Enantioselective allylation of aldehydes with allyltrichlorosilane promoted by new chiral dipyridylmethane N-oxides

Article abstract of DOI:10.1016/j.tetlet.2007.04.028

New chiral dipyridine N-monoxides and N,N′-dioxides, which possess an isopropylidene backbone between two pyridine rings, have been prepared from naturally occurring monoterpenes. Their utility as organocatalysts has been demonstrated in the enantioselect

Full text of DOI:10.1016/j.tetlet.2007.04.028

Tetrahedron Letters 48 (2007) 4037–4041
Enantioselective allylation of aldehydes with allyltrichlorosilane
promoted by new chiral dipyridylmethane N-oxides
a
b,
b
Giorgio Chelucci,a,* Nicola Belmonte, Maurizio Benaglia and Luca Pignataro
*
a
`
Dipartimento di Chimica, Universita di Sassari, via Vienna 2, I-07100 Sassari, Italy
`
Dipartimento di Chimica Organica e Industriale, Universita degli Studi di Milano, via Golgi 19, I-20133 Milano, Italy
b
Received 30 January 2007; revised 4 April 2007; accepted 4 April 2007
Available online 11 April 2007
Abstract—New chiral dipyridine N-monoxides and N,N0-dioxides, which possess an isopropylidene backbone between two pyridine
rings, have been prepared from naturally occurring monoterpenes. Their utility as organocatalysts has been demonstrated in the
enantioselective addition of allyltrichlorosilane to aldehydes. Enantioselectivities up to 85% ee have been obtained.
Ó 2007 Elsevier Ltd. All rights reserved.
1. Introduction
ting a high enantioselectivity in the allylation.4 Thus, for
instance, Malkov and Kocovsky showed that in the allyl-
ation of the benzaldehyde with allyltrichlorosilane, the
terpene-derived bipyridine N-monoxide PINDOX 4a
(Fig. 1) afforded high enantioselectivity (up to 92% ee)
and was more selective than the related C2-symmetric
bipyridine N,N0-dioxide 4b that produced the opposite
enantiomer in only 41% ee.5 Moreover, 4a was found
to be only a little less enantioselective than the related
axially chiral bipyridine N-monoxide 4c (98% ee) that
was in turn much more successful than the correspond-
ing C2-symmetric axially chiral N,N0-dioxide 4d (up to
14% ee).5
Chiral Lewis base-catalyzed stereoselective allylation of
aldehydes with allyltrichorosilanes has been recognised
to be one of the most efficient methods of obtaining
homoallylic alcohols with high enantioselectivity
(Scheme 1).1,2 The generally good selectivity obtained
with several types of bases indicates that the reaction
likely proceeds via closed cyclic chair-like transition
states involving hypervalent silicates (TSs A, Scheme 1).
Among chiral bases used to promote this process, het-
eroaromatic amine N-oxides have recently become a
popular choice of oxygen donors for developing new
chiral oxygen-containing ligands.3 Although good
results have been initially obtained with C2-symmetric
biquinoline and bipyridine N,N0-dioxides, recent studies
have shown that neither the presence of a stereogenic
axis nor C2-symmetry is an absolute prerequisite for get-
To explain the high selectivity obtained in the PINDOX-
promoted allylation, Malkov and Kocovsky have
proposed the transition structure B (Fig. 1), in which
the ligand functions as an N,O-bidentate ligand at hexa-
coordinated silicon. In a similar fashion, the related
LB*
R1
OH
R1
R1
Lewis
base*
O
H
SiCln
R2
SiCl3
*
+
R2
R
*
R
H
O
R3
R2
R
R3
R3
1
2
3
A
Scheme 1.
Keywords: Chiral N-oxides; Enantioselective organocatalysis; Allylation; Dipyridylmethane ligands; Allyltrichlorosilanes.
