-deprotonation
(irreversible)
Key words: Michael addition, a-aminonitriles, indoles, deprotona-
tion
Y
NH
C-deprotonation
(reversible)
O
Strecker products derived from ammonia or primary
amines and aromatic, heteroaromatic, or non-enolizable
a,b-unsaturated aldehydes can serve as readily accessible
starting materials for the generation of stabilized a-amino
carbanions. Their quantitative deprotonation with potassi-
um bis(trimethylsilyl)amide in tetrahydrofuran at low
temperatures will furnish the corresponding potassium
keteneiminates without inducing the competing retro-
Strecker reaction.1–6 If, on the other hand, weaker bases
such as potassium tert-butoxide are employed, reprotona-
tion of the carbanion obtained can not be prevented. This
ultimately leads to the elimination of HCN from the ami-
nonitrile to furnish the corresponding imine. Only if the
keteniminate anion is consumed in a fast consecutive re-
action, clean a-substitution of an aminonitrile can be
achieved under ‘thermodynamic’ deprotonation condi-
tions. An example of such a process is the one-pot synthe-
sis of substituted indoles from a-aminonitriles derived
from 2-aminocinnamic acid esters and amides.7 In this
case, the 5-exo-trig-cyclization of the carbanion 3 is fast
enough to furnish the indole 5 via the 2-cyanoindoline 4
in up to quantitative yield (Scheme 1).
NC
R
1
Y
Y = OR, NR2
NH
·
R
3
–N
5-exo-trig
(fast)
+ H+
O
O
Y
Y
– HCN
CN
R
R
N
N
H
H
5
4
Scheme 1 One-pot synthesis of indoles from a-aminonitriles
become an important task with respect to antiproliferative
chemotherapy.13 While paullone (11) itself is active in the
low micromolar range, some close derivatives inhibit cy-
clin-dependent kinases at nanomolar or even subnanomo-
lar concentrations.14
If the reaction is performed in an aprotic solvent, the inter-
mediates 4 can be isolated and characterized, whereas in
protic solvents, the elimination of HCN is fast and indoles
5 are obtained directly. Here, we report on the application
of this method to the synthesis of the CDK inhibitor
paullone (11),8 which represents the prototype of a class
of potent inhibitors of cyclin-dependent kinases (CDKs)
that have attracted much attention in recent years.9–12 The
CDKs are a family of protein kinases that are involved in
the regulation of the cell cycle. As a large fraction of hu-
man tumors exhibit aberrant CDK activity, the design of
inhibitors of CDKs that induce arrest of the cell cycle has
For the synthesis of 11, 2-aminobenzyl alcohol was Boc-
protected and converted into aldehyde 6 by Swern oxida-
tion. Presumably due to the bulky ortho-substituent, the
Strecker reaction of 6 with ethyl 2-aminocinnamate (7)
proceeded sluggishly and required repeated addition of
potassium cyanide and acetic acid. Surprisingly, the pro-
longed reaction times and elevated reaction temperatures
already led to the formation of substantial amounts (13%)
of indole 9; aminonitrile 8 was obtained in 47% yield
along with imine 10 (5%). Cyclization of aminonitrile 8
with potassium tert-butoxide in ethanol gave indole 9 in
74% yield. Again, imine 10 (17%) was obtained as a side
product. Thus, the combined yield for the conversion of
aminocinnamate 7 into indole 9 amounted to 48%. Aci-
dolytic removal of the Boc group and subsequent forma-
tion of the seven-membered lactam ring by heating the
SYNTHESIS 2008, No. 24, pp 3941–3944
x
x.
x
x
.
2
0
0
8
Advanced online publication: 01.12.2008
DOI: 10.1055/s-0028-1083250; Art ID: T11408SS
© Georg Thieme Verlag Stuttgart · New York

3942
T. Opatz, D. Ferenc
PAPER
F
254, E. Merck). Flash column chromatography was carried out on
1) Boc2O
OH
NH2
O
silica gel (32–63 mm, 60 Å, MP Biomedicals GmbH).
