Scalable Synthesis of Mycocyclosin

Article abstract of DOI:10.1021/acs.orglett.8b00894

We report herein the scalable total synthesis of the secondary metabolite, mycocyclosin, initially isolated from Mycobacterium tuberculosis. Mycocylosin bears a highly strained 3,3′-dityrosine biaryl system which arises biosynthetically from an intramolec

Full text of DOI:10.1021/acs.orglett.8b00894

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Scalable Synthesis of Mycocyclosin
Xu Zhu, Christopher C. McAtee, and Corinna S. Schindler*
Department of Chemistry, Willard Henry Dow Laboratory, University of Michigan, 930 North University Avenue, Ann Arbor,
Michigan 48109, United States
S
* Supporting Information
ABSTRACT: We report herein the scalable total synthesis of
the secondary metabolite, mycocyclosin, initially isolated from
Mycobacterium tuberculosis. Mycocylosin bears a highly strained
3,3′-dityrosine biaryl system which arises biosynthetically from
an intramolecular oxidative dehydrogenative cross-coupling of
cyclo(L-Tyr-L-Tyr) (cYY) catalyzed by the P450 enzyme
CYP121. CYP121 is found exclusively in M. tuberculosis.
Scalable access to mycocyclosin and related derivatives via a
Pd(II)-catalyzed macrocyclization is anticipated to facilitate the
biological evaluation of these compounds as novel tuberculosis antimicrobials.
he human pathogen Mycobacterium tuberculosis is the
Torganism responsible for the development of tuberculosis
(TB), a chronic but curable infectious disease that is associated
with up to 3 million deaths annually.1 This incongruity between
treatment and global health threat can be attributed to the rise
of drug-resistant and multidrug-resistant strains for which the
current front- and second-line antitubercular treatments have
proven ineffective.2 The genome of M. tuberculosis encodes a
significant amount of cytochrome P450 enzymes, while only a
select few are essential for M. tuberculosis virulence.3
Furthermore, interest in these enzymes was enhanced by
studies implicating them as targets for several azole-derived
compounds that were previously identified as effective
antimicrobial molecules.4 Unfortunately, the use of azole-
derived pharmacaphores often results in significant toxicity due
to cross-reactivity with other cytochrome P450 enzymes within
the host.
Figure 1. Biogenesis of mycocyclosin (2) from cYY (1).
The gene rv2276 encodes CYP121, a cytochrome P450
enzyme with a distinct metabolic role found exclusively in
strains of M. tuberculosis, thereby making it one of the most
logical candidates for evaluation as a potential drug target.4a,5,6
Specifically, CYP121 converts cyclic dipeptide cYY (1) in a
dehydrogenative cross-coupling reaction of the tyrosine
subunits, accessing a highly strained 3,3′-dityrosine biaryl
system, to form mycocyclosin (2) (Figure 1).5 Moreover, cYY
(1) is the only known substrate of CYP121 activity. As a result,
the distinct scaffold of mycocyclosin can be used as a platform
to design selective inhibitors of CYP121 that exhibit low
toxicity to the host.7 The first total synthesis of mycocyclosin
(2) was reported by Hutton in 2012, in which the target
molecule was assembled in eight synthetic transformations.8 In
the key step, a Suzuki−Miyaura cross-coupling effected the
desired macrocyclization in 42% yield on a 50 mg scale.8 With
an interest in developing potent inhibitors of CYP121 based on
structural analogy to mycocyclosin, we planned to take
advantage of this previously reported route. To facilitate the
biological evaluation of mycocyclosin and related derivatives as
small-molecule drug candidates for the inhibition of CYP121,
we required a scalable and robust synthesis to access the
constrained cyclophane. Unfortunately, when employing
previously reported macrocyclization conditions, we observed
difficulties executing the Suzuki−Miyaura cross-coupling on
scales larger than 50 mg. Herein, we report a synthesis of
mycocyclosin and derivatives relying on a Pd(II)-catalyzed
cross-coupling which can be carried out on up to gram scale.
Benzylated diketopiperazine (DKP) 3 is readily accessible
from 3-iodo- L-tyrosine and was chosen as our initial substrate
to evaluate reaction conditions for the scalable synthesis of
strained cyclophane 4 via a Pd(II)-catalyzed macrocyclization
(see the Supporting Information for a complete summary of
reaction conditions). When 3 was subjected to reaction
Received: March 19, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b00894
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
parameters initially reported by Hutton (Pd(dppf)Cl2·CH2Cl2
(20 mol %), K2CO3 (6 equiv), B2pin2 (1 equiv) and DMSO at
