Asymmetric synthesis of functionalized 3,4-dihydronaphthalenes via an organocatalytic domino nitroalkane-Michael/aldol condensation reaction

Article abstract of DOI:10.1055/s-0029-1217380

An organocatalytic domino nitroalkane-Michael addition/aldol condensation reaction has been developed. This process provides an efficient asymmetric synthesis of trisubstituted 3,4-dihydronaphthalenes in moderate to good yields (40-75%) and high stereosel

Full text of DOI:10.1055/s-0029-1217380

LETTER
1777
Asymmetric Synthesis of Functionalized 3,4-Dihydronaphthalenes via an
Organocatalytic Domino Nitroalkane-Michael/Aldol Condensation Reaction
A
symmetric Synth
i
esis of
e
Functional
t
ized 3,4
e
-Dihydrona
r
phthalenes Enders,*a Chuan Wang,a Jan W. Batsb
a
Institute of Organic Chemistry, RWTH Aachen University, Landoltweg 1, 52074 Aachen, Germany
Fax +49(241)8092127; E-mail: enders@rwth-aachen.de
b
Institute of Organic Chemistry and Chemical Biology, University Frankfurt, Marie-Curie-Str. 11, 60439 Frankfurt am Main, Germany
Received 26 March 2009
was achieved in every solvent used. The optimal result
with respect to yield (77%) and stereoselectivity (>98%
de, 96% ee) was obtained when the domino reaction was
performed in diethyl ether (Table 1, entry 5). Under a low-
er catalysis load (5 mol%), no significant decrease in yield
or stereoselectivity was observed (Table 1, entry 6).
Abstract: An organocatalytic domino nitroalkane-Michael addi-
tion/aldol condensation reaction has been developed. This process
provides an efficient asymmetric synthesis of trisubstituted 3,4-di-
hydronaphthalenes in moderate to good yields (40–75%) and high
stereoselectivities (de >98%, ee = 91 to >99%).
Key words: dihydronaphthalenes, organocatalysis, domino reac-
tion, Michael addition, aldol condensation
Ph
Ph
O
O
R
N
O
H
OTMS
(S)-4 (5 mol%)
After intense research in recent years, the field of
organocatalysis1 has grown with breathtaking speed and is
now considered as one of the main branches of asymmet-
ric synthesis. Currently, a focal point at the forefront is the
development of organocatalytic domino reactions,2 which
are characterized by the efficient and stereoselective con-
struction of complex molecules from simple precursors in
a single operation, thereby avoiding common drawbacks
of classical synthesis. Consequently, organocatalytic
domino reactions have been intensively investigated dur-
ing the past few years and a large number of interesting
reactions have been developed.3,4 For example, our group
reported recently a secondary amine catalyzed domino
Michael/nitroalkane-Michael/aldol condensation reaction
for the stereoselective synthesis of tri- and tetrasubstituted
cyclohexene carbaldehydes.4c,e,n In a continuation of our
investigations in this field, we envisaged 2-(nitrometh-
yl)benzaldehyde (1) and a,b-unsaturated aldehydes 2 as
potential substrates for a domino nitroalkane-Michael
addition5–7/aldol condensation8–10 reaction, which should
provide access to 3-substituted 3,4-dihydronaphthalenes 3
with two stereogenic centers and synthetically valuable
aldehyde and nitro groups (Scheme 1).
+
solvent
R
NO2
NO2
1
2
3
(1.0 equiv)
(1.1 equiv)
Scheme 1 Asymmetric synthesis of 3,4-dihydronaphthalenes 3
from 2-(nitromethyl)benzaldehyde (1) and a,b-unsaturated aldehydes
2
Table 1 Solvent Screening of the Organocatalytic Domino Reactiona
Entry
Solvent
toluene
EtOH
MeCN
THF
Yield (%)b de (%)c
ee (%)d
94
1
2
3
4
5
6e
40
49
61
71
77
75
>98
>98
>98
>98
>98
>98
91
82
93
Et2O
96
Et2O
96
a Reaction conditions: 1 (1.0 mmol), cinnamaldehyde (2a; 1.1 mmol,
1.1 equiv), (S)-4 (10 mol%), solvent (2 mL), stirring, r.t., 5 h.
b Yield of isolated product 3a.
In the first instance, we performed the reaction between 2-
(nitromethyl)benzaldehyde (1) and cinnamaldehyde (2a)
in toluene at room temperature utilizing (S)-diphenylpro-
c Determined by NMR spectroscopy.
d Determined by HPLC analysis on a chiral stationary phase.
linol TMS-ether11 [(S)-4; 10 mol%] as catalyst. The reac- e Reaction carried out with 5 mol% catalyst (S)-4.
tion was found to be completed within five hours,
affording dihydronaphthalene 3a in moderate yield (40%)
Based on these results, we investigated the scope of the
and high diastereo- and enantioselectivity (>98% de, 94%
ee, Table 1, entry 1). Encouraged by this initial result, a
brief solvent-screening was undertaken and it was found
that high stereoselectivity (>98% de, 82–96% ee, Table 1)
reaction by varying the structure of enal 2. In the case of
substituted aromatic enals, the dihydronaphthalenes 3b,
3c and 3d were readily synthesized at room temperature in
good yields (57–75%) and excellent stereoselectivities
(>98% de, 94 to >99% ee, Table 2). The electronic fea-
tures and the position of the substituent on the aromatic
ring had no significant influence on the stereoselectivity
of the reaction. Employing the heteroaromatic furyl-
SYNLETT 2009, No. 11, pp 1777–1780
x
x
.x
x
.2
0
0
9
Advanced online publication: 15.06.2009
DOI: 10.1055/s-0029-1217380; Art ID: G09709ST
© Georg Thieme Verlag Stuttgart · New York

