Enantioselective synthesis of quinolizidines and indolizidines via a catalytic asymmetric hydrogenation cascade

Article abstract of DOI:10.1055/s-0030-1259955

A catalytic enantioselective synthesis of a new class of quinolizidines and indolizidines is presented. An asymmetric Bronsted acid catalyzed hydrogenation cascade as well as a sequential Bronsted acid/metal catalyzed hydrogenation protocol of 2-substituted quinolines yields benzofused quinolizidines and indolizidines in good yields with high diastereo- and enantioselectivities. Georg Thieme Verlag Stuttgart - New York.

Full text of DOI:10.1055/s-0030-1259955

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1243
Enantioselective Synthesis of Quinolizidines and Indolizidines via a Catalytic
Asymmetric Hydrogenation Cascade
E
nantioselective Synth
a
e
sis of
Q
ui
g
a
n
nd Indolizidi
unes s Rueping,* Lukas Hubener
Institute of Organic Chemistry, RWTH Aachen University, Landoltweg 1, 52074 Aachen, Germany
Fax +49(241)8092665; E-mail: magnus.rueping@rwth-aachen.de
Received 31 January 2011
Abstract: A catalytic enantioselective synthesis of a new class of
quinolizidines and indolizidines is presented. An asymmetric Brøn-
sted acid catalyzed hydrogenation cascade as well as a sequential
Brønsted acid/metal catalyzed hydrogenation protocol of 2-substi-
[H]
N
N
H
O
n
n
– H2O
O
tuted quinolines yields benzofused quinolizidines and indolizidines
in good yields with high diastereo- and enantioselectivities.
R
R
Key words: organocatalysis, domino reaction, amines, asymmetric
reduction, Hantzsch ester
[H]
N
N
n
n
In the still growing field of organocatalysis1 the concept
of domino or cascade reactions2 has drawn increasing in-
terest in recent years. Organocatalysts proved to be pow-
erful promoters for these reactions,3 enabling either
simultaneous or sequential activations of the substrates
and combining usually high levels of asymmetry with
green chemistry aspects like atom and step economy.4 Al-
though the field of organocatalyzed domino reactions is
dominated by the use of Lewis base catalysts, for example
secondary amines,5 recent reports have shown that
Brønsted acid catalysts6,7 are also highly effective in such
transformations.
R
R
Scheme 1 Devised reduction cascade; [H] = hydride source
aldol condensation with subsequent reduction to give
amine 5 (Scheme 2).13
Subsequently, a variety of chiral BINOL- and [H8]-
BINOL-based phosphoric acids 6a–f and N-triflyl phos-
phoramides 6g–i were employed in the above cascade
reaction14,15 in order to evaluate their effect on the enantio-
selectivity and product distribution. The results are sum-
marized in Table 1. Despite showing promising
enantioselectivites, phosphoric acid catalysts 6a–f fur-
nished the products in low yields (Table 1, entries 1–6). In
contrast, the more acidic N-triflyl phosphoramide cata-
lysts 6g–i gave very good yields but exhibited only low
levels of enantiocontrol (Table 1, entries 7–9). Regarding
the influence of the substituents in the 3,3¢-positions,
bulky residues led to superior results in terms of enanti-
oselectivity as well as minimization of side-product for-
mation (Table 1, entries 1 and 6). Changing the solvent to
chloroform resulted in a slight increase in yield (Table 1,
entry 6 vs. 10). Addition of molecular sieves to remove
the water liberated in the condensation step turned out to
Among the quinoline-derived alkaloids, quinolizidines
and indolizidines are common structural motifs in natural
products, and considerable attention has been devoted to
their enantioselective synthesis.8,9 Inspired by our success
in the organocatalytic cascade transfer hydrogenation of
2-substituted quinolines,10,11 we questioned whether this
methodology could be extended to a more complex cas-
cade reaction which takes advantage of the generated sec-
ondary amine moiety. For this purpose a substituent
bearing a carbonyl functionality that subsequently under-
goes a reductive amination with the amine was envisioned
(Scheme 1).
Initial studies with the easily accessible 5-(2-quinolinyl)-
2-pentanone (1),12 Hantzsch dihydropyridine 2 as hydride
donor, and catalytic amounts of an achiral diaryl phos-
phate 3 revealed that the reaction can well be performed.