90 °C), the anticipated biaryl product 4 was isolated in 39%
yield on 50 mg scale in accordance with the literature (Table 1,
diboronate in which trace contamination of air allows for
productive catalysis. In an attempt to improve cross-coupling
efficiency, we next evaluated the addition of air to the reaction.
Notably, under aerobic conditions, catalysis was not inhibited
as 3 converted to 4 in an identical yield of 63% as under an
inert nitrogenous atmosphere on 500 mg scale (Table 2, entry
Table 1. Reaction Optimization for the Conversion of 3 to
a
4.
a
Table 2. Evaluating the Addition of Exogenous Air
a
Conditions: biaryl 3 (50 mg, 0.066 mmol to 500 mg, 0.66 mmol),
Pd(dppf)2Cl2·CH2Cl2 (20 mol %) K2CO3 (6 equiv), B2pin2 (1.0−6.0
equiv as noted) and DMSO (N2 sparged, ACROS Chemicals, 99.7%,
b
0.001 M) at 90 °C for 19 h. DMSO from solvent dispensing system.
c
a
DMSO/H2O = 100:1.
Conditions: biaryl 3 or 5 (500 mg,1 equiv), Pd(dppf)2Cl2•CH2Cl2
(20 mol %) K2CO3 (6 equiv), B2pin2 (6.0 equiv) and DMSO/H2O =
entry 1).8 Interestingly, switching the DMSO source from
Acros (anhydrous 99.7%) to DMSO dried over alumina via a
solvent-dispensing system led to no isolable yield of the desired
product in our hands (Table 1, entry 2). Nevertheless, the yield
of 4 could be further improved to 61% by increasing the
amount of B2pin2 (Table 1, entries 3−4). While increasing the
B2pin2 ratio proved beneficial on smaller scale (50 mg),
carrying out the biaryl coupling on 100 and 500 mg scales,
under otherwise identical reaction conditions, led to diminished
yields of 4 of 44% and 25%, respectively (Table 1, entries 5 and
6). Inspired by Denmark’s mechanistic insight into the Suzuki−
Miyaura transmetalation step,9 we hypothesized that water may
play a critical role in facilitating the biaryl coupling. Karl Fischer
titration of our reaction solvent provided additional support for
this proposal. Specifically, we observed a stark difference in
water content of Acros DMSO (anhydrous 99.7%) and that
from our solvent system (464.8 and 197.2 ppm, respectively).
Based on this analysis, we evaluated the mixed solvent system
DMSO/H2O (100:1). Water proved to be beneficial and
allowed for consistent formation of 4 in 63% isolated yield on
50 mg scale (Table 1, entry 7). Moreover, an identical yield was
obtained on half gram scale with 6 equiv of B2pin2 (Table 1,
entry 8).
100:1 (N2 sparged, ACROS Chemicals, 99.7%, 0.001 M) at 90 °C for
b
19 h. 5 mL air added after 30 min.
2). In comparison, biaryl substrate 5 underwent the macro-
cyclization in a modest 51% yield under inert conditions (Table
2, entry 3). Examination of the crude reaction mixture after 30
min by HRMS led us to observe the m/z ratio consistent with
diboronate 7 (see the Supporting Information for details).
Subsequently, subjecting 5 to the cross-coupling conditions
with air as exogenous oxidant, under otherwise identical
conditions, afforded 6 in an enhanced 80% isolated yield.
Thus, depending on the biaryl substrate being evaluated under
the cross-coupling conditions, the addition of air to the reaction
can be advantageous.
With suitable reaction conditions in hand to access the
desired cyclophane motif on half gram scale, we next sought to
investigate the functional group tolerance on the DKP and aryl
subunits (Table 3). While the OBn-NH DKP scaffold (3)
proved to be a viable cross-coupling substrate under either
aerobic or anaerobic atmosphere, the corresponding OMe-NH
DKP 8 proved to be sluggish under either set of reaction
conditions (Table 3, entries 3 and 4 versus 5 and 6).12
Conversely, when substrates bearing acid-sensitive MOM
ethers (12) or oxidatively labile PMB groups (14) were
exposed to the Pd(II)-catalyzed cross-coupling reaction, the
isolated yield of the anticipated [8.2.2] polycycle (13 and 15,
respectively) were only modestly affected upon the incorpo-
ration of air (Table 3, entries 9−12). As a means to provide a
metric for molecular strain, the arene displacement angles (α),
which depict how distorted a benzene ring is from planarity,
were determined for cyclophane 11 by acquiring a single-crystal
Observing that excess B2pin2 leads to increased reaction
efficiency, we considered that an alternative reaction pathway to
the Suzuki−Miyaura coupling may be plausible.10 Recently,
Jasti elegantly demonstrated that preformed diboronic esters
readily undergo Pd(II)-catalyzed intramolecular homocoupling
in air to form strained macrocycles.11 Similarly, we hypothe-
sized that under our optimized reaction conditions for
accessing 4 the transformation may be proceeding via a
B
DOI: 10.1021/acs.orglett.8b00894
Org. Lett. XXXX, XXX, XXX−XXX