*
Corresponding authors. Tel.: +39 079 229539; fax: +39 079 229559 (G.C.); tel.: +39 0250315171; fax: +39 0250314159 (M.B.); e-mail addresses:
chelucci@uniss.it; Maurizio.Benaglia@unimi.it
0040-4039/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2007.04.028

4038
G. Chelucci et al. / Tetrahedron Letters 48 (2007) 4037–4041
+
R
R
-
Cl
Cl
Si
N
N
R
N
+
R
O X
O
O
R2
Ph
N
+
4a-d
Cl
R1
H
a: X= no atom, R= H; b: X= O, R= H;
c: X= no atom, R= Me; d: X= O, R= Me
B
Figure 1.
66e (Scheme 2). These ligands were then assessed in the
asymmetric addition of allyltrichlorosilane to benzalde-
hyde, that is, used as the testing ground for new chiral
nucleophilic catalysts.
N
N
O X
A typical experiment involved the use of 0.1 mol equiv
of the catalyst, 1.2 mol equiv of allyl(trichloro)silane
and 3 mol equiv of diisopropylethylamine (DIPEA) in
acetonitrile at different temperatures.10 Isolated yields
and ees, as determined by HPLC, are collected in Table
1; the (S) absolute configuration was assigned to the pre-
dominant isomer by comparison of optical rotation.11
While mono N-oxide 7 showed to be a poor catalyst,
dipyridylmethane N,N0-dioxide 8 catalysed the allyla-
tion affording (S)-3 (Scheme 1, R = Ph, R1 = R2 =
R*
R*
Figure 2. New organocatalysts of type 5 (X = O or no atom).
N,N0-dioxide 4b would act as a O,O-bidentate ligand at
silicon.2 Therefore, the most relevant difference between
these ligands is the chelate ring size of silicon. In fact,
PINDOX 4a would generate a six-membered ring,
whereas chelation with N,N0-dioxide 4b would lead to
a seven-membered ring. Since to obtain an effective
transfer of the chiral information from the catalyst to
the product a careful evaluation of the ligand bite angle
would be contemplated in the ligand design,6 we have
considered the possibility to extend the chelate ring size
of these systems.7 Therefore we decided to prepare new
chiral organocatalysts containing two pyridine rings
connected by a proper, tunable spacer.
Table 1. Allylation of benzaldehyde with allyltrichlorosilanea
O
OH
Cat.*, DIPEA,
SiCl3
+
Ph
H
Ph
CH3CN
Entry Catalyst Temperature Time Yieldb eec Configu-
(°C)
(h)
(%)
(%) rationd
1
2
3
4
5
6e
7
8
9
7
7
ꢀ40
0
48
48
67
67
67
67
72
72
72
—
17
37
58
97
98
30
24
22
—
6
—
—
S
Herein, we report the preparation of new enantiomeri-
cally pure dipyridine N-monoxides and N,N0-dioxides
of type 5 (Fig. 2), from naturally occurring monoter-
penes, which possess an isopropylidene backbone
between two pyridine rings. Their efficiency as organo-
catalysts2c has also been investigated in the enantio-
selective addition of allyltrichlorosilane to aromatic,
heteroaromatic and nonaromatic aldehydes.
8
ꢀ40
85
83
77
64
4
8
0
S
8
25
0
0
0
0
S
8
S
10
12
14
S
30
35
S
S
a The reaction was carried out at a 0.3 mmol scale in CH3CN (2 ml)
with allyl(trichloro)silane (1.2 equiv), catalyst (10 mol %), DIPEA
(3 equiv) for 60 h.
b Isolated yields after flash chromatography.
2. Results and discussion
c Determined by HPLC on a Chiracel OD column (hexane:isopropanol
95:5; flow rate 0.8 ml/min; k 220 nm): tR = 10.4 min, tS = 11.3 min.
d Assigned by comparison of optical rotation.
Starting our investigation, N-monoxide 78 and N,N0-
dioxide 88 were prepared by MCPBA oxidation9 in
CH2Cl2 of the known parent dipyridylmethane ligand
e Reaction run in CH2Cl2.
a
b
N
N
N
N
N
N
69%
76%
O
O O
7
8
6
Scheme 2. Reagents and conditions: (a) MCPBA, CH2Cl2, excess of 6, rt 24 h; (b) MCPBA (excess), CH2Cl2, rt 24 h.

