Cross-Dehydrogenative Cyclization-Dimerization Cascade Sequence for the Synthesis of Symmetrical 3,3′-Bisoxindoles

Article abstract of DOI:10.1021/acs.orglett.1c01799

The synthesis of symmetrical 3,3′-bisoxindoles from simple acyclic β-oxoanilides is reported. The described method forges three new C-C bonds in a single step via a sequential Mn(OAc)3·2H2O mediated oxidative radical cyclization-fragmentation-dimerization process. The scope of this reaction is demonstrated in the preparation of a variety of 3,3′-bisoxindoles, as well as its application toward the formal synthesis of the Calycanthaceae alkaloid, (±)-folicanthine.

Full text of DOI:10.1021/acs.orglett.1c01799

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Letter
Cross-Dehydrogenative Cyclization−Dimerization Cascade
Sequence for the Synthesis of Symmetrical 3,3′-Bisoxindoles
Farhaan Dobah,† C. Munashe Mazodze,† and Wade F. Petersen*
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ABSTRACT: The synthesis of symmetrical 3,3′-bisoxindoles from simple
acyclic β-oxoanilides is reported. The described method forges three new
C−C bonds in a single step via a sequential Mn(OAc)3·2H2O mediated
oxidative radical cyclization-fragmentation−dimerization process. The
scope of this reaction is demonstrated in the preparation of a variety of
3,3′-bisoxindoles, as well as its application toward the formal synthesis of
the Calycanthaceae alkaloid, ( )-folicanthine.
isoxindoles represent a particularly exciting class of
Bcompounds for synthetic organic chemists. Not only do
they possess interesting biological activities,1 but they serve as
important synthetic intermediates in the construction of
complex natural products.2 Specifically, they are perfectly
poised to forge the contiguous quaternary stereogenic centers
found in cyclotryptamine alkaloids (Figure 1),3,4 which
themselves have significant biological profiles.
Figure 1. Cyclotryptamine alkaloids.
An obvious disconnection in visualizing the formation of the
quaternary bisoxindole framework identifies the strategy of
Figure 2. (a−c) Previously reported approaches for the synthesis of
simply combining two oxindole derived fragments together.
Therefore, it is not surprising that the overwhelming majority
of reports toward the synthesis of bisoxindoles utilize this
approach, namely, additions to isatin derivates5 and the direct
dimerization of oxindole intermediates (Figure 2a),6 promoted
either chemically or photochemically. Other methods involving
already cyclic starting materials include C3/C3′ functionaliza-
tion of monosubstituted bisoxindoles,7 as well as the
functionalization of 3-indolyl-oxindoles.8 By contrast, methods
for the synthesis of bisoxindoles from acyclic precursors are
rare and typically involve a double cyclization protocol starting
from bis-anilides (Figure 2b).9 Rarer still are methodologies
that construct the bisoxindole framework from simple
monomeric anilides through a sequential cyclization-dimeriza-
tion process; the only such report is by Zhang who made use of
a copper-catalyzed arylation−dimerization sequence as the key
bisoxindoles.
step toward the total synthesis of various cyclotryptamine
alkaloids (Figure 2c).9d Herein, we report a one-step oxidative
cross-dehydrogenative cyclization−fragmentation−dimeriza-
tion sequence for the synthesis of symmetrical bisoxindoles
(Figure 3), with the key cyclization step proceeding via a
formal C(sp2)−H/C(sp3)−H activation, and its application
Received: May 31, 2021
Published: July 7, 2021
https://doi.org/10.1021/acs.orglett.1c01799
© 2021 American Chemical Society
Org. Lett. 2021, 23, 5466−5470
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toward the synthesis of the Calycanthaceae alkaloid,
( )-folicanthine.
