Total Synthesis of Pactalactam, an Imidazolidinone-Type Pactamycin Analogue

Article abstract of DOI:10.1021/acs.orglett.9b00905

The first total synthesis of pactalactam was accomplished using substrate-controlled stereoselective aziridination and regioselective aziridine ring-opening to construct three continuous amino groups on an octasubstituted cyclopentane core. The cyclopentane framework was obtained by ring-closing metathesis and aldol coupling using a l-threonine-derived oxazoline compound. Cyclic urea formation, m-acetylphenyl group introduction by Chan-Lam coupling, and primary alcohol-selective acylation yielded the reported pactalactam structure. The presence of pactalactam in the fermentation broth of pactamycin-producing bacteria was also confirmed.

Full text of DOI:10.1021/acs.orglett.9b00905

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Total Synthesis of Pactalactam, an Imidazolidinone-Type
Pactamycin Analogue
Taejung Kim,†,‡ Shohei Matsushita,† So Matsudaira,† Tsuyoshi Doi,† Shinji Hirota,†
Young-Tae Park,‡ Masayuki Igarashi,§ Masaki Hatano,§ Noriko Ikeda,§ Jungyeob Ham,*,‡
Masaya Nakata,*,† and Yoko Saikawa*,†
†Department of Applied Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama
223-8522, Japan
‡Natural Products Research Institute, Korea Institute of Science and Technology (KIST), 679 Saimdang-ro, Gangneung 25451,
Republic of Korea
§Institute of Microbial Chemistry (BIKAKEN), 3-14-23 Kamiosaki, Shinagawa-ku, Tokyo 141-0021, Japan
S
* Supporting Information
ABSTRACT: The first total synthesis of pactalactam was
accomplished using substrate-controlled stereoselective azir-
idination and regioselective aziridine ring-opening to construct
three continuous amino groups on an octasubstituted
cyclopentane core. The cyclopentane framework was obtained
by ring-closing metathesis and aldol coupling using a L-
threonine-derived oxazoline compound. Cyclic urea formation, m-acetylphenyl group introduction by Chan−Lam coupling, and
primary alcohol-selective acylation yielded the reported pactalactam structure. The presence of pactalactam in the fermentation
broth of pactamycin-producing bacteria was also confirmed.
actamycin (1) is a representative bioactive amino-
Pactalactam (2) was reported in a review paper published in
1980 by Rinehart et al. as a minor component of the
metabolite of the same genus culture broth as that of 1, but no
data were available to confirm its structure (Figure 1).2b,e Its
reported structure formula is similar to that of 1, except for the
imidazolidinone moiety; it is also analogous to that of
pactamycate, an oxazolidinone derivative obtained by the
acidic treatment of 1 where the 7-hydroxy group attacked the
carbamoyl group.7 Although 2 and pactamycate were reported
to be inactive against bacteria, these shelved analogues deserve
reexamination regarding their structures and coexistence in the
culture broth. Herein, we report the synthesis of the reported
structure of 2 and reveal its existence in nature.
P
cyclopentitol isolated from the culture broth of
Streptomyces pactum var. pactum (Figure 1).1 Its attractive
Despite the presence of many reports8 regarding synthetic
studies toward 1, the first total synthesis by Hanessian’s group9
and a concise total synthesis by Johnson’s group10 are the only
two reported total syntheses of 1 to date. Notably, the
aforementioned pactamycin analogues have not been synthe-
sized yet. Thus, we envisioned devising a strategy to access
cyclopentane core 3, which might be applicable to access other
pactamycin analogues (Scheme 1). For the synthesis of 2, we
envisioned late-stage formation of cyclic urea, m-acetylphenyl
group introduction by Chan−Lam coupling,11 and primary
alcohol-selective acylation using Hanessian’s procedure9 to
yield 2 from core 3. Azidoamine 3 would be constructed by a
regioselective aziridine-opening reaction with an azide group
Figure 1. Chemical structures of pactamycin (1) and pactalactam (2).
bioactivity and intriguing chemical structure based on a
densely functionalized cyclopentane core drove the isolation
of several related bioactive aminocyclitols, such as 7-
deoxypactamycin2 and 8″-hydroxypactamycin,2b−e as well as
the recently isolated jogyamycin.3 Although the development
of 1 as a clinical drug has been curtailed by its broad and
potent cytotoxicity, these related analogues exhibit antimicro-
bial,4 antitumor,5 and antiprotozoal6 activities, among others.
However, their structure−activity relationships are not
completely understood. Thus, expanding the scope of
pactamycin analogues may lead to the discovery of more
acceptable safety profiles with well-characterized bioactivity.
