Synthesis and evaluation of phenylalanine-derived trifluoromethyl ketones for peptide-based oxidation catalysis

Article abstract of DOI:10.1016/j.bmc.2016.07.012

We report the synthesis of phenylalanine-derived trifluoromethyl ketones for the in situ generation of dioxiranes for the purpose of oxidation catalysis. The key features of this synthesis include the use of a masked ketone strategy and a Negishi cross-coupling to access the parent amino acid. The derivatives can be readily incorporated into a peptide for use in oxidation chemistry and exhibit good stability and reactivity.

Full text of DOI:10.1016/j.bmc.2016.07.012

Bioorganic & Medicinal Chemistry xxx (2016) xxx–xxx
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry
journal homepage: www.elsevier.com/locate/bmc
Synthesis and evaluation of phenylalanine-derived trifluoromethyl
ketones for peptide-based oxidation catalysis
⇑
Aaron L. Featherston, Scott J. Miller
Department of Chemistry, Yale University, PO Box 208107, New Haven, CT 06520-8107, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
We report the synthesis of phenylalanine-derived trifluoromethyl ketones for the in situ generation of
dioxiranes for the purpose of oxidation catalysis. The key features of this synthesis include the use of a
masked ketone strategy and a Negishi cross-coupling to access the parent amino acid. The derivatives
can be readily incorporated into a peptide for use in oxidation chemistry and exhibit good stability
and reactivity.
Received 17 June 2016
Revised 5 July 2016
Accepted 6 July 2016
Available online xxxx
Ó 2016 Elsevier Ltd. All rights reserved.
This manuscript is dedicated to Professor
William L. Jorgensen on the occasion of his
receiving the Tetrahedron Prize
Keywords:
Peptide
Catalysis
Oxidation
Dioxirane
Amino acid
1. Introduction
replacements for the aliphatic ketones we had studied previously.
Tuning of the arene was a particular advantage we coveted, and,
Since oxidation remains a fundamental maneuver in organic
synthesis, the development of new approaches to oxidation
catalysis remains an intensely pursued research area. In the area
of olefin epoxidation catalysis, an impressive list of biocatalysts,1
metal-based catalysts,2 and organocatalysts3 seems to be growing
without any indication of ebb. Among the most widely employed
epoxidation catalysts is the remarkable carbohydrate-derived Shi
ketone,4 which has proven to be a seminal contribution (Fig. 1a).
We recently explored the possibility that peptide-based ketones5,6
might also be effective catalysts for oxidation, perhaps for both
epoxidation and C–H hydroxylation reactions.7,8 In an earlier study,
for example, we found that a peptide-based trifluoromethyl ketone
exhibited good catalytic properties, such that certain olefins could
undergo epoxidation with enantiomeric ratios of up to 91:9 and
good yields with use of 10 mol% of the catalyst (Fig. 1b).6 However,
certain limitations were encountered, including the compatibility
of an N-terminal amino acid residue as the moiety carrying the tri-
fluormethyl ketone as the catalytic side chain. These tribulations
led us to speculate about potential advantages of new ketones,
such as arene-substituted trifluoromethyl ketones9 as possible
thus, we set our sights on catalysts of type 1 (Fig. 1c). We imagined,
for example, that arene substitution could modulate reaction rates.
p-Fluoroarene substitution, for example, had been demonstrated by
Hilinski to be advantageous in ketone-catalyzed C–H hydroxylation
reactions.7 The successful synthesis of these unusual compounds,
and the demonstration of their suitability as competent catalysts
for olefin epoxidation is the subject of this report.
2. Results and discussion
The synthesis of phenylalanine derivatives has been studied
extensively, culminating in numerous methods to access a wide
variety of analogues.10 Our retrosynthetic analysis of our proposed
catalyst culminated in a projected Negishi cross-coupling of a pre-
functionalized aryl bromide and a suitably protected b-iodoalanine
as the key step (Scheme 1). Our synthesis thus began with the
nucleophilic trifluoromethylation of commercial 2-bromo-4-
fluorobenzaldehyde 3 using CF3Si(CH3)3.11 Oxidation of trifluoro-
carbinol 4a to ketone 5 using IBX, followed by a Wittig olefination
provided trifluoromethyl styrene derivative 6 in 76% yield, which
was designed to serve as a masked ketone precursor.12 Notably,
our initial attempts at direct Negishi cross-coupling of 4a and
Boc-b-iodoalanine methyl ester were unsuccessful, resulting in
⇑
Corresponding author. Tel.: +1 203 432 9885; fax: +1 204 436 4900.
