Facile one-pot synthesis of 2,3-dihydro-1H-indolizinium derivatives by rhodium(iii)-catalyzed intramolecular oxidative annulation via C-H activation: application to ficuseptine synthesis

Article abstract of DOI:10.1039/c7cc00008a

Various substituted indolizidinium, quinolizinium and pyrido[1,2-a]azepinium salts synthesized from benzaldehydes (or α,β-unsaturated aldehydes) and alkyne-amines catalyzed by rhodium complexes via C-H activation are demonstrated. The reaction was carried

Full text of DOI:10.1039/c7cc00008a

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc3N@C2n (2n 68 and 80). Synthesis of the
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Facile One-Pot Synthesis of 2,3-Dihydro-1H-indolizinium
Derivatives by Rhodium(III)-Catalyzed Intramolecular Oxidative
Annulation via C–H Activation: Application to Ficuseptine
Synthesis
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Nitinkumar Satyadev Upadhyay,a Jayachandran Jayakumar,a and Chien-Hong Chenga,*
www.rsc.org/
Abstract. Various substituted indolizidinium, quinolizinium and quarternary ammonium salts via C–H activation were
pyrido[1,2-a]azepinium salts synthesized from benzaldehydes (or intensively investigated. Recently, rhodium- and ruthenium-
α,β-unsaturated aldehydes) and alkyne-amines catalyzed by catalyzed C–H activation and intramolecular oxidative coupling
rhodium complex via C–H activation is demonstrated. The reaction of alkyne tethers as key steps for synthesis of N-hetrocycles
was carried out under mild reaction conditions using was developed by several groups. Park and co-workers
Cu(BF4)2⋅6H2O as the oxidant and the anion source and discovered a Rh-catalyzed intramolecular annulation of alkyne-
inexpensive oxygen as co-oxidant. A reaction mechanism involving tethered hydroxamic esters to afford isoquinolones.10 Later,
imine formation followed by an ortho C−H activation, alkyne Mascarenas and Gulias et al. reported a Rh-catalyzed synthesis
isertion and reductive elimination via a 7-membered rhodacycle is of tricyclic isoquinolines by intramolecular annulations of
proposed. The present method is successfully applied to the alkyne-tethered benzamides.11 At the same time, Jia and Liu
synthesis of natural product, ficuseptine.
reported a Rh(III)-catalyzed intramolecular amidoarylation of
alkynes via C–H activation to give fused tricyclic indole
scaffolds.12 Reddy and co-workers developed a microwave
assisted Ru-catalyzed intramolecular annulation of alkyne-
tethered benzamides to afford tricyclic isoquinolinones.13 Very
recently, Hua and co-workers revealed a mild Rh(III)-catalyzed
inter- and intra-molecular annulations of alkyne-tethered
ketones and amines to afford polyhetrocycles via cascade
oximation.14
The indolizidine, quinolizidine and pyrido[1,2-a]azepinium
alkaloids are important hetrocycles and are found in many
naturally occuring alkaloids.1 Such as louludinium chloride,
ipalbidium,
ficuseptine,
juliprosine,
tylophorine-D,
dehydrotylophorine, clathryimine (B and C), and CCR5
antagonist anibamine C (Figure 1).2 Among these alkaloids,
ficuseptine, isolated from Ficus Septica, shows interesting
antibacterial and antifungal activities and. Bracher and Daab
first reported a five-step synthesis of ficuseptine via Suzuki and
Sonogashira couplings followed by Sandmeyer iodination
reaction.3 Later, Snider and Neubert reported a one-pot
synthesis of ficuseptine by using biomimic intramolecular
Chichibabin pyridine synthesis.4 However, these methods
suffered from harsh and tedius reaction conditions.
However, most of the above methods have not been applied
to the synthesis of natural product. Recently, we reported a
mild Rh-catalyzed intramolecular alkyne tethered imines
providing a direct access to protoberberine alkaloids.15a We
also developed various methods for the synthesis of
isoquinolinium, cinnolinium, quinolizinium, and pyridinium
salts via metal-catalyzed C–H activation.15 Our continued
interest in the application of metal-catalyzed C–H activation to
natural product synthesis prompts us to investigate the
synthesis of 2,3-dihydro-1H-indolizinium salts from the
Transition metal-catalyzed C–H functionalization has been a
promising method for the synthesis of various N-containing
heterocycles,5 particularly, quaternary ammonium salts,6
which are important key intermediates found in many natural
products and medicinal ingredients.1 Rh-7 Ru-8 and Co-
catalyzed9 intermolecular oxidative annulation of a variety of
arylmethan-imines, arylpyridines, diazoarenes, N-hetrocyclic
carbenes (NHC) and arylpyridinium salts with alkynes to give
reaction of benzaldehydes and
α,β-unsaturated aldehydes
with alkyne-amines using Rh(III) species as the catalyst. Herein,
we report the results of this study and the application of this
methodology to the synthesis of 2,3-dihydro-1H-indolizinium
alkaloid, ficuseptine.
