Cascade reactions of indigo with oxiranes and aziridines: efficient access to dihydropyrazinodiindoles and spiro-oxazocinodiindoles

Article abstract of DOI:10.1039/c8ob00865e

The base-initiated alkylation of the abundant natural dye indigo 1 with ring-strained electrophiles results in the unprecedented, one-pot synthesis of functionalised dihydropyrazino[1,2-a:4,3-a′]diindoles, dihydroepoxy[1,5]oxazocino[5,4-a:3,2-b′]diindoles, and dihydrodiazepino[1,2-a:4,3-a′]diindoles, resulting from intramolecular ring opening-expansion cyclisation processes of their parent oxiranes and aziridines. Regiochemical and stereochemical aspects of the reactions are reported together with integrated mechanistic proposals. This new indigo cascade chemistry should have broad applicability in the synthesis of chemical architectures, not readily-accessible by other means. The three-step synthesis of the useful synthetic precursor (R)-2-(chloromethyl)-1-tosylaziridine 14 is also described. Initial biological activity investigations into these new 2,2′-dindolyl-based heterocyclic derivatives revealed potent, selective antiplasmodial activity in vitro for several isolated structures, with IC₅₀ values as low as 76.6 nM for (±)-8, while demonstrating low human cell toxicity.

Full text of DOI:10.1039/c8ob00865e

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ARTICLE
Cascade reactions of indigo with oxiranes and aziridines: efficient
access to dihydropyrazinodiindoles and spiro‐oxazocinodiindoles
Nicholas M. Butler,a Rudi Hendra,a John B. Bremner,a Anthony C. Willis,b Leonardo Lucantoni,c Vicky
Received 00th January 20xx,
Accepted 00th January 20xx
M. Avery,c and Paul A. Kellera*
DOI: 10.1039/x0xx00000x
The base‐initiated alkylation of the abundant natural dye indigo 1 with ring‐strained electrophiles results in the
unprecedented,
one‐pot
synthesis
of
functionalised
dihydropyrazino[1,2‐a:4,3‐a']diindoles,
www.rsc.org/
dihydroepoxy[1,5]oxazocino[5,4‐a:3,2‐b']diindoles, and dihydrodiazepino[1,2‐a:4,3‐a']diindoles, resulting from
intramolecular ring opening‐expansion cyclisation processes of their parent oxiranes and aziridines. Regiochemical and
stereochemical aspects of the reactions are reported together with integrated mechanistic proposals. This new indigo
cascade chemistry should have broad applicability in the synthesis of chemical architectures, not readily‐accessible by other
means. The three‐step synthesis of the useful synthetic precursor (R)‐2‐(chloromethyl)‐1‐tosylaziridine 14 is also described.
Initial biological activity investigations into these new 2,2'‐dindolyl‐based heterocyclic derivatives revealed potent, selective
antiplasmodial activity in vitro for several isolated structures, with IC50 values as low as 76.6 nM for (±)‐8, while
demonstrating low human cell toxicity.
We have previously reported the N‐alkylation of indigo with
allylic and propargylic halides, which initiate a complex domino
process where the electron‐rich C=C or C≡C bond reacts with
Introduction
There has been a resurgence in the fields of both discovery‐ and
diversity‐oriented synthesis as tools for the realisation of new
molecular architectures, particularly through multi‐bond‐
forming cascade reactions.1‐4 Our fortuitous discovery of the
complex cascade chemistry of indigo (
allowed access to a range of new 2,2 ‐diindole‐based systems,
after initial base‐induced N‐alkylation of indigo in procedurally‐
simple, one‐pot reactions (Scheme 1).5‐8
The synthesis of 2,2ꞌ‐biindole derivatives is of particular
importance, as they constitute a broad family of molecules with
diverse bioactivity profiles, including as kinase inhibitors,
antimicrobials and antiparasitic agents.9‐14 Conceptually, indigo
would represent an ideal starting point for the synthesis of such
molecules as it is produced annually on kilo‐tonne scales for the
dyeing of denim and other textiles, however despite its
popularity and colourful history, there is a surprising dearth of
understanding about the accessible chemistry of indigo.
the indigo nucleus, where the nature of the terminal π‐bond
plays a defining role in the pathway direction as the cascade
proceeds. Spirocyclic products predominate with the use of
allylic bromides, resulting from nucleophilic attack of the allyl
C=C bond on the indigo C2 position, while the rigidity of the
propargyl pendant served to suppress this pathway, instead
leading to products resulting from either further N‐alkylation
and subsequent cyclisation, or strain‐driven ring‐expansion
(Scheme 1). Encouraged by the promising antiplasmodial
activity of several derived species from these initial
investigations,6 we wished to further explore this pathway
divergence, with a view to accessing spirocyclic scaffolds
incorporating new heteroatoms to assess their biological
relevance. Thus we investigated the reactions of indigo in the
presence of base with alkyl halides incorporating a terminal
oxirane or aziridine group. We hypothesised that ring‐opening
of these strained, electrophilic systems by the indigo core could
be utilised as a trigger for further reactions with the newly‐
exposed nucleophilic heteroatom, promoting the cascade
process. Furthermore, the prevalence and availability of chiral
oxiranes and aziridines would allow access to these geometries
in a stereospecific fashion. Therefore, as a direct three‐carbon
analogue to previously‐explored propargylic and allylic
substrates, we explored the reactivity of indigo with
(halomethyl)oxiranes and aziridines, as well as selected four‐
carbon homologues in their dimeric bioxirane and biaziridine
1) and its family has
ꞌ
a. School of Chemistry, University of Wollongong, Wollongong, NSW 2522,
Australia.