G. Chelucci et al. / Tetrahedron Letters 48 (2007) 4037–4041
4039
a
N
N
N
N
N
N
N
N
79%
O O
10
9
a
N
N
60%
O O
11
12
a
N
N
O O
24%
13
14
Scheme 3. Reagents and conditions: (a) MCPBA (excess), CH2Cl2, rt, 24 h.
R3 = H) in 37% yield and 85% ee at ꢀ40 °C (Table 1,
entry 3). Raising the temperature at 0 °C and 25 °C
brought about a higher yield (58 and 98%, respectively)
with only a slight decrease in the enantioselectivity (83%
and 77% ee, respectively; entries 4,5 vs 3). Changing the
solvent from MeCN to CH2Cl2 led to a quantitative
yield, but had a negative effect on the enantioselectivity
(entry 6 vs 4).10 It is worth mentioning that 8 showed a
selectivity similar to that of bipyridine N-monoxide 1a,
but clearly higher than that of bipyridine N,N0-dioxide
1b.
O
a
H3C
CH3
+
13
b
O
O
15
16
Scheme 4. Reagents and conditions: (a) 15, LDA, THF, ꢀ78 °C, 2 h,
then 16 from ꢀ78 °C to slowly rt; (b) AcOH, AcONH4, THF, reflux,
5 h.
The interesting results obtained with 8 prompted us to
prepare its other congeners. Thus, N,N0-dioxides 10,
12 and 148 were prepared by MCPBA oxidation9 in
CH2Cl2 of the parent dipyridylmethane ligands 9, 11
and 13 (Scheme 3). Whereas 9 and 11 were known com-
pounds,6e dipyridylmethane 138 was prepared in a two-
step sequence from a,b-unsaturated ketone 16, available
from (+)-2-carene in several steps.12 Thus, the conjugate
addition of the lithium enolate of 3,3-dimethylpentane-
2,4-dione 1513 (generated by treatment with lithium
diisopropylamine (LDA) at ꢀ78 °C for 2 h) with 17
(from ꢀ78 °C to room temperature) was performed, fol-
lowed by the aza-annulation of the unisolated 1,5-dicar-
bonyl intermediate with the ammonium acetate/acetic
acid system (AcOH, AcONH4, reflux, 5 h)6e (Scheme
4). However, in the allylation reaction, carried out under
the usual conditions (0 °C, 60 h), all new catalysts exhib-
ited both lower yields and enantioselectivities than its
congener 8 (Table 1, entries 7–9).
Table 2. Allylation of aldehydes with allyltrichlorosilane with 8 as the
catalyst (Scheme 1, R1 = R2 = R3 = H)a
O
OH
Cat. 8, DIPEA,
SiCl3
+
R
H
Ph
CH3CN, 0 ˚C
Entry
R
Yieldb (%) eec (%) Configurationd
1
2
3
4
5
6
7
Ph
58
68
65
83
70
80
75
73
18
4
S
S
S
S
S
S
S
p-O2NC6H4
p-MeOC6H4
2-Thiophenyl 60
3-Thiophenyl 61
Ph–CH@CH
Ph–CH2CH2
41
46
a The reaction was carried out at a 0.3 mmol scale in CH3CN (2 ml)
with allyl(trichloro)silane (1.2 equiv), catalyst (10 mol %), DIPEA
(3 equiv) at 0 °C for 60 h.
b Isolated yields after flash chromatography.
c Determined by HPLC on a chiral column.
d Assigned by comparison of optical rotation.
Having thus identified catalyst 8 as the more efficient
one, its use was extended to the addition of allyltrichlo-
rosilane to other aromatic aldehydes (Table 2). The
reported data show that ee’s equal to or greater than
70% and yields constantly higher than 58% could be
obtained (entries 1–5).