E-mail address: scott.miller@yale.edu (S.J. Miller).
http://dx.doi.org/10.1016/j.bmc.2016.07.012
0968-0896/Ó 2016 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Featherston, A. L.; Miller, S. J. Bioorg. Med. Chem. (2016), http://dx.doi.org/10.1016/j.bmc.2016.07.012

2
A. L. Featherston, S. J. Miller / Bioorg. Med. Chem. xxx (2016) xxx–xxx
Scheme 2. Synthesis of trifluoromethyl ketone catalyst 14. (i) CH3I, K2CO3, DMF, rt;
(ii) I2, imidazole, PPh3, CH2Cl2, 0 °C to rt; (iii) 6 (1.3 equiv), Zn dust, I2 (30 mol%),
SPhos (10 mol%), Pd2(dba)3 (5 mol%), DMF, 55 °C; (iv) LiOH, H2O/THF, 0 °C to rt; (v)
H-Pro-D-Val-(R)-a-MbaꢀHCl, EDCꢀHCl, HOBtꢀH2O, i-Pr2EtN, CH2Cl2, 0 °C to rt; (vi) O3,
CH2Cl2, ꢁ78 °C then (CH3)2S, ꢁ78 °C to rt. Cbz, carboxybenyzl; Mba,
methylbenzylamine; EDC, 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide; HOBt,
hydroxybenzotriazole.
turnover, leading to only 12% conversion of 15 to the desired epox-
ide 16 (Eq. 1). These modest results led to an investigation into the
stability of catalyst 14. Intriguingly, 19F NMR studies revealed the
formation of 17 (4:1 dr) wherein the nitrogen of the carbamate
had undergone cyclization with the ketone to generate a proposed
hemiaminal species14 as a result of the electrophilic nature of the
trifluoromethyl ketone (Fig. 2). In an attempt to reverse the
cyclization reaction, the mixture obtained after reversed phase
purifications was suspended in CHCl3 with 4 Å molecular sieves;
yet, no change in the relative amounts of 14 and 17 was observed.
We had previously observed that trifluoromethyl ketone hydrates
may revert to the ketone form under these conditions.6 Addition-
ally, 19F NMR analysis performed after the catalyst was subjected
to the reaction conditions, in the presence or absence of substrate,
revealed a mixture of 17 (4:1 dr) and 18 (2:1 dr). Based on these
findings, we concluded that the catalyst would not be suitable
for our intended applications.
In order to circumvent this issue, we sought to synthesize a new
analogue wherein the N-terminus was replaced with a simple
methyl group, in analogy to the strategy we had used in previous
catalyst designs with no deleterious effects.6,15 The synthesis began
with an Appel iodination of methyl (S)-(+)-3-hydroxy-2-methyl-
propionate ester 19 to afford iodide 20. Negishi cross-coupling with
bromide 6 and subsequent saponification provided the free acid 22
in 65% yield over three steps (Scheme 3). In a departure from our
previous synthetic strategy, we hypothesized that ozonolysis prior
to peptide coupling would be preferable, as it would provide the
fully deprotected residue in turn reducing the number of manipula-
tions to the catalyst. Thus we found that ozonolysis of olefin 22 pro-
vided the trifluoromethyl ketone monomer 23 in 55% yield. The
phenylalanine analogue was then incorporated into a peptide scaf-
fold using HCTU as the coupling reagent to afford 24 in good yield.
Additionally, each step of the synthesis has been demonstrated to
be scalable, providing access to >6 g of carboxylic acid 22 and
>250 mg of catalyst 24 in a single batch.
Figure 1. (a) Asymmetric olefin epoxidation with the fructose-derived ketone
catalyst of Shi. (b) Previous alkyl trifluoromethyl ketone peptide-based catalyst for
enantioselective olefin epoxidation. (c) Proposed second-generation phenylalanine
derived trifluoromethyl ketones as potential oxidation catalysts. DMM, dimethox-
ymethane; EDTA, ethylenediaminetetraacetic acid.
only recovered starting material and proto-dehalogenation, lead-
ing to alanine derivatives. Moreover, when TBS ether derivative
4b was subjected to the Negishi cross-coupling conditions, product
7b was not observed, which may be attributed to the steric bulk of
the silyl group inhibiting oxidative addition. However, when the
ketone was masked as the olefin (as in compound 6), ortho cross-
coupling was successful (vide infra), perhaps due to the decrease
in size relative to 4b.
Thus, the revised monomer synthesis began with esterification
of Cbz-Ser-OH (8) and subsequent Appel iodination to afford the
requisite b-iodoalanine methyl ester 10 (Scheme 2).13 Subsequent
Negishi cross-coupling of bromide 6 with iodide 10 gave the fully
protected monomer 11 in 54% yield. Saponification of methyl ester
11 provided the free carboxylic acid 12, which was then efficiently
incorporated into a peptide scaffold through standard EDC/HOBt
coupling methods to give the ketone precursor 13. Lastly, the
styrene moiety was converted to the ketone through ozonolysis
providing the desired catalyst 14 in 26–34% yield.
