Modular Chemoenzymatic Synthesis of GE81112 B1 and Related Analogues Enables Elucidation of Its Key Pharmacophores

Article abstract of DOI:10.1021/jacs.0c13424

The GE81112 complex has garnered much interest due to its broad antimicrobial properties and unique ability to inhibit bacterial translation initiation. Herein we report the use of a chemoenzymatic strategy to complete the first total synthesis of GE81112 B1. By pairing iron and α-ketoglutarate dependent hydroxylases found in GE81112 biosynthesis with traditional synthetic methodology, we were able to access the natural product in 11 steps (longest linear sequence). Following this strategy, 10 GE81112 B1 analogues were synthesized, allowing for identification of its key pharmacophores. A key feature of our medicinal chemistry effort is the incorporation of additional biocatalytic hydroxylations in modular analogue synthesis to rapidly enable exploration of relevant chemical space.

Full text of DOI:10.1021/jacs.0c13424

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Modular Chemoenzymatic Synthesis of GE81112 B1 and Related
Analogues Enables Elucidation of Its Key Pharmacophores
Christian R. Zwick, III, Max B. Sosa, and Hans Renata*
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ABSTRACT: The GE81112 complex has garnered much interest due
to its broad antimicrobial properties and unique ability to inhibit
bacterial translation initiation. Herein we report the use of a
chemoenzymatic strategy to complete the first total synthesis of
GE81112 B1. By pairing iron and α-ketoglutarate dependent
hydroxylases found in GE81112 biosynthesis with traditional synthetic
methodology, we were able to access the natural product in 11 steps
(longest linear sequence). Following this strategy, 10 GE81112 B1
analogues were synthesized, allowing for identification of its key
pharmacophores. A key feature of our medicinal chemistry effort is the
incorporation of additional biocatalytic hydroxylations in modular
analogue synthesis to rapidly enable exploration of relevant chemical
space.
INTRODUCTION
RESULTS AND DISCUSSION
■
■
GE81112 is a complex of three related tetrapeptides identified
in 2006 during a screen for new inhibitors of bacterial protein
synthesis (Figure 1A).1 Structurally, GE81112 consists of
several unusual amino acids, including 3-hydroxy-L-pipecolic
acid (AA1), 4-hydroxy-L-citrulline or O-carbamoyl-2-amino-
dihydroxyvaleric acid (AA2), 2-amino-L-histidine (AA3), and
β-hydroxy-2-chloro-L-histidine (AA4). Each GE81112 tetra-
peptide demonstrates broad antimicrobial properties conveyed
by a unique mechanism of action: inhibition of bacterial
translation initiation.1 Subsequent studies have confirmed that
GE81112 binds to the 30S ribosome, inhibiting the P-site
decoding of an mRNA’s initiation codon by fMet-tRNA’s
anticodon.2 Although GE81112 displays broad-spectrum
activity in minimal media, its activity is significantly attenuated
against bacteria grown in rich media.1 Subsequent mechanistic
studies indicate that illicit transport of GE81112 into its
bacterial target is mediated by the peptidyl transporter
oligopeptide permease (Opp), but competition with peptides
found in rich media results in reduced efficacy.3 Based on these
findings, development of GE81112 analogues that can
permeate the bacterial cell membrane without the aid of
Opp presents a potential solution. To date, there is only one
reported total synthesis of the GE81112s (GE81112 A, 2018)
though a minimal structure−activity relationship (SAR) has
been established for this tetrapeptide.4 Additionally, the
reported synthesis suffers from high step count (7−8 steps
per fragment) and poor stereocontrol (Figure 1A). In light of
these shortcomings, we sought to develop a tractable
chemoenzymatic synthesis of GE81112 B1 that will also be
amenable for analogue generation for SAR analysis.
Our synthetic design combines biocatalytic and contemporary
chemical approaches to construct each fragment with minimal
step count.5 To balance these synthetic paradigms, we devised
a biocatalytic approach for the preparation of AA1 and AA2
and a traditional chemical approach for AA3 and AA4. We
envisioned utilizing C−H hydroxylation reactions present in
GE81112 biosynthesis to construct AA1 and AA2 (Figure 1B).
