The total synthesis of K-252c (staurosporinone) via a sequential C-H functionalisation strategy

Article abstract of DOI:10.1039/c5sc04399a

A synthesis of the bioactive indolocarbazole alkaloid K-252c (staurosporinone) via a sequential C-H functionalisation strategy is reported. The route exploits direct functionalisation reactions around a simple arene core and comprises of two highly-select

Full text of DOI:10.1039/c5sc04399a

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The total synthesis of K-252c (staurosporinone) via
a sequential C–H functionalisation strategy†
Cite this: DOI: 10.1039/c5sc04399a
*
J. C. Fox,‡ R. E. Gilligan,‡ A. K. Pitts, H. R. Bennett and M. J. Gaunt
A synthesis of the bioactive indolocarbazole alkaloid K-252c (staurosporinone) via a sequential C–H
functionalisation strategy is reported. The route exploits direct functionalisation reactions around
a simple arene core and comprises of two highly-selective copper-catalysed C–H arylations, a copper-
catalysed C–H amination and a palladium-catalysed C–H carbonylation, which build up the structural
complexity of the natural product framework.
Received 17th November 2015
Accepted 12th January 2016
DOI: 10.1039/c5sc04399a
www.rsc.org/chemicalscience
The indolocarbazole alkaloids K-252a–d (1–4) are microbial we report a concise synthesis of the indolocarbazole alkaloid K-
metabolites rst isolated in 1986 from culture broths of 252c starting from a commercial toluidine starting material.
Nocardiopsis sp. K-252 and Nocardiopsis sp. K-290 (Fig. 1a).1 The Seven direct functionalisations are used to transform C–H
compounds were found to be potent inhibitors of protein bonds of the aniline into the substituents required for the
kinase C, an enzyme family known to play a critical role in hexasubstituted arene core of the natural product architecture.
a myriad of signal transduction pathways associated with Despite the increasingly complex architecture that results from
metabolism, gene expression, membrane transport and cell each step, a notable feature of this synthesis is the exquisite
proliferation.2 Consequently, the protein kinase C family of selectivity of each C–H transformation, thereby highlighting the
proteins has become an important therapeutic target for several efficacy of sequential C–H functionalisation strategies.9
disease classes, particularly in oncology, resulting in sustained
interest in the design and development of inhibitors as poten- toluidine, a simple commercial aniline, could be used to launch
tial pharmaceutical agents.3
a sequential C–H functionalisation strategy for the synthesis of
At the outset of our design stage, we questioned whether p-
In light of their intriguing biological properties and potential K-252c, 3, (Scheme 1). Many of the previous syntheses use
as pharmaceutical agents, the indolocarbazole alkaloids have a common strategy to assemble the indolocarbazole framework
attracted considerable interest from the synthetic community. that is based on the union of two indole molecules with
In particular K-252c, (3, staurosporinone),4 the potential a synthetic precursor to the lactam ring system, followed by an
biosynthetic precursor and aglycone of staurosporine 5,5 has oxidative electrocyclisation to form the poly(hetero)aromatic
attracted a number of elegant syntheses. We were attracted to framework.4,5 Most elegant among these syntheses is the work
these natural products because of their interesting biological of Wood and co workers who developed a concise approach to
activity and the unusual functional pattern of the hexasub-
stituted arene framework. Our group has a long standing
interest in the synthesis of natural products that rely on strat-
egies based on the sequential functionalisation of the C–H
bonds in simple aromatic building blocks (Fig. 1b).6–8 We
envisaged that K-252c, 3, could provide a platform to further
extend this total synthesis strategy by orchestrating a series of
direct functionalisations on a readily available aniline to form
the fully substituted benzene core of the target molecule.
Furthermore, such a modular strategy would be amenable to
the preparation of analogues of this important scaffold. Herein
Department of Chemistry, Department of Cambridge, Lenseld Road, Cambridge, CB2
1EW, UK. E-mail: mjg32@cam.ac.uk
† Electronic supplementary information (ESI) available: 1H and 13C NMR spectra,
and crystallographic data. CCDC 1431476. For ESI and crystallographic data in CIF
or other electronic format see DOI: 10.1039/c5sc04399a
‡ JCF and REG contributed equally to the work in this manuscript.