Received: March 13, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b00905
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
center at C5 was conveniently isomerized to the desired
stereoisomer 9 in 68% yield through oxidation by Dess−
Martin periodinane (DMP) and subsequent NaBH4 reduction.
The secondary alcohol of 9 was protected with TBSCl, giving
silyl ether 10 in 99% yield. Next, the methyl ester was
converted into a terminal olefin in three steps. The subsequent
selective deprotection of the Tr group was achieved using a
large excess of ZnBr2 in a CH2Cl2−MeOH (1:1) solvent
system. Dess−Martin oxidation provided aldehyde 11; only
one chromatographic separation was needed after the above
five steps (from 10 to 11) at the scaled-up synthesis. After the
Grignard reaction with vinylmagnesium bromide, the inter-
Scheme 1. Synthetic Plan toward Pactalactam (2) and Core
3
1
mediate mixture of allylic alcohols (3:1 ratio by its H NMR
spectrum) was subjected to RCM using a second-generation
Grubbs catalyst to produce the separable cyclopentenols 12a
and 12b in 74% and 19% yields, respectively, over two steps.
The stereochemistries of the C-4 and C-5 positions having
oxygen functional groups were determined on the basis of
NOE experiments using 12a, 12b, and their C-5 diastereomers
from 8.14 Finally, oxidation of the allylic alcohol furnished
cyclopentenone 13.
We next focused on stereoselective aziridination to construct
the three continuous chiral centers with amino groups of core
3 via a Gabriel−Cromwell method15 starting from cyclo-
pentenone 13 (Scheme 3). The conventional iodination16 of
and the sequential construction of two continuous tetrasub-
stituted carbons by dihydroxylation and addition of a methyl
group. These could be conducted from the upper face of an
idealized intermediate of pentenone 4, whose lower face would
be shielded by the methyl group in the oxazoline ring.
Stereoselective aziridination would be controlled by the rigid
spirostructure and the 5R-alkoxy group (OP2), which would
avoid conflict with the incoming aziridinating reagent. Finally,
the preparation of 5 could be traced to L-threonine-derived
oxazoline methyl ester 6, utilizing ring-closing metathesis
(RCM) to construct the five-membered ring with a face-
selective coupling reaction using a chiral oxazoline enolate.
Stepwise construction of the potentially functionalized
cyclopentenone 13 began with oxazoline methyl ester 6
(Scheme 2), derived from L-threonine in three known steps.12
The nitrogen-containing stereogenic carbon center was formed
by face-selective coupling using the chiral oxazoline enolate
with a known aldehyde 713 derived from glycerol in two steps.
Our procedure yielded the separable adducts 8 (37%) and 9
(40%). Secondary alcohol 8 with the undesired stereogenic
Scheme 3. Stereoselective Aziridinations of Cyclopentenone
13 via a Gabriel−Cromwell Reaction
13 using iodine and pyridine in CCl4 proceeded smoothly to
provide α-iodoenone 14 in 96% yield. Attempts at aziridine
formation using p-methoxybenzylamine and Cs2CO3 in xylene
according to Maycock’s conditions17 induced decomposition
of the substrate upon heating (95 °C), whereas no reaction
occurred at room temperature. To our delight, aziridination
proceeded smoothly in DMF at room temperature to produce
the desired N-PMB aziridine 16, through 15, as a single isomer
in good yield (86%).18
Scheme 2. Synthesis of Cyclopentenone 13 for
Stereoselective Aziridination
Next, aziridine 16 was converted to the key intermediate 19
with three continuous stereogenic tetrasubstituted carbons and
then to octasubstituted cyclopentane core 21 (Scheme 4). The
Wittig reaction of 16 yielded exo-olefin 17, the stereochemistry
of which was confirmed by X-ray crystallographic analysis.
