Total Synthesis of Ajudazol A by a Modular Oxazole Diversification Strategy

Article abstract of DOI:10.1021/acs.orglett.0c02188

The total synthesis of the potent respiratory chain inhibitor ajudazol A was accomplished by a concise strategy in 17 steps (longest linear sequence). The modular approach was based on a direct oxazole functionalization strategy involving a halogen dance

Full text of DOI:10.1021/acs.orglett.0c02188

pubs.acs.org/OrgLett
Letter
Total Synthesis of Ajudazol A by a Modular Oxazole Diversification
Strategy
̈
Philipp Wollnitzke, Sebastian Essig, Jan Philipp Golz, Karin von Schwarzenberg, and Dirk Menche*
Cite This: https://dx.doi.org/10.1021/acs.orglett.0c02188
Read Online
sı
*
Metrics & More
Article Recommendations
Supporting Information
ACCESS
ABSTRACT: The total synthesis of the potent respiratory chain inhibitor
ajudazol A was accomplished by a concise strategy in 17 steps (longest
linear sequence). The modular approach was based on a direct oxazole
functionalization strategy involving a halogen dance reaction for selective
halogenation in combination with a challenging combination of sp2−sp2 and
sp2−sp3 Negishi cross coupling reactions. The applicability of this strategy
for analogue synthesis was demonstrated by the synthesis of a simplified as
well as stabilized ajudazol analogue.
exomethylene group next to the oxazole, ajudazol B (2) has a
s exemplified by the ajudazols, potent respiratory chain
Ainhibitors of myxobacterial origin, oxazoles are central methyl group at this position. In combination with a very
potent biological profile, characterized by selective inhibition
of the mitochondrial NADH-dehydrogenase,7 these intriguing
molecular architectures have attracted a great deal of interest
from the synthetic community,5,8 culminating in one total
synthesis of ajudazol B, which has been accomplished in 24
steps (longest linear sequence, 42 steps in total), previously
described by our group.5 In contrast, a direct oxazole
functionalization would enable a much more convergent and
modular approach and directly allows for application to
analogue synthesis. Herein, we report the design, development,
and application of a modular oxazole diversification strategy
and the implementation of this approach for the total synthesis
of ajudazol A (1) and the preparation of a simplified, stabilized,
and much more readily available analogue.
As shown in Figure 1, our retrosynthetic strategy was based
on challenging sp2−sp2 and sp2−sp3 oxazole cross coupling
reactions in combination with selective functionalizations of
each oxazole position. This involved the effective use of the
different heterocyclic pKa values,9 with H2 being the most
acidic and H4 being the least acidic position, in combination
with an adventurous halide rearrangement protocol. Con-
sequently, lithiation of unsubstituted oxazole 6 selectively
occurs at the C2 site, giving intermediate 7 that was trapped
with diphenyl disulfide toward thioether 8 [91% (Scheme
1)].10 Subsequent deprotonation at the C5 position and
structural features in a large number of natural products and
bioactive agents.1 Commonly, such types of heterocycles are
prepared by a conventional cyclodehydration strategy, which
involves condensation of an α-hydroxy amine with a
carboxylate followed by oxidation of the resulting oxazoline.
This biomimetic approach, as largely developed by the Wipf
group,2 guarantees mild reaction conditions and has been
successfully applied in a wide variety of complex natural
product total syntheses,3 including ajudazol B.4,5 However, the
required linear sequence considerably increases the number of
overall steps and complicates direct adaptability for useful
analogue synthesis. As shown in Figure 1, the ajudazols are
distinguished by a central oxazole core that is decorated by an
unusual hydroxy-isochromanone moiety and a side chain with
a (Z,Z)-diene and a terminal methoxybutenoic acid methyl-
amide as characteristic features.6 While ajudazol A (1) bears an
Received: July 2, 2020
Figure 1. Retrosynthetic approach to the ajudazols by an oxazole
diversification strategy and acidities of oxazole protons.
https://dx.doi.org/10.1021/acs.orglett.0c02188
© XXXX American Chemical Society
Org. Lett. XXXX, XXX, XXX−XXX
A

Organic Letters
pubs.acs.org/OrgLett
Letter
particular, only a slow process has been found in sp3−sp2 cross
couplings with heteroaromatics, and no examples of oxazole
alkyl cross couplings have been developed,16,17 demonstrating
the synthetic challenge of this key bond formation.