Previous reports established GetF and GetI as iron and α-
ketoglutarate dependent dioxygenase enzymes (Fe/αKG)
responsible for hydroxylation of L-pipecolic acid (L-Pip) and
L-citrulline (L-Cit), respectively (Figure 1B).6 Next, a chemo-
selective azo coupling would establish a “masked” 2-amino-
imidazole motif present within AA3, aiding purification and
imparting chemoselectivity in subsequent transformations.7 An
asymmetric aldol transform would establish AA4 while also
controlling the syn-1,2-amino alcohol stereochemistry. Con-
vergent assembly of each fragment followed by protecting
group removal would afford GE81112 B1. This strategy was
also designed with modularity in mind, as further analogues of
GE81112 B1 could be prepared by tapping into the wealth of
sequence diversity of Fe/αKG hydroxylases for preparing
additional AA1 and AA2 units and by using alternative
Received: December 29, 2020
Published: January 8, 2021
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was possible by utilizing 20 mM substrate concentration, pH =
8 kPi buffer, and clarified lysate (Scheme 1A and Table SI1).
Scheme 1. (A) GetF-Catalyzed Hydroxylation of L-Pip and
Subsequent Boc Protection; (B) GetI-Catalyzed
Hydroxylation of L-Cit and Subsequent Protecting Group
Manipulations; (C) Peptide Coupling between 6 and 7
Followed by Saponification
Following ion-exchange purification, the resulting crude
material was converted to the corresponding N-Boc derivative,
affording 6 in 74% yield over two steps (Scheme 1A).
Similarly, L-Cit was combined with αKG, FeSO4, asc, and
lysate of E. coli BL21(DE3) expressing GetI. Optimization of
lysis conditions and headspace volume proved vital for
obtaining complete conversion consistently on the gram scale
with 20 mM substrate concentration (Table SI2). Ion-
exchange purification as well as Boc and TBS protections
followed by methyl esterification resulted in a 41% yield of 10
over four steps (Scheme 1B). Because of the acid labile nature
of the 2° TBS group, removal of the Boc group necessitated
the use of TESOTf buffered by 2,6-lutidine.10a,b These
conditions presumably proceed through the intermediacy of
a silyl carbamate, which is hydrolyzed upon work-up. The
resulting amine 7 was carried on crude to peptide coupling
with 6, yielding 4 after saponification (Scheme 1C).
With access to 4, construction of dipeptide 5 could now
commence (Scheme 2). Initially, we tested previously reported
conditions for 2-azotization of Boc-His-OMe (HCl, NaNO2,
pH = 10.5 Na2B4O7 buffer, 4-aminobenzoate, 65%), which
unfortunately gave 6% yield in our hands.7 After testing a
variety of azo coupling conditions (Table SI3), we eventually
discovered that modulating the initially tested conditions by
switching the coupling partner (4-aminobenzoate to 4-
bromoaniline) gave 48% yield of our desired 2-azoimidazole
(Scheme 2A). Saponification of the methyl ester gave acid 8,
which was used crude in subsequent peptide coupling. To
construct AA4, we initially attempted Myers’ asymmetric aldol
by using pseudophenamine glycinamide as a chiral auxiliary,
but the resulting aldol adduct proved unstable to purification
Figure 1. Structures of the GE81112 tetrapeptide complex. (A)
Known biosynthesis of its monomers. (B) Our retrosynthetic analysis
of GE81112 B1 (3).
aldehyde partners for preparing additional AA4 units. While
seemingly straightforward, successful implementation of our
strategy would still require careful choreography of protecting
group manipulations and condition selections to avoid
chemoselectivity issues. For example, late-stage hydrogenative
unmasking of the 2-azoimidazole functionality in the presence
of reduction-prone 2-chloroimidazole could present a major
concern. Here, we rationalized that introduction of a suitable
protecting group on the 2-chloroimidazole unit would allow for
chemoselective azo hydrogenation based on differing steric
environments.
We first sought to establish GetF and GetI as robust
biocatalysts for the preparation of monomers AA1 and AA2.