2) Swern
NHBoc
96% (crude)
2-(tert-Butoxycarbonylamino)benzaldehyde (6)19,20
6
O
To a soln of 2-aminobenzyl alcohol (2.50 g, 20.3 mmol) in THF (10
mL) was added Boc2O (4.60 g, 21.1 mmol). The soln was stirred
overnight at r.t. and then heated to 50 °C for 5 h. After removal of
the solvent in vacuo, the residue was filtered over silica gel (cyclo-
hexane–EtOAc, 6:1). Concentration in vacuo furnished crude 2-
(tert-butoxycarbonylamino)benzyl alcohol (4.54 g) as a yellow oil
which was subjected to oxidation without further purification.
1H NMR (300 MHz, CDCl3): d = 7.88 (br d, J = 8.0 Hz, 1 H, H3),
7.65 (br s, 1 H, NH), 7.30 (d pseudo t, Jt = 7.5 Hz, Jd = 1.7 Hz, 1 H,
H4), 7.15 (dd, J = 7.5, 1.7 Hz, 1 H, H6), 7.01 (dt, Jt = 7.5 Hz,
Jd = 1.1 Hz, 1 H, H5), 4.65 (s, 2 H, CH2), 2.30 (br s, 1 H, OH), 1.52
(s, 9 H, t-Bu).
OEt
KCN, AcOH, EtOH
NH
O
47%
NC
OEt
BocHN
NH2
8
7
KOtBu, EtOH
74%
5%
13%
O
O
OEt
OEt
A soln of oxalyl chloride (732 mL, 8.39 mmol) in anhyd CH2Cl2 (5
mL) was cooled to –78 °C under an argon atmosphere. After the ad-
dition of DMSO (1.79 mL, 25.2 mmol), the soln was stirred for 45
min. A soln of the crude amino alcohol (2.00 g) in anhyd CH2Cl2
(8.3 mL) was added and the mixture was stirred at –78 °C for 30
min. Anhyd Et3N (7.00 mL, 50.3 mmol) was added and the mixture
was stirred at –78 °C for 45 min and at –20 °C for 30 min.21 H2O (20
mL) was added and the mixture was partitioned between H2O and
Et2O. The aqueous phase was extracted with Et2O, the combined or-
ganic layers were washed with 1 M KHSO4 soln and brine, dried
(Na2SO4), and the solvent was removed in vacuo to furnish 6 (1.89
g, 96% over 2 steps) as a yellowish oil that solidified upon standing
and was used without further purification; mp 50–51 °C (crude)
N
N
H
BocHN
BocHN
10
9
O
73%
NH
1) TFA, EtSMe, CH2Cl2
2) AcOH, dioxane, 80 °C
N
H
paullone (11)
1H NMR (300 MHz, CDCl3): d = 10.39 (br s, 1 H, NH), 9.88 (s, 1
H, CHO), 8.44 (d, J = 8.5 Hz, 1 H), 7.61 (dd, J = 7.7, 1.7 Hz, 1 H),
7.55 (ddd, J = 8.5, 7.3, 1.7 Hz, 1 H), 7.12 (pseudo t, J = 7.5 Hz, 1
H), 1.51 (s, 9 H, t-Bu).
Scheme 2 Synthesis of paullone (11) from aminocinnamate 7
aniline with acetic acid in dioxane furnished paullone (11)
in 73% yield.
