Total synthesis of calothrixins and their analogues via formal [3+2] cycloaddition of arynes and 2-aminophenanthridinedione

Article abstract of DOI:10.1016/j.tetlet.2016.06.091

Bioactive indolo[3,2-j]phenanthridine alkaloids, calothrixin A, B, and their analogues have been synthesized via formal cycloaddition of arynes and 2-aminophenanthridinedione as the key step, which proceeds through successive C–C/C–N bond formation and su

Full text of DOI:10.1016/j.tetlet.2016.06.091

Tetrahedron Letters 57 (2016) 3481–3484
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Total synthesis of calothrixins and their analogues via formal [3+2]
cycloaddition of arynes and 2-aminophenanthridinedione
y
y
⇑
Jian Guo , I. N. Chaithanya Kiran , Jiangsheng Gao, R. Santhosh Reddy, Yun He
School of Pharmaceutical Sciences and Innovative Drug Research Centre, Chongqing University, 55 Daxuecheng South Rd., Shapingba, Chongqing City 401331, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Bioactive indolo[3,2-j]phenanthridine alkaloids, calothrixin A, B, and their analogues have been synthe-
sized via formal cycloaddition of arynes and 2-aminophenanthridinedione as the key step, which pro-
ceeds through successive C–C/C–N bond formation and subsequent oxidation under transition-metal-
free and mild conditions in the final stage of the synthesis.
Received 1 June 2016
Revised 20 June 2016
Accepted 21 June 2016
Available online 23 June 2016
Ó 2016 Elsevier Ltd. All rights reserved.
Keywords:
Total synthesis
Calothrixins
Analogues
Arynes
Calothrixin A (1a) and B (2a) are quinone-based natural prod-
ucts isolated from cell extracts of Calothrix cyanobacterium in
1999.1 They contain indolo[3,2-j]phenanthridine framework with
a unique assembly of highly important pharmacophores including
indole, quinoline, and quinone (Fig. 1). These compounds act as
human DNA topoisomerase I poisons, inhibiting in vitro growth
of chloroquine-resistant strain of the human parasite Plasmodium
falciparum and also exhibiting lethal effects on human cancer cell
lines at low nano-molar concentrations.2
Owing to their unique indolo[3,2-j]phenanthridine scaffold and
promise as lead compounds for drug discovery, great efforts have
been made for the total synthesis of calothrixins. Several
approaches have been reported for their synthesis,3 utilizing
strategies such as ortho-lithiation,4 Pd, or Cu-catalyzed coupling,5
electrocyclization,6 Friedel–Crafts acylation/alkylation,7 and radi-
cal reactions.8 These strategies rely upon functionalized indole,
quinoline, or carbazole as synthetic precursors, but a greater chal-
lenge lies in developing a practical and flexible synthetic strategy
for generating a library of calothrixin analogues, which would be
invaluable for structure–activity relationship (SAR) studies. Cas-
cade reactions based on arynes is a powerful tool for organic syn-
thesis and have been used in many total synthesises of natural
products.9 Especially, the introduction of 2-(trimethylsilyl)-aryl
triflates as aryne precursors allows the reaction to proceed at tran-
sition-metal-free and mild condition.10 Herein, we report a simple
and practical approach for the synthesis of calothrixins based on
cascade reaction of arynes, which has the flexibility to modulate
the functional groups on ring A at the final stage of their synthesis.
As a part of our research program on the synthesis of bioactive
heterocycles and their analogues employing aryne chemistry,11
calothrixins were identified as important synthetic targets. Based
on the knowledge of reaction between aryne and 2-amino-
quinone,11 their retrosynthetic analysis is outlined in Scheme 1.
It was hypothesized that ring A of calothrixin B (2a) could be pre-
pared via a formal [3+2] cycloaddition reaction of aryne 3a0 with 2-
aminophenanthridinedione 4, and variable aryne precursors could
also be used in the final stage to provide diverse analogues of calo-
thrixin B. Intermediate amine 4 could be obtained by oxidation of
compound 5, which in turn could be achieved by the reductive
amination of aldehyde 7 with o-iodoaniline (6) followed by palla-
dium-catalyzed intramolecular coupling.
