Enantioselective Synthesis of N-PMP-1,2-dihydropyridines via Formal [4 + 2] Cycloaddition between Aqueous Glutaraldehyde and Imines

Article abstract of DOI:10.1021/acs.orglett.5b02744

A simple and highly practical one-pot formal [4 + 2] cycloaddition approach for the enantioselective synthesis of N-PMP-1,2-dihydropyridines (DHPs) is described. This chemistry involves an amino-catalytic direct Mannich reaction/cyclization followed by IBX-mediated chemo- and regioselective oxidation sequence between readily available aqueous glutaraldehyde and imines under very mild conditions. A series of N-PMP-1,2-DHPs have been prepared in high yields and excellent enantioselectivity. This method also gives access to both enantiomers of 1,2-DHPs in surplus amount by shifting the catalyst configuration.

Full text of DOI:10.1021/acs.orglett.5b02744

Letter
pubs.acs.org/OrgLett
Enantioselective Synthesis of N‑PMP-1,2-dihydropyridines via Formal
[4 + 2] Cycloaddition between Aqueous Glutaraldehyde and Imines
Panduga Ramaraju,† Nisar A. Mir,† Deepika Singh,‡ Vivek K. Gupta,§ Rajni Kant,§ and Indresh Kumar*,†
†Department of Chemistry, Birla Institute of Technology and Science, Pilani 333 031, Rajasthan India
‡Instrumentation Division, IIIM-CSIR Laboratory, Jammu 180 001, Jammu and Kashmir, India
§X-ray Crystallography Laboratory, Post-Graduate Department of Physics & Electronics, University of Jammu, Jammu 180 006,
Jammu and Kashmir, India
S
* Supporting Information
ABSTRACT: A simple and highly practical one-pot formal [4 + 2]
cycloaddition approach for the enantioselective synthesis of N-PMP-
1,2-dihydropyridines (DHPs) is described. This chemistry involves an
amino-catalytic direct Mannich reaction/cyclization followed by IBX-
mediated chemo- and regioselective oxidation sequence between
readily available aqueous glutaraldehyde and imines under very mild
conditions. A series of N-PMP-1,2-DHPs have been prepared in high
yields and excellent enantioselectivity. This method also gives access
to both enantiomers of 1,2-DHPs in surplus amount by shifting the catalyst configuration.
ihydropyridines (DHPs) are frequently encountered in
natural and synthetic compounds that possess many
Scheme 1. Involvement of Imines in Cycloaddition
Approaches to 1,2-DHPs
D
interesting biological activities.1 In particular, 1,2-DHPs are
important building blocks to synthesize a wide range of organic
molecules, such as piperidines, pyridines, indolizidines, and
quinolizidines.2 This structural unit has also been considered as
a suitable substrate to prepare isoquinuclidines,3 an important
structural motif for a variety of complex natural products.4
Owing to the high synthetic and biological importance of 1,2-
DHPs, a number of methods have been developed for their
synthesis.5 More strictly, these synthetic efforts can be broadly
divided into two main categories: (i) Nucleophilic addition to
activated pyridines (path 1, Figure 1),6 and (ii) 6π-electro-
Figure 1. General routes to 1,2-dihydropyridines.
cyclization of 1-azatrienes (path 2, Figure 1).7 Alternatively,
methods involving imines as suitable unsaturated partners to
synthesize 1,2-DHPs have had some success.8 Initially, the
Ogoshi group developed a metal-catalyzed [2 + 2 + 2]
cycloaddition reaction between alkynes and N-sulfonyl-8a,b or
N-arylimines.8c A similar cycloaddition was independently
explored by Yoshikai and co-workers using N-pyridylimines.8d
These approaches were mainly restricted to produce racemic
1,2-DHPs (eq 1, Scheme 1). Recently, Gandon and co-workers
developed the first asymmetric variant of this metal-catalyzed [2
+ 2 + 2] cycloaddition between diynes and N-sulfonimines (eq
2, Scheme 1).9 Previously, a very few graceful asymmetric
syntheses for 1,2-DHPs were reported, although they possess
experimental limitations like tedious preactivation.10 Despite
these efforts, the development of a simple and efficient
enantioselective method to access 1,2-DHPs presents a difficult
task and is still in high demand. Herein, we report a
straightforward one-pot strategy for the enantioselective
synthesis of N-PMP-1,2-DHPs (PMP = p-OMeC6H4) from
inexpensive materials with high yields and enantioselectivity
Received: September 22, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.5b02744
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
under an organocatalytic system (eq 3, Scheme 1). To the best
of our knowledge, there is no report on the organocatalytic
approach for 1,2-DHPs in asymmetric fashion.
