Enantioselective synthesis of densely functionalized pyranochromenes via an unpredictable cascade Michael-oxa-Michael-tautomerization sequence

Article abstract of DOI:10.1002/chem.201002490

A surprising example of enantioselective cascade Michael-oxa-Michael- tautomerization reactions of malononitrile and benzylidenechromanones has been developed. In this case, malononitrile functions as both nucleophile and electrophile. Meanwhile, a simple

Full text of DOI:10.1002/chem.201002490

DOI: 10.1002/chem.201002490
Enantioselective Synthesis of Densely Functionalized Pyranochromenes via
an Unpredictable Cascade Michael–Oxa-Michael–Tautomerization Sequence
Qiao Ren, Yaojun Gao, and Jian Wang*[a]
Cascade synthetic strategy has been developed as a
uniquely powerful tool for synthetic connections and plays a
pivotal role in modern organic synthesis.[1] Given its wide-
spread use, the development of new synthetic cascade reac-
tions for the construction of complex molecules is an impor-
tant goal of research carried in both academic and industrial
laboratories.[2] In particular, asymmetric organocatalytic cas-
cade processes[3] are even more appearing because of their
advantages, such as operational simplicity, environment
friendliness, and rapid one-pot entries to molecular com-
plexity via atom, step and redox economic or protecting-
group-free protocols. Herein, we disclose a new enantiose-
lective organocatalytic cascade reaction with a Michael–oxa-
Michael–tautomerization sequence that generates highly
functionalized chiral pyranochromene complexes in a con-
cise manner (Scheme 1). Notably, the features of the strat-
Scheme 1. Strategy for enantioselective synthesis of densely functional-
ized pyranochromenes
egy include 1) the cascade process efficiently catalyzed by a
new indane amine–thiourea catalyst via hydrogen-bonding
catalysis in good to excellent yields (up to 99%); 2) the first
utilization of malononitrile both as nucleophile and electro-
phile in asymmetric organocatalytic Michael addition; 3) the
efficient assembly of highly functionalized pyranochromene
complexes with high to excellent enantioselectivities (up to
99% ee) in a one-pot reaction.
nitroalkanes.[8] Extending the scope of either the acceptors
or the donors employable in the asymmetric Michael addi-
tion is an important advance in order to have access to di-
versely functionalized and synthetic useful chiral building
blocks. The utilization of malononitrile as the nucleophile in
Michael additions has received less attention, although the
nitrile group can undergo further transformations to 1,3-di-
carbonyl derivatives, or imines. Quite recently, the Jacobsen
and Kanemasa groups independently reported the enantio-
selective Michael additions of malononitrile to a,b-unsatu-
rated imides catalyzed by metal catalyst.[9] Later on, the
Takemoto group reported the first example of enantioselec-
tive Michael reactions of malonontrile to a,b-unsaturated
ꢀ
The catalytic asymmetric C C bond formation represents
one of the most interesting and challenging fields in organic
chemistry. A number of asymmetric Michael additions to
a,b-unsaturated carbonyl acceptors catalyzed by chiral orga-
nocatalysts have been reported.[4] However, the nucleophiles
employed in the asymmetric Michael addition reactions are
restricted to malonate esters,[5] diketones,[6] ketoesters[7] and
imides in the presence of
a
chiral organocatalyst
[a] Q. Ren, Dr. Y. Gao, Prof. Dr. J. Wang
Department of Chemistry
(Scheme 2).[10] More recently, the Deng,[11] and Lattanzi[12]
groups also independently reported a cinchona alkaloid cat-
alyzed enantioselective Michael addition on malononitrile
to enones (Scheme 2). Furthermore, the Zhao group also
successfully incorporated malononitrile into three-compo-
nent reaction for the synthesis of pyranopyrazoles.[16] How-
National University of Singapore
3 Science Drive 3, Singapore 117543
Fax : (+65) 6516-1691
E-mail: chmwangj@nus.edu.sg
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201002490.
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Chem. Eur. J. 2010, 16, 13594 – 13598

COMMUNICATION
Table 1. Evaluation of the bifunctional organocatalyst.[a]
Entry
Catalyst
7a
Yield [%][b]
8a
Yield [%][b]
d.r. [%][c]
ee [%][d]
1
2
3
4
5
6
1
65
41
78
72
72
21
0.7:1
1.5:1
1.3:1
1.9:1
1.5:1
0.4:1
21
12
16
18
26
72
65
65
45
67
-72
79
2a
2b
2c
3
Scheme 2. Organocatalyst promoted cascade reactions of malononitrile
with a,b-unsaturated ketones.