Table 1. Selected Optimization Results of the One-Step
Cyclization-Dimerization Cascade Sequence
a
X =
yield of 10a
yield of 11a
entry
H/OH
oxidant
equiv
0.50
(%)
(%)
b
1
H
Mn(OAc)3·
2H2O
Mn(OAc)3·
2H2O
Mn(OAc)3·
2H2O
Mn(OAc)3·
2H2O
53
37
92
81
32
11
−
b
2
OH
H
0.50
3.0
cd
,
3
ce
,
4
OH
3.0
−
a
Isolated yields as an ∼1:1 mixture of separable meso:( )-D,L-
Figure 3. Our approach for the one-step synthesis of bisoxindoles
from acyclic precursors.
b
c
d
diastereomers. Under air. Under argon. ACN as the solvent, reflux.
e
THF as the solvent (vigorous reflux).
In conceptualizing our one-step strategy, we identified three
key disconnections (Figure 3). Namely, the formation of the
desired bisoxindole 10 from the homocoupling of methine
radical 9 (Figure 3a), generated in turn via a homolytic
fragmentation of a suitable oxindole 7/8 (Figure 3b), that
could be obtained using an oxidative cross-dehydrogenative
cyclization reaction (Figure 3c). The cross-dehydrogenative
cyclization10 and the homocoupling steps6 are well-established
as independent transformations in the literature and therefore
the only consideration remaining was selecting an appropriate
group Z (Figure 3), as a delayed radical precursorstable
enough to promote the oxidative cross-dehydrogenative
cyclization, but labile enough to afford radical intermediate 9
in the subsequent step of the cascade sequence. This led us to
identify oxoanilides 5 and 6 (where Z = formyl (5) and
carboxylic acid (6)), respectively, Figure 3), as key starting
materials since both cross-dehydrogenative cyclizations of
related oxoanilides,10 as well as oxidative deformylation11,12
and decarboxylation13 processes, have been reported in the
literature. Furthermore, we considered the relative stability of
starting materials 5 and 6, and by extension accessing the
related oxindoles 7 and 8, to be more attractive throughout the
entire envisage cascade, when compared to other functional
groups for the homolytic generation of radical 9.14 It is
noteworthy to mention that aldehydes 5 are remarkably
stableup to 6 months refrigerated and up to 1 week of bench
storageand their syntheses are often shorter than the related
acid 6 (see the Supporting Information).
Table 1 shows selected results of our optimization studies
using oxoanilides 5a and 6a as model substrates (see Table S1
in the Supporting Information for full data). We initially set
out to develop a metal-catalyzed process as either via a
deformylative or decarboxylative strategy and using air as the
terminal oxidant to regenerate the active catalyst15 for each
requisite oxidation step.
In the event, using a 50 mol % catalyst loading of
Mn(OAc)3·2H2O (∼17 mol % per oxidation step) produced
the desired bisoxindole 10a in 53% and 37% yield, from the
corresponding aldehyde (5a) and acid (6a), respectively,
together with a 32% and 11% yield of hydroxyoxindole 11a,
respectively (Table 1, entries 1 and 2). It quickly became clear
that the interception of methine radical 9a (Figure 3) by O2
under the aerobic conditions in the formation of 11a, rather
than the desired homocoupling, was a significant competing
process. Gratifyingly, switching to an anaerobic system and
using a stoichiometric amount of oxidant, albeit 1 equiv per
requisite oxidation step, ultimately (and following solvent
optimization; Table S2 in the Supporting Information),
produced the ideal set of conditions for both aldehyde 5a
and carboxylic acid 6a, affording 10a in 92% and 81% yield,
respectively (Table 1, entries 3 and 4). With these conditions
in hand, we explored the substrate scope of the cascade
sequence utilizing both starting materials (Scheme 1), paying
attention to any distinct advantages between the deformylative
(5) and decarboxylative (6) strategies.
Various alkyl groups (R1) were well-tolerated, producing
bisoxindoles 10a−10h in yields of 56%−93%. Modification of
the protecting group (R2) on the aniline afforded the N-benzyl
protected bisoxindole 10i in yields of 90% and 91%, from the
corresponding aldehyde and carboxylic acid, respectively.
Variation of the substituents around the aromatic ring (R3)
utilizing halides, electron-withdrawing, and electron-releasing
groups, afforded 10j−10q in 70%−96% yield as well as
disubstituted bisoxindoles 10r−10s in 59%−95% yield. All
things considered, these two strategies, namely, via a
deformylative or decarboxylative fragmentation, appear largely
complementary. A specific point of difference encountered
during the course of our work, however, was found in the
synthesis of bisoxindole 10t, which was produced in 65% yield
from acid 6t. Conversely, 10t could not be accessed via the
corresponding aldehyde 5t as its synthesis via our prescribed
formylation failed.