1778
D. Enders et al.
LETTER
acrolein led to the formation of 3e in good yield (57%) and A plausible catalytic cycle is given in Scheme 2. The
excellent stereoselectivity (>98% de, 93% ee).
reaction starts with iminium activation of the enal 2 by the
chiral amine catalyst (S)-4. In the next step, the nitroalde-
hyde 1 performs an intermolecular Michael addition to the
generated iminiumion 5. The resulting enamine 6 under-
goes an intramolecular aldol reaction affording intermedi-
ate 7. Subsequent hydrolysis regenerates the catalyst (S)-
4 for the next catalytic cycle and the alcohol 8 dehydrates
to the 3,4-dihydronaphthalene products 3.
When aliphatic enals 2f and 2g were used as the Michael
acceptors the reactions were performed at –5 °C affording
the desired domino products 3f and 3g in moderate yields
(40–41%) and high stereoselectivities (>98% de, 91–93%
ee).
Table 2 Yield and Stereoselectivities of the Organocatalytic Domi-
no Reactionsa
H2O
3
R
Time (h) Yield (%)b de (%)c
ee (%)d
96
O
a
Ph
5
5
75
74
67
58
57
40
41
>98
>98
>98
>98
>98
>98
>98
b
4-MeOC6H4
2-MeOC6H4
2-O2NC6H4
furan-2-yl
n-Bu
94
R
ce
df
e
5
>99
>99
93
NO2
OH
O
R
3
5
O
5
Ph
NO2
fg
gg
12
12
91
Ph
8
R
N
H2O
2
H
OTMS
n-Pr
93
catalyst (S)-4
a Reaction conditions: 1 (1.0 mmol), enal 2a (1.1 mmol, 1.1 equiv),
(S)-4 (5 mol%), solvent (2 mL), Et2O, stirring, r.t.
b Yield of isolated product 3.
Ph
Ph
OTMS
Ph
Ph
OTMS
N
O
c Determined by NMR spectroscopy.
d Determined by HPLC analysis on a chiral stationary phase.
e Workup B.
N
catalytic
cycle
f Workup C.
R
OH
g Carried out at –5 °C. Yield at r.t.: 45–49%, de >98%, ee = 87–90%.
NO2
R
5
O
7
The 3,4-dihydronaphthalene 3c was determined to be
trans-configured by both Röntgen crystal structure analy-
sis (Figure 1)12 and NOESY measurements. According to
the obtained results in other (S)-diphenylprolinol TMS-
ether [(S)-4] catalyzed Michael additions to enals under
iminium activation,4,6,7 the absolute configuration of the
C-3 stereogenic center was always assigned to be R, if C-
4, C-2 and C-1¢ possess a descending priority according to
the CIP-rules. Based on these results, the absolute config-
uration of the 3,4-dihydronaphthalenes 3 was assigned as
3R,4S.
Ph
Ph
OTMS
N
O
enamine activation
H2O
NO2
1
iminium activation
R
NO2
6
Scheme 2 Proposed catalytic cycle of the domino reaction
In summary, we have developed a domino nitroalkane–
Michael addition/aldol condensation reaction of 2-(ni-
tromethyl)benzaldehyde (1) and a,b-unsaturated alde-
hydes 2. This process is efficiently catalyzed by (S)-
diphenylprolinol TMS-ether [(S)-4; 5 mol%] to afford
functionalized 3,4-dihydronaphthalenes 3 with two ste-
reogenic centers in moderate to good yields (40–75%) and
excellent stereoselectivities (>98% de, 91 to >99% ee).13
Acknowledgment
This work was supported by the Deutsche Forschungsgemeinschaft
(priority program Organocatalysis) and the Fonds der Chemischen
Industrie. We thank the former Degussa AG and BASF AG for the
donation of chemicals.
Figure 1 Röntgen crystal structure of 3c12
Synlett 2009, No. 11, 1777–1780 © Thieme Stuttgart · New York

