DDQ-Mediated Oxidative Radical Cycloisomerization of 1,5-Diynols: Regioselective Synthesis of Benzo[b]fluorenones under Metal-Free Conditions

Article abstract of DOI:10.1021/acs.orglett.5b03533

A regio- and chemoselective oxidative cycloisomerization reaction of acyclic 1,5-diynols has been developed. The reaction proceeds under metal-free reaction conditions with high efficiency and broad functional group tolerance, which offers a general and straightforward access to benzo[b]fluorenones under metal-free conditions. The preliminary mechanistic studies revealed the possible involvement of a Meyer-Schuster rearrangement combined with an oxidative radical cyclization.

Full text of DOI:10.1021/acs.orglett.5b03533

Letter
pubs.acs.org/OrgLett
DDQ-Mediated Oxidative Radical Cycloisomerization of 1,5-Diynols:
Regioselective Synthesis of Benzo[b]fluorenones under Metal-Free
Conditions
Hui Zhu and Zhiyuan Chen*
Key Laboratory of Functional Small Organic Molecules, Ministry of Education, and College of Chemistry & Chemical Engineering,
Jiangxi Normal University, Nanchang, Jiangxi 330022, China
S
* Supporting Information
ABSTRACT: A regio- and chemoselective oxidative cycloisomeriza-
tion reaction of acyclic 1,5-diynols has been developed. The reaction
proceeds under metal-free reaction conditions with high efficiency and
broad functional group tolerance, which offers a general and
straightforward access to benzo[b]fluorenones under metal-free
conditions. The preliminary mechanistic studies revealed the possible
involvement of a Meyer−Schuster rearrangement combined with an oxidative radical cyclization.
luorenone is an important carbocycle that can be frequently
found in many biologically active molecules, natural
Despite the critical importance of the benzofluorene
F
derivatives, only a handful of synthetic methods have been
11
products, and drug candidates.1 It can also serve as a building
block in the synthesis of optoelectronic advanced materials.
Because of these features, developing novel and efficient methods
for constructing fluorenone derivatives has received considerable
attention, including direct oxidation of fluorenes,2 Friedel−
Crafts reactions,3 metal-catalyzed cyclizations or cycloadditions,4
and others.5
reported. In 2000, Dominguez and Saa
́
reported thermal
́
cycloaromatizations of nonconjugated aryldiacetylenes, such as
benzotriynes and benzodiynes, to produce benzo[b]fluorene and
benzo[c]fluorene skeletons, respectively. It is known that
benzo[b]fluorenones and benzo[c]fluorenones can be realized
in the thermal cyclization of 1-[2-(arylethynyl)phenyl]-3-
trimethylsilylpropynones (Scheme 1, eq 1).12 However, due to
On the other hand, the benzofluorenesconsisting of fused
fluorene skeletons with arenesare very close to optoelectronic
materials or related devices due to their highly rigid π-conjugated
systems.6 In addition, families of these linearly polycyclic
aromatic hydrocarbons (PAHs) also display very interesting
medicinal applications (Figure 1). For example, 5-diazobenzo-
Scheme 1. Benzo[b]fluorenone Synthesis via
Cycloisomerization Protocols
the inherent radical reorganization processes and the high
temperature conditions, the separation of constitutional isomers
and the functional tolerance always remain problematic.
Recently, we developed a facile approach for the AgBF4-catalyzed
tandem electrophilic cycloisomerization of 1,6-diynols with NXS
(X = Br or I) to selectively afford halo-substituted benzo[a]-
fluorenols under very mild conditions.13
Figure 1. Bioactive substances containing a benzofluorene moiety.
[b]fluorene is known as kinamycin D and was isolated from
Streptomyces proteinase in the 1970s.7 It is a naturally occurring
diazo compound with potent antibiotic activity against Gram-
positive organisms.8 Stealthin C is isolated from an S.
