Versatile domino rearrangement of diphenylhomobenzoquinone epoxides induced by CF3SO3H

Article abstract of DOI:10.1002/ejoc.201200455

CF₃SO₃H-catalyzed reaction of 5-alkyl-(R)-substituted diphenylhomobenzoquinone epoxides (R = Me, iPr, tBu) provided indenoquinones, a cyclopenta[b]chromene-2-carbaldehyde, and furan-3(2H)-ones through novel cationic domino rearrangements depending on the R substituents. The mechanisms of these reactions were described by a combination of various key steps involving (i) transannular cyclization of the endo-phenyl group, (ii) epoxide and cyclopropane ring opening, (iii) ring contraction of the original quinone frame, (iv) dehydration and intramolecular S_E2/S_N2-Ar cyclization, as well as (v) possible pseudopericyclic cheletropic decarbonylation.

Full text of DOI:10.1002/ejoc.201200455

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SHORT COMMUNICATION
DOI: 10.1002/ejoc.201200455
Versatile Domino Rearrangement of Diphenylhomobenzoquinone Epoxides
Induced by CF3SO3H
Haruyasu Asahara,[a] Satoshi Matsui,[a] Yoshiro Masuda,[a] Naohiko Ikuma,[a] and
Takumi Oshima*[a]
Keywords: Domino reactions / Rearrangement / Epoxides / Decarbonylation / Pseudopericyclic reaction
CF3SO3H-catalyzed reaction of 5-alkyl-(R)-substituted di-
tion of various key steps involving (i) transannular cyclization
phenylhomobenzoquinone epoxides (R = Me, iPr, tBu) pro- of the endo-phenyl group, (ii) epoxide and cyclopropane ring
vided indenoquinones,
a
cyclopenta[b]chromene-2-carb-
opening, (iii) ring contraction of the original quinone frame,
(iv) dehydration and intramolecular SE2/SN2-Ar cyclization,
as well as (v) possible pseudopericyclic cheletropic decarb-
onylation.
aldehyde, and furan-3(2H)-ones through novel cationic dom-
ino rearrangements depending on the R substituents. The
mechanisms of these reactions were described by a combina-
(Table 1, Entry 1).[4] In this paper, we wish to report that
CF3SO3H (TfOH) brought about the intriguing multipath
domino rearrangements of 1a leading to new products 3a–
Introduction
The use of domino reactions in synthetic organic chemis-
try is fascinating, especially with regard to the preparation
of structurally defined highly complex molecules and the
construction of biologically important natural substances.[1]
In view of the ability to form several bonds in one sequence
without isolating any intermediates, changing the reaction
conditions, or adding new reagents, such reactions can be
economically and environmentally favorable because they
reduce reagent/solvent and human resources. In addition,
the strategic design of the starting compounds and the
mechanistic elucidation of the sequential process provide
useful information to the development of synthetic organic
chemistry.
Table 1. Product distribution in the acid-catalyzed domino re-
arrangements of 1a–c and 2a in CDCl3 at 30 °C.
In this connection, we have previously synthesized highly
strained tricyclic diones as molecular building blocks for
the construction of various polycyclic compounds in acid-
catalyzed systems.[2] The designed tricyclic diones contain-
ing both a cyclopropane and a cyclobutene ring were syn-
thesized from the [2+2] photocycloaddition of alkynes to
diphenylhomobenzoquinones.[2] The BF3-catalyzed reac-
tions were found to give several bicyclic and tricyclic com-
pounds through domino rearrangements involving sequen-
tial opening of the cyclobutene and cyclopropane rings.[3]
Very recently, we found that the BF3-catalyzed reaction
of newly prepared diphenylhomobenzoquinone epoxide 1a
(R = Me) undergoes π-aryl participated epoxide ring open-
ing to provide quantitatively tricyclic diketo alcohol 2a
Entry Compd.