Substrate-dependent (the neighboring siloxy group and the
methyl group on oxazoline) dihydroxylation of 17 using OsO4
without chiral ligands provided an inseparable mixture of two
diols at the C-4 position. The diols were treated with 2,2-
dimethoxypropane to give acetonides. Removal of the TBS
group, followed by oxidation of the secondary alcohol,
produced separable ketone 18 in 72% yield along with its C-
4 diastereomer in 13% yield over four steps. The stereo-
selective addition of a methyl anion to 18 using MeLi gave 19
in low yield (<26%) with poor selectivity (ratio of 19/isomer =
2:1), whereas by using MeMgBr adduct 19 was obtained in
good yield with high selectivity (19/83%; isomer: 13%). N-(p-
Nitrobenzenesulfonyl) aziridine (Ns-aziridine) 20 as the active
B
DOI: 10.1021/acs.orglett.9b00905
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 4. Synthesis of Azidoamine 21 Containing
Octasubstituted Cyclopentane Core
Scheme 5. Completion of the Total Synthesis of the
Reported Structure of Pactalactam (2)
substrate for the nucleophilic ring-opening reaction was
produced by the deprotection of the PMB group of 19 with
CAN, followed by protection with NsCl at −40 °C in 73%
yield over two steps. The stereochemistry of the three
continuous tetrasubstituted carbons in 20 was confirmed by
X-ray crystallographic analysis. The regioselective aziridine
opening of 20 with NaN3 in the presence of stoichiometric
19
amounts of BiCl3 proceeded with the anticipated exclusive
regioselectivity to form the desired structure of octasubstituted
cyclopentane core 21, as confirmed by NOE experiments.
Without BiCl3, only deprotection of the Ns group was
observed.
separation. Deprotection of the acetonide was accomplished by
treatment with 80% aqueous AcOH to provide slightly water-
soluble tetraol 29. This left a highly reactive primary alcohol
for ketene-mediated selective acylation using cyanomethyl
ester 30 to provide pactalactam (2, total 31 steps (2.3% overall
yield) from 6).
To complete the natural product synthesis, core 21 was
converted to pactalactam (2) using the reaction sequence
displayed in Scheme 5. For the construction of the cyclic urea
unit, oxazoline opening reaction of 21 in the presence of a
NaBH3CN/AcOH mixture, followed by protection of the TIPS
group in the resulting secondary alcohol, was performed to
obtain benzylamine 22 in 77% overall yield. To access the
cyclic urea, the vicinal diamine skeleton was installed next.
Oxidation of the hindered benzylamine in 22 with diisopropyl
azodicarboxylate (DIAD)20 proceeded cleanly to yield the
putative imidazolidine intermediate as a mixture with the
excess DIAD, which was treated under the optimized acid
hydrolysis conditions to give diamine 23 as a precursor of the
cyclic urea. Treatment of 23 with triphosgene efficiently
furnished cyclic urea 24 containing the important main
skeleton of the natural product in quantitative yield.
Deprotection of the Ns group of the urea moiety in 24,
followed by the reduction of the azide group by zinc powder
and NH4Cl, afforded amine 25 as a substrate for the Chan−
Lam coupling. Next, introduction of the m-acetylphenyl group
to amine 25 was attempted using various m-acetylphenylboron
compounds under coupling conditions (see the Supporting
Information for details). The coupling of 25 with neopentyl
glycol borane 26 afforded an inseparable mixture of m-
acetylaniline 27 and unidentifiable products derived from the
fragmentation of the borane reagent. Treatment of this mixture
with TBAF gave pure alcohol 28 after chromatographic
Although no data of pactalactam exists for comparison, and
both acidic and basic conditions could cause the migration of
the imidazolidinone carbonyl group to the 4- or 7-OH group,
the two characteristic NH signals and their heteronuclear
multiple-bond correlations corroborate that the imidazolidi-
none is retained. With the desired compound 2 in hand, we
proceeded to confirm the existence of 2 in the culture broth of
pactamycin-producing actinomycetes, Streptomyces sp. MJ375-
46F6. In the liquid chromatography−mass spectrometry
(LCMS), the authentic synthetic 2 was detected at 2.90 min
(m/z 514.22), and a peak with same retention time as the
authentic sample was detected by the selected ionization
monitoring (SIM) of m/z 514.22,21 albeit a minor peak.7 Using
the specific peak corresponding to pactalactam as an index, we
finally isolated the natural pactalactam from 20 L of culture
broth. The analytical data of natural pactalactam was identical
with those of the synthetic sample.21 In conclusion, we have
achieved the first total synthesis and structural confirmation of
pactalactam as a minor metabolite of pactamycin-producing
actinomycetes. The success of the realization of a structural
formula in a 40-year-old report demonstrates the potential to
discover new bioactive pactamycin analogues.
C
DOI: 10.1021/acs.orglett.9b00905
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Herner, A. E. Biochemistry 1969, 8, 1327−1335. (c) Kappen, L. S.;
Goldberg, I. H. Biochemistry 1976, 15, 811−818. (d) Mankin, A. S. J.
Mol. Biol. 1997, 274, 8−15. (e) Brodersen, D. E.; Clemons, W. M., Jr.;
Carter, A. P.; Morgan-Warren, R. J.; Wimberly, B. T.; Ramakrishnan,
V. Cell 2000, 103, 1143−1154.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b00905.
■
S
(5) 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.
