Harnessing the Substrate Promiscuity of Dioxygenase AsqJ and Developing Efficient Chemoenzymatic Synthesis for Quinolones

Article abstract of DOI:10.1021/acscatal.1c01150

Nature has developed complexity-generating reactions within natural product biosynthetic pathways. However, direct utilization of these pathways to prepare compound libraries remains challenging because of limited substrate scopes, involvement of multiple-step reactions, and moderate robustness of these sophisticated enzymatic transformations. Synthetic chemistry offers an alternative approach to prepare natural product analogues. However, because of complex and diverse functional groups appended on the targeted molecules, dedicated design and development of synthetic strategies are typically required. Herein, by leveraging the power of chemoenzymatic synthesis, we report an approach to bridge the gap between biological and synthetic strategies in the preparation of quinolone alkaloid analogues. Leading byin silicoanalysis, the predicted substrate analogues were chemically synthesized. The AsqJ-catalyzed asymmetric epoxidation of these substrate analogues was followed by a Lewis acid-triggered ring contraction to complete the viridicatin formation. We evaluated the robustness of this method in gram-scale reactions. Lastly, through chemoenzymatic cascades, a library of quinolone alkaloids is effectively prepared.

Full text of DOI:10.1021/acscatal.1c01150

Made available for a limited time for personal research and study only License.
pubs.acs.org/acscatalysis
Letter
Harnessing the Substrate Promiscuity of Dioxygenase AsqJ and
Developing Efficient Chemoenzymatic Synthesis for Quinolones
Haoyu Tang, Yijie Tang, Igor V. Kurnikov, Hsuan-Jen Liao, Nei-Li Chan,* Maria G. Kurnikova,*
Yisong Guo,* and Wei-chen Chang*
Cite This: ACS Catal. 2021, 11, 7186−7192
Read Online
sı
*
Metrics & More
Article Recommendations
Supporting Information
ACCESS
ABSTRACT: Nature has developed complexity-generating reactions within natural
product biosynthetic pathways. However, direct utilization of these pathways to
prepare compound libraries remains challenging because of limited substrate scopes,
involvement of multiple-step reactions, and moderate robustness of these sophisticated
enzymatic transformations. Synthetic chemistry offers an alternative approach to
prepare natural product analogues. However, because of complex and diverse
functional groups appended on the targeted molecules, dedicated design and
development of synthetic strategies are typically required. Herein, by leveraging the
power of chemoenzymatic synthesis, we report an approach to bridge the gap between
biological and synthetic strategies in the preparation of quinolone alkaloid analogues.
Leading by in silico analysis, the predicted substrate analogues were chemically
synthesized. The AsqJ-catalyzed asymmetric epoxidation of these substrate analogues
was followed by a Lewis acid-triggered ring contraction to complete the viridicatin
formation. We evaluated the robustness of this method in gram-scale reactions. Lastly, through chemoenzymatic cascades, a library
of quinolone alkaloids is effectively prepared.
KEYWORDS: Fe/2OG enzyme, AsqJ, chemoenzymatic synthesis, rearrangement, quinolone
and methylation to form (4′-methoxy)cyclopeptin. An iron
and 2-oxoglutarate (Fe/2OG) dependent oxygenase, AsqJ,
catalyzes sequential desaturation and asymmetric epoxidation
to furnish (4′-methoxy)cyclopenin (2) via (4′-methoxy)-
dehydrocyclopeptin (1).15 Finally, cyclopenase, AsqI, triggers
ring contraction to complete the biosynthesis of (4′-methoxy)-
viridicatin (3) (Figure 1A).16 In comparison with the
enzymatic synthesis, 1 and 5 can be chemically prepared
through Perkin condensation (Figure 1B).9 However, chemical
asymmetric epoxidation of 1 remains a synthetically challeng-
ing task. Herein, we demonstrated that utilization of AsqJ
catalyzed epoxidation represents an attractive approach to
prepare cyclopenin analogues (6), particularly as the
epoxidation requires only molecular oxygen and 2-oxoglutarate
to enable the catalysis under mild conditions and generates
succinate and CO2 as coproducts. Using in silico analysis, we
demonstrated that a Lewis acid can effectively convert
cyclopenin to viridicatin (2 → 3 and 6 → 7), which is
analogous to the cyclopenase-catalyzed reaction where a zinc
uinoline and quinolone alkaloids are secondary
Q metabolites produced by several organisms with broad
biological properties including antiallergic, antibacterial,
antitumor, and antiviral activities.1−4 In both chemical and
biological syntheses, quinoline and quinolone core serve as key
intermediates in the preparation of these bioactive molecules.5
Among them, the skeleton of 3-hydroxyquinolin-2(1H)-one,
viridicatin, has been recognized as a valuable scaffold in
numerous natural products as well as synthetic compounds.3,6,7
Quite a few synthetic strategies have been developed for the
construction of this skeleton including ring expansion of isatin
and aryldiazomethane, Knoevenagel reaction followed by
cyclization, condensation using 2-aminobenzyldehyde with
chloroacetic anhydride or N-phenylacetoacetamide derivative,
transition metal-catalyzed cross-coupling, and ring contraction
of cyclopenin.8−14 In nature, viridicatin is biosynthesized from
O-methyl-L-tyrosine/phenylalanine and anthranilic acid
through a series of enzymatic transformations involving
condensation, methylation, desaturation, epoxidation, and
rearrangement.15 Although the aforementioned strategies
have their merits, the limitations of these methods to generate
viridicatin analogues led us to consider developing an efficient
method combining the tools employed by chemistry and
nature.