dehydrogenate carbonyls into the corresponding α,β-unsatu-
rated carbonyls.15 Here, the exclusive production of 1,2-DHPs
5 could be justified through chemo- and regioselective
oxidation of intermediate 4 by using bulky IBX at a less
substituted allylic position C4 (path a, Scheme 2). While
oxidation at the more substituted position C2 (path b, Scheme
2) was not observed, enhancement in the yield (87%) and
enantioselectivity (93:7 er) was observed when amino-catalyzed
direct Mannich/cyclization sequence was carried out at 10 °C
followed by in situ IBX oxidation at 40 °C. Further decreasing
the reaction temperature during IBX oxidation (entry 5 and 6,
Table 1) and catalyst loading (entry 7, Table 1) led to extended
reaction time with reduced yields without variation in
enantioselectivity. Thus, we preferred to perform this one-
pot, two-step sequence to N-PMP-1,2-DHPs 5 with optimized
conditions (entry 4, Table 1).
Conversely, organocatalysis is considered an important eco-
friendly toolbox to develop several asymmetric, nonasymmetric
transformations and also to contribute proficient one-pot
cascade processes.11 Recently, linear dialdehydes have been
established as suitable bifunctionalized substrates for amino-
catalyzed cascade transformations to synthesize important
bioactive carbo- and heterocyclic compounds.12 In this context,
Xu and co-workers have utilized glutaraldehyde 2 and imines 3
for enantioselective tetrahydropyridines (THPs),13 while chiral
piperidines were synthesized through in situ reduction of THPs
4 by our group, independently.14 We further envisioned that in
situ site-selective oxidation of THP compound 4 could lead to
the synthesis of DHPs (Scheme 2). Interestingly, the oxidation
The scope of this method was then investigated to confirm
its robustness with regard to a variety of preformed imines 3
under optimized conditions, and the results are summarized in
Table 2. The reaction proceeded well with good yields and
enantioselectivity in almost all cases when electron-withdrawing
groups (EWG) (e.g., −NO2, −F, −Cl, −Br, and −CN) were
substituted at the ortho-, meta-, or para-positions on the
arylimines (entries 1−16, Table 2). However, the reactions
were rather slow in the case of arylimines substituted at the
Scheme 2. IBX-Mediated Chemo- and Regioselective
Dehydrogenative Oxidation of THP 4
Table 2. Substrate Scope for Formal [4 + 2] Cycloaddition
of 4 could occur at two different places, which could
subsequently give 1,2-DHPs 5 (path a, Scheme 2) or 1,4-
DHPs 6 (path b, Scheme 2). Keeping this idea in mind, we
quickly established the reaction conditions for one-pot
asymmetric synthesis of 1,2-DHPs as shown in Table 1.