Experimental procedures and characterization data
(PDF)
(6) 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.
(7) Wiley, P. F.; Jahnke, H. K.; Mackellar, F.; Kelly, R. B.;
Argoudelis, A. D. J. Org. Chem. 1970, 35, 1420−1425.
Accession Codes
CCDC 1900776 and 1900778 contain 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.
(8) (a) Tsujimoto, T.; Nishikawa, T.; Urabe, D.; Isobe, M. Synlett
2005, 433−436. (b) Knapp, S.; Yu, Y. Org. Lett. 2007, 9, 1359−1362.
(c) Haussener, T. J.; Looper, R. E. Org. Lett. 2012, 14, 3632−3635.
(d) Yamaguchi, M.; Hayashi, M.; Hamada, Y.; Nemoto, T. Org. Lett.
2016, 18, 2347−2350. (e) Matsumoto, N.; Nakazaki, A.; Nishikawa,
T. Synlett 2017, 28, 2303−2306. (f) Goto, A.; Yoshimura, S.; Nakao,
Y.; Inai, M.; Asakawa, T.; Egi, M.; Hamashima, Y.; Kondo, M.; Kan,
T. Org. Lett. 2017, 19, 3358−3361. (g) Rodrigues, R.; Lazib, Y.;
Maury, J.; Neuville, L.; Leboeuf, D.; Dauban, P.; Darses, B. Org. Chem.
Front. 2018, 5, 948−953. (h) Su, J. Y.; Olson, D. E.; Ting, S. I.; Du
Bois, J. J. Org. Chem. 2018, 83, 7121−7134.
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: ham0606@kist.re.kr.
*E-mail: masayanakata@keio.jp.
*E-mail: saikawa@applc.keio.ac.jp.
(9) (a) Hanessian, S.; Vakiti, R. R.; Dorich, S.; Banerjee, S.;
ORCID
̂
Lecomte, F.; DelValle, J. R.; Zhang, J.; Deshenes-Simard, B. Angew.
Taejung Kim: 0000-0002-9449-4763
Young-Tae Park: 0000-0003-1787-1577
Masaki Hatano: 0000-0002-9602-6197
Jungyeob Ham: 0000-0003-3046-9267
Masaya Nakata: 0000-0001-8211-8878
Yoko Saikawa: 0000-0002-4571-1821
Notes
Chem., Int. Ed. 2011, 50, 3497−3500. (b) Hanessian, S.; Vakiti, R. R.;
̂
Dorich, S.; Banerjee, S.; Deshenes-Simard, B. J. Org. Chem. 2012, 77,
9458−9472.
(10) (a) Malinowski, J. T.; Sharpe, R. J.; Johnson, J. S. Science 2013,
340, 180−182. (b) Sharpe, R. J.; Malinowski, J. T.; Johnson, J. S. J.
Am. Chem. Soc. 2013, 135, 17990−17998.
(11) (a) Chan, D. M. T.; Monaco, K. L.; Wang, R.-P.; Winters, M. P.
Tetrahedron Lett. 1998, 39, 2933−2936. (b) Lam, P. Y. S.; Clark, C.
G.; Saubern, S.; Adams, J.; Winters, M. P.; Chan, D. M. T.; Combs, A.
Tetrahedron Lett. 1998, 39, 2941−2944. (c) Evans, D. A.; Katz, J. L.;
West, T. R. Tetrahedron Lett. 1998, 39, 2937−2940.
(12) Seebach, D.; Aebi, J. D.; Gander-Coquoz, M.; Naef, R. Helv.
Chim. Acta 1987, 70, 1194−1216.
(13) Kleinbeck, F.; Fettes, G. J.; Fader, L. D.; Carreira, E. M. Chem. -
Eur. J. 2012, 18, 3598−3610.
(14) For the synthesis and the structure determination of C-5
diastereomers (epi-12a and epi-12b) and epi-13, see the Supporting
Information.
(15) (a) Nagel, D. L.; Woller, P. B.; Cromwell, N. H. J. Org. Chem.
1971, 36, 3911−3917. (b) Tarburton, P.; Woller, P. B.; Badger, R. C.;
Doomes, E.; Cromwell, N. H. J. Heterocycl. Chem. 1977, 14, 459−464.
(c) Filigheddu, S. N.; Taddei, M. Tetrahedron Lett. 1998, 39, 3857−
3860.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank Dr. Ryuuichi Sawa (Institute of Microbial
Chemistry) for analyzing the culture medium extract by LC/
MS/MS, Dr. Jae Kyun Lee (Korea