Received: March 15, 2021
Revised: May 14, 2021
Published: June 6, 2021
AsqK, a methyl transferase domain-containing NRPS
(nonribosomal peptide synthetase), catalyzes the cyclization
https://doi.org/10.1021/acscatal.1c01150
© 2021 American Chemical Society
ACS Catal. 2021, 11, 7186−7192
7186

ACS Catalysis
pubs.acs.org/acscatalysis
Letter
Figure 1. (A) Biosynthetic pathway of viridicatin and (B) preparation of quinolone analogues through chemoenzymatic transformations.
ion located at the active site triggers the rearrangement. Lastly,
a cross-coupling reaction using halogenated viridicatins (7a−
7d) yields a quinolone library (Figure 1B).
S83−S86). In addition, calculations using alchemical ligand
transformation21,22 and the Bennett Acceptance Ratio
method23,24 suggest that the differences between the binding
free energies of all compounds are relatively small. Compared
with the native substrates (1a and 1b), the binding energies for
5b and 5d are 4.7 and 2.3 kcal/mol higher. On the other hand,
the binding energies for 5a and 5c are lowered by 1.4 kcal/mol.
More importantly, the distances between the iron center and
the C10 of all compounds are kept in a narrow range of ∼4.0−
5.5 Å (Figure 2). According to the recent study of the AsqJ
catalyzed epoxidation,20 the iron-oxo of the ferryl intermediate
reacts with the olefin moiety at the C10 position of the
substrate. Thus, the Fe−C10 distances predicted by the MD
simulations strongly suggest that 5a−5d would be suitable for
AsqJ catalysis.
Given the potential substrate promiscuity assessed through
in silico analysis, we began by synthesizing the halogenated
dehydrocyclopeptin analogues (5a−5d, see Supporting In-
formation (SI) for details). In parallel, AsqJ was heterologously
expressed in Escherichia coli and purified using metal-affinity
chromatography. Analytical-scale reactions with each substrate
were carried out. Analogous to the native substrates, complete
substrate consumption of 5c and 5d was detected in 4 h (Table
1, entries 5 and 6). Moderate conversions (∼71−75%) were
observed in the reactions using 5a and 5b (entries 3 and 4).
To harness the advantages provided by AsqJ, challenges
including substrate scope, selectivity, and effectiveness toward
substrate analogues have to be addressed. By inspecting the
previously published crystallographic data on AsqJ,17−20 we
found that the binding of 1 and analogues (5) are highly
conserved. The two carbonyl oxygens from the diazepine-2,5-
dione moiety of the substrate participate in hydrogen bonding
interactions with the N atom of Asn70 and the backbone N
atom of Met137, respectively. The aryl ring of diazepine-2,5-
dione has hydrophobic interactions with Leu79, Met118,
Met122, and Thr227. These four residues are part of the
double stranded β-helix structure commonly present in Fe/
2OG enzymes. In addition, the phenyl moiety has a π−π
interaction with His134 and is also stabilized by hydrophobic
interactions with Val72, Phe139, and Pro132 (Figure 2). To
efficiently build a quinolone library through cross-coupling
reactions, we consider brominated viridicatins (7) as our
targeted molecules. We performed molecular dynamics (MD)
simulations to assess the binding and potential steric effects. As
revealed by 10 ns MD trajectory simulations carried out using
1a, 1b, and 5a−5d, all compounds exhibit a similar binding
configuration in the MD equilibrated structures, which closely
resembles those in the AsqJ crystal structures (Figures 2 and
7187
https://doi.org/10.1021/acscatal.1c01150
ACS Catal. 2021, 11, 7186−7192

ACS Catalysis
pubs.acs.org/acscatalysis
Letter
Figure 2. Top: Overlap of 1b (green) in the AsqJ active site (PDB: 6K0E) with 5c (wheat) (Panel A) or with 5d (yellow) (Panel B) obtained from
the MD equilibrated structures. The protein residues surrounding 1b are highlighted. Bottom: The Fe−C10 distance distribution among 1a, 5a,
and 5b (Panel C), and among 1b, 5c, and 5d (Panel D) based on MD simulations.
