Sequencing [4 + 1]-Cycloaddition and Aza-Michael Addition Reactions: A Diastereoselective Cascade for the Rapid Access of Pyrido[2′,1′:2,3]/Thiazolo[2′,3′:2,3]imidazo[1,5- a]quinolone Scaffolds as Potential Antibacterial and Anticancer Motifs

Article abstract of DOI:10.1021/acs.joc.9b01919

The design and synthesis of a quality compound library containing a small number of skeletally diverse scaffolds, whose members rapidly deliver new chemical probes active against multiple phenotypes, is paramount in drug discovery. In this context, an eff

Full text of DOI:10.1021/acs.joc.9b01919

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Article
Sequencing [4+1]-Cycloaddition and Aza-Michael Addition
Reactions: A Diastereoselective Cascade for the Rapid Access
of Pyrido[2',1':2,3]/Thiazolo[2',3':2,3]imidazo[1,5-a]quinolone
Scaffolds as Potential Antibacterial and Anticancer Motifs
Vunnam Srinivasulu, Monther A. A Khanfar, Hany A. Omar, Raafat ElAwady, Scott McN. Sieburth,
Anusha Sebastian, Dana M. Zaher, Farah Al Marzooq, Fatema Hersi, and Taleb H. Al-Tel
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.9b01919 • Publication Date (Web): 21 Oct 2019
Downloaded from pubs.acs.org on October 21, 2019
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Sequencing [4+1]-Cycloaddition and Aza-Michael Addition Reactions: A
Diastereoselective
Cascade
for
the
Rapid
Access
of
as
Pyrido[2',1':2,3]/Thiazolo[2',3':2,3]imidazo[1,5-a]quinolone
Potential Antibacterial and Anticancer Motifs
Scaffolds
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†
‡
†,£
†, £
Vunnam Srinivasulu, Monther Khanfar, Hany A. Omar, Raafat ElAwady, Scott McN
§
†
†
†
†
Sieburth, Anusha Sebastian, Dana M. Zaher, Farah Al-Marzooq, Fatema Hersi and Taleb H.
Al-Tel*†,£
†Sharjah Institute for Medical Research, University of Sharjah, P.O. Box 27272, Sharjah, UAE
‡Department of Chemistry, University of Jordan, 11942, Amman, Jordan
£College of Pharmacy, University of Sharjah, P.O. Box 27272, Sharjah, UAE
§Temple University, Department of Chemistry, 201 Beury Hall, Philadelphia, PA 19122, US
taltal@sharjah.ac.ae
One-Pot
[4+1]-Cycloaddition/Aza-Michael addition
O
O
O
Y
S
N
H
H
S
N
NH2
Y
O
N
NH2
N
N
N
X
X
X
H
N
N
=
CH or O
NH
X
NH
2
3
CF SO3
CF SO
3
3
Sc(OTf)
3
Sc(OTf)
3
Atom-, Step-Economic, Wide Substrate Scope, Complete Stereocontrol
>
99% d.r.
> 99% d.r.
ABSTRACT: The design and synthesis of a quality compound library containing a small
number of skeletally diverse scaffolds, whose members rapidly deliver new chemical probes
active against multiple phenotypes, is paramount in drug discovery. In this context, an efficient
3
one-pot strategy for the synthesis of
hexahydropyrido[2',1':2,3]imidazo[1,5-a]quinolinium
a
mini
and
library
of
sp
enriched
hexahydrothiazolo[2',3':2,3]
imidazo[1,5-a]quinolinium architectures, is described. This new one-pot method features a
combination of Sc(OTf) -catalyzed [4+1]-cycloaddition with aza-Michael addition reactions.
3
The cascade results in a rapid and diastereoselective formation of these scaffolds via
desymmetrization of the oxidative-dearomatization products of phenols. Phenotypic screening
of the mini library against multidrug resistant bacteria and a panel of cancer cell lines identified
potential antibacterial and anticancer lead drug candidates. Further investigation of the anticancer
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leads, indicated their activity as tubulin-polymerization inhibitors, representing a promising
approach for cancer therapy.
INTRODUCTION
The last decade has witnessed a burgeoning interest in the establishment of diversity-oriented
synthesis (DOS) strategies for the assembly of nature-inspired and drug-like scaffolds needed for
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drug discovery campaigns. One such strategy is the use of a single pluripotent starting material
that can be ornamented via reactions with several reagents, thereby enabling the construction of
skeletally and stereochemically diverse compound collections.2 On the other side,
multicomponent reactions (MCRs) hold a privileged position in DOS, allowing the formation of
3
many new bonds and bringing together more than two reactants in one-step. A particularly
attractive DOS strategy, is a reaction that combines both strategies in one. Such a strategy should
produce a large set of skeletally and stereochemically diverse molecular scaffolds, not achievable
by either strategy alone. Within our group, we have developed several starting material- and
4
MCR-based DOS approaches utilizing few pluripotent building blocks. In continuation of a
program aiming at the development of efficient synthetic methodologies needed for our drug
discovery campaign, we herein present a new strategy for the efficient and diastereoselective
synthesis of pyrido[2',1':2,3]imidazo[1,5-a]quinolone and analogues thereof. The latter class of
compounds are among the biologically valuable frameworks that are found in a large number of
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natural products and pharmaceutically active compounds (compounds A-H; Figure 1). These
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include antiviral, antimicrobial, antitumor, and neuroactive pharmaceuticals (Figure 1). Other
interesting isosters, are the natural products ellipticine, pazellipticine and 6-methylimidazo[1,2-
7
8
a:5,4-b’]dipyridin-2-amine (Glu-P-1). These were reported to exhibit cytotoxic, genotoxic, and
9
mutagenic activities. Given that structures in this class display an array of biological activities
(
Figure 1),5-7 the development of a modular, step-, and atom-economic methodology for their
access would be a significant achievement.7a,7b,10 Such a protocol would facilitate a
comprehensive SAR studies for the discovery of new chemical probes for multiple disease states.
Only a few synthetic methods for the access of pyrido[2',1':2,3]imidazo[1,5-a]quinolone
scaffolds have been reported in the literature. Among others, are the elegant contributions from
Anil,11a Roman,11b Daniel11c research groups. Unfortunately, none of these reports introduced 3D-
3
shapes into the core scaffolds of these compounds. The need for high sp content (3D-shapes)
containing compounds, has recently been suggested in many reports as a favorable way of
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decreasing clinical toxicity and compound attrition rate in clinical trials.12 Therefore, we
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envisioned an efficient and modular strategy for the access of sp -enriched imidazo[1,5-
a]quinoline (type VII) could be developed via a cascade process combining [4+1]-cycloaddition
with aza-Michael addition utilizing synthons of type III (Figure 2, vide infra). Herein, we report
on the synthesis details of this cascade and the biological screening of the designed mini library
as potential anticancer and antibacterial lead drug candidates.
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CF3
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F
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OH
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HOOC
F
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Et
Et
Ph
N
CH3
N
R
O
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R
Et
N
N
A: antibacterial activity5d
B: antibacterial activity5e
: antibacterial activity5f D: antiviral activity5g
C
NH2
N
N
O
N
N
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N CH3
N
N
N
N
S
N
HCl
S
NEt2.
CH3
N
N
OH
: GSK812397
E: Glu-P-1
anticancer activity7a
G: Levamisole
anthelmintic activity7c
F
H: antitumor activity7d
anti-HIV drug7b
CH3
Figure 1. Representative examples of biologically active imidazo[1,5-a]quinolone and
imidazopyridines/thiazole derivatives.
RESULTS AND DISCUSSION
Rational Design and Synthesis. The oxidative-dearomatization products of phenols, used in this
study, were utilized by many groups which resulted in the construction of complex nature-
inspired scaffolds.13 Recently, our group has reported three-directional routes utilizing the
pluripotent building block III for the construction of skeletally and stereochemically diverse
compound collections (Figure 2).14a In this report, we described a one-pot build/couple/pair
scheme that distinctively allows access to diverse collections of complex scaffolds of the types
IV-VI, with complete diastereocontrol. These complex molecular shapes were accessed via
desymmetrization of the oxidative-dearomatization products of phenols. In brief, the pluripotent
building block II was synthesized via the oxidative-dearomatization of phenol I. The aldehyde
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group in II, served as a key branching point which was transformed to the imine III (Figure 2).
