Enantioselective synthesis of 8-azabicyclo[3.2.1]octanes: Via asymmetric 1,3-dipolar cycloadditions of cyclic azomethine ylides using a dual catalytic system

Article abstract of DOI:10.1039/c8cc09224a

The first example of asymmetric 1,3-dipolar cycloadditions between diazo imine-derived cyclic azomethine ylides and acryloylpyrazolidinone using a rhodium(ii) complex/chiral Lewis acid binary system is reported. The asymmetric cycloadditions afforded opti

Full text of DOI:10.1039/c8cc09224a

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Enantioselective synthesis of
8-azabicyclo[3.2.1]octanes via asymmetric
1,3-dipolar cycloadditions of cyclic azomethine
ylides using a dual catalytic system†
Cite this:DOI: 10.1039/c8cc09224a
Received 22nd November 2018,
Accepted 12th December 2018
Hiroyuki Suga, *a Masahiro Yoshiwara,a Takaaki Yamaguchi,a Takashi Bando,a
DOI: 10.1039/c8cc09224a
bc
Mizuki Taguchi,a Ayano Inaba,a Yuichi Goto,a Ayaka Kikuchi,a Kennosuke Itoh
a
rsc.li/chemcomm
and Yasunori Toda
The first example of asymmetric 1,3-dipolar cycloadditions between
diazo imine-derived cyclic azomethine ylides and acryloylpyrazoli-
dinone using a rhodium(II) complex/chiral Lewis acid binary system
is reported. The asymmetric cycloadditions afforded optically active
8-oxabicyclo[3.2.1]octanes with high diastereo- and enantioselectivities
(up to 499 : 1 dr, 99% ee). A switch of exo/endo-selectivity was observed
depending on the diazo substrates.
The 8-azabicyclo[3.2.1]octane framework, the core of tropane
derivatives, is of considerable interest because it is a common
scaffold among a wide range of biologically active compounds.1
Tropanes often play a key role in neurological and psychiatric
diseases. Notably, hundreds of tropane alkaloids, such as
(ꢀ)-cocaine, (ꢀ)-bao gong teng A, and (+)-darlingine, are known
to occur in natural sources as optically active compounds
(Scheme 1a).2,3 General asymmetric synthetic routes to function-
alized tropane derivatives can be classified according to their
starting materials: the chiral pool, prochiral substrates using
chiral auxiliaries or chiral reagents, and optical resolution.1b
Although some applications of stereoselective [4+3] cycloadditions
have been reported for the asymmetric synthesis of tropanes, a
straightforward approach using chiral catalysts has rarely been
demonstrated.4 In 2013, Antonchick and Waldmann reported the
Cu(I)-catalyzed diastereo- and enantioselective [3+2] cycloaddition
reactions of 1,3-fused cyclic azomethine ylides with nitroalkenes
(Scheme 1b).5 This reaction features the formation of a
Scheme 1 Synthesis of optically active tropane derivatives based on the
strategy of enantioselective [3+2] cycloaddition reactions.
On the other hand, 1,3-dipolar cycloadditions between cyclic
Cu-associated azomethine ylide, leading to both activation of carbonyl ylides and dipolarophiles are one of the most efficient
the substrate and enantiofacial control of the ylide.
transformations for the synthesis of epoxy-bridged heterocycles
such as 8-oxabicyclo[3.2.1]octanes.6 Chiral Rh(II) catalysis has
enabled these cycloadditions to proceed in an enantioselective
fashion.7,8 In contrast, we have disclosed that the use of chiral
Lewis acid catalysts is crucial not only for accelerating the
cycloadditions but also for achieving high levels of asymmetric
induction.9 Thus, we conceived a catalytic asymmetric 1,3-dipolar
cycloaddition reaction of diazo imine-derived cyclic azomethine
ylides with electron-deficient alkenes by a Rh(II) complex/chiral
Lewis acid binary system (Scheme 1d). To date, there is one example
a Department of Materials Chemistry, Faculty of Engineering, Shinshu University,
Wakasato, Nagano 380-8553, Japan
b Laboratory of Medicinal Chemistry, School of Pharmacy, Kitasato University,
5-9-1 Shirokane, Minato-ku, Tokyo 108-8641, Japan
c Medicinal Research Laboratories, School of Pharmacy, Kitasato University,
5-9-1 Shirokane, Minato-ku, Tokyo 108-8641, Japan
† Electronic supplementary information (ESI) available: Experimental details,
spectral data, and copies of NMR charts. CCDC 1875446. For ESI and crystal-
lographic data in CIF or other electronic format see DOI: 10.1039/c8cc09224a
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Table 1 The cycloaddition reaction of the cyclic azomethine ylide generated
from O-methyloxime 1aa
of an enantioselective reaction involving cyclic azomethine ylide
generated by Rh(II) catalysis (Scheme 1c).10,11 This may be attributed
to the substantial challenges that must be overcome, including:
(1) lower reactivity of cyclic azomethine ylides toward dipolarophiles
than cyclic carbonyl ylides; (2) highly basic character of the imino
nitrogen that can deactivate the Lewis acid catalysts; (3) selective
activation of dipolarophiles by chiral Lewis acids; (4) regiochemical
outcome of the cycloaddition; and (5) control of diastereo- and
enantioselectivities.
