Total Synthesis of K777: Successful Application of Transition-Metal-Catalyzed Alkyne Hydrothiolation toward the Modular Synthesis of a Potent Cysteine Protease Inhibitor

Article abstract of DOI:10.1021/acs.orglett.5b03535

We report the total synthesis of K777 and a series of analogues via alkyne hydrothiolation catalyzed by Wilkinson's complex (ClRh(PPh₃)₃). The alkyne hydrothiolation reactions proceeded with excellent regio- and diastereoselectivity

Full text of DOI:10.1021/acs.orglett.5b03535

Letter
pubs.acs.org/OrgLett
Total Synthesis of K777: Successful Application of Transition-Metal-
Catalyzed Alkyne Hydrothiolation toward the Modular Synthesis of a
Potent Cysteine Protease Inhibitor
Erica R. Kiemele, Matthew Wathier, Paul Bichler, and Jennifer A. Love*
Department of Chemistry, 2036 Main Mall, The University of British Columbia, Vancouver, BC, Canada, V6T 1Z1
S
* Supporting Information
ABSTRACT: We report the total synthesis of K777 and a series of analogues via alkyne hydrothiolation catalyzed by
Wilkinson’s complex (ClRh(PPh3)3). The alkyne hydrothiolation reactions proceeded with excellent regio- and
diastereoselectivity to generate the desired E-linear vinyl sulfides in high yield. The use of Ellman’s auxiliary generates the
requisite propargyl amines in excellent enantiomeric excess (ee) and obviates the use of L-homophenylalanine, an expensive
unnatural amino acid. The vinyl sulfone derivatives exhibit a large difference in rate toward Michael addition. Kinetic data are
consistent with rate-limiting nucleophilic attack to generate the carbanion intermediate.
This approach possesses several advantages over the reported
preparation of K777,11 most notably in eliminating L-
homophenylalanine as a chiral building block. Although our
approach does involve the use of a rhodium catalyst and the
Ellman auxiliary (vide infra), both have been shown to be
recyclable.24,36,37 Additionally, analogues of K777 can be readily
synthesized via alkyne hydrothiolation by simply varying the
thiol employed. In comparison, Horner−Wadsworth−Emmons
(HWE) olefination is used in the reported synthesis, which, in
addition to low atom efficiency, requires the synthesis of each
phosphonate reagent.21,25
We anticipated that an appropriately protected propargyl
amine (4) could serve as a chiral building block en route to
K777 (Scheme 1). The asymmetric synthesis of 4 could be
accomplished via a diastereoselective ethynylation of a
sulfinimide derived from Ellman’s auxiliary.26,27 To this end,
condensation of hydrocinnamaldehyde (5) with (S)-N-tert-
butanesulfinamide (6) gave the corresponding sulfinimide (7)
in 92% yield (Scheme 2). Addition of (trimethylsilylethynyl)-
magnesium chloride at low temperature provided 8 as a single
diastereomer in 73% yield by NMR.38 Desilylation with tert-
butylammonium fluoride (TBAF), hydrolysis of the sulfina-
mide, and diprotection of the free amine as a tert-
butylcarbonate (Boc) provided the target propargyl amine
(4) in 79% yield over three steps.
he design of peptidomimetic molecules for the inhibition
Tof cysteine proteases is a popular strategy for the
treatment of a variety of neglected diseases.1−6 In particular,
K777, a cysteine protease inhibitor, is gaining considerable
attention for the treatment of American Trypanosomiasis
(Chagas disease), a potentially fatal parasitic infection endemic
in Latin American countries.7,8 The vinyl sulfone of K777
irreversibly inhibits Cruzain, the major cysteine protease of
Trypansoma cruzi, the causative organism of Chagas
disease.7−10 Inhibition occurs upon Michael addition of the
active site cysteine residue to the electrophilic vinyl sulfone
“warhead”, resulting in a covalently bound enzyme complex.11
K777 has shown comparable efficacy in preclinical studies to
drugs currently in use without the multitude of significant side
effects of those drugs.9,12−16 As a consequence of the specificity
of vinyl sulfone peptidomimetics, molecules with this functional
group are becoming attractive drug candidates for the treatment
of cancer and neglected global diseases, such as Malaria and
African sleeping sickness.2,4,5,17
Given our interest in the development of catalytic methods
for the formation of vinyl sulfides,18−20 we elected to test such a
strategy in the synthesis of K777. We anticipated that alkyne
hydrothiolation with Wilkinson’s catalyst would generate the E-
linear vinyl sulfide, which upon oxidation would provide the
requisite vinyl sulfone (3). Subsequent peptide coupling with
the known fragment 2 would produce K777 (Scheme 1).