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[RhCp*Cl2]2
3
i-propanol 46
[RhCp*Cl2]2
[RhCp*Cl2]2
[RhCp*Cl2]2
[RhCp*Cl2]2
[RhCp*Cl2]2
[Cp*Rh(MeCN)3][BF4]2
[Ru(p-cymene)Cl2]2
Pd(OAc)2
4
TFE
70
5
methanol
DME
81
6
65
7
THF
70
8
DCE
66
9
t-butanol
t-butanol
t-butanol
t-butanol
t-butanol
t-butanol
t-butanol
86
10
11
12
13
14
15
45
NR
NR
Trace
NR
70c
[RhCl(PPh3)]
Co(OAc)2.2H2O
-
Figure 1. Representative naturally occurring and synthetic quaternary
[RhCp*Cl2]2
indolizinium, quinolizinium and pyrido[1,2-a]azepinium alkaloids.
a
Unless otherwise mentioned, all reactions were carried out using
benzaldehyde 1a (0.36 mmol), alkyne-amine 2a (0.30 mmol),
[Cp*RhCl2]2 (2.0 mol %), Cu(BF4)2 6H2O (0.30 mmol), O2 (1 atm, ca 1.5
To find the optimized conditions for the reaction of
benzaldehyde 1a with alkyne-amine 2a to give 2,3-dihydro-1H-
indolizinium salt 3aa, we examined the effect of catalyst, and
solvent used (see Table 1) on the yield of 3aa. Treatment of 1a
⋅
b
L) and solvent (2.0 mL) at 90 ºC for 6 h. Yields were measured by H
1
c
NMR, using mesitylene as an internal standard. Air was used instead
of O2.[NR = No Reaction]
(0.36 mmol), 2a (0.30 mmol) and Cu(BF4)2⋅6H2O (0.30 mmol) in
the presence of [RhCp*Cl2]2 (2 mol %) in t-amylOH (2.0 mL)
under O2 (1 atm) for 6 h gave indolizinium salt 3aa consisting
of BF4- as the anion in 76% isolated yield (Table 1, entry 1). The
To evaluate the scope of the present reaction, first, we
examined the reactions of para-substituted benzaldehydes 1a–1f
with alkyne-amine 2a under the optimized reaction conditions
(entry 6, Table 1). Thus, 2a reacted with p-bromo 1b, phenyl 1c,
-
presence of BF4 anion and the structure of 3aa were
1
confirmed by its H, 13C, 19F and 11B NMR spectra and HRMS ethyl 1d, methoxy 1e and nitro benzaldehydes 1f to afford the
expected indolizinium salts 3ba–3fa in 81-91% yields. The
substituents, either electron-donating or -withdrawing, at the para
position of 1 all appear to give excellent product yield. The
structure of 3aa was further confirmed by the results of single-
crystal X-ray diffraction.17 We also tested the reactivity of ortho-
substitued benzaldehydes with 2a. Both 2-methyl- 1g and 2-
nitrobenzaldeyhydes 1h reacted with 2a nicely under the standard
conditions to provide salts 3ga and 3ha in 78 and 75% yields,
respectively. Further, it is interesting to know that the
data. The other solvents including t-butanol, i-propanol, TFE,
methanol, DME, THF and DCE were also tested to give the
expected product 3aa in moderate to excellent yields (entries
2-8). Among these solvents t-butanol was found to be the best
solvent for the reaction affording 3aa in 95% yield (entry 2),
while methanol also gave 3aa in 81% yield (entry 5). To find
the most suitable catalyst for the reaction, we investigated
several metal catalysts in which [Cp*Rh(MeCN)3][BF4]2 and
[Ru(p-cymene)Cl2]2 affords 3aa in 86 and 45% yields,
respectively. In contrast, Pd(OAc)2, [RhCl(PPh3)] and Co(OAc)
regioselectivity
of
m-substituted
and
m,p-disubstituted
benzaldehydes were investigated. Thus, 3-methoxybenzaldehyde 1i
reacted with alkyne 2a efficiently to afford regioisomeric products
3ia and 3ia‘ in a 50:50 ratio in 84% yield. These results suggest that
the C-H bond activation and annulation reaction occurred nearly
equally at both the ortho positions of benzaldehyde groups.