Initially, 3-acryloyl-2-oxazolidinones were employed as the
dipolarophiles for the reaction of diazo O-methyloxime 1a, but
considerable amounts of regioisomers were obtained (see ESI†).
The cycloaddition reactions between N-benzyl acryloylpyrazoli-
dinone (2a: Ar = Ph)12 and the cyclic azomethine ylide derived
from O-methyloxime 1a were therefore investigated as shown in
Table 1. Chiral Lewis acid catalysts were prepared by mixing
Ni(ClO4)2ꢁ6H2O with (R)-N,N0-bis(2-quinolylmethylene)-1,10-
binaphthyl-2,20-diamines (ligands A–D) in accordance with the
reported procedure from our laboratory.13 The reactions were
generally performed by adding a solution of 1a to a solution of
the in situ prepared Lewis acid catalysts (20 mol%), Rh2(OAc)4
(2 mol%), and pyrazolidinone 2a dropwise over a period of 3 h at
26 1C. As a result of the initial ligand screening, exo- and endo-3a
were obtained in 71–77% combined yields with 92 : 8–82: 18 exo-
selectivities (Table 1, entries 1–4).14 Importantly, no formation of
regioisomers and moderate ee values (43–63% ee) were observed
for exo-3a. Use of acryloylpyrazolidinone 2b (Ar = 1-naphthyl) led
to higher enantioselectivity of 71% ee (Table 1, entry 5).12c To
improve both exo- and enantioselectivities, (R)-1,10-binaphthyl-
2,20-diamine ligand E in combination with several metal per-
chlorates was used as the chiral Lewis acid (Table 1, entries 6–9).
To our delight, the combined use of ligand E and Zn(ClO4)2ꢁ
6H2O (exo/endo = 98: 2, 91% ee) or Cu(ClO4)2ꢁ6H2O (exo/endo =
98: 2, 98% ee) were found to be effective in obtaining the exo
isomer with high enantioselectivities (Table 1, entries 8 and 9).
Finally, switching the Cu salt to Cu(OTf)2 gave exo-3b as the sole
product in high yield (95–97%) and excellent enantioselectivity
(97–99% ee) (Table 1, entry 10).15,16
Entry
M
Ligand
Yield (%)
exo : endob
eec (%)
1d
2d
3d
4d
5
Ni
Ni
Ni
Ni
Ni
Ni
Co
Zn
Cu
Cu f
A
B
C
D
A
E
E
E
E
E
71 (3a)
73 (3a)
77 (3a)
73 (3a)
68 (3b)
84 (3b)
76 (3b)
86 (3b)
81 (3b)
95–97 (3b)
92 : 8
62
63
56
43
71
50
64
91
82 : 18
82 : 18
82 : 18
88 : 12
99 : 1
96 : 4
98 : 2
6
7
8e
9
98 : 2
499 : 1
98
97–99
10e
a
The reaction was carried out by adding a solution of 1a (0.5 mmol)
over a period of 3 h to a solution of 2 equivalents of 2 (1.0 mmol),
Rh2(OAc)4 (0.01 mmol), and a chiral Lewis acid (0.1 mmol). See ESI for
details. Determined by 1H NMR analysis after hydrogenation to 6.
b
c
d
Determined by chiral-phase HPLC after hydrogenation to 6. In
e
f
CH2Cl2. At 35 1C. Cu(OTf)2 was used instead of Cu(ClO4)2ꢁ6H2O.