Our investigation commenced with the synthesis of key inter-
mediate 4 as shown in Scheme 2. Regioselective bromination of
commercially available 2,5-dimethoxyaniline (8) followed by the
protection of primary amine with (Boc)2O provided compound 10
in good yields. Bromide 10 readily underwent lithium halogen
exchange, and quenching of the resulting anion with DMF gave
the corresponding aldehyde 7. Reductive amination of aldehyde 7
with o-iodoaniline (6) using NaCNBH3 and acetic acid provided
amine 11 in 82% yield.8b Acetylation of compound 11 with Ac2O
in the presence of catalytic amount of DMAP, followed by
intramolecular
palladium-catalyzed
cyclization
afforded
⇑
compound 5 in 85% yield.8b Compound 5 on treatment with CAN
in CH3CN/H2O (2:1) furnished phenanthridinedione 13 in 64%
Corresponding author.
E-mail address: yun.he@cqu.edu.cn (Y. He).
Contributed equally.
y
http://dx.doi.org/10.1016/j.tetlet.2016.06.091
0040-4039/Ó 2016 Elsevier Ltd. All rights reserved.

3482
J. Guo et al. / Tetrahedron Letters 57 (2016) 3481–3484
2
OCH3
TMS
OTf
TMS
OTf
TMS
OTf
1
3
4
TMS
OTf
O
O
O
C
E
N
13
H
N
H
N
B
12
11
D
6
N
5
O-
10
A
3a
3b
3c
3d
7
8
O
9
Figure 2. Aryne precursors 3a–3d.
Calothrixin A (1a)
Calothrixin B (2a)
Figure 1. Structures of calothrixins.
O
O
R
H2N
N
TBAT
3a + 4
N
N
+
O
O
O
O
THF
H
N
H2N
+
O
O
N
N
2a, R = H, 26%
14, R = Ph, 5%
15, 4%
4
Calothrixin B (2a)
3a'
Scheme 3. Preliminary results of formal [3+2] cycloaddition. Conditions: 3a
(0.64 mmol), 4 (0.4 mmol), TBAT (1.28 mmol), THF (8.0 mL), N2, 25 °C.
TMS
OTf
uncyclized C-arylated 15 (4%), and other several unidentifiable
by-products (Scheme 3). Based on the literature precedents, the
nitrogen atom in quinoline moiety could induce side reaction by
reacting with aryne,14 which might account for the low yield of
the desired product. It is noteworthy that other fluoride sources
(KF and CsF) led to decrease in desired product 2a and significant
increase of side products such as 14 and 15. Oxidation of calo-
thrixin B with mCPBA in CH2Cl2 afforded calothrixin A in 71% yield
(Table 1). Utilizing the same strategy, the synthesis of calothrixin
analogues was conducted. Under the optimized reaction condition,
key intermediate 4 was subjected to transition-metal-free cascade
reaction with symmetric and unsymmetrical aryne precursors
(3b–3d) to provide calothrixin B analogues (2b–2d) in moderate
yields. Calothrixin A analogues (1b–1d) were obtained in good
yield using the same oxidizing procedure with 1a, and the results
are summarized in Table 1.
When unsymmetrical aryne precursor 3c was used as the aryne
source, calothrixin analogue 2c was obtained regioselectively. To
establish the regioselectivity of the reaction, compound 1c was
treated with CH3I/NaH, followed by quenching the reaction mix-
ture with methanol, leading to the formation of derivative 17 in
88% yield, presumably through intermediate 16. Structure of com-
pound 17 was confirmed by NOE correlations (Scheme 4).
3a
OCH3
OCH3
I
BocHN
BocHN
+
NAc
H2N
CHO
OCH3
OCH3
7
6
5
Scheme 1. Retrosynthetic analysis of calothrixin B.
yield.12 Deprotection of Boc with TFA provided 2-aminophenan-
thridinedione 4 in quantitative yield.