Ethyl (E)-2-({[2-(tert-Butoxycarbonylamino)phenyl]cyano-
methyl}amino)cinnamate (8) and Ethyl (E,E)-2-{[2-(tert-But-
oxycarbonylamino)benzylidene]amino}cinnamate (10)
In summary, 7 was converted into paullone (11) in four
steps in 35% overall yield. In contrast to the classical
preparation of the 7,12-dihydroindolo[3,2-d][1]benz-
To
a soln of 6 (1.13 g, 5.11 mmol) and ethyl (E)-2-
azepin-6(5H)-one core from 3,4-dihydro-1H-1-benz- aminocinnamate22,23 (7, 813 mg, 4.25 mmol) in EtOH (5 mL) was
added AcOH (100 mL, 1.75 mmol) and the mixture was stirred at 50
°C for 3 h. After the addition of KCN (747 mg, 11.5 mmol) and
AcOH (779 mL, 13.6 mmol), stirring was continued at 60 °C for 18
h. Another portion of KCN (277 mg, 4.25 mmol) and AcOH (243
mL, 4.25 mmol) was added and the mixture was stirred at 60 °C for
5 h. The mixture was partitioned between H2O and CH2Cl2, the or-
ganic layer was washed with sat. aq NaHCO3 soln, dried (Na2SO4),
and concentrated in vacuo. As the 1H NMR spectrum of the crude
product revealed incomplete conversion, the material was dissolved
in EtOH (5 mL), KCN (747 mg, 11.5 mmol), and AcOH (779 mL,
13.6 mmol) were added and the mixture was stirred at 60 °C for 3
h. After partitioning between H2O and CH2Cl2, the organic layer
was washed with sat. aq NaHCO3 soln, dried (Na2SO4), and concen-
trated in vacuo to furnish the crude product (1.86 g). Flash chroma-
tography (silica gel, PE–t-BuOMe, 6:1) furnished 8 (843 mg, 47%)
along with imine 10 (87 mg, 5%), indole 9 (223 mg, 13%), and un-
reacted aldehyde 6 (142 mg, 642 mmol).
azepine-2,5-dione and phenylhydrazine in a Fischer
indole synthesis,8 the presented route avoids the use of
harsh reaction conditions. As the preparation of indoles by
the cyclization of deprotonated a-aminonitriles allows the
introduction of substituents to both benzene rings, a wide
variety of paullone derivatives should be accessible. In
terms of simplicity and efficiency, the method compares
favorably with other alternative approaches to the
paullones, e.g. by palladium-catalyzed reactions15,16 or
radical cyclizations.17,18
1H and 13C NMR spectra were recorded on a Bruker AC-300 or
Avance-II 400 spectrometer, chemical shifts were referenced to the
residual solvent signal (CDCl3: dH = 7.26, dC = 77.0). FD-MS spec-
tra were measured on a Finnigan MAT-95 spectrometer. ESI-
HRMS spectra were measured on a Waters Q-TOF-Ultima 3
equipped with a LockSpray interface (trihexylamine as external ref-
erence). IR spectra were recorded on a Perkin-Elmer 1760X FTIR
spectrophotometer. The melting points were measured on a Dr. Tot-
toli apparatus (Büchi) and are uncorrected. CH2Cl2 and Et3N were
freshly distilled from CaH2 under argon, EtOH was distilled from
Mg(OEt)2 under argon. All other solvents and reagents were pur-
chased from commercial suppliers and were used without further
purification. Petroleum ether (PE) had a boiling range of 40–70 °C.
TLC was performed on aluminum sheets coated with silica gel (60
Aminocinnamate 8
Yellow foam; mp 63–65 °C.
IR (KBr): 3405 (br), 2980, 1703, 1630, 1604, 1519 (sh), 1454, 1368,
1317, 1248 (sh), 1161 (sh), 1049 (sh), 753 cm–1.
1H NMR (300 MHz, CDCl3): d = 7.66–7.78 (m, 3 H) [contained in
this multiplet: 7.76 (d, J = 15.6 Hz, 1 H, Hb)], 7.37–7.50 (m, 3 H),
7.24 (dd, J = 7.6, 1.4 Hz, 1 H), 6.97–7.06 (m, 3 H), 6.33 (d, J = 15.6
Hz, 1 H, Ha), 5.58 (d, J = 9.0 Hz, 1 H, CHN), 4.50 (d, J = 9.0 Hz, 1
Synthesis 2008, No. 24, 3941–3944 © Thieme Stuttgart · New York

PAPER
Synthesis of the CDK-Inhibitor Paullone
3943
H, NH), 4.20 (q, J = 7.2 Hz, 2 H, OCH2), 1.44 (s, 9 H, t-Bu), 1.29
(t, J = 7.2 Hz, 3 H, CH3).