The initial scope of O-methyloximes 1 is summarized in on the parent benzene ring of 4 (R1 = Br, phenylalkynyl, and Me)
Scheme 2 (upper half). Although increasing the catalyst loading were tolerated, and other substituents on the isoxazoline ring
and raising the reaction temperature to 50 1C were required for (R2 = Ph and Et) were also applicable to provide good to high
relatively inactive cyclic azomethine ylides, all reactions proceeded enantioselectivities (84–96% ee). The cyclic azomethine ylide
under similar conditions to afford only exo-cycloadducts 3c–3e in without fused benzene ring also showed moderate enantioselectivity.
good to moderate yields (66–77%) with high enantioselectivities
Cycloadduct exo-3b could be easily transformed to the
(91–95%). It is noteworthy that a phenylacetylene group was corresponding alcohol 6 in 90% yield as a single isomer, in
tolerated in the reaction because triple bonds tend to undergo which the naphthylmethyl group of the pyrazolidinone of 3b
[3+2] cycloaddition with the azomethine ylide. In order to explore was cleaved during hydrogenation with Pd(OH)2/C in MeOH
the reaction scope further, we attempted to employ diazo isoxazo- (Scheme 3). Alcohol 6 was then converted to p-bromobenzyl
lines 4 as azomethine ylide precursors (Scheme 2, lower half). ester 7 in 84% yield by DCC condensation with p-bromobenzoic
Unfortunately, the Cu(OTf)2-ligand E catalyst did not show any acid. The absolute and relative configuration of 7 was confirmed
satisfactory results, affording 5a in low yield with poor enantio- by X-ray crystallographic analysis.17 On the other hand, the endo
selectivity. After several trials (see ESI†), it was found that stereochemistry of cycloadducts 5 was determined by NOESY
20 mol% of Ni(BF4)2ꢁ6H2O-ligand A catalyst in CHCl3 at 50 1C experiments on endo-5a, in which NOEs were observed between the
furnished cycloadduct endo-5a as the sole endo diastereomer methine proton of the substituted carbon with the acylpyrazoli-
with 96% ee (67% yield) in the reaction of diazo substrate 4a dinone moiety as well as the methylene proton and the methyl group
(R1 = Br, R2 = Me) with pyrazolidinone 2b. Several substituents on the isoxazolidine ring (Fig. 1). Furthermore, the absolute
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Fig. 1 The endo stereochemistry of endo-5a determined by NOE
experiments.
for 1 and endo-selectivity for 4, could be explained by substrate
control in which the bulky isoxazoline ring prevents the exo
approach toward the Lewis acid-activated 2.10
In conclusion, we have established the first example of
highly enantioselective cycloadditions of diazo imine-derived
cyclic azomethine ylides with acryloylpyrazolidinones, which
are efficiently catalyzed by a rhodium(II) complex/chiral Lewis
acid binary system. We observed a switch of exo/endo-selectivity
to realize the stereodivergent-type synthesis of 8-azabicyclo[3.2.1]-
octanes. In the reactions of diazo O-methyloximes 1, the Cu(OTf)2-
binaphthyldiamine catalyst was effective in achieving high levels of
asymmetric induction, affording only exo-cycloadducts as sole dia-
stereomers. In contrast, the Ni(BF4)2ꢁ6H2O-binaphthyldiimine
catalyst showed the best result of the reactions of diazo isoxazolines
4 in terms of high enantioselectivities, providing endo diastereomers
as sole cycloadducts. Synthetic applications based on the
methodology described herein are currently under investigation
in our laboratory.
Scheme 2 Substrate scope.
This work was supported in part by the Japan Society for the
Promotion of Science (JSPS) through Grant-in-Aid for Scientific
Research (C) (Grant No. JP15K05497, Grant No. JP17KT0096) and
Grant-in-Aid for Young Scientists (B) (Grant No. JP17K14483). We
are grateful to Akira Ohnishi for the preparation of substrates.
Conflicts of interest
There are no conflicts to declare.
Notes and references
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(also see ESI†).
15 Preparation of the chiral Cu(II) catalyst and the cycloaddition under
oxygen atmosphere showed satisfactory results in terms of reproducibility
(see ESI†).
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this paper†.
˜ ´
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