Increasing synthetic efficiency via direct C-H functionalization: Formal synthesis of an inhibitor of botulinum neurotoxin

Article abstract of DOI:10.1039/c1cc10755k

A new and efficient scheme for the synthesis of one of the best known inhibitors of botulinum neurotoxin serotype A (BoNTA) is reported herein. The synthetic route involves two palladium-catalyzed C-H functionalization reactions, formally activating three C-H bonds. The Royal Society of Chemistry 2011.

Full text of DOI:10.1039/c1cc10755k

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Increasing synthetic efficiency via direct C–H functionalization: formal
synthesis of an inhibitor of botulinum neurotoxinwz
Shathaverdhan Potavathri, Abhishek Kantak and Brenton DeBoef*
Received 8th February 2011, Accepted 25th February 2011
DOI: 10.1039/c1cc10755k
A new and efficient scheme for the synthesis of one of the best
known inhibitors of botulinum neurotoxin serotype A (BoNTA)
is reported herein. The synthetic route involves two palladium-
catalyzed C–H functionalization reactions, formally activating
three C–H bonds.
Botulinum neurotoxin serotype A (BoNTA) is a protein
produced by a spore-forming bacteria called Clostridium
botulinum. It is the world’s most poisonous protein having a
median lethal dose of approximately 1 ng/Kg.1 The toxin is
Fig. 1 Structure of thiophene-indole BoNTA inhibitors.
associated with numerous food-borne illnesses and is a potential
We and others have recently developed catalytic methods for
directly arylating heterocycles involving either one or two C–H
functionalizations.11–16 These methods offer the benefit of
greater step economy by using readily available hydrocarbon
starting materials.17,18 Herein, we present a novel method for
synthesizing inhibitor 1 using two key palladium catalyzed
C–H activation steps.
bioweapon. Consequently, the synthesis of small molecule
inhibitors of BoNTA is of high importance.2,3 Previous work
has shown that development of BoNTA inhibitors is particularly
challenging because the enzyme-substrate interface is unusually
large. Recently, Pang computationally docked potential
inhibitors into the zinc endopeptidase active site of BoNTA
and then synthesized and tested the best drug candidates
(Fig. 1).4–7 The best inhibitors consisted of two heterocyclic
halves: a 2-phenlyindole and a 2-phenylthiophene (1, Ki =
15 mM).6 Further computational and synthetic studies recently
showed that further elaboration of the 2-aryl thiophene moiety
provided increased BoNTA inhibition (2, Ki = 5.4 mM).4
In general, two synthetic strategies can be envisioned for the
synthesis of these aryl substituted heterocycles (Scheme 1): the
biaryl bond could be synthesized via an organometallic
coupling or the heterocycle could be constructed via annulation
of an aryl-substituted linear substrate. Pang and co-workers
employed both of these paths by cyclizing a 2-alkylaniline
to form the 2-arylindole substructure and by arylating a
2-bromothiophene in a Suzuki reaction.4
We have recently shown that electron-rich aromatic hetero-
cycles and benzene-derived arenes can be oxidatively cross-
coupled by palladium catalysis. For example, both we and the
Fagnou group have shown that N-acetylindoles can be regio-
selectively coupled with benzene at either their 2- or 3-position
depending on the nature of the oxidant chosen for the
reaction.19–23 We have extended this methodology to include
N-alkylindoles, a considerably more challenging class of
compounds due to their tendency to decompose in the oxidative
reaction conditions. In particular, we have found that
N-alkylindoles bearing electron-withdrawing groups provide
high levels of regioselectivity, favoring the 2-aryl-product.19
Consequently, 1 appeared to be an ideal target for the
application of our oxidative coupling technology.
Contrastingly, Itahara developed a method for oxidatively
coupling heteroarenes such as indoles, pyrroles and furans
with benzene in the early 1980’s.8–10 These original reactions
required superstoichiometric amounts of palladium acetate.
We commenced our study by attempting the oxidative
arylation of the butylphthalimide protected ester indole 13,
but low regioselectivity was observed (14; 3 : 1, 2-Ph : 3-Ph),
and the two regioisomers were inseparable by flash chromato-
graphy (Scheme 2). However, selective oxidative arylation of
the SEM-protected indole 15,24 using catalytic palladium
acetate (PdOAc2) and silver acetate (AgOAc) as an oxidant,
afforded 16 in a good yield with a 9 : 1 regioselectivity favoring
arylation at the indole’s 2-position, and these regiomers could
be separated via flash chromatography. The optimal conditions
contained a slight excess of AgOAc (3 equiv.) compared
to pivalic acid (PivOH, 2.5 equiv.), which likely prevents the
Department of Chemistry, University of Rhode Island,
51 Lower College Road, Kingston, RI 02881, USA.
E-mail: bdeboef@chm.uri.edu
w This article is part of a ChemComm web-based themed issue on new
advances in catalytic C–C bond formation via late transition metals.
z Electronic supplementary information (ESI) available: Procedures
for the synthesis of all new compounds and their characterization
(1H-NMR, 13C-NMR, and mass spectra). See DOI: 10.1039/
c1cc10755k
c
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Chem. Commun., 2011, 47, 4679–4681 4679