The latter was subjected to three cascades of reactions. The first included desymmetrization of
III through coupling with various tryptamine derivatives under acid catalyzed conditions, to
generate the nature-inspired octahydroindolo[2,3-a]quinolizine IV (Figure 2). In the second
route, a novel one-step transformation leading to the diastereoselective preparation of oxocane
scaffolds of type V was discovered. The third round involved the employment of amino acids as
one of the reaction components which allowed for the rapid access of polysubstituted
piperazinones of type VI with complete enantio-control (Figure 2). Inspired by these cascades,
we envisioned that the aldehyde group in intermediate II, could be utilized as a branching point
for a [4+1]-cycloaddition reaction to generate intermediate 5 (Scheme 1). The latter sets the stage
for Michael addition reaction to deliver the tetracyclic product of type VI. This initiative
commonality with the many reports from our group that described the rapid synthesis of diverse
collection of privileged and nature-inspired architectures.4c,14 The ultimate goal of these
initiatives is to establish a quality and diverse compound collections needed for our drug
discovery campaign. In this context, we envisioned to synthesize a pilot library of
polyfunctionalized and conformationally constrained imidazo[1,5-a]quinoline analogues.
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Multidirectional
Type V
Build
Couple
Pairing
H
H
R
X
X
O
N
Type I
Type II
Type III
O
N
OH
previous work
Pluripotent Building Blocks
MeO C
2
N
VI
Type
O
H
N
present work
N
X
NH
CF SO
3
3
Type VII
Figure 2. Three directional build/couple/pair protocol for the rapid construction of diverse molecular shapes.
Scheme 1. One-pot synthesis of pyrido[2,1:2,3]imidazo[1,5-a]quinolones
Y
O
OH
O
O
O
Y
N
N
NH2
H
Y
step-2a
4
step-1a
N
N
OH
OH
O
R
N
R
R
R
X
X
1
X
CF SO3
X
X
3
NH
H
NH
2
3
5
6
a
Reaction conditions: step-1- Aldehyde (3, 0.5 mmol), Amine (4, 0.5 mmol), DCM, MS, -78 °C 1h, and then
(
20 mol%), tert-Butyl isocyanide (0.55 mmol), -78 °C to rt, 5h; step-2- Sc(OTf) (20 mol%), rt, 10h
Sc(OTf)
3
3
Borrowing inspiration from previous reports while keeping an eye on the various biological
activities of imidazo[1,5-a]quinoline analogues5d-g, it was envisioned that reacting the 4-
oxocyclohexa-2,5-dienyloxy)acetaldehyde 3, and derivatives thereof, with various amino-azines
and isocyanides would promote the Groebke–Blackburn–Bienaymé MCR reaction to produce
the imidazopyridine intermediate 5. The latter is suitably poised with a Michael adduct, which
would facilitate the subsequent desymmetrization process leading to the formation of the
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polycyclic system 6 (Scheme 1). To test this hypothesis, compound 3 was first prepared by
oxidative-dearomatization reaction of the 3-hydroxy-1-propyl phenol (1) catalyzed by
hyperiodinate reagent followed by Dess-Martin oxidation of the resulting primary alcohol 2
(
Scheme 1). Thus, reaction of 2-aminopyridine (4) with 3 employing 20 mole% of Sc(OTf) as a
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catalyst at -78 °C delivered intermediate 5. To promote the aza-Michael addition step, a wide set
of conditions were examined, however, most were unsuccessful. Nevertheless, we were
delighted to find that treating intermediate 5 with an additional 20 mole% of Sc(OTf) ), rapidly
3
furnished the pyrido[2, 1:2,3] imidazo[1,5-a]quinolinium system 6a in 52% yield with complete
stereocontrol (>99% dr, Scheme 2). To optimize the reaction conditions, other lanthanides such
as Yb(Otf) was also found to be similarly effective and delivered compound 6a in 48% yield.
3
Furthermore, the loading efficiency of the catalyst was systematically explored. Thus, screening
the catalyst loading from 1 to 20 mol% indicated that a 20 mol% of the catalyst is the optimal
equivalence needed in each steps for complete consumption of the starting materials.
To briefly comment on the diastereoselectivity of the cascade, the Groebke-Blackburn-
Bienaymé/aza-Michael addition sequence provided a single diastereoisomer, presumably through
2
the cis-addition of the imidazo sp -hybridized nitrogen on the enone group of intermediate 5,
guided by intramolecularity, as shown in Scheme 1. Apparently, in the tethered intermediate 5,
2
the sp -hybridized nitrogen atom can adopt the right disposition enforces the nucleophilic
addition on the α,β-unsaturated system from the energetically favored disposition leading to a
single diastereoisomer (compounds 6/8).
Scheme 2. Diastereoselective synthesis of various pyrido/thiazo[2,1:2,3]imidazo[1,5-a]quinolone
derivatives
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O
O
R1
Y
S
N
O
R1
R2
NH2
H
Y
H
S
N
N
4
NH2
R2
N
N
7
O
N
O
a
O
conditionsa
4a: Y = CH
4b: Y = N
conditions
=
Me, R = Ph
O
NH
NH
7a: R1
b: R1
c: R1
d: R1
= Me, R2
CF
SO
CF SO
H
3
3
3
3
7
7
7
2= 4-Br-Ph
3a
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=
R = Ph
4
c: 2-aminoquinoline
2
8
a-d; >99% dr
= R = 4-MeO-Ph
6a-c; >99% dr
2
O
O
O
O
H
CH3
H
H
N
H
S
N
N
N
N
N
N
N
N
O
O
O
O
NH
NH
NH
NH
CF SO
CF SO3
CF
3
SO
3
3
CF SO3
3
3
3
b
c
8a, 61%
6
a, 52%, 48% , 13%
6b, 48%
6c, 45%
O
O
O
O
O
H
H
CH3
H
N
S
N
S
N
S
N
N
O
O
O
N
NH
NH
CF SO3 NH
Br
3
CF SO3
CF SO
3
3
3
ORTEP structure of
8
b, 63%
8c, 63%
8d, 59%
compound 8a
a
Reaction conditions: step-1- Aldehyde (3a, 0.5 mmol), Amine (4/7, 0.5 mmol), DCM, MS, -78 °C 1h, and then
(
20 mol%), tert-Butyl isocyanide (0.55 mmol), -78 °C to rt, 5h; step-2- Sc(OTf) (20 mol%), rt, 10h.
Sc(OTf)
3
3
was used as a catalyst.
b
c5 mol% of Sc(OTf)
Yb(OTf)
3
3
With the optimized conditions in hand, the 2-aminopyrazine (4b) and 2-aminoquinoline (4c)
were reacted with aldehyde 3a, to deliver compounds 6b and 6c each as a single diastereoisomer
in 48 and 45 % yield, respectively, scheme 2. To enrich the pilot library with scaffolds bearing
additional classes of fused heterocyclic rings, the reaction scope was extended by employing
various derivatives of 2-aminothiazole 7a-d as one of the reaction components. These reactions
proceeded
smoothly
to
produce
4
new
family
members
of
the
hexahydrothiazolo[2',3':2,3]imidazo[1,5-a]quinolinium systems (8a-d) in good over all yields
and complete diastereocontrol. To unambiguously confirm the structures of these compounds,
X-ray crystallographic analysis of compound 8a was as a representative example (vide supra,
Scheme 2).
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In addition to the structural diversity of the aminoazines used thus far, we incorporated a second
diversity element into the pilot library by employing the oxygen-tethered aldehyde 3b. Hence,
we envisioned that applying the same chemistry to this system (Scheme 3), would allow for
additional ring-fusion structural diversity within the mini library. To demonstrate this, the
aldehyde 3b was reacted with 2-aminopyridine (4a) under the optimized reaction conditions to
give rise to the corresponding novel ring system, benzo[d]pyrido[2',1':2,3]imidazo[5,1-
b][1,3]oxazinium (9a) in 50% yield with complete diastereocontrol (Scheme 3). Similarly, when
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
2
-aminothiazoles 7a-d, were used as the aminoazine entries, compounds 10a-d were produced in
good yields and with complete stereocontrol.