GetF was recently used to hydroxylate 3-methylproline in the
synthesis of 3-hydroxy-3-methylproline, although a large-scale
reaction with L-Pip has not been reported.8 GetF was initially
found to suffer from poor soluble expression when expressed as
N-His6-tagged protein, but coexpression of chaperones
GroES/GroEL resulted in a dramatic improvement in soluble
enzyme expression.8,9 For large-scale reactions, L-Pip was
combined with αKG, FeSO4, ascorbic acid (asc), and lysate of
E. coli BL21(DE3) coexpressing GetF and GroES/GroEL
chaperone proteins. Complete conversion on a 500 mg scale
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Scheme 2. (A) Azo Coupling on Boc-L-His-OMe to Prepare the Masked 2-Aminohistidine Motif and Subsequent Hydrolysis;
(B) Diastereoselective Synthesis of 9 via Franck Aldol between 12 and 13 Followed by Azide Reduction; (C) Synthesis of Key
a
Dipeptide 5 by Peptide Coupling between 8 and 9 Followed by Boc Deprotection
a
Blue inset: substrate preference of Franck aldol with 13. Teal inset: model study for chemoselective reduction of the 2-azoimidazole moiety.
or subsequent transformations, in line with their previous
report.11 As an alternative, we investigated TiCl4-mediated
aldol conditions previously reported by Franck for the
synthesis of syn-1,2-azido alcohols from azidoacetyl-4-phenyl-
thiazolidin-2-thione (13, Scheme 2B).12 To our dismay, 2-
chloro-5-formyl-BOM-imidazole (14) showed only a minor
product by LC-MS analysis and no observable product upon
work-up (Scheme 2, blue inset). Switching the BOM
protecting group to Trt (15) showed no reaction by LC/MS
or NMR. On the basis of these findings, we speculated that
increased steric congestion proximal to the aldehyde might
block the titanium enolate’s approach. In line with this
hypothesis, 4-formyl-Trt-imidazole (16), which places the Trt
protecting group distal to the aldehyde, gave 7:1 dr and 33%
isolated yield after methanolysis. The use of 2-chloro-4-formyl-
Trt-imidazole (12, accessible from known SI4 in one step) in
the reaction gave the syn-1,2-azido alcohol product in 59%
yield as a single diastereomer on the gram scale. Subsequently,
azide reduction proved nontrivial, resulting in trityl depro-
tection, chloride reduction, or nonspecific degradation under
the conditions tested (Table SI4). A report by Boger used
gaseous hydrogen sulfide (H2S) for azide reduction in the
preparation of related histidine analogues.13 As hydrogen
sulfide is toxic and not widely available, we instead tried
aqueous ammonium sulfide ((NH4)2S), which very cleanly
yielded the desired amino alcohol 9 without the need for
subsequent purification (Table SI4, entry 9).14 To the best of
our knowledge, this is the first reported use of these conditions
for the reduction of alkyl azides.
18 to reaction with H2 and PtO2 gratifyingly yielded
chemoselective azo hydrogenation.7 To confirm that chemo-
selectivity was imparted by the Trt, we also submitted an
equimolar ratio of 17 and des-Trt-alcohol 19 to the same
hydrogenation conditions, which gave a 4:1 mixture of 19 to
des-chloro 21. These observations suggest that Trt protection
imparts chemoselectivity between the azo and chloro
functionality and that control of the steric environment around
the 2-chloroimidazole would allow for late-stage removal of the
azo functionality en route to GE81112 B1.
With conditions for late-stage azo removal in hand, we next
targeted dipeptide 5 (Scheme 2C). Careful temperature
control (e.g., slowly warming from −40 °C to rt) proved
necessary to avoid epimerization when coupling 8 and 9.
Under these conditions, diasteriomerically pure product could
be obtained in 79% over two steps.4 Similar to 10, Boc
deprotection required exceptionally mild conditions to avoid
Trt deprotection; therefore, we tested silyltriflates buffered by
2,6-lutidine (Table SI5). While TBSOTf and TESOTf resulted
in stable TBS carbamate and Trt deprotection, respectively,
TMSOTf cleanly afforded 5 in 77% isolated yield.10 With
fragments 4 and 5 in hand, we proceeded to construct the core
tetrapeptide of GE81112 B1 (Scheme 3). Slow addition of 4 to
a solution of 5, EDC, Oxyma, and NaHCO3 cleanly afforded
22 as a single diastereomer in 66% isolated yield.4 In line with
our model hydrogenation, 22 was chemoselectively reduced to
amine 23 in 66% yield under the action of H2 and PtO2.
Subsequent methyl ester hydrolysis followed by global
deprotection and HPLC purification yielded GE81112 B1 in
36% over two steps as the TFA salt.