b. School of Chemistry, The Australian National University, Canberra, ACT 2601,
Australia.
c. Eskitis Institute for Drug Discovery, Griffith University, Brisbane Innovation Park,
Don Young Road, Nathan 4111, Australia.
† Footnotes relaꢀng to the ꢀtle and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [Full experimental procedures counterparts, in order to probe their utility toward the
and 1H and 13C NMR spectra for all compounds and HPLC traces and crystallographic
data for .]. See DOI: 10.1039/x0xx00000x
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O
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DOI: 10.1039/C8OB00865E
H
3
O
N
O
N
N
5
6'
2'
1
N
H
7
4'
O
Indigo
1
2
Br
Br
IC50 0.85
M
O
N
•
O
N
N
N
N
O
O
O
N
O
N
N
H
O
3
4
5
6
IC50 8.9
M
IC50 9.9 M
IC50 9.4
M
IC50 6.3 M
Scheme 1. Selected examples of molecules isolated from cascade reactions of indigo with allylic and propargylic bromides, with associated IC50 data in vitro against P. falciparum
(K1 strain).
construction of new poly‐heterocyclic frameworks, and to lead to major changes in the outcome of the reaction.7
probe the antiplasmodial potential of these compounds.
Therefore, under optimised conditions, a warmed, sonicated
suspension of indigo ( ) in DMF was reacted with pre‐dried and
1
ground cesium carbonate (3.7 eq.), and the mixture heated for
one hour in an oil bath preheated to strictly 85–87 ˚C under an
inert nitrogen atmosphere. The nitrogen flow was cut, then (S)‐
Results and Discussion
Reactions of indigo with 2‐(halomethyl)oxiranes and 2,2ꞌ‐bioxirane
epichlorohydrin (
mixture stirred vigorously at 85–87 ˚C for 30 minutes, resulting
in the isolation of three compounds (Table 1) – the N,N ‐cyclised
7, optimised to 5.0 eq.) injected, and the
Our previous experience with the cascade reactions of indigo
demonstrated that small variations in reaction conditions can
ꞌ
Scheme 2. Base‐mediated cascade reaction of indigo producing heterocycles 8‐10, with ORTEP depiction of compounds 9, and 10. Adjacent to compounds 8‐10 is pictured a
solution of each in dichloromethane, highlighting the colour. Yields reflect Table 1, Entries 1 and 6 respectively. The synthesis of compound 10 was variable and was isolated
with a small amount of grease contaminant present, as analysed by NMR, which could not be removed despite repeated attempts. For the structure of 11, please see Scheme
3. Note: The X‐ray structure atom numbers are different from the systematic atom numbers.
2 | J. Name., 2012, 00, 1‐3
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Indigo
1
8
, and N,O
yield (Scheme 2).
The major product was the dark blue N,N
dihydropyrazinodiindole , which was isolated in 44% yield by
ꞌ‐spirocyclised products 9 and 10 – in a combined 95%
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DOI: 10.1039/C8OB00865E
1. DMF, N2, 60 °C, ))), 1 h
ꞌ
‐cyclised
2. Cs2CO3, 3Å sieves,
8
85-87 °C, 1 h
O
recrystallisation from the crude reaction mixture. Analysis of
the HRESI mass spectrum indicated a peak at m/z 341.0888,
assigned as [C19H14N2O3Na]+, revealing the addition of a single
, 2 h
O
3.
Cl
O
3'
1
glycidyl subunit to the indigo core. Analysis of the H NMR
O
spectrum showed a broad singlet resonance at 5.28 ppm which
exchanged with D2O and showed no direct C‐H attachment
through gHSQC analysis, and was assigned as a hydroxyl group.
Analysis of the 13C‐APT spectrum indicated the retention of both
carbonyls of the parent scaffold, assigned to resonances at
179.9 and 179.7 ppm. The gHMBC spectrum revealed strong,
three‐bond correlations between methine H6 and both C4a,
and C13b; and methylene H7 with both C8a, and C13a,
respectively, and the alkyl chain was therefore determined to
bridge both indigo nitrogen atoms.
