Base-catalyzed tandem michael/dehydro-diels-alder reaction of a,a-dicyanoolefins with electron-deficient 1,3-conjugated enynes: A facile entry to angularly fused polycycles

Article abstract of DOI:10.1002/chem.201302960

Angularly fused carbocyclic frameworks and their heteroatom-substituted analogues exist in many natural products that display a broad and interesting range of biological activities. Preparation of polycyclic products by cycloaddition reactions have been the long-standing hot topic in the synthetic community. Dehydro-Diels- Alder (DDA) reactions are one class of dehydropericyclic reactions that are derived conceptually by systematic removal of hydrogen atom pairs. A base-promoted tandem Michael addition and DDA reaction of a,a-dicyanoolefins with electron-deficient 1,3-conjugated enynes was realized in which a DDA reaction takes place between the arylalkynes and electron-deficient tetrasubstituted olefin. The control experiments support the stepwise anionic reaction pathway rather than the concerted reaction pathway.

Full text of DOI:10.1002/chem.201302960

DOI: 10.1002/chem.201302960
Communication
&
Organic Synthesis
Base-Catalyzed Tandem Michael/Dehydro-Diels–Alder Reaction of
a,a-Dicyanoolefins with Electron-Deficient 1,3-Conjugated
Enynes: A Facile Entry to Angularly Fused Polycycles
Mingrui Zhang[a] and Junliang Zhang*[a, b]
Abstract: Angularly fused carbocyclic frameworks and
their heteroatom-substituted analogues exist in many nat-
ural products that display a broad and interesting range
of biological activities. Preparation of polycyclic products
by cycloaddition reactions have been the long-standing
hot topic in the synthetic community. Dehydro-Diels–
Figure 1. Natural products bearing 5/5/5 and 5/5/6 angularly fused frame-
works.
Alder (DDA) reactions are one class of dehydropericyclic
reactions that are derived conceptually by systematic re-
moval of hydrogen atom pairs. A base-promoted tandem
Michael addition and DDA reaction of a,a-dicyanoolefins
with electron-deficient 1,3-conjugated enynes was realized
in which a DDA reaction takes place between the aryl-
alkynes and electron-deficient tetrasubstituted olefin. The
control experiments support the stepwise anionic reaction
pathway rather than the concerted reaction pathway.
standing hot topic in the synthetic community. Dehydro-Diels–
Alder (DDA) reactions[4] are one class of dehydropericyclic reac-
tions[5] that are derived conceptually by systematic removal of
hydrogen atom pairs. In past years, the DDA reactions occur-
ring between enynes, arylalkynes, and alkynes under photo-
chemical or harsh conditions, leading to benzenes or naphtha-
lenes have been extensively established (Scheme 1a),[6] but
DDA reactions between enynes or arylalkynes with olefins
leading to dihdrobenzenes or 1,2-dihydronaphthalenes have
The development of domino reactions to rapidly access com-
plex molecules with multiple stereogenic centres has attracted
considerable attention in both academic and industrial re-
search. Angularly fused carbocyclic frameworks and their
heteroatom-substituted analogues exist in many natural prod-
ucts that display a broad and interesting range of biological
activities (Figure 1).[1–3] One of the challenges in the synthesis
of these natural products and their analogues for biological
and medicinal studies is usually related to exploring one
domino reaction to construct the polycyclic skeleton bearing
a multiple consecutive stereogenic carboncenter with a quater-
nary one.
Cycloadditions with the character of 100% atom economy,
which can well address the challenging selective preparation
of cyclic (as well as bicyclic and even polycyclic) products with
all the substituents in the right position, have been a long-
[a] M. Zhang, Prof. J. Zhang
Shanghai Key Laboratory of Green Chemistry
and Chemical Processes. Department of Chemistry
East China Normal University
Shanghai 200062 (P. R. China)
E-mail: jlzhang@chem.ecnu.edu.cn
[b] Prof. J. Zhang
State Key Laboratory of Organometallic Chemistry
Shanghai Institute of Organic Chemistry
Shanghai 200032 (P. R. China)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201302960.
Scheme 1. Previous DDA reactions and our present work.