Table 1. Substrate Consumption Monitored by LC-MS
entry
substrate
4 h (%)
20 h (%)
1
2
3
4
5
6
1a
1b
5a
5b
5c
5d
90
100
75
71
100
96
100
100
93
92
100
97
For all substrates, longer reaction time led to almost complete
substrate consumption.
Next, we assessed the reaction outcome and scalability of
this biotransformation. Although AsqJ delivers 2a and 2b as
the sole products, it is possible that non-native substrates may
tilt the reaction outcomes. Fe/2OG enzymes have been
reported to produce alternative products when substrate
analogues are used. For example, in NapI and SyrB2, the
reaction outcomes change from native desaturation and
halogenation to hydroxylation.25,26 Therefore, we optimized
the reaction conditions to achieve ∼1000 turnovers for 5a, 5c,
and 5d and 250 turnovers for 5b. Currently, the reason for the
lower turnover using 5b is unknown. In the scaled-up
reactions, the loading of enzyme is at 0.1−0.4% molar ratio
to the substrate. All reaction products are purified and
characterized by detailed NMR analysis (Figure 3 and SI for
Figure 3. Structural characterization by selective correlations of
1
HMBC and H−1H COSY and the X-ray structure of 6c.
1
details). As revealed by H and 13C NMR, 6a−6d display
similar resonances as those of 2a and 2b. The up-shifting of the
13C signal (C7 or C8) is due to the introduction of the
bromine. Based on HMBC and 1H−1H COSY correlations, the
structures of 6a−6d are determined. Furthermore, on the basis
of the X-ray structure of 6c, the newly installed chiral centers at
C3 and C10 have R and S configurations (Figure 3). These
results are consistent with the substrate binding configuration
observed in the AsqJ structures and the aforementioned MD
simulation predictions where the oxygen atom transfer by the
ferryl intermediate is stereospecific.
Additionally, we determined the steady-state kinetics of the
AsqJ catalyzed reactions using 1a, 1b, 5a−5d (Figure S24 and
Table S2). Comparing the steady-state kinetics of the two
native substrates, 1b has higher catalytic efficiency (kcat/Km of
7188
https://doi.org/10.1021/acscatal.1c01150
ACS Catal. 2021, 11, 7186−7192

ACS Catalysis
pubs.acs.org/acscatalysis
Letter
Figure 4. Reaction coordinate for the conversion of 2a (green) and 2b (black) to 3a and 3b in the presence of BF3. The numerical values indicate
the calculated free energy differences of various species. The molecular structures of reactant, intermediate, product, and transition states of 2a →
3a are shown.
1b/1a is ∼5). When analogues (5a−5d) are used, Km values
are increased by 5−10 times. Among 5a−5d, the Km values of
7-brominated analogues (5a and 5c) are ∼2 times smaller than
8-brominated analogues (5b and 5d). The kcat values of 5a and
5c are similar or even slightly higher than those of the native
substrates. Thus, the catalytic efficiency is not perturbed.