1'
O
1
4
13
15
N
N
O
10
O
6
8
(±)-11 (X-ray)
11
6%
+
+
+
(±)-8
(±)-9
(±)-10
44%
6%
7%
The spiroketal compound 9 was isolated as orange plate crystals
Scheme 3. Reaction of indigo with (±)‐epichlorohydrin (7) to give spirocycle (±)‐11, and
the ORTEP drawing of racemic 11. Yields reflect Entry 3, Table1. Adjacent to the structure
is a picture of a solution of 11 in dichloromethane, showing its colour in ambient light.
in 30% yield, and showed brilliant yellow‐green fluorescence in
CH2Cl2 solution, both in ambient light, and under UV light (365
nm). Analysis of the HRESI mass spectrum revealed the addition
of a single glycidyl subunit by a peak at m/z 341.0900, assigned
as the [M+Na]+ molecular ion. The rigid, spirocyclic alkyl tether
was assigned by analysis of the gHMBC spectrum, which
showed a strong three‐bond correlation between three proton
resonances at 4.26, 4.37, and 5.08‐5.16 ppm (assigned as H8a,
H8b and H7, respectively) to a quaternary carbon at 110.9 ppm,
assigned as C9a. 13C‐APT spectral analysis revealed a single
carbonyl resonance at 188.1 ppm, indicating that one carbonyl
remained intact. There was no evidence of peak doubling in the
13C‐spectrum, suggesting the compound to be free of multiple
diastereomers, and x‐ray crystallography confirmed the
catalyst (entry 5) resulted in mainly polymeric baseline
materials. Product distribution correlated with the reaction
time when using (halomethyl)oxiranes; longer reaction times
typically led to diminished overall recovery and increased
isolation of polymeric baseline materials. The N,O
11), not seen in the original reaction (Scheme 2, Table 1, Entry
1), was isolated as orange plate crystals in 6% yield from the
reaction of indigo with (±)‐epichlorohydrin at 85–87 ˚C for 2 h
(Scheme 3). Subsequent optimisation gave 11 in 20% yield using
(S)‐7 (Table 1, Entry 6).
Table 1. Optimisation studies of the reaction of indigo with epichlorohydrin (
ꞌ‐spirocycle
(
7),
and other (halomethyl)oxiranes.
structure of
9, as well as the relative stereochemical
Baseline
material
(%mass)
8
(%)
9
(%)
10
(%)
11
(%)
configuration at the C7 and C9a carbon atoms.
Halide Time Additives
Compound 10 was isolated as orange plate crystals in 21% yield.
Analysis of the gHSQC spectrum indicated three methylene
carbons at 52.2, 61.9, and 67.6 ppm, which were assigned as C6,
30
Cl (7)
1
2
44
41
44
43
5
30
9
21
min
C3
ꞌ
, and C8, respectively by analysis of the zTOCSY spectrum,
3 Å
sieves
3 Å
sieves
3 Å
sieves
b
b
Cl†
Cl†
Cl†
Cl
1 h
2 h
30
46
9
1
which showed long‐range coupling between H resonances at
3.74 and 4.27 ppm, assigned as H6a and H8, respectively.
Further analysis of the 1H‐spectrum revealed the presence of a
pair of spin‐coupled resonances at 6.07 and 6.87 ppm, assigned
3
6
7
8
6c
30
min
30
min
30
min
30
min
30
4
17
to H2
ꞌ
and H1
ꞌ
respectively in the trans‐enamine moiety, which
‐methylene by COSY and zTOCSY
b
coupled with the C3
ꞌ
5
KIa
84
experiments. The multiplet ranging from 5.01–5.05 ppm was
assigned as H7 by gCOSY analysis, which showed correlations to
both H6 and H8, allowing complete assignment of the
spirocyclic ring, as confirmed by x‐ray crystallography. Despite
its isolation from numerous reactions during the optimisation
process, its synthesis was variable.
6
Cl
41
32
84
76
30
21
20
7d
8
Cl
6
Br
min
10
Dihydropyrazinodiindole
8 was the major product for all
9
OTs
min
reactions of indigo with epichlorohydrin derivatives, and
interestingly, the substitution of the chloride leaving group for
the more reactive bromide (Table 1, entry 8) or tosylate (entry
a 0.1 eq. (based on electrophile). b Detected by ESI‐MS/TLC analyses of the crude
mixture, but not isolated. c See Scheme 3. Entry 1 describes outcomes as illustrated
in Scheme 2. d Reaction performed using acetonitrile as solvent at reflux. † racemic
chloride used.
9) groups resulted in the sole isolation of
8 in high yield (84%
and 79% respectively), while in contrast, the addition of a KI
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the product moreover confirms that a potential Payne
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rearrangement of the α‐hydroxyepoxidDeOrIe: 1s0u.1lt0i3n9g/Cf8rOomB00r8in65gE‐
opening16 – which would produce the corresponding trans‐
dioxolane – had not occurred, and the stereogenic integrity of
the precursor bioxirane had been retained.
Compound 13, a N‐substituted derivative of 7, was isolated as a
pale yellow‐green, amorphous solid in 19% yield. Analysis of the
1H‐NMR spectrum of 13 revealed geminal proton resonances at
δ 2.32 and δ 2.57, assigned as H4
oxirane substituent, which TOCSY experiments revealed to be
distinct from the N,O ‐spiro tether. This secondary bioxirane
unit was confirmed to be a result of N ‐alkylation by analysis of
the HMBC spectrum, which showed a correlation between the
C1 methylene protons and C13a.