Consequently, we first studied more readily available simplified
surrogate 21, having a similar substitution pattern, including
the chiral β-methyl center of the original fragment. As shown
in Scheme 2, it was obtained in three steps from commercial
isopulegol 19 involving TBS protection, diastereoselective
hydroboration/oxidation, and Appel reaction of derived 20.
Scheme 1. Selective Oxazole Functionalization
Scheme 2. sp3−sp2 Oxazole Cross Coupling Studies
reaction of intermediate 9 with either NBS or iodine give C5-
halides 10 and 11. Finally, to access the C4 position selectively,
a base-mediated halide displacement strategy was imple-
mented. Such types of oxazole 1,2-halide rearrangements
were first reported in 1953,11 and these so-called halogen
dance reactions involve treatment of 5-bromoxazoles with an
excess of LDA, leading to migration of the bromine atom to
the less accessible C4 position, based on the higher acidity of
the C5 site.12 While this procedure could be readily adopted to
effectively move the bromine atom from position 5 to 4
(89%),13 only low degrees of conversion were observed for the
corresponding iodide 12 (Table 1, entry 1, in Scheme 1). As
this more reactive halide was required for the ensuing cross
coupling reactions (vide infra), a variant of the original
protocol had to be devised. Presumably, the low yield arises
from an undesired lithium−iodide exchange competing with
the required C4 deprotonation. Finally, this selectivity issue
could be resolved by the addition of catalytic amounts of
oxazole 10, which suppresses undesired lithium−iodide
exchange,9 resulting in a clean rearrangement of iodide 11
toward 4-iodinated isomer 12 in high yields (88%, entry 2).
Thioether oxidation to sulfone 13 was then realized in good
yields with ammonium molybdate/H2O2. These sulfones may
be directly replaced with organolithium compounds allowing
for a facile introduction of Eastern subunits.10 However, such
reagents are not compatible with the authentic ajudazol
isochromanone.13
a
Compound 22 was obtained by lithium−iodide exchange of the
corresponding iodide 21 with tert-butyllithium (2.00 equiv) and 9-
BBN-OMe. See the Supporting Information for details. Compound
23 was obtained by insertion of Zn into the corrresponding iodide 21
with commercially available Zn powder in the presence of LiCl. See
the Supporting Information for details. The reaction mixture was
heated to 70 °C for 18 h. The reaction mixture was heated to 60 °C
for 2 h.
b
c
Therefore, as an alternative a cross coupling strategy was
pursued.14
d
As shown in Scheme 1b, for implementation of a cross
coupling strategy lithiated oxazole 7 was transmetalated with
ZnCl2 and a Negishi cross coupling of resulting organyle 14
with vinyl iodide 15 was developed.14 Subsequent C5
iodination and halogen dance reaction of derived 16 toward
4-iodooxazole 18 proceeded smoothly following our previously
developed protocols (74%, two steps), demonstrating the
general usefulness of these procedures for selective oxazole
derivatization. In general, alkyl cross coupling reactions
continue to present formidable synthetic challenges.15 In
Inspired by a few examples in complex natural product
syntheses,18 we first turned our attention to Suzuki reactions
(Table 2, top part, in Scheme 2). As it soon became apparent
that bromides were too unreactive, the coupling of iodide 13
with simplified Western subunit 21 was studied. To realize this
coupling, Pd(dtbpf)Cl2 proved to be a suitable catalyst for our
requirements among those evaluated.17 We assumed the
reluctance of halooxazoles to oxidatively add to Pd(0) is one
B
https://dx.doi.org/10.1021/acs.orglett.0c02188
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
pubs.acs.org/OrgLett
Letter
of the barriers to cross coupling the substrates and required
heating to 70 °C (entry 1). Because β-hydride elimination
occurs upon warming at 50 °C,19 we investigated lower
temperatures and discovered the ability of Pd(dtbpf)Cl2 to
forge oxazole 24 readily at room temperature (entry 2). The
yield could be slightly increased in neat THF (entry 3), while
lower degrees of efficiency were observed in more polar
solvents like dioxane (entry 4) or DMF, giving mainly
decomposition independent of the base (entries 5 and 6).
Finally, useful yields were obtained with Pd(dtbpf)Cl2 in the
presence of Cs2CO3 in neat THF (58%, entry 7), which could
be further increased by the addition of AsPh3 (entry 8).20
As an alternative, an oxazole alkyl Negishi cross coupling was
evaluated. Such an approach would involve a direct insertion of
zinc into the organic halide, thus avoiding a lithiation process
as required above.21 Consequently, the coupling of elaborate
oxazole 18 with iodide 21 was studied (entries 9−15). After
initial unsuccessful attempts with conditions similar to those
described above (entry 9) as well as related protocols with a
large number of equivalents of LiCl to favor formation of
heteroleptic zinc compounds (entry 10),22 1,3-dimethyl-2-
imidazolidinone (DMI) as a highly polar co-solvent was added.