With the required key intermediate 4 and aryne precursors 3a–
3d (Fig. 2) (synthesized following the reported procedures)13 in
hand, the formal [3+2] cycloaddition for constructing ring A of
calothrixin B was explored. We systematically examined the effect
of solvent (THF, Et2O, CH3CN, CH2Cl2, and TolH), fluoride source
(KF, CsF, TBAF, and TBAT), temperature (0 °C, 25 °C, and 60 °C)
and additive (18-crown-6) (see Supporting Information,
Table S1). When TBAT was used as fluoride source and THF as reac-
tion solvent at room temperature,11 calothrixin
obtained in 26% yield (43% brsm), along with N-arylated 14 (5%),
B (2a) was
Two plausible mechanisms for this cascade reaction are
depicted in Scheme 5. In Mechanism I, quinone amine nitrogen
OCH3
OCH3
OCH3
OCH3
BocHN
BocHN
H2N
RHN
n-BuLi, DMF
6
I
-100 oC to -80 o
C
AcOH, NaCNBH3
Br2
R
N
Br
CHO
76%
86%
82%
OCH3
OCH3
OCH3
OCH3
7
11, R = H
8
9, R = H
Ac2O, DMAP
94%
(Boc)2O
96%
10, R = Boc
12, R = Ac
OCH3
O
O
O
BocHN
BocHN
H2N
CH2Cl2/TFA
Pd(OAc)2, PPh3, K2CO3
85%
CAN
N
N
N
quant.
CH3CN:H2O (2:1)
64%
OCH3
O
O
5
13
Scheme 2. Synthesis of aminophenanthridinedione 4.
4

J. Guo et al. / Tetrahedron Letters 57 (2016) 3481–3484
3483
Table 1
Synthesis of calothrixin A (1a), calothrixin B (2a) and their analoguesa
O
O
O
O
R1
O
H
N
H
N
R1
R1
R2
R3
TMS
OTf
H2N
ii) mCPBA
i) TBAT
THF
+
N
N
N
R2
R2
CH2Cl2
O
O
R3
R3
4
2a, 2b, 2c, 2d
1a, 1b, 1c, 1d
3a, 3b, 3c, 3d
Entry
Aryne precursor
R1
R2
R3
Yield of 2 (%)
Yield of 1 (%)
1
2
3
4
3a
3b
3c
3d
H
H
OCH3
H
H
CH3
H
H
CH3
H
2a (26, 43b)
2b (22, 38b)
2c (25, 41b)
2d (25, 40b)
1a (71)
1b (70)
1c (76)
1d (74)
–(CH2)3–
a
Conditions: (i) 3 (0.64 mmol), 4 (0.4 mmol), TBAT (1.28 mmol), THF (8.0 mL), N2, 25 °C. (ii) 2 (0.05 mmol), mCPBA (0.25 mmol), CH2Cl2 (50 mL), reflux.
Yield based on brsm.
b
NOE
formal [3+2] cycloaddition of arynes and 2-aminophenanthridine-
dione as the key reaction. Various aryne precursors could be used
to quickly prepare ring-A modified calothrixins at the final stage of
their synthesis. Application of this methodology in synthesizing a
library of complex calothrixins and biological evaluations of the
synthetic calothrixins are currently underway in our laboratory
and will be reported in due course.
H3CO OCH3
H3C
N
H3CO
NaH, DMF, CH3I, 2 h
2c
N
then quenched with CH3OH
88%
CH3OH
O
17
OCH3
H3C
N
H3CO
N
Mechanism I:
O
O
TMS
OTf
H2N
F-
R1
R1
+
16
Scheme 4. Synthesis of 17 and NOE correlations.
N
O
3
3'
4
C-N bond
formation
in 4 could serve as a nucleophile to initiate a cascade reaction by
attacking the intermediately generated aryne 30, leading to N-ary-
lated anion intermediate 18. The newly generated aryl carbanion
may undergo subsequent Michael addition onto the quinone moi-
ety followed by in situ oxidation to furnish 20. In Mechanism II, the
carbon of enamine moiety acts as a nucleophile to attack aryne 30
to initiate the cascade reaction,15 leading to the zwitterionic inter-
mediate 180.16 The newly generated aryl carbanion attacks the
electron-deficient nitrogen (path a), followed by aromatization to
provide 2. The electron-withdrawing character of naphthalene-
dione makes the iminium nitrogen of 180 to behave as an elec-
trophile, resulting in C–N bond formation.17 Compound 2 could
react further with arynes to provide arylated compound 14,
demonstrating potential expandability of this method in forming
N-arylated analogues. Alternatively, the zwitterionic intermediate
180 could abstract a proton in an intra-molecular fashion, followed
by imine–enamine tautomerization to form 15 (path b). Based on
the fact that the nucleophilic attack usually favors meta position
to the methoxy group of the aryne generated in situ from unsym-
metrical aryne precursor 3c16,18 and the regioselectivity of
obtained product 2c, it is presumed that the cascade reaction pro-
ceeds through mechanism II, in line with our previous studies.11
The formation of by-product 15 also support Mechanism II, favour-
ing C-arylation over N-arylation.
