Baylis-Hillman acetates in organic synthesis: Convenient one-pot synthesis of α-carboline framework - A concise synthesis of neocryptolepine

Article abstract of DOI:10.1039/c2ob26339d

A convenient, facile, and one-pot methodology for the synthesis of α-carbolines from Baylis-Hillman (BH) acetates, involving three steps (reactions), (1) mono alkylation of 2-nitroarylacetonitriles with BH-acetates, (2) reduction of nitro group into amino group using Fe/AcOH and (3) formation of two (five and six membered) rings, is presented. This methodology is successfully applied to the synthesis of bioactive alkaloid neocryptolepine.

Full text of DOI:10.1039/c2ob26339d

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BaylisꢀHillman Acetates in Organic Synthesis:
Convenient OneꢀPot Synthesis of αꢀCarboline Framework ꢀ A Concise
Synthesis of Neocryptolepine†
5
Deevi Basavaiah* and Daggula Mallikarjuna Reddy
Received (in XXX, XXX) Xth XXXXXXXXX 200X, Accepted Xth XXXXXXXXX 200X
First published on the web Xth XXXXXXXXX 200X
DOI: 10.1039/b000000x
with BHꢀacetates 2) reduction of nitro group into amino group
using Fe/AcOH 3) formation of two (five and six membered)
rings in an operationally simple oneꢀpot procedure. We have also
successfully applied this protocol to the synthesis of
50 neocryptolepine (2), an important bioactive alkaloid.
10 A convenient, facile, and oneꢀpot methodology for synthesis of
αꢀcarbolines from the BaylisꢀHillman (BH) acetates, involving
three steps (reactions), that is, 1) mono alkylation of 2ꢀ
nitroarylacetonitriles with BHꢀacetates 2) reduction of nitro
group into amino group using Fe/AcOH 3) formation of two
15 (five and six membered) rings, is presented. This methodology
is successfully applied to the synthesis of bioactive alkaloid
neocryptolepine (2).
The BaylisꢀHillman reaction has been and continues to attract the
attention of various synthetic chemists because it provides diverse
classes of multifunctional molecules.5 In fact, due to the close
proximity of functional groups the BHꢀadducts have become an
Development of facile and convenient protocols for synthesis1 of 55 attractive source/ substrates for
a
number of organic
20 pyrido[2,3ꢀb]indole (αꢀcarboline) framework has received
increasing attention from synthetic chemists due to the fact that
several molecules containing this structural organization show
various biological activities such as antiꢀinflammatory,2a
transformations often leading to the synthesis of several bioactive
compounds and important frameworks of medicinal relevance.5, 6
We have recently reported interesting oneꢀpot synthetic protocols
for the facile conversion of BaylisꢀHillman adducts into
anxiolytic,2b and cytotoxic.2c Also a number of natural products 60 substituted [1,8]naphthyridinꢀ2ꢀones,7a triꢀ/tetracyclic azocines,7b
25 [mescengricin (1a),3a grossularineꢀ1 (1b),3b grossularineꢀ2 (1c)3b
and neocryptolepine (2) (also known as cryptotackieine)3c (Figure
1)] possess this tricyclic skeleton as their core structure.
3ꢀbenzoylquinolines,7c tetrahydroacridines7d and quinolines.7e In
all these strategies, the in situ reduction of nitro group using
Fe/AcOH into an amino group and subsequent cyclization ( mono
or bis) are the key steps.7 This experience has led us to envision
65 that BaylisꢀHillman acetates can be in principle transformed into
pyrido[2,3ꢀb]indole framework via the mono alkylation (SN2') of
2ꢀnitroarylacetonitriles (3) with BHꢀacetates (4) followed by in
situ reduction of nitro group and then two successive cyclizations
(first one forming five membered ring and the second one
70 producing the six membered ring) according to the retroꢀsynthetic
strategy as shown in Scheme 1.
HO
OH
NMe2
N
O
HN
O
O
O
O
Me
N
N
N
H
HO
N
H
HO
N
H
1b
1a
R2
NMe2
N
HN
R2
O
R1
cyclization
N
O
N
R1
CN
NH2
N
N
H
Me
N
R
R
N
H
2
OH
1c
6
30
Figure 1. Natural products containing αꢀcarboline core
reduction
R2
In continuation of our ongoing long term major research program
in the area of BaylisꢀHillman reaction and its applications,4 we
35 herein report a convenient, facile, and oneꢀpot methodology for
synthesis of pyrido[2,3ꢀb]indole derivatives from appropriate
BaylisꢀHillman (BH) acetates. This protocol involves three steps
(reactions), that is, 1) mono alkylation of 2ꢀnitroarylacetonitriles
CN
NO2
OAc
O
R
3
R1
R2
R1
alkylation
(SN2')
O
CN
NO2
4
R
40
5
School of Chemistry, University of Hyderabad, Hyderabad, 500 046,
India. E-mail: dbsc@uohyd.ernet.in; Fax: (+91) 40-23012460
†Electronic Supplementary Information (ESI) available: Representative
experimental procedures, with all spectral data of 2, 5a, 6aꢀq, 8ꢀ11,
45 crystal data and ORTEP diagram of 6n & 8 See DOI: 10.1039/b000000x.
