Iodine-Promoted N-H/α,β-C(sp3)-Trifunctionalization of l -Proline: Access to 3,4-Dihydrobenzo[ b][1,7]naphthyridines via Consecutive Decarboxylation/Ring Opening/Dicyclization

Article abstract of DOI:10.1021/acs.orglett.9b01277

A N-H/α,β-C(sp³)-trifunctionalization of l-proline, proceeding through an iodine-promoted consecutive decarboxylation/ring-opening/dicyclization process, is achieved. This strategy affords structurally diverse fused N-heterocycles in good yields with a wide substrate scope. Preliminary mechanistic studies indicate that catabolism of l-proline might be involved in this cascade reaction and the in situ generated intermediate 4-aminobutanal was identified as the key intermediate. Notably, this domino strategy enriches the reactivity of versatile l-proline in the synthesis of fused heterocycles.

Full text of DOI:10.1021/acs.orglett.9b01277

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Iodine-Promoted N−H/α,β-C(sp3)‑Trifunctionalization of L‑Proline:
Access to 3,4-Dihydrobenzo[b][1,7]naphthyridines via Consecutive
Decarboxylation/Ring Opening/Dicyclization
Xiao Geng,† Can Wang,† Peng Zhao, You Zhou, Yan-Dong Wu,* and An-Xin Wu*
Key Laboratory of Pesticide & Chemical Biology, Ministry of Education, College of Chemistry, Central China Normal University,
Wuhan 430079, P. R. China
S
* Supporting Information
ABSTRACT: A N−H/α,β-C(sp3)-trifunctionalization of L-proline, proceeding through an iodine-promoted consecutive
decarboxylation/ring-opening/dicyclization process, is achieved. This strategy affords structurally diverse fused N-heterocycles
in good yields with a wide substrate scope. Preliminary mechanistic studies indicate that catabolism of L-proline might be
involved in this cascade reaction and the in situ generated intermediate 4-aminobutanal was identified as the key intermediate.
Notably, this domino strategy enriches the reactivity of versatile L-proline in the synthesis of fused heterocycles.
Scheme 1. Ring-Opening of L-Proline
L-Proline and its derivatives are abundant biologically active
natural products1 and have been recognized as a versatile
catalyst2 as well as important synthons3−6 in organic synthesis.
Many organic chemists have reported the preparation of
diverse pyrrolo-based compounds via decarboxylative annula-
tions3 or decarboxylation−coupling of L-proline.4 More
importantly, studies employing L-proline as a chain synthon
via ring-opening reactions have been reported, but remain
rare.5,6 For example, Grigg and co-workers5a disclosed an
interesting example of the N−H/α-C-difunctionalization of L-
proline giving 1-azacyclooctadiene via a decarboxylative ring-
opening reaction under mild conditions (Scheme 1a).
Recently, the Yan group5b disclosed an appealing multi-
component reaction for the construction of unique eight
polyheterocyclic compounds. Notably, L-proline and isatin
rings were both opened in this reaction. Unfortunately, only
five compounds have been reported (Scheme 1b). Further-
more, Kundu’s group6 disclosed a novel decarboxylative
cyclization/ring expansion N−H/α-C-difunctionalization re-
action for the construction of various fused-heterocycles from
preprepared 2-alkynyl benzaldehydes and L-proline without
using any additives (Scheme 1c). Although these studies
provide straightforward and efficient processes for the
construction of fused heterocyclic compounds, they have
focused on the N−H/α-C-difunctionalization of L-proline via
ring-opening processes. However, methods for the N−H/α,β-
C(sp3)-trifunctionalization of L-proline to construct fused
heterocycles via a ring-opening process remain unexploited.
Therefore, it is a challenging and meaningful goal in the area of
organic synthesis.
Multicomponent bicyclization reactions (MBRs), represent-
ing an unmatched opportunity to expand the structural
diversity and molecular complexity of desired products, are
regarded to be among the most powerful and reliable tools for
the synthesis of fused heterocyclic scaffolds.7 Although, several
excellent MBR studies have been reported,8 the development
Received: April 12, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b01277
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a b
,
of a new convenient and environmentally friendly MBR from
commercially available and simple substrates is urgent and
significant in organic synthesis. As part of our continuing
research interest in the preparation of diverse heterocycles,9 we
now report iodine-promoted MBRs for the preparation of 3,4-
dihydrobenzo[b][1,7]naphthyridines from commercially avail-
able aryl methyl ketones, arylamines, and L-proline via a
decarboxylation/ring-opening/dicyclization process under
mild conditions (Scheme 1d).