ꞌa and H4ꞌb of the pendant
ꞌ
ꞌ
ꞌ
Reactions of indigo with N‐sulfonyl‐2‐(halomethyl)aziridines, N‐
tosyl‐2‐phenylaziridine and N,Nꞌ‐ditosyl‐2,2ꞌ‐biaziridine
N‐Sulfonylaziridines show comparable reactivity to oxiranes
with respect to rates of ring‐opening and Lewis acid‐mediated
1,3‐dipolar cycloadditions.17 The reactions of both activated and
non‐activated aziridines have been extensively reviewed.18‐20 It
was anticipated that the use of the corresponding N‐sulfonyl‐2‐
(halomethyl)aziridines would provide similar outcomes when
reacted with indigo to those obtained from reactions with 2‐
(halomethyl)oxiranes, to give spiro‐oxazolidine‐containing
heterocycles, therefore a small series of simple aziridine
precursors was investigated.21,22 Aziridinyl chloride 14 (which
until now had not been synthesised chirally) was prepared
Scheme 4. One‐pot cascade reaction of indigo with (2S,2'S)‐2,2'‐bioxirane to produce
compounds 12 and 13, and the ORTEP drawing of compound 12. Below compound 12
is a picture of a solution in acetone, showing its fluorescence under UV light (365 nm);
a solution of 13 in dichloromethane (below) shows its colour under ambient light.
HRESI mass spectral analysis revealed the incorporation of both
two glycidyl subunits and a carboxylate fragment, evidenced by
a peak assigned as [M+H]+ at m/z 419.1257, corresponding to
the molecular formula C23H18N2O6. Analysis of the 1H‐spectrum
of 11 revealed a methine resonance at 5.49 ppm assigned to
conveniently from (S)‐epichlorohydrin 7 in 64% yield over three
steps (see experimental section for details). This was then
reacted with indigo under the same optimised conditions as for
H2
resonances at 5.20, 4.30, 4.28 and 4.11 ppm assigned to H3
H3 b, H1 a and H1 b of the cyclic carbonate pendant,
respectively. X‐ray crystallographic analysis confirmed the
structure of 11, as well as the relative diastereomeric
configuration of the three stereogenic carbons.
ꞌ
, which showed correlations in the gCOSY spectrum with
epichlorohydrin, affording the N,Nꞌ‐cyclised product 15 in 46%
ꞌ
a,
yield as a glassy blue solid (Scheme 5), in addition to a mixture
of several indigo adducts which proved inseparable by flash
chromatography, or fractional crystallisation. Subsequent
preparative RP‐HPLC separation of this fraction afforded
compounds 16 and 17 in 8% and 9% yield, respectively, in
addition to a fraction deemed to contain a variety of polymeric
ꞌ
ꞌ
ꞌ
In an extension of these oxirane reactivity studies, a mixture of
indigo and (2S,2ꞌS)‐2,2ꞌ
‐bioxirane15 was reacted for 30 min, and
indigo adducts. As with their oxygenated analogue
8, solutions
upon workup gave a complex mixture, from which two major
products (12 and 13, Scheme 4) were isolated in a combined
81% yield. Interestingly, both were determined to be products
of compounds 15 17 showed characteristic dark‐purple
colouration in a variety of solvents in ambient light.
It was anticipated that replacement of the chloro with a bromo
leaving group would lead to greater selectivity toward cyclic
‐
of N,Oꞌ‐spirocyclisation, with no N,Nꞌ‐cyclised products
observed. Both compounds were also determined to be
modestly photosensitive to ambient light in solution.
products 15‐16, however the combined high reactivity of this
aziridinyl bromide, and the demonstrated nucleophilicity of the
liberated sulfonamide, led to uncontrollable polymerisation
over longer reaction times. Optimal conditions were established
by reacting indigo with (±)‐N‐tosyl‐2‐(bromomethyl)aziridine22
for 5 min, to afford (±)‐15 in 52% yield, and a complex mixture
of polymeric materials. Moderation of the sulfonyl activator to
The substituted spiro‐dioxolane 12 was isolated as bright red‐
orange plate crystals in 62% yield, and in solution showed
brilliant yellow‐green fluorescence under ambient and UV (365
nm) light. The relative stereochemistry of the dioxolane unit
was determined by analysis of the ROESY spectrum, which
revealed strong through‐space correlations between the
resonances at 4.74 and 5.00 ppm, assigned as protons H8 and
H7, respectively, consistent with a cis‐arrangement of the C6‐C7
bond and the hydroxymethyl pendant to the dioxolane ring.
This configuration was confirmed by X‐ray crystallographic
analysis, and the absence of peak doubling in the 13C spectrum
indicated only one diastereomer. The stereochemical purity of
the
smaller N‐(methanesulfonyl)aziridine dramatically
increased the sensitivity of the product heterocycles, producing
only uncharacterisable polymeric baseline materials.