75
Scheme 1 Retroꢀsynthetic strategy for the synthesis of
αꢀcarboline framework
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Accordingly, we have first selected 4ꢀacetoxyꢀ3ꢀmethyleneꢀ4ꢀ
89% isolated yield. Subsequent treatment of 5a with Fe /AcOH
phenylbutanꢀ2ꢀone (4a), acetate of BaylisꢀHillman alcohol 50 at reflux temperature for 1 h, furnished 3ꢀbenzylꢀ2ꢀmethylꢀ9Hꢀ
(obtained via the reaction between benzaldehyde and methyl
vinyl ketone), as substrate for alkylation of 2ꢀ
nitrophenylacetonitrile (3a). Best results in this direction were
obtained when we treated BHꢀacetate 4a (2 mmol) with 2ꢀ
pyrido[2,3ꢀb]indole (6a) in 61% isolated yield after the usual
workup followed by column chromatography (Scheme 2) (in
overall 54% yield for both the steps).
a
5
With a view to understand the feasibility of oneꢀpot process we
nitrophenylacetonitrile 3a (2 mmol) in the presence of K2CO3 (2 55 have directly treated the in situ generated 5a (after removal of
mmol) in THF (6 mL) at room temperature for 12 h to provide
(E)ꢀ3ꢀbenzylidineꢀ5ꢀcyanoꢀ5ꢀ(2ꢀnitrophenyl)pentanꢀ2ꢀone (5a) in
THF under reduced pressure) with Fe/AcOH under reflux
temperature for 1h to afford the desired compound 6a in 52%
overall isolated yield (Table 1). Since the yields are comparable
in both procedures we have selected the oneꢀpot procedure for
60 understanding the generality of this reaction. We have prepared
representative BaylisꢀHillman acetates (4bꢀg) from the
corresponding alcohols (derived from selected benzaldehydes and
alkyl vinyl ketones). The acetates (4bꢀg) were then subjected to
similar synthetic sequence as in the case of 4a to provide the
65 desired αꢀcarbolines (6bꢀn) in 50ꢀ67% isolated yields (Table 1,
entries 2ꢀ14). With a view to understand the applicability of this
strategy to BaylisꢀHillman acetates containing ortho substituted
aryl groups9 we have also used allyl acetates 4hꢀj (containing
aryl groups with 2ꢀ chloro, 2ꢀbromo, and 2ꢀmethyl substitution
70 respectively). These reactions worked well and the resulting αꢀ
carbolines 6oꢀq were obtained in reasonable (48ꢀ52 %) yields.
In order to further confirm the formation of tricyclic ring system
we also obtained single crystal for compound 6n and established
its structure by Xꢀray data analysis (Figure 2).8
10
OAc
O
CN
K2CO3, THF
rt, 12 h, 89%
Me
Me
+
CN
NO2
O
NO2
3a
4a
5a
Fe / AcOH,
reflux,1h, 61%
Me
N
N
H
6a
Scheme 2 Stepwise synthesis of 6a
15 Table 1. Synthesis of pyrido[2,3ꢀb]indoles via the reaction of 4aꢀ
j with 3a, 3b or 3c a followed by reductive cyclization
75
O
R2
CN
NO2
i) K2CO3/THF
rt, 48 h
OAc
+
OAc
O
i) K2CO3/THF
rt, 4-30 h
CN
O
8
R1
NO2
CN
+
3a
R1
N
7
R2
R
N
R
NO2
ii) Fe/AcOH
reflux, 1 h
48-67%
H
ii) Fe/AcOH
reflux, 1 h
R = H
(3a)
6a-q
R = Me (3b)
R = OMe (3c)
40%
4a-j
20 Entry NAANb Acetate
R1
R2
Productc Yield
(%)
N
N
+
+
N
N
(9+ 10)
H
H
6%
10
9
22%
12%
1
2
3
4
5
6
7
8
3a
3a
3a
3a
3a
3a
3a
3b
3b
3b
3b
3b
3b
3c
3a
3a
3a
4a
4b
4c
4d
4e
4f
4g
4a
4b
4c
4d
4e
4f
Me
Et
H
H
4ꢀMe
4ꢀCl
4ꢀBr
3ꢀBr
3ꢀOMe 6g
H
H
4ꢀMe
4ꢀCl
4ꢀBr
3ꢀBr
H
2ꢀCl
2ꢀBr
2ꢀMe
6a
6b
6c
6d
6e
6f
52
67
53
55
59
57
50
56
62
50
53
58
55
60
51
48
52
25
Me
Me
Me
Me
Me
Me
Et
Me
Me
Me
Me
Et
1, 4 - Dioxane, DDQ
reflux, 8 h, 68 %
THF, MeI
reflux, 12 h, 82 %
30
6h
6i
6j
6k
6l
6m
6nd
6o
6p
6q
N
N
N
H
9
N
Me
10
11
12
35 13
14
15
16
17
40
2
11
Scheme 3 A simple route for synthesis of compound 2
4b
4h
4i
80 Then our attention was directed towards the application of this
methodology for synthesis of bioactive compound
Me
Me
Me
neocryptolepine (2),1a,10 containing the pyrido[2,3ꢀb]indole
skeleton. We have in fact achieved synthesis of 2 (Scheme 3) in
22% overall yield in three steps, starting from 2ꢀ
85 acetoxymethylcyclohexꢀ2ꢀenone (7), acetate of BaylisꢀHillman
alcohol (obtained via the reaction between formaldehyde and
cyclohexꢀ2ꢀenone). Thus treatment of the acetate 7 with 3a
