Asymmetric Synthesis and Biological Activities of Pactamycin-Inspired Aminocyclopentitols

Article abstract of DOI:10.1021/acs.orglett.7b03681

Pactamycin is a structurally unique aminocyclitol antibiotic with broad-spectrum cell growth inhibitory activity. To explore the bountiful activity of the aminocyclitol core of pactamycin, an efficient, modular, and asymmetric synthesis of aminocyclopentitols resembling the pactamycin pharmacophore has been developed employing a SmI₂-mediated imino-pinacol coupling strategy. Two of the compounds exhibited antitumor activity against A375 melanoma cells.

Full text of DOI:10.1021/acs.orglett.7b03681

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Asymmetric Synthesis and Biological Activities of Pactamycin-
Inspired Aminocyclopentitols
Corey J. Brumsted,† Evan L. Carpenter,‡ Arup K. Indra,‡ and Taifo Mahmud*,†,‡
‡
†Department of Chemistry and Department of Pharmaceutical Sciences, Oregon State University, Corvallis, Oregon 97333, United
States
S
* Supporting Information
ABSTRACT: Pactamycin is a structurally unique aminocyclitol antibiotic
with broad-spectrum cell growth inhibitory activity. To explore the
bountiful activity of the aminocyclitol core of pactamycin, an efficient,
modular, and asymmetric synthesis of aminocyclopentitols resembling the
pactamycin pharmacophore has been developed employing a SmI2-
mediated imino-pinacol coupling strategy. Two of the compounds exhibited antitumor activity against A375 melanoma cells.
Scheme 1. Retrosynthesis of TM-118
atural products continue to play a vital role in drug
Ndiscovery. Approximately two-thirds of small molecule
pharmaceuticals currently on the market are natural products or
derived from natural product structures.1 Among the plethora of
bioactive natural products is pactamycin (Figure 1), a potent
antitumor antibiotic produced by the soil bacterium Streptomyces
pactum.2 Pactamycin is a member of the aminocyclopentitol
family of microbial secondary metabolites whose structures
commonly derive from carbohydrates.3 Aminocyclopentitols
cover a broad swathe of biological activity, some of which include
glycosidase inhibitors and antitumor, antimicrobial and antiviral
agents (Figure 1).4 Unfortunately, the biological activity
displayed by pactamycin spans across all three phylogenetic
domains,2,5−7 where its indiscriminate cytotoxicity toward
mammalian cells has suppressed its development toward
therapeutic application. Nevertheless, the pactamycin pharma-
cophore is believed to be a wellspring of promising biological
activity that is waiting to be harnessed. For example, we have
demonstrated, through biosynthetic manipulations, production
of new pactamycin analogs that are more active against malarial
parasites, Plasmodium falciparum, than toward bacteria and
mammalian cells.8−11 We have subsequently elucidated the
tailoring steps in pactamycin biosynthesis11,12 and obtained
intermediate compounds that also showed potent biological
activities. Continuing our efforts to draw further on the bountiful
activity of the aminocyclitol core of pactamycin, we have taken a
synthetic approach to access the aminocyclopentitol ring, which
could open up a diverse library of biologically active compounds.
The intricate aminocyclopentitol structure of pactamycin,
harboring 6-contiguous stereocenters, has garnered considerable
attention from the synthesis community13−19 over the years,
which only more recently culminated in the landmark total
synthesis by Hanessian in 2011,20 followed by the elegant and
efficient total synthesis of Johnson in 2013.21
The structurally complex aminocyclitol core is expected to be
the source of the bioactivity of pactamycin and its congeners. As
such, we set out to synthesize the aminocyclopentitol core
lacking the aromatic rings, methyl groups, and the N,N-dimethyl
Figure 1. Chemical structures of bioactive natural products and
clinically used drugs that contain aminocyclopentitol units (red).