4
[a] Reaction was conducted on 0.2 mmol scale in DCM (0.2 mL) at RT
for 4 d, and the ratio of 5a/6 is 1:1.5. [b] Yield of isolated product. [c] De-
termined by 1H NMR analysis of the crude product. [d] Enantiomeric
excess (ee) was determined by HPLC analysis.
ever, all above-mentioned cases demonstrated that malono-
nitrile was only utilized as nucleophile.
To extend the synthetic utility of malononitrile as electro-
phile or both electrophile and nucleophile in asymmetric
catalysis, we wish to undertake an investigation of new pro-
tocols. We hypothesized that the new Michael reaction
based process might facilitate a new synthetic pathway to
construct a complex molecule instead of a simple Michael
adduct. Surprisingly, the results of our new reaction explora-
tion envisioned that the change of common enones to ben-
zylidenechromanones efficiently drove the reaction to inter-
act with malononitrile to generate new chiral pyranochro-
mene complexes (Scheme 2). Herein, malononitrile worked
as electrophile and nucleophile. Notably, these pyranochro-
mene complexes might possess a broad application in me-
dicinal chemistry.[13]
nomenon implied that malononitrile could be utilized both
as nucleophile and electrophile, especially in asymmetric or-
ganocatalysis. Inspired by this new discovery, we then fo-
cused our attention to discover a proportional powerful
chiral organocatalyst which could efficiently promote reac-
tion to selectively synthesize 8a and also highly control the
stereochemistry of 8a.
Recently, our group developed a series of new bifunction-
al indane amine–thiourea catalysts which were approved to
be efficient chiral catalysts for Michael-type reactions.#
Based on these unpublished results, we found that indane
catalysts exhibited a number of amazing properties, such as
high reactivity and excellent stereocontrol in many cases.
Thereby, a test of our indane catalystꢂs efficiency in this re-
action was performed. To this end, a small library of our
group synthesized indane catalysts 2–4 (see above) was ap-
plied to this reaction. The library includes 1) catalyst 2a, 2b
and 2c with modifications on the conformation of amine
functional group; 2) catalyst 4 with a specific alternation on
the relative orientation of amine and thiourea functional
groups in comparison with catalyst 2c; 3) catalyst 3 with an
adjustment of the relative positions of amine and thiourea
functional groups in contrast to catalyst 4. Based on our
best knowledge, we believe the dihedral angel between two
functional groups is one of the key issues for stereocontrol.
We envisioned that the change of dihedral angel between
the functional groups of amine and thiourea might introduce
an ideal and stable transition state to assist the formation of
an excellent stereoselectivity. It should be noted that the
above discussion is largely speculative and qualitative. Even-
tually, the new catalyst 4 was discovered as a relatively ideal
promoter to catalyze this cascade process under the same
condition we stated above (Table 1, entry 6, 72% and 79%
ee for 8a). In the catalyst screening, we observed that the
geometry and the relative positions of amine and thiourea
functional groups played a critical role in promoting reac-
tion and controlling the enantioselectivity. If there was anti-
geometry between amine and thiourea functional groups,
the reactivity of catalysts (2a–c) was relative weak and the
To explore the feasibility of the proposed catalytic cas-
cade Michael–oxa-Michael–tautomerization process, reac-
tions of (E)-3-benzylidenechroman-4-one (5a) with malono-
nitrile (6) were performed in CH2Cl2 at room temperature
in the presence of cinchona alkaloid amine–thiourea catalyst
1, independently developed by the Connon[5e] and Soꢁs[14]
groups (see below). As shown in Table 1, the major Michael
adduct 7a was afforded by catalyst 1 with 65% yield and
0.7:1 d.r. (Table 1, entry 1). Interestingly, a small amount of
unpredicted complex 8a was also formed by a 21% yield
1
and a moderate 65% ee. The H and 13C NMR results of
complex 8a demonstrated that it has completely different
structure in comparison with Michael adduct 7a. This phe-
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13595