murayamaensis culture.9 The Kinobscurinone and O5,N-
dimethylstealthin C are useful advanced intermediates in the
biosynthesis of kinamycins.8a,10
The Meyer−Schuster rearrangement is an attractive approach
for the generation of α,β-unsaturated carbonyl compounds.14
A
Received: December 13, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.5b03533
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
formal 1,3-hydroxyl shift to form the allenol intermediate and
tautomerization was proposed as the key step in the rearrange-
ment.15 Zhang and co-workers reported an interesting Au−Mo
cocatalyzed iodo Meyer−Schuster rearrangement for the
synthesis of α-iodo-α,β-unsaturated aldehydes or ketones.16
Reddy and co-workers developed the I2/NIS-promoted Meyer−
Schuster rearrangement of 3-alkoxypropargyl alcohols under
metal-free conditions.17 The avoidance of using transition metals
in an organic transformation is significant because the metal
species are highly toxic, presenting an intrusive limitation to
industrial scale use, especially in the last synthetic stage of the
purification of optoelectronic materials or pharmaceutical
ingredients. Inspired by the previous achievements of the
benzo[b]fluorenone synthesis using ynones as the starting
materials (Scheme 1, eq 1) and our recent achievements in the
metal-catalyzed cycloisomerization of 1,n-benzoendiynyls,13,18
we envisioned that by employing the Meyer−Schuster rearrange-
ment as a key step, a facile metal-free synthesis of benzo[b]-
fluorenones would be possible when using diynols as starting
materials under oxidative conditions (Scheme 1, eq 2).
To verify the hypothesis, we employed benzodiynol 1a as the
substrate in a model reaction. To our delight, treatment of 1a at
80 °C under open air in anhydrous acetonitrile resulted in a new
product that was identified as the expected benzo[b]fluorenone
2a, albeit in very low yield. The yield of 2a was improved to 59%
when the reaction was carried out under an oxygen atmosphere
(Table S1, entries 1 and 2). This metal-free and regioselective
oxidative cycloisomerization reaction aroused our interest and
inspired us to further investigate the reaction conditions. Worse
results were observed when a peroxide oxidant such as H2O2 or
tBuOOH was used. A strong oxidant like K2S2O8 performed even
worse. Pleasingly, a good yield (71%) was obtained when
benzoquinone was used (Table S1, entry 6). We next studied the
reaction conditions using the benzoquinone analogues as the
optimal oxidants. electron-rich benzoquinones such as TMQ
(2,3,5,6-tetramethylcyclohexa-2,5-diene-1,4-dione) and A
(3,3′,5,5′-tetra-tert-butyl-[1,1′-bi(cyclohexylidene)]-2,2′,5,5′-
tetraene-4,4′-dione) were less effective for the formation of
product 2a. However, electron-deficient benzoquinones such as
DCQ (2,5-dichlorocyclohexa-2,5-diene-1,4-dione) and TCQ
(2,3,5,6-tetrachlorocyclohexa-2,5-diene-1,4-dione) gave im-
proved yields of 83 and 84%, respectively (Table S1, entries
7−10). Good results were obtained when DDQ (4,5-dichloro-
3,6-dioxocyclohexa-1,4-diene-1,2-dicarbonitrile) was added as
the test oxidant. The yield could be improved to 89% when 1.0
equiv of DDQ was employed. Increasing the oxidant loading to
2.0 equiv only led to a drastic decrease in efficiency (Table S1,
entries 11 and 12). The reaction is easily handled because only
the catalyst and solvent are required to facilitate the expected
oxidative cycloisomerization.
Table 1. DDQ-Mediated Oxidative Cycloisomerizations of
Benzodiynol 1 To Form Benzo[b]fluorenone 2
a
a
entry
R1
R2 (1)
Ph (1a)
yield of 2 (%)
1
2
H
H
H
H
H
H
H
H
H
H
H
H
H
89 (2a)
94 (2b)
87 (2c)
87 (2d)
94 (2e)
95 (2f)
96 (2g)
91 (2h)
69 (2i)
97 (2j)
99 (2k)
75 (2l)
41 (2m)
65 (2n)
88 (2o)
91 (2p)
97 (2q)
p-MeC6H4 (1b)
p-MeOC6H4 (1c)
p-nBuC6H4 (1d)
p-tBuC6H4 (1e)
p-FC6H4 (1f)
3
4
5
6
7
p-ClC6H4 (1g)
p-CNC6H4 (1h)
p-NO2C6H4 (1i)
p-COOMeC6H4 (1j)
3-thienyl (1k)
8
9
10
11
12
13
14
15
16
17
tBu (1l)
cyclopropyl (1m)
p-MeC6H4 (1n)
p-MeC6H4 (1o)
p-MeC6H4 (1p)
p-MeC6H4 (1q)
4-MeO
5-Me
4-F
5-CF3O
a
Benzodiynol 1 (0.20 mmol), DDQ (0.20 mmol), CH3CN (4.0 mL),
80 °C, 10−12 h. Isolated yield based on 1.