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Scheme 1 Comparison of synthetic schemes for constructing arylated heterocycles.
acid-promoted oxidative decomposition of the indole substrate
and products (Scheme 3).19
Alkaline hydrolysis of 16 afforded the free acid 17 in a good
yield. In situ conversion of 17 to the acid chloride, followed by
Friedel–Crafts acylation, as described by Pang,4 was attempted,
but the desired ketone was not observed. Later, the acid
chloride was reacted with the anion of methyl-3-thiophene-
acetate to afford the desired product 18 in a modest 38%
isolated yield. The isomer which is coupled at the thiophene’s
2-position (proximal to the ester), was isolated as a minor
product (6% yield). We attribute the selectivity favoring the
desired product to steric factors (Scheme 3).
In an effort to expand our methodology to form biaryl C–C
bonds by activating two C–H bonds, we initially attempted to
oxidatively couple benzene to thiophene 18 using our earlier
Scheme 2 Oxidative arylation of N-alkyl protected indole.
Scheme 3 Synthetic scheme for BoNTA inhibitor 1.
c
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4680 Chem. Commun., 2011, 47, 4679–4681

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optimization studies as a guide,19,22 but the starting material
failed to convert. This may be the result of the ability of
the thiophene to deactivate the palladium catalyst via
coordination of its sulfur atom. However, using Mori’s
conditions, the C–H bond of the thiophene 18 was selectively
arylated using both palladium and silver catalysts and iodo-
benzene as the arene source.25 The SEM protecting group of
compound 19 was cleaved by treatment with tetrabutyl-
ammonium fluoride (TBAF) in DMF to afford 20 and
subsequent N-alkylation was carried out with N-(4-bromobutyl)-
phthalimide in the presence of potassium tert-butoxide to
afford the N-alkylindole 21. Finally, the BoNTA inhibitor 1
was synthesized following the procedure of Pang by treating
21 with excess hydroxylamine, which simultaneously converts
the methyl ester and phthalimide to hydroxamic acid and
amine, respectively (Scheme 3).4
Notes and references
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8 T. Itahara, J. Chem. Soc., Chem. Commun., 1981, 254.
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In conclusion, we have developed a novel synthetic route for
the synthesis of an inhibitor of botulinum neurotoxin serotype
A (BoNTA). The 4.6% overall yield is nearly identical to that
which was reported by Pang, but the step economy of the
overall synthesis has been increased due to the incorporation
of reactions which do not rely on prefunctionalized starting
materials; rather the biaryl C–C bonds were directly formed
from the C–H bonds of simple arenes and heteroarenes. It should
also be noted that the C–H functionalizing reactions are two of the
highest yielding steps in this synthesis. While our initial attempts to
form biaryl C–C bonds with thiophene-type substrates such as 18
were not successful, efforts in our laboratory are currently focused
on expanding our oxidative coupling methods to encompass these
and other classes of heteroarenes for application to the synthesis of
more high-value targets.
21 D. R. Stuart, E. Villemure and K. Fagnou, J. Am. Chem. Soc.,
2007, 129, 12072–12073.
22 T. A. Dwight, N. R. Rue, D. Charyk, R. Josselyn and B. DeBoef,
Org. Lett., 2007, 9, 3137–3139.
23 D. R. Stuart and K. Fagnou, Science, 2007, 316, 1172–1175.
This work was supported by the National Science Founda-
tion (CAREER 0847222) and a Pilot/Feasibility Grant
from the Rhode Island INBRE Program (NIH-NCRR
P20RR016457).
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c
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Chem. Commun., 2011, 47, 4679–4681 4681