Scheme 3. Diastereoselective synthesis of various pyrido/thiazo[2,1:2,3]imidazo[1,5-
a]morpholine derivatives
O
O
O
R1
S
N
O
NH2
H
R
1
H
S
N
N
NH2
R2
N
7
4a
N
H C
conditionsa
O
H
3
C
N
3
R2
a
O
conditions
H C
3
O
=
Me, R = Ph
H
NH
NH
7a: R1
b: R1
7c: R1
= Me, R2
CF SO
3 3
CF SO
3b
3
3
7
2= 4-Br-Ph
=
R = Ph
>99% dr
2
1
0; >99% dr
= R = 4-MeO-Ph
9;
7
d: R1
2
O
O
O
H
H
CH3
H
CH3
S
S
N
N
N
N
H C
N
H3
C
N
H C
3
3
O
O
O
NH
NH
NH
Br
CF SO
CF SO
CF SO
3
3
3
3
3
3
9a, 50%
10a, 61%
10b
, 63%
O
O
O
a
Reaction conditions: step-1- Aldehyde
, 0.5 mmol), Amine ( , 0.5 mmol),
H
H
S
N
S
N
3b
4/7
(
N
N
DCM, MS, -78 °C 1h, and then
Sc(OTf) (20 mol%), tert-Butyl
isocyanide (0.55 mmol), -78 °C to rt,
H C
H
C
3
3
O
O
SO
3
NH
NH
O
(20 mol%), rt, 10h
CF SO
CF
3
3
5h; step-2- Sc(OTf)
3
3
3
10c, 64%
10d, 62%
3
To further expand the mini library to include sp -enriched molecular shapes, the hexahydro-7-
oxonaphthalen-4a-yloxy)acetaldehyde (3c) was synthesized and reacted with various 2-
aminoazoles (compounds 4a and 7a-d; scheme 4). Under the optimized conditions, this reaction
enabled the regioselective synthesis of the pentacyclic imidazo[1,5-d][1,4]oxazinium systems
11a and 12a-d in good yields and >99% dr (Scheme 4). The structure of compound 12c was
unambiguously deduced from its X-ray crystallographic analysis (Scheme 4).
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Scheme 4. Diastereoselective synthesis of polycyclic imidazo[1,5-d][1,4]oxazinium scaffolds
O
O
O
R1
S
N
NH2
H
H
R1
S
N
4a
NH2
R2
N
7
N
O
N
N
conditionsa
O
conditionsa
R2
O
O
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
H
= Me, R = Ph
= Me, R2
= R = Ph
NH
NH
3c
7
7
a: R1
b: R1
7c: R1
d: R1
3
CF SO3
CF SO
3
3
2= 4-Br-Ph
>99% dr
1
2; >99% dr
11;
2
= R = 4-MeO-Ph
7
2
O
O
O
H
H
CH3
H
CH3
S
N
S
N
N
N
O
N
O
N
O
NH
NH
NH
CF SO
CF SO
Br
3
3
3
3
CF SO3
3
41%
11a,
12a, 46%
12b
, 48%
O
O
O
H
N
H
S
N
S
N
N
O
O
NH
NH
O
CF SO
CF SO3
3
3
3
12c, 43%
12d, 41%
ORTEP structure of
compound 12c
a
Reaction conditions: step-1- Aldehyde (3c, 0.5 mmol), Amine (4/7, 0.5 mmol), DCM, MS, -78 °C 1h, and then
(
20 mol%), tert-Butyl isocyanide (0.55 mmol), -78 °C to rt, 5h; step-2- Sc(OTf) (20 mol%), rt, 10h
Sc(OTf)
3
3
Scheme 5. Diastereoselective synthesis of polycyclic imidazo[1,5-d][1,4]oxazinium scaffolds
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1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
O
O
R1
S
N
O
NH2
S
N
NH2
R1
R
2
N
7
4a
N
O
O
a
a
N
N
conditions
conditions
O
O
R2
CF SO
3 3
CF SO
3
3
= Me, R = Ph
Me, R2
R = Ph
2
HN
HN
7
7
a: R1
b: R1
7c: R1
H
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
3d
=
=
2= 4-Br-Ph
14, >99% dr
13, >99% dr
O
O
O
O
S
S
S
H C
3
H C
N
O
3
N
N
N
O
O
O
N
N
N
N
CF SO
CF SO
CF SO
3 3
3
3
CF SO
3
3
3
3
HN
HN
14a, 58%
HN
14b, 56%
HN
Br
13a, 43%
14c, 57%
aReaction conditions: step-1- Aldehyde (3d, 0.5 mmol),
Amine (4/7, 0.5 mmol), DCM, MS, -78 °C 1h, and then
(
20 mol%), tert-Butyl isocyanide (0.55 mmol), -
Sc(OTf)
8 °C to rt, 5h; step-2- Sc(OTf)3
3
(
20 mol%), rt, 10h
7
ORTEP structure of
compound 14c
Interestingly, when the tetrahydro-5-oxo-1H-inden-7a-yloxy)acetaldehyde (3d) was used as a
starting material, the regioselectivity was altered relative to that found for the cylohexadienone
congener 3c. Accordingly, the regioisomer 14, was delivered in fair yield and >99% dr (Scheme
5
). The structure of compound 14c was unequivocally confirmed through 1D-, 2D-NMR spectra
(
S35-S38; SI) and X-ray analysis (Scheme 5). When the same reaction was performed with the
aminopyridine 4a, similar regio- and diastereo-selectiviy, was observed. Thus, the bridgehead
pentacyclic oxazinium system 13 was delivered in 43% yield with complete diastereocontrol.
The observed regiochemistry when using aldehyde 3d as a starting material is notorious
compared to that of compound 3c when reacted with aminoazines 7a-d. Obviously, the aza-
Michael addition reaction of the Greobuck-Blackburn intermediate to deliver compound 12,
proceeded at the least hindered position of the α,β-unsaturated system producing a single
regioisomer with complete stereocontrol. While for compounds 13 and 14, the attack of the sp2
hybridized nitrogen atom of the azole ring on the α,β-unsaturated system proceeded from the
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tetra-substituted carbon rather than from the least hindered tertiary carbon. These findings could
be rationalized as contemplated in Figure 3. Clearly, the inherent steric bias contained in
intermediates V and VI (Figure 3), dictated the observed regioselectivity. Apparently, the
congested strain in the fused cyclohexane ring in intermediate III, hindered the Michael addition
at the quaternary center of the α,β-unsaturated system, whereas, the steric compression contained
in conformer VI prevented the attack at the least hindered position of the α,β-unsaturated ketone
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
(Figure 3). However, the minimal steric strain present in conformer VIII, promoted the aza-
Michael addition at the more substituted site leading to the bridgehead compounds 13 and 14.
S
N
S
S
S
N
N
N
N
H
N
O
O
H
HN
N
HN
N
O
HN
O
HN
O
O
O
O
II
I
III
obseved product
steric clash
S
O
O
O
O
N
H
N
N
H
O
HN
HN
HN
O
O
N
S
N
S
N
N
S
N
O
N
H
not observed
IV
V
VI
O
O
S
O
O
N
N
N
HN
HN
HN
N
S
N
S
O
O
N
N
N
O
N
H
O
S
obseved product
VII
VIII
IX
Figure 3. Proposed regiochemical analysis for the formation of the aza-Michael addition
products 12/14.
Phenotypic screening. Multidrug resistant bacteria (MDRB) represent a serious threat to the
global health. If not seriously addressed, this silent epidemic is projected to kill up to 300 million
1
5
people by 2050 with an estimated economic burden of $100 trillion. Due to misuse and overuse
of antibiotics, new MDRB species continue to rise, while the discovery of first-in-class
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2
2
2
2
2
2
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3
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3
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5
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5
5
6
antibiotics to combat these superbugs has drastically declined. On the other side, cancer is the
second leading cause of death world-wide after cardiovascular diseases. Although tens of drugs
for the treatment of cancer are available in the market, yet the life-span improvement of cancer
patients is still very low relative to the amount of time and costs spent on their discovery.
0
1
2
3
4
5
6
7
8
9
0
1
2
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7
8
9
0
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4
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6
7
8
9
0
Having established a robust en route to the aforementioned compound collections, we aimed to
profile these compounds for their activities as potential antiproliferative and antibacterial motifs.
Of the 21 compounds tested, compound 8d showed the most potent antiproliferative activity
upon dose-response analysis (Table 1), suggesting that this compound could be a starting point
for the development of first-in-class anticancer lead drug candidate. To briefly comment on the
structure-activity relationship of the tested pilot compound library, we observed that when the
benzo[b]imidazo[1,5-d][1,4]oxazinium core scaffold is fused to thiazole ring (compounds 8d,
10c-d, and 14a-b), the anticancer activity is enhanced when compared to the pyridine fused
analogues (compounds 6, 9, 11 and 13). Furthermore, when the core scaffold,
benzo[b]imidazo[1,5-d][1,4]oxazinium, bears a diphenylthiazole ring (compounds 8d, 10c and
1
0d), the anticancer potency is sharply enhanced. Interestingly, a di(methoxyphenyl)thiazole
group anchored on the basic framework delivered the most potent anticancer motifs (compounds
d and 10d). Further investigation of the anticancer best hits indicated that these compounds are
8
potential tubulin polymerization destabilizers. The tubulin inhibition dynamics were tested at
their IC50 concentrations (2.1-4.0 µM) and found to be comparable to that of colchicine, a
microtubule-polymerization inhibitor but not analogous to the effect of the microtubule-stabilizer
paclitaxel (Figure S6, SI). This finding supports the notion that these compounds might exert
their antiproliferative effects through the inhibition of tubulin assembly and suppression of
microtubule formation.
