Catalytic Arylhydroxylation of Dehydroalanine in Continuous Flow for Simple Access to Unnatural Amino Acids

Article abstract of DOI:10.1002/chem.201804094

This report discloses the first example of catalytic arylhydroxylation of dehydroalanine with aryldiazonium salts. Aryldiazonium salts, which are generated from aniline precursors under partially aqueous conditions in continuous flow, efficiently reacted with dehydroalanine in the presence of 10–15 mol % ferrocene to furnish α-hydroxyarylalanine derivatives (up to 82 % yield). The reactions proceeded with regioselectivity, broad functional group tolerance, and without polymerization of the dehydroalanine. Furthermore, the products were used to access α-unnatural amino acids, important targets with application in drug development.

Full text of DOI:10.1002/chem.201804094

A Journal of
Accepted Article
Title: Catalytic Arylhydroxylation of Dehydroalanine in Continuous Flow
for Simple Access to Unnatural Amino Acids
Authors: Stephen L. Buchwald, R. Kashif M. Khan, Yang Zhao, and Tal
Scully
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To be cited as: Chem. Eur. J. 10.1002/chem.201804094
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COMMUNICATION
Catalytic Arylhydroxylation of Dehydroalanine in Continuous
Flow for Simple Access to Unnatural Amino Acids
R. Kashif M. Khan,[a]‡ Yang Zhao,[a]‡ Tal D. Scully, and Stephen L. Buchwald*
[a]
[a]
Abstract: This report discloses the first example of catalytic
arylhydroxylation of dehydroalanine with aryldiazonium salts.
Aryldiazonium salts, which are generated from aniline precursors
under partially aqueous conditions in continuous flow, efficiently
react with dehydroalanine in the presence of 10-15 mol% ferrocene
to furnish hydroxyarylalanine derivatives (up to 82% yield). The
reactions proceed with regioselectivity, broad functional group
tolerance, and without polymerization of the dehydroalanine.
Furthermore, the products can be used to access unnatural amino
acids, important targets with application in drug development.
related arylation step (in an overall reductive arylation process)
has been reported with aryldiazonium salts in the presence of
excess TiCl
subsequent addition of H
3
reducing agent. 14 Upon the 1e-oxidation of VIII and
O, the desired product IX would be
2
formed. However, a major complication could arise if free
radical-based polymerization of VII were to occur. Indeed,
polymerization of dha has been reported in the literature. 15
Moreover, the use of ArN Cl reagents, which are prone to
2
spontaneous and violent decomposition, could pose serious
safety hazards.16
Dehydroalanine (dha), a terminal alkene-based amino acid, is
1
found in a large number of naturally occurring peptides. Owing
2
to the high propensity of dha to undergo conjugate addition, it
serves as an excellent electrophile in cysteine-mediated peptide
cyclization, a key step during posttranslational modification
3
(
PTM) of peptides to form lantibiotics. In addition to its role as a
natural synthon, dha is routinely used as a building block in
various synthetic schemes for the preparation of unnatural
amino acids (UAAs) 4 — an important class of molecules
employed
pharmaceuticals
in
the
development
of
peptide-based
Although dha
5
6
and PTM of proteins.
functionalization strategies permit variation in the UAA
architecture, there exists a dearth of methods to access the
related UAA subclass. Notably, the discoveries of CS
and CO linked residues in antimicrobial peptides (i.e., I and II,
7
8
respectively) as well as their relevance to drug development
have underlined the need for techniques to access heteroatom-
substituted UAAs (Figure 1a). Within the aforementioned
category, the O-substituted variants can be readily transformed
Figure 1. Significance of O-based UAAs and proposal for their formation
through catalytic arylhydroxylation of dehydroalanine (VII).
9
into several types of UAAs (IVVI, Figure 1b). Despite the high
In order to provide synthetically useful and operationally viable
access to UAAs, we set out to develop an integrated
continuous flow reactor (CFR) setup (Figure 2). We anticipated
some challenges associated with the use of a CFR to perform
this transformation. For instance, the potential precipitation of
ArN Cl in a multistep protocol and the release of N (g) could
2 2
hamper the overall efficiency as well as the safety profile of the
process.
potential for synthetic utility, structures similar to III, only a few
methods for their preparation are available and they are mainly
accessed through the chemical 10 and electrochemical 11
oxidation of the corresponding phenylalanine derivatives. In this
context, the development of robust catalytic methods for the
facile conversion of dha to the corresponding UAAs (VII to
IX, Figure 1c) would be of high value.12
Our group previously reported that treatment of enol ethers
with ArN
generates acetaldehydes.13 Similarly, we reasoned that dha
VII) would rapidly undergo coupling with an aryl radical
2
Cl in the presence of a catalytic amount of ferrocene
(
accessed from the 1e-reduction of the aryldiazonium partner to
form a highly stabilized intermediate (VIII). It is noteworthy that a
[
a]
Dr. R. K. M. Khan, Dr. Y. Zhao, T. D. Scully, Prof. S. L. Buchwald
Department of Chemistry
Figure 2. Catalyzed arylhydroxylation of dehydroalanine (1) with
aryldiazonium salts promoted by ferrocene in continuous flow.
Massachusetts Institute of Technology
Our studies commenced with the development of
homogeneous reaction conditions in batch on a small scale. In a
one-pot procedure under air, the aryldiazonium salt 3b was
77 Massachusetts Avenue, Cambridge, MA 02139 (USA)
E-mail: sbuchwal@mit.edu
These authors contributed equally.
[‡]
17
Supporting information for this article is given via a link at the end of
the document.
generated from anline 3a in situ at 0 °C and subsequently
allowed to react with alkene 1 (2.5 equivalents) in the presence
of 10 mol% ferrocene for one hour (entry 1, Table 1). Upon
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workup with aqueous bicarbonate, 10% of the desired product
R
flow-equilibration over two total residence times (i.e., 2 x t = 74
1
3c was detected. Notably, the H NMR analysis of the unpurified
min), the resulting stream of the crude product mixture was
collected under air in a pre-cooled (0 °C) test tube followed by
the treatment with aqueous bicarbonate.17 Upon purification by
silica gel chromatography, 3c was delivered in 82% yield on 1.0
mmol scale. It is important to note that while 10 mol% ferrocene
was sufficient for 81% yield in batch (cf. entry 7 in Table 1), 15
mol% loading was necessary in the CFR protocol. The
difference may arise from a low effective concentration of the
catalyst caused by being in a slug-flow regime (i.e., liquid-gas
reaction mixture revealed >98% consumption of 3a and the
generation of p-chlorobenzene (7%) along with an unknown
colored side-product.18 Furthermore, no polymerization of dha 1
was detected. The diazotization time (t
led to the improved formation of 3c (i.e., 17% with t
1
) was increased, which
= 15 min,
1
entry 3). Nonetheless, the overall transformation continued to
suffer from low efficiency. Remarkably, the admixture of the
aryldiazonium salt with 1 (at t = 5 min) prior to the addition of
2
ferrocene resulted in a 71% yield of 3c (entry 4). The reaction
2
segmentation) upon N evolution in the CFR. Furthermore, the
was further improved by increasing the pre-mixing time (i.e.,
employment of a 20 psi back-pressure regulator to discourage
the slug-flow behavior led to significantly diminished yield (36%
3c).
2
81% with t = 20 min, entry 7). Moreover, whereas the reaction
efficiency was slightly improved using 15 mol% ferrocene (84%,
entry 8), significant attenuation with 5 mol% loading was
observed (50%, entry 9).
Table 1. Optimization of
arylhydroxylation of 1.
2
a single-pot ArN Cl generation and catalytic
2
Scheme 1. Rapid ArN Cl generation and catalyzed arylhydroxylation of
dehydroalanine in CFR. See supporting information for details.
Next, we examined the scope with anilines involving various
substitution patterns and containing functional groups (4a-16a,
Table 2). In line with our results in Scheme 1, the transformation
with 4-bromoaniline furnished 4c in 81% yield. Similarly, the use
of a 4-tert-butyl ester based substrate led to 66% of 5c. The
relative decrease in efficiency could be attributed to the lability of
With the batch-scale conditions in hand, adaptation to a
continuous flow protocol was begun. As shown in Scheme 1, we
designed an integrated flow setup involving three reactors (R ,
1