However, for 5b and 5d, the overall enzyme efficiency is
compromised. Thus, Br-substituent at 7 position (5a and 5c)
has minimal influence on AsqJ efficiency, whereas C8-
bromination (5b and 5d) reduces the efficiency by ∼10−100
fold. These results are consistent with the MD results where
the binding affinity of 5b and 5d are ∼4.7 and 2.3 kcal/mol
higher than native substrates. Taken together, these observa-
tions demonstrate that AsqJ is flexible toward the 7- and 8-
substitutions and is capable of catalyzing effective asymmetric
epoxidation, which is compared favorably to the reported
chemical epoxidation of dehydrocyclopeptin.10
Toward preparation of the quinolone library, our next goal is
to develop an efficient method to convert cyclopenin to
viridicatin (2 → 3 and 6 → 7). In the reported AsqI catalysis,
the active site Zn ion serves as a Lewis acid to activate the
epoxide and to facilitate an anti-Baldwin-type epoxide-opening
6-endo-tet cyclization.16 Followed by the elimination of methyl
isocyanate, a keto−enol tautomerization leads to viridicatin
formation (Figure S93). On the other hand, Groll et al. and
Gulder et al. reported that the rearrangement is spontaneous
once cyclopenin (2) is released from the active site of
AsqJ.17,27 We first assess the plausible rearrangement pathways
through in silico analysis. The DFT calculated reaction
coordinate using 2a and 2b largely follows the above-
mentioned mechanism (see the SI for the details). The C−C
bond formation between C10 and C5a accompanied by
epoxide ring opening generates a cyclized intermediate.
Subsequently, elimination of a methylisocyanate and keto−
enol tautomerization completes the reaction. Interestingly, in
both aqueous and organic conditions (calculated by using the
Polarizable Continuum Model),28 2a and 2b alone are unlikely
to rearrange to 3a and 3b spontaneously. The calculated free
energy of the transition state connecting 2a/2b to the cyclized
intermediate is ∼25−30 kcal/mol (ΔG‡) higher than the
substrate state (Figure S87, S88). This result is consistent with
the DFT calculations reported by Borowski et al.29 and is less
consistent with the reported spontaneous rearrangement
phenomenon. To explore the possibility of using an Lewis
acid as a potential catalyst, we calculated the effect of Lewis
acids on the rearrangement reaction. The DFT calculations
predict that a Lewis acid, such as BF3, would strongly interact
with the epoxide oxygen of cyclopenin to enable epoxide ring
opening in organic solvent, such as dichloromethane (Figure
4). Release of the ring strain of the epoxide leads to a better
π−π stacking interaction between the two aryl rings of the
substrate, which in turn significantly reduces the distance
between C10 and C5a from ∼3.3 to ∼2.7 Å. As a consequence,
the BF3 bound reactant is structurally similar to the transition
state structure (the C10 and C5a distance is ∼2.0 Å) and
dramatically reduces the barrier (ΔG‡) of the C10−C5a bond
formation from ∼25−30 to ∼5 kcal/mol for 2b (Figure 4). For
the subsequent methylisocyanate elimination step, the reaction
barrier is ∼6 kcal/mol. Compared with 2b, the methoxy group
of 2a seems to increase the transition state barriers for both
steps to ∼8−9 kcal/mol. Besides, other commonly used Lewis
acids, such as AlCl3, show a similar effect (Figure S89) and
brominated analogues (6a−6d) do not affect the overall
reaction coordinate and energetics (Figures S90, S91). It is
worthwhile to mention that 2b obtained in the AsqJ crystal
structure and 6c have very different three-dimentional
conformations (Figure S92). Because of the geometric
constraints enforced by AsqJ, only the flat, or chairlike, form
is retained in the active site. The folded, or boat-like, form
becomes a dominant species once the epoxide is released from
the active site of AsqJ. Our DFT calculations suggest that the
folded conformation is energetically favorable by ∼2 kcal/mol
for all the compounds used in this study (Table S3).
We then set out to test the rearrangement reaction by
1
monitoring the conversion of 2a and 2b using H NMR.
Within 120 min, no obvious substrate consumption can be
7189
https://doi.org/10.1021/acscatal.1c01150
ACS Catal. 2021, 11, 7186−7192

ACS Catalysis
pubs.acs.org/acscatalysis
Letter
Figure 5. A library of quinolone analogues is prepared via chemoenzymatic reactions. The products synthesized using XI are shown in the SI
(Figure S94).
observed under pH 5−8.5 (Figures S25, S26). These
observations indicate the need of an acid for efficient
rearrangement. To our delight, when using BF3 in dichloro-
methane, through a thin-layer-chromatographic analysis, we
observed a complete consumption of the substrates (2a and
2b) and the formation of (4′-methoxy)viridicatin (3a and 3b).