Table 1. Cytotoxicity data of active compounds against a panel of human cancer cell lines
IC50 (μM)a
Compound
MCF7
2.1
A549
4.4
HeLa
2.88
5.4
HCT116
1.7
8
d
1
0c
3.1
4.5
3.64
1
0d
2.5
7.4
1.2
3.0
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1
4a
6.9
11.2
14.6
0.12
2.1
5.1
6.1
9.4
14b
9.1
Doxorubicin
0.17
0.29
0.09
aBreast cancer (MCF-7), lung cancer (A549), cervical cancer (HeLa), colon cancer (HCT116).
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
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3
3
3
3
3
3
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5
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5
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0
1
2
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7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
IC50 = Half-maximal inhibitory concentration.
Furthermore, phenotypic screening of the designed library against three species of multidrug
resistant Staphylococcus aureus (Gram-positive pathogen) (Table S4) and E. Coli (Gram-
negative), identified compound 8d as a potential antibacterial lead drug candidate. Compound 8d
demonstrated high growth inhibition activity against the four species under investigation, with
MIC values ranges from 12.5 to 25 μg/mL. However, none of the tested compounds was active
against E. Coli (Table 2).
Table 2. Antibacterial activity of compound 8d against Staphylococcus aureus (Gram-positive
bacteria), Escherichia coli (Gram-negative bacteria) and three multidrug resistant strains of
methicillin-resistant Staphylococcus aureus (MRSA-1, MRSA-2 and MRSA-3)
MICs (µg/ml)
Staphylococcus
Escherichia
Compound
MRSA- MRSA-
aureus
(strain
MRSA-3
coli
(strain
1
2
ATCC 25923)
ATCC 25922)
8d
25
12.5
32
12.5
16
25
2
Non-active
0.008
Ciprofloxacin 0.125
While an in-depth assessment of the biological findings of these motifs is required, the
encouraging results support a promising application of these compounds as potential lead drug
candidates for the development of antibacterial and anticancer agents.
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2
2
2
2
2
2
2
2
2
3
3
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4
4
4
4
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6
CONCLUSION
In summary,
hexahydrothiazolo[2',3':2,3]imidazo[1,5-a]quinolinium heterocycles and anologues thereof,
through a one-pot combination of formal Sc(OTf) -catalyzed [4+1]-cycloaddition with aza-
we
have
developed
a
highly
efficient
synthesis
of
3
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
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7
8
9
0
1
2
3
4
5
6
7
8
9
0
Michael addition reactions. The use of commercial catalysts, readily available substrates, and a
simple procedure bode well for practical application of this strategy. The described protocol
allowed for the access of quality compounds with complete stereocontrol, step- and atom-
economic control. This approach offered interesting skeletal and stereochemical complexity not
achievable by either approach alone. Subsequent phenotypic screening of the mini library
identified unique chemotypes, compound 8d, which effectively suppressed proliferation of
cancer cells, and inhibit tubulin polymerization, representing a promising approach for the
development of first-in-class lead drug candidates for potential treatment of cancer. In another
aspect, the designed mini library, offered a potential opportunity to develop the designed motifs
as potent antibiotics against multidrug resistant bacteria. Finally, the established chemistry is
projected to enable the discovery of new chemical probes and work along these lines is presently
in progress in our laboratories.
EXPERIMENTAL SECTION
General. Chemical reagents and anhydrous solvents were purchased from Sigma-Aldrich and
were used without further purification. Solvents for extraction and column chromatography were
distilled prior to use. TLC analysis were performed with silica gel plates (0.25 mm, E. Merck, 60
1
F
254) using iodine and a UV lamp for visualization. H and 13C NMR experiments were
performed on a 500 MHz instrument. Chemical shifts are reported in parts per million (ppm)
downstream from the internal tetramethylsilane standard. Spin multiplicities are described as s
(singlet), bs (broad singlet), d (doublet), dd (doubledoublet), t (triplet), q (quartet), or m
(multiplet). Coupling constants are reported in Hertz (Hz). ESI mass spectrometry was
performed on a Q-TOF high resolution mass spectrometer or Q-TOF Ultim LC-MS. Single-
crystal X-ray diffraction data were collected using an Oxford Diffraction XCalibur, equipped
with (Mo) X-ray Source (λꢀ=ꢀ0.71073ꢀÅ) at 293(2)ꢀK.
General Reaction Procedure for the Preparation of Compounds 3a-d. Compounds 3a-d were
prepared according to a precedent procedure and the spectroscopic data are matching that
published in reference 14a.14a
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General reaction procedure for the preparation of compounds 6 and 8-14. Aldehyde (3, 0.5
mmol, 1.0 eq) and molecular sieves were mixed in DCM (2 mL, 0.25M) and a solution of amine
(4/7, 0.5 mmol, 1.0 eq) in DCM (2.0 mL, 0.25M) was added dropwise at -78 °C. After 1h,
scandium triflate (20 mol%, 0.2 eq) and a solution of tert-butyl isocyanide (0.55 mmol, 1.1 eq) in
MeOH (0.5 mL, 1.1M) were introduced and stirring was continued for 5h. After completion of
step-1, an additional 20 mol% (0.2 eq) of scandium triflate was added and the reaction was
continued at rt for 10h. After completion, solvent was removed under vacuum and the crude was
purified on flash column chromatography, using a gradient of MeOH/DCM as an eluent to
deliver pure products.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Reaction procedure for the preparation of compound 6a at 2mmol scale. Aldehyde (3a, 360
mg, 2.0 mmol) and molecular sieves were mixed in DCM (8.0 mL, 0.25M) and a solution of
amine (4a, 188 mg, 2.0 mmol) in DCM (8.0 mL, 0.25M) was added dropwise at -78 °C. After
1
h, scandium triflate (196 mg, 20 mol%) and a solution of tert-butyl isocyanide (183 mg, 2.2
mmol) in MeOH (2.0 mL, 1.1M) were introduced and stirring was continued for 5h. After
completion of step-1, an additional 20 mol% (196 mg) of scandium triflate was added and the
reaction was continued at rt for 10h. After completion, solvent was removed under vacuum and
the crude was purified on flash column chromatography, using a gradient of 3% MeOH in DCM
as an eluent to deliver compound 6a (548 mg, 56% yield).
(4aS,13aS)-7-(Tert-butylamino)-4a-methoxy-2-oxo-1,2,4a,5,6,13a-
hexahydropyrido[2',1':2,3]imidazo[1,5-a]quinolin-13-ium trifluoromethanesulfonate (6a).
1
White solid (127 mg, 52% yield, eluent: 3% MeOH in DCM). mp 92−94 °C. H NMR (500
MHz, CDCl
3
) δ 8.74 (d, J = 6.7 Hz, 1H), 7.98 – 7.90 (m, 1H), 7.90 – 7.83 (m, 1H), 7.41 (t, J =
6
3
.8 Hz, 1H), 7.01 (d, J = 10.4 Hz, 1H), 6.23 (d, J = 10.4 Hz, 1H), 5.48 – 5.39 (m, 1H), 3.48 –
.42 (m, 1H), 3.42 (s, 3H), 3.18 – 3.09 (m, 2H), 2.95 – 2.86 (m, 1H), 2.36 – 2.30 (m, 2H), 1.28
13
(
s, 9H). C{1H} NMR (125 MHz, CDCl
3
) δ 193.5, 150.2, 135.7, 133.1, 131.6, 128.5, 126.1,
125.8, 120.6 (q, 1JC-F = 318.7 Hz), 117.1, 110.1, 73.4, 56.2, 52.9, 51.5, 41.3, 30.7, 25.6, 17.6.
HRMS (ESI-TOF) m/z: [M-CF SO 340.2025, found 340.2029.
