Cyano Group Removal from Cyano-Promoted Aza-Diels-Alder Adducts: Synthesis and Structure-Activity Relationship of Phenanthroindolizidines and Phenanthroquinolizidines

Article abstract of DOI:10.1021/acs.orglett.5b03395

Phenanthroindolizidines and phenanthroquinolizidines were concisely synthesized by the reductive decyanization of cyano-promoted intramolecular aza-Diels-Alder cycloadducts followed by aryl-aryl coupling. Cyano groups were removed from α-aminoacrylonitriles via treatment with sodium borohydride in 2-propanol in almost quantitative yields; a possible mechanism was proposed and examined using D-labeling experiments. A systematic study of the effects of the phenanthrene substitution pattern on the anticancer activity against three human cancer cell lines was discussed. (Chemical Equation Presented).

Full text of DOI:10.1021/acs.orglett.5b03395

Letter
pubs.acs.org/OrgLett
Cyano Group Removal from Cyano-Promoted Aza-Diels−Alder
Adducts: Synthesis and Structure−Activity Relationship of
Phenanthroindolizidines and Phenanthroquinolizidines
Chi-Fen Chang,† Chien-Fu Li,‡ Chia-Chen Tsai,‡ and Ta-Hsien Chuang*,‡,§
†Department of Anatomy, School of Medicine, China Medical University, Taichung 40402, Taiwan
‡School of Pharmacy, China Medical University, Taichung 40402, Taiwan
§Research Center for Chinese Herbal Medicine, China Medical University, Taichung 40402, Taiwan
S
* Supporting Information
ABSTRACT: Phenanthroindolizidines and phenanthro-
quinolizidines were concisely synthesized by the reductive
decyanization of cyano-promoted intramolecular aza-Diels−
Alder cycloadducts followed by aryl−aryl coupling. Cyano
groups were removed from α-aminoacrylonitriles via treatment
with sodium borohydride in 2-propanol in almost quantitative
yields; a possible mechanism was proposed and examined
using D-labeling experiments. A systematic study of the effects
of the phenanthrene substitution pattern on the anticancer
activity against three human cancer cell lines was discussed.
ntramolecular aza-Diels−Alder (IADA) reactions are useful
Ifor the synthesis of nitrogen heterocycles such as
indolizidines and quinolizidines. However, it is difficult to
react an unsubstituted 1-azadiene with an unreactive carbon−
carbon double bond. In a normal (HOMOdiene-controlled)
electron-demand IADA reaction, the introduction of a
carbethoxy group on the dienophile would promote the
IADA reaction.1 Alternatively, the presence of an electron-
synthetic strategies to access other positions (especially C-1, C-
withdrawing group (e.g., acyl, diethoxyphosphoryl, or cyano
group) on 1-azadiene would also increase its reactivity toward
enophiles by lowering the lowest unoccupied molecular orbital
(LUMO) of 1-azadiene in the inverse electron-demand IADA
reaction.2−4 Although these functional groups promote the
IADA reaction, their removal after the reaction presents a
challenge that limits their applications in the total synthesis of
natural products.
Phenanthroindolizidines and phenanthroquinolizidines ex-
hibit several interesting biological activities (e.g., anticancer and
antiamoebic activities, etc.), and several synthetic methods with
access to these pentacyclic alkaloids for structure−activity
relationship (SAR) studies have been developed.5 Although
several previously reported routes were concise, the syntheses
and anticancer SARs focused only on specific positions (C-2, C-
3, C-6, and C-7) of the phenanthrene ring owing to the
limitations of the synthetic methods.6 These limitations include
commercially unavailable starting materials and sensitivity of
the formation of the phenanthrene ring by intramolecular aryl−
aryl coupling of stilbenes to the substituents on the phenyl
rings and the double bond.7 Notably, Niphakis and Georg
reported a VOF3-mediated aryl−alkene coupling to prepare C5-
substituted phenanthroindolizidines (eq 1).8 However, versatile
4, and C-8) on phenanthroindolizidines and phenanthro-
quinolizidines for anticancer SAR studies regarding the
phenanthrene substitution pattern are urgently needed.