To determine whether homocyclisation would occur in the
absence of a halomethyl tether, indigo was alkylated with (±)‐
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1. DMF, N2, 60 °C, ))), 1 h
2. Cs2CO3, 85-87 °C, 1 h
O
3.
H
Ts
N
N
Cl
, 30 min
N
14
H
O
Indigo
1
O
N
O
O
N
O
N
O
O
N
N
N
NHTs
Cl
NHTs
NTs
NTs
NTs
15
46%
16
8%
17
9%
Scheme 5. Synthesis of (tosylamidomethyl)dihydropyrazinodiindoles 15‐17 from the reaction of indigo with aziridinyl chloride 14. Solutions of compounds 15, 16, and 17 (left to right)
in dichloromethane are pictured, showing their colour under ambient light. Although compound 16 was synthesised on multiple occasions, its successful isolation from the large pool
of additional material (including polymers) was achieved only once.
N‐tosyl‐2‐phenylaziridine (18), and yielded exclusively acyclic nucleophile, the reaction of (2R,2
ꞌR) N,Nꞌ‐ditosyl‐2,2ꞌ‐biaziridine
adducts 19 and 20 in a combined yield of 98% (Scheme 6). with indigo afforded the diazepinodiindole 21 in excellent yield
The mono‐alkylated indigo derivatives 19 and 20 were isolated (Scheme 7).
as intense, dark‐blue powders in 62% yield and 36% yield, Compound 21 was isolated as a dark blue powder in 93% yield,
respectively, resulting from either terminal (19) or benzylic (20
)
which showed intense deep‐purple colouration in
ring‐opening of the aziridine. Interestingly, despite the dichloromethane solution. Analysis of the 13C‐NMR spectrum
proximity of the nucleophilic sulfonamide to the adjacent revealed a pair of carbon resonances at 180.8 and 180.9 ppm,
carbonyl, no evidence for homocyclisation of either 19 or 20 suggesting both carbonyl units of the parent indigo had
was observed, even upon prolonged heating under basic remained intact. Analysis of the ROESY spectrum revealed a
conditions, supporting the hypothesis that further cyclisation through‐space correlation between the H6 and H7 proton
under these conditions is dependent on the presence of resonances at δ 4.95 and 3.62 ppm, suggesting a cis orientation,
suitably‐electrophilic functionalities on the N‐alkyl substituent. inferring that epimerisation of the parent aziridine had been
We have recently utilised N,N
ꞌ
‐ditosyl‐ and N,N
ꞌ
‐diBoc‐2,2
ꞌ
‐
avoided under these conditions.
biaziridine (derived from either D‐ or L‐tartaric acid) as
convenient precursors to small libraries of enantiopure vicinal
diamines and imidazolidinones by ring‐opening using a diverse
range of nucleophiles.23‐25 Using the indigo anion as a 1,4‐di‐
Mechanistic discussion
Reactions of indigo with (halomethyl)oxiranes and 2,2ꞌ‐bioxirane
Indigo
Epihalohydrins in polar aprotic solvents undergo preferential
nucleophilic attack by ring‐opening prior to halide
displacement,26,27 with epoxide ring‐opening being up to 200
times more favourable than direct SN2 attack at the halide.
Therefore, the proposed mechanisms for these reactions
considered the enthalpic benefit of ring‐opening as the
1
1. DMF, N2, 60 °C, ))), 1 h
2. Cs2CO3, 85-87 °C, 1 h
Ts
3.
N
Ph
, 10 min
7'
18
O
O
4
H
N
7'
4'
4
H
3
3
N
2'
1. DMF, N2, 60 °C, ))), 1 h
2. Cs2CO3, 85-87 °C, 1 h
O
O
2'
13
1
+
6
6
N
N
4'
3.
O
O
, 20 min
NTs
TsN
Indigo
1
N
3
N
Ph
Ph
2'''
1'''
10
6
NHTs
NHTs
8
93%
NHTs
6a
19
62%
20 (X-ray)
36%
NHTs
21
20
Scheme 7. Reaction of indigo with (2R,2ꞌR)‐2,2'‐biaziridine to give compound 21. Below
the structure is pictured a solution of 21 in dichloromethane.
Scheme 6. Synthesis of acyclic aziridine adducts 19 and 20 from indigo by either terminal
(19) or benzylic (20) ring‐opening, and ORTEP drawing of benzylic product 20. Adjacent to
each is depicted a solution in acetonitrile of 19 (left, below) and 20 (right, below).