electron-withdrawing properties (1f−1j) were well-tolerated
and afforded the corresponding products 2b−2j in good to
excellent yields (Table 1, entries 1−9). The structure of
compound 2b was identified unambiguously by X-ray diffraction
analysis.19 The strong electron-deficient functional groups such
as fluoro (2f), cyano (2h), nitro (2i), and ester groups (2j) were
tolerated without difficulty. The heterocycle 3-thienyl furnished
the desired product 2k in almost quantitative yield. The
efficiency of this reaction was not compromised by the alkyl-
substituted substratesthis was demonstrated by the tert-butyl
and cyclopropyl-substituted products (2l and 2m) that were
isolated in serviceable yields (Table 1, entries 11 and 12).
For the various substitutions (R1) that were attached at the
benzonoid core structure of compound 1, experiments revealed
good to excellent yields of the corresponding products 2n−2q
whenever the substitutions had electron-donating or fluorine-
containing properties. For instance, good yields were observed
when methoxyl or methyl groups were attached at the benzonoid
position. Corresponding products 2n or 2o were isolated in 65
and 88% yields, respectively. The R1 group with fluorine (1p) or
trifluoromethoxy (1q) substituents underwent the reaction
smoothly and afforded the corresponding fluorinated products
2p and 2q in excellent yields. The fluoro- and trifluoromethoxyl-
substituted benzo[b]fluorenones (2p, 2q) were especially
noticeable because it is well-known that the fluorinated PAHs
can usually exhibit interesting physicochemical and biological
properties. These are recently used in the pharmaceutical,
agrochemical, and materials-related fields.20
With optimized reaction conditions in hand (Table S1, entry
11), we next investigated the scope of this oxidative cyclo-
isomerization reaction. Since the oxidative cyclization seemed to
selectively occur on the aryl group of the starting materials, two
phenyl groups on the carbon bearing an OH group in compound
1 were planted. A variety of benzo[b]fluorenones 2 could be
easily produced in good to excellent yields through this method
(Table 1). The influence of the electronic effects attached at the
alkyne moiety (R2) in compound 1 (R1 = H) was first examined.
The reaction displayed excellent functional group compatibility
in terms of the substituents attached to the aromatic rings of 1.
Versatile functional arenes with attached substitutions with
electron-donating characterisitics (1a−1e) and those with
The effect of electronic variation of the propargyl alcohol
moiety was next examined under optimized conditions (Table
2). For the aromatic functional groups that were located at the
alcoholic carbon position of compound 3, various substitutions
including methyl (3a), methoxyl (3b), fluorine (3c), chlorine
B
DOI: 10.1021/acs.orglett.5b03533
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Organic Letters
Letter
Table 2. Effect of Electronic Variation in the R3 and R4
Positions of Benzodiynol 3
Scheme 2. Formation of Thienyl-Containing Fused
Tetracyclic Ketones
a
a
entry
R3, R4 (3)
yield of 4 + 5 (%)
4/5
1
2
Me, Me (3a)
MeO, MeO (3b)
F, F (3c)
91 (4a)
87 (4b)
3
86 (4c)
4
Cl, Cl (3d)
Br, Br (3e)
MeO, H (3f)
Me, H (3g)
H, F (3h)
88 (4d)
5
73 (4e)
6
95 (4f + 5f)
96 (4g + 5g)
96 (4h + 5h)
99 (4i + 5i)
86 (4j + 5j)
2.90:1
1.61:1
1.76:1
1.69:1
1.60:1
7
8
9
H, Cl (3i)
deficient one. Thus, we speculated that a radical pathway should
be involved in the key cyclization step. To gain insight into the
reaction mechanism, 2.0 equiv of TEMPO was added to the
standard reaction condition of compound 1a (Scheme 2, eq 5). A
sluggish conversion was observedtetracyclic product 2a was
finally isolated in 13% yield.
To further confirm that an organic radical species is involved in
the overall process, an electron paramagnetic resonance (EPR)
experiment was conducted to trap the carbon radical. When
compound 1b was employed as a starting material under the
standard conditions for 30 min, we successfully observed a signal
from the carbon radical, as shown in Figure 2. This experiment
10
MeO, F (3j)
a
Benzodiynol 3 (0.20 mmol), DDQ (0.20 mmol), CH3CN (4.0 mL),
80 °C, 10−12 h. Isolated yield based on 3.