(1 d). The mixture was poured into sat. aq NaHCO3 and after addi-
tion of EtOAc, the organic layer was separated, dried (Na2SO4), and
concentrated in vacuo to furnish a colorless foam (61 mg). The
crude aniline was dissolved in dioxane (2 mL) and after addition of
AcOH (250 mL, 4.37 mmol), the mixture was stirred at 80 °C until
TLC indicated complete conversion (2 d). The mixture was parti-
tioned between sat. aq NaHCO3 and EtOAc and the organic layer
was separated, dried (Na2SO4), and concentrated in vacuo. Tritura-
tion of the residue (EtOAc–PE) furnished 11 (46 mg, 73%) as pale
yellow crystals; mp >260 °C (dec.)16
13C NMR (75.5 MHz, CDCl3): d = 166.7 (CO2Et), 153.3 (OCONH),
142.6 (C2), 138.8 (Cb), 136.4 (C2¢), 131.5, 130.6, 128.4, 128.3,
125.3, 124.7 (2C), 123.4, 121.5, 120.6, 117.1 (CN), 114.6 (C3),
81.1 [C(CH3)3], 60.6 (OCH2), 48.2 (CHN), 28.1 [C(CH3)3], 14.2
(CH3).
MS (FD): m/z (%) = 421.5 (100) [M]+.
Anal. Calcd for C24H27N3O4: C, 68.39; H, 6.46; N, 9.97. Found: C,
68.12; H, 6.43; N, 9.66.
IR (KBr): 3436 (br), 3230 (br), 2925, 1646, 1576 (sh), 1491 (sh),
1464, 1423, 1399, 743 cm–1.
(Benzylideneamino)cinnamate 10
Yellowish crystals; mp 137–138 °C.
1H NMR (300 MHz, DMSO-d6): d = 11.59 (br s, 1 H, indole-NH),
10.10 (br s, 1 H, amide-NH), 7.74 (dd, J = 7.7, 1.6 Hz, 1 H), 7.66
(br d, J = 7.7 Hz, 1 H), 7.44 (dt, Jd = 8.0 Hz, Jt = 0.9 Hz, 1 H), 7.37
(ddd, J = 8.0, 7.4, 1.6 Hz, 1 H), 7.24–7.30 (m, 2 H), 7.17 (ddd,
J = 8.1, 7.0, 1.2 Hz, 1 H), 7.07 (ddd, J = 8.0, 7.0, 1.0 Hz, 1 H), 3.50
(br s, 2 H, CH2).
13C NMR (75.5 MHz, DMSO-d6): d = 171.6 (C6), 137.4, 135.4,
132.5 (C4a, C11a, C12a), 128.0, 126.9 (C1, C3), 126.6 (C7b), 123.7
(C2), 122.9 (C12b), 122.3, 122.1, 119.1, 118.0 (C4, C8, C9, C10),
111.5 (C11), 107.6 (C7a), 31.6 (C7).
IR (KBr): 3424 (br), 2982, 1722, 1708, 1632, 1619, 1583, 1531,
1481, 1540, 1366, 1317, 1272, 1245, 1197, 1179, 1159, 1161, 1030
(sh), 771, 760 cm–1.
1H NMR (300 MHz, CDCl3): d = 11.85 (br s, 1 H, NH), 8.52 (d,
J = 8.3 Hz, 1 H), 8.46 (s, 1 H, CH=N), 8.14 (d, J = 16.0 Hz, 1 H,
Hb), 7.65 (dd, J = 7.7, 1.5 Hz, 1 H), 7.41–7.50 (m, 3 H), 7.29 (pseu-
do t, J = 7.6 Hz, 1 H), 7.00–7.10 (m, 2 H), 6.46 (d, J = 16.0 Hz, 1 H,
Ha), 4.25 (q, J = 7.1 Hz, 2 H, OCH2), 1.51 (s, 9 H, t-Bu), 1.31 (t,
J = 7.1 Hz, 3 H, CH3).