However, the reaction of 3-bromo-4-methoxybenzaldehyde 1j and
3,4-dimethoxybenzaldehyde 1k with 2a proceeded at C-6 position
to afford 3ja, and 3ka in high regioisomeric ratios of 98:2 and 90:10
in 82 and 85% combined yields, respectively. The C-H bond
activation and annulation reaction occurred more favorably at the
6-position of the m,p-dimethoxy group. In contrast, the reaction of
benzo[d][1,3]dioxole-5-carbaldehyde 1l with 2a proceeded
favorably at C-2 position to afford regioisomers 3la in a 90:10 ratio
in 78% combined yield.15a The regioselectivity of the major product
is opposite to most of the m,p-disubstituted benzaldehydes. Furan-
2-carbaldehyde 1m and thiophene-2-carbaldehyde 1n also
underwent C-H bond activation and annulation with alkyne-amine
2a to afford the corresponding indolizinium salts 3ma and 3na in 75
2⋅2H2O gave only a trace or no 3aa (entries 11-13). Notably,
when the reaction was carried out without catalyst, no
product 3aa was formed (entry 14). When the reaction was
performed in air, 3aa was obtained in 70% yield (entry-15).
Alkyne-amines
2 was prepared from commercially available 5-
chloropent-1-yne in three steps in 60–75 yields (see
Supporting Information).16
Table 1. Optimization studies for the reaction of benzaldehyde
1a with alkyne-amine 2a a
.
-
BF4
O
H2N
N
[RhCp*Cl2]2 (2 mol %)
Cu(BF4)2.6H2O/ O2
Ph
Ph
solvent, 90 o
C
3aa
1a
2a
Entr
y
Catalyst
Solvent
Yield
[%]b
and 84% yields, respectively
.
[RhCp*Cl2]2
1
t-amylOH
t-butanol
76
[RhCp*Cl2]2
2
95
2 | J. Name., 2012, 00, 1-3
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Table 2. Results of Rh-catalyzed reaction of aldehydes and amino-
aford the expected salt product 3af in 75% yield. In addition, the
present Rh-catalyzed C–H activation reaction also can be applied to
α,β-unsaturated aldehydes. Thus, the reaction of acrylaldehyde 1q
and (E)-2-methyl-3-phenylacrylaldehyde 1r with akyne-amine 2a
afforded the corresponding dihydroindolizinium salts 3qa and 3ra in
72 and 85% yields, respectively. These results stimulate us to find
and design a novel synthesis of natural product.
alkynes.a,b
-
BF4
O
H2N
N
(2 mol %)
[RhCp*Cl2]2
R
Cu(BF4)2.6H2O / O2
t-BuOH,, 90 oC, 6 h.
R'
R
R'
3
1
2
The significance of this Rh-catalyzed intra-molecular C–H
activation/annulation reaction is further demonstrated by its
application to the direct synthesis of
a indolizinium alkaloid
R
-
BF4
ficuseptine (Scheme 1). Thus, the reaction of 5-(4-
methoxyphenyl)pent-4-yn-1-amine 2g (0.40 mmol) with 2-(4-
methoxyphenyl)acrylaldehyde[18] 1t (0.48 mmol) in the presence of
[RhCp*Cl2]2 (2.0 mol %) and Cu(BF4)2⋅6H2O (0.45 mmol) in t-butanol
under 1 atm of O2 at 90 °C for 6 h afforded ficuseptine 4 with BF4− as
the counter anion in 84% isolated yield.[4]
-
BF4
3aa: R = H, 91%
N
N
3ba: R = Br, 86%
3ca: R = Ph, 90%
3da: R = Et, 88%
3ea: R = OMe, 85%
3fa: R = NO2, 81%
R
3aa
Ph
Ph
3ga: R = Me, 78%
3ha: R = NO2, 75%
-
-
BF4
6
-
BF4
BF4
2
MeO
2
MeO
Br
N
-
N
N
BF4
2
N
MeO
H2N
O
[RhCp*Cl2]2 (2 mol %)
MeO
6
6
OMe Ph
Ph
Ph
Cu(BF4)2.6H2O / O2
t-BuOH, 90 oC, 6 h.
3ja, 82% (98:2)
3ia:3ia' = 50:50, 84%
MeO
4,
-
84%
-
1t
2g
BF4
BF4
-
BF4
OMe
2
6
MeO
X
N
N
N
Scheme 1. One-pot synthesis of ficuseptine by Rh-catalyzed C–H activation/
MeO
6
annulation reaction.