Because late-stage chloride reduction was of concern, we
designed a model system to probe the viability of performing a
chemoselective 2-azoimidazole hydrogenation in the presence
of a 2-chloroimidazole-containing molecule (Scheme 2, teal
inset). Submitting an equimolar ratio of 17 and AA4 surrogate
Having established a tractable route to GE81112 B1 (3), we
set out to determine its key pharmacophores that significantly
contribute to its antimicrobial activity. Prior efforts by
Fabbretti et al. have resulted in a solved crystal structure of a
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seems operative, given many analogous literature reactions that
proceed under standard radical chlorination conditions.17b,c
Another possibility involves concerted homolysis of the C−S
and S−Cl bonds in 31, followed by recombination of the
resulting carbon and chlorine radicals to 32.17a,d However, this
mechanism seems less likely given the high bond dissociation
energy (BDE) of C−S and S−Cl bonds (∼67 kcal/mol).17d
Further studies are underway to elucidate the mechanism of
this transformation.
Scheme 3. Peptide Coupling of 4 and 5 Followed by
Deprotection Sequence to Complete the Synthesis of
GE81112 B1
Further difficulty in the synthesis of 27 was experienced
when the trityl-protected derivative of 32 proved to be more
labile than 9, posing significant challenge in selective Boc
deprotection after coupling with 8. As a workaround, linear
assembly to a tripeptide intermediate consisting of 6, 7, and 8
was first prepared and submitted to a final peptide coupling
with SI32, which provided analogue 27 after deprotection. Our
synthesis of 27 delineates an alternative assembly strategy to
the GE81112 core that is additionally more well-suited to
diversity-oriented synthesis in the production of large numbers
of analogues. Finally, the use of a Franck aldol on aldehyde 16
and selective azide reduction provided protected β-hydroxy-L-
histidine 33, albeit in diminished yield and diastereoselectivity.
Nevertheless, 33 could be submitted to the same synthetic
sequence described in Schemes 2 and 3 to afford analogue 28.
Analogues 24−28 were next subjected to antibacterial
assays, consisting of minimal inhibitory concentration (MIC)
measurements against E. coli strain MG1655 in minimal media
with 3 and gentamycin as a control. The MIC obtained for 3
was in close agreement with previous data in the literature.1,3,4
Analogue 24 was found to be completely inactive. In
conjunction with previous finding from Sanofi, this result
suggests that the syn-β-hydroxy amino acid unit in AA1 is
highly crucial for inhibitory activity.4 Analogue 25, containing
unmodified L-Cit at AA2, was found to maintain the
antibacterial activity of the parent natural product. This
finding points to the extraneous nature of the γ-OH unit in
AA2 for activity and also the possibility of further developing
simplified analogue(s) of 3 with equipotent antibacterial
activity. Removal of the C2-NH2 moiety in AA3 or C2−Cl
in AA4 only reduced the activity of the resulting compounds
by 5−15-fold, while removal of the β-OH in AA4 led to a more
than 500-fold reduction in activity. These data show that the β-
OH motif is the most crucial peripheral modification within
the AA3−AA4 dipeptide for antibacterial activity.
In light of the above data, we next sought to further
investigate the structural requirements for antibacterial activity
at the AA1 and AA2 positions (Scheme 4B). The importance
of the syn-β-hydroxy amino acid unit in AA1 begged the
question whether this property is highly specific to 3-hydroxy-
L-pipecolic acid or whether substitution with other hydroxy-
lated amino acids, either cyclic or acyclic, is tolerated at this
position. To this end, we synthesized three analogues
containing cis-3-hydroxy-L-proline, cis-4-hydroxy-L-proline,
and cis-3-hydroxy-L-norleucine at AA1 (34, 35, and 36,
respectively). The corresponding AA1 building blocks for
these analogues were all prepared through the use of enzymatic
hydroxylations with the appropriate Fe/αKGs. Thus, L-Pro was
submitted to hydroxylation with two different Fe/αKG proline
hydroxylases, P3H18a,b,c,d and P4H,19 to afford 37 and 38. To
synthesize 39, we leveraged a previous report describing a two-
enzyme system20 that converts N-succinylated aliphatic amino
acids to their β-hydroxylated free amino acid counterparts via
hydroxylation with the Fe/αKG SadA, followed by N-
3:30S complex.2b However, we noted a lack of electron density
around the ligand in this structure, which likely renders it
unreliable for structure-based analogue design. Conversely,
SAR investigation between 3 and its antimicrobial activity
would shed light onto the contribution of each structural motif
to the inhibitory activity of the tetrapeptide and aid future
efforts in designing superior analogue(s) or simplified
analogue(s) with comparable activity. First, we sought to
assess the importance of the five peripheral modifications for
inhibitory activity (Scheme 4A). To this end, we prepared five
simplified analogues of 3 (24−28), each containing one fewer
peripheral modification than the parent natural product.