13C NMR (75.5 MHz, CDCl3): d = 166.6 (CO2Et), 163.9 (CH=N),
153.3 (OCONH), 149.8 (C2), 141.4 (C2¢), 140.7 (Cb), 134.6, 132.9,
131.2, 128.7 (C1), 127.5, 126.5, 121.2, 120.1, 120.0 (C1¢), 119.2,
118.0, 80.2 [C(CH3)3], 60.4 (OCH2), 28.1 [C(CH3)3], 14.3 (CH3).
MS (FD): m/z (%) = 248.1 (100) [M]+.
HRMS (ESI): m/z [M + Na]+ calcd for C16H12NaN2O: 271.0847;
found: 271.0858.
MS (FD): m/z (%) = 394.5 (100) [M]+.
Acknowledgment
Anal. Calcd for C23H26N2O4: C, 70.03; H, 6.64; N, 7.10. Found: C,
69.79; H, 6.49; N, 6.98.
This work was supported by the Deutsche Forschungsgemeinschaft.
We thank H. Kolshorn (Mainz) for the NMR spectroscopic analyses
and Dr. N. Hanold (Mainz) for mass spectrometry.
Ethyl {2-[2-(tert-Butoxycarbonylamino)phenyl]-1H-indol-3-
yl}acetate (9)
To a soln of 8 (300 mg, 712 mmol) in anhyd EtOH (6 mL) was added
KOt-Bu (88 mg, 784 mmol) and the soln was stirred at r.t. for 30
min. The mixture was partitioned between sat. aq NaHCO3 and
CH2Cl2, the organic layer was dried (Na2SO4) and concentrated in
vacuo to furnish an orange foam (279 mg). Flash chromatography
(silica gel, PE–t-BuOMe, 20:1) furnished 9 (209 mg, 74%) as col-
orless crystals; mp 113–115 °C. Imine 10 (47 mg, 17%) was also
obtained.
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IR (KBr): 3405 (br), 2980, 2933, 1730 (sh), 1586, 1519, 1461, 1446,
1393, 1369, 1300, 1242, 1158 (sh), 1049, 1027, 766, 745 cm–1.
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1H NMR (300 MHz, CDCl3): d = 8.32 (br s, 1 H, indole-NH), 8.16
(d, J = 8.3 Hz, 1 H), 7.66 (d, J = 8.3 Hz, 1 H), 7.37–7.45 (m, 2 H),
7.34 (dd, J = 7.5, 1.7 Hz, 1 H), 7.17–7.31 (m, 2 H), 7.12 (pseudo t,
J = 7.5 Hz, 1 H), 6.93 (br s, 1 H, NHBoc), 4.12 (q, J = 7.1 Hz, 2 H,
OCH2), 3.63 (s, 2 H, CH2CO), 1.46 (s, 9 H, t-Bu), 1.22 (t, J = 7.1
Hz, 3 H, CH3).
13C NMR (75.5 MHz, CDCl3): d = 171.8 (CO), 153.2 (C2¢), 137.3,
136.1, 132.3 (C2, C2¢, C7a), 131.3 (CH), 129.9 (CH), 128.1, 122.9
(CH), 122.6 (CH), 121.6, 120.2 (CH), 119.1 (CH), 111.2 (C7),
108.0 (C3), 80.7 [C(CH3)3], 60.8 (OCH2), 30.7 (ArCH2), 28.2
[C(CH3)3], 14.1 (CH3).
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69.97; H, 6.66; N, 7.08.
Paullone [7,12-Dihydroindolo[3,2-d][1]benzazepin-6(5H)-one,
11]8
To a soln of 9 (100 mg, 254 mmol) in anhyd CH2Cl2 (2 mL) was add-
ed EtSMe (229 mL, 2.54 mmol) and TFA (195 mL, 2.54 mmol) and
the soln was stirred at r.t. until TLC indicated complete conversion
Synthesis 2008, No. 24, 3941–3944 © Thieme Stuttgart · New York

3944
T. Opatz, D. Ferenc
PAPER
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Synthesis 2008, No. 24, 3941–3944 © Thieme Stuttgart · New York