Facile domino access to chiral bicyclo[3.2.1]octanes and discovery of a new catalytic activation mode

Article abstract of DOI:10.1021/ol1007795

(Figure presented) A highly enantio- and diastereoselective organocatalytic domino Michael-Henry process for the preparation of synthetically unique and medicinally important bicyclo[3.2.1]octane derivatives with four stereogenic centers including two quaternary stereocenters has been developed. Theoretical DFT calculations on the transition states have been carried out to reveal origins of the excellent stereoselectivities. A novel dual model was thus proposed.

Full text of DOI:10.1021/ol1007795

ORGANIC
LETTERS
2010
Vol. 12, No. 12
2682-2685
Facile Domino Access to Chiral
Bicyclo[3.2.1]octanes and Discovery of a
New Catalytic Activation Mode
Bin Tan, Yunpeng Lu, Xiaofei Zeng, Pei Juan Chua, and Guofu Zhong*
DiVision of Chemistry and Biological Chemistry, School of Physical and Mathematical
Sciences, Nanyang Technological UniVersity, 21 Nanyang Link, Singapore 637371
guofu@ntu.edu.sg
Received April 5, 2010
ABSTRACT
A highly enantio- and diastereoselective organocatalytic domino Michael-Henry process for the preparation of synthetically unique and
medicinally important bicyclo[3.2.1]octane derivatives with four stereogenic centers including two quaternary stereocenters has been developed.
Theoretical DFT calculations on the transition states have been carried out to reveal origins of the excellent stereoselectivities. A novel dual
model was thus proposed.
Bicyclo[3.2.1]octane skeletons are found in a number of
interesting natural products and pharmaceuticals1 such as
Trichorabdal B, Kadsurenin C and L, Piersformoside, Platen-
simysin, and Platencin (Figure 1). For example, Platencin which
was isolated from Streptomyces platensis MA7339, can inhibit
the biosynthesis of bacterial fatty acids through binding with
the initiation condensing and elongation condensing enzymes
FabH.2 In 2008, Nicolaou2b,c and Rawal2d reported the total
synthesis of this compound, involving a metal-catalyzed
Diels-Alder reaction to construct the bicyclo[3.2.1]octane
skeletons. The stereochemical importance of this structural
Figure 1. Natural products containing bicyclo[3.2.1]octanes.
motif in biological activity is significant, and it represents a
considerable synthetic challenge that remained to be ad-
dressed in preparative studies.3 It is therefore crucial to create
an efficient synthetic route to this skeleton.
Domino reactions serve as a powerful tool for the rapid
and efficient assembly of complex structures from simple
starting materials with minimized waste production.4 Orga-
nocatalytic5 enantioselective domino processes6 are particu-
(1) (a) Fujita, K.; Node, M.; Sai, M.; Fujita, E.; Shingu, T.; Watson,
W. H.; Grossie, D. A.; Zabel, V. Chem. Pharm. Bull 1989, 37, 1465. (b)
Han, G.; Dai, P.; Xu, L.; Wang, S.; Zheng, Q. Chin. Chem. Lett. 1992, 3,
521. (c) Li, L.; Weng, Z.; Huang, S.; Pu, J.; Li, S.; Huang, H.; Yang, B.;
Han, Y.; Xiao, W.; Li, M.; Han, Q.; Sun, H. J. Nat. Prod. 2007, 70, 1295.
(d) Ma, Y.; Han, G.; Liu, Z. Yaoxue Xuebao 1993, 28, 307. (e) Filippini,
M. H.; Rodriguez, J. Chem. ReV. 1999, 99, 27.
10.1021/ol1007795  2010 American Chemical Society
Published on Web 05/14/2010

larly appealing because of their operational simplicity and
environmental friendliness. Although much progress has been
made in the organocatalytic domino reactions,7 the construc-
tion of bicyclo[3.2.1]octane skeletons still remains elusive,
and the development of new methodologies for the generation
of molecules with multiple stereogenic carbons including
quaternary centers8 in a cascade manner remains a big
challenge at the forefront of synthetic organic chemistry.
The Michael and Henry reactions are widely recognized
as among the most important C-C bond-formation processes
in organic chemistry, as they are versatile tools to assemble
multisubstituted carbon skeletons9 and transform nitroaldol
products into a number of nitrogen- and oxygen-containing
derivatives.10 However, to our knowledge, there is no report
describing the formation of two quaternary centers in the
asymmetric synthesis of the bicyclo[3.2.1] motif using a
domino Michael-Henry reaction strategy with good results.
Herein, we discovered an organocatalytic enantioselective
domino Michael-Henry reaction to afford highly function-
alized bicyclo[3.2.1]octanes with four stereogenic centers
including two quaternary carbons.
Readily accessible cinchona alkaloids and derivatives,
which were developed recently in several research groups,
have been identified as efficient bifunctional organocatalysts
in an asymmetric Michael reaction,11 Henry reactions,12 and
domino Michael-Henry reactions.13 These results prompted
us to explore the feasibility of employing quinine amine
catalyst I to catalyze the domino Michael-Henry reaction
involving a nitroolefin and a designed carbon nucleophiles
1a. To our delight, the desired product was obtained in good
yield and with moderate enantioselectivity (40% ee). Encour-
aged by this initial results, several cinchona alkaloid derived
catalysts (Figure 2) were investigated and displayed note-
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A.; Genilloud, O.; Teresa Diez, M.; Vicente, F.; Gonzalez, I.; Salazar, O.;
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Barbas, C. F., III Angew. Chem., Int. Ed. 2008, 47, 42. (d) Dondoni, A.;
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Kano, T.; Maruoka, K. Chem. Commun. 2008, 5465.
Figure 2. Structures of cinchona alkaloid derived catalysts.
worthy effects on the outcome of the domino reaction.
Thiourea catalysts generally afforded better results in yields
and stereoselectivities. Various solvents were screened, and
it was found that the more polar solvent benzonitrile resulted
in a higher enantioselectivity while maintaining the activity
of the catalyst. We next investigated the influence of the
reaction temperature and catalyst loading. It was observed
that the reactions were not affected by changing reaction
temperatures, unlike most reactions where lower temperatures
usually led to higher enantioselectivies and lower reaction
rates (Table 1, entries 11 and 12). However, lower catalyst
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Org. Lett., Vol. 12, No. 12, 2010
2683