Notably, this constitutes the first use of transition-metal-
catalyzed alkyne hydrothiolation in total synthesis.23
Received: December 13, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.5b03535
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Organic Letters
Letter
Scheme 1. Alkyne Hydrothiolation Approach to K777
Scheme 2. Synthesis of K777
With alkyne 4 in hand, we were poised to investigate alkyne
hydrothiolation with Wilkinson’s catalyst. Initial studies
revealed that double protection of the amine was required for
the highly selective formation of the E-linear vinyl sulfide; a
monoprotected amine resulted in poor selectivity for the E-
linear product.28
Gratifyingly, Boc-protected propargyl amine 4 underwent
alkyne hydrothiolation with excellent regio- and stereo-
selectivity, in the presence of 3 mol % ClRh(PPh3)3 in DCE
at room temperature, to produce the E-linear vinyl sulfide (10)
in 89% yield (25:1 E-linear/branched). Subsequent oxidation
with m-CPBA provided vinyl sulfone 3 in 80% yield and 97%
ee. Lastly, deprotection of vinyl sulfone (3) with TFA, followed
by amide coupling with the known peptide 2,22 afforded K777
(1) in 64% yield. The overall yield of this 10-step synthesis is
21% and is competitive with Palmer’s reported yield of 34%,21
considering the extra steps involved to generate the
homophenylalanine stereocenter. Given the success of this
strategy, we elected to study a series of other arene thiols as a
means to generate analogues of K777.
respectively. Sulfonamides were found to be poor inhibitors of
cruzain. By comparing 11, 12b, and 12c, the size of the P1′
derivative is seen to play a role in inhibition of cruzain, likely
due to changes in binding affinity (KI). However, comparing
12a to 12b reveals a large increase in reactivity for the more
electrophilic sulfonate ester, likely due to an increase in ki.29
We selected a series of para-substituted arene thiols, ranging
from electron-rich to electron-poor, all providing good yields
and excellent selectivity for the E-linear isomers 13−17 (Table
1). Notably, both p-chloro- and p-bromobenzene thiol were
well-tolerated, without detectable cleavage of the C−Cl and C−
Br bonds, respectively (entries 3 and 4). The vinyl sulfides 13−
17 were then converted to the corresponding sulfones 18−22
upon oxidation with mCPBA. The vinyl sulfones 18−22 were
subsequently deprotected with TFA and coupled with peptide
2, providing analogues 23−27 in good yield.
Next we sought to understand the potential effect of varying
the para-substituent (R) on cruzain inhibition. Following the
work of Roush and co-workers,31 we modeled the expected
first-order rates of inhibition of cruzain (ki) by measuring the
solution phase rates of Michael addition for the series of
substituted vinyl sulfones with a model thiol nucleophile (Table
2).
In polar, protic solvents such as methanol, addition of thiols
to Michael acceptors involves two steps: nucleophilic attack to
generate an enolate ion, followed by protonation.33 Relative
rates are highly dependent on the electrophilicity of the β-
carbon and stabilization of the emerging enolate.34,35
Consequently, Roush and co-workers found vinyl sulfonate
ester 28 to be the most reactive Michael acceptor, reacting 25
times faster than vinyl sulfone 29 (Figure 2). This mimics the
reactivity difference in cruzain inhibition between 12a and 12b.
Unsurprisingly, vinyl sulfones with electron-withdrawing
substituents reacted much faster than those bearing electron-
donating substituents. Compound 22 (R = CF3, σ = 0.54)32
reacted 16 times faster than compound 3 (R = H), and 49
times faster than 18 (R = OCH3, σ = −0.27).32 Comparison of
half-lives is much more dramatic: whereas the half-life for
compound 18 is 6 h at room temperature, the half-life for the
CF3 substituted vinyl sulfone is a mere 7 min.