Synthesis of analogues 24, 25, and 26 proceeded in a routine
fashion, requiring the use of readily available building blocks
Boc-L-Pip-OH, L-Cit-OMe, and Boc-L-His-OMe, respectively,
in a route that is almost identical with Schemes 1−3.
Previously, we reported the synthesis of Cl-L-His-OMe 32
for functional characterization of GetI.6c While useful for
analytical scale preparation of 32, our reported approach
suffers from low yield and limited scalability. In turn,
preparation of analogue 27 necessitated the development of
a novel strategy that remedies these synthetic shortcomings
toward 2-chloro-L-histidine. Direct chlorination of Boc-L-His-
OMe gave only C4 chlorination using SEAr conditions (NCS
or PIDA/NaCl) or the ketone following lithiation (Figure
SI4).15 An effective ANRORC reaction with phenyl chlor-
oformate afforded oxo-Boc-L-His-OMe (SI29) in 85% yield,
although only minor amounts of 32 were observed while
testing various chlorination conditions from this intermedia-
te.16a Success was finally achieved by developing an analogous
ANRORC reaction utilizing phenyl thionochloroformate (29)
to produce thio-Boc-L-His-OMe 30 in 90% yield.16b Chemo-
selective oxidation of 30 with NCS afforded the intermediate
sulfonyl chloride (31), which was then converted to 32
following desulfination under gentle heating.17a This novel
reaction sequence proceeds on a >500 mg scale without any
observable loss in isolated yield, solving the key limitations
observed in our previous route. The unique desulfinative
chlorination from 31 to 32 may proceed via the intermediacy
of a chlorine radical that participates in the ipso substitution of
the sulfonyl chloride, regenerating the chlorine radical
following sulfur dioxide extrusion.17b Although the exact
precursor to a chlorine radical is puzzling, this mechanism
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Scheme 4. (A) Synthesis of Simplified Analogues 24−28 and Their MIC Measurements; (B) Synthesis of Analogues
Containing Alternative Hydroxylated Amino Acids at AA1 (34−36) and Alternative Amino Acids at AA2 (40, 41) and Their
a
MIC Measurements
a
MIC values are provided as a range obtained from three measurements.
desuccinylation with LasA. In our hands, modifications to the
originally reported procedure were found necessary (see Table
SI7 for details). First, the reported one-pot protocol proved
problematic in our hands. We elected instead for a two-step
procedure consisting of β-hydroxylation of N-Suc-L-Nle with
SadA, followed by C18 purification, and then subjection of the
hydroxylated product to desuccinylation with LasA to afford
39. Additionally, adjustments to several key reaction
parameters, such as relative enzyme concentrations, reaction
time, and temperature, for both SadA and LasA had to be
made as well for robust production of 39. Conversion of 37,
38, and 39 to 34, 35, and 36 proceeded in a routine fashion
following previously established route (Schemes 1−3). MIC
measurements revealed that 34 is equipotent to 3 while 35 is
completely inactive, suggesting strict requirement for syn-β-
hydroxy amino acid at the AA1 position, which cannot be
rescued even by syn-γ-hydroxy amino acid. On the other hand,
analogue 36 displayed almost 100-fold loss in activity. This
observation shows the importance of cyclic amino acid motif at
the same position, likely due to its superior conformational
rigidity relative to its acyclic counterpart. It is worth
emphasizing that the generation of these key SAR data is
greatly facilitated by the use of biocatalytic hydroxylation, as
some of the building blocks needed are not readily accessible
by any other means. For example, 37 is only available
commercially at $400/g and asymmetric aldol to prepare 39 by
using Franck’s method12 would be problematic as the method
is known to be incompatible with aliphatic aldehydes.