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not been well studied and both the scope of olefins is often
limited to electron-rich olefins (Scheme 1b).[7] On the other
hand, despite the fact that the DDA reaction has been discov-
ered for more than a century,[8] there is still an ongoing debate
in the scientific community on the exact reaction mechanism,
that is, by means of the concerted 1,5-hydrogen shift or step-
wise anionic mechanism.[6c,9] Herein, we wish to report our
new findings that a base-promoted tandem Michael addition
and DDA reaction of a,a-dicyanoolefins with electron-deficient
1,3-conjugated enynes, in which the DDA reaction takes place
between the arylalkynes and an electron-deficient tetrasubsti-
tuted olefin, occurs by a stepwise anionic reaction pathway.
Our previous studies showed that electron-deficient 1,3-con-
jugated enynes are good Michael acceptors that can undergo
tandem addition reactions with nucleophiles, such as 1,3-dicar-
bonyl compounds, hydroxylamines, and hydrazines, leading to
various acyclic as well as heterocyclic compounds.[10] These re-
actions are believed to proceed by an 1,2-allene intermediate
by tandem inter- and intramolecular nucleophilic addition.
Moreover, a,a-dicyanoolefins are another kind of readily avail-
able compound accessible by simple Knoevenagel condensa-
tion of the corresponding carbonyl compounds with malononi-
trile,[11] and have been widely used in organic synthesis acting
as both Michael acceptors[12] as well as vinylogous nucleo-
philes.[13] With the continuous interest in developing new
transformations of electron-deficient 1,3-conjugated enynes,
we envisaged that compound 4, generated from the Michael
addition a,a-dicyanoolefins to electron-deficient enynes 2,
might undergo an intramolecular DDA reaction between the
arylallene moiety and electron-deficient olefin moiety
(Scheme 2), which provides a metal-free, atom- and step-eco-
nomic shortcut to angularly polycyclic compounds.[14]
Table 1. Screening of reaction conditions.[a]
Entry
Base
Solvent
T [h]
Yield [%][b]
1
2
Cs2CO3
CsF
THF
THF
28
26
26
46
58
58
20
10
10
14
11
12
24
12
12
43
trace
0
57
76
65
50
63
77
78
39
90
68
78
89
3
DIPEA
tBuOK
K2CO3
TBAF
DBU
THF
THF
THF
THF
4[c]
5[c]
6[d]
7
THF
8
9
DBU
DBU
DBU
DBU
DBU
DBU
DBU
DBU
DMF
MeCN
toluene
MeOH
DCE
DCE
DCE
DCE
10
11
12
13[e]
14[f]
15[g]
[a] Unless specifically noted, the reactions were carried out with a,a-di-
cyanoolefins 1a (0.8 mmol) and enynes 2a (0.4 mmol), base (20 mol%),
4.0 mL of solvent at RT. [b] Isolated yield. [c] RT, 28 h and then 508C, 18 h.
[d] RT, 28 h, and 508C, 18 h then 708C, 12 h. [e] 15 mol% of DBU.
[f] 1.5 equivalents of 1a. [g] 2.5 equivalents of 1a. DIPEA=N,N-diisopro-
pylethylamine; DBU=1,8-diazabicyclo[5,4,0]undecen-7-ene; TBAF=tetra-
n-butylammonium fluoride.
temperature in the presence of a catalytic amount of Cs2CO3
(20 mol%), the reaction indeed afforded the desired 3aa in
43% isolated yield after 28 h (Table 1, entry 1). The structure of
3aa was confirmed by NMR spectroscopy and single-crystal X-
ray diffraction analysis. (Figure 2) Other organic and inorganic
Figure 2. X-ray crystal structure of 3aa.
Scheme 2. Reaction design for the tandem Michael addition and DDA reac-
tion.
bases, such as N,N-diisopropylethylamine (DIPEA), 1,8-
diazabicyclo[5.4.0]undec-7-ene (DBU), CsF, tBuOK, and Cs2CO3,
were then investigated. The reaction under the catalysis of CsF
is quite messy (entry 2) and DIPEA shows no catalytic activity
at all (entry 3). tBuOK, K2CO3, and TBAF could catalyze this
transformation but require a higher temperature, leading to
the products in moderate yields (entires 4–6). When DBU was
used as the base, the product was isolated in 50% yield at
To test this hypothesis, a,a-dicyanoolefin 1a and enyne 2a
were selected as the model substrates. With the knowledge
that a,a-dicyanoolefins easily undergo self-dimerization, by
means of intermolecular vinylogous addition and ring closing
processes, under basic conditions,[15] 2.0 equivalents of 1a
were used, referring to 1.0 equivalent of 2a. To our delight,
when these two substrates were subjected to THF at ambient
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room temperature in 20 h (entry 7). 1,2-Dicholoethane (DCE)
was found to be the best solvent among those solvents tested
(THF, DMF, CH3CN, toluene, MeOH, and DCE) to give the high-
est yield (90%) as a single diastereoisomer (entries 8–12). Thus,
the standard reaction conditions chosen were DBU (20 mol%)
as the base, DCE as the solvent, and room temperature as the
reaction temperature (entry 12). Reducing the catalyst loading
from 20 to 15 mol% led to a lower yield (68%, entry 13). Fur-
thermore, variation of the equivalent of the a,a-dicyanoolefin
also resulted in a lower yield (entries 14–15).