However, beside the desired tertiary amine 4 small
amount of the tetrahydroquinoline 5 was obtained. Most
probably the enamine intermediate formed in the reduc-
tion of the quinoline ring undergoes an intramolecular
H
H
ArO
ArO
EtOOC
COOEt
O
P
N
O
2
OH
N
H
2
3
1
2 (3.3 equiv)
3 (5 mol%)
1
+
toluene
60 °C
N
N
H
SYNLETT 2011, No. 9, pp 1243–1246
x
x.
x
x.
2
0
1
1
4
5
Advanced online publication: 18.04.2011
DOI: 10.1055/s-0030-1259955; Art ID: Y03411ST
© Georg Thieme Verlag Stuttgart · New York
Scheme 2 Cascade reduction of quinolinylpentanone 1

1244
M. Rueping, L. Hubener
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be crucial for the yield of the reaction (Table 1, entry 11). Friedel–Crafts acylation of different arenes with 3-bro-
Under the optimized conditions the desired product 4 was mopropionyl chloride led to 3-bromopropanones 10 in
isolated in high yield with excellent enantioselectivity, moderate to good yields (39–71%). The ketals 12 ob-
while the side-product formation was almost completely tained upon reaction with ethylene glycol were unstable
suppressed (Table 1, entry 11).
on silica; however, filtration through an alumina plug fur-
nished them in acceptable purity.17 Although most exam-
ples contained a considerable amount of elimination
product (up to 10%), these impurities did not hamper the
subsequent reaction with lithiated quinaldine and the sub-
stitution products 14 were isolated in moderate yields.
Ketal deprotection with 1 M HCl and acetone yielded the
desired ketones 16 in almost quantitative yields for most
examples.
Table 1 Catalyst Evaluation for the Reduction Cascade of 1a
6a X = OH, Ar = 9-phenanthryl
Ar
O
6b X = OH, Ar = 2-naphthyl
6c X = OH, Ar = 1-naphthyl
6d X = OH, Ar = phenyl
6e X = OH, Ar = 1-naphthyl, [H8]
6f X = OH, Ar = 9-anthracenyl
6g X = NHTf, Ar = 9-phenanthryl
6h X = NHTf, Ar = 1-naphthyl
6i X = NHTf, Ar = 1-naphthyl, [H8]
O
X
P
O
Ar
We were also interested in synthesizing ketones with a
shorter alkyl chain. However, a-bromoketal 11, derived
from bromoacetophenone 9, showed very poor reactivity
in the reaction with lithiated quinaldine. Accordingly, 3-
(2-quinolinyl)propiophenone 15 was obtained in low
overall yield (Scheme 3).
Entry Catalyst Solvent
Yield Ratio of
dr 4c
ee 4
(%)b
46
49
40
30
37
41
79
79
76
43
84
4/5c
(%)c,d
1
2
6a
6b
6c
6d
6e
6f
PhMe
PhMe
PhMe
PhMe
PhMe
PhMe
PhMe
PhMe
PhMe
CHCl3
CHCl3
93:7
3.3:1
4.2:1
5.8:1
3.7:1
3.7:1
3.8:1
9:1
88/86
–12/–20
40/40
2:1
3:2
3
O
O
O
4
1:2
–38/–38
38/36
OH
Br
Br
HO
p-TSA, C6H6, Δ
n
n
R
R
5
3:2
9 n = 0
10 n = 1
11 n = 0: 96%
12 n = 1: 45–70%
6
96:4
100:0
100:0
100:0
97:3
86/88
7
6g
6h
6i
6/10
8
9:1
–38/4
N
N
1 M HCl
acetone
n
n
7, n-BuLi
THF, – 78 °C
9
9:1
–38/–54
84/82
100 °C (MW)
1 h
O
10
11e
6f
4.4:1
O
R
O
R
6a
99.5:0.5 3.7:1
97/97
13 n = 0: 25%
14 n = 1: 25–70%
15 n = 0: 94%
a Reaction conditions: 1 (0.15 mmol), solvent (0.15 M), 2 (3.3 equiv),
6 (5 mol%), 60 °C, 16 h.
