H8-BINOL chiral imidodiphosphoric acids catalyzed enantioselective synthesis of dihydroindolo-/-pyrrolo[1,2- a ]quinoxalines

Article abstract of DOI:10.1021/ol502965p

The first enantioselective synthesis of 5,6-dihydroindolo[1,2-a]quinoxalines is achieved by using a newly developed H₈-BINOL-type imidodiphosphoric acid catalyst with low catalyst loading through efficient Pictet-Spengler-type reactions of indolyl anilines with ketones. This methodology also generates phenyl-4,5-dihydropyrrolo[1,2-a]quinoxalines with high yields and excellent enantioselectivities. Moreover, this method was utilized to synthesize an HIV-1 inhibitor with high yield and good enantioselectivity through a one-step procedure.

Full text of DOI:10.1021/ol502965p

Letter
pubs.acs.org/OrgLett
H8‑BINOL Chiral Imidodiphosphoric Acids Catalyzed Enantioselective
Synthesis of Dihydroindolo-/-pyrrolo[1,2‑a]quinoxalines
Yan-Sen Fan,† Yi-Jun Jiang,*,† Dong An,† Di Sha,† Jon C. Antilla,‡ and Suoqin Zhang*,†
†College of Chemistry, Jilin University, 2699 Qianjin Street, Changchun 130012, China
‡Department of Chemistry, University of South Florida, 4202 East Fowler Avenue, CHE205A, Tampa, Florida 33620, United States
S
* Supporting Information
ABSTRACT: The first enantioselective synthesis of 5,6-dihydroindolo[1,2-a]quinoxalines is achieved by using a newly
developed H8-BINOL-type imidodiphosphoric acid catalyst with low catalyst loading through efficient Pictet−Spengler-type
reactions of indolyl anilines with ketones. This methodology also generates phenyl-4,5-dihydropyrrolo[1,2-a]quinoxalines with
high yields and excellent enantioselectivities. Moreover, this method was utilized to synthesize an HIV-1 inhibitor with high yield
and good enantioselectivity through a one-step procedure.
uinoxalines are relatively common nitrogen-containing
mitigate drug toxicity,9 the enantioselective synthesis of chiral
dihydroindolo-/-pyrrolo[1,2-a]quinoxalines is rare. Only Tian
and co-workers have reported the chiral boron Lewis acid-
catalyzed enantioselective synthesis of dihydropyrrolo[1,2-a]-
quinoxalines very recently.10 To the best of our knowledge, the
enantioselective synthesis of dihydroindolo[1,2-a]quinoxalines
remains unexplored. Thus, developing strategies toward highly
enantioselective synthesis of dihydroindolo-/-pyrrolo[1,2-a]-
quinoxalines is an unmet challenge.
1
Q heterocycles found in numerous pharmaceuticals.
Among them, dihydroindolo-/-pyrrolo[1,2-a]quinoxalines and
their derivatives exhibit a wide range of important physiological
and biological properties, including anticancer2 (Figure 1,
Chiral imidodiphosphoric acids, which are composed of two
chiral BINOL scaffolds, have shown impressive catalytic
efficiency and stereocontrolled ability in several highly
enantioselective transformations.11 However, the structural
diversity of chiral imidodiphosphoric acids has remained
undeveloped. As part of our ongoing interest in developing
new organocatalysts, we have reported VAPOL-type chiral
imidodiphosphoric acid in our previous work,11d which showed
complementary catalytic performance compared to BINOL type
catalysts. To fully exploit the great structure advantage of the
chiral imidodiphosphoric acids, introducing other fascinating C2-
symmetric scaffolds is a worthwhile endeavor. H8-BINOL
scaffolds, which were first reported by Cram and co-workers in
1978,12 have showed attractive chiral controlled ability in many
enantioselective transformations when applied as chiral phos-
phoric acids13 and metallic organophosphates.14 Inspired by the
above work, we developed H8-BINOL-type chiral imidodiphos-
Figure 1. Biologically active dihydroindolo-/-pyrrolo[1,2-a]-
quinoxalines and their derivatives.