Applying this method also affords brominated viridicatins
analogues (7a−7d). Although the origin of anti-Baldwin
rearrangement remains to be carefully investigated, these
results highlight the advantage of our approach which allows
for the effective preparation of viridicatins through tandem
enzymatic and chemical reactions.
scale reactions can serve as a bottleneck to the enzymatic
transformations being embraced for synthetic applications. We
envisioned transitioning from sequential reactions/purification
to a scalable platform could benefit synthetic usefulness.
Toward this goal, reactions using AsqJ-containing lysate were
investigated. Although complete consumption of the substrate
was achieved, the components originated from the crude lysate
severely affect the rearrangement efficiency. Taking advantage
of robust expression and stability of AsqJ (∼38 mg protein per
liter of culture), the scale-up reactions were conducted using
the purified enzyme. After AsqJ completed epoxidation, the
crude product obtained via a simple ethyl acetate extraction
was subjected to BF3 catalyzed rearrangement to afford 7a−7d
in gram scale with an overall yield of 57−91%.
While enzymatic reactions are typically carried out on
submilligram to miligram quantities to identify and character-
ize the reaction products, a potential challenge of preparative-
7190
https://doi.org/10.1021/acscatal.1c01150
ACS Catal. 2021, 11, 7186−7192

ACS Catalysis
pubs.acs.org/acscatalysis
Letter
To build quinolone library, we sought to leverage the
reactivity of brominated viridicatins via cross-coupling
reactions. We anticipated that a series of compounds can be
accessed in one-step by reacting with cross-coupling partners.
The 6-substituted vinyl quinolone skeleton is found in
aflaquilonones, aspoquinolones, 4-phenyl-3,4-dihydroquino-
lones, and yaequilonones.30−33 However, the naturally
occurring 7-vinyl substituted quinolones are rare. Herein, we
explored the synthesis of a library of 6- and 7-vinyl substituted
quinolones. To access the 6-substituted vinyl quinolones, 7a
and 7c were reacted with substituted alkenes under Heck
reaction conditions34 to afford the desired product in
reasonable yields (Figure 5 and Figure S94, 8a-(I-XI), 8c-(I-
X)). Analogously, the 7-substituted vinyl quinolones were
prepared using 7b and 7d as the substrates (9b-(I-XI), 9d-(I-
X)). It is worthwhile to mention that using 1-vinylpyrrolidin-2-
one and D-limonene as the coupling partners (X and XI)
produces the anticipated products as well as the terminal olefin
products (10a-(X-XI), 10b-(X-XI), 10c-X, and 10d-X).
Production of 10a-d-X likely originates from regioselectivity
at the migratory insertion. Furthermore, formation of 10a-XI
and 10b-XI is likely due to the regio-selectivity of the beta-
hydride elimination.
In summary, the results described herein demonstrated the
potential to combine the current tools employed by chemistry
and nature for the preparation of important molecules. The
substrate diversity available from chemical synthesis, the
robustness of AsqJ catalyzed asymmetric epoxidation, a simple
Lewis Acid-catalyzed rearrangement, and the cross-coupling
reactions have provided a platform for developing a suit of
quinolone analogues. As enabled by mechanistic and in silico
analyses, the catalytic efficiency of AsqJ paired with the mild
and environmentally friendly reaction conditions provide an
opportunity to apply this biocatalyst in chemoenzymatic
reaction cascades. Along with the development of a biomimetic
Lewis acid method of ring contraction, viridicatins can be
prepared in gram-scale quantity. Overall, this work sets an
example to use tools employed by synthetic chemistry and
nature for the effective preparation of quinolone alkaloids. It
would be interesting to test the biological activities of newly
synthesized compounds in future studies.
Maria G. Kurnikova − Department of Chemistry, Carnegie
Mellon University, Pittsburgh, Pennsylvania 15213, United
States; orcid.org/0000-0002-8010-8374;
Email: kurnikova@cmu.edu
Nei-Li Chan − Institute of Biochemistry and Molecular
Biology, College of Medicine, National Taiwan University,
Taipei 100, Taiwan; orcid.org/0000-0003-0139-6513;
Email: nlchan@ntu.edu.tw
Authors
Haoyu Tang − Department of Chemistry, North Carolina
State University, Raleigh, North Carolina 27695, United
States
Yijie Tang − Department of Chemistry, Carnegie Mellon
University, Pittsburgh, Pennsylvania 15213, United States;
orcid.org/0000-0003-2628-6230
Igor V. Kurnikov − Department of Chemistry, Carnegie
Mellon University, Pittsburgh, Pennsylvania 15213, United
States
Hsuan-Jen Liao − Institute of Biochemistry and Molecular
Biology, College of Medicine, National Taiwan University,
Taipei 100, Taiwan
Complete contact information is available at:
https://pubs.acs.org/10.1021/acscatal.1c01150
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the grant from the National
Institutes of Health (GM125924 to Y.G., W.-c.C, and M.K.).
