Hydride-induced anionic cyclization: An efficient method for the synthesis of 6- H -phenanthridines via a transition-metal-free process

Article abstract of DOI:10.1021/acs.orglett.5b00544

A novel procedure for hydride-induced anionic cyclization has been developed. It includes the reduction of a biaryl bromo-nitrile with a nucleophilic aromatic substitution (S_NAr). A range of polysubstituted 6-H-phenanthridines were so obtained in moderate to good yield with good substrate tolerance. This method involves a concise transition-metal-free process and was applied to synthesize natural alkaloids.

Full text of DOI:10.1021/acs.orglett.5b00544

Letter
pubs.acs.org/OrgLett
Hydride-Induced Anionic Cyclization: An Efficient Method for the
Synthesis of 6‑H‑Phenanthridines via a Transition-Metal-Free Process
Wei-Lin Chen, Chun-Yuan Chen, Yan-Fu Chen, and Jen-Chieh Hsieh*
Department of Chemistry, Tamkang University, New Taipei City, 25137, Taiwan
*
S Supporting Information
ABSTRACT: A novel procedure for hydride-induced anionic cycliza-
tion has been developed. It includes the reduction of a biaryl bromo-
nitrile with a nucleophilic aromatic substitution (S Ar). A range of
N
polysubstituted 6-H-phenanthridines were so obtained in moderate to
good yield with good substrate tolerance. This method involves a
concise transition-metal-free process and was applied to synthesize
natural alkaloids.
henanthridines, particularly those of the 6-H-phenanthri-
general ppm. Thus, the loading amount and the metal removal
P
dine variety (Figure 1), are important structural motifs of
in each single synthetic step are crucial. Anionic ring closure
reactions can potentially proceed without transition metals,
but are rarely reported for the construction of poly aromatic
1
11
many bioactive natural alkaloids and medically relevant
2
compounds. 6-H-Phenanthridine derivatives often exhibit
1
2
good inhibition of DNA topoisomerase I and are of potential
value to antitumor therapy. Although numerous methods have
been developed to access phenanthridine derivatives, specific
routes to 6-H-phenanthridines are still limited to photolysis,
compounds via nucleophilic aromatic substitution (S Ar). In
N
3
2
S Ar reactions, an electron-deficient group activating the Csp −
N
4
13
X bond is generally required. Among all the examples, the
5
2
14
Csp −F has the fastest reaction rate in S Ar reactions, and
N
6
15
microwave-assisted cyclization, oxidation of 5,6-dihydrophe-
most of the reports involve the cleavage of C−F bonds. Other
7
8
2
nanthridine, and intramolecular condensation. In addition,
most of these synthetic methods can only be applied to prepare
products with a narrow scope of structural diversity. Therefore,
a novel method that provides a general route to many different
Csp −X bonds such as C−Cl, C−Br and C−I are more
commonly activated by an oxidative addition with late
16
transition-metal complexes. These reasons make the preload-
ing of a fluoride in substrate important for application to the
6-H-phenanthridines is still desired.
S Ar reaction. However, the fluorination of aromatic
N
17
compounds is still developing and not as easy to perform
as the well developed bromination. Herein, we report the first
example of an efficient synthesis of 6-H-phenanthridines
involving hydride-induced anionic cyclization by an S Ar
N
reaction, with the cleavage of the C−Br bond, and not
requiring transition metals.
We used 2′-bromo-[1,1′-biphenyl]-2-carbonitrile (1a) as a
model substrate (Table 1, entry 1) for our initial studies, which
was treated with 1.5 equiv of LiAlH in 0.5 mL of benzene at
4
1
00 °C for 36 h; the corresponding 6-H-phenanthridine (2a)
was obtained in only 11% NMR yield. Product 2a was
1
13
confirmed by the H NMR, C NMR, and HRMS analysis.
After working up the reactions we obtained messy crude NMR
spectra, and the starting substrate that remained was observed
with the desired product and several unidentified byproducts.
To optimize the reaction conditions, the effect of solvents,
hydride sources, and reaction temperatures was investigated
(Table 1). We first evaluated the effect of solvent on this
reaction (entries 1−8) and found that it was significant. When
aromatic solvents such as toluene, o-xylene, and 1,2-
dichlorobenzene were used, the reaction proceeded but gave
Figure 1. Examples of 6-H-phenanthridine alkaloids.
We have recently developed a method for the synthesis of 6-
alkyl/aryl/acetyl-phenanthridines via a Cu(I/III) catalytic cycle
9
involving the transformation of a nitrile. This, with the success
of our previous works involving transition-metal-catalyzed
1
0
coupling reactions of nitriles, motivated us to further
investigate the possibility of synthesizing 6-H-phenanthridines
by an annulation that is more environmentally friendly and
efficient.
Transition-metal-free reactions are an important research
target in organic synthesis, especially in the pharmaceutical
industry. The standard limit for the metal content of drugs is in
Received: February 21, 2015
Published: March 12, 2015
©
2015 American Chemical Society
1613
DOI: 10.1021/acs.orglett.5b00544
Org. Lett. 2015, 17, 1613−1616