J. Wang et al.
major product was Michael adduct 7a (entries, 2–4: 41, 78
and 72%, respectively). Nevertheless, catalyst 4 manifested
a relatively higher level of catalyst efficiency and stereocon-
trol with a formation of desired product 8a. In terms of cat-
alyst 4 s structure, we found that a syn-geometry between
amine and thiourea functional groups in catalyst 4 was a sig-
nificant factor. Surprisingly, a slight interchange of the rela-
tive positions of amine and thiourea functional groups (from
4 to 3) definitely caused the loss of desired compound 8a
(entry 5, 72% yield for major Michael adduct 7a and only
26% yield for 8a). In contrast to other catalysts, we envi-
sioned that catalyst 4 could further reduce the activation
energy of oxa-Michael step for promoting this cascade se-
quence.
In view of high enantioselectivity, further optimization ef-
forts were performed by examining other parameters, such
as solvents, temperature and reaction concentrations
(Table 2). In contrast to other solvents, toluene was revealed
as the best media (Table 2, entry 9, 95% and 87% ee, 3 d).
In the hope of higher enantioselectivities, we decreased the
reaction concentration from 1.0 to 0.5m. As a result, an ex-
cellent enantiomeric excess was achieved (entry 13, 95%,
92% ee) with a negligible time increase (entry 13, reaction
completed in 4 d). If the concentration was reduced to 0.2m,
95% ee was achieved, but with an increased reaction time
of 9 d (entry 14).
reliable synthetic process for the preparation of densely
functionalized pyranochromenes (Table 3). Significantly, the
new stereocenter was efficiently created in good to excellent
Table 3. Substrate scope of the reaction.[a]
Entry
X
Y
R
Yield [%][b]
ee [%][c]
1[f]
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
CH2
S
H
H
H
H
H
H
H
H
H
H
H
H
Ph (8a)
95
93
92
88
83
99
72
90
85
91
90
88
86
88
91
60
85
86
75
92
88
90
92
87
86
93
99
90
92
87
88
81
92
94
80
81
86
87
2[e]
3-ClC6H4 (8b)
4-ClC6H4 (8c)
3,4-Cl2C6H3 (8d)
2-F-4-ClC6H3 (8e)
2-BrC6H4 (8 f)
4-NO2C6H4 (8g)
4-MeOC6H4 (8h)
4-allyloxyC6H4 (8i)
2-allyloxyC6H4 (8j)
3-PhOC6H4 (8k)
4-iPrC6H4 (8l)
2-thiophenyl (8m)
Ph (8n)
3[e]
4[e]
5[e]
6[e]
7[d]
8[g]
9[g]
10[e]
11[e]
12[f]
13[g]
14[e]
15[e]
16[g]
17[g]
18[g]
19[g]
H
6-Cl
6-Me
H
H
H
Ph (8o)
Ph (8p)
Ph (8q)
cyclohexyl (8r)
iPr (8s)
O
O
H
[a] Unless specified, see the Experimental section for reaction conditions.
[b] Yield of isolated product. [c] ee determined by HPLC analysis. [d] Re-
action time: 2 d. [e] 3 d. [f] 4 d. [g] 5 d.
Table 2. Influence of solvent and concentration on the enantioselective
reaction.[a]
enantioselectivities in one-pot reaction. Moreover, the pro-
cess not only afforded a pyranochromene complex, and also
allowed for a diversely structural variation of (E)-3-benzyli-
denechroman-4-ones 5 (Table 3, entries 1–17, when R=aryl;
entries 18 and 19, when R=alkyl). The process also tolerat-
ed a replacement of “O” by “S” and “CH2” to introduce dif-
ferent heterocycles (entries 16 and 17). For examples, the
catalyst 4-promoted cascade process smoothly afforded
moderate to excellent yields (60–99%) and high to excellent
enantioselectivities (80–99% ee). The efficiency of catalyst 4
allowed the reaction to bear a diverse structure of benzyli-
denechroman-4-one substrates which possess neutral func-
tional groups (entries 1, 14–17, 60–95%, 80–94% ee), elec-
tron-withdrawing groups (entries 2–7, 72–99%, 86–93% ee)
and electron-donating groups (entries 8–12, 85–91%, 87–
99% ee) in backbone. Noticeably, the bulky alkyl group in-
volved benzylidenechroman-4-ones also worked to afford
the desired product with good yields (86 and 75%, respec-
tively, entries 18 and 19) and high enantioselectivities (86%
and 87% ee, entries 18 and 19). When a heterocyclic thio-
phene was introduced to benzylidenechroman-4-one back-
bone, the reaction still provided a good enantioselectivities
(entry 13, 86%, 81% ee). The absolute configuration of the
pyranochromene 8 f was unequivocally determined by
single-crystal X-ray diffraction analysis (Figure 1).[15]
Entry Solvent
7a
8a
Yield [%][b] d.r. [%][c] Yield [%][b] ee [%][d]
1
2
3
CH2Cl2
CHCl3
DCE
21
13
9
0.4:1
0.8:1
0.7:1
0.9:1
2.6:1
3.0:1
1.8:1
2.2:1
n.d.[g]
n.d.[g]
1.9:1
1.1:1
n.d.[g]
n.d.[g]
72
83
86
81
56
61
73
68
95
92
33
34
95
90
79
75
77
67
62
67
6
4
Et2O
10
23
35
25
25
<5
<5
59
5
THF
6
7
8
9
10
11
12
13[e]
14[f]
dioxane
MeOH
iPrOH
toluene
xylenes
DMF
cyclohenane 10
toluene
toluene
34
87
88
7
85
92
95
<5
<5
[a] Unless specified, see the Experimental section for reaction conditions.