From these preliminary results it seems that an electron-
withdrawing group at the para-position of benzaldehyde
exhibits a negative impact on the enantioselectivity (en-
try 2), whereas the substrate bearing an electron-donat-

4040
G. Chelucci et al. / Tetrahedron Letters 48 (2007) 4037–4041
Acknowledgements
+
-
Cl
Financial support from MIUR (PRIN 2005035123, Re-
gio- and enantioselective reactions mediated by transi-
tion metal catalysts for innovative processes in fine
chemicals synthesis) and from the University of Sassari
is gratefully acknowledged by G.C. Financial support
from MIUR (Rome) within the national project ‘Nuovi
metodi catalitici stereoselettivi e sintesi stereoselettiva di
molecole funzionali’ is gratefully acknowledged by M.B.
+
N
Cl
Si
O
R2
Ph
+
N
O
O
Cl
R1
H
C
References and notes
Figure 3.
1. For a general review on the catalytic asymmetric allyl-
ation, see: Denmark, S. E.; Fu, J. Chem. Rev. 2003, 103,
2763.
2. For general reviews on chiral Lewis base-promoted
asymmetric allylation of aldehydes with allyltrichorosil-
anes, see: (a) Orito, Y.; Nakajima, M. Synthesis 2006,
1391; (b) Oestreich, M.; Rendler, S. Synthesis 2005, 1727;
(c) Dalko, P. I.; Moisan, L. Angew. Chem., Int. Ed. 2004,
43, 5138.
3. For reviews on the use N-oxides as chiral organocatalysts,
see: (a) Malkov, A. V.; Kocovsky, P. Eur. J. Org. Chem.
2007, 29; (b) Chelucci, G.; Murineddu, G.; Pinna, G. A.
Tetrahedron: Asymmetry 2004, 15, 1373, See also Refs. 1
and 2.
ing group at the para-position does not seem to affect the
stereoselectivity (entry 3). A marginal difference in selec-
tivity and yield was observed for the two isomeric thio-
phene–carboxaldehydes (entries 4 and 5) showing the
compatibility of the new catalyst in the reaction with
heteroaromatic systems. Finally, both a,b-unsaturated
and saturated aldehydes such as cinnamaldehyde and
dihydrocinnamaldehyde, respectively, afforded the cor-
responding homoallylic alcohols with low enantiomeric
excesses (entries 6 and 7).14
The comparison between a homogeneous series of N-
monoxides and N,N0-dioxides derived from bipyridine
and dipyridylmethane ligands suggests that both cata-
lytic activity and stereoselectivity of the allylation reac-
tion appear to depend on the chelate ring size of
silicon in the hypothesized transition state. Bipyridine
N-monoxide 4a (87% ee at ꢀ40 °C) is more stereoselec-
tive than bipyridine N,N0-dioxide 4b (41% ee at
ꢀ90 °C), which is in turn a better catalyst than dip-
yridylmethane N-monoxide 7 (no activity at ꢀ40 °C),
but less selective than dipyridylmethane N,N0-dioxide
8 (85% ee at ꢀ40 °C), which shows similar selectivity
of 4a. In other words, it seems that in the dipyridine
series, ligands forming an even-membered chelate ring
(4a and 8) are more stereoselective than those generat-
ing an odd-membered ring (4b and 7). On the basis of
these observations and previous studies from other
groups on O-based bidentate ligands,1,2 a possible
transition state model C compatible with the stereoselec-
tion observed for the new organocatalysts such as 8 is
proposed in Figure 3, as a working hypothesis for future
studies.
4. For recent examples of C1-symmetric N-oxides as organo-
catalysts without stereogenic axis, see: (a) Traverse, J. F.;
Zhao, Y.; Hoveyda, A. H.; Snapper, M. L. Org. Lett.
2005, 7, 3151; (b) Pignataro, L.; Benaglia, M.; Annunzi-
ata, R.; Cinquini, M.; Cozzi, F. J. Org. Chem. 2006, 71,
1458, and references cited therein.
5. Malkov, A. V.; Bell, M.; Orsini, M.; Pernazza, D.; Massa,
A.; Herrmann, P.; Meghani, P.; Kocovsky, P. J. Org.
Chem. 2003, 68, 9659.