To the best of our knowledge, few analogues have been
reported in which the vinyl sulfone moiety was modified,
despite this functionality being the reactive site in cysteine
protease inhibition (Figure 1, P1 and P1′ fragments).22,29,30 The
second-order inactivation constants of cruzain (ki/KI) have
been reported for these analogues, including the sulfonate ester
12a, and the benzyl and phenethyl sulfones 12b and 12c,
Figure 1. Second-order (ki/KI) inactivation rates of cruzain for K777
Analogues. a Reference 22. b Reference 29.
B
DOI: 10.1021/acs.orglett.5b03535
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Organic Letters
Letter
Table 1. Synthesis of para-Substituted Vinyl Sulfones
a
b
a
c
a
d
entry
R
vinyl sulfide yield (%) E-linear: branched
vinyl sulfone yield (%) ee (%) final product yield (%) overall yield (%)
1
2
3
4
5
6
H
10
13
14
15
16
17
89
52
81
88
80
84
25:1
50:1
50:1
50:1
50:1
50:1
3
80
81
79
68
79
61
96
92
98
99
90
98
1
69
42
78
37
72
41
21
8
OCH3
CH3
Cl
18
19
20
21
22
23
24
25
26
27
21
9
Br
19
9
CF3
a
b
c
1
Isolated yield following column chromatography. Determined by H NMR spectroscopy. Determined by chiral HPLC separation. Details are
d
provided in the Supporting Information. Linear yield from hydrocinnamaldehyde. Yield of 2 is 90%.
The successful application of alkyne hydrothiolation in the
Table 2. Rates of Michael Addition of 2′-(Phenethyl)thiol
a
synthesis of K777 and analogues 23−27 illustrates the potential
of this method for the formation of biologically active cysteine
protease inhibitors. The use of Ellman sulfinamides in the
diastereoselective synthesis of propargyl amines helps avoid the
use of the expensive L-homophenylalanine starting material that
had been used in the other reported synthesis. Kinetic
measurements demonstrate that variation in the P1′ moiety of
K777 can have a significant impact in the reactivity of the vinyl
sulfone as a Michael acceptor. Our methodology provides a
facile method for varying the substitution of the vinyl sulfone
using commercially available and inexpensive thiols. This
approach offers a direct avenue to tailor the pharmacodynamic
properties of K777 derivatives. The Hammett plot for the
reaction between a series of para-substituted vinyl sulfones and
2-phenethanethiol revealed a large positive ρ value of 2.0. This
is consistent with rate-limiting generation of the α-carbanion
succeeding nucleophilic attack of the thiol.
with para-Substituted Vinyl Sulfones
b
entry
R
σ
krel
t1/2 (min)
1
2
3
4
5
6
OCH3 (18)
CH3 (19)
H (3)
−0.27
−0.17
0
0.34
0.52
1
360
231
121
35
Cl (20)
0.23
0.23
0.54
3.49
1.35
Br (21)
90
CF3 (22)
16.4
7
a
Pseudo-first-order reaction rates were determined by 1H NMR
b
spectroscopy. Reactions performed in triplicate. Reference 32.
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.5b03535.
Figure 2. Selection of Michael acceptors assayed by Roush and co-
workers (ref 31).
Experimental details, characterizations, H, 13C, and 19F
1
NMR spectra for new compounds, and HPLC spectra for
ee % determinations (PDF)
Measuring the rates of reaction for a series of para-
substituted compounds allowed us to generate a Hammett
plot for the reaction of 2′-(phenethyl)thiol with vinyl sulfones
(Figure 3). The high positive ρ value of 2.0 is consistent with
rate-limiting nucleophilic attack of the thiol.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: jenlove@chem.ubc.ca.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors thank NSERC (Discovery, Research Tools, and
Instrumentation Grants; CGS-M and CGS-D to E.R.K.) and
the University of British Columbia for support of this work. We
would like to thank Dr. Jason Hein (UBC) for use of their
HPLC and Diana Yu (UBC) for performing the HPLC
separations. We would like to thank Malte Leverenz
(Technische Universitat Munchen) for his assistance in
synthesizing material for the kinetic assays.
̈
̈
Figure 3. Hammett plot for the reaction of 2′-(phenethyl)thiol with
para-substituted vinyl sulfones.
C
DOI: 10.1021/acs.orglett.5b03535
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Letter
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