]+ calcd for C20
H
26
N
3
O
2
3
3
(4aS,13aS)-7-(tert-butylamino)-4a-methoxy-2-oxo-1,2,4a,5,6,13a-
hexahydropyrazino[2',1':2,3]imidazo[1,5-a]quinolin-13-ium
trifluoromethanesulfonate
1
(
6b). White solid (117 mg, 48% yield, eluent: 4% MeOH in DCM). mp 196−198 °C. H NMR
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3
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(
500 MHz, CD
3
OD) δ 9.59 (s, 1H), 8.85 (d, J = 3.5 Hz, 1H), 8.55 (d, J = 4.5 Hz, 1H), 7.25 (d, J
= 10.4 Hz, 1H), 6.33 (d, J = 10.4 Hz, 1H), 5.95 (dd, J = 12.3, 5.4 Hz, 1H), 3.43 (s, 3H), 3.38 –
.34 (m, 1H), 3.31 – 3.26 (m, 2H), 3.04 – 2.93 (m, 1H), 2.48 – 2.28 (m, 2H), 1.32 (s, 9H).
13C{1H} NMR (125 MHz, CD
OD) δ 195.0, 152.2, 137.5, 135.8, 132.4, 132.0, 131.2, 128.3,
21.7 (q, 1JC-F = 313.7 Hz),119.3, 74.3, 58.0, 54.5, 51.7, 43.1, 30.6, 26.0, 18.4. HRMS (ESI-
TOF) m/z: [M-CF SO 341.1977, found 341.1984.
3
3
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
]+ calcd for C19
H
25
N
4
O
2
3
3
(7aS,11aS)-14-(tert-butylamino)-11a-methoxy-9-oxo-7a,8,9,11a,12,13-
hexahydroimidazo[1,2-a:3,4-a']diquinolin-7-ium trifluoromethanesulfonate (6c). Brownish
1
solid (121 mg, 45% yield, eluent: 3% MeOH in DCM). mp 189−191 °C. H NMR (500 MHz,
CD
9.5 Hz, 1H), 7.96 (t, J = 7.6 Hz, 1H), 7.81 (t, J = 7.6 Hz, 1H), 7.26 (d, J = 10.4 Hz, 1H), 6.32
d, J = 10.4 Hz, 1H), 5.79 (dd, J = 12.4, 5.4 Hz, 1H), 3.45 (s, 3H), 3.31 – 3.18 (m, 3H), 2.97 –
3
OD) δ 10.09 (d, J = 8.8 Hz, 1H), 8.36 (d, J = 9.4 Hz, 1H), 8.18 (d, J = 7.8 Hz, 1H), 8.03 (d, J
=
(
1
3
2.89 (m, 1H), 2.49 – 2.42 (m, 1H), 2.39 – 2.26 (m, 1H), 1.30 (s, 9H). C{1H} NMR (125 MHz,
CD OD) δ 195.2, 152.4, 137.0, 136.1, 135.1, 132.3, 132.2, 131.6, 131.1, 128.8, 127.5, 126.6,
20.9 (q, 1JC-F = 317.8 Hz), 120.5, 108.8, 74.6, 58.1, 54.0, 51.7, 42.5, 30.2, 26.3, 18.2. HRMS
ESI-TOF) m/z: [M-CF SO 390.2181, found 390.2184.
3
1
(
3
3
]+ calcd for C24
H
28
N
3
O
2
(4aS,12aS)-7-(tert-butylamino)-4a-methoxy-10-methyl-2-oxo-9-phenyl-1,2,4a,5,6,12a-
hexahydrothiazolo[2',3':2,3]imidazo[1,5-a]quinolin-12-ium trifluoromethanesulfonate (8a).
1
White solid (178 mg, 61% yield, eluent: 2% MeOH in DCM). mp 193−195 °C. H NMR (500
MHz, acetone-d
J = 10.4 Hz, 1H), 5.50 (dd, J = 12.0, 5.7 Hz, 1H), 3.42 (s, 1H), 3.32 (s, 3H), 3.10 (dd, J = 16.2,
.2 Hz, 2H), 3.05 – 2.96 (m, 1H), 2.89 – 2.78 (m, 1H), 2.36 (s, 3H), 2.30 – 2.22 (m, 2H), 0.61 (s,
6
) δ 7.57 – 7.50 (m, 2H), 7.50 – 7.42 (m, 3H), 7.08 (d, J = 10.4 Hz, 1H), 6.10 (d,
5
1
3
9H). C{1H} NMR (125 MHz, acetone-d
6
) δ 193.5, 151.1, 151.0, 141.4, 132.7, 131.9, 130.9,
1
2
4
29.6, 129.2, 129.1, 128.0, 127.5, 121.2 (q, 1JC-F = 315.0 Hz), 74.0, 55.7, 55.3, 51.6, 40.3, 30.0,
6.0, 18.3, 13.3. HRMS (ESI-TOF) m/z: [M-CF
3
SO
3
]+ calcd for C25
H
30
N
3
2
O S 436.2058, found
36.2047
(4aS,12aS)-9-(4-bromophenyl)-7-(tert-butylamino)-4a-methoxy-10-methyl-2-oxo-
1
,2,4a,5,6,12a-hexahydrothiazolo[2',3':2,3]imidazo[1,5-a]quinolin-12-ium
trifluoromethanesulfonate (8b). White solid (204 mg, 63% yield, eluent: 2.5% MeOH in
1
DCM). mp 200−202 °C. H NMR (500 MHz, acetone-d
6
) δ 7.79 (d, J = 8.2 Hz, 2H), 7.66 (d, J =
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.2 Hz, 2H), 7.22 (d, J = 10.4 Hz, 1H), 6.24 (d, J = 10.4 Hz, 1H), 5.64 (dd, J = 12.2, 5.6 Hz, 1H),
3.71 (s, 1H), 3.46 (s, 3H), 3.30 – 3.20 (m, 2H), 3.20 – 3.10 (m, 1H), 3.03 – 2.94 (m, 1H), 2.52 (s,
13
3
1
H), 2.43 – 2.35 (m, 2H), 0.79 (s, 9H). C{1H} NMR (125 MHz, acetone-d
6
) δ 193.5, 151.1,
51.0, 141.5, 134.7, 132.1, 131.9, 131.6, 129.6, 128.2, 127.2, 124.7, 121.0 (q, 1JC-F = 316.5
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
]+ calcd for
Hz), 74.0, 55.7, 55.3, 51.6, 40.3, 26.0, 18.3, 13.3. HRMS (ESI-TOF) m/z: [M-CF
3
SO
3
C
25
H29BrN
3
O
2
S514.1163, found 514.1153.
(4aS,12aS)-7-(tert-butylamino)-4a-methoxy-2-oxo-9,10-diphenyl-1,2,4a,5,6,12a-
hexahydrothiazolo[2',3':2,3]imidazo[1,5-a]quinolin-12-ium trifluoromethanesulfonate (8c).
White solid (194 mg, 63% yield, eluent: 2.5% MeOH in DCM). mp 198−200 °C. 1H NMR (500
MHz, CDCl
Hz, 2H), 6.95 (d, J = 10.4 Hz, 1H), 6.28 (d, J = 10.4 Hz, 1H), 5.03 – 4.97 (m, 1H), 3.45 (s, 3H),
.40 – 3.32 (m, 1H), 3.29 – 3.21 (m, 2H), 3.11 – 3.01 (m, 1H), 2.48 – 2.39 (m, 1H), 2.37 – 2.29
3
) δ 7.52 – 7.41 (m, 3H), 7.40 – 7.34 (m, 2H), 7.33 – 7.29 (m, 3H), 7.18 (d, J = 7.2
3
1
3
(m, 1H), 0.72 (s, 9H). C{1H} NMR (125 MHz, CDCl
3
) δ 193.4, 148.8, 140.0, 132.1, 131.6,
1
30.4, 130.2, 129.9, 129.8, 129.48, 129.47, 129.2, 128.9, 128.65, 128.64, 126.7, 120.8 (q, 1JC-F
]+
=
320.0 Hz),73.1, 55.8, 54.6, 51.3, 39.2, 30.1, 25.6, 18.3. HRMS (ESI-TOF) m/z: [M-CF
3
SO
3
calcd for C30
H
32
N
3
O
2
S 498.2215, found 498.2228.