Acid
Time
[h][c]
Yield [%][a]
(equiv.)[b]
2
3
4
5
6
1
2
3
4
5
6
7
8
9
10
1a
1a
1a
1a
1a
BF3 (3)[d]
MsOH (3)
TfOH (1)
TfOH (3)
TfOH (10)
TfOH (3)
TfOH (3)
TfOH (3)
TfOH (10)
TfOH (20)
24
2
100
99
90
10
0
0
0
–
–
0
0
2
52
53
45
0
53
78
98
0
0
0
3
0
0
0
47
21
1
0
0
7
23
0
50
99
0
0
0
0
11
46
4
0
0
0
0
1.5
1.5
1.5
1.5
1.5
1.5
1
1b
1c[e]
2a
2a
2a
0
0
1
–
[a] Department of Applied Chemistry,
Graduate School of Engineering, Osaka University,
Suita, Osaka 565-0871, Japan
[a] Based on the amount of 1 or 2a used, and calculated by 1H
NMR spectroscopy. [b] MsOH = MeSO3H, TfOH = CF3SO3H.
[c] Reactions were complete except for BF3 catalyst (Entry 1; 65%
conversion). [d] See ref.[4a] [e] CO gas evolution was analytically
confirmed (GASTEC GV-100S).
Fax: +81-6-6879-4593
E-mail: oshima@chem.eng.osaka-u.ac.jp
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201200455.
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H. Asahara, S. Matsui, Y. Masuda, N. Ikuma, T. Oshima
SHORT COMMUNICATION
6a (Table 1, Entry 4). We also extended the reaction to more
congested 1b (R = iPr) and 1c (R = tBu) to estimate their
steric effects on these reaction features.
Results and Discussion
The reactions of 1a–c and 2a with the requisite number
of equivalents of TfOH were carried out in CDCl3 at 30 °C
according to a previous report.[4] The reaction conditions
and the product distributions are collected in Table 1. The
1
structures of 3–6 were deduced from their H NMR, 13C
1
NMR, HSQC, HMBC, H–1H COSY, and IR spectra, in
addition to HRMS (see the Supporting Information). The
structure of 5c was also confirmed by X-ray crystal struc-
tural analysis (Figure 1). It is noteworthy that the MS data
of these compounds suggested a loss of water for 3 and 4
and a loss of carbon monoxide for 5 and 6.
Scheme 1. TfOH-catalyzed rearrangement of 2a,b into 3a,b.
By contrast, the formation of 4a was only observed for
the Me-substituted homologue. It was also found that the
original quinone framework was completely degraded dur-
ing the transformation into 4a. This rearrangement seems
to proceed through a reaction sequence involving initial
ring-contraction of 2a by 1,2-shift of the o-phenylene-
bridged carbon atom onto the acid-activated carbonyl car-
bon atom[6] with simultaneous formylation and dehydration
associated with cyclopropane ring cleavage and intramolec-
ular ipso-SN2-Ar displacement by the carbonyl oxygen atom
followed by deprotonation and electron reorganization into
4a (Scheme 2). The inability to form 4b (R = iPr) may be
attributable to the steric hindrance of the iPr group, which
would inhibit the initial ring contracted 1,2-shift. Moreover,
the large excess amount of TfOH dramatically suppressed
the formation of 4a probably because of the additional pro-
tonation of the intervening OH group, which reduces the
1,2-shift (Table 1, Entries 8–10).
Figure 1. ORTEP drawing of compound 5c.
Some noticeable points can be extracted from Table 1:
(1) BF3- or MeSO3H-catalyzed reaction of 1a gave only tri-
cyclic diketo-alcohol 2a through ring opening of the epox-
ide and transannular cyclization of the endo-phenyl ring
(Table 1, Entries 1 and 2). (2) CF3SO3H catalysis brought
about further rearrangement to afford new products 3a–
6a,[5] and the distributions were dependent upon the
amount of acid (Table 1, Entries 3–5). (3) iPr-Substituted 1b
provided 3b and 5b without the formation of 2b (Table 1,
Entry 6), and moreover, tBu-substituted 1c yielded 5c as the
sole product (Table 1, Entry 7). (4) The reaction of 2a did
not produce 5a or 6a, but 3a/4a instead, and the amount of
3a formed increased with an increase in the amount of acid
added (Table 1, Entries 8–10).
Scheme 2. TfOH-catalyzed rearrangement of 2a into 4a.