We hoped to have a beneficial effect by enhancing the
formation of higher-order zincates (RZnX32−) as the putative
active transmetalating agent.23 This indeed proved to be
effective, giving the desired alkyloxazole 25 with useful degrees
of conversion (entry 11), which could be further improved by
exchanging DMI with N-methylpyrrolidinone (NMP) (entry
12).24 For further enhancement, we considered the possibility
that each catalytic cycle would liberate one ZnX2 molecule by
transmetalation, which may then sequester halide ions from
solution, due to their higher Lewis acidity,25 and therefore, the
Schlenk equilibrium would shift away from the formation of
higher-order species required for coupling.23 Indeed, the
consequential increase in the number of LiCl equivalents had
a beneficial effect (entry 13). In contrast, removal of AsPh3
leads to incomplete conversion (entry 14), suggesting this co-
catalyst is relevant for oxidative addition.20 Also, N-
methylimidazole (NMI), which had been described as an
auxiliary reagent in previous alkyl−alkyl Negishi couplings,24
had an inhibitory effect in our studies (entry 15), despite its
structural similarities to DMI.
Scheme 3. Implementation of Previous Protocols for
Analogue Synthesis
a
a
Compounds 26 and 27 were obtained in a manner analogous to that
of 16. See the Supporting Information for details.
Scheme 4. Total Synthesis of Ajudazol A
After having established these protocols, we turned our
attention to application of these approaches for more elaborate
target synthesis and first evaluated the preparation of simplified
analogue 33 (Scheme 3). Notably, it contains the authentic
butenamide motif from crocacin and the myxalamids,26 which
was considered to be part of the pharmacophore. Accordingly,
oxazole 27 was elaborated toward 29,27 involving the two-step
sequence established above, which proceeded uneventfully.
Negishi cross coupling with surrogate 21 then procceded in
high yield (69%), supporting the usefulness of the protocol for
complex alkyl cross coupling of oxazoles.28 Completion of the
analogue synthesis involved vinyl iodide 30, which was
obtained by selective removal of the primary TES ether
under acidic conditions, IBX oxidation, and Stork−Zhao
olefination of the intermediate aldehyde, which proceeded with
full Z selectivity.29 Finally, Suzuki coupling with known Eastern
fragment 3130 using Pd(dppf)Cl2 in the presence of Cs2CO3
and deprotection with buffered HF·pyridine provided synthetic
analogue 33 in 32% yield over two steps.31
Starting from isochromanone 34,30 which was obtained
following tactics and protocols previously developed by our
group,5,13,32 the required Western building block 35 was
obtained by TBS protection, oxidative alkene cleavage by an
improved Lemieux−Johnson oxidation,33 NaBH4 reduction of
the corresponding intermediate aldehyde, and Appel reaction
of the derived alcohol in 92% yield over these three steps.
Cross coupling with oxazole 27 likewise proceeded in high
yield (66%), considering the functional complexity of the
substrates and the great synthetic challenge posed by this
fusion. Following our previously established protocols for our
previous synthesis of analogue 33, dialkylated oxazole 37 was
then coupled with Eastern fragment 31, to give synthetic
ajudazol A (1). 1H and 13C NMR data were in agreement with
the published data.6
Importantly, the first biological evaluation of ajudazol A (1)
and simplified surrogate 33 revealed both compounds
effectively inhibit T cell leukemia cells at micromolar
concentrations (Jurkat cells),34 confirming our hypothesis of
This approach could then be successfully adopted for the
total synthesis of ajudazol A (1), as shown in Scheme 4.
C
https://dx.doi.org/10.1021/acs.orglett.0c02188
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
pubs.acs.org/OrgLett
Letter
the terminal Eastern butenamide motif to be part of the
pharmacophore. The simplified analogue 26 was even more
potent (IC50 = 2.83 μM) than the parent natural product (IC50
= 11.2 μM).
ACKNOWLEDGMENTS
■
The authors thank the DFG (ME-2756/9-1 and TRR 261) for
generous funding and Andreas J. Schneider (University of
Bonn) for excellent HPLC support.