6. For the effect of changing the bite angle of bidentate
ligands on metal-catalyzed processes, see for general
accounts: (a) Hageman, J. A.; Westerhuis, J. A.; Fruhaus,
¨
H.-W.; Rothenberg, G. Adv. Synth. Catal. 2006, 348, 361;
(b) Burello, E.; Rothenberg, G. Adv. Synth. Catal. 2005,
347, 1969; For pyridine-based ligands: (c) Zhang, F.;
Prokopchuk, E. M.; Broczkowski, M. E.; Jennings, M. C.;
Puddephatt, R. J. Organometallics 2006, 25, 1583; (d)
Bolm, C.; Frison, J.-C.; Le Paih, J.; Moessner, C.; Raabe,
G. J. Organomet. Chem. 2004, 689, 3767; (e) Chelucci, G.;
Loriga, G.; Murineddu, G.; Pinna, G. A. Tetrahedron
Lett. 2002, 43, 8599; For phosphine-based ligands: (f)
Shimizu, H.; Nagasaki, I.; Saito, T. Tetrahedron 2005, 61,
5405; (g) Raghunath, M.; Zhang, X. Tetrahedron Lett.
2005, 47, 8213; (h) Freixa, Z.; Van Leeuwen, P. W. N. M.
Dalton Trans. 2003, 10, 1819; For oxazoline-based ligands:
(i) Desimoni, G.; Faita, G.; Jorgensen, K. A. Chem. Rev.
2006, 106, 3561; (j) McManus, H. A.; Guiry, P. J. Chem.
Rev. 2004, 104, 4151; (k) Helmchen, G.; Pfaltz, A. Acc.
Chem. Res. 2000, 33, 336; (l) Johnson, J. S.; Evans, D. A.
Acc. Chem. Res. 2000, 33, 325; (m) Ghosh, A. K.;
Mathiyanan, P.; Cappiello, J. Tetrahedron: Asymmetry
1998, 9, 1.
In conclusion, the first examples of N-monoxides and
N,N0-dioxides based on the framework of dipyri-
dylmethane ligands have been prepared and their poten-
tiality as organocatalysts has been demonstrated in
the addition of allyltrichorosilanes to aldehydes. More-
over, the information gained by these O-ligands gives
some new insights into the relationship between
catalytic activity and/or stereoselectivity and ligand
bite angle for the examined stereoselective allylation
process.
7. For the role played by a proper spacer between two
ligands, see for the allylation of aldehydes with allyltri-
chlorosilanes using bisphosphoramide-based ligands: (a)
Denmark, S. E.; Fu, J. J. Am. Chem. Soc. 2001, 123, 9488;
For the allylation of N-acylhydrazones with allyltrichlo-
rosilanes using bisphosphine oxides: (b) Ogawa, C.;
Konishi, H.; Sugiura, M.; Kobayashi, S. Org. Biomol.
Chem. 2004, 2, 446.
Further studies are in progress to explore this trend and
to assess these new ligands in other enantioselective
organocatalyzed reactions.

G. Chelucci et al. / Tetrahedron Letters 48 (2007) 4037–4041
4041
8. 2-Bis[(6R,8R)-6,8-methano-7,7-dimethyl-5,6,7,8-tetrahydro-
1H NMR (300 MHz, CDCl3): d 7.34 (d, 2H, J = 8.4 Hz),
7.11 (d, 2H, J = 8.4 Hz), 2.89–2.74 (m, 2H), 2.28–2.08 (m,
4H), 1.80 (s, 6H), 1.39–1.26 (m, 4H), 1.25 (d, 6H,
J = 6.6 Hz), 1.22 (s, 6H), 0.62 (s, 6H). 13C NMR (CDCl3):
d 153.80, 147.47, 138.37, 121.75, 118.87, 88.96, 41.84,
32.24, 29.82, 27.52, 24.75, 24.02, 23.24, 22.40, 17.98, 15.15.
Anal. Calcd for C29H38N2O2: C, 77.99; H, 8.58; N, 6.27.