As seen in Table 1 (Entries 1–3), the product ratios of 5a
On the basis of these notions, the mechanisms of these and 6a on varying the amount of TfOH imply that 6a is
versatile domino rearrangements leading to compounds 3– derived from 5a through 1,2-Me shift (Scheme 3, path c)
6 is clearer. The formation of indenoquinones 3 can be ex- promoted by protonation at the alkenyl β-carbon atom. In-
plained by considering the transformation of 2 by several deed, preliminary treatment of 5a with an excess amount of
processes: acid-induced cyclopropane ring opening, dehy- TfOH in CDCl3 for 8 h brought about its complete trans-
dration followed by double bond migration, and trans- formation into 6a.[7] Obtained 6a is assigned as the E-iso-
annular ipso-SE2-Ar electrophilic aromatic substitution mer, which is more stable than the Z-isomer by 1.76 kcal/
(Scheme 1).
mol on the basis of the optimized geometries calculated at
2
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Domino Rearrangement of Diphenylhomobenzoquinone Epoxides
the B3LYP/6-311+G** level. Regarding the mechanism for
the formation of 5, it is very informative that carbon mon-
oxide was expelled in the reaction of 1c but not in the reac-
tion of 2a, as confirmed by a CO gas analyzer (GASTEC
GV-100S).
Figure 2. Schematic orbital interactions having two orbital discon-
nections in a plausible pseudopericyclic decarbonylation of 1. The
epoxide and cyclopropane rings offer p character Walsh orbitals
for the out-of-plane π-orbital array. There are two orbital discon-
nections where the out-of-plane (arbitrary phases) and in-plane sets
of orbitals meet at the carbon atoms of the epoxide and cyclopro-
pane rings.
calculations, pseudopericyclic decarbonylation reactions
have been invoked for several cyclic carbonyl compounds
such as 2,3-furanone and 2,3-pyrroledione.[13,15]
Scheme 3. TfOH-catalyzed rearrangement of 1 to 5 by two possible
decarbonylation pathways.
Conclusions
What is the mechanism of acid-induced decarbonylation
from epoxides 1? Two possibilities are outlined in Scheme 3
(paths a and b), although the later stage leading to 5 can
be interpreted by the favorable 5-endo-trig cyclization[8] of
intermediate allyl cation II. Path a involves stepwise hetero-
lytic bond cleavage to generate acyl cation I, which will un-
dergo CO evolution and cyclopropane ring cleavage to af-
ford II. On the other hand, path b is a concerted pericyclic
reaction such as cheletropic decarbonylation.[9] However,
epoxides 1 do not satisfy the cyclic loop of interacting π-
orbitals predicted by the Woodward and Hoffman rule.[10]
This contradiction may be resolved by relying on pseudo-
pericyclic reactions.[11] Since the original definition by Le-
mal et al.,[12] pseudopericyclic reactions were developed by
Birney et al.[13] to include pericyclic reactions in which the
cyclic array of overlapping orbitals is fulfilled by consider-
ing the orbital disconnections. The general characteristics
of the thermal pseudopericyclic reactions can be summa-
rized as follows: (1) A pseudopericyclic reaction may be or-
bital symmetry allowed regardless of the number of elec-
trons involved. (2) Pseudopericyclic reactions will have
planar transition states. (3) Energy barriers to pseudoperi-
In summary, we have found that TfOH-catalyzed reac-
tions of diphenylhomobenzoquinone epoxides 1a–c with 5-
Me, iPr, and tBu substituents brought about domino re-
arrangements to give indenoquinones 3a and 3b, cy-
clopenta[b]chromene-2-carbaldehyde 4a, and furanones 5a–
c, respectively, depending on the 5-alkyl substituents. The
mechanistic investigations of these reactions show a variety
of fundamental features of organic reactions involving a
possible pseudopericyclic cheletropic decarbonylation and
offer a very fascinating methodology in synthetic organic
chemistry. As one of the general criteria of pseudopericyclic
reactions (point 3), molecular orbital analysis is underway
to estimate the possible low energy barrier to the decarb-
onylation process.
Experimental Section
General Procedure for the Synthesis of Homobenzoquinone Epoxides
1b and 1c: To a mixture of homobenzoquinone (0.5 mmol) and
30% H2O2 (0.75 mmol) in DMSO (1 mL) was added dropwise
(10 min) a solution of Bu4NF (1 m in THF, 0.5 mmol) at room
cyclic reactions can be very low. Satisfying points (1) and temperature. After the addition of the reagent, the reaction mixture
was stirred for 2 h. Then, epoxide 1 was extracted with ethyl acetate
(3ϫ3 mL). The organic layer was washed with water (3ϫ3 mL)
and dried with magnesium sulfate. The solvent was evaporated in
vacuo. Epoxide 1 was purified by column chromatography on silica
gel (benzene) and recrystallized (hexane/diethyl ether). The struc-
tures of all epoxides were deduced by 1H NMR, 13C NMR, and
IR spectroscopy.