Organic Letters
Letter
a
Table 1. Optimization of Reaction Conditions
Table 2. Reaction for the Substrates with Different Halides
and Pseudohalides
a
temp
(°C)
[H−]
n
solvent
yield (%)
b
entry
b
entry
X
t (h)
yield (%)
1
2
3
4
5
6
7
8
9
LiAlH4
1.5 benzene
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
11
9−12
27
16
12−17
31
0
1
2
3
4
5
6
7
8
9
F
48
48
48
48
48
48
48
24
24
24
24
91
87
92
34
46
43
3
c
LiAlH4
LiAlH4
LiAlH4
LiAlH4
LiAlH4
LiAlH4
LiAlH4
1.5 aromatic solvents
1.5 pentane
Cl
Br
1.5 hexane
I
d
1.5 ethers
OTf
OTs
OMe
F
1.5 THF
1.5 tBuOH
e
1.5 polar solvents
0
82
56
53
31
NaAlH4
1.5 THF
1.5 THF
1.5 THF
1.5 THF
1.5 THF
1.5 THF
3.0 THF
38
43
37
0
Cl
10
11
12
13
14
15
16
DIBAL-H
1
0
Br
LiAlH(OtBu)
3
11
I
f
MBH4
a
Reactions were carried out using 0.1 mmol (1.0 equiv) of substrate 1
with 1.1 equiv of Li(Et) BH in 0.5 mL of THF at 100 °C for t h.
NMR yield based on internal standard mesitylene.
Li(Et) BH
69
52
17
74
b
1
3
H
3
LiBH(secBu)3
NaH
Li(Et) BH (0.5)/NaH THF
3
(3.0)
was found that, with a 48 h reaction time, reactions for all the
different substrates were completed. The performance for a
fluoride and bromide as leaving groups is similar, slightly better
than that of a chloride. When an iodo-substrate was introduced
into the reaction, the crude spectrum was messy and a much
lower yield of 2a was detected. By GC-MS analysis, the major
side reaction was confirmed as protonation of the iodide.
Pseudohalides such as triflate and tosylate also worked, but in
lower yields. A methoxy group in this reaction did not work
well and provided 2a in only 3% NMR yield. Almost all of the
substrate remained. It is noteworthy that when the reaction
time was 24 h (entries 8−11), the fluoro-substrate afforded 2a
in the highest yield, and only the reaction of iodo-substrate was
completed.
17
18
19
20
Li(Et) BH
1.5 THF
1.5 THF
1.1 THF
1.1 THF
120
80
78
54
3
Li(Et) BH
3
g
i
h
Li(Et) BH
100
100
92 (87)
58
3
Li(Et) BH
3
a
Reactions were carried out using 0.1 mmol (1.0 equiv) of 2′-bromo-
1,1′-biphenyl]-2-carbonitrile (1a) with a hydride source (n equiv) in
.5 mL of solvent at t °C for 36 h. H NMR yield based on internal
standard mesitylene. Toluene, o-xylene, and 1,2-dichlorobenzene
were used. Et O, 1,4-dioxane, and DME were used. DMF, DMSO,
[
0
b
1
c
d
e
2
f
g
h
and NMP were used. M = Na, Li, K. 48 h. Isolated yield in 0.4
mmol scale. Under air.
i
the product in very low yields (entry 2). Better yields were
obtained in nonpolar solvents, such as pentane and hexane
Our hydride-induced anionic cyclization reaction was
successfully extended to various bromo-substrates 1, and the
results are listed in Scheme 1. In most cases, the reaction
required at least 48 h to fully consume the substrates (1).
However, longer reaction times were occasionally associated
with overreacting of the hydride and reduction of the 6-H-
phenanthridines to 5,6-dihydrophenanthridines. Investigations
into the effect of electron density then were made. As indicated,
these reactions worked very well for both electron-withdrawing
(2b−2e) and electron-donating groups (2f, 2g) on the
benzonitrile moiety. Reaction of the substrate with a methyl
group on the para position of nitrile (2f) gave a lower yield,
which might be caused by the abstraction of a proton on the
methyl group. Substituents ortho to the nitrile group did not
impede the addition of hydride and provided the corresponding
products 2h and 2i in good yields. The electron density on the
moiety of the aryl bromide (2j−2n) also did not significantly
influence the reaction. However, when the substrates with the
benzodiozole subunit were used in the reaction, some
undefined side products were observed alongside the desired
products (2n, 2A, 2B, and 2J). For a substrate with a
naphthalene subunit, the reaction proceeded smoothly and
provided the corresponding product 2o in good yield.
Heteroaromatic compounds such as pyridine (2p) and
quinoline (2q) were tolerated as well. However, some side
(
entries 3, 4). Ether-type solvents, such as 1,4-dioxane, DME,
and Et O, slightly improved the yield of 2a (entry 5). Among
all the tests, THF provided the best yield for this reaction
2
(
entry 6). It was found that the reaction is unable to proceed in
protic and highly polar solvents (entries 7, 8). The hydride
source is also crucial for this reaction. Different aluminum
hydrides afforded similar results for this reaction (entries 9−
1
1). NaBH , LiBH , and KBH were functionless (entry 12).
4 4 4
But Li(Et) BH, a so-called “Super Hydride,” effectively
improved the yield of 2a to 69% (entry 13). Interestingly, an
excess amount of NaH also induced the reaction (entry 15),
and the combination of Li(Et) BH (0.5 equiv of H ) with NaH
afforded an even better yield of the desired product (entry 16).
The reaction proceeded smoothly at 80 to 120 °C (entries 17,
3
−
3
1
8). When the reaction was conducted at 120 °C, higher yields
of 2a were observed; however, overreaction occurred as well.
We thus reduced the amount of hydride and increased the
reaction time to avoid this, and got a very good yield of the
desired product 2a (entry 19). The hydride-induced anionic
cyclization could also proceed under air in a lower yield (entry
2
0). This is due to the moisture interference in this reaction.
After optimizing the reaction conditions, the capacity of this
hydride-induced cyclization reaction to work with different
halides and pseudohalides was then investigated (Table 2). It
1
614
DOI: 10.1021/acs.orglett.5b00544
Org. Lett. 2015, 17, 1613−1616