Scheme 2. Scope of Aryl Methyl Ketones
Initially, acetophenone (1a), p-toluidine (2a), and L-proline
(3) were reacted in the presence of I2 in DMSO at 90 °C. As
our first attempt, we were happy to notice that product 4a was
obtained in 72% yield (Table 1, entry 1), with the structure of
Table 1. Representative Optimization of the Dicyclization
a
Reaction
a
entry
I2 (equiv)
3 (equiv)
temp (°C)
additive
yield (%)
1
2
3
4
5
6
7
8
1.6
1.6
1.6
1.6
1.6
1.6
0.1
0.5
1.0
2.0
1.6
1.6
1.6
1.6
1.6
1.0
1.0
1.0
1.0
1.5
2.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
90
72
76
74
69
71
68
trace
27
65
72
77
75
72
75
71
100
110
120
100
100
100
100
100
100
100
100
100
100
100
a
b
1.0 mmol scale. Isolated yield of products 4.
9
10
11
12
13
14
15
oxone
TBHP
TFA
HI
subatrates were also reactive and provided fused heterocycle
products in moderate yields (34−50%, 4p−4r). Gratifyingly,
heteroaryl ketones were tolerated under the optimal
conditions, leading to the fused heterocycles products (72%
and 61%, 4s and 4t).
HAc
a
Reaction conditions: 1a (0.5 mmol), 2a (0.5 mmol), 3 (x mmol), I2
To further examine the generality of this domino process,
various arylamines were employed in this iodine-promoted
dicyclization reaction, with the results shown in Scheme 3.
Arylamines with electron-rich (−OMe, −OEt, −C(CH3)3,
−CH(CH3)2), halogen (F, Cl, Br, I), or electron-deficient
(−CO2Et) substituents at the para-position exhibited moder-
ate to good reactivity under the optimized conditions (57−
79%, 5a−5i). Furthermore, a range of polysubstituted
arylamines (3,5-dimethyl, 2,3-dimethyl, 3,4,5-trimethoxyl)
successfully underwent the dicyclization process to deliver
the desired products (55−67%, 5j−5l). Unfortunately, other
α-amino acids were not compatible with this trifunctionaliza-
tion process. We assume that the desired products are unstable
or the in situ generated intermediates of thiazolidine-4-
carboxylic acid and 4-hydroxypyrrolidine-2-carboxylic acid
might not be compatible in the first cyclization reaction. As
for piperidine-2-carboxylic acid, the desired product can be
detected by GC-MS in a trace amount.
(x mmol), additive (1.0 equiv), indicated temperature, DMSO 2 mL,
2 h, unless otherwise noted. Isolated yields.
b
4a confirmed by X-ray crystallographic analysis (see Support-
ing Information). Even though the initial yield was acceptable,
we aimed for further improvements. First, various temperatures
were screened for this consecutive decarboxylation/ring-
opening/dicyclization process, with 100 °C affording the best
yield (Table 1, entries 1−4). Increasing the amount of L-
proline led to a decreased yield of 4a (Table 1, entries 5 and
6). Next, the effect of the iodine amount was investigated, with
1.6 equiv of iodine giving the best yield (Table 1, entries 7−
10). Finally, diverse additives were tested, but no significant
improvements were obtained (Table 1, entries 11−15).
With the optimized conditions in hand, we sought to
investigate different substituents on the benzene ring of aryl
methyl ketone. As shown in Scheme 2, substrates containing
electron-rich substituents on the aromatic ring were smoothly
executed giving the dicyclization products in satisfactory yields
(53−79%, 4a−4g). Halogen functional groups F, Cl, Br, also
yielded the desired compounds in satisfied yields (61−77%,
4h−4m). Notably, 1-naphthyl or 2-naphthyl methyl ketones
could react in this reaction, affording products (4n, 4o) in 67%
and 52% yield, respectively. Moreover, electron-deficient
Further demonstrating the synthetic practicality of this
dicyclization process, a 10 mmol scale synthesis of 7-methyl-1-
phenyl-3,4-dihydrobenzo[b][1,7]naphthyridine (4a) was car-
ried out, giving 4a in 54% yield (Scheme 4a). Next, late
oxidative aromatization was conducted on 4a to obtain a 7-
methyl-1-phenylbenzo[b][1,7]naphthyridine compound (7a)
in 87% yield (Scheme 4b).
B
DOI: 10.1021/acs.orglett.9b01277
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a b
,
Scheme 3. Scope of Anilines
Scheme 5. Control Experiments
cyclization. Fortunately, when 3b was used as a surrogate of 4-
aminobutanal, the desired product was obtained in 74% yield
(Scheme 5e). To further verify this hypothesis, desired product
4a was formed with a yield of 79%, when 3c was mixed with 1a
and 2a under standard conditions (Scheme 5f).11
a
b
1.0 mmol scale. Isolated yield of products 5.