Table 2. Variation of the a,a-dicyanoolefin component 1.[a]
With the optimal reaction conditions in hand, we turned our
attention to study the reaction scope of this tandem transfor-
mation by variation of the a,a-dicyanoolefin component
1 (Figure 3) and the results are summarized in Table 2. In con-
Figure 3. Structures of various a,a-dicyanoolefins 1.
trast to those a,a-dicyanoolefins derived from cyclic aliphatic
ketones, those reactions of the a,a-dicyanoolefins derived
from cyclic aryl ketones proceeded more smoothly to afford
the desired products in better yields. Gratifyingly, angularly
fused polycyclic skeletons with one heterocycle, such as 3ba,
3ca, 3ea, 3 fa, and 3 fi, could be successfully constructed from
the corresponding heterocyclic substrates 1b and 1e contain-
ing one oxygen atom as well as 1c and 1 f containing one
sulfur atom, respectively. Furthermore, 4-(tert-butyl)cyclohexa-
none-derived 1g gave a better yield of the desired product
3ga (74%) than the product 3da (46%) from the correspond-
ing cyclohexanone-derived 1d without sterically hindered tert-
butyl substituent. It is most likely that the tert-butyl group acts
as an anchor kept in an equatorial position and thus sup-
presses unfavorable ring flipping of the cyclohexylidene ring.
To our delight, the a,a-dicyanoolefin 1h derived from cyclo-
heptanone also showed good reactivity to provide the fused
6/5/7 tricyclic ring system 3ha in 56% yield with 3:1 diastereo-
selectivity. However, unfortunately, the fused 6/5/5 tricyclic
ring system cannot be constructed from the a,a-dicyanoolefin
derived from cyclopentanone, which showed no reactivity in
this reaction.
[a] Isolated yield. The mixture of substrates 1 and 2 was dissolved in 4 mL
of solvent and was added to the reaction system by a syringe pump
(0.5 mLhÀ1). [b] 20 mol% of Cs2CO3. [c] The d.r. was determined by
1H NMR spectroscopy.
Figure 4. Structures of various enynes 2.
To expand the generality of this process, other various sub-
stituted electron-deficient 1,3-conjugated enynes 2 (Figure 4)
were next examined and the results were summarized in
Table 3. In general, a diverse array of complex polycyclic frame-
works could be efficiently constructed. The substituent effect
of R2 on the aryl moiety was first investigated. In general,
enynes with electron-donating R2 on the aryl moiety showed
higher reactivity than those with electron-withdrawing ones.
For instance, 4-Me (2b) and 4-OMe (2c) substituted enynes
gave the desired products 3ab and 3ac in 89 and 88% yields,
respectively, whereas the 4-F substituted one (2 f) afforded the
product 3af in a relatively lower yield (72%). Various functional
groups on the aryl ring including CÀF (3af), CÀCl (3ad), and
even CÀBr (3ae) bonds are tolerated, which provide an oppor-
tunity for further modification. Besides the substituted phenyl
ring, naphthyl can also be well introduced to give the corre-
sponding product 3ak in 70% yield. It is noteworthy that the
thiophene on the alkyne is also compatible to this tandem re-
action and the desired product 3al could be isolated in 80%
yield as a single stereoisomer. The substituent effect of R1 on
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Table 3. Variation of the enyne component 2.
alkene moiety was studied next. The alkene with electron-rich
MeOC6H4 (2g) would reduce the reactivity leading to the re-
sulting 3ag in 55% yield after 22 h, whereas 3aa was obtained
in 90% yield in a shorter reaction time. Other electron-with-
drawing groups, such as ketone (2i) or cyano (2j), are applica-
ble to this transformation, producing the desired products 3ai
and 3aj in good yields.
Scheme 3. Plausible mechanism.
should be noted that no reaction occurs without DBU [Eq. (2)].