(3d), and bromine (3e) were introduced into the R3 and R4
positions to probe the reactivity and chemoselectivity (Table 2).
The reaction showed very good functional group compatibility,
and corresponding products 4a−4e were successfully isolated in
good to excellent yields (∼73−91%).
Chemoselectivity of the reaction was observed when the arene
ring in the propargyl position of 3 was attached to substituents
with different electronic characterisitics (R3 and R4). The
dehydrogenative cyclization process was always more likely to
occur in the relatively electron-rich phenyl group. For instance,
for the reaction of compound 3f, in which R3 was an electron-
donating methoxyl and R4 was an electron-neutral H atom, the
product 4f containing the MeO group in the core structure of the
tetracyclic ring system was isolated as the major outcome as
opposed to its isomer 5f (Table 2, entry 6). A similar reaction was
observed in the reaction of alkyl-substituted compound 3g (R3 =
Me, R4 = H). Product 4g was formed through a cyclization on the
4-methylphenyl groupthis was isolated as the major product
(Table 2, entry 7). On the other hand, when the R4 group in
compound 3 was an electron-withdrawing halogen group (3h, 3i,
and 3j, in Table 2, entries 8−10) and the phenyl group that
linked with the R3 group (H or MeO) was more electron-rich
than that of the R4-phenyl moiety, the cyclization process was
more favored in the electron-rich R3 arene moiety. The 4h, 4i,
and 4j species were isolated as major products. The structure of
compound 4j was firmly established via X-ray diffraction
analysis.19
Figure 2. EPR experiment.
suggested that the organic radical is initially generated by DDQ
and reacts with the 1,5-diyols in the reaction system to induce
subsequent oxidative annulation sequences.
To demonstrate the utility of the DDQ-mediated oxidative
cycloisomerization method, we finished the synthesis of the
heterocyclic-containing fused tetracyclic ketones 7 and 9
(Scheme 2, eqs 3 and 4). The thienyl group was incorporated
into a rigid π-conjugated molecule because it can dramatically
enhance the photochromic and the fluorescence emission
properties of the optoelectronic materials.21
The results in Table 2 (entries 6−10) indicate that the
reactivities of the aromatic rings which were attached at the
hydroxyl carbon were different; the electron-rich arene
(substituent with the R3 group) was higher than the electron-
We proposed a plausible mechanism for this cascade
cycloisomerization reaction, as depicted in Scheme 3. The
Meyer−Schuster rearrangement of compound 1 would be
induced to occur first by trace amounts of water or acidic
substances under heated conditions that formed an allenol
intermediate I. This was then oxidized by DDQ to form a DDQH
radical and allenol radical II.22 The subsequent intramolecular
cascade radical addition with alkene and an alkyne group in
species II led to the five-membered ring intermediate III and the
benzo[b]fluorenone radical IV.23 Finally, the hydrogen radical
from IV was trapped by the DDQH radical to produce the final
C
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Organic Letters
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Interestingly, to our knowledge, this is the first example of
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In conclusion, we have developed an efficient and selective
synthesis of benzo[b]fluorenones via DDQ-mediated oxidative
cycloisomerizations of easily accessible acyclic 1,5-diynols with
excellent functional group tolerance. No metal species were
required in the cyclization process, which are significant in the
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ASSOCIATED CONTENT
* Supporting Information
■
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S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.5b03533.
1
Experimental details, characterizations, and copies of H
and 13C NMR spectra for new compounds (PDF)
(16) (a) Ye, L.; Zhang, L. Org. Lett. 2009, 11, 3646. (b) Yu, M.; Zhang,
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AUTHOR INFORMATION
Corresponding Author
■
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2014, 356, 3809.
(19) CCDC 1414214 (2b) and CCDC 1414249 (4j) contain the
supplementary crystallographic data for this paper. These data can be
obtained free of charge from the Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
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*E-mail: zchen@jxnu.edu.cn.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the National Natural Science Foundation of China
(21462022) for financial support. We thank Prof. Aiwen Lei and
Dr. Hong Yi from Wuhan University for facilitation of the EPR
experiments.
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DOI: 10.1021/acs.orglett.5b03533
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