Scheme 4. Gram-Scale Experiments and Late Oxidation
Aromatization
On the basis of the above experimental investigation and
previous reports,12 a plausible mechanism for the construction
of 3,4-dihydrobenzo[b][1,7]naphthyridines was proposed, as
shown in Scheme 6. The transformation begins with the
formation of phenylglyoxal 1ab via iodination and Kornblum
oxidation. Intermediate 1ab then undergoes a dehydration
reaction with p-toluidine 2a to produce imine intermediate A.
Meanwhile, a cascade reaction (L-proline catabolism) proceeds
through an I2-triggered sequential decarboxylation, oxidation,
and hydrolysis reaction to give ring-opening product D
To gain some insight into the plausible mechanism for this
novel consecutive decarboxylation/ring-opening/dicyclization
process, a set of control experiments were conducted. Initially,
phenylglyoxal 1ab and hydrated species 1ac were obtained in
quantitative yield when acetophenone (1a, 1.0 mmol) was
heated with I2 (1.6 mmol) in DMSO at 100 °C (Scheme 5a).
Next, when acetophenone 1a was replaced with hydrated
species 1ac for the reaction with 2a and 3 under the optimized
conditions, this reaction proceeded smoothly, but without any
dicyclization products 4a obtained in the absence of I2
(Scheme 5b). As expected, desired product 5a was obtained
in 88% yield when C-acylimine 6a and 3 were mixed under
standard conditions (Scheme 5c). These results confirmed that
hydrated species 1ac and C-acylimine 6a were intermediates in
this domino reaction and highlighted the vital role of iodine in
this multicomponent cascade reaction. However, 3a did not
react smoothly with substrates 1a and 2a to afford product 4a
(Scheme 5d). This result implied that 3a might not be an
intermediate in this cascade dicyclization reaction. Inspired by
our previous work,10 we speculated that L-proline might
initially undergo catabolism to generate chain intermediate 4-
aminobutanal, which would then participate in the cascade
Scheme 6. Proposed Mechanism
C
DOI: 10.1021/acs.orglett.9b01277
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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(deceted by GC-MS; see the Supporting Information), which
then reacts with 2a to generate enamine intermediate E
through dehydration and tautomerization processes. In situ
generated intermediate E and A undergo a Povarov-type
reaction to afford intermediate B, which then undergoes
oxidative aromatization and deamination12a,c to form C.
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dehydration condensation.
In summary, we have developed an iodine-promoted cascade
strategy for the synthesis of 3,4-dihydrobenzo[b][1,7]-
naphthyridines. This represents the first example of a N−H/
α,β-C(sp3)-trifunctionalization of L-proline via a consecutive
decarboxylation/ring-opening/dicyclization process. Further-
more, this novel strategy enriches the reactivity of L-proline.
Notably, this study reports an excellent multicomponent
bicyclization reaction (MBR) under mild conditions using
commercially available starting materials. Further studies
toward applications of such N−H/α,β-C(sp3)-trifunctionaliza-
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ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b01277.
Experimental procedures, product characterizations,
1
crystallographic data for 4a, and copies of the H and
13C NMR spectra (PDF)
Accession Codes
(5) (a) Ardill, H.; Grigg, R.; Sridharan, V.; Malone, J. J. Chem. Soc.,
Chem. Commun. 1987, 0, 1296−1298. (b) Cao, J.; Yang, F.; Sun, J.;
Huang, Y.; Yan, C. G. J. Org. Chem. 2019, 84, 622−635.
CCDC 1901642 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
(6) Samala, S.; Singh, G.; Kumar, R.; Ampapathi, R. S.; Kundu, B.
Angew. Chem., Int. Ed. 2015, 54, 9564−9567.
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AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: chwuyd@mail.ccnu.edu.cn.
*E-mail: chwuax@mail.ccnu.edu.cn.
́
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2012, 41, 3969−4009. (f) Estevez, V.; Villacampa, M.; Menendez, J.
C. Chem. Soc. Rev. 2014, 43, 4633−4657.
ORCID
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(d) Rajarathinam, B.; Vasuki, G. Org. Lett. 2012, 14, 5204−5206.
(e) Feng, X.; Wang, J. J.; Xun, Z.; Zhang, J. J.; Huang, Z. B.; Shi, D. Q.
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An-Xin Wu: 0000-0001-7673-210X
Author Contributions
†X.G. and C.W. contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the National Natural Science
Foundation of China (Grants 21472056, 21602070, and
21772051). This work was supported by “The Fundamental
Research Funds for the Central Universities” (CCNU15Z-
X002and CCNU18QN011). This work was also supported by
the 111 Project B17019.
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