16 n = 1: 72–96%
b Yield of 4 and 5 after column chromatography.
c Determined by chiral GC.
Scheme 3 Synthetic route to aryl-substituted quinolinyl ketones
d Enantioselectivities of major/minor diastereomers.
e Powdered MS 4Å (1 weight equiv) were added.
3-Quinolinyl-propiophenone 15 turned out to be a very
reactive substrate for the reductive cyclization, and the
corresponding benzoindolizidine could be isolated in
quantitative yield but only with moderate enantioselectiv-
ity (Table 2, entry 1). The reduction of the aryl-3-quinoli-
nylbutanones 16 was incomplete even after prolonged
reaction time, due to the higher stability of the aromatic
enamines formed as intermediates. However, the insepa-
rable enamine–amine mixtures 18 (enamine/amine ratio
usually around 5:1) could easily be isolated and subjected
to metal-catalyzed hydrogenation to give the desired
amines 17. Using palladium on charcoal as the catalyst led
to byproduct formation. In contrast, iridium nanoparticles,
reductively deposited on carbon nanotubes, a heteroge-
neous hydrogenation catalyst recently developed in our
group,18 led to full conversion without noticeable loss of
product. Thus, using the optimized reaction conditions in
this triple hydrogenation sequence, arylbutanones 16 all
furnished the corresponding benzoquinolizidines 17 in
good yields and with high enantioselectivities (Table 2,
entries 3–9). Furthermore, for these substrates bearing
In order to extend the scope of this reaction, we aimed to
synthesize a number of differently substituted (2-quinoli-
nyl)ketones. While the iodoketal 8, required for the syn-
thesis of quinolinylpentanone 1 (Equation 1), was easily
available from methyl vinyl ketone in a one-pot reac-
tion,16 this procedure could not be applied to the synthesis
of iodoketals from other enones.
1. n-BuLi (1.05 equiv)
THF, −78 °C
8 (1.4 equiv)
O
O
1
2. 1 M HCl
100 °C, 1 h
N
I
7
8
Equation 1 Synthesis of 1 from lithioquinaldine and iodoketal 8
Thus, ketones bearing aryl substituents have been ob-
tained according to the synthetic route outlined in
Scheme 3.
Synlett 2011, No. 9, 1243–1246 © Thieme Stuttgart · New York

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Enantioselective Synthesis of Quinolizidines and Indolizidines
1245
Typical Procedure
1-p-Tolyl-hexahydro-1H-pyrido[1,2-a]quinoline
bulkier substituents, virtually no second diastereomer
could be observed. The absolute stereochemistry of 17
was assigned in analogy to our previous studies on the re-
duction of 2-substituted quinolines9 in combination with
NOE measurements.
1-Tolyl-3-(2-quinolinyl)butanone (43.4 mg, 0.15 mmol), Hantzsch
dihydropyridine (125 mg, 0.48 mmol), activated MS 4Å (43 mg,
powder) and 3,3¢-diphenanthryl-BINOL-phosphate (5.3 mg, 7.5
mmol, 0.05 equiv) were dissolved in dry CHCl3 (1 mL), purged with
argon and sealed with a rubber septum. The mixture was stirred at
60 °C for 36 h and directly purified by flash chromatography
through a short (!) silica column (toluene–EtOAc, 100:1) to isolate
the unpolar enamine–amine mixture [TLC cyclohexane–EtOAc =
5:1; Rf (amine) = 0.8, Rf (enamine) = 0.35]. The combined fractions
were concentrated to dryness and the mixture transferred to a 10 mL
round-bottom flask, dissolved in EtOAc (1.5 mL) and iridium nano-
particles on carbon nanotubes (2 mol%) were added. A three-way