(2), epoxides 1 will attain a pseudopericyclic orbital top-
ology with two orbital disconnections at the departing CO
bonded two carbon atoms (Figure 2). Of interest is that
both the epoxide and the cyclopropane ring of 1 can offer
p character Walsh orbitals[14] for the construction of the
out-of-plane π orbital array in the quinone fragment.
In Figure 2, the departing CO is drawn hybridized as car-
bon monoxide and therefore the carbon lone pair n (HO)
and CO π* (LU) orbitals can interact nucleophilically and
electrophilically with epoxide σ* (LU) and cyclopropane σ
(HO) orbitals, respectively. These in-plane sets of orbitals
have orthogonal orbital interactions with the above out-of-
plane π-orbital array (arbitrary phases), having a planar
General Procedure for the BF3-Catalyzed Reaction of Homobenzo-
quinone Epoxides: To a CDCl3 solution (600 μL) of 1 (0.02 mmol)
in an NMR tube was added the requisite amount of acid at room
temperature under an atmosphere of N2 by using a microsyringe.
The progress of the reaction was monitored by 1H NMR spec-
troscopy. After the requisite time, the reaction solution was trans-
ferred into a separatory funnel, diluted with chloroform (10 mL),
transition state. Recently, on the basis of molecular orbital and then washed with water (3ϫ3 mL). The aqueous layer was
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H. Asahara, S. Matsui, Y. Masuda, N. Ikuma, T. Oshima
SHORT COMMUNICATION
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extracted with chloroform (2ϫ5 mL). The combined organic layer
was washed with water (3ϫ3 mL) and then dried with calcium
chloride. After evaporation of the solvent in vacuo, the residue was
submitted to 1H NMR spectroscopic analysis to determine the
product distribution. The products were separated by column
chromatography on silica gel (hexane/ethyl acetate). The structures
of all products were deduced from 1H NMR, 13C NMR, and IR
spectroscopy and HRMS.
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General Procedure for the TfOH-Catalyzed Transformation of 5a
Into 6a: To a CDCl3 solution (600 μL) of 5a (5.8 mg, 0.02 mmol)
in an NMR tube was added the requisite amount of TfOH at room
temperature under an atmosphere of N2 by using a microsyringe.
The progress of reaction was monitored by 1H NMR spectroscopy.
After the requisite time, the reaction solution was transferred into
a separatory funnel, diluted with chloroform (10 mL), and then
washed with aq. NaCl solution (3ϫ3 mL). The combined organic
layer was then dried with MgSO4. After evaporation of the solvent
in vacuo, the residue was submitted to 1H NMR spectroscopic
analysis to determine the product distribution. The products were
separated by column chromatography on silica gel (hexane/ethyl
acetate). The structure of the product was deduced from 1H NMR,
13C NMR, and IR spectroscopy and HRMS.
Supporting Information (see footnote on the first page of this article):
Compound data and NMR spectra for 1b and 1c and products 3–6.
Acknowledgments
This work was supported by a Grant-in-Aid for Scientific Research
(C) from the Ministry of Education, Culture, Sports, Science and
Technology, Japan (No. 20550040). The authors express their deep
gratitude to Dr. K. Fukushima (Wakayama Medical University) for
helpful discussions on pseudopericyclic reactions.
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1
few drops of D2O (by H NMR spectroscopy).
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Received: April 10, 2012
Published Online: ᭿
4
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Domino Rearrangement of Diphenylhomobenzoquinone Epoxides
Domino Reaction
Acid-catalyzed reaction of various 5-alkyl-
H. Asahara, S. Matsui, Y. Masuda,
substituted
diphenylhomobenzoquinone
N. Ikuma, T. Oshima* ...................... 1–5
epoxides provided various products
through novel cationic domino rearrange-
ments and decarbonylation depending on
the acid (BF3/MeSO3H/TfSO3H) and the
alkyl substituents (R = Me/iPr/tBu).
Versatile Domino Rearrangement of Di-
phenylhomobenzoquinone Epoxides In-
duced by CF3SO3H
Keywords: Domino reactions / Rearrange-
ment
/
Epoxides
/
Decarbonylation
/
Pseudopericyclic reaction
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