Herein, we report a concise strategy to construct
pyrrolizidine and indolizidine systems by a cyano-group-
promoted IADA reaction followed by reductive decyanization.
A series of phenanthroindolizidines 1 and phenanthro-
quinolizidines 2 were synthesized from decyanization products
3 and 4 via aryl−aryl oxidative coupling reactions (Scheme 1).
This reaction sequence provides a new synthetic approach to
synthesize pentacyclic alkaloids with three to five methoxyl
groups on the C-1 to C-8 positions of the phenanthrene ring.
First, (E)-2,3-diphenylacrylaldehydes 9a−i were synthesized
by the Knoevenagel condensation of commercially available
benzaldehydes 11v−z with phenylacetonitriles 12v−z, followed
by DIBAL-H reduction and hydrolysis under acidic conditions
(for the yields of these two reactions see the Supporting
Information).9 Next, α-iminonitrile 7i was synthesized in a low
yield, probably because of poor solubility, by the one-pot
reaction of acrylaldehyde 9i, pent-4-enylamine, trimethylsilyl
Received: November 26, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.5b03395
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Letter
delight, the cycloaddition of 7i could be carried out in a sealed
tube by heating at 160 °C overnight, affording trans-5i (64%
yield) and cis-5i (7% yield). The IADA reactions of 2-
(alkenylimino)-3,4-diphenyl-(3E)-butenenitriles 7a−i and 8a−i
were conducted under the above-mentioned optimized
conditions, and the yields are shown in Table 1. The IADA
Scheme 1. Strategy for Synthesis of Alkaloids 1 and 2
Table 1. Yields of IADA Cycloadducts 5 and trans-6
a
b
c
entry
7
8
substituent
5 (%)
6 (%)
1
2
3
4
5
6
7
8
9
7a
7b
7c
7d
7e
7f
8a
8b
8c
8d
8e
8f
R1 = R2 = R3 = R6 = R7 = OCH3
R3 = R6 = R7 = OCH3
5a 60
5b 73
5c 78
5d 67
5e 62
5f 70
5g 70
5h 78
5i 71
6a 84
6b 90
6c 91
6d 90
6e 86
6f 90
6g 92
6h 93
6i 92
R2 = R6 = R7 = OCH3
cyanide (TMSCN), 2-iodoxybenzoic acid (IBX), and
tetrabutylammonium bromide (TBAB) in CH3CN following
the method described by Zhu et al.10 Nevertheless, a two-step
method was developed to synthesize 2-(alkenylimino)-3,4-
diphenyl-(3E)-butenenitriles 7a−i and 8a−i in good yields via
Schiff base formation (Scheme 2).
R2 = R3 = R4 = R6 = R7 = OCH3
R2 = R3 = R5 = R6 = R7 = OCH3
R2 = R3 = R7 = OCH3
R2 = R3 = R6 = OCH3
R2 = R3 = R6 = R7 = R8 = OCH3
R2 = R3 = R6 = R7 = OCH3
7g
7h
7i
8g
8h
8i
a
b
The substituents not mentioned are hydrogens. Trans/cis
c
Scheme 2. Synthesis of IADA Precursors 7a−i and 8a−i
diastereomeric mixtures 5 were used in the next step. Diastereomers
trans-6a−i were obtained predominantly.