The authors thank Dr. Roger Sommer for helpful discussion.
Crystallography was performed by the Molecular Education,
Technology and Research Innovation Center at NC State
University Funding for D8 VENTURE acquisition was
provided by North Carolina Biotechnology Center (2019-
IDG-1010).
REFERENCES
■
(1) Heguy, A.; Cai, P.; Meyn, P.; Houck, D.; Russo, S.; Michitsch,
R.; Pearce, C.; Katz, B.; Bringmann, G.; Feineis, D.; Taylor, D. L.;
Tyms, A. S. Isolation and characterization of the fungal metabolite 3-
O-methylviridicatin as an inhibitor of tumour necrosis factor alpha-
induced human immunodeficiency virus replication. Antivir. Chem.
Chemother. 1998, 9 (2), 149−55.
ASSOCIATED CONTENT
* Supporting Information
■
sı
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acscatal.1c01150.
(2) Mizutani, N.; Aoki, Y.; Nabe, T.; Ishiwara, M.; Yoshino, S.;
Takagaki, H.; Kohno, S. Effect of TA-270, a novel quinolinone
derivative, on antigen-induced nasal blockage in a guinea pig model of
allergic rhinitis. Eur. J. Pharmacol. 2009, 602 (1), 138−42.
(3) Duplantier, A. J.; Becker, S. L.; Bohanon, M. J.; Borzilleri, K. A.;
Chrunyk, B. A.; Downs, J. T.; Hu, L. Y.; El-Kattan, A.; James, L. C.;
Liu, S.; Lu, J.; Maklad, N.; Mansour, M. N.; Mente, S.; Piotrowski, M.
A.; Sakya, S. M.; Sheehan, S.; Steyn, S. J.; Strick, C. A.; Williams, V.
A.; Zhang, L. Discovery, SAR, and pharmacokinetics of a novel 3-
hydroxyquinolin-2(1H)-one series of potent D-amino acid oxidase
(DAAO) inhibitors. J. Med. Chem. 2009, 52 (11), 3576−85.
(4) Suchaud, V.; Bailly, F.; Lion, C.; Tramontano, E.; Esposito, F.;
Corona, A.; Christ, F.; Debyser, Z.; Cotelle, P. Development of a
series of 3-hydroxyquinolin-2(1H)-ones as selective inhibitors of HIV-
1 reverse transcriptase associated RNase H activity. Bioorg. Med.
Chem. Lett. 2012, 22 (12), 3988−92.
Experimental details, Figures S1−S94, and Tables S1−
S3 (PDF)
DFT_xyz (ZIP)
X-ray data (CIF)
AUTHOR INFORMATION
Corresponding Authors
■
Wei-chen Chang − Department of Chemistry, North Carolina
State University, Raleigh, North Carolina 27695, United
States; orcid.org/0000-0002-2341-9846;
Email: wchang6@ncsu.edu
Yisong Guo − Department of Chemistry, Carnegie Mellon
University, Pittsburgh, Pennsylvania 15213, United States;
orcid.org/0000-0002-4132-3565; Email: ysguo@
andrew.cmu.edu
(5) Muthukrishnan, I.; Sridharan, V.; Menendez, J. C. Progress in
the Chemistry of Tetrahydroquinolines. Chem. Rev. 2019, 119 (8),
5057−91.
7191
https://doi.org/10.1021/acscatal.1c01150
ACS Catal. 2021, 11, 7186−7192

ACS Catalysis
pubs.acs.org/acscatalysis
Letter
(6) Sit, S. Y.; Ehrgott, F. J.; Gao, J. N.; Meanwell, N. A. 3-hydroxy-
quinolin-2-ones: Inhibitors of [H-3]-glycine binding to the site
associated with the NMDA receptor. Bioorg. Med. Chem. Lett. 1996, 6
(5), 499−504.
(7) Chen, M. H.; Fitzgerald, P.; Singh, S. B.; O’Neill, E. A.;
Schwartz, C. D.; Thompson, C. M.; O’Keefe, S. J.; Zaller, D. M.;
Doherty, J. B. Synthesis and biological activity of quinolinone and
dihydroquinolinone p38 MAP kinase inhibitors. Bioorg. Med. Chem.