connection with a hydrogen balloon and a vacuum outlet were at-
tached, and the flask was repeatedly evacuated and purged with hy-
drogen. The reaction was stirred for 8 h at r.t., filtered through a
celite plug, concentrated, and flashed through a very short silica col-
umn (cyclohexane–EtOAc = 50:1) to yield 33 mg (79%) of the pure
benzoquinolizidine. 1H NMR (400 MHz, C6D6): d = 7.24–7.20 (m,
3 H), 7.01–6.96 (m, 3 H), 6.92–6.87 (m, 1 H), 6.67 (t, 1 H, J = 7.3
Hz), 6.46 (d, 1 H, J = 8.3 Hz), 4.36 (t, 1 H, J = 5.1 Hz), 3.12–3.03
(m, 1 H), 2.78–2.69 (m, 1 H), 2.68–2.60 (m, 1 H), 2.10 (s, 3 H),
2.08–1.98 (m, 1 H), 1.70–1.62 (m, 3 H), 1.49–1.42 (m, 2 H), 1.34–
1.21 (m, 2 H) ppm. 13C NMR (100.6 MHz, CDCl3): d = 147.3,
142.4, 135.9, 129.5, 128.4, 127.1, 126.4, 125.5, 117.4, 114.4, 60.4,
52.7, 32.1, 31.9, 31.5, 28.0, 21.0, 18.0 ppm MS (EI): m/z
(%) = 278.2 (28.1), 277.2 (100) [M+], 276.2 (37), 186.2 (81.7),
158.1 (28.5), 145.1 (17.5), 132.1 (55.4), 130.1 (32.4), 117.1 (25.6),
105.1 (9.8), 91.2 (19.6). IR: n ∼3018, 2935, 2858, 1902, 1601, 1497,
1453, 1282, 1052, 814, 747 cm–1. Chiral GC (Hydrodex-b, 2 K/
min): major enantiomer tR = 41.07 min (95.4%), minor enantiomer
tR = 41.81 min (4.6%), 91% ee. HRMS (ESI, in MeOH, acidified):
m/z calcd for C20H23NK: 316.14621; found: 316.14600.
Table 2 Scope of the Reductive Cyclization Cascadea
for 1 (n = 1, R = Me)
and 15 (n = 0, R = Ph):
N
N
2 (3.3 equiv), 6a (5 mol%)
n
n
R
4, 17
O
R
for 16 (n = 1,
R = aryl
2 (3.3 equiv)
6a (5 mol%)
H2
N
Ir@CNT (2 mol%)
R
18
Entry R
n
0
1
1
1
1
1
1
1
1
Yield (%)b dr
ee (%)c
41
1
2
3
4
5
6
7
8
9
Ph
95
84
9:1c
Me
3.7:1c 97
>98:2 93
Ph
78
79
70
67
73
80
73
4-MeC6H4
4-MeOC6H4
3,4-Me2C6H3
3,4-(MeO)2C6H3
4-FC6H4
4-ClC6H4
>98:2
91
90
93
86
86
89
>98:2
>98:2
>98:2
>98:2
>98:2
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synlett. It contains
experimental procedures for preparation of the starting materials
and characterizations of the (2-quinolinyl)ketones and products.
a Reaction conditions: ketone (0.15 or 0.2 mmol), CHCl3 (0.15 M), 6a
(5 mol%), Hantzsch dihydropyridine 2 (3.3 equiv), powdered MS 4Å
(1 weight equiv), 60 °C, 24–36 h. For 16 (n = 1, R = aryl): isolation
of enamine–amine mixture, EtOAc (2 mL), iridium nanoparticles on
carbon nanotubes (2 mol%), H2 (1 atm), r.t., 6–8 h.
Acknowledgment
Financial support by the DFG (SPP 1179) is gratefully acknowled-
ged.
b Yield after flash chromatography.
c Determined by chiral GC (Hydrodex b) or SFC (OJH column).
References and Notes
In conclusion, we have developed a reduction cascade that
yields a new class of chiral benzofused quinolizidines and
indolizidines starting from 2-substituted quinolines. In the
case of aryl-substituted benzoquinolizidines a sequential
protocol was necessary. In general, the selectivity of the
final reduction step is controlled by the previously estab-
lished stereocenter and conformational restraints. There-
fore, we established that no chiral catalyst is required for
high diastereoselectivities. The yields are generally good
and diastereo- and enantioselectivities are high. Thus, this
new cascade reaction resembles an efficient protocol for
the synthesis of quinolizidine and indolizidines natural
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Synlett 2011, No. 9, 1243–1246 © Thieme Stuttgart · New York