(
4aS,12aS)-7-(tert-butylamino)-4a-methoxy-9,10-bis(4-methoxyphenyl)-2-oxo-
1
,2,4a,5,6,12a-hexahydrothiazolo[2',3':2,3]imidazo[1,5-a]quinolin-12-ium
trifluoromethanesulfonate (8d). White solid (208 mg, 59% yield, eluent: 3.0% MeOH in
1
DCM). mp 190−192 °C. H NMR (500 MHz, CDCl
3
) δ 7.54 – 7.34 (m, 2H), 7.08 (d, J = 8.7 Hz,
2
1
H), 7.00 – 6.87 (m, 3H), 6.80 (d, J = 8.7 Hz, 2H), 6.24 (d, J = 10.4 Hz, 1H), 5.00 – 4.92 (m,
H), 3.83 (s, 3H), 3.78 (s, 3H), 3.42 (s, 3H), 3.36 – 3.27 (m, 1H), 3.25 – 3.10 (m, 3H), 3.08 –
1
3
2.97 (m, 1H), 2.42 – 2.24 (m, 2H), 0.72 (s, 9H). C{1H} NMR (125 MHz, CDCl
3
) δ 193.4,
1
61.1, 160.6, 149.0, 139.7, 131.6, 131.4, 131.1, 130.1, 129.8, 129.4, 128.9, 121.2, 120.8 (q, 1JC-
F = 318.7 Hz),118.8, 114.7, 114.1, 73.1, 55.59, 55.57, 55.5, 54.7, 51.3, 39.2, 30.2, 25.6, 18.2.
+
HRMS (ESI-TOF) m/z: [M-CF
3
SO
3
] calcd for C32
H N
36 3
4
O S558.2426, found 558.2416.
(4aR,13aS)-7-(tert-butylamino)-4a-methyl-2-oxo-1,4a,5,13a-tetrahydro-2H-
benzo[d]pyrido[2',1':2,3]imidazo[5,1-b][1,3]oxazin-13-ium trifluoromethanesulfonate (9a).
1
White solid (118 mg, 50% yield, eluent: 3.5% MeOH in DCM). mp 138−140 °C. H NMR (500
MHz, CDCl ) δ 8.75 (d, J = 6.7 Hz, 1H), 7.97 (d, J = 8.8 Hz, 1H), 7.92 – 7.84 (m, 1H), 7.44 (t, J
3
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2
2
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2
2
2
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3
3
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3
3
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5
5
5
5
5
5
5
6
=
6.8 Hz, 1H), 6.78 (d, J = 10.2 Hz, 1H), 6.18 (d, J = 10.2 Hz, 1H), 5.26 – 5.10 (m, 3H), 2.96 (d,
13
J = 7.5 Hz, 2H), 1.65 (s, 3H), 1.23 (s, 9H). C{1H} NMR (125 MHz, CDCl
3
) δ 193.8, 147.7,
1
5
36.5, 133.9, 130.9, 126.4, 124.6, 123.6, 120.6 (q, 1JC-F = 317.5 Hz),117.6, 110.7, 69.7, 57.4,
+
6.3, 54.7, 39.9, 30.5, 21.8. HRMS (ESI-TOF) m/z: [M-CF
3
SO
3
] calcd for C19
H
24
3
N O2 326.1868,
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
found 326.1874.
4aR,12aS)-7-(tert-butylamino)-4a,10-dimethyl-2-oxo-9-phenyl-1,2,4a,12a-tetrahydro-6H-
(
benzo[b]thiazolo[2',3':2,3]imidazo[1,5-d][1,4]oxazin-12-ium
trifluoromethanesulfonate
1
(10a). White solid (174 mg, 61% yield, eluent: 3% MeOH in DCM). mp 170−172 °C. H NMR
(500 MHz, acetone-d ) δ 7.71 – 7.65 (m, 2H), 7.65 – 7.59 (m, 3H), 7.12 (dd, J = 10.4, 1.3 Hz,
6
1
3
H), 6.21 (d, J = 10.4 Hz, 1H), 5.18 – 5.06 (m, 3H), 3.61 (s, 1H), 3.47 (dd, J = 17.6, 4.0 Hz, 1H),
13
.29 (dd, J = 17.6, 5.2 Hz, 1H), 2.51 (s, 3H), 1.81 (s, 3H), 0.71 (s, 9H). C{1H} NMR (126
) δ 192.9, 149.8, 142.7, 132.7, 132.5, 132.0, 131.1, 129.3, 128.2, 128.1, 127.7,
MHz, acetone-d
6
126.3, 122.4 (q, 1JC-F = 320.0 Hz),72.6, 59.7, 58.3, 55.6, 38.4, 23.7, 13.2. HRMS (ESI-TOF) m/z:
+
[
M-CF
3
SO
3
] calcd for C24
H
28
N
3
2
O S422.1902, found 422.1892.
(4aR,12aS)-9-(4-bromophenyl)-7-(tert-butylamino)-4a,10-dimethyl-2-oxo-1,2,4a,12a-
tetrahydro-6H-benzo[b]thiazolo[2',3':2,3]imidazo[1,5-d][1,4]oxazin-12-ium
trifluoromethanesulfonate (10b). White solid (204 mg, 63% yield, eluent: 2.5% MeOH in
1
DCM). mp 160−162 °C. H NMR (500 MHz, CDCl
3
) δ 7.67 (d, J = 5.5 Hz, 2H), 7.60 – 7.31 (m,
2
H), 6.88 (d, J = 10.1 Hz, 1H), 6.20 (d, J = 10.1 Hz, 1H), 5.14 – 4.93 (m, 2H), 4.90 – 4.78 (m,
1
3
1H), 3.35 – 3.15 (m, 2H), 2.37 (s, 3H), 1.74 (s, 3H), 0.66 (s, 9H). C{1H} NMR (125 MHz,
CDCl ) δ 192.7, 149.3, 141.2, 131.8, 131.2 (2C), 130.7, 127.5, 127.2, 125.8, 125.1, 125.0, 120.5
q, 1JC-F = 316.2 Hz), 71.5, 58.7, 57.6, 54.6, 38.0, 29.6, 23.4, 13.1. HRMS (ESI-TOF) m/z: [M-
3
(
+
CF
3
SO
3
] calcd for C26
H29BrN
3
2
O S526.1163, found 526.1155.
(4aR,12aS)-7-(tert-butylamino)-4a-methyl-2-oxo-9,10-diphenyl-1,2,4a,12a-tetrahydro-6H-
benzo[b]thiazolo[2',3':2,3]imidazo[1,5-d][1,4]oxazin-12-ium
trifluoromethanesulfonate
1
(10c). White solid (202 mg, 64% yield, eluent: 3% MeOH in DCM). mp 186−188 °C. H NMR
(500 MHz, CDCl
J = 7.3 Hz, 2H), 7.21 (d, J = 7.3 Hz, 2H), 6.92 (d, J = 10.2 Hz, 1H), 6.26 (d, J = 10.2 Hz, 1H),
.19 – 5.03 (m, 2H), 4.94 – 4.83 (m, 1H), 3.40 (d, J = 16.8 Hz, 1H), 3.26 (d, J = 16.8 Hz, 1H),
3
) δ 7.77 – 7.56 (m, 1H), 7.55 – 7.41 (m, 4H), 7.36 (d, J = 7.1 Hz, 1H), 7.31 (d,
5
1
3
2.93 (s, 1H), 1.80 (s, 3H), 0.68 (s, 9H). C{1H} NMR (125 MHz, CDCl ) δ 192.8, 149.5, 142.0,
3
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31.4, 131.1, 131.0, 130.7, 130.1, 130.0, 129.0, 129.2, 129.1, 128.4, 127.7, 126.7, 125.5, 120.7
(q, 1JC-F = 318.7 Hz),71.7, 59.0, 57.9, 55.0, 38.2, 29.8, 23.8. HRMS (ESI-TOF) m/z: [M-CF
3
SO
]+
3
calcd for C29
H
30
N
3
O
2
S 484.2058, found 484.2049.
(
4aR,12aS)-7-(tert-butylamino)-9,10-bis(4-methoxyphenyl)-4a-methyl-2-oxo-1,2,4a,12a-
tetrahydro-6H-benzo[b]thiazolo[2',3':2,3]imidazo[1,5-d][1,4]oxazin-12-ium
trifluoromethanesulfonate (10d). (White solid, 214 mg, 62% yield). mp 156−158 °C. H NMR
500 MHz, CDCl ) δ 7.56 – 7.33 (m, 2H), 7.14 (d, J = 6.6 Hz, 2H), 7.04 – 6.95 (m, 2H), 6.91 (d,
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
(
3
J = 9.8 Hz, 1H), 6.83 (d, J = 6.6 Hz, 2H), 6.25 (d, J = 9.8 Hz, 1H), 5.17 – 5.00 (m, 2H), 4.88 (d,
J = 14.4 Hz, 1H), 3.87 (s, 3H), 3.81 (s, 3H), 3.39 (d, J = 16.0 Hz, 1H), 3.23 (d, J = 16.0 Hz, 1H),
1
3
13
3
.01 (s, 1H), 1.80 (s, 3H), 0.72 (s, 9H). C NMR (125 MHz, CDCl
3
) δ C NMR (125 MHz,
CDCl
3
) δ 192.8, 161.4, 160.8, 149.6, 141.6, 131.3, 130.6, 130.4, 127.7, 125.3, 120.8 (q, 1JC-F =
3
18.7 Hz), 120.7, 118.6, 114.7, 114.5, 71.7, 59.2, 58.1, 55.6, 55.5, 55.1, 38.3, 29.9, 23.9. HRMS
(ESI-TOF) m/z: [M-CF SO S 544.2270, found 544.2253.