Fig. 1 Strategy for the synthesis of K-252 indolocarbazole alkaloids.
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Scheme 1 The synthesis of K-252c.
this class of natural products.5c Despite the simplicity of this four out of seven direct functionalisations on the aromatic
type of strategic disconnection, it remains difficult to engineer framework of this complex molecule.
an unsymmetrical indolocarbazole framework from such
The total synthesis of K-252c commenced from the readily
a convergent route. As part of our synthetic strategy towards available dibenzyl p-toluidine 7, available in multigram quan-
staurosporinone, we envisaged that a sequential C–H func- tities (Scheme 1). The rst C–H functionalisation of the arene
tionalisation approach might benet from late-stage carbazole core involved the union of the commercial diphenyliodium
formations in order to modulate the electron density of the triate with 7 through the action of a simple copper(II) catalyst,
central arene core (Fig. 2). The lactam would originate from a biaryl coupling strategy recently developed by our group.10 The
a C–H carbonylation directed by the benzylamine motif, which ortho-selective arylation proceeded in good yield and on a mul-
itself could be introduced through a radical-mediated benzylic tigram scale to form 8; we did not detect any of the corre-
oxidation. These disconnections would reveal a teraryl frame- sponding di-ortho-phenylation product, which is oen
work 6, displaying an amine motif that would control the a problem with comparative metal-catalysed arylations. We
selective installation of the adjacent nitro group and a series of believe that the selectivity of this process is due to the low
ortho- and meta-C–H arylations, leading back to p-toluidine. reactivity of the product to subsequent arylations. A clash
Taken together, the amine motif of the aniline directly controls between the ortho-phenyl substituent and the N,N-dibenzyl
group forces the amino group to rotate to alleviate the steric
interaction; the lone pair on the nitrogen rotates out of conju-
gation with the arene, reducing the arene nucleophilicity and
making it less reactive towards a second arylation.
To install the second aryl group, we envisaged the use of
a copper-catalysed meta-arylation that has also been recently
developed in our laboratory. To achieve this, however, the N-
benzyl substituents needed to be switched to a carbonyl-con-
taining group, which is important to impart the desired meta-
selective arylation.11 The nature of the carbonyl group needed to
Fig. 2 Retrosynthetic analysis of K-252c 3.
carefully considered with respect to both the effectiveness in the
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meta-arylation, its stability to subsequent reaction conditions possibly due to the deactivating effect of the resident nitro
and its compatibility with the oxidative C–H amination that group. To overcome these difficulties, a selective radical bis-
would be required to form the carbazole at a later point in the bromination of the benzylic position (to 12a, Scheme 2),16a fol-
synthesis. We had previously shown that meta-arylation was lowed by treatment with DMF and K2CO3 afforded the desired
best effected by a pivalamide group, but there are no examples aldehyde functionality with concomitant cleavage of the
of C–H aminations using amines derived with this bulky group. carbazole N-protecting group.16b This sequence could be
In contrast, meta-arylation directed by simple acetamides was combined into a one-pot procedure to supply the desired alde-
oen low yielding due to competitive amide cleavage, although hyde in 54% yield on a gram scale.
there are a number of C–H aminations that can be initiated by
While we reasoned that reductive amination of aldehyde 13
this simple amide motif. In balancing these factors we elected with a suitable amine would lead to a precursor for the
to investigate the propionyl group in the hope that the extra size proposed C–H carbonylation to form the g-lactam, we were
of the ethyl substituent would render the amide less liable to conscious that primary amines coordinate strongly to metals
cleavage, but would still prove effective in the carbazole such as palladium and oen inhibit catalytic activity. Therefore,
formation step. Therefore, a one-pot hydrogenolysis and we elected to carry out the reductive amination with a benzylic
subsequent carbamoylation with propionyl chloride enabled amine with the intention of adopting Orito's C–H carbonylation
the formation of anilide 9 in excellent 96% yield on a multi- to the desired protected g-lactam.17 However, the choice of
gram scale. Pleasingly, a slight modication to our copper-cat- amine required further consideration as the use of benzyl
alysed meta-arylation worked well and treatment of anilide 9 amine would lead to selectivity problems in the carbopallada-
with diphenyliodonium triate in the presence of copper(I) tion step of the C–H carbonylation (selectivity between I and II,
iodide, silver(I) triate (providing an in situ source of copper(I)
triate) and solid sodium hydrogen carbonate (to buffer the
reaction in light of the formation of triic acid as a by-product)
produced the teraryl intermediate 10 in 71% yield on a 4 g scale
(the mass balance was unreacted starting material).