reactions of compounds 7b−i with a three-atom spacer
afforded cycloadducts 5b−i in trans/cis ratios of ca. 10:1, as
determined by their crude 1H NMR spectra. Interestingly,
when IADA precursors 8a−i with four-atom spacers were used,
only cycloadducts trans-6a−i were isolated in high yields. Thus,
the efficiency of the IADA reactions and the trans/cis
diastereomeric ratios of the IADA cycloadducts were affected
by the spacer length. The IADA reactions of 7 and 8 could
proceed through the more stable exo transition state with less
steric hindrance, affording trans-cycloadducts 5 and 6 as the
main products (Scheme 3).3b
Scheme 3. Exo and Endo Transition States of IADA
Subsequently, 3,4-bis(3,4-dimethoxyphenyl)-2-(4-pentenyl-
imino)-(3E)-butenenitrile 7i was selected as the initial model
to investigate the feasibility of the IADA reaction. Conventional
heating was selected over microwave heating, because the
cycloaddition reaction of 7i did not reach completion under
microwave conditions. A 0.05 M solution of 7i in toluene was
heated in a sealed tube at 165 °C under the maximum power
250 W for 3 h using a focused microwave reactor. A solution of
compound 7i in toluene was refluxed for 72 h, affording
cycloadduct 5i in 58% yield. A similar result was obtained by
heating the mixture in a sealed tube at 130 °C for 48 h. To our
With a series of 6,7-diphenylindolizine-5-carbonitriles (5a−i)
and trans-2,3-diphenylquinolizine-4-carbonitriles (trans-6a−i)
in hand, an efficient method was developed for the removal of
the cyano group from the α-aminoacrylonitriles. A mixture of
cycloadducts 5a−i (and trans-6a−i) and 10 equiv NaBH4 in 2-
propanol was heated in a sealed tube at 100 °C for 24 h. The
decyanization products, 6,7-diphenylindolizines 3a−i and 7,8-
diphenylquinolizines 4a−i, were obtained in nearly quantitative
yields (Table 2). The reductive decyanization of α-amino-
acrylonitriles is of critical importance, because this key step
helps to construct indolizidine and quinolizidine ring systems
B
DOI: 10.1021/acs.orglett.5b03395
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Letter
with three or four methoxyl groups could be smoothly
synthesized under Park’s conditions (method A, entries 2, 3,
6, 7, and 9 in Table 3).11 However, an attempt to prepare
Table 2. Reductive Decyanization of Cycloadducts 5 and 6
Table 3. Aryl−Aryl Coupling of cis-Stilbenes 3 and 4
a
entry
5
6
substituent
3 (%)
4 (%)
b
1
5a
5b
5c
5d
5e
5f
6a R1 = R2 = R3 = R6 = R7 = OCH3 3a 92
4a 95
2
3
4
5
6
7
8b
9
6b R3 = R6 = R7 = OCH3
6c R2 = R6 = R7 = OCH3
3b 100
3c 100
4b 100
4c 100
4d 98
4e 100
4f 100
4g 100
4h 97
4i 100
6d R2 = R3 = R4 = R6 = R7 = OCH3 3d 96
6e R2 = R3 = R5 = R6 = R7 = OCH3 3e 100
c
entry
3
4
Substituent
1 (%)
2 (%)
b
1
3a
3b
3c
3d
3e
3f
4a
4b
4c
4d
4e
4f
R1 = R2 = R3 = R6 = R7 = OCH3
R3 = R6 = R7 = OCH3
1a 70
1b 82
1c 86
1d 89
1e 85
1f 85
1g 86
1h 54
1i 85
2a 77
2b 88
2c 84
2d 90
2e 88
2f 86
2g 88
2h 71
2i 92
6f
6g R2 = R3 = R6 = OCH3
R2 = R3 = R7 = OCH3
3f 100
a
2
5g
5h
5i
3g 100
a
3
R2 = R6 = R7 = OCH3
6h R2 = R3 = R6 = R7 = R8 = OCH3 3h 94
6i 3i 100
R2 = R3 = R6 = R7 = OCH3
b
4
R2 = R3 = R4 = R6 = R7 = OCH3
R2 = R3 = R5 = R6 = R7 = OCH3
R2 = R3 = R7 = OCH3
R2 = R3 = R6 = OCH3
R2 = R3 = R6 = R7 = R8 = OCH3
R2 = R3 = R6 = R7 = OCH3
b
5
a
b
a
The substituents not mentioned are hydrogens. Reactions were
6
a
carried out at 120 °C.
7
3g
3h
3i
4g
4h
4i
b
8
a
via the formation of a cyano-group-promoted IADA adduct. A
possible mechanism for the reductive decyanization is shown in
Scheme 4. First, allylic deprotonation of 5 and 6 under basic,
9
a
Method A: a 0.04 M solution of 3 or 4 (0.2 mmol) in anhydrous
CH2Cl2 (5 mL) was added to VOF3 (1.0 mmol) at 0 °C and the
mixture was stirred for 15 min. TFA (2.8 mmol) was added and the
b
Scheme 4. A Possible Reductive Decyanization Mechanism
mixture was stirred at 0 °C for 1 h. Method B: a 0.04 M solution of 3
or 4 (0.2 mmol) in anhydrous CH2Cl2 (5 mL) was added to VOF3
(0.4 mmol) at −20 °C and the mixture was stirred for 15 min. TFA
(2.8 mmol) was added and the mixture was stirred at −20 °C for 1 h.
c
The substituents not mentioned are hydrogens.
phenanthroquinolizidine 2a with five methoxyl groups under
the same conditions resulted in extensive oxidative decom-
position. To our delight, phenanthroindolizidines 1a, 1d, 1e,
and 1h as well as phenanthroquinolizidines 2a, 2d, 2e, and 2h
with five methoxyl groups could be obtained in good to
moderate yields (entries 1, 4, 5, and 8 in Table 3) with
complete regiospecificity under mild conditions (method B: 2
equiv VOF3, −20 °C). The total synthesis of phenanthro-
indolizidines 1a−i and phenanthroquinolizidines 2a−i from
benzaldehydes with phenylacetonitriles was achieved in six
steps in 8.8−42.1% and 19.3−63.5% overall yields, respectively.
Finally, the cytotoxic activities of 18 compounds, 1a−i and
2a−i, were evaluated against three human cancer cell lines,
breast carcinoma (MCF-7), lung carcinoma (H1299), and
prostate carcinoma (DU145), by using tylophorine (1i) and 7-
methoxycryptopleurine (2i) for comparison. The cytotoxic
activities of phenanthroindolizidines 1a−i and phenanthro-
quinolizidines 2a−i are shown in Table 4. Phenanthro-
quinolizidines 2a−i were more active than the corresponding
phenanthroindolizidines 1a−i.6h In the phenanthroindolizidine
series, the IC50 ratio of compounds 1a−h to tylophorine (1i)
followed the order 1a ∼ 1h ∼ 1c > 1f ≫ 1b ∼ 1e > 1d > 1g;
however, in the phenanthroquinolizidine series, the IC50 ratio
of compounds 2a−h to 7-methoxycryptopleurine (2i) followed
the order 2c ∼ 2a ∼ 2h ∼ 2b ≫ 2f ∼ 2d > 2e > 2g. The data
indicate that when the C-1 and C-8 substituents were converted
from methoxy to hydrogen and the C-3 substituent was
converted from hydrogen to methoxy, the cytotoxicity
increased dramatically (entries 1, 3, and 8 vs entry 9).
Interestingly, C-6 methoxylation was more important than C-
2 methoxylation for cytotoxicity of the phenanthroindolizidine
protic conditions could result in the shift of the double bound
into stilbene A. Then, elimination of a cyanide ion from A
would lead to the formation of iminium ion B, followed by a
hydride attack on the iminium carbon to produce diphenyl-
tetrahydropyridine derivatives 3 and 4.
The mechanism was evaluated using three D-labeling
experiments. Reduction of 5i with NaBH4 in 2-propanol-d8
(99+ atom% D) and NaBD4 (98 atom % D) in 2-propanol
under the same conditions (100 °C, 24 h) afforded one-
deuterium product d-3i, indicating that one of the methylene
hydrogens of the allylamine in 3i was derived from the protic
solvent and the other hydrogen came from the reductive agent.
Again, only d2-3i was obtained via treatment of NaBD4 in 2-
propanol-d8 at 100 °C for 24 h (Figure 1).
Next, an efficient oxidizing agent, vanadium oxytrifluoride
(VOF3), was chosen to examine the oxidative aryl−aryl
coupling of 6,7-diphenylindolizines 3a−i and 7,8-diphenyl-
quinolizines 4a−i. Phenanthroindolizidines 1b, 1c, 1f, 1g, and
1i as well as phenanthroquinolizidines 2b, 2c, 2f, 2g, and 2i
Figure 1. Products of D-labeling reductive decyanization.
C
DOI: 10.1021/acs.orglett.5b03395
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
(3) (a) Hwang, Y. C.; Fowler, F. W. J. Org. Chem. 1985, 50, 2719.
(b) Cheng, Y. S.; Lupo, A. T., Jr.; Fowler, F. W. J. Am. Chem. Soc.
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Table 4. IC50 Values of 1a−i and 2a−i against Three Human
Cancer Cell Lines
a
compd
IC50 (nM)
H1299
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T. S. Bioorg. Med. Chem. 2008, 16, 6233. (f) Gao, W.; Bussom, S.;
Grill, S. P.; Gullen, E. A.; Hu, Y. C.; Huang, X.; Zhong, S.; Kaczmarek,
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entry
1 {2}
MCF-7
DU145
1
2
3
4
5
6
7
8
9
1a {2a}
1b {2b}
1c {2c}
1d {2d}
1e {2e}
1f {2f}
>2500 {276.4}
304.9 {272.4}
2481.0 {392.5}
61.2 {61.9}
>2500 {251.9}
235.2 {210.6}
>2500 {314.4}
54.1 {54.4}
>2500 {271.9}
175.6 {184.5}
2152.0 {278.9}
58.8 {35.7}
223.9 {19.3}
1909.0 {68.7}
42.4 {11.3}
165.3 {27.6}
2017.0 {57.3}
32.5 {9.0}
150.1 {23.8}
1510.0 {41.8}
30.3 {4.6}
1g {2g}
1h {2h}
1i {2i}
>2500 {277.4}
41.1 {10.7}
>2500 {232.4}
30.9 {7.1}
>2500 {214.9}
28.3 {4.5}
a
The values in the brackets show the IC50 of compounds 2.
series, whereas a reverse trend was observed in the phenanthro-
quinolizidine series (entries 2 and 6 vs entry 9). Moreover, C-4
and C-5 methoxylation showed only slightly deleterious effects
on the cytotoxicity (entries 4 and 5 vs entry 9). Notably, the
absence of a methoxy group at the C-7 position did not
significantly affect the cytotoxicity in the two series of alkaloids
(entry 7 vs entry 9).
In summary, we developed a concise synthetic strategy for
the construction of phenanthroindolizidines and phenanthro-
quinolizidines by the reductive decyanization of α-amino-
acrylonitriles, followed by an efficient aryl−aryl coupling. The
reductive decyanization was investigated using D-labeling
experiments. The development of more polar and water-
soluble phenanthroquinolizidines to minimize the central
nervous system toxicity by decreasing diffusion through the
blood−brain barrier is underway.
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10, 1509.
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9372.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.5b03395.
Full experimental and characterization data for all
compounds (PDF)
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: thchuang@mail.cmu.edu.tw.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This project was financially supported by the Ministry of
Science and Technology, Taiwan (MOST 104-2113-M-039-
001), China Medical University (CMU 104-N-01), and, in part,
by a grant from the Chinese Medicine Research Center, China
Medical University (the Ministry of Education, the Aim for the
Top University Plan).
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D
DOI: 10.1021/acs.orglett.5b03395
Org. Lett. XXXX, XXX, XXX−XXX