Initial studies with the N-protected peptide catalyst as a
dioxirane precursor for olefin epoxidation showed low catalyst
Scheme 1. Synthesis of aryl bromide 5. Reagents and conditions: (i) CF3Si(CH3)3,
K2CO3 (1 mol%), DMF, rt then Bu4NF; (ii) IBX, EtOAc, 77 °C; (iii) n-BuLi, CH3PPh3Br,
THF, 0 °C to rt; (iv) Boc-b-iodoalanine methyl ester, Zn dust, I2 (30 mol%), SPhos
(7.5 mol%), Pd2(dba)3 (3.75 mol%), DMF, 60 °C; (v) TBSCl, imidazole, DMAP, DMF, rt.
IBX, 2-iodoxybenzoic acid; SPhos, 2-dicyclohexylphosphino-20,60-dimethoxy-
biphenyl; dba, dibenzylideneacetone; TBSCl, tert-butyldimethylsilyl chloride;
DMAP, 4-dimethylaminopyridine.
ð1Þ
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A. L. Featherston, S. J. Miller / Bioorg. Med. Chem. xxx (2016) xxx–xxx
3
13
A
19F, 376 MHz, CDCl3
O
CH3
O
CH3
13
CH3
O
CH3
O
N
HN
N
HN
14
B
O
O
CbzHN
F
CbzHN
HN
Ph
HN
Ph
CH3
CH3
19F, 376 MHz, CDCl3
F
CH2
O
14 H2O
CF3
CF3
14
14
14 H2O
13
67.04 (CF3)
14
O
CH3
C
14
14
17
17
19F
19F
70.44 (CF3)
CH3
O
N
HN
19F, 376 MHz, CDCl3
O
CbzHN
HN
17
CH3
Ph
OH
F
OH
D
CF3
19F, 376 MHz, CDCl3
14 H2O
84.29 (CF3)
19F
17
17
O
CH3
O
CH3
CH3
O
CH3
O
14
14
N
N
HN
HN
Cbz
H
O
O
E
N
N
HO
F3
HO
HN
Ph
HN
Ph
14
17
C
F3C
CH3
CH3
18
19F, 471 MHz, CDCl3
F
F
18
17
18
19F
19F
80.27 (CF3, minor)
80.51 (CF3,major)
19F
19F
82.43 (CF3, major)
82.71(CF3, minor)
Figure 2. 19F NMR analysis of 13, 14, 17, and 18. (A) Pure compound 13. (B) Mixture of compounds 13, 14 and 14ꢀH2O obtained after first round of purification (see
Supplementary Information). (C) Mixture of compounds 14 and 17 obtained after second round of purification (see Supplementary Information). (D) Products from C with
activated 4 Å molecular sieves overnight. No disappearance of 19F signals at d ꢁ80.27 and ꢁ80.51 ppm suggests that the peaks are not related to a hydrated species. (E) Crude
analysis of catalyst decomposition under the oxidation reaction conditions in the absence of substrate demonstrating appearance of 17 and 18 during the course of the
reaction as a part of the catalyst decomposition pathway. Similar results were also obtained in the presence of substrate.
catalyst, <5% conversion was observed, suggesting that the ketone
is indeed generating a dioxirane in situ as the active catalyst
species, in analogy to many other reports.16 It is also interesting
to note that while the ketone catalyst is obtained as a mixture
of the ketone and hydrated species, there appear to be no negative
effects observed on the reactivity and turnover of the catalyst,
presumably since either may enter into the necessary catalytic
cycle. For example, when purified via reversed-phase
chromatography, the catalyst is obtained as a 1:2 mixture of the
ketone and hydrate. However, when purified via normal phase
chromatography, the catalyst was isolated as a >20:1 ketone:
hydrate mixture. Both mixtures performed equally well under
the reaction conditions.
Scheme 3. Synthesis of methyl analogue 24. (i) I2, imidazole, PPh3, CH2Cl2, 0 °C to
rt; (ii) 6 (1.0 equiv), Zn dust, I2 (30 mol%), SPhos (10 mol%), Pd2(dba)3 (5 mol%),
ð2Þ
DMF, 55 °C; (iii) KOH, H2O/EtOH, 0 °C to rt; (iv) O3, CH2Cl2, ꢁ78 °C then (CH3)2S,
ꢁ78 °C to rt; (v) H-
D
-Pro-Aic-
D
-Phe-N(CH3)2ꢀHCl, HCTU, NMM, CH2Cl2, ꢁ10 °C to rt.