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Scheme 8. Proposed mechanism for the formation of 8 from indigo and (S)‐epichlorohydrin. Cesium counter‐ions omitted for clarity.
energetic driving force for subsequent cyclisation reactions with reflected by the increase in yield from 44% to 84% by
indigo (Scheme 8). It was observed that moderation of the replacement of X=Cl with X=Br.
leaving group from Cl to Br suppressed spiroketalisation, It is proposed that spiro‐oxazocinodiindoles 9‐11 are derived
suggesting that the identity of the halide plays a key role in from intermediate A where it instead follows Path B (Scheme
determination of the mechanistic pathway of the reaction. It is 9), where the chloride is eliminated by the oxygen of the indigo
proposed
(halomethyl)oxirane by indigo produces the key intermediate A, rigid 8‐membered ring of intermediate C. The rigidity of this
from which Path A (Scheme 8) and Path B (Scheme 9) diverge. macrocycle impedes attack of the pendant alkoxide to the
Compound is derived from Path A (Scheme 8), where adjacent indoleninyl C3‐position, and directs the nucleophile to
intermediate A undergoes halide elimination by the liberated the re‐face, thereby defining the C9a stereochemistry of
alkoxide, to reform the epoxide. Prototropic tautomerisation spirocycle . Further alkylation with additional epichlorohydrin
that
nucleophilic
ring‐opening
of
the core via a dearomative 8‐exo‐tet cyclisation to forge the semi‐
8
9
about the indigo double bond occurs readily in the presence of may occur to give D, allowing nucleophilic attack by cesium
cesium carbonate,6 which subsequently affords the stabilised carbonate, to ring‐open the epoxide pendant affording
cis anion B, and a 6‐exo‐tet displacement of the more‐ intermediate E, which undergoes subsequent dehydration to
substituted carbon of the epoxide results in inversion at C6 and afford cyclic carbonate ester 11. There have been several
formation of the dihydropyrazinodiindole
8. This process would reported examples of direct addition of oxiranes to ketones to
become more favourable with better leaving groups, as
Scheme 9. Proposed mechanism for the formation of compounds 9 and 11 from intermediate A via Path B.
6 | J. Name., 2012, 00, 1‐3
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halide. Reactions with (halomethyl)aziridines likely follow an
View Article Online
analogous pathway to that in Scheme D8O, I:w10h.e10r3e9/rCin8gO‐Bc0lo0s8u65reE
leads to N‐alkylindigo intermediate G which undergoes
prototropic tautomerisation of the central bond, and N‐
alkylation to afford compound 15 (Scheme 12). Subsequent
alkylation of the sulfonamide with excess chloroaziridine 14
affords halosulfonamide 17, which in the presence of base
undergoes a 3‐exo‐tet cyclisation to afford aziridine 16
.
Compounds 19 and 20 (see Scheme 6, vide supra) are the result
of direct nucleophilic attack of the indigo nitrogen on a single
unit of 2‐phenylaziridine, with the absence of a leaving group
Scheme 10. E2 elimination from intermediate D afforded compound 10.
form ketals28‐30 though these typically proceed via activation of quickly terminating the cascade process. Interestingly, the two
the oxirane with strong Lewis‐ or Brönsted acid catalysts. The represent differing site‐selectivities for nucleophilic attack,
formation of cyclic carbonates from the addition of CO2 to where ring‐opening at the less‐hindered position is favoured
oxiranes is also known,31 however this typically requires the use over benzylic ring‐opening in a 1.7:1 ratio. Preferential
of high pressures and/or elevated temperature. The low yield electrophilicity of the benzylic position of N‐tosyl‐2‐
(6%) of 11 reported here is attributed to the weakly‐ arylaziridines has been previously reported in the presence of
nucleophilic character of the carbonate or bicarbonate ions. Lewis acids eg. BF3.OEt2, or Pd(II) complexes, resulting in
Compound 10 could similarly be derived from D with cesium spontaneous ring‐opening to externally‐stabilised 1,3‐dipoles –
carbonate acting as a base, via an E2‐displacement (Scheme 10). under such conditions, the sulfonamide‐Lewis acid complex
The reaction of indigo (1) with (2S,2ꞌS)‐2,2ꞌ‐bioxirane, where the bears a formal negative charge, and a benzylic carbocation is
halide is replaced with a second oxirane unit, results in generated.32‐34 In the absence of a Lewis acid, the 32% yield of
suppression of Path A. The reaction instead favours Path B the benzylic product 20 is attributed to the minor steric effect
(Scheme 11), producing hydroxymethyl‐substituted spirocycle of the phenyl substituent providing only modest site‐selectivity,
12 by an analogous mechanism to that proposed for the leading to a mixture of both terminal and benzylic adducts.
formation of compound 9, whereby the rigid 8‐membered ring In our hands, N‐substituted‐2,2
ꞌ‐biaziridines have been shown
(intermediate F) restricts nucleophilic attack at the spiro‐carbon to undergo facile, sequential ring‐opening reactions at the less‐
to the si‐face. The stereochemical configuration of the bioxirane hindered C1 and C4 positions, with mono‐nucleophiles.23‐25
moiety is conserved in the cis‐spirodioxolane product, with the Contrary to these previous reports, the use of indigo as a di‐
formation of a trans‐dioxolane via a Payne rearrangement not nucleophile (Scheme 14), results in biaziridine exclusively
observed under these conditions.16 Subsequent N‐alkylation of undergoing 1,3‐nucleophilic di‐ring‐opening, driven by the
12 with a second unit of the bioxirane material affords formation of the stable 7‐membered diazepinodiindole 21, with
compound 13
.