Found: C, 77.82; H, 8.57; N, 6.25. 2-Bis[(5R,7R,8S)-7,
8-methano-5,11,11-trimethyl-5,6,7,8-tetraidroquinolin-2-yl]-
25
quinolin-2-yl]propane N-oxide (7): mp 85–90 °C; ½aꢁD
+35.7 (c 0.83, CHCl3); 1H NMR (300 MHz, CDCl3): d
7.31 (d, 1H, J = 3.9 Hz), 7.28 (d, 1H, J = 3.3 Hz), 7.05 (d,
1H, J = 8.1 Hz), 6.98 (d, 1H, J = 7.8 Hz), 3.90 (t, 1H, J =
5.4), 2.94 (s, 2H), 2.85 (s, 2H), 2.73 (t, 1H, J = 11.1 Hz),
2.61–2.52 (m, 2H), 2.28–2.21 (m, 2H), 1.84 (s, 3H), 1.72 (s,
3H), 1.37 (s, 3H), 1.33 (s, 3H), 1.18–1.31 (m, 2H), 0.63 (s,
3H), 0.58 (s, 3H). 13C NMR (75.4 MHz, CDCl3): d 164.36,
161.84, 156.15, 154.37, 135.14, 130.99, 125.94, 124.48,
120.68, 116.12, 50.04, 45.25, 40.16, 39.99, 39.65, 38.99,
38.93, 31.18, 30.85, 30.61, 30.19, 28.06, 26.04, 25.65, 25.28,
21.02, 20.96. Anal. Calcd for C27H34N2O: C, 80.55; H,
8.51; N, 6.96. Found: C, 80.50; H, 8.52; N, 6.98.
2-Bis[(6R,8R)-6,8-methano-7,7-dimethyl-5,6,7,8-tetrahydro-
20
propane (14): oil; ½aꢁD ꢀ26.5 (c 0.61, CHCl3); 1H NMR
(300 MHz, CDCl3): d 7.26 (d, 2H, J = 6 Hz), 6.78 (d, 2H,
J = 6 Hz), 2.92–2.81 (m, 2H), 2.21–2.13 (m, 2H), 1.80 (d,
2H, J = 6.3 Hz), 1.77 (s, 6H), 1.36–1.22 (m, 4H), 1.23 (s,
6H), 1.19 (d, 6H, J = 5.4 Hz), 0.62 (s, 6H). 13C NMR
(75.4 MHz, CDCl3): d 164.71, 155.39, 134.79, 131.27,
118.43, 47.22, 32.27, 30.28, 28.67, 28.14, 26.97, 23.67,
23.01, 18.06, 16.26. Anal. Calcd for C29H38N2: C, 84.01;
H, 9.24; N, 6.76. Found: C, 84.00; H, 9.26; N, 6.75.
9. In order to obtain the selective formation of N-monoxide
7, a large excess of dipyridylmethane ligand 6 with respect
to MCPBA was used. The unconverted starting material
and 7 were recovered by flash chromatography. On the
other hand, N,N0-dioxides were prepared using an excess
of MCPBA.
25
quinolin-2-yl]propane N,N0-dioxide (8): mp 82–84 °C; ½aꢁD
ꢀ35.9 (c 0.22, CHCl3); 1H NMR (300 MHz, CDCl3): d
7.33 (d, 2H, J = 8.1 Hz), 7.06 (d, 2H, J = 8.1 Hz), 3.83 (t,
2H, J = 2.7 Hz), 2.90 (dq, 4H, J = 17.1 Hz, J = 2.7 Hz),
2.66–2.54 (m, 2H), 2.27–2.18 (m, 2H), 1.88 (s, 6H), 1.36 (s,
6H), 1.80 (s, 2H), 0.63 (s, 6H). 13C NMR (75.4 MHz,
CDCl3): d 158.11, 154.87, 153.52, 130.01, 125.70, 119.99,
42.45, 40.14, 39.86, 39.04, 31.17, 29.91, 25.75, 24.31, 21.01.
Anal. Calcd for C27H34N2O2: C, 77.48; H, 8.19; N, 6.69.