Lett. 2008, 18 (6), 2222−6.
(8) Huntress, E. H.; Bornstein, J.; Hearon, W. M. An Extension of
the Diels-Reese Reaction. J. Am. Chem. Soc. 1956, 78 (10), 2225−8.
(9) Martin, P. K.; Rapoport, H.; Smith, H. W.; Wong, J. L. Synthesis
of Cyclopenin and Isocyclopenin. J. Org. Chem. 1969, 34 (5), 1359−
63.
(10) Smith, H. W.; Rapoport, H. Mechanism of Transformation of
Cyclopenin to Viridicatin. J. Am. Chem. Soc. 1969, 91 (22), 6083−9.
(11) Kobayashi, Y.; Harayama, T. A concise and versatile synthesis
of viridicatin alkaloids from cyanoacetanilides. Org. Lett. 2009, 11 (7),
1603−6.
(12) Paterna, R.; Andre, V.; Duarte, M. T.; Veiros, L. F.; Candeias,
N. R.; Gois, P. M. P. Ring-Expansion Reaction of Isatins with Ethyl
Diazoacetate Catalyzed by Dirhodium(II)/DBU Metal-Organic
System: En Route to Viridicatin Alkaloids. Eur. J. Org. Chem. 2013,
2013 (28), 6280−90.
(13) Yuan, Y.; Yang, R.; Zhang-Negrerie, D.; Wang, J.; Du, Y.; Zhao,
K. One-pot synthesis of 3-hydroxyquinolin-2(1H)-ones from N-
phenylacetoacetamide via PhI(OCOCF3)2-mediated alpha-hydrox-
ylation and H2SO4-promoted intramolecular cyclization. J. Org.
Chem. 2013, 78 (11), 5385−92.
(14) Tangella, Y.; Manasa, K. L.; Krishna, N. H.; Sridhar, B.; Kamal,
A.; Nagendra Babu, B. Regioselective Ring Expansion of Isatins with
In Situ Generated alpha-Aryldiazomethanes: Direct Access to
Viridicatin Alkaloids. Org. Lett. 2018, 20 (12), 3639−42.
(15) Ishikawa, N.; Tanaka, H.; Koyama, F.; Noguchi, H.; Wang, C.
C.; Hotta, K.; Watanabe, K. Non-heme dioxygenase catalyzes atypical
oxidations of 6,7-bicyclic systems to form the 6,6-quinolone core of
viridicatin-type fungal alkaloids. Angew. Chem., Int. Ed. 2014, 53 (47),
12880−4.
(16) Kishimoto, S.; Hara, K.; Hashimoto, H.; Hirayama, Y.;
Champagne, P. A.; Houk, K. N.; Tang, Y.; Watanabe, K. Enzymatic
one-step ring contraction for quinolone biosynthesis. Nat. Commun.
2018, 9 (1), 2826.
(24) Shirts, M. R.; Pande, V. S. Comparison of efficiency and bias of
free energies computed by exponential averaging, the Bennett
acceptance ratio, and thermodynamic integration. J. Chem. Phys.
2005, 122 (14), 144107.
(25) Matthews, M. L.; Neumann, C. S.; Miles, L. A.; Grove, T. L.;
Booker, S. J.; Krebs, C.; Walsh, C. T.; Bollinger, J. M., Jr. Substrate
positioning controls the partition between halogenation and
hydroxylation in the aliphatic halogenase, SyrB2. Proc. Natl. Acad.
Sci. U. S. A. 2009, 106 (42), 17723−8.
(26) Dunham, N. P.; Chang, W.-c.; Mitchell, A. J.; Martinie, R. J.;
Zhang, B.; Bergman, J. A.; Rajakovich, L. J.; Wang, B.; Silakov, A.;
Krebs, C.; Boal, A. K.; Bollinger, J. M., Jr. Two Distinct Mechanisms
for C-C Desaturation by Iron(II)- and 2-(Oxo)glutarate-Dependent
Oxygenases: Importance of alpha-Heteroatom Assistance. J. Am.
Chem. Soc. 2018, 140 (23), 7116−26.
(27) Einsiedler, M.; Jamieson, C. S.; Maskeri, M.; Houk, K. N.;
Gulder, T. A. M. Fungal Dioxygenase AsqJ Is Promiscuous and
Bimodal: Substrate-Directed Formation of Quinolones versus
Quinazolinones. Angew. Chem., Int. Ed. 2021, 60, 8297−302.
