Effective C5-Arylation of Peptide-Integrated Oxazoles: Almazole D

Article abstract of DOI:10.1002/adsc.202001262

An efficient functionalization approach of oxazoles by using Pd-catalyzed cross-coupling reactions is reported. Oxazolone formation and subsequent sulfamoylation in situ are facilitated by employing N,N-diethylsulfamoyl imidazolium triflate. Cross-couplings provide both oxazole-arenes and oxazole-heteroarenes and were compatible with sensitive and epimerization-prone substrates, as exemplified by the total synthesis of the natural product almazole D. (Figure presented.).

Full text of DOI:10.1002/adsc.202001262

Title: Effective C5-Arylation of Peptide-Integrated Oxazoles: Almazole
D
Authors: Ansgar Oberheide and Hans-Dieter Arndt
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To be cited as: Adv. Synth. Catal. 10.1002/adsc.202001262
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10.1002/adsc.202001262
Advanced Synthesis & Catalysis
UPDATE
DOI: 10.1002/adsc.202((will be filled in by the editorial staff))
Effective C5-Arylation of Peptide-Integrated Oxazoles: Almazole D
Ansgar Oberheide and Hans-Dieter Arndta*
a
Friedrich-Schiller-Universität Jena, Institut für Organische Chemie und Makromolekulare Chemie, Humboldstr. 10, D-
07743 Jena, *E-mail: hd.arndt@uni-jena.de.
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.((Please
delete if not appropriate))
For highly substituted oxazoles some de novo
Abstract. An efficient functionalization approach of
oxazoles by using Pd-catalyzed cross-coupling reactions is syntheses have been described that allow for their
assembly,[17] but are difficult to apply for late-stage
diversification. Moreover, (catalytic) C–H activation
of the oxazole ring has been reported,[18–20] such as
C5-functionalization by imidation,[21] alkylation,[22]
acylation,[23] alkenylation,[24] or arylation.[25] Several
protocols for regioselective C–H activation have been
developed,[19,20,26,27] but typically were applied to
structurally simple oxazoles stabilized by a C2-
(hetero)aryl group. Alternatively, transition metal-
mediated cross-couplings of 5-bromooxazoles[27–29]
and oxazolyl-5-ethers[30] have been realized, but often
suffer from limited substrate scope as well as modest
stability of the oxazole ring such activated.
(Hetero)aryl esters,[31] carbonates,[32] carbamates,[32,33]
sulfonates,[34] and sulfamates[32,33,35] have been
explored as C–O electrophiles in selected cross-
couplings. Applications of these transformations to
complex substrates remain rare and are likely to be
highly substrate dependent.
To facilitate accessing highly functionalized
oxazoles more generally, we envisioned to swiftly
generate suitable oxazole electrophiles from open
chain precursors (Scheme 1, top), as substrates for
cross-couplings. Oxazolyl-triflates[36,37] have been
applied for C2- and C4-functionalization of oxazoles,
but 5-triflyloxy-oxazoles have been reported to be
unstable.[36,37] Only a single case of a Sonogashira
coupling is reported.[37]
reported.
Oxazolone
formation
and
subsequent
sulfamoylation in situ are facilitated by employing N,N-
diethylsulfamoyl imidazolium triflate. Cross-couplings
provide both oxazole-arenes and oxazole-heteroarenes and
were compatible with sensitive and epimerization-prone
substrates, as exemplified by the total synthesis of the
natural product almazole D.
Keywords: Cross-coupling; peptides; natural products;
total synthesis; heterocycles.
Oxazoles are a prominent scaffold in both, natural
products[1] and drug discovery.[2] In natural products,
oxazoles are typically derived from serine or
threonine by cyclodehydration and oxidation,[3]
resulting in C5-H or C5-Me substitution. Oxazoles
with C5-aryl substituents may be produced by -
oxidation and cyclization of phenylalanine, tyrosine,
histidine, or tryptophan residues.[4] Examples are
breitfussin A (1, Figure 1) and B (2),[5] almazole D
(3),[6] martefragin A (4),[7] and diazonamide (5),[8] all
featuring
indoles,
or
the
phenyl-oxazole
urukthapelstatin A (6).[9] In medicinal chemistry,
oxazoles have been found to show antibiotic,[10]
antitubercular,[11] cytotoxic,[12] neuroprotective,[13]
anti-inflammatory,[14] and fungicidal[15] activity. Also,
arylated oxazoles feature interesting fluorescence
properties and have hence been used as probes.[16]
Toward identifying more broadly useful 5-oxazolyl
electrophiles, both an efficient access to 5-(4H)-
oxazolones and their functionalization was required,
in order to allow their reliable cross-coupling. For
identifying a combination of matching reagents and
derivatization, various functionalized oxazoles 9a–g
were scouted by using modified Erlenmeyer
azlactone synthesis conditions (Scheme 1), starting
from carboxylic acid 7 (15-87% yield). Additionally,
oxazolyl ether 9h was prepared by dehydration of
diethylamido malonate 10 (66% yield). The library of
functionalized oxazoles such obtained (9a–h) was
then subjected to cross-coupling studies with
PhB(OH)2, by varying the catalyst, ligand, solvent,
and reaction temperature for the different substrates.