the high yield (93%) suggesting this to be a highly favourable
process. Presumably, the reason for this contrary site‐selectivity
– via a 1,3‐di‐alkylation rather than 1,4‐di‐ring‐opening – is the
Reactions of indigo with aziridines
There was no evidence of spiro‐oxazolidine formation from flexibility of the mono‐ring‐opened chain, whereby steric
reactions of indigo with aziridine‐containing heterocycles. This repulsion between the indigo core and the tosyl activator
is largely due to the requirement of strong N‐activating groups, positions the pendant aziridine distally from the N‐nucleophile,
which both stabilise the liberated heteroanion and provides restricting attack at the terminal C4 carbon. Though
considerable steric encumbrance to O‐displacement of the macrocyclisation of indigo toward 7 and 8‐membered rings has
Scheme 11. Proposed mechanism for the formation of hydroxymethylspirodioxolanes 12 and 13 from indigo via intermediate F. Cesium counterions are omitted for clarity.
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Journal Name
O
O
H
N
O
View Article Online
DOI: 10.1039/C8OB00865E
H
H
N
N
Path A
14
Cs2CO3
3-exo-tet
N
N
N
H
O
O
O
TsN
Cl
TsN
G
1
Protoisomerisation,
then 6-exo-tet
O
O
N
O
O
N
O
O
N
Cs2CO3
14
Cs2CO3
3-exo-tet
N
N
N
NTs
NTs
NHTs
Cl
NHTs
NTs
16
15
17
Scheme 12. Formation of pyrazinodiindoles 15‐17 from the reaction of indigo with (halomethyl)aziridines via Path A.
been reported previously in low yield with alkyl dichlorides,35 these complex multicyclic frameworks can be selectively
evidently this steric effect provides sufficient impedance to the synthesised in one pot and in good yields from the
formation of diazocinodiindole 22 that this otherwise‐typical exceptionally‐cheap starting material indigo. Also of note are
1,4‐di‐ring opening pathway is not observed, and the more instances where cyclisation occurs between the indigo nitrogen
entropically‐favourable heptacycle is exclusively obtained.
The reactions of indigo with strained‐ring electrophiles (both
oxiranes and aziridines) are scalable up to at least half‐gram
quantities. In comparison to our previous experiences of the
chemistry of indigo (see Scheme 1), π‐nucleophiles invoke
cascade reactions by nucleophilic attack at the indole C2 with
N‐allyl substituents,5,7 or ring‐expand the biindole nucleus to
indolo‐naphthiridinones with N‐propargyl substituents,6 while
the strained‐ring electrophiles explored in this study have
largely retained the integrity of the indigo core, allowing its
integration of new spiro‐heterocyclic frameworks. Importantly,
these reactions proceed with the stereochemical identity of the
parent oxiranes and aziridines intact, suggesting that these
cascading processes are the result of sequential nucleophilic
displacements, rather than radical chain‐reactions.
Figure 1. Dihydropyrazinodiindoles
Staurosporine analogue MP072, a potent checkpoint kinase inhibitor with activity
against the HT‐29 (colorectal adenocarcinoma) cell line.41
8 and 15 synthesised from indigo, and
The formation of both substituted and non‐substituted epoxy‐
oxazocinodiindolones 13 is unprecedented, and represents
the first report of these poly‐heterocyclic systems. Importantly,
9
‐
Scheme 13. Comparison of reaction outcomes between previous reports,23‐25 and those reported here. Sequential 1,3‐di‐ring‐opening of biaziridine affords diazepinodiindole 21,
rather than the anticipated diazocinodiindole 22.
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atoms, affording the core dihydropyrazinodiindole scaffold of the human HEK 293 cell line, with SI values rangingVfireowmArt9icl8e‐O1n2lin0e.
staurosporine analogue MP072 (Figure 1) in a single step. The The activity of most compounds against DthOeI:d10ru.1g03‐r9e/Csi8sOtaBn0t08D6d52E
presence of malleable functional groups such as hydroxyl‐ and strain followed a similar trend as against 3D7, with IC50 ratios
indolyl‐functionalities could allow for the incorporation of these ranging between 1.2 (11) and 2.6 (±)‐8 (only the weakly active
heterocycles into other complex architectures, allowing compound 15 showed an IC50 ratio of 4.5, still within the
structural diversification over two steps. The encouraging accepted limits of equipotency).