More detailed biological studies will follow. In conclusion,
this first total synthesis of ajudazol A highlights the potential of
commercially available oxazole as a direct starting material for
complex oxazole-containing natural products. It was accom-
plished in 17 steps (longest linear sequence), which compares
favorably to a previous approach to this natural product class
involving a conventional oxazole condensation strategy (24
steps). Highlights of this innovative strategy include specific
oxazole functionalizations by the halogen dance reaction as
well as sp2−sp2 and sp2−-sp3 oxazole cross couplings. The
general usefulness of this approach for rapid and modular
analogue synthesis was demonstrated by the facile preparation
of a simplified and stabilized ajudazol analogue, which was
obtained in only nine steps. It retains potent biological
activities, suggesting the Eastern subunit is an essential part of
the pharmacophore. This strategy may help to enable more
detailed biological evaluations of the biological potential.
Furthermore, the various strategies and tactics developed
within this endeavor may be beneficial for the synthesis of
various other complex oxazole architectures.
REFERENCES
■
́
́
(1) Riego, E.; Hernandez, D.; Albericio, F.; Alvarez, M. Synthesis
2005, 2005, 1907−1922.
(2) (a) Wipf, P.; Venkatraman, S. J. Org. Chem. 1995, 60, 7224−
7229. (b) Wipf, P.; Miller, C. P. J. Org. Chem. 1993, 58, 3604−3606.
(3) Yeh, V. S.C. Tetrahedron 2004, 60, 11995−12042.
(4) Essig, S.; Schmalzbauer, B.; Bretzke, S.; Scherer, O.; Koeberle,
A.; Werz, O.; Mu
1357.
(5) Essig, S.; Bretzke, S.; Mu
2012, 134, 19362−19365.
̈
ller, R.; Menche, D. J. Org. Chem. 2016, 81, 1333−
̈
ller, R.; Menche, D. J. Am. Chem. Soc.
̈
(6) Jansen, R.; Kunze, B.; Reichenbach, H.; Hofle, G. Eur. J. Org.
Chem. 2002, 2002, 917−921.
(7) Kunze, B.; Jansen, R.; Hofle, G.; Reichenbach, H. J. Antibiot.
̈
2004, 57, 151−155.
(8) (a) Birkett, S.; Ganame, D.; Hawkins, B. C.; Meiries, S.; Quach,
T.; Rizzacasa, M. A. J. Org. Chem. 2013, 78, 116−123. (b) Birkett, S.;
Ganame, D.; Hawkins, B. C.; Meiries, S.; Quach, T.; Rizzacasa, M. A.
Org. Lett. 2011, 13, 1964−1967. (c) Egan, B. A.; Paradowski, M.;
Thomas, L. H.; Marquez, R. Tetrahedron 2011, 67, 9700−9707.
(d) Egan, B. A.; Paradowski, M.; Thomas, L. H.; Marquez, R. Org.
Lett. 2011, 13, 2086−2089. (e) Hobson, S. J.; Parkin, A.; Marquez, R.
Org. Lett. 2008, 10, 2813−2816. (f) Krebs, O.; Taylor, R. J. K. Org.
Lett. 2005, 7, 1063−1066.
ASSOCIATED CONTENT
■
sı
* Supporting Information
(9) Proust, N.; Chellat, M.; Stambuli, J. Synthesis 2011, 2011, 3083−
3088.
(10) Williams, D. R.; Fu, L. Org. Lett. 2010, 12, 808−811.
(11) Vaitiekunas, A.; Nord, F. F. J. Am. Chem. Soc. 1953, 75, 1764−
1768.
(12) Stanetty, P.; Spina, M.; Mihovilovic, M. D. Synlett 2005, 1433−
1434.
(13) Attempts to lithiate the authentic isochromanone led to
intramolecular lactone opening. See the Supporting Information for
details.
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c02188.
Detailed experimental procedures, characterization, and
1
copies of H and 13C NMR spectra of new compounds,
including full details for all intermediates, synthesis of
starting compounds (26, 27, 31, and 34) as well as
additional experimental information (see refs 13, 26, and
27) (PDF)
(14) Reeder, M. R.; Gleaves, H. E.; Hoover, S. A.; Imbordino, R. J.;
Pangborn, J. J. Org. Process Res. Dev. 2003, 7, 696−699.
(15) Geist, E.; Kirschning, A.; Schmidt, T. Nat. Prod. Rep. 2014, 31,
441−448.
AUTHOR INFORMATION
■
Corresponding Author
(16) Molander, G. A.; Yun, C.-S. Tetrahedron 2002, 58, 1465−1470.