followed by reductive cyclization (with Fe /AcOH) provided 3,4ꢀ
dihydroꢀ6Hꢀindolo[2,3ꢀb]quinoline (9, 12%) along with 1,2,3,4ꢀ
90 tetrahydroꢀ6Hꢀindolo[2,3ꢀb]quinoline (10, 22%) in over all 40%
yield (including their mixture 6%). We have converted the
mixture [9 + 10 + (9+10)] into aromatic product (11)11 via the
4j
aAll reactions were carried out on 1 mmol scale of BaylisꢀHillman acetate
(4aꢀj) with 1 mmol of NAAN (3aꢀc) in the presence of K2CO3 (1 mmol)
in THF at room temperature followed by reductive cyclization with Fe (6
eq.)/ AcOH. bNAAN = 2ꢀnitroarylacetonitriles. cAll compounds were
45 fully characterized (see Electronic Supplementary Information).
dStructure of this molecule was further confirmed by single crystal Xꢀray
data (see Electronic Supplementary Information).8
2
| Journal Name, [year], [vol], 00–00
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treatment with DDQ.10b This molecule 11 was then transformed
into the natural product (2)11 via alkylation with MeI following
the known procedure10c (Scheme 3).
OAc
O
Me
OAc
O
S
N2'
4a
Me
CN
CN
NO2
NO2
3a
-AcO
Figure 3 ORTEP diagram of compound 8
25
Fe/AcOH
A plausible mechanism for the formation of pyrido[2,3ꢀb]indole
(αꢀcarboline) framework [taking 4a (BaylisꢀHillman acetate) and
3a as a model case) is presented in Scheme 4. The first step
involves alkylation (SN2') to provide trisubstituted alkene (5a).
C
N
O
NH2
Me
O
NC
Me
NO2
5a
30 The second step proceeds through the reduction of nitro group to
provide amine followed by the simultaneous formation (third
step) of five and six membered heterocycles (Scheme 4).
In case of the acetate 7 the first step proceeds through SN2
alkylation to afford trisubstituted alkene (8). The second and third
35 steps proceed as in the case of the acetates 4 to give the
compounds 9 and 10 which on aromatization with DDQ provided
the molecule 11 according to Scheme 5.
H
-H2O
H
Me
OH
Me
N
N
N
H
N
H
isomerization
Me
N
AcO
N
H
6a
5
7
SN
2
Fe/AcOH
O
O
Scheme 4 Plausible mechanism for synthesis of αꢀcarboline core
O
CN
NO2
-OAc
N
CN
NH2
We have in fact in a separate experiment isolated the alkylated
compound (8) (SN2 product) (in the case of acetates 4 we
NO2
3a
8
H+
10 obtained exclusively SN2' products) and confirmed this structure
by single crystal Xꢀray data analysis (Figure 3).8
H
H
H
-H2O
-H2O
Although the overall yield of the bioactive molecule,
neocryptolepine (2) is not that high, this protocol is still
15 interesting in the sense that this strategy demonstrates the
importance of the BHꢀacetates in the synthesis of heterocyclic
compounds.
OH
N
N
N
N
N
N
H
H
H
isomerization
isomerization
C19
C20
-H2
N
N
NH
C14
C15
N
H
N
H
N
H
10
C18
9
C2
C17
C13
C16
DDQ
N
C3
C10
C4
C9
C1
N1
C12
C5
N
H
C11
N2
11
C8
O1
C6
C7
40
Scheme 5 Plausible path way for the formation of 9 and 10
C21
20
In conclusion, we have developed a convenient, operationally
simple, oneꢀpot procedure for synthesis of pyrido[2,3ꢀb]indole (αꢀ
Figure 2 ORTEP diagram of compound 6n
45 carbolines) from the BHꢀacetates. This strategy has also been
successfully employed to the synthesis of bioactive compound
neocryptolepine 2.
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Union Road, Cambridge CB2 1EZ, UK for compounds 6n (CCDC #
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Acknowledgements
We thank the DST (New Delhi) for funding this project. D.M.R
thanks CSIR (New Delhi) for his research fellowship. We thank
the UGC (New Delhi) for support and providing some
instrumental facilities. We are grateful to the National Singleꢀ
Crystal Xꢀray and HRMS facility funded by the DST. We also
thank Professor S. Pal, School of Chemistry, University of
Hyderabad for helpful discussions regarding Xꢀray data analysis.
75
80
5
7
10
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