Received: November 28, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b03681
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 2. Synthesis of Aminocyclopentitols
Figure 3. Growth inhibitory assay of TM-117−TM-122 against
melanoma cells in vitro. Dose response curves indicate A375 melanoma
cells are sensitive to TM-121, TM-122, and the previously characterized
analog of pactamycin (TM-026). All other synthetic analogs exhibited
no inhibitory activity at the tested doses. Data represent mean SD with
N = 2.
unique aminocyclopentitol, that itself or a direct precursor could
be derivatized to suit further SAR studies.
Our retrosynthetic analysis to obtain the desired amino-
cyclopentitol TM-118 (1) began with deprotection of 2
(Scheme 1). An orthogonally protected diamine 2 would allow
further functionalization/derivatization at nearly any position,
imparting a high degree of synthetic flexibility to support future
SAR studies. Diamine 2 could be constructed in a key imino-
pinacol cyclization step from a 1,5-keto-aldoxime ether 3.24,25
The keto-oxime ether would come from a suitably protected
carbohydrate derived from D-(+)-glucosamine. Aside from the
cost benefit, glucosamine offers multiple advantages, e.g., it
contains most of the desired functionality and 3 out of 5 of the
necessary stereocenters in the product, and the stereochemistry
at C-2 and C-4 could be critical for obtaining the desired
diastereoselection in the key imino-pinacol coupling step.26−29
The forward synthesis (Scheme 2) commenced with Cbz-
protection of the glucosamine nitrogen under aqueous
conditions, followed by benzylidene acetalization of the 4,6-
hydroxy groups, then bis-silylation of the remaining anomeric
and the C-3 hydroxyls with 2 equiv of TBSCl/imidazole
providing the bis-TBS benzylidene acetal 4 as a single
diastereomer in 54% yield over three steps. After considerable
experimentation and screening a number of known methods, we
did not find suitable conditions to affect the reductive
benzylidene ring opening to expose the free C-6 hydroxyl.30,31
Instead, we found Et3SiH/TFA to be a high yielding reagent
combination for this substrate that selectively exposed the C4-
OH (5, Scheme 2). Although we later developed new conditions
that exclusively provided the exposed C6-hydroxyl intermediate
10 (Scheme 3), our first attempts at synthesizing the amino-
cyclopentitol ring proceeded through the 4-OH intermediate (5,
Scheme 2).
Scheme 3. Synthesis of TM-117−TM-120
Initial efforts to access a protected aminocyclopentitol 2 by
reductive benzylidene ring opening of acetal 4 with Et3SiH/TFA
gave the 4-OH glucopyranose intermediate 5 in good yield
(Scheme 2). However, the C4 hydroxyl group on the pyranose
ring proved particularly unreactive as a nucleophile. We found
benzylation ineffective under a variety of conditions, and even O-
acylation with benzoyl chloride and stoichiometric quantities of
DMAP was challenging with this substrate.31 However,
acetylation gave the 4-OAc compound in high yield. While this
protecting group could be problematic during the imino-pinacol
cyclization, due to potential elimination of the alpha acetoxy
group, we decided to carry the 4-OAc substrate forward to the
imino-pinacol coupling stage for several reasons. First, in
Figure 2. Key NOE correlations of the imino-pinacol products TM-119
and TM-120.
urea, all of which have been shown to affect the cytotoxicity of
pactamycin or its analogs to some extent.7,9,11,22,23 The
overarching goal was to provide modular access to a structurally
B
DOI: 10.1021/acs.orglett.7b03681
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Organic Letters
Letter
addition to the mild radical promoter SmI2, there are other
reagents capable of generating ketyl radicals and effective at
promoting pinacol coupling reactions that could be attemp-
ted.24,32−37 Second, we reasoned if deoxygenation were to occur,
the linear deoxy-byproduct might undergo cyclization and
produce a structurally unique monodeoxy-aminocyclitol product
that would be an interesting substrate for later SAR studies.