a
b
c
d
entry
R
time (h)
5
yield (%)
er
1
2-NO2C6H4
3-NO2C6H4
4-NO2C6H4
2-FC6H4
9
8
5a
5b
5c
5d
5e
5f
68
83
87
69
71
77
68
70
73
68
74
79
72
70
72
70
66
67
71
73
67
71
87
nr
99:1
98:2
93:7
96:4
93:7
88:12
96:4
95:5
90:10
98:2
93:7
91:9
95:5
97:3
84:16
94:6
88:12
94:6
84:16
92:8
88:12
87:13
>99:1
nd
Table 1. Optimization of Reaction Conditions
2
3
6
4
9
5
3-FC6H4
8
6
4-FC6H4
8
7
2-ClC6H4
3-ClC6H4
4-ClC6H4
2-BrC6H4
3-BrC6H4
4-BrC6H4
3-Br-4-FC6H3
3,4-Cl2C6H3
3-CNC6H4
4-CNC6H4
Ph
9
5g
5h
5i
a
b
c
8
8
entry
conditions
yield (%)
er
9
8
1
2
3
4
5
6
DMSO, rt, 5 h/DDQ, rt, 12 h
DMSO, rt, 5 h/SeO2, rt, 6 h
10
11
12
13
14
15
16
17
18
19
20
21
22
10
9
5j
5k
5l
DMSO, rt, 5 h/IBX, 70 °C, 3 h
DMSO, 10 °C, 6 h/IBX, 40 °C, 4 h
DMSO, 10 °C, 6 h/IBX, rt, 6 h
DMSO, 10 °C, 6 h/IBX, 10 °C, 9 h
DMSO, 10 °C, 12 h/IBX, 40 °C, 4 h
76
87
76
65
58
88:12
93:7
93:7
93:7
93:7
9
8
5m
5n
5o
5p
5q
5r
8
9
d
7
9
a
Unless otherwise indicated, the reaction was carried out with (i) 3c
9
(0.3 mmol), 2 (25% aqueous sol., 0.9 mmol), L-proline 1 (20 mol %),
2-pyridyl
8
b
solvent (3.0 mL); (ii) oxidant (120 mol %). Isolated yield of 5 refers
c
d
3-pyridyl
9
5s
5t
to 3c. Determined using stationary chiral columns. Catalyst 1 (10
4-pyridyl
8
mol %).
2-thiophene
2-(5-NO2)furan
CO2Et
9
5u
5v
5w
5x
8
Our initial attempts failed to give any DHPs (5 or 6) through
in situ oxidation of THP intermediate 4 in DMSO (entries 1
and 2, Table 1). Gratifyingly, N-PMP-1,2-DHP 5c was obtained
as the sole product with good yield (76%) and enantiose-
lectivity (88:12 er) when IBX (2-Iodoxybenzoic acid) was used
as oxidizing agent at 70 °C in the same flask (entry 3, Table 1).
This one-pot transformation was made feasible by taking
advantage of IBX solubility in DMSO as well as the ability to
e
23
6
24
4-OMeC6H4
20
a
Unless otherwise indicated, the reaction was carried out with (i) 3
(0.3 mmol), 2 (25% aqueous sol., 0.9 mmol), L-proline 1 (20 mol %),
DMSO (3.0 mL); (ii) IBX (120 mol %), 40 °C, 4 h. Time for direct
Mannich/cyclization reaction. Isolated yield. Determined using
b
c
d
e
stationary chiral columns. Reaction was carried out without water.
B
DOI: 10.1021/acs.orglett.5b02744
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Letter
ortho-position (entries 1, 4, 7, and 10, Table 2), possibly due to
steric effects. A similar result was obtained when imine derived
from benzaldehyde was employed (entry 17, Table 2).
Pleasingly, heteroaromatic imines also gave the corresponding
products 5r−v in good yields and enantioselectivity (entries
18−22, Table 2) under these optimized conditions. A clean
transformation to highly functionalized N-PMP-1,2-DHP 5w
was observed with high yields (90%) and excellent selectivities
(>99:1 er) when activated imine 3w was utilized without water
(entry 23, Table 2). This reaction is limitated to electron-
deficient imines as the reaction failed in the case of
electronically rich arylimine (entry 24, Table 2).