compound A and D), anti-HIV3 (compound B), and polycystic
kidney disease inhibition4 (compound C) activities. The most
common approach for generating dihydroindolo-/-pyrrolo[1,2-
a]quinoxalines is an acid-catalyzed Pictet−Spengler-type reac-
tion,2b,5,6 which is more economical and less toxic when
compared with precious-metal-mediated cascade reactions7 and
N-arylation reactions.8 Although the demand for constructing
the absolute configuration of the pharmacological active
molecules is growing rapidly to maximize treatment effects or
Received: October 8, 2014
© XXXX American Chemical Society
A
dx.doi.org/10.1021/ol502965p | Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
phoric acid catalysts 1 (Figure 2) for the first time to further
expand the application scope of the newly emerging
Table 1. Optimization of the Reaction Conditions for
DHIQs
a
Figure 2. H8-BINOL-type imidodiphosphoric acids 1.
imidodiphosphoric acid catalyst. Herein we report the highly
enantioselective synthesis of quaternary carbon-centered 5,6-
dihydroindolo[1,2-a]quinoxalines (DHIQs) and phenyl-4,5-
dihydropyrrolo[1,2-a]quinoxalines (PDHPQs) via a Pictet−
Spengler-type reaction. The chemistry employs an H8-BINOL
type chiral imidodiphosphoric acid catalyst 1d and shows
excellent catalytic activity (up to 98% ee, 98% yield) and high
efficiency (as low as 2 mol %) in this capacity.
As shown in Figure 2, H8-BINOL-type chiral imidodiphos-
phoric acids 1a−d were synthesized successfully with modified
procedures reported in our previous work11c,d by using
corresponding H8-BINOL phosphoryl chlorides and phosphor-
amides.15 Unexpectedly, better chemoselectivities and higher
yields were observed through the entire synthetic routes
compared with the synthesis of BINOL type imidodiphosphoric
acid catalysts (see the Supporting Information).11c Optically
active imidodiphosphoric acids 1a−d were then tested for the
enantioselective synthesis of DHIQs and PDHPQs.
b
c
cat.
temp
yield
(%)
ee
entry
(mol %)
solvent
toluene
(°C)
time
(%)
1
1a (2)
1b (2)
1c (2)
1d (2)
5 (2)
rt
rt
12 h
12 h
12 h
12 h
12 h
12 h
2 d
61
73
69
96
87
90
52
95
95
89
94
96
34
37
33
83
75
8
2
toluene
toluene
toluene
toluene
toluene
diethyl ether
anisole
3
rt
4
rt
5
rt
6
6 (2)
rt
7
1d (2)
1d (2)
1d (5)
1d (1)
1d (2)
1d (2)
rt
85
89
89
87
91
93
8
rt
12 h
8 h
9
anisole
rt
10
11
12
anisole
rt
2 d
anisole
0
1 d
anisole
−20
2 d
a
Reactions were performed with 0.1 mmol of 2a, 0.1 mmol of 3a, 70
mg 5 Å MS, and 1 mL of solvent. Yield of the isolated product.
Determined by HPLC analysis on Chiralcel AD-H or OD-H columns.
b
c
Our preliminary studies revealed that equimolar ratios of
unprotected indolyl aniline 2a and activated ketone pyruvate 3a
were viable substrates for the Pictet−Spengler-type reaction to
generate enantiomeric excess of DHIQ product 4a containing a
quaternary stereogenic center in the presence of 2 mol % of
imidodiphosphoric acid catalysts 1. The 3,5-(CF3)2-phenyl
substituted H8-BINOL catalyst 1d gave higher yield and
enantioselectivity (96% yield, 83% ee, Table 1, entry 4)
compared with other phenyl- or naphthyl-substituted H8-
BINOL imidodiphosphoric acids (Table 1, entries 1−4). By
comparison, 3,5-(CF3)2-phenyl-substituted BINOL catalyst 5,
which was similar to H8-BINOL catalyst 1d in structure, was also
tested and found to be less effective and led to an apparent
decrease of stereochemical induction (Table 1, entry 5). It might
be because the dihedral angel of H8-BINOL scaffold is slightly
larger than that of the BINOL scaffold,16 which makes the chiral
environment of 1d more rigid and stereoselective than 5.