HCTU, 2-(6-Chloro-1H-benzotriazole-1-yl)-1,1,3,3-tetramethylaminium hexafluo-
rophosphate; NMM, N-methylmorpholine.
Finally, during the course of our studies, we were able to obtain
an X-ray crystal structure of ketone 24 (Fig. 3).17 The catalyst pre-
sents a folded secondary structure in the solid state, wherein the
methyl group of the catalytic residue adopts a conformation driven
by minimization of allylic strain.18 The X-ray structure also shows
that the trifluoromethyl ketone is orientated away from the pep-
tide scaffold, which signals a ground state conformer that may
not be favorable for intermolecular interactions that could lead
to highly enantioselective epoxidations. Second-generation cata-
lysts that include the possibility for enhanced interactions, either
through catalyst dynamics, or explicit targeting of preferred con-
formations in solution, could be the paths to achieve stereoselec-
tivity in the future.
With a robust synthesis of the monomer and the ability to
incorporate the catalytic residue into a suitable peptide scaffold,
the stage was set for investigating its potential utility and reactiv-
ity. As a benchmark for reactivity, we turned our attention to low
reactivity olefins, such as substituted styrene derivatives, using
previously developed reaction conditions.6,9 The aryl trifluo-
romethylketone catalyst 24 was able to catalyze the epoxidation
of substrate 25 to epoxide 26 in >99% conversion with no appar-
ent catalyst degradation (Eq. 2). Minimal enantioselectivity was
observed in this experiment, although the stage is now set to
explore incorporation of this catalytic ketone into an expanded
library of peptides. Importantly, in the absence of the ketone
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4
A. L. Featherston, S. J. Miller / Bioorg. Med. Chem. xxx (2016) xxx–xxx
for the preparation of substrate 15 and we thank Anthony J.
Metrano for helpful discussions. We especially thank Dr. B. Q. Mer-
cado for acquiring the X-ray crystallographic data at the Advanced
Light Source at Lawrence Berkeley National Laboratory. The
Advanced Light Source is supported by the Director, Office of
Science, Office of Basic Energy Sciences, of the U.S. Department of
Energy under Contract No. DE-AC02-05CH11231.
A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.bmc.2016.07.012.
References and notes
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Liu, Z.-M.; Su, K.-X. Chem. Rev. 2005, 105, 1603.
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11224; (b) Shi, Y. Acc. Chem. Res. 2004, 37, 488.
Figure 3. The X-ray crystal structure of 24.19
5. Blank, J. T.; Miller, S. J. Biopolymers (Pept. Sci.) 2006, 84, 38.
6. Romney, D. K.; Miller, S. J. Org. Lett. 2012, 14, 1138.
3. Conclusions
7. Pierce, C. J.; Hilinski, M. K. Org. Lett. 2014, 16, 6504. and references therein.
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M.; Rasul, G.; Mathew, T.; Olah, G. A. J. Org. Chem. 2006, 71, 6806.
12. Cheng, H.; Pei, Y.; Leng, F.; Li, J.; Liang, A.; Zou, D.; Wu, Y.; Wu, Y. Tetrahedron
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15. Jakobsche, C. E.; Peris, G.; Miller, S. J. Angew. Chem., Int. Ed. 2008, 47, 6707.
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We have developed a robust and scalable route to access triflu-
oromethyl ketone-functionalized phenylalanine derivatives and
analogues for studies in dioxirane-mediated oxidations. With the
broad applicability of trifluoromethyl ketones within medicinal
chemistry and chemical biology, it is our hope that the methodol-
ogy developed herein could be of value beyond the context of our
specific goals.20 Nevertheless, for our local objectives, preliminary
studies have shown that the catalysts in which the amino function-
ality of the catalytic residue is replaced with a methyl group are
both highly reactive towards olefin epoxidation and demonstrate
increased stability. We are currently optimizing the catalyst
designs further with hopes of developing an improved peptide-
based catalyst.
17. Crystallographic data are deposited with the Cambridge Crystallographic Data
Centre under the accession number CCDC 1490745 (24).
18. Hoffman, R. W. Chem. Rev. 1989, 89, 1841.
19. CYLview, 1.0b; C. Y. Legault, Université de Sherbrooke, 2009, http://
www.cylview.org.
Acknowledgments
20. Kelly, C. B.; Mercadante, M. A.; Leadbeater, N. E. Chem. Commun. 2013, 49,
11133.
This work was supported by the National Institutes of Health
(NIH R01-GM096403). We also acknowledge Dr. David Romney
Please cite this article in press as: Featherston, A. L.; Miller, S. J. Bioorg. Med. Chem. (2016), http://dx.doi.org/10.1016/j.bmc.2016.07.012