LETTER
Asymmetric Synthesis of Functionalized 3,4-Dihydronaphthalenes
(6) For selected examples of organocatalytic nitroalkane–
1779
References and Notes
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LETTER
(12) Compound 3c: CCDC-725063 contains the supplementary
crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/
cif
(13) General Procedure: To a solution of 2-(nitromethyl)-
benzaldehyde (1; 1.0 mmol) and a,b-unsatured aldehyde
2 (1.1 mmol, 1.1 equiv) in Et2O (2 mL), was added (S)-di-
phenylprolinol TMS-ether [(S)-4; 0.05 mmol, 5 mol%]. The
reaction mixture was stirred at the temperature and for the
time displayed in Table 2.
(vs), 1489 (s), 1460 (s), 1435 (m), 1399 (s), 1358 (s), 1327
(m), 1289 (s), 1273 (s), 1245 (vs), 1192 (w), 1158 (vs), 1105
(s), 1052 (s), 1028 (s), 961 (w), 924 (s), 855 (m), 819 (m),
755 (vs), 704 (s) cm–1; 1H NMR (400 MHz, CDCl3): d = 3.95
(s, 3 H, OCH3), 5.48 (br s, J = <2 Hz, 1 H, H-3), 5.62 (br s,
J = <2 Hz, 1 H, H-4), 6.60 (dd, J = 7.6, 1.6 Hz, 1 H, H-6¢),
6.65 (td, J = 7.6, 0.8 Hz, 1 H, H-5¢), 6.91 (d, J = 7.6 Hz, 1 H,
H-3¢), 7.19 (td, J = 7.6, 1.6 Hz, 1 H, H-4¢), 7.36–7.42 (m,
2 H, H-5,7), 7.48–7.54 (m, 2 H, H-6,8), 7.68 (s, 1 H, H-1),
9.73 (s, 1 H, CHO); 13C NMR (101 MHz, CDCl3): d = 34.5
(C-3), 55.6 (OCH3), 86.4 (C-4), 110.8 (C-3¢), 120.3 (C-5¢),
122.5 (C-1¢), 126.9 (C-6¢), 128.0 (C-9), 129.0 (C-4¢), 129.4
(C-8), 130.9 (C-6), 131.3 (C-5), 131.6 (C-7), 131.7 (C-10),
137.8 (C-2), 144.1 (C-1), 156.7 (C-2¢), 190.7 (CHO); MS
(EI, 70 eV): m/z (%) = 309.4 (5.8) [M+], 277.3 (2.2), 263.3
(74), 245.3 (50), 235.4 (100), 231.4 (24), 202.3 (65), 189.3
(19), 176.3 (2.7), 165.3 (8.8), 155.3 (7.5), 152.3 (2.7), 127.3
(9.5), 117.6 (9.1), 101.3 (19), 94.8 (7.2), 83.1 (4.7), 77.4
(10), 57.4 (2.2), 51.4 (4.0), 43.3 (2.2); Anal. Calcd for
C18H15NO4: C, 69.89; H, 4.89; N, 4.53. Found: C, 69.92; H,
4.94; N, 4.50.
Workup A: Direct purification of the reaction mixture by
flash chromatography afforded 3,4-dihydronaphthalenes 3
(pentane–Et2O, 2–10:1).
Workup B: Direct suction through a funnel followed by
washing with Et2O afforded 3c.
Workup C: The reaction mixture was suctioned through a
funnel and washed with Et2O. Purification of the obtained
solid by flash chromatography (silica gel, pentane–Et2O,
1:3) afforded 3d.
(3R,4S)-3-(2-Methoxyphenyl)-4-nitro-3,4-dihydro-
naphthalene-2-carbaldehyde (3c; Figure 2): Isolated as a
colorless solid (206 mg, 67%). The ee (>99%) was
determined by HPLC on a chiral stationary phase [Chiralcel
OD; n-heptane–i-PrOH (8:2); 1.0 mL/min, tR = 9.04
min(major), 10.29 min (minor, based on the racemic
mixture)]; mp 182 °C; [a]D20 –482 (c 1.0, CHCl3); IR (KBr):
3310 (w), 3000 (w), 2946 (w), 2823 (m), 2728 (w), 2324 (w),
2268 (w), 2184 (w), 2048 (w), 1989 (w), 1942 (w), 1735 (w),
1701 (w), 1663 (vs), 1627 (s), 1599 (m), 1570 (m), 1538
8
5
1
CHO
2
9
7
OMe
2'
1'
6
3
3'
4'
10
4
NO2
6'
5'
Figure 2
Synlett 2009, No. 11, 1777–1780 © Thieme Stuttgart · New York