Figure 1. Naturally occuring C5-arylated oxazoles.
1
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10.1002/adsc.202001262
Advanced Synthesis & Catalysis
acids was apparently depending on sterics (30–62%,
11f–h).
Electron-rich,
heteroaromatic
2-
furanylboronic acid and protected 3-indolylboronic
acid could be employed as well (11i, j). By using KF
instead of K3PO4 in higher boiling iAmOH, yields for
the heteroaromatic boronic acids were improved (11i,
j), but not for 4-nitrophenylboronic acid (11e). Since
N,N-diethylsulfamate 9i proved to be a more
productive electrophile, its coupling efficiency was
further evaluated (11k–p). Electron-rich substrates
readily underwent the cross-coupling giving yields up
to 89% (11k, l, 80% on 1 mmol scale). A
trifluoromethylated substrate was coupled in 85%
yield (71% on 1 mmol scale, 11m). A benzyl ether
was found labile, resulting in a diminished yield of
35% (11n), whereas alkenylation proceeded smoothly
(11o, 73% yield). Coupling of a thiophene proved to
be surprisingly difficult, giving a poor yield when
using either K3PO4 or KF in tBuOH (14%, 11p).
Taken together, these data indicated that K3PO4 as
base was superior for cross-couplings to aryl boronic
acids, whereas KF was more suitable for heteroaryl
boronic acids.
Scheme 1. Synthesis of different oxazole C–O
electrophiles. aReagents and conditions: RCOCl (4 equiv.),
NMM (5 equiv.), THF, or RSO2Cl (3-5 equiv.), NMI (4-5
equiv.),
THF.
bPivCl;
cEtOCOCl;
dEt2NCOCl;
eC5H6ClN3O2; fMsCl; gTsCl; hMe2NSO2Cl.
While a spectrum of Ni- and Pd-based catalysts were
studied, the delicate character of this transformation
became readily apparent (Table 1 and S.I.). To our
delight, sulfamate 9g emerged from this screening
effort as a promising electrophile when submitted to
Pd(OAc)2/XPhos in tBuOH and K3PO4 (Table 1 and
S.I.).
Table 1. Selected cross-coupling conditions for oxazolyl-
sulfamates.
complexa
ligand
solvent
yieldb (%)
1
2
3
4
5
6
7
8
NiCl2(PCy3)2
NiCl2(dppe)
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
-
-
toluened
0
0
<5
54 (51)g
98 (96)g
62 (59)g
<5
toluened
Mo-Phosc tBuOHe
BrettPhosc tBuOHe
XPhosc
XPhosc
XPhosc
XPhosc
tBuOHe
dioxanef
DMFf
Scheme 2. Scope of the Suzuki–Miyaura-type cross-
a
b
c
couplings. 9g was used; 9i was used; KF and iAmOH at
110 °C were used to couple sulfamate 9g; dKF and iAmOH
DMEe
48
e
a[Ni]: 10 mol %; [Pd]: 2 mol % Pd(OAc)2 and 4 mol %
at 110 °C were used to couple sulfamate 9i; reaction was
performed on 1 mmol scale; fisopropenylboronic acid
pinacol ester was used.
ligand; by HPLC; see S.I.; d110 °C; 85 °C; 100 °C;
b
c
e
f
gisolated yield in brackets.
This profiling of a test substrate set the stage to
implement the sulfamate cross-coupling chemistry to
structurally more sophisticated peptidic substrates.
Now effective access to oxazolyl sulfamates became
important. Unfortunately, O-sulfamoylation of
oxazolones by using commonly available sulfamoyl
chloride reagents remained sluggish. We therefore
We then explored the scope of this cross-coupling
reaction by studying 2-phenyloxazolyl-sulfamates 9g
and 9i and different boronic acids (Scheme 2). The
coupling of regular and electron rich aryl boronic
acids performed smoothly (80-96%, 11a, b, k, l).