(17) (a) Vechorkin, O.; Proust, V.; Hu, X. J. Am. Chem. Soc. 2009,
131, 9756−9766. (b) McDaniel, S. W.; Keyari, C. M.; Rider, K. C.;
Natale, N. R.; Diaz, P. Tetrahedron Lett. 2011, 52, 5656−5658.
(c) Herkommer, D.; Thiede, S.; Wosniok, P. R.; Dreisigacker, S.;
Tian, M.; Debnar, T.; Irschik, H.; Menche, D. J. Am. Chem. Soc. 2015,
137, 4086−4089. (d) Schmink, J. R.; Tudge, M. T. Tetrahedron Lett.
2013, 54, 15−20.
́
Dirk Menche − Kekule-Institute for Organic Chemistry and
Biochemistry, University of Bonn, D-53121 Bonn, Germany;
orcid.org/0000-0002-4724-8383; Email: dirk.menche@
uni-bonn.de
Authors
́
Philipp Wollnitzke − Kekule-Institute for Organic Chemistry
(18) Chemler, S. R.; Trauner, D.; Danishefsky, S. J. Angew. Chem.,
Int. Ed. 2001, 40, 4544−4568.
and Biochemistry, University of Bonn, D-53121 Bonn, Germany
(19) Kirchhoff, J. H.; Netherton, M. R.; Hills, I. D.; Fu, G. C. J. Am.
Chem. Soc. 2002, 124, 13662−13663.
́
Sebastian Essig − Kekule-Institute for Organic Chemistry and
Biochemistry, University of Bonn, D-53121 Bonn, Germany
(20) Farina, V.; Krishnan, B. J. Am. Chem. Soc. 1991, 113, 9585−
9595.
́
̈
Jan Philipp Golz − Kekule-Institute for Organic Chemistry and
Biochemistry, University of Bonn, D-53121 Bonn, Germany
Karin von Schwarzenberg − Department of Pharmacy-Center
for Drug Research, Ludwig-Maximilians-University of Munich,
D-83177 Munich, Germany
(21) Krasovskiy, A.; Malakhov, V.; Gavryushin, A.; Knochel, P.
Angew. Chem., Int. Ed. 2006, 45, 6040−6044.
(22) McCann, L. C.; Organ, M. G. Angew. Chem., Int. Ed. 2014, 53,
4386−4389.
(23) Hunter, H. N.; Hadei, N.; Blagojevic, V.; Patschinski, P.;
Achonduh, G. T.; Avola, S.; Bohme, D. K.; Organ, M. G. Chem. - Eur.
J. 2011, 17, 7845−7851.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c02188
(24) Zhou, J.; Fu, G. C. J. Am. Chem. Soc. 2003, 125, 12527−12530.
(25) Achonduh, G. T.; Hadei, N.; Valente, C.; Avola, S.; O’Brien, C.
J.; Organ, M. G. Chem. Commun. 2010, 46, 4109−4111.
Notes
The authors declare no competing financial interest.
D
https://dx.doi.org/10.1021/acs.orglett.0c02188
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
pubs.acs.org/OrgLett
Letter
̈
(26) (a) Kunze, B.; Jansen, R.; Hofle, G.; Reichenbach, H. J.
Antibiot. 1994, 47, 881−886. (b) Gerth, K.; Jansen, R.; Reifenstahl,
̈
G.; Hofle, G.; Irschik, H.; Kunze, B.; Reichenbach, H.; Thierbach, G.
J. Antibiot. 1983, 36, 1150−1156.
(27) Compounds 26 and 27 were obtained from 18, or in analogy to
18, demonstrating the generality of these protocols. See the
Supporting Information.
(28) C5 alkylation by cross coupling was also successful. See the
Supporting Information for details.
(29) Stork, G.; Zhao, K. Tetrahedron Lett. 1989, 30, 2173−2174.
(30) For synthesis, see the Supporting Information for details.
(31) Scheeff, S.; Menche, D. Org. Lett. 2019, 21, 271−274.
(32) Essig, S.; Menche, D. J. Org. Chem. 2016, 81, 1943−1966.
(33) Yu, W.; Mei, Y.; Kang, Y.; Hua, Z.; Jin, Z. Org. Lett. 2004, 6,
3217−3219.
(34) Takeuchi, Y.; McClure, M. O.; Pizzato, M. J. Virol. 2008, 82,
12585−12588.
E
https://dx.doi.org/10.1021/acs.orglett.0c02188
Org. Lett. XXXX, XXX, XXX−XXX