bicyclo[3.2.1]octane derivatives but also serves as a facile
approach for the preparation of a range of substituted bicycles
containing four chiral centers in excellent enantiomeric
exesses (90-96% ee) and diastereoselectivities (>99:1 dr in
all cases) (Table 2). The VI-promoted domino Michael-Henry
process takes place with a variety of nitroolefin Michael
acceptors, which possess neutral, electron-donating, electron-
withdrawing groups in the phenyl ring (Table 2, entries 1-8
and 11-12). It appeared that substituents’ electronic and
steric natures have minimal impact on efficiencies, enanti-
oselectivities, and diastereoselectivities of the Michael-Henry
reactions. Not only aromatic groups but also heteroaromatic
groups such as furyl and thienyl could be successfully
employed to afford the respective cyclopentane rings with
excellent stereoselectivities (Table 2, entries 9 and 10).
Notably, only one Michael-Henry adduct was obtained from
the reaction of nitrodiene 2m in 86% ee (Scheme 1).
Table 1. Organocatalytic Domino Michael-Henry Reactions of
Trisubstituted Carbon Nucleophiles and trans-ꢀ-Nitrostyrenea
entry catalyst
solvent
toluene
time (h) yield (%)b drc ee (%)d
1
I
12
12
4
73
78
85
90
87
86
89
88
91
91
89
86
91
83
>99:1 -40
>99:1 50
2
II
toluene
3
4
5
III
IV
V
toluene
toluene
toluene
>99:1 -80
>99:1 -89
>99:1 -89
4
4
6
7
8
9
10
11e
12e
13f
14g
VI
VI
VI
VI
VI
VI
V
toluene
Et2O
6
4
4
4
4
8
8
6
>99:1
>99:1
>99:1
>99:1
>99:1
>99:1
>99:1 -92
>99:1
>99:1
92
92
90
90
94
94
m-xylene
nitrobenzene
benzonitrile
benzonitrile
benzonitrile
benzonitrile
benzonitrile
Scheme 1
.
Domino Michael-Henry Reactions of 1a with 2m
and 1b with 2a
VI
VI
94
93
10
a Unless otherwise specified, the reactions were carried out using 1a
(0.2 mmol, 2.0 equiv) and 2a (0.1 mmol, 1.0 equiv) with 10 mol % of
catalysts in 0.5 mL of solvent at room temperature (23 °C). b Isolated yields.
c Determined by NMR and HPLC analysis. d Determined by chiral HPLC
analysis. e Reaction at 4 °C. f 5 mol % catalyst. g 3 mol % catalyst.
loadings did lead to longer reaction times (Table 1, entries
13 and 14), and using V or VI could produce both
enantiomers.
In our exploratory effort, this new methodology not only
provides a facile access to a range of multisubstituted
Although both ꢀ- and δ-positions of 2m can possibly be
theoretically attacked because of the two congruous double
bonds, the formation of only one adduct showed the great
regioselectivity and stereoselectivity of this methodology.
Furthermore, the domino reaction also proceeded smoothly
when 1a was replaced by 1b, giving good enantio- (85%
ee) and diastereoselectivities (>99:1 dr) as displayed in
Scheme 1.
Table 2. Domino Michael-Henry Reactions of 1a and
Nitroolefins 2 Catalyzed by Catalyst VIa
According to the dual activation model, the two substrates
involved in the reaction are activated simultaneously by
catalyst14 as shown in Figure 3a. Nitroolefins have been
assumed to interact with the amine moiety of the thiourea
entry
R
3
time (h) yield (%)b
drc
ee (%)d
1
2
3
4
5
6
7
8
Ph
3a
3b
3c
3d
3e
3f
3g
3h
3i
6
10
10
8
93
84
79
91
87
86
93
88
80
84
85
77
>99:1
>99:1
>99:1
>99:1
>99:1
>99:1
>99:1
>99:1
>99:1
>99:1
>99:1
>99:1
94
95
90
93
94
96
95
94
92
92
94
93
3-OMe-Ph
4-OMe-Ph
4-Me-Ph
4-Br-Ph
2-Cl-Ph
4-Cl-Ph
4-F-Ph
2-thienyl
2-furyl
1-naphthyl 3k
6
6
6
6
8
6
9
10
11
12
3j
12
12
4-NO2-Ph 3l
a All the reactions were carried out using 1a (0.2 mmol, 2 equiv) and
2 (0.1 mmol, 1.0 equiv) in the presence of 5 mol % of VI at rt with
benzonitrile (0.5 mL). b Isolated yields. c Determined by NMR and HPLC
analysis. d Determined by chiral HPLC analysis.
Figure 3
. Proposed activation modes of the catalyst and substrates
before (a) and after (b) the DFT calculations.
2684
Org. Lett., Vol. 12, No. 12, 2010