Scheme 4. Synthetic Applications
Next, we decided to prepare N-PMP-1,2-DHPs in a
reasonably good amount and in stable form. The presence of
a −CHO group at C3 provides stabilization, as electron-
withdrawing substituents stabilize DHPs. To fulfill the first
objective and to demonstrate the practical utility of our
protocol, we prepared both enantiomers of 1,2-DHP at gram
scale by utilizing both L- and D-proline as catalysts. While a
somewhat longer reaction time was required, 5c and ent-5c
were obtained without much reduction in yields and with the
same selectivity from 3c (1.0 g) as shown in Scheme 3. Easy
Scheme 3. Gram-Scale Synthesis of Both Enantiomers of N-
PMP-1,2-DHP (5c and ent-5c)
group using acidic NaBH4 in 85% yield. Next, the alcohol group
was converted into −OTs under standard conditions, and
catalytic hydrogenation using Pd/C and (H2)(g) was performed
with crude material without purification. The reduction of
−NO2 and alkene groups takes place under these conditions as
expected. Interestingly, intramolecular cyclization of in situ
generated amine furnished polycyclic alkaloid-type product 8
having syn-stereochemistry at two chiral centers in the same pot
(see the SI). Our initial efforts to convert 5a directly to 8
through one-pot reduction of all alkene bonds and a −NO2
group, followed by reductive cyclization with aldehyde under
different reaction conditions, failed. After PMP removal from 8,
a fused product 9 was obtained that contained tetrahydroquino-
line and piperidines moieties. These units are key elements
found in numerous biologically active natural products and
synthetic pharmaceuticals.17 N-PMP deprotection from 1,2-
DHP 5 failed, while this deprotection was successfully
demonstrated from N-PMP-tetrahydropyridine 10 to highly
functionalized tetrahydropyridine 11 as shown in Scheme 4 (b).
This compound can be further transformed to various
piperidine derivatives.
availability of starting materials, glutaraldehyde 2, and imines 3
and metal-free access to both enantiomers of N-PMP-1,2-DHP
make this approach quite practical and attractive.
Single-crystal X-ray study of 5b and 5w further confirmed the
stereochemical outcome at C2 (Figure 2), as expected through
In conclusion, we have developed an operationally simple,
metal-free enantioselective synthesis of N-PMP-1,2-dihydropyr-
idines via one-pot formal [4 + 2] cycloaddition between readily
available aldimines and aqueous glutaraldehyde. This reaction
proceeds through proline-catalyzed direct Mannich/cyclization,
followed by an IBX-mediated site-selective dehydrogenative−
oxidation sequence with high yields and selectivity. The
viability of this method was established through (i) gram-
scale synthesis of both enantiomers of N-PMP-1,2-DHPs and
(ii) quick synthesis of fused chiral tetrahydroquinoline-based
important alkaloid skeleton. This novel strategy sets the stage
for the synthesis of chiral isoquinuclidines and their
applications in natural product synthesis. Further study in
this direction is in progress.
Figure 2. Single-crystal X-ray analysis of 5b and 5w.
L-proline 1 catalyzed syn-Mannich reaction16/intramolecular
cyclization/oxidation sequence. The absolute stereochemistry
of all other products was assigned through analogy. A complete
plausible mechanism has been proposed to rationalize the high
stereochemical outcome of this transformation, as shown in the
Supporting Information.
In order to show the synthetic application of our strategy, we
quickly converted compound 5a into fused polycyclic
compound 8 using the sequence of synthetic transformation
as shown in Scheme 4 (a). Initially, 5a was reduced to
tetrahydropyridine (THP) 7 constituted with an allylic alcohol
C
DOI: 10.1021/acs.orglett.5b02744
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́
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.5b02744.
■
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Experimental procedures, spectra, and X-ray data for
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AUTHOR INFORMATION
Corresponding Author
■
*E-mail: indresh.chemistry@gmail.com, indresh.kumar@pilani.
bits-pilani.ac.in.
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was made possible by support from DST−New
Delhi and BITS Pilani. P.R. thanks UGC−New Delhi for a
Senior Research Fellowship.
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