Electron-poor substrates with CF3 and CN
substituents were tolerated, too (60-67%, 11c, d). A
nitro group in the para-position was less productive
(30% yield) but the N,N-diethylsulfamate substrate 9i
provided a slightly better yield (11e). Coupling
efficiency for hindered, ortho-substituted boronic
employed
the
more
electrophilic
N,N-
dialkylsulfamoyl imidazolium triflates 14a and 14b
(Scheme 3), which were easily synthesized in high
yield and on multi-gram scale from sulfamoyl
2
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10.1002/adsc.202001262
Advanced Synthesis & Catalysis
chlorides 12a and 12b by substitution with 2-
methylimidazole und quaternization.[38] The reagents
14a and 14b provided sulfamates 9g and 9i in good
yields of 53% and 58%, respectively, starting from
carboxylic acid 7 (Scheme 3 and S.I.). Moreover, the
target oxazolyl-sulfamates could be either formed by
sequentially using pivaloyl chloride and the
sulfamoyl imidazolium triflate, or -carboxy amides
– i.e. peptide acids – could be directly transformed to
C-terminal oxazolones in one step by exposing them
to an excess of reagents 14a or 14b.
elimination of the thioether was observed. Using KF
as base suppressed this side reaction, to provide the
coupling products 22a–c (61-72%). Excellent yields
were obtained for the coupling of proline-derived
sulfamate 18 with various aromatic boronic acids
(23a–c, 80-88%). Heteroaryl couplings were then
investigated for sulfamate 19. Both 2-benzofuranyl-
and 2-furanyl-boronic acid could be coupled, to give
the bi-heteroarenes 24a and 24b (44% and 48%). To
test whether amide groups at the C4-position were
tolerated, the oxazolyl-sulfamates 20 and 21 were
studied. They gave successful cross-couplings with
yields up to 81% (25a-c, 26a-e), again with more
variable efficiency for heterocyclic boronic acids
(26d, e). Importantly, Cα-epimerization was not
observed in any of the cases studied (1H-NMR).
Scheme 3. Synthesis of N,N-dialkylsulfamoyl imidazolium
triflates for oxazolone formation and O-sulfamoylation.
In order to demonstrate the scope of these novel,
operationally simple condition, a series of N,N-
diethylsulfamoyloxy-oxazoles 15–21 was prepared
from peptide acid precursors in 40-71% yield
(Scheme 4). Gratifyingly, substituents at the C2-
position of the oxazole derived from protected
cysteine, proline, phenylalanine, or threonine were
tolerated equally well, as was a peptide-type amide at
the C4-position. Surprisingly, oxazolyl-sulfamates 15
and 17 remained unproductive in cross-couplings
(Scheme 5), indicating the STr group to be
incompatible with this chemistry. The more stable
tert-butyl thioether 16 proceeded well,[29] but some -
Scheme 5. Substrate scope and functional group tolerance
a
of sulfamate couplings. Pd(OAc)2, XPhos, KF, tBuOH,
85 °C; bPd(OAc)2, XPhos, K3PO4, tBuOH, 85 °C;
cPd(OAc)2, XPhos, KF, iAmOH, 110 °C.
To further prove the utility of the oxazolyl sulfamate
cross-coupling, we applied this method to a synthesis
of the natural product almazole D (3, Scheme 6).[6]
Oxazolyl-sulfamate 19 was easily obtained from an
open-chain precursor peptide in 55% yield (see S.I.).
Subsequent cross-coupling of sulfamate 19 with
protected
3-indolylboronic
acid
performed
excellently and gave oxazolyl-indole 27 in 90% yield.
Boc group cleavage and dimethylation provided
dimethylamine 28 in 51% yield. Finally, alkaline
ester hydrolysis and N-desulfonylation with nBu4NF
completed a facile total synthesis of almazole D (3,
67% yield),[39] with perfectly matching analytical data,
by employing the newly developed method as a key
step.
In conclusion, we have shown that 5-oxazolyl-
sulfamates are privileged electrophiles for Pd-
catalyzed late-stage C5-arylations of oxazoles, which
may even be peptide-embedded. Competent 5-
oxazolyl-sulfamates can be directly accessed from
Scheme 4. Facile one-pot synthesis of oxazolyl-N,N-
diethylsulfamates 15–21.
3
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10.1002/adsc.202001262
Advanced Synthesis & Catalysis
peptides by using N,N-dialkylsulfamoyl imidazolium
triflates. The developed couplings were shown to be
racemization-free and applicable to a broad range of
functionalized substrates, as exemplified by the total
synthesis of the antibacterial natural product almazole
D (3). This methodology is hence expected to
considerably facilitate natural product synthesis,
medicinal chemistry, and materials chemistry
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General Procedure: iAmOH (0.05 mol/L) was added to
the corresponding oxazolyl-sulfamate (1 equiv.), boronic
acid (2 equiv.), Pd(OAc)2 (0.1 equiv.), Xphos (0.2 equiv.),
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Acknowledgements
This work was supported by the Carl-Zeiss-Foundation (PhD
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Advanced Synthesis & Catalysis
UPDATE
Effective C5-Arylation of Peptide-Integrated
Oxazoles: Almazole D
Adv. Synth. Catal. Year, Volume, Page – Page
Ansgar Oberheide, Hans-Dieter Arndt*
6
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