With teraryl 10 in hand, efforts next focused on the addition
of the second nitrogen substituent. It was envisaged that
installation of a nitro group, which could be subsequently
employed in a late-stage reductive cyclisation to a carbazole,
would be the optimal strategy. Treatment of 10 with concen-
trated nitric acid in a cooled solution of triuoroacetic anhy-
dride and triuoroacetic acid yielded the desired product in
Scheme 2 Telescoped synthesis of aldehyde 13.
60% yield, although attempts to scale-up the reaction were
unsuccessful, resulting in lower yields. Interestingly, we found
that replacing the triuoroacetic anhydride with tri-
uoromethanesulfonic anhydride,
a modication of an
underutilised aromatic nitration protocol developed by Hill and
co-workers,12 led to signicant improvements in both the yield
and selectivity. Furthermore, the modied conditions were
scalable, enabling an exquisitely selective and gram scale reac-
tion to produce the nitro-arene 11 in 92% yield.
With both nitrogenous groups installed on the central arene
framework, attention was turned to the C–H amination to form
one of the two-carbazole units. Several metal catalysed C–H
aminations to carbazoles have been developed in recent years,13
but unfortunately, we found that palladium-catalysed methods
for carbazole formation were either capricious or resulted in
none of the desired product when applied to our system.14
Pleasingly, the copper-catalysed oxidative C–H amination
protocol developed by Chang and co-workers employing phe-
nyliodide bis-triuoroacetate and copper(II) triate effected the
carbazole formation (to 12) in good yield (70% on 3.58 g scale,
88% on 100 mg scale).15
We next turned to the installation of the g-lactam motif that
we speculated could arise from an advanced benzaldehyde.
Despite the availability of a number of methods to affect to the
oxidation of the benzylic methyl group to a benzaldehyde, all
attempts to directly access 13 proved unsuccessful. This is Scheme 3 Optimisation of protecting group for C–H carbonylation.
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Scheme 3a). Mindful of this, we prepared a number of benzyl-
amine variants displaying ortho-substituents to block the
competitive C–H activation (Scheme 3b). We were pleased to
nd that 2,6-dimethylbenzylamine performed well in both the
reductive amination and the palladium-catalysed C–H carbon-
ylation to form g-lactam 15a.
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ꢀ
min at 210 C), 16 could be isolated in 85% yield without the
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afford K-252c 3 proved to be challenging, but aer a survey of
a range of standard N-debenzylation techniques we found that
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data for which were consistent with those reported in the
literature.
Conclusions
In summary, we have successfully applied a sequential C–H
bond functionalisation strategy to the synthesis of K-252c. The
overall yield if the synthesis from a readily available aniline (7)
is 12.7%. The sequence of direct functionalisations follows
a logical order of reactivity that sees the amine group of the
starting toluidine control four of the C–H functionalisations.
The synthesis showcases transformations comprising of two
selective copper-catalysed C–H arylations, a highly selective
electrophilic nitration, two C–H amination protocols to form
carbazoles, a benzylic methyl oxidation and a palladium-cata-
lysed C–H carbonylation. The synthesis is amenable to scale-up,
providing access to intermediates on gram or multi-gram scale.
Furthermore, it is envisaged that this modular approach could
be used to provide rapid access to analogues of this biological
important class of molecule, as well as complex hexasubstituted
benzenes.20 Current research in our laboratory is focused on
applying this sequential C–H functionalisation strategy to the
synthesis of staurosporinone analogues and other complex
natural products that could benet from such a strategy.
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Acknowledgements
We are grateful to EPSRC and ERC for funding. Mass spec-
trometry data were acquired at the EPSRC UK National Mass
Spectrometry Facility at Swansea University.
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