O
O
O
i. C-C bond
formation
H
H
N
N
ii. oxidation
N
N
R1
O
R1
2'
18
Mechanism II:
O
TMS
H2N
F-
R1
R1
+
N
OTf
O
O
3
3'
4
C-C bond
formation
O
i. C-N bond
formation
H
N
H2N
b
ii. oxidation
a
N
N
a
R1
O
O
R1
2
18'
3'
b
In summary, we have developed a novel and flexible strategy
for the synthesis of calothrixins A, B, and their analogues.
Calothrixins A and B have been achieved in 4.8% (ten steps) and
6.8% (nine steps) overall yield involving transition-metal-free
14
15
Scheme 5. Plausible mechanisms of the cascade reaction.

3484
J. Guo et al. / Tetrahedron Letters 57 (2016) 3481–3484
Kusurkar, R. S. Tetrahedron Lett. 2014, 55, 155–162; (h) Ramkumar, N.;
Acknowledgments
Nagarajan, R. RSC Adv. 2015, 5, 87838–87840; (i) Ramkumar, N.; Nagarajan,
R. Org. Biomol. Chem. 2015, 13, 11046–11051.
We are grateful for the financial support from the National Nat-
ural Science Foundation of China (Nos. 21572027 and 21372267)
and the Chongqing Postdoctoral Research Grant (Nos.
xm2014079 and xm2015097).
6. (a) Tohyama, S.; Choshi, T.; Matsumoto, K.; Yamabuki, A.; Ikegata, K.; Nobuhiro,
J.; Hibino, S. Tetrahedron Lett. 2005, 46, 5263–5264; (b) Ramalingam, B. M.;
Saravanan, V.; Mohanakrishnan, A. K. Org. Lett. 2013, 15, 3726–3729; (c)
Saravanan, V.; Ramalingam, B. M.; Mohanakrishnan, A. K. Eur. J. Org. Chem.
2014, 1266–1279.
7. (a) Bernardo, P. H.; Chai, C. L. L.; Elix, J. A. Tetrahedron Lett. 2002, 43, 2939–
2940; (b) Bernardo, P. H.; Chai, C. L. L. J. Org. Chem. 2003, 68, 8906–8909.
8. (a) Bennasar, M.-L.; Roca, T.; Ferrando, F. Org. Lett. 2006, 8, 561–564; (b) Xu, S.;
Nguyen, T.; Pomilio, I.; Vitale, M. C.; Velu, S. E. Tetrahedron 2014, 70, 5928–
5933.
9. (a) Tadross, P. M.; Stoltz, B. M. Chem. Rev. 2012, 112, 3550–3577; (b) Gampe, C.
M.; Carreira, E. M. Angew. Chem., Int. Ed. 2012, 51, 3766–3778; (c) Bhunia, A.;
Yetra, S. R.; Biju, A. T. Chem. Soc. Rev. 2012, 41, 3140–3152; (d) Okuma, K.
Heterocycles 2012, 85, 515–544; (e) Wu, C.; Shi, F. Asian J. Org. Chem. 2013, 2,
116–125; (f) Dubrovskiy, A. V.; Markina, N. A.; Larock, R. C. Org. Biomol. Chem.
2013, 11, 191–218.
Supplementary data
Supplementary data (experimental procedures and 1H NMR and
13C NMR spectra) associated with this article can be found, in the
online version, at http://dx.doi.org/10.1016/j.tetlet.2016.06.091.
References and notes
10. (a) Himeshima, Y.; Sonoda, T.; Kobayashi, H. Chem. Lett. 1983, 1211–1214; for a
modified procedure, see (b) Peña, D.; Pérez, D.; Cobas, A.; Guitián, E. Synthesis
2002, 1454–1458.
11. Guo, J.; Kiran, I. N. C.; Reddy, R. S.; Gao, J.; Tang, M.; Liu, Y.; He, Y. Org. Lett.
2016, 18, 2499–2502.
12. Feng, Y.; Liu, J.; Carrasco, Y. P.; MacMillan, J. B.; De Brabander, J. K. J. Am. Chem.
Soc. 2016, 138, 7130.