A regioselective desilylation of 6 exposed the anomeric
hydroxyl group in high yield providing both alpha and beta
diastereomers, which could be allowed to equilibrate to the alpha
diastereomer, then subjected to dehydrative ring opening to form
the 5-hydroxyaldoxime 7 in moderate, but useful yield over two
steps. Oxidation to the 5-keto-1-aldoxime ether would provide
the imino-pinacol precursor. This was achieved using a modified
Swern oxidation [(COCl)2, DMSO, THF, then DIPEA, −78 to
−20 °C] to give the desired 5-ketoaldoxime 8.38 As mentioned
above, the instability of 8 could limit the reagent system that
could be employed for the imino-pinacol cyclization to those
capable of forming the requisite ketyl radical at low temperatures
such as SmI2 or Et3B/O2. Gratifyingly, a one-pot Swern
oxidation/SmI2-mediated reductive cyclization produced the
desired aminocyclopentitol TM-122, albeit in low yield (two
steps, 14%), and the linear des-acetoxy ketone 9 (35% yield)
(Scheme 2). We observed the N-cyclized compound was formed
when the 5-ketoaldoxime 8 from the Swern oxidation step was
warmed to room temperature. While the pyrrolidine byproduct
could be suppressed by keeping the Swern oxidation below −15
°C, after the addition of DIPEA, this resulted in a modest
increase in yield to 24%, formation of the linear des-acetoxy 9
product could not. However, 9 was isolated and resubjected to
the reductive cyclization conditions, which provided the deoxy-
aminocyclopentitol TM-121. Interestingly, we also found that
the des-acetoxy-aminocyclopentitol TM-121 could be obtained
directly from the Swern oxidation/SmI2 reductive coupling
sequence simply by switching the proton donor from t-BuOH to
MeOH.39
The liberated C-6 alcohol 10 was eventually obtained through
a regioselective reductive benzylidene ring opening (Scheme 3).
By switching TFA-Et3SiH in our first route (Scheme 2) to a
Lewis acid system, Bu2BOTf-Et3SiH, we were able to reverse the
benzylidene ring opening selectivity and obtained the desired,
free C-6 alcohol 10 in high yield (87%). To our knowledge, this
reagent combination is new for this type of transformation.30,31
Benzylation of the C-6 hydroxyl provided benzylated
intermediate 11 in modest yield (55%), but suffered from
competitive N-benzylation of the 6-O-benzyl product with the
starting material. N-Benzylation could be suppressed completely
by keeping the reaction below −20 °C and quenching at 50%
conversion. Selective removal of the anomeric-TBS group
afforded the corresponding free lactol. Literature methods for
this transformation use AcOH-TBAF to buffer the basicity of the
fluoride anion, but we found the reaction sluggish, requiring over
48 h to reach completion.40 By omitting AcOH, the reaction was
complete within 10 min, selective, and high yielding (90%).
Dehydrative ring opening of the lactol, provided both E/Z-
isomers of the 5-hydroxy-aldoxime ether 12 in 64% yield. After
screening a number of oxidation conditions to obtain the 1,5-
keto-aldoxime as the imino-pinacol precursor, we settled on
modified Swern conditions (COCl2, DMSO, DIPEA, −78 °C)
but observed that the product ketone 13 was unstable above 0
°C.38 As the reductive keto-aldoxime cyclization was expected to
occur at low temperatures, we therefore attempted to telescope
the Swern and imino-pinacol coupling steps to overcome the
instability issue of the intermediate ketone.26 To our delight, the
Swern/imino-pinacol coupling successfully produced the
orthogonally protected aminocyclopentitols TM-119 and TM-
120 (Figure 2) as a mixture of two out of four possible
diastereomers (75% yield). We have successfully repeated this
two-step conversion on gram scale. Notably, Chiara and co-
workers attempted similar imino-pinacol couplings with
protected 2-amino-carbamate 1-aldoxime ethers but reportedly
suffered from N-cyclization and a mixture of many products.41 In
our hands we found the N-cyclized compound 14 was the major
decomposition product from the ketone starting material if
allowed to warm above 0 °C. Importantly, this finding grants
convenient access to differentially protected cyclopentyl
diamines utilizing an imino-pinacol coupling strategy. While
formation of the desired stereoisomer TM-120 can be
rationalized by the stereoelectronic effects noted in Figure S1,
the formation of diastereomer TM-119 is less straightforward.