Found: C, 77.50; H, 8.15; N, 6.68. 2-Bis[(5S,7S)-5,
7-methano-6,6-dimethyl-5,6,7,8-tetrahydroquinolin-2-yl]-
10. The use of other solvents (toluene, DME, THF) led to
lower yields and ee.
25
propane N,N0-dioxide (10): mp 202–205 °C; ½aꢁD ꢀ7.28 (c
11. Typical procedure: to a stirred solution of the catalyst
(0.03 mmol) in acetonitrile (2 ml) kept under nitrogen, an
aldehyde (0.3 mmol) and diisopropylethylamine (DIPEA,
0.154 ml, 0.9 mmol) were added in this order. The mixture
was then cooled to 0 °C and allyl(trichloro)silane
(0.054 ml, 0.36 mmol) was added dropwise by means of
a syringe. After 48 h stirring at 0 °C the reaction was
quenched by the addition of a saturated aqueous solution
of NaHCO3 (1 ml). The mixture was allowed to warm up
to room temperature and water (2 ml) and EtOAc (5 ml)
were added. After usual work up the crude products were
purified by flash chromatography with different hex-
ane:EtOAc mixtures as eluents.
12. Adam, W.; Heidenfelder, T.; Sahin, C. Synthesis 1995,
1163.
13. Malkov, A. V.; Pernazza, D.; Bell, M.; Bella, M.; Massa,
A.; Teply, F.; Meghani, P.; Kocovsky, P. J. Org. Chem.
2003, 68, 4727.
14. Thus, the reactivity trend observed with new type 5
catalyst (aromatic aldehydes > a,b-unsaturated aldehydes
and aliphatic aldehydes) does not differ very much from
that of most of the other N-oxides described in the
literature.
0.11, CHCl3); 1H NMR (300 MHz, CDCl3): d 7.31 (d, 2H,
J = 8.1), 6.89 (d, 2H, J = 8.1 Hz), 2.89 (dq, 4H,
J = 18.9 Hz, J = 2.7 Hz), 2.73 (t, 2H, J = 5.4 Hz), 2.63–
2.53 (m, 2H), 2.36–2.28 (m, 2H), 1.89 (s, 6H), 1.37 (s, 6H),
1.36–1.18 (m, 2H), 0.62 (s, 6H). 13C NMR (75.4 MHz,
CDCl3): d 153.94, 145.16, 141.56, 123.22, 119.51, 45.64,
41.90, 39.28, 39.26, 31.10, 30.69, 25.74, 24.42, 20.80. Anal.
Calcd for C27H34N2O2: C, 77.48; H, 8.19; N, 6.69. Found:
C, 77.51; H, 8.16; N, 6.66. 2-Bis[(5R,7R,8S)-5,7-methano-
6,6,8-trimethyl-tetrahydroquinolin-2-yl]propane N,N0-diox-
25
ide (12): mp185–188 °C; ½aꢁ +91.9 (c 0.43, CHCl3); 1H
NMR (300 MHz, CDCl3): dD7.23 (d, 2H, J = 8.1 Hz), 6.86
(d, 2H, J = 8.1 Hz), 3.23–3.09 (m, 2H), 2.71 (t, 2H,
J = 5.7), 2.52–2.40 (m, 2H), 2.10–2.01 (m, 2H), 1.84 (s,
6H), 1.41 (m, 2H), 1.37 (s, 6H), 1.22 (d, 6H, J = 6.6 Hz),
0.58 (s, 6H). 13C NMR (75.4 MHz, CDCl3): d 155.06,
148.16, 141.06, 123.13, 118.55, 47.11, 46.43, 41.81, 41.38,
34.27, 28.14, 25.83, 24.98, 20.40, 14.12. Anal. Calcd for
C29H38N2O2: C, 77.99; H, 8.58; N, 6.27. Found: C, 77.98;
H, 8.58; N, 6.28. 2-Bis[(5R,7R,8S)-5,7-methano-6,6,8-
trimethyl-5,6,7,8-tetrahydroquinolin-2-yl]propane
N,N0-
dioxide (14): mp 94–97 °C; ½aꢁD +151.2 (c 0.36, CHCl3);
25