(28) Tomasi, J.; Mennucci, B.; Cammi, R. Quantum mechanical
continuum solvation models. Chem. Rev. 2005, 105 (8), 2999−3093.
(29) Wojdyla, Z.; Borowski, T. On how the binding cavity of AsqJ
dioxygenase controls the desaturation reaction regioselectivity: a
QM/MM study. JBIC, J. Biol. Inorg. Chem. 2018, 23 (5), 795−808.
(30) Uchida, R.; Imasato, R.; Tomoda, H.; Omura, S. Yaequino-
lones, new insecticidal antibiotics produced by Penicillium sp. FKI-
2140. II. Structural elucidation. J. Antibiot. 2006, 59 (10), 652−8.
(31) Scherlach, K.; Hertweck, C. Discovery of aspoquinolones A-D,
prenylated quinoline-2-one alkaloids from Aspergillus nidulans,
motivated by genome mining. Org. Biomol. Chem. 2006, 4 (18),
3517−20.
(32) Neff, S. A.; Lee, S. U.; Asami, Y.; Ahn, J. S.; Oh, H.; Baltrusaitis,
J.; Gloer, J. B.; Wicklow, D. T. Aflaquinolones A-G: secondary
metabolites from marine and fungicolous isolates of Aspergillus spp. J.
Nat. Prod. 2012, 75 (3), 464−72.
(33) An, C. Y.; Li, X. M.; Luo, H.; Li, C. S.; Wang, M. H.; Xu, G. M.;
Wang, B. G. 4-Phenyl-3,4-dihydroquinolone derivatives from
Aspergillus nidulans MA-143, an endophytic fungus isolated from
the mangrove plant Rhizophora stylosa. J. Nat. Prod. 2013, 76 (10),
1896−901.
(34) Heck, R. F.; Nolley, J. P. Palladium-Catalyzed Vinylic
Hydrogen Substitution Reactions with Aryl, Benzyl, and Styryl
Halides. J. Org. Chem. 1972, 37 (14), 2320−2.
(17) Brauer, A.; Beck, P.; Hintermann, L.; Groll, M. Structure of the
Dioxygenase AsqJ: Mechanistic Insights into a One-Pot Multistep
Quinolone Antibiotic Biosynthesis. Angew. Chem., Int. Ed. 2016, 55
(1), 422−6.
(18) Liao, H. J.; Li, J.; Huang, J. L.; Davidson, M.; Kurnikov, I.; Lin,
T. S.; Lee, J. L.; Kurnikova, M.; Guo, Y.; Chan, N. L.; Chang, W.-c.
Insights into the Desaturation of Cyclopeptin and its C3 Epimer
Catalyzed by a non-Heme Iron Enzyme: Structural Characterization
and Mechanism Elucidation. Angew. Chem., Int. Ed. 2018, 57 (7),
1831−5.
(19) Mader, S. L.; Brauer, A.; Groll, M.; Kaila, V. R. I. Catalytic
mechanism and molecular engineering of quinolone biosynthesis in
dioxygenase AsqJ. Nat. Commun. 2018, 9 (1), 1168.
(20) Li, J.; Liao, H. J.; Tang, Y.; Huang, J. L.; Cha, L.; Lin, T. S.; Lee,
J. L.; Kurnikov, I. V.; Kurnikova, M. G.; Chang, W.-c.; Chan, N. L.;
Guo, Y. Epoxidation Catalyzed by the Nonheme Iron(II)- and 2-
Oxoglutarate-Dependent Oxygenase, AsqJ: Mechanistic Elucidation
of Oxygen Atom Transfer by a Ferryl Intermediate. J. Am. Chem. Soc.
2020, 142 (13), 6268−84.
(21) Straatsma, T. P.; Mccammon, J. A. Computational Alchemy.
Annu. Rev. Phys. Chem. 1992, 43, 407−35.
(22) Mobley, D. L.; Klimovich, P. V. Perspective: Alchemical free
energy calculations for drug discovery. J. Chem. Phys. 2012, 137 (23),
230901.
(23) Bennett, C. H. Efficient Estimation of Free-Energy Differences
from Monte-Carlo Data. J. Comput. Phys. 1976, 22 (2), 245−68.
7192
https://doi.org/10.1021/acscatal.1c01150
ACS Catal. 2021, 11, 7186−7192