These results indicated that a reverse reaction (elimination) of
the addition reaction is competing with the cycloaddition reac-
tion under the basic conditions and the DDA reaction indeed
needs the base for reaction promotion. To gain further insight
into the protontropic shift process, the designed deuterium-
labeled substrates [D2]1a and [D5]2a were then prepared.
When [D2]1a and 2a [Eq. (3)] were subjected to the standard
reaction conditions, the product [D3]3aa was isolated in 76%
yield, in which the olefinic positions are deuterium-labeled at
42%, the a-position of the ester in 33%, and the corner posi-
tion in 60%, respectively. These results indicated that the de-
protonation step of 1d is reversible under the basic conditions
and the hydrogen transfer of the DDA reaction is more possi-
ble through an anionic pathway (path d) rather than the con-
certed intramolecular [1,5]hydrogen shift or two [1,3]hydrogen
shifts (path e). The low level of deuterium label in the product
[D6]3aa at the a-position of the ester and the olefinic position
also support that hydrogen transfer proceeds by means of pre-
dominantly (if not the only route) a stepwise anionic reaction
pathway [Eq. (4)]. Another control experiment in CDCl3 showed
the possibility of abstraction of a proton from the solvent
[Eq. (5)], in which both a- and olefinic-positions are deuterium-
labeled at 27 and 16%, respectively. This result again provided
strong support for a stepwise dehydrogenation step and an in-
termolecular anionic reaction pathway (path d)
Based on the results described above, a plausible mecha-
nism for this tandem transformation into angular polycyclic
frameworks 3 was proposed. To make it simple, compounds
1d and 2a were selected as the substrates to depict the mech-
anism (Scheme 3, to simplify, the enolate forms of IB, ID, and
IH were not shown). The addition of a nucleophilic anion IA,
generated from the deprotonation of the a,a-dicyanoolefin 1d
in the presence of DBU, to electron-deficient enyne 2a afford-
ed an intermediate structure as the propargylic anion IB and
its resonance form 1,2-allenic anion IC, which undergoes sub-
sequent protonation (from the substrate 1d or the conjugate
acid DBUÀH+ of the base DBU) to give 1,2-allenyl ester 4da.
Then, three possible pathways (path a, b and c) from IB, IC,
and compound 4da are proposed for the further DDA step to
afford the intermediates ID, IE, or IF, respectively. Upon proto-
nation, intermediate IE would give intermediate IF, which
would produce the final product 3da by path d or e. Path d is
the stepwise anionic pathway by aromatization (deprotona-
tion) and subsequent protonation, and path e is the concerted
[1,5]hydrogen shift (or two concerted [1,3]hydrogen shifts).
To probe the mechanism, some control experiments were
designed and carried out. An allene product 4aa can be ob-
tained in 27% yield by running the reaction at À208C in DCE
for 10 h with 10 mol% of DBU as the catalyst [Eq. (1)]. Treat-
ment of compound 4aa with 20 mol% of DBU in DCE at room
temperature could give the product 3aa in 56% yield along
with enyne 2a in 18% yield as a single stereoisomer, and it
In conclusion, we have developed a tandem Michael addi-
tion/dehydro-Diels–Alder reaction sequence, which provided
a facile access to angularly fused polycycles with multiple con-
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Scholar at Shanghai Institutions of Higher Learning, and
STCSM (12XD1402300) for financial support.
Keywords: Diels–Alder reactions · domino reactions · enynes ·
Michael reactions · polycycles
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Experimental Section
General procedure for the base-catalyzed synthesis of polycyclic
structures: An oven-dried reaction tube was charged with the elec-
tron-deficient 1,3-conjugated enyne 2 (0.4 mmol), a,a-dicyanoole-
fin 1 (0.8 mmol, 2.0 equiv), catalyst DBU or Cs2CO3 (0.08 mmol,
20 mol%), and dry DCE (4 mL, 0.1m). The resulting mixture was
stirred at room temperature until the enyne 2 was completely con-
sumed, as determined by TLC analysis. The reaction mixture was
concentrated under reduced pressure and the residue was purified
by flash chromatography on silica gel (eluent: petroleum ether/di-
ethyl ether/dichloromethane, 8:1:1) to afford the corresponding
product 3.
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Acknowledgements
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We are grateful to 973 Program (2011CB808600), Fok Ying
Tung Education Foundation (121014), the Program of Eastern
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Received: July 27, 2013
Revised: September 9, 2013
Published online on December 2, 2013
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