bioactivities of these scaffolds demonstrate these to be new
and effective starting points for the design and synthesis of new
biologically‐active small molecules. The unique absorptive and
Conclusions
fluorescent properties of these and other indigo derivatives
may also warrant further investigation – specifically into their
materials chemistry applications, as has been previously
noted.36‐40
We report here a series of new cascade reactions of the
chemically‐overlooked dye indigo via base‐mediated alkylation
with ring‐strained electrophiles. The ability of indigo to act as
both nucleophile and electrophile throughout the duration of
the reaction – combined with the bifunctional activity of
epoxides and aziridines – initiates, and then propagates a
cascade process, accessing unprecedented polycyclic
architectures from simple, one‐pot reactions. Importantly,
divergent access to a variety of both heteroatom‐substituted
dihydropyrazinodiindoles and dihydrodiazepinodiindoles via
Biological Testing
Given the promising antiplasmodial potential of the indigo core
(see Scheme 1),6,7 indigo derivates
8,11,13, 15‐17, and 19‐21
were subjected to in vitro antiplasmodial testing against the
malarial parasite Plasmodium falciparum (Table 2); compounds
9
and 10 were also tested but they were inactive. All tested
N,N
examples of both substituted and unsubstituted
oxazocinodiindoles via N,O ‐spirocyclisation. In particular, the
potent antiplasmodial activity of some of the synthesised N,N
ꞌ‐cyclisation is possible, as well as the first documented
compounds containing either the N,N
ꞌ
‐cyclic scaffold (
8,
15‐
17,
and 21) or the mono‐N‐substituted indigo scaffold (19 and 20
)
ꞌ
revealed notable antiplasmodial activity in the micromolar
range against both the drug‐susceptible 3D7 and drug‐resistant
Dd2 strains of P. falciparum, sufficient for each to be considered
potential leads for further development. In comparison with the
ꞌ
‐
cycles with good selective toxicity warrants further
investigation as new leads for the treatment of malaria. That
this diverse array of chemical transformations can be achieved
from the cheap and readily‐available precursor indigo
demonstrates the significant potential in the efficient
construction of novel fused‐heterocyclic frameworks from this
multi‐faceted natural dye. We are currently pursuing the
discovery of the next‐generation of cascade reactions of indigo,
aiming towards the development of a greater understanding of
the cascade chemistry of indigo, to investigate the chemical,
biochemical, and photophysical characteristics of these new
polycyclic architectures.
previous lead compound 2 (IC50 0.85 μM), (±)‐8, (S)‐8, and (R)‐8
displayed exceptional and equivalent potencies (3D7 IC50 0.077,
0.1, and 0.09 μM, respectively), showing significantly greater
activity than 2.6 Compounds of subset
8 in particular are more
than an order of magnitude more potent than all other
compounds disclosed herein. Importantly, these new
compounds show diminished cytotoxicity toward the human
HEK 293 cell line, with SI (selectivity index; 3D7/HEK293) ratios
>10 in most cases, and for the stereoisomers of
8, all show two
orders‐of‐magnitude‐greater activity against P. falciparum than
Table 2. In vitro antiplasmodial and human cell toxicity data of selected indigo derivatives from this work.
Compound ID
IC50 3D7 (nM) IC50 Dd2 (nM) IC50 HEK293 (nM)a Dd2/3D7 IC50 ratio SI (3D7/HEK293 IC50 ratio)
(±)-8
76.6 ± 4.0
88.6 ± 16.3
105.6 ± 37.4
2767 ± 731
3075 ± 600
3698 ± 731
3976 ± 801
4653 ± 2472
5625 ± 1775
9411 ± 2082
9922 ± 3476
1.09 ± 0.03
5.81 ± 0.13
0.40 ± 0.05
141.9 ± 12.7
4.00 ± 0.05
201.7 ± 7.4
164.5 ± 32.2
195.5 ± 0.2
4594 ± 1623
3869 ± 2121
6009 ± 2406
8118 ± 55
8127 ± 5605
25209 ± 5459
16076 ± 4192
21501 ± 5817
1.74 ± 0.50
94.6 ± 24.9
0.57 ± 0.08
147.4 ± 23.3
IA
7726 ± 467
10708 ± 1090
10361 ± 1028
14677 ± 2000
66% at 80 µM
IA
2.6
1.9
1.9
1.7
1.3
1.6
2.0
1.7
4.5
1.7
2.2
1.6
16.3
1.4
101
120
98
5.3
> 11
> 21
> 20
> 17
> 28
> 2
(R)-8
(S)-8
21
16
19
20
IA
IA
IA
17
15
13
64% at 80 µM
IA
71% at 10 µM
67% at 40 µM
62% at 10 µM
1202 ± 3
11
> 8
Artesunate
Chloroquine
Dihydroartemisinin
Puromycin
Pyrimethamine
> 2000
> 3000
> 10000
8.47
> 1250
1.0
(resistant)
73% @ 40 µM
aPercent inhibition at highest concentration is given if an IC50 value could not be calculated due to dose‐response curve not reaching full inhibition plateau; IA = inactive
(i.e. < 50% inhibition at the highest concentration).
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DOI: 10.1039/C8OB00865E
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Conflicts of interest
There are no conflicts to declare.
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Acknowledgements
We acknowledge the Australian Government for the award of
Australian Government Research Training Program Award to
NB. We also thank the University of Wollongong Centre for
Molecular and Medical Bioscience, the Australian National
University, Griffith University, and the Griffith Institute for Drug
Discovery for their support in preparation of this manuscript.
The authors wish to thank and acknowledge the Australian Red
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