O
2
Ph
Ph
O
Ph
3ma: X = O, 75%
3na: X = S, 84%
3ka, 85% (90:10)
3la, 78% (90:10)
To compare the reactivity of alkyne-amine 2 with an alkyne
which cannot directly link to the aldehyde group of 1, equal molar
amounts of benzaldehyde 1a, alkyne-amine 2a, and 4-octyne
2a‘ were dissolved in t-butanol and allowed to react under the
standard reaction conditions. Interestingly, the reaction gave only
the intramolecular annulation product 3aa. The intermolecular
annulation product 3aa‘ was not formed. These results suggest
that the intramolular annulation is much faster than the
intermolecular reaction (Scheme 2).
-
-
-
BF4
BF4
BF4
N
N
N
Cl
R
Ph
Ph
R'
3od: 88%
3ob: R = Cl, 88%
3pb: R = Me, 86%
3ac: R' = 4-Me-C6H4, 93%
-
BF4
-
-
BF4
BF4
R
N
N
N
R''
Ph
-
Ph
BF4
Me
3qa: R R'' = H, 72%
3ra: R = Me, R'' = Ph, 85%
3ae, 0%
N
H2
N
3af, 75%
O
[a]
[RhCp*Cl2]2 (2 mol %)
3aa, 85%
aUnless otherwise mentioned, all reactions were carried out using alkyne-
amines 2 (0.40 mmol), aldehydes 1 (0.48 mmol), [Cp*RhCl2]2 (2.0 mol %),
Cu(BF4)2⋅6H2O (0.45 mmol), O2 (1 atm, ca 1.5 L) and t-butanol (2.5 mL) at 90
ºC for 6 h. b Isolated yields.
Ph
Ph
2a
-
Cu(BF4)2.6H2O / O2
t-BuOH,, 90 oC, 6 h.
BF4
N
Pr
Pr
Ph
1a
2a'
Pr
Pr
3aa',
0%
Scheme 2. Inter- and intra-molecular Rh-catalyzed annulation
reaction.[a] Standard reaction conditions.
Next, we examined the reactions of benzaldehydes with
ArC≡C(CH2)nNH2 (2) by varying the alkyl chain length under standard
reaction conditions. Thus, 2b (n
= 4) reacted with 4-chloro
Based on the known transition-metal catalyzed directing group-
assisted inter- and intra-molecular C–H activation/annulation
reactions,[5–15] a possible reaction mechanism is proposed for this
catalytic reaction (Scheme 3). The catalytic cycle is probably
initiated by the coordination of the imine nitrogen generated in situ
from 1a and 2a to Rh(III), and is followed by the ortho C–H bond
activation to form five-membered rhodacycle I and the release of
H3O+. Next, the coordination of the intramolecular alkynyl group in I
to the Rh center affords intermediate II. Further insertion of the
alkynyl group into the Rh–C bond gives a bicyclic six- and seven-
membered rhodacycle III. Reductive elimination of III affords the
final salt, 3aa, and Rh(I). The Rh(I) species is reoxidized by Cu(II) or
O2 to regenerate the active Rh(III) species for the next catalytic
cycle.
benzaldehyde 1o and 4-methylbenzaldehyde 1p smoothly to afford
1,2,3,4-tetrahydro-pyrido-isoquinolinium salts 3mb and 3nb in 88
and 86% yields, respectively. Similarly, alkyne-amine 2c where Ar is
p-tolyl, reacted with benzaldehyde 1a smoothly to afford the
corresponding tetrahydro-pyrido-isoquinolinium salts 3ac in 93%
yield. In a similar manner, 4-chloro benzaldehyde 1o also reacted
with 7-phenylhept-6-yn-1-amine 2d efficiently to afford salt product
3od containing a fused azacycloheptane ring in 88% yield. In
contrast, the reaction of 4-phenylbut-3-yn-1-amine 2e with
benzaldehyde 1a did not give the expected 4-membered ring salt
product, due to the large ring strain of the expected product.
Similarly, we examined the reactions of benzaldehydes 1a with
MeC≡C(CH2)3NH2 2g under the standard reaction conditions to
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Chem. Soc. Rev., 2011, 40, 5068-5083; (g) X. Huang, Y. Wang,
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Scheme 3. Proposed catalytic cycle.
Conclusions
In summary, we have successfully developed an efficient
Rh(III)/Cu/O2 system for the one-pot synthesis of bi- tri- and
hetro-cyclic fused quaternary ammonium salts from
substituted benzaldehydes or α,β-unsaturated aldehydes and
alkyne-amines via C
applied to the synthesis of natural product ficuseptine
excellent yields. Notably, the present method is suitable for
the convenient synthesis of variety of indolizidinium,
−
H activation. This protocol is succuessfully
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a
7
8
quinolizinium and pyrido[1,2-a]azepinium functionalized
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product synthesis is underway.
Acknowledgement
We thank the Ministry of Science and Technology of the
Republic of China (MOST 104-2633-M-007-003) for support of
this research.
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