The proposed mechanism of the reaction, using the
formation of 10a as a representative example, is shown in
Scheme 2 (see Supporting Information for full details). The
oxidative cross-dehydrogenative cyclization step likely pro-
ceeds in accordance with current mechanistic thinking,
involving conversion to oxindoles 7a/8a, from 5a/6a,
respectively, via two sequential oxidative single electron
transfer (SET) processes.10 Following deprotonation of 8a to
the corresponding carboxylate anion, a third oxidative SET
process generates methine radical 9a with concomitant loss of
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Scheme 1. Substrate Scope and Application to the Synthesis of ( )-Folicanthine
formal synthesis in only seven steps, starting from N-
methylaniline (Scheme 1). Meso-10h was also suitable for
this transformation, producing meso-12 in 66% yield (after
recrystallization), which can similarly be envisaged toward the
total synthesis of meso-folicanthine.
Scheme 2. Proposed Mechanism
In summary, we have developed a Mn(OAc)3·2H2O
mediated one-step synthesis of symmetric bisoxindoles enabled
by an oxidative cross-dehydrogenative cyclization−dimeriza-
tion cascade sequence through the use of aldehydes and
carboxylic acids as delayed radical precursors. The utility of
this reaction was demonstrated by the synthesis of a diverse
library of bisoxindoles as well as its application to the formal
synthesis of ( )-folicanthine.
ASSOCIATED CONTENT
■
sı
* Supporting Information
CO2,14 which subsequently homocouples to afford bisoxindole
10a. The mechanism for the generation of radical 9a from 7a is
envisaged to proceed in accordance known deformylation
processes mediated by polynuclear metal complexes.11 We had
considered an alternative pathway occurring via oxidation of
aldehyde 7a to carboxylic acid 8a, followed by subsequent
generation of radical 9a. In this scenario, however, an
additional oxidation step would be required and would imply
a maximum theoretical yield of 75% over the cascade sequence
(with 3 equiv of oxidant). As our yields obtained were
generally above this threshold, it would suggest that this
pathway is unlikely, but we cannot rule this out in all cases.
TEMPO trapping experiments with both 5a and 6a failed to
produce any of the desired dimer 10a (see Scheme S1 in the
Supporting Information). In the case of acid 6a, ketoamide 15a
was the major product isolated in 45% yield, while 5a afforded
ketoamide 15a in 33% yield together with hydroxyoxinolde
11a in 19% yield  overall supporting our proposed cascade
sequence.
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.1c01799.
Experimental procedures and compound characteriza-
tion data (PDF)
AUTHOR INFORMATION
■
Corresponding Author
Wade F. Petersen − Department of Chemistry, University of
Cape Town, Cape Town 7700, South Africa; orcid.org/
0000-0003-3215-5560; Email: wade.petersen@uct.ac.za
Authors
Farhaan Dobah − Department of Chemistry, University of
Cape Town, Cape Town 7700, South Africa
C. Munashe Mazodze − Department of Chemistry, University
of Cape Town, Cape Town 7700, South Africa
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.1c01799
To further demonstrate the synthetic utility of this reaction,
bisoxindole ( )-D,L-10h was easily converted into azide
( )-D,L-12 in 71% yield (after recrystallization), which is a
key intermediate in Ghosh’s total synthesis of the Calycantha-
ceae alkaloid ( )-folicanthine,5b and thus completed the
Author Contributions
†These authors contributed equally.
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We would like to acknowledge the Central Analytical Facilities
(Stellenbosch University, South Africa) for their assistance in
obtaining the HRMS data. We would also like to thank the
Royal Society and the African Academy of Sciences (No. FLR
\R1\190531, to F.D., C. M. M., and W.F.P.), the Royal Society
of Chemistry (No. RF19-2196), the National Research
Foundation (No. UID117747, to W.F.P.) and the University
of Cape Town for their funding contributions.
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