Interestingly, a similar erosion of stereoinduction was reported in
an imino-pinacol coupling with an O-acetate vicinal to the radical
accepting aldoxime.42 Their reported result with the O-Ac was an
anomaly in a series of otherwise completely diastereoselective
imino-pinacol couplings. Carbonyl oxygens are known to be
capable of directing SmI2-mediated reactions.43 We hypothesize
that coordination of an organosamarium with the NH-Cbz group
is likely responsible for overriding the otherwise inherent
stereoelectronic bias imposed by allylic strain in conformation
C of Figure S1 and producing TM-119.
At this stage in the synthesis, the protected aminocyclopentitol
TM-120 was used as a branching point for derivatization/SAR
studies, but for our immediate purposes we desired the free
aminocyclopentitol TM-118. However, a number of debenzyla-
tion conditions failed to deliver any desired product even with
extended reaction times and higher temperatures (Table S1).
Interestingly, the one electron reductant, lithium 4,4′-ditert-
butylbiphenylide (LiDBB), furnished the putative desired
intermediate TM-117 but left the highly water-soluble amino-
cyclitol as an intractable mixture of mostly inorganic impurities
after aqueous workup.31 A number of nonaqueous workup/
purification conditions also failed to provide TM-117. It is
noteworthy, however, that LiDBB has not been previously
demonstrated to cleave the N−O bond in hydroxylamines.
Finally, using a 1:1 mixture of Pd(OH)2/C and Pd/C, which was
reported to possess superior hydrogenation reactivity than either
catalyst alone,44 we successfully debenzylated the amino-
cyclopentitol at 0 °C (H2, EtOH, conc. HCl, 5.2 equiv) in less
than 3 h to provide the desired TM-117 as the sole product (99%
yield). Encouraged by this result, we hoped to debenzylate and
remove the TBS group in one pot by switching from EtOH to
MeOH as the hydrogenation solvent, but this was unsuccessful.45
However, we observed that after full conversion to TM-117,
followed by a filtration, solvent exchange to MeOH, and
additional HCl (11.7 M, 36 equiv), full desilylation had occurred
providing the desired final product TM-118 in nearly
quantitative yield (99%).
Preliminary biological testing of TM-117−TM-122 against
A375 human melanoma cells revealed that the deoxy-amino-
cyclopentitol TM-121 and the acetate TM-122 in a dose-
dependent manner inhibited growth of cancer cells and exhibited
antitumor activity, on par with the activity observed with the
pactamycin derivative TM-026 (Figure 3). The IC50 values (95%
CI) for TM-026, TM-121, and TM-122 are 1.405 (0.9474 to
2.084) μM, 1.619 (1.393 to 1.881) μM, and 4.861 (3.300 to
7.161) μM, respectively. The lack of activity of TM-118 suggests
C
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(19) Goto, A.; Yoshimura, S.; Nakao, Y.; Inai, M.; Asakawa, T.; Egi, M.;
Hamashima, Y.; Kondo, M.; Kan, T. Org. Lett. 2017, 19, 3358−3361.
(20) Hanessian, S.; Vakiti, R. R.; Dorich, S.; Banerjee, S.; Lecomte, F.;
DelValle, J. R.; Zhang, J.; Deschenes-Simard, B. Angew. Chem., Int. Ed.
2011, 50, 3497−3500.
that additional side chains to the core aminocyclitol are necessary
for the compounds to show appreciable cell inhibitory activity.