VAPOL-type imidodiphosphoric acid 6 was also evaluated under
the same conditions but gave inferior enantioselectivity (Table 1,
entry 6). Further screening of solvents showed that diethyl ether
could improve the stereoselectivity slightly but decrease the
reaction rate and yield dramatically (Table 1, entry 7). To our
delight, switching the solvent to anisole further enhanced both
the ee and yield (Table 1, entry 8). Increasing the catalyst loading
of 1d to 5 mol % did not improve the catalytic profile or
stereochemical outcome, while decreasing the catalyst loading to
1 mol % led to depressed chemo- and stereoselectivity (Table 1,
entries 9 and 10). The ee value was further increased as the
reaction temperature was lowered, although longer reaction time
was required (Table 1, entries 11 and 12). The best
enantioselectivity was achieved when the reaction was run at
−20 °C in anisole by using 2 mol % of catalyst 1d (Table 1, entry
12). In addition, molecular sieves were essential to this reaction
for the dehydration step.
Using the optimized reaction conditions (Table 1, entry 12),
the substrate scope was subsequently investigated. The synthesis
of DHIQs 4 from indolyl anilines 2 and activated ketones 3 was
shown to tolerate a variety of different substitution patterns. As
summarized in Scheme 1, substrate 2 with a methyl group on 3-
position of the indole moiety gave rise to DHIQ 4b in high yield
with excellent enantioselectivity (92% yield, 95% ee). Changing
ethyl pyruvate 3a to phenyl pyruvate 3b afforded DHIQ 4c with
even better enantioselectivity (98% ee). Substrate 2c with a
bromo-group on 4-position of the indole moiety and phenyl
pyruvate 3b provided the corresponding product 4d in 96% yield
and 94% ee, and its absolute configuration was further confirmed
by X-ray crystallography (Scheme 1).17 Substrate 2 bearing
either electron-donating, electron-withdrawing, or aryl groups on
the 5-position of the indole moiety reacted with either ethyl
pyruvate 3a or phenyl pyruvate 3b to furnish the chiral DHIQs
4e−j with high yields and enantioselectivities (74−97% yield,
84−93% ee). Modification of the aniline moiety on substrate 2
was also examined, and DHIQ 4k was obtained with the best
result in Scheme 1 (97% yield and 98% ee). Then we investigated
the tolerance for substrate 3. Benzyl, 2-naphthyl, or tert-butyl
pyruvates 3c−e were employed and smoothly underwent the
standard conditions to give DHIQs 4l−n with excellent yields
and enantioselectivities (78−92% yield, 92−96% ee). In addition
to pyruvates, butanedione was also used to give the
corresponding DHIQ 4o in high yield with excellent
enantioselectivity. Furthermore, this transformation could be
performed on gram scale (Scheme 1, 4c) without any loss of yield
(96%) but with slightly decreased enantioselectivity (97% ee).
Aldehydes were also screened for this reaction. However, only
moderate enantioselectivities were achieved in most cases.
B
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Organic Letters
Letter
a
work, we have constructed two kinds of PDHPQ frameworks
containing a crowded quaternary chiral carbon center by the
newly developed H8-BINOL imidodiphosphoric acid catalyst 1d
with low catalyst loading (2 mol %). The reaction conditions
were reoptimized, and the best result was obtained in 1,4-dioxane
at 15 °C. When benzyl pyruvate 3c was selected as the activated
ketone, substrate 7 with a range of aryl groups on the 2- or 3-
position of pyrrole ring reacted smoothly to give 3- or 5-
PDHPQs 8 with high yields and enantioselectivities. The C-5
position of the pyrrole ring in 2-phenylpyrrolyl substrates 7a−g
attacked their intramolecular imine intermediates and produced
5-PDHPQ 8a−g as the products. The introduction of electron-
donating groups on the phenyl moiety was found to slightly
decrease the ee value (Scheme 3, 8b and 8c). On the other hand,
Scheme 1. Pictet−Spengler−Type Reaction for DHIQs
Scheme 3. Pictet−Spengler-Type Reaction for PDHPQs and
a
NDHPQs
a
Reactions were performed with 0.1 mmol of 2, 0.1 mmol of 3, 70 mg
5 Å MS, and 2 mol % of catalyst 1d in 1 mL of anisole at −20 °C.