3
3
]+ calcd for C31
H
34
N
3
O
4
(7aR,16aS)-10-(tert-butylamino)-2-oxo-1,4,5,6,7,16a-hexahydro-2H,9H-naphtho[8a,1-
b]pyrido[2',1':2,3]imidazo[1,5-d][1,4]oxazin-16-ium
trifluoromethanesulfonate
(11a).
1
(White solid, 41% yield, eluent: 3% MeOH in DCM). mp 103−105 °C. H NMR (500 MHz,
CDCl ) δ 8.78 (d, J = 4.6 Hz, 1H), 7.92 – 7.86 (m, 1H), 7.51 – 7.43 (m, 1H), 7.33 – 7.25 (m,
3
1
H), 6.05 (s, 1H), 5.30 – 5.12 (m, 2H), 5.03 – 4.90 (m, 1H), 3.01 – 2.84 (m, 2H), 2.78 – 2.65 (m,
1H), 2.59 – 2.47 (m, 2H), 2.05 – 1.96 (m, 1H), 1.78 – 1.67 (m, 2H), 1.63 – 1.52 (m, 2H), 1.27 (s,
1
3
9
1
2
H). C {1H} NMR (125 MHz, CDCl ) δ 193.1, 159.6, 136.3, 133.7, 126.6, 126.4, 124.5, 123.6,
3
20.4 (q, 1JC-F = 317.5 Hz), 117.5, 110.3, 71.1, 56.6, 56.2, 54.7, 39.7, 32.3, 32.0, 30.4, 26.2,
+
0.5. HRMS (ESI-TOF) m/z: [M-CF
3
SO
3
] calcd for C22
H
28
3
N O2 366.2181, found 366.2178.
(7aR,15aS)-10-(tert-butylamino)-13-methyl-2-oxo-12-phenyl-1,4,5,6,7,15a-hexahydro-
2
H,9H-naphtho[8a,1-b]thiazolo[2',3':2,3]imidazo[1,5-d][1,4]oxazin-15-ium
trifluoromethanesulfonate (12a). (White solid, 140 mg, 46% yield, elent: 2% MeOH in DCM).
1
mp 162−164 °C. H NMR (500 MHz, CDCl
3
) δ 7.70 – 7.34 (m, 5H), 6.01 (s, 1H), 4.94 (d, J =
1
5.4 Hz, 1H), 4.79 – 4.65 (m, 3H), 3.10 – 2.94 (m, 2H), 2.87 (s, 1H), 2.63 – 2.45 (m, 2H), 2.37
d, J = 13.4 Hz, 1H), 2.31 (s, 3H), 2.02 – 1.93 (m, 1H), 1.86 – 1.67 (m, 2H), 1.51 – 1.42 (m, 1H),
.59 (s, 9H). 13C{1H} NMR (125 MHz, CDCl
) δ 192.8, 161.1, 141.0, 131.8, 130.5, 128.9,
127.2, 126.7, 126.6 (2C), 126.4, 124.8, 120.6 (q, 1JC-F = 318.7 Hz), 72.5, 57.5, 56.6, 54.7, 38.4,
(
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2
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2
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3
3
3
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3
3
3
3
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4
4
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4
5
5
5
5
5
5
5
5
5
5
6
+
3
4.3, 32.7, 29.6, 27.6, 20.5, 13.2. HRMS (ESI-TOF) m/z: [M-CF
3
SO
3
] calcd for C27
H
32
N
3
O S
2
462.2215, found 462.2200.
(
7aR,15aS)-12-(4-bromophenyl)-10-(tert-butylamino)-13-methyl-2-oxo-1,4,5,6,7,15a-
hexahydro-2H,9H-naphtho[8a,1-b]thiazolo[2',3':2,3]imidazo[1,5-d][1,4]oxazin-15-ium
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
trifluoromethanesulfonate (12b). (White solid, 165 mg, 48% yield, eluent: 2% MeOH in
1
DCM). mp 202−204 °C. H NMR (500 MHz, CDCl
3
) δ 7.61 (d, J = 7.1 Hz, 2H), 7.52 – 7.24 (m,
2
2
H), 6.00 (s, 1H), 4.97 (d, J = 15.4 Hz, 1H), 4.83 – 4.60 (m, 2H), 3.09 – 2.86 (m, 3H), 2.62 –
.53 (m, 1H), 2.49 (d, J = 13.1 Hz, 1H), 2.38 (d, J = 13.1 Hz, 1H), 2.30 (s, 3H), 2.01 – 1.94 (m,
1
3
1H), 1.85 – 1.66 (m, 2H), 1.50 – 1.41 (m, 1H), 0.63 (s, 9H). C NMR (125 MHz, CDCl
3
) δ
1
92.7, 161.0, 140.9, 131.8, 131.0, 127.4, 126.9, 126.7, 126.6, 125.4, 125.2, 125.0, 120.6 (q, 1JC-
F = 312.5 Hz), 72.2, 57.5, 56.6, 54.6, 38.4, 34.0, 32.7, 29.7, 27.4, 20.5, 13.2. HRMS (ESI-TOF)
+
m/z: [M-CF
3
SO
3
] calcd for C27
H31BrN
3
2
O S540.1320, found 540.1306.
1
0-(tert-butylamino)-2-oxo-12,13-diphenyl-1,4,5,6,7,15a-hexahydro-2H,9H-naphtho[8a,1-
b]thiazolo[2',3':2,3]imidazo[1,5-d][1,4]oxazin-15-ium trifluoromethanesulfonate (12c).
1
White solid (107 mg, 43% yield, eluent: 2.5% MeOH in DCM). mp 192−194 °C. H NMR (500
MHz, CDCl
3
) δ 7.44 – 7.31 (dd, J = 10.6, 6.5 Hz, 4H), 7.30 – 7.20 (m, 4H), 7.14 – 7.11 (m, 2H),
.05 –6.02 (m, 1H), 4.99 (d, J = 15.5 Hz, 1H), 4.79 – 4.71 (m, 2H), 3.13 – 3.07 (m, 1H), 3.05 –
.99 (m, 1H), 2.94 (s, 1H), 2.63 – 2.52 (m, 2H), 2.39 (d, J = 13.4 Hz, 1H), 2.02 – 1.95 (m, 1H),
6
2
1
1
3
.86 – 1.67 (m, 3H), 1.52 – 1.41 (m, 1H), 0.61 (s, 9H). C{1H} NMR (125 MHz, CDCl ) δ
3
192.8, 161.2, 141.6, 131.1, 130.6, 130.5, 129.9, 129.8, 129.7, 129.1, 128.9, 128.4, 127.7, 126.7,
1
2
26.5, 124.9, 120.6 (q, 1JC-F = 321.2 Hz) 72.4, 57.6, 56.7, 54.8, 38.3, 34.2, 32.8, 29.7, 27.6,
+
0.5. HRMS (ESI-TOF) m/z: [M-CF
3
SO
3
] calcd for C32
H
34
N
3
O S524.2371, found 524.2367.
2
(
7aR,15aS)-10-(tert-butylamino)-12,13-bis(4-methoxyphenyl)-2-oxo-1,4,5,6,7,15a-
hexahydro-2H,9H-naphtho[8a,1-b]thiazolo[2',3':2,3]imidazo[1,5-d][1,4]oxazin-15-ium
trifluoromethanesulfonate (12d). White solid (150 mg, 41% yield, eluent: 3% MeOH in DCM).