[b] Yield of isolated product. [c] Determined by 1H NMR analysis of the
crude mixture. [d] ee determined by HPLC analysis. [e] 0.5m, 4 d.
[f] 0.2m, 9 d. [g] Not determined.
Having established the optimal reaction conditions, we
next examined the generality of this catalytic process. Re-
markably, this enantioselective cascade reaction served as a
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Enantioselective Synthesis
COMMUNICATION
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Figure 1. X-ray crystal structure of compound 8 f.
In conclusion, we have developed a cascade Michael–oxa-
Michael–tautomerization reaction of malononitrile to
enones mediated by an easily accessible chiral bifunctional
indane amine-thiourea catalyst 4. The densely functionalized
chiral pyranochromenes were obtained in a one-pot reaction
in good to high yields (up to 99%) and high to excellent
enantioselectivities (up to 99% ee). Moreover, this catalytic
system also tolerated many synthetic useful functional
groups, such as nitrile, nitro, amino groups which might be
manipulated for accessing more sophisticated heterocyclic
compounds. Further application of the catalytic system to
other new reactions is under investigation in our laboratory.
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Experimental Section
General procedure: To a solution of malononitrile 2a (10 mg, 0.15 mmol)
in toluene (0.2 mL) was added (E)-3-benzylidenechroman-4-one (1a,
24 mg, 0.1 mmol) at room temperature, followed by catalyst 4 (4.9 mg,
0.01 mmol). The mixture was stirred at room temperature for 4 d. The
crude product was purified by column chromatography on silica gel,
eluted by hexane/EtOAc 8:1 then 4:1 to afford the desired product 8a as
yellow solid (28.7 mg, 95%). 1H NMR (500 MHz, CDCl3): d=7.39–7.32
(m, 3H), 7.27 (q, J=6.0 Hz, 3H), 7.23–7.15 (m, 1H), 6.95 (t, J=7.5 Hz,
1H), 6.79 (d, J=8.1 Hz, 1H), 4.64 (s, 2H), 4.63 (d, J=13.2 Hz, 1H), 4.43
(d, J=13.8 Hz, 1H), 4.03 ppm (s, 1H); 13C NMR (125 MHz, CDCl3): d=
158.77, 154.12, 140.87, 138.01, 130.30, 129.02, 127.92, 127.88, 121.26,
121.04, 119.26, 116.66, 115.94, 105.05, 66.48, 61.16, 39.71 ppm; HRMS
(EI): m/z: calcd for C19H14O2N2: 302.1055, found: 302.1048; HPLC (Chir-
alpak IC, isopropanol/hexane 10:90, flow rate 1.0 mLminꢀ1, l=254 nm):
tR (major)=27.5 min, tR (minor)=15.9 min, ee=92%; ½aꢁ3D0 =ꢀ64.1 (c=
0.98 in CHCl3).
Acknowledgements
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The authors acknowledge the National University of Singapore for finan-
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and R143000443112).
Keywords: asymmetric catalysis
·
cascade reaction
·
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Received: August 28, 2010
Published online: November 9, 2010
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Chem. Eur. J. 2010, 16, 13594 – 13598