In conclusion, we have developed efficient asymmetric routes
to the aminocyclopentitol TM-118 and its analogs. Further, we
have demonstrated that some of our pactamycin-inspired
synthetic compounds have appreciable biological activity and
display promising antitumor activity.
(21) Malinowski, J. T.; Sharpe, R. J.; Johnson, J. S. Science 2013, 340,
180−182.
(22) Sharpe, R. J.; Malinowski, J. T.; Sorana, F.; Luft, J. C.; Bowerman,
C. J.; DeSimone, J. M.; Johnson, J. S. Bioorg. Med. Chem. 2015, 23,
1849−1857.
(23) Hanessian, S.; Vakiti, R. R.; Chattopadhyay, A. K.; Dorich, S.;
Lavallee, C. Bioorg. Med. Chem. 2013, 21, 1775−1786.
(24) Shono, T.; Kise, N.; Fujimoto, T. Tetrahedron Lett. 1991, 32,
525−528.
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.7b03681.
(25) Burchak, O. N.; Py, S. Tetrahedron 2009, 65, 7333−7356.
(26) Chiara, J. L.; Marco-Contelles, J.; Khiar, N.; Gallego, P.; Destabel,
C.; Bernabe, M. J. Org. Chem. 1995, 60, 6010−6011.
Experimental procedures, [α]D, IR, NMR, and MS data for
compounds 4−7, 9−12, and TM-117−TM-122, and their
1D and 2D NMR spectra (PDF)
́
(27) Marco-Contelles, J.; Gallego, P.; Rodriguez-Fernandez, M.; Khiar,
N.; Destabel, C.; Bernabe, M.; Martinez-Grau, A.; Chiara, J. L. J. Org.
Chem. 1997, 62, 7397−7412.
AUTHOR INFORMATION
■
(28) Storch de Gracia, I.; Dietrich, H.; Bobo, S.; Chiara, J. L. J. Org.
Chem. 1998, 63, 5883−5889.
Corresponding Author
*E-mail: taifo.mahmud@oregonstate.edu.
ORCID
(29) Chiara, J. L.; Garcia, A. Synlett 2005, 17, 2607−2610.
(30) Ohlin, M.; Johnsson, R.; Ellervik, U. Carbohydr. Res. 2011, 346,
1358−1370.
(31) Wuts, P. G. M. Greene’s Protective Groups in Organic Synthesis, 5th
ed.; John Wiley & Sons, 2014.
Taifo Mahmud: 0000-0001-9639-526X
Notes
(32) Corey, E. J.; Pyne, S. G. Tetrahedron Lett. 1983, 24, 2821−2824.
(33) Naito, T.; Tajiri, K.; Harimoto, T.; Ninomiya, I.; Kiguchi, T.
Tetrahedron Lett. 1994, 35, 2205−2206.
(34) Hutton, T. K.; Muir, K. W.; Procter, D. J. Org. Lett. 2003, 5, 4811−
4814.
(35) Suzuki, K.; Tamiya, M. In Comprehensive Organic Synthesis, 2nd
ed.; Knochel, P., Molander, G. A., Eds.; Elsevier: 2014; pp 580−620.
(36) Gansauer, A.; Bluhm, H. Chem. Rev. 2000, 100, 2771−2788.
(37) Shi, L.; Fan, C.-A.; Tu, Y.-Q.; Wang, M.; Zhang, F.-M. Tetrahedron
2004, 60, 2851−2855.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the OSU College of Pharmacy
General Research Funds. The authors thank Gitali Ganguli-Indra
in the OSU College of Pharmacy for helping with interpretation
and analysis of the in vitro assay in human melanoma cells.
(38) Mancuso, A. J.; Swern, D. Synthesis 1981, 1981, 165−185.
(39) Procter, D. J.; Flowers, R. A.; Skrydstrup, T. Organic Synthesis
Using Samarium Diiodide: A Practical Guide; The Royal Society of
Chemistry: Cambridge, U.K., 2009.