13. (a) Shen, C.; Yang, G.; Zhang, W. Org. Lett. 2013, 15, 5722; (b) Rossini, A. F.;
Muraca, A. C.; Casagrande, G. A.; Raminelli, C. J. Org. Chem. 2015, 80, 10033; (c)
Hermann, K.; Pratumyot, Y.; Polen, S.; Hardin, A. M.; Dalkilic, E.; Dastan, A.;
Badjic´, J. D. Chem. Eur. J. 2015, 21, 3550–3555.
14. (a) Bhunia, A.; Porwal, D.; Gonnade, R. G.; Biju, A. T. Org. Lett. 2013, 15, 4620–
4623; (b) Bhunia, A.; Roy, T.; Pachfule, P.; Rajamohanan, P. R.; Biju, A. T. Angew.
Chem., Int. Ed. 2013, 52, 10040–10043.
15. Ramtohul, Y. K.; Chartrand, A. Org. Lett. 2007, 9, 1029–1032.
16. Liu, Z.; Larock, R. C. J. Org. Chem. 2006, 71, 3198–3209.
17. (a) Ciganek, E. Org. React. 2008, 72, 1–366; (b) Brachi, J.; Rieker, A. Synthesis
1977, 708–711; (c) Fiaud, J.-C.; Kagan, H. B. Tetrahedron Lett. 1970, 11, 1813–
1816.
1. Rickards, R. W.; Rothschild, J. M.; Willis, A. C.; de Chazal, N. M.; Kirk, J.; Kirk, K.;
Saliba, K. J.; Smith, G. D. Tetrahedron 1999, 55, 13513–13520.
2. (a) Bernardo, P. H.; Chai, C. L. L.; Heath, G. A.; Mahon, P. J.; Smith, G. D. J. Med.
Chem. 2004, 47, 4958–4963; (b) Bernardo, P. H.; Chai, C. L. L.; Guen, M. L.;
Smith, G. D.; Waring, P. Bioorg. Med. Chem. Lett. 2007, 17, 82–85; (c) Khan, Q. A.;
Lu, J.; Hecht, S. M. J. Nat. Prod. 2009, 72, 438–442; (d) Hatae, N.; Satoh, R.; Chiba,
H.; Osaki, T.; Nishiyama, T.; Ishikura, M.; Abe, T.; Hibino, S.; Choshi, T.; Okada,
C.; Yoyota, E. Med. Chem. Res. 2014, 23, 4956–4961.
3. (a) Choshi, T.; Hibino, S. Heterocycles 2009, 77, 85–97; (b) Parvatkar, P. T. Curr.
Org. Chem. 2016, 20, 839–855; (c) Xu, S.; Nijampatnam, B.; Dutta, S.; Velu, S. E.
Mar. Drugs 2016, 14, 17.
4. (a) Kelly, T. R.; Zhao, Y.; Cavero, M.; Torneiro, M. Org. Lett. 2000, 2, 3735–3737;
(b) Ramkumar, N.; Nagarajan, R. J. Org. Chem. 2014, 79, 736–741; (c) Mal, D.;
Roy, J.; Biradha, K. Org. Biomol. Chem. 2014, 12, 8196–8203.
5. (a) Bernardo, P. H.; Fitriyanto, W.; Chai, C. L. L. Synlett 2007, 1935–1939; (b)
Abe, T.; Ikeda, T.; Yanada, R.; Ishikura, M. Org. Lett. 2011, 13, 3356–3359; (c)
Abe, T.; Ikeda, T.; Choshi, T.; Hibino, S.; Hatae, N.; Toyata, E.; Yanada, R.;
Ishikura, M. Eur. J. Org. Chem. 2012, 5018–5027; (d) Bhosale, S. M.; Gawade, R.
L.; Puranik, V. G.; Kusurkar, R. S. Tetrahedron Lett. 2012, 53, 2894–2896; (e)
Ramkumar, N.; Nagarajan, R. J. Org. Chem. 2013, 78, 2802–2807; (f)
Kaliyaperumal, S. A.; Banerjee, S.; Kumar, U. K. S. Org. Biomol. Chem. 2014, 12,
6105–6113; (g) Bhosale, S. M.; Momin, M. A.; Kunjir, S.; Rajamohanan, P. R.;
18. Tadross, P. M.; Gilmore, C. D.; Bugga, P.; Virgil, S. C.; Stoltz, B. M. Org. Lett. 2010,
12, 1224–1227.