1
mp 185−187 °C. H NMR (500 MHz, CDCl
3
) δ 7.67 – 7.34 (m, 2H), 7.14 (d, J = 7.3 Hz, 2H),
7.03 – 6.92 (m, 2H), 6.83 (d, J = 7.3 Hz, 2H), 6.12 (s, 1H), 5.08 (d, J = 15.4 Hz, 1H), 4.88 – 4.76
(
m, 2H), 3.86 (s, 3H), 3.81 (s, 3H), 3.23 – 3.02 (m, 3H), 2.77 – 2.58 (m, 2H), 2.48 (d, J = 12.4
13
Hz, 1H), 2.13 – 2.02 (m, 1H), 1.99 – 1.75 (m, 2H), 1.60 – 1.49 (m, 1H), 0.74 (s, 9H). C NMR
(125 MHz, CDCl ) δ 192.8, 161.2, 160.7, 141.1, 131.1, 130.6, 129.9, 127.8, 126.7 (2C), 124.6,
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20.7, 120.6 (q, 1JC-F = 317.5 Hz), 118.4, 115.0 (2C), 114.3, 72.4, 57.5, 56.7, 55.5, 55.4, 54.9,
+
38.3, 34.2, 32.7, 29.7, 27.5, 20.5. HRMS (ESI-TOF) m/z: [M-CF
3
SO
3
] calcd for C34
H
38
N
3
4
O S
584.2583, found 584.2555.
(4aS,13aR)-7-(tert-butylamino)-2-oxo-1,2-dihydro-6H-4a,13a-
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
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5
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7
8
9
0
propanobenzo[b]pyrido[2',1':2,3]imidazo[1,5-d][1,4]oxazin-13-ium
trifluoromethanesulfonate (13a). White solid (148 mg, 43% yield, eluent: 4.5% MeOH in
1
DCM). mp 156−158 °C. H NMR (500 MHz, acetone-d
6
) δ 9.06 (d, J = 6.8 Hz, 1H), 8.44 (d, J =
9
.2 Hz, 1H), 8.17 – 8.10 (m, 1H), 7.72 – 7.67 (m, 1H), 7.23 (d, J = 10.4 Hz, 1H), 6.23 (dd, J =
10.4, 0.9 Hz, 1H), 5.32 (d, J = 16.1 Hz, 1H), 5.15 (d, J = 16.1 Hz, 1H), 4.57 (s, 1H), 3.66 (d, J =
7.2 Hz, 1H), 3.47 (d, J = 17.2 Hz, 1H), 2.80 – 2.75 (m, 1H), 2.58 – 2.48 (m, 1H), 2.48 – 2.37
1
1
3
(
m, 2H), 2.22 – 2.09 (m, 2H), 1.25 (s, 9H). C{1H} NMR (125 MHz, acetone-d
48.7, 148.6, 137.8, 134.6, 132.0, 127.6, 125.1, 120.6 (q, 1JC-F = 322.5 Hz), 118.3, 113.3, 83.6,
70.4, 58.1, 57.0, 44.6, 37.6, 36.4, 30.1, 21.9. HRMS (ESI-TOF) m/z: [M-CF SO
]+ calcd for
2 352.2025, found 352.2030.
6
) δ 193.8,
1
3
3
C
21
H
26
3
N O
(4aS,12aR)-7-(tert-butylamino)-10-methyl-2-oxo-9-phenyl-1,2-dihydro-6H-4a,12a-
propanobenzo[b]thiazolo[2',3':2,3]imidazo[1,5-d][1,4]oxazin-12-ium
trifluoromethanesulfonate (14a). White solid (173 mg, 58% yield, eluent: 2.5% MeOH in
1
DCM). H NMR (500 MHz, CDCl
3
) δ 7.97 – 7.65 (m, 1H), 7.63 – 7.31 (m, 4H), 7.00 (d, J = 10.4
Hz, 1H), 6.23 (d, J = 10.4 Hz, 1H), 5.08 (d, J = 15.2 Hz, 1H), 4.74 (d, J = 15.2 Hz, 1H), 3.31 (d,
J = 16.9 Hz, 1H), 3.22 – 3.04 (m, 2H), 2.67 – 2.54 (m, 1H), 2.53 – 2.42 (m, 1H), 2.37 (s, 3H),
13
2
1
3
.35 – 2.17 (m, 3H), 2.06 – 1.91 (m, 1H), 0.60 (s, 9H). C NMR (125 MHz, CDCl
3
) δ 193.0,
48.2, 139.8, 132.5, 131.3, 130.6 (2C), 128.8, 127.8, 126.9, 126.3, 125.1, 120.8 (q, 1JC-F =
21.3 Hz), 81.7, 69.7, 57.9, 54.5, 43.0, 35.8, 35.6, 29.7, 21.1, 13.3. HRMS (ESI-TOF) m/z: [M-
+
CF
3
SO
3
] calcd for C26
H N
30 3
2
O S 448.2058, found 448.2046.
(4aS,12aR)-9-(4-bromophenyl)-7-(tert-butylamino)-10-methyl-2-oxo-1,2-dihydro-6H-
a,12a-propanobenzo[b]thiazolo[2',3':2,3]imidazo[1,5-d][1,4]oxazin-12-ium
4
trifluoromethanesulfonate (14b). White solid (189 mg, 56% yield, eluent: 2.5% MeOH in
1
DCM). H NMR (500 MHz, CDCl
3
) δ 8.08 – 7.81 (m, 1H), 7.82 – 7.47 (s, 3H), 7.03 (d, J = 9.4
Hz, 1H), 6.26 (d, J = 9.4 Hz, 1H), 5.27 – 5.03 (m, 1H), 4.90 – 4.67(m, 1H), 3.31 (d, J = 16.2 Hz,
H), 3.25 – 3.01 (m, 2H), 2.82 – 2.62 (m, 1H), 2.57 – 2.46 (m, 1H), 2.40 (s, 3H), 2.36 – 2.21 (m,
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2
2
2
2
2
2
2
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3
3
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5
5
5
5
5
5
5
5
5
6
13
3
H), 2.07 – 1.92 (m, 1H), 0.68 (s, 9H). C NMR (125 MHz, CDCl
3
) δ 193.0, 148.2, 139.9,
131.8, 131.4, 131.3 (2C), 127.7, 127.5, 125.5, 125.1, 125.0, 120.5 (q, 1JC-F = 318.7 Hz) 81.7,
9.7, 57.8, 54.3, 49.7, 42.9, 35.7, 29.7, 21.0, 13.3. HRMS (ESI-TOF) m/z: [M-CF SO
]+ calcd
S 500.1007, found 500.1003.
6
3
3
for C24
H27BrN
3
O
2
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
(4aS,12aR)-7-(tert-butylamino)-2-oxo-9,10-diphenyl-1,2-dihydro-6H-4a,12a-
propanobenzo[b]thiazolo[2',3':2,3]imidazo[1,5-d][1,4]oxazin-12-ium
trifluoromethanesulfonate (14c). Off white solid (187 mg, 57% yield, eluent: 3% MeOH in
1
DCM). mp 202−204 °C. H NMR (500 MHz, CDCl
3
) δ 7.57 – 7.37 (m, 4H), 7.38 – 7.28 (m, 4H),
7.22 (d, J = 7.2 Hz, 2H), 7.06 (d, J = 10.4 Hz, 1H), 6.29 (d, J = 10.4 Hz, 1H), 5.19 (d, J = 15.4
Hz, 1H), 4.80 (d, J = 15.4 Hz, 1H), 3.40 (d, J = 16.9 Hz, 1H), 3.31 (s, 1H), 3.15 (d, J = 16.9 Hz,
1
H), 2.80 – 2.69 (m, 1H), 2.61 – 2.51 (m, 1H), 2.39 – 2.24 (m, 3H), 2.07 – 1.93 (m, 1H), 0.65 (s,
1
3
9
H). C{1H} NMR (125 MHz, CDCl ) δ 192.9, 148.3, 140.2, 131.9, 131.1, 130.49, 130.46,
3
129.9, 129.8, 129.11, 129.07, 128.7, 128.4, 128.2, 126.3, 125.3, 120.7 (q, 1JC-F = 322.5 Hz)
1.5, 69.6, 57.8, 54.3, 42.9, 35.7, 35.3, 29.6, 21.0. HRMS (ESI-TOF) m/z: [M-CF SO
]+ calcd for
S 510.2215, found 510.2209.
8
3
3
C
31
H
32
N
3
O
2
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of Charge on the ACS Publications website at:
DOI: X-ray diffraction data, biological activity data and copies of NMR spectra (PDF).
AUTHOR INFORMATION
Corresponding Author
*
Email: taltal@sharjah.ac.ae.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGEMENTS
This work was supported by generous grants from the Research Funding Department, University
of Sharjah-UAE (15011101007 and 15011101002).
REFERENCES
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