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Table 3. Functionalized Mycocyclosin Derivatives Obtained
Figure 2. Arene displacement angles (α) of 11.
Intrigued by our observation that H2O and air are critical
reaction additives in order to obtain reproducible yields for the
Pd(II)-catalyzed macrocyclization on scale, we considered two
complementary mechanistic regimes generalized in Figure 3.
Figure 3. General formation of 19 from 16.
Following initial Miyaura borylation of the bisiodide 16 leading
to 17, we hypothesize that (Ar)PdIII can readily undergo anion
metathesis with hydroxide to form (Ar)PdIIOH, which allows
for facile oxopalladium transmetalation with the aryl boronate
(Figure 3, path A).13 Additionally, two consecutive borylations
of 16 can provide 18, which under oxidative conditions can
undergo Pd(II)-catalyzed homocoupling to form 19 (Figure 3,
path B).11 If the rate of Suzuki−Miyaura coupling (17 → 19,
k1) is faster than the second borylation event (17 → 18, k2),
then the addition of oxidant to the reaction will not affect the
overall reaction yield (see Table 3, entries 11 and 12),
Conversely, if k2 is greater than k1, exogenous oxidant should be
advantageous (see Table 2, entries 7 and 8).
With these considerations in mind, we propose that two
catalytic cycles are operative, and complementary, for the
formation of macrocycle 19 (Scheme 1A). Under an anaerobic
atmosphere (Scheme 1A, path A), monoboronate 17 can
engage the Suzuki−Miyaura catalytic cycle by undergoing
oxidative addition with LPd(0) (L= dppf) to 20.14 Conversion
of 20 to palladium alkoxy 21 facilitates the incipient
transmetalation event to 23 proceeding through a transient
intermediate like 22, initially identified by Denmark,9 followed
by reductive elimination to 19. Additionally, on the basis of the
experimentally and computationally proposed mechanism by
Adamo,15 under oxidative conditions (Scheme 1A, path B),
LPd(0) may also be oxidized with O2 to the corresponding
palladium(II) peroxy intermediate 24. Next, coordination of an
aryl boronate ester (18) to 24 affords 25, which facilitates the
a
Conditions: biaryl (1.0 equiv), Pd(dppf)2Cl2·CH2Cl2 (20 mol %)
K2CO3 (6 equiv), B2pin2 (6.0 equiv) and DMSO/H2O = 100:1 (N2
sparged, ACROS Chemicals, 99.7%, 0.001 M) at 90 °C for 19 h.
mL of air added via syringe after 30 min.
b
5
X-ray structure (Figure 2).11b Indeed, there proved to be
significant variation between α1, α2, and α3, which ranged from
5.7° to 9.7°.
C
DOI: 10.1021/acs.orglett.8b00894
Org. Lett. XXXX, XXX, XXX−XXX

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Letter
a
Scheme 1. (A) Proposed Catalytic Cycles for the Formation of 19. (B) Synthesis of Mycocyclsin
a
Conditions: see the Supporting Information for complete reaction details.
first transmetalation event leading to boronic peroxo 26. Attack
of water on peroxide 26 generates palladium(II) hydroxy
complex 21, which intercepts the Suzuki−Miyaura cycle
(Scheme 1A, path A).
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b00894.
■
S
Careful optimization of the palladium(II)-catalyzed macro-
cyclization allowed for a scalable synthesis of mycocyclsin
(Scheme 1B). 3-Iodo-L-tyrosine 27 was smoothly converted to
28 in three steps with no column chromatography. At this
stage, ester hydrolysis or Boc deprotection afforded 29 or 30,
respectively, in quantitative yields. Next, peptide coupling of 29
and 30 with HBTU, followed by acid-promoted intramolecular
cyclization, provided DKP 3 in up to eight grams.8 Diiodide 3
was readily transformed to cyclophane 4 by employing the
optimized Pd(II)-catalyzed oxidative coupling conditions on
both half gram and gram scale in yields up to 63%. Finally,
benzyl ether removal to expose the free phenol afforded
mycocyclosin 2 in excellent yield.16
In conclusion, we have developed a scalable approach to the
strained macrocycle, mycocyclosin. Careful evaluation of the
reaction conditions for the palladium(II) catalyzed cross-
coupling reaction led us to observe that both water and air
are critical reaction additives to promote efficient reactivity.
Experimental support for the formation of a diboronate
intermediate suggests that oxidative homocoupling to the
strained biaryl architecture is plausible. We anticipate that this
scalable entry to mycocyclosin will facilitate expedient access to
potential M. tuberculosis antimicrobials.
Experimental details and spectroscopic data for all
intermediates, reactants, and products (PDF)
Accession Codes
CCDC 1837911 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_
request@ccdc.cam.ac.uk, or by contacting The Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2
1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: corinnas@umich.edu.
ORCID
Corinna S. Schindler: 0000-0003-4968-8013
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the David and Lucile Packard Foundation and the
Sloan Foundation for financial support. C.C.M. thanks the
National Science Foundation for a predoctoral fellowship
D
DOI: 10.1021/acs.orglett.8b00894
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Letter
(16) Okano, K.; Okuyama, K.-i.; Fukuyama, T.; Tokuyama, H. Synlett
2008, 2008, 1977.
(Grant No. 1256260). We thank Dr. Jeff W. Kampf for X-ray
analysis.
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DOI: 10.1021/acs.orglett.8b00894
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