2
CO t-Bu under the acidic reaction conditions. Moreover, the
reactions with 3-CN and 2-OPh based anilines proceeded
readily to give the corresponding products (6c: 73% and 7c:
R
2
, and R
3
) composed of 0.04’’ inner-diameter PFA tubing to
68%). Subsequently, arylamines with multiple substitution
carry
out
the
diazotization-premixing-arylhydroxylation
patterns were tested. In this regard, we noticed that the lack of
ortho-substitution led to relatively higher yield in 10c (vs 8c and
sequence, respectively. The reagent delivery was conducted
through four syringes (each at 50 L/min flow rate) connected
with 0.02’’ inner-diameter PFA feeding lines. In order to
maximize the homogeneity of the reaction mixture which is
necessary for efficient flow at 04 °C, the CFRs were placed in a
constantly sonicated ice-bath. Under the CFR setup, mixing of
the acidic solution of 3a (syringe 1) with aqueous sodium nitrite
9c). Furthermore, reaction with the mildly electron-releasing 3-
iodo-4-methylaniline took place with lower efficiency (11c: 53%).
The CFR protocol also permitted the facile formation of products
derived from 4,5-dimethyl-2-nitroaniline and 6-aminoquinoline
(
12c: 72% and 13c: 74%). The related heteroarylamine-derived
UAAs (14c and 15c) were also assembled in 59% and 64%
(
1
syringe 2, 1.05 equiv) in R
0 minutes. Two check-valves were employed to prevent any
backward flow of 3b, which could potentially occur due to N gas
evolution in the arylhydroxylation step. Subsequently, the
solution was mixed with a stream of 1 (syringe 3, 2.5 equiv) in R
for 17 minutes. At this point, ferrocene (syringe 4, 15 mol%) was
introduced and allowed to react over 10 minutes in R . After the
1
led to complete diazotization within
yield, respectively. Most notably, sulfamethoxazole, an aniline-
based antibiotic, was successfully employed in this procedure
2
(
16c: 57%). We then examined the reaction efficiency with
NCbz protected dha (vs NBoc in 1). Under the same
conditions used for 3c (cf. Scheme 1), the NCbz protected
2