on the catalyst via multiple H-bondings, thus enhancing the
electrophilic character of the reacting carbon center. How-
ever, the enolic form is assumed to interact with the tertiary
amine group, and a subsequent Henry reaction results in a
stereocontolled product. The absolute configuration of 3f was
determined by X-ray analysis (see Supporting Information),
presenting in accordance with our prediction the relative
structure anticipated from the catalytic mechanism.
To provide theoretical insight on the high diastereo- and
enantioselectivity of this reaction, a systematic conforma-
tional analysis of transition states were investigated with
density functional theory (DFT) calculations. The relative
energies of the different transition state were then used to
predict the product ratios (see Supporting Information).15
There should be many possible conformational isomers
for the transition state due to its flexibility. Based on a
systematic conformational search (see Figure in Supporting
Information), eight possible transition states have been
optimized for the Michael addition catalyzed by the catalyst
VI. The best transition state is TS-6 (Figure 4 right), which
consistent with the previous theoretical calculation results.16
More importantly, there was a hydrogen bond between
hydrogen in the phenyl group and ester group (O···H 2.058
Å). Because of the existence of the two CF3 groups, the
acidity of the hydrogen atom in the phenyl group was
increased significantly, and they were keener to form
hydrogen bonds with nucleophilic atoms, such as oxygen.
We found out that in the TS-6 enolic ester substrate was
firmly bound to catalyst VI and the proton in the tertiary
amine was largely transferred to the nitro group as is shown
by the N-H bond distance (1.070 Å). The formation of the
(R,S)- enantiomer was favored by 5.71 kcal/mol (Table 3 in
Supporting Information) when compared to the (S,R)-
enantiomer, which supported the high stereoselectivity
observed in the experiment. Transition state TS-1 (Figure 4
left) was coincided with the “dual activation model”,17 as
the enolic ester part was bound to the tertiary amine group
and the nitro group was bound to the thiourea unit. However,
TS-6 not only has the most number of hydrogen bonding
between the thiourea moiety and the enolic ester part when
compared to the other transition states, it also exhibits
hydrogen bonding between hydrogen in the phenyl group
and ester group that plays a crucial role in determining the
stereoselectivities of the Michael adduct. In addition, we find
that a proton transfer from the coordinated enol to the amine
group of the catalyst can easily take place, which is displayed
clearly in Figure 4.
We thus proposed a new catalytic model for this domino
Michael-Henry reaction (Figure 3b). In this model, the
thiourea group and an acidic proton in the phenyl ring
activates the 1,3-dicarbonyl substrates together, and at the
same time a tertiary amine activates the nitro group, which
promotes the domino reaction smoothly with excellent
stereoselectivity. The novel model may direct the future
design of bifunctional catalysts.
Figure 4. Typical transition states (TS-1 and TS-6) using thiourea
catalyst VI by DFT calculation.
is in accordance with the major product isolated during the
experiment. In TS-6, both the 1,3-dicarbonyl group and
nitroolefin were bonded to the catalyst VI simultaneously
through hydrogen bondings. However, unlike the “dual
activation model” in Figure 3a, the nitro group was activated
by the tertiary amine group, and the enolic ester part was
activated by the thiourea group. The enolic ester part and
the thiourea unit showed an almost coplanar structure, and
this gave rise to a concerted hydrogen bonding network
where the steric hindrance is minimized. This was also
Acknowledgment. Research support from the Ministry
of Education in Singapore (ARC12/07, no. T206B3225) is
gratefully acknowledged. Y.L. thanks CBC at NTU for
providing the computational resource for the DFT studies.
We greatly appreciate the helpful suggestions from Professor
Zhi-Xiang Yu at Peking University and Professor Dawei
Zhang at NTU.
Note Added after ASAP Publication. Published ASAP
May 14, 2010. Reference 1e was added to version reposted
on May 21, 2010.
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Supporting Information Available: Experimental pro-
cedures, characterization, spectra, chiral HPLC conditions
and X-ray crystallographic data (CIF file of 3f). This material
is available free of charge via the Internet at http://pubs.acs.org.
OL1007795
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Org. Lett., Vol. 12, No. 12, 2010
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