REFERENCES
■
(1) Newman, D. J.; Cragg, G. M. J. Nat. Prod. 2016, 79, 629−661.
(2) Bhuyan, B. K. Appl. Microbiol. 1962, 10, 302−304.
(3) Mahmud, T.; Flatt, P. M.; Wu, X. J. Nat. Prod. 2007, 70, 1384−
1391.
(40) Lu, Y.; Krische, M. J. Org. Lett. 2009, 11, 3108−3111.
(41) Chiara, J. L.; Garcia, A.; Cristobal-Lumbroso, G. J. Org. Chem.
2005, 70, 4142−4151.
(4) Flatt, P. M.; Mahmud, T. Nat. Prod. Rep. 2007, 24, 358−392.
(5) White, F. R. Cancer Chemother. Rep. 1962, 24, 75−78.
(6) Taber, R.; Rekosh, D.; Baltimore, D. J. Virol. 1971, 8, 395−401.
(7) Otoguro, K.; Iwatsuki, M.; Ishiyama, A.; Namatame, M.; Nishihara-
Tukashima, A.; Shibahara, S.; Kondo, S.; Yamada, H.; Omura, S. J.
Antibiot. 2010, 63, 381−384.
(42) Bobo, S.; Storch de Gracia, I.; Chiara, J. L. Synlett 1999, 1999,
1551−1554.
(43) Matsuda, F.; Kawatsura, M.; Hosaka, K.; Shirahama, H. Chem. -
Eur. J. 1999, 5, 3252−3259.
(44) Li, Y.; Manickam, G.; Ghoshal, A.; Subramaniam, P. Synth.
Commun. 2006, 36, 925−928.
(8) Lu, W.; Roongsawang, N.; Mahmud, T. Chem. Biol. 2011, 18, 425−
431.
(45) Ikawa, T.; Hattori, K.; Sajiki, H.; Hirota, K. Tetrahedron 2004, 60,
6901−6911.
(9) Almabruk, K. H.; Lu, W.; Li, Y.; Abugreen, M.; Kelly, J. X.;
Mahmud, T. Org. Lett. 2013, 15, 1678−1681.
(10) Guha, G.; Lu, W.; Li, S.; Liang, X.; Kulesz-Martin, M. F.; Mahmud,
T.; Indra, A. K.; Ganguli-Indra, G. PLoS One 2015, 10, e0125322.
(11) Abugrain, M. E.; Lu, W.; Li, Y.; Serrill, J. D.; Brumsted, C. J.;
Osborn, A. R.; Alani, A.; Ishmael, J. E.; Kelly, J. X.; Mahmud, T.
ChemBioChem 2016, 17, 1585−1588.
(12) Abugrain, M. E.; Brumsted, C. J.; Osborn, A. R.; Philmus, B.;
Mahmud, T. ACS Chem. Biol. 2017, 12, 362−366.
(13) Tsujimoto, T.; Nishikawa, T.; Urabe, D.; Isobe, M. Synlett 2005,
2005, 433−436.
(14) Knapp, S.; Yu, Y. Org. Lett. 2007, 9, 1359−1362.
(15) Haussener, T. J.; Looper, R. E. Org. Lett. 2012, 14, 3632−3635.
(16) Yamaguchi, M.; Hayashi, M.; Hamada, Y.; Nemoto, T. Org. Lett.
2016, 18, 2347−2350.
(17) Gerstner, N. C.; Adams, C. S.; Grigg, R. D.; Tretbar, M.; Rigoli, J.
W.; Schomaker, J. M. Org. Lett. 2016, 18, 284−287.
(18) Matsumoto, N.; Tsujimoto, T.; Nakazaki, A.; Isobe, M.;
Nishikawa, T. RSC Adv. 2012, 2, 9448−9462.
D
DOI: 10.1021/acs.orglett.7b03681
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