hydroxy-(4-chlorophenyl)alanine derivative (17c)17 was
3
isolated in 78% yield. This result is potentially useful for
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COMMUNICATION
protection group alterations that are occasionally necessary in
the enantioselective hydrogenation of dehydroarylalanine
precursors (cf. Scheme 3).
With a simple and reliable method to make UAAs in a
CFR, we next explored their conversion to the valuable UAAs.
In this context, a straightforward dehydrationenantioselective-
hydrogenation protocol was developed. As shown in Scheme 3,
Table 2. Substrate scope with complex aryl and heteroaryl primary amines for
UAAs.
3
(
c was converted to the corresponding dehydroarylalanine
3e) in 69% yield over two steps. The Rh-(R,R)-Et-Duphos
catalyzed enantioselective hydrogenation of 3e under
pressure (20 bar) furnished 3f 25 in high efficiency and
H
2
selectivity (i.e., >98% yield, >98% ee). The absolute
stereochemical configuration of the UAA 3f was determined
through
stereochemical
correlation
with
authentic
enantiomerically pure sample. Similarly, high yields were
attained even with the bulkier and electron-deficient 8f and 9f
(
i.e., 3 steps: 88% and 80% yields, respectively). However, the
attenuation in ee values (8f: 86% and 9f: 84%) could be due to
the ortho-substitution in both cases. Consequently, higher
enantioselectivity with 10f (97% ee) was achieved.
A
preliminary mechanistic proposal for the catalytic
transformation is put forth in Scheme 2. The aryldiazonium 3b is
reduced through single-electron transfer (SET) arising from
Scheme 3. Efficient and enantioselective derivatization protocol for conversion
to UAAs.
II
III 19
oxidation of Fe to Fe . The resulting aryl radical rapidly reacts
with dehydroalanine to form a captodative 20 intermediate A.
Subsequently, the radical polar-crossover, 2 which occurs
In summary, we report the first examples of a continuous flow
process for the synthesis of hydroxyarylalanines. The strategy
features a catalytic arylhydroxylation of dehydroalanine with
transiently generated aryldiazonium salts in a controlled and
safe manner. The reactions proceed with a broad range of
electron-deficient aryl and heteroaryl based anilines in high yield,
and do not require inert conditions. The CFR setup, which
consists of three reactors powered by two syringe pumps, is
simple to assemble and allows product generation within 37
minutes. Furthermore, a functionalization strategy for accessing
1
III 22
II
through SET with mildly oxidizing [Fe ] to regenerate [Fe ],
provides a reactive iminium B thus allowing the addition of
water23 to form the corresponding UAA (3c). The proposed
scenario involving the intermediacy of iminium is supported by
the isolation of 18c in the presence of methanol. A radical chain
mechanism has not been ruled out.24
UAAs in high enantioselectivity is also described. In light of
this investigation, efforts are ongoing for the development of new
peptide based conjugation strategies. Detailed studies to
understand the mechanism are also underway.
Acknowledgements
Scheme 2. Plausible mechanism involving catalyzed arylhydroxylation of
dehydroalanine with Cp
2
Fe.
The authors would like to thank Novartis AG for providing funds
for this research. Discussions with Dr. Benjamin Martin, Dr.
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Berthold Schenkel, and Dr. Gerhard Penn were most helpful.
TDS was an undergraduate researcher and UROP scholar. Dr.
Mycah Uehling and Dr. Nicholas White (MIT) provided helpful
feedback during the manuscript preparation.
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[
Keywords: captodative • dehydroalanine • aryldiazonium salts •
arylhydroxylation • UAAs • redox catalysis • continuous flow
2
4
[
1
[
•
UAAs
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