In similar fashion, two other analogues, 40 and 41, were
synthesized to probe the structural requirements at the AA2
position, specifically with regards to the functional group at its
δ position. In light of prior SAR data with analogue 25, no γ-
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OH group was introduced at the AA2 position of these
analogues, and their synthesis was accomplished in a
straightforward fashion from suitably protected L-Arg and L-
Orn. We initially presumed that a ureido or a carbamate motif
on AA2 is needed to elicit favorable polar or hydrogen-bonding
interactions in the binding based on the antibacterial profile of
all three natural congeners of GE81112. Interestingly,
analogues 40 and 41 were found to be just as potent as 3 in
MIC tests with E. coli MG1655. These findings suggest that
several alternative functional groups can be introduced at the δ
position of AA2 without any deleterious effects on antibacterial
activity against E. coli.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/jacs.0c13424.
■
sı
Experimental details, analytical data, H and 13C NMR
data (PDF)
1
AUTHOR INFORMATION
Corresponding Author
■
Hans Renata − Department of Chemistry, The Scripps
Research Institute, Jupiter, Florida 33458, United States;
orcid.org/0000-0003-2468-2328; Email: hrenata@
scripps.edu
CONCLUSION
■
This work describes the development of a chemoenzymatic
strategy to arrive at the first synthesis of GE81112 B1 and the
application of this strategy for the synthesis of 10 analogues. By
combining biocatalytic hydroxylation and traditional chemical
methods, we developed a strategy that requires only 2−5 steps
(3 steps average) to synthesize each AA fragment. Additionally,
our hybrid approach allows each key fragment to be prepared
with high levels of stereocontrol. Thus, our strategy not only
constitutes a distinct departure from contemporary approaches
to morphed peptide synthesis but also provides a robust
solution to some of the shortcomings encountered in previous
synthetic studies on GE81112 A.4
Authors
Christian R. Zwick, III − Department of Chemistry, The
Scripps Research Institute, Jupiter, Florida 33458, United
States
Max B. Sosa − Department of Chemistry, The Scripps
Research Institute, Jupiter, Florida 33458, United States
Complete contact information is available at:
https://pubs.acs.org/10.1021/jacs.0c13424
Notes
The authors declare no competing financial interest.
Similar to our previous work,8,9,21 this report highlights the
strategic benefits of using enzymatic C−H oxidation to
streamline access to complex, bioactive natural products. Our
efficient strategy to access GE81112 B1 also enabled the
preparation of 10 analogues, which in turn allowed us to
elucidate the key pharmacophores of GE81112 B1 and obtain
the most exhaustive SAR insights into its inhibitory activity to
date. Some of the key SAR insights obtained include the
paramount importance of the syn-β-hydroxy amino acid at the
AA1 position and the β-OH on AA4 for antibacterial activity
and the surprising tolerance of the scaffold toward sub-
stitutions at the AA2 position. Though five of the analogues
described in this work are just as active as the parent natural
product in tests against E. coli, it is possible that some may
display different antibacterial profiles against other pathogens.
Additionally, some of these analogues may exhibit an
alternative uptake mechanism, and the laxed SAR profile at
the AA2 position may provide an opportunity to develop pro-
drug or delivery strategies that bypass the need for Opp-
mediated transport. Continued investigation on these issues is
ongoing in our laboratory.
From a strategic perspective, this work provides a robust
validation for the use of other Fe/αKG amino acid
hydroxylases in generating unnatural analogues of GE81112
B1 through a chemoenzymatic approach, which should
compare favorably to efforts relying solely on synthetic
chemistry or biosynthetic engineering. As illustrated in our
synthesis of 34−36, these hydroxylations can be applied in a
modular fashion to generate analogues that are not readily
accessible otherwise, thereby enabling efficient exploration of
novel chemical space. The strategy outlined here is expected to
be broadly applicable to the medicinal chemistry exploration of
other highly oxidized peptides wherein a pool of Fe/αKG
hydroxylases could be employed to rapidly perform SAR
studies on a lead molecule to elucidate key pharmacophores
and identify superior analogues.
ACKNOWLEDGMENTS
■
This work was supported, in part, by the National Institutes of
Health Grants GM128895 (H.R.) and S10 OD021550 (for
600 MHz NMR instrumentation). We thank Prof. Ian Seiple
for useful discussions on asymmetric aldol reactions. We thank
Drs. Andrew Steele and Christiana Teijaro for providing
assistance with MIC measurements. We acknowledge the
Roush, Bannister, Kodadek, and Shen laboratories for generous
access to their reagents and instrumentation.
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J. Am. Chem. Soc. 2021, 143, 1673−1679

Journal of the American Chemical Society
pubs.acs.org/JACS
Article
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J. Am. Chem. Soc. 2021, 143, 1673−1679