Isolated yields. The ee value was determined by HPLC analysis on
b
Chiralcel AD-H or OD-H column. 1.11 g of product (3.02 mmol,
97% ee, 96% yield) was obtained when the gram-scale reaction was
c
d
carried out. Reactions were run at 5 °C. Reactions were run at 25
°C.
a
Reactions were performed with 0.1 mmol of 7, 0.1 mmol of 3c, 70
Fortunately, when methylindolyl aniline 2b and furaldehyde 3f
were employed as the substrates, DHIQs 4p (compound B in
Figure 1) was obtained with high yield and good enantiose-
lectivity (Scheme 2). Notably, DHIQ 4p has recently been
discovered as a new HIV-1 inhibitor in vitro, which showed
promising anti-HIV-1 activities.3
mg of 5 Å MS, and 2 mol % of catalyst 1d in 1 mL of 1,4-dioxane at 15
°C. Isolated yields. The ee value was determined by HPLC analysis on
b
a Chiralcel AD-H or OD-H column. 1.05 g of product (2.45 mmol,
98% ee, 96% yield) was obtained when the gram-scale reaction was
carried out.
the presence of electron-withdrawing substituents on the phenyl
moiety were favored, and 5-PDHPQs 8d and 8e were formed
with excellent yields (94−95%) and enantioselectivities (94−
97% ee). 1-Naphthyl- and 2-naphthyl-substituted substrates
could also give satisfying results and 5-naphthyl-4,5-
dihydropyrrolo[1,2- a]quinoxalines (5-NDHPQs) 8f and 8g
were obtained with excellent yields (89−97%) and enantiose-
lectivities (91−92% ee). 3-Phenylpyrrolyl substrates 7h and 7i
were also tested, and the more crowded C-2 position of the
pyrrole ring attacked their intramolecular imine intermediates
and produced 3-PDHPQ 8h and 8i as the main products with
excellent yields and remarkable enantioselectivities (94% yield
and 98% ee in both cases). The absolute configuration of the
optically active 8i was established by single-crystal X-ray
structure analysis (Scheme 3).18 Moreover, 8i could also be
obtained through gram-scale reaction without any loss of chemo-
and stereoselectivity (Scheme 3).
Scheme 2. One-Step Enantioselective Synthesis of HIV-1
Inhibitor 4p
Encouraged by successful enantioselective synthesis of chiral
DHIQs 4, we next turned our attention to enantioselective
synthesis of chiral PDHPQs. Although similar work has been
reported by Tian and co-workers,10 the substrates with
substituents on the pyrrole ring were not investigated and only
tertiary chiral carbon centers were constructed in their products.
In addition, their catalyst loading was high (10 mol %). In our
C
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Organic Letters
Letter
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In conclusion, we have developed H8-BINOL chiral
imidodiphosphoric acids, which enabled the highly efficient
and enantioselective synthesis of quaternary carbon-centered
DHIQs and PDHPQs with low catalyst loading. These two
Pictet−Spengler-type reactions were both amenable to gram
scale, which provided practical and convenient ways for the
further biological activity studies of DHIQs and PDHPQs. In
addition, a straightforward one-step procedure has been
developed for the rapid synthesis of promising HIV-1 inhibitor
DHIQ 4p with high yield and good enantioselectivity. Further
applications of H8-BINOL chiral imidodiphosphoric acid
catalysts and the transformations mentioned above are currently
being investigated.
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(17) CCDC 1008679 (compound 4d) contains the supplementary
crystallographic data for this paper. These data can be obtained free of
charge from the Cambridge Crystallographic Data Centre via www.ccdc.
cam.ac.uk/data_request/cif.
(18) CCDC 1011722 (compound 8i) contains the supplementary
crystallographic data for this paper. These data can be obtained free of
charge from the Cambridge Crystallographic Data Centre via www.ccdc.
cam.ac.uk/data_request/cif.
ASSOCIATED CONTENT
* Supporting Information
Experimental procedures and characterization data for all new
compounds and CIF files for compounds 4d and 8i. This
material is available free of charge via the Internet at http://pubs.
acs.org.
■
S
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: jiangyijun@jlu.edu.cn.
*E-mail: suoqin@jlu.edu.cn.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the National Natural Science Foundation of China
(21202059), the China Postdoctoral Science Foundation
(2013M541287), and the Jilin Province Science & Technology
Development Program (20100538, 20110436, 201215033) for
financial support.
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