Organic Letters
Letter
Scheme 1. Scope of 6-H-Phenanthridinesa,b
Scheme 2. Syntheses of Trisphaeridine, Bicolorine, 5,6-
Dihydrobicolorine, and the Analogues of Trispharidine
9,10,12−15
Based on previous reports
tentative reaction pathway can be proposed as below (Scheme
). The reaction is likely to be initiated by the chelating of
and the above results, a
3
Scheme 3. Proposed Reaction Pathway
a
Reactions were carried out using 0.4 mmol (1.0 equiv) of substrate 1
with 1.1 equiv of Li(Et) BH in 2.0 mL of THF at 100 °C for 48 h.
compound 1 to lithium (complex A). Subsequent addition of
hydride to nitrile provides the iminyl lithium complex (complex
B). A subsequent nucleophilic aromatic substitution then takes
place to generate the anionic intermediate C. Elimination of
halide or pseudohalide affords desired product 2 and precipitate
LiX.
In conclusion, we have developed a novel method for
hydride-induced anionic ring closure to obtain 6-H-phenan-
thridines in moderate to good yields with tolerance of a wide
variety of substrates. In addition, the present procedure can be
applied to the syntheses of the three natural alkaloids,
trisphaeridine, bicolorine, and 5,6-dihydrobicolorine. Further
studies to explore other applications are currently underway.
3
b
Isolated yield.
products were observed and the desired products were isolated
only in moderate yields. The combination of various subunits
to establish polysubstituted 6-H-phenanthridines (2A−2I) and
polyaromatic compounds (2J, 2K) was also easily achieved in
moderate to good yields by the present methodology.
Our standard procedure was also applied to the synthesis of
1
8
natural alkaloid trisphaeridine (T1, Scheme 2), which
19
represents the basic skeleton of the Amaryllideceae alkaloids
and appears in a wide range of natural alkaloids and bioactive
compounds. The further N-methylation and reduction afforded
20
two other alkaloids, bicolorine and 5,6-dihydrobicolorine.
ASSOCIATED CONTENT
Supporting Information
■
Both of these two alkaloids were isolated from the bulbs of
*
S
Narcissus bicolor, and the bicolorine even exhibits a high
3a
Experimental procedures, characterization, spectral data, and
copies of all compounds. This material is available free of
charge via the Internet at http://pubs.acs.org.
anticancer effect. Our synthetic process provides trisphaer-
idine in 29% overall yield in four steps from the commercial
source, bicolorine in 25% (six steps) and 5,6-dihydrobicolorine
in 20% (seven steps). In addition, the analogues of
trisphaeridine (T2−T5) could be also obtained in moderate
yields. This demonstrated the high convenience of this method
AUTHOR INFORMATION
Corresponding Author
■
3
for use in medicinal chemistry.
*E-mail: jchsieh@mail.tku.edu.tw.
1
615
DOI: 10.1021/acs.orglett.5b00544
Org. Lett. 2015, 17, 1613−1616

Organic Letters
Letter
Notes
2450. (c) Zhao, J.; Zhao, Y.; Fu, H. Angew. Chem., Int. Ed. 2011, 50,
3
769. (d) Ottersbach, P. A.; Elsinghorst, P. W.; Hac
̈
ker, H.-G.;
The authors declare no competing financial interest.
Gutschow, M. Org. Lett. 2010, 12, 3662. (e) Behr, J.-B.; Kalla, A.;
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Schwarzbach, J. L. Org. Lett. 2007, 9, 4893. (e) Kristensen, J. L.; Vedsø,
ACKNOWLEDGMENTS
■
We thank the Ministry of Science and Technology of Republic
of China (MOST 103-2113-M-032-009-MY2) for financial
support of this research.
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DOI: 10.1021/acs.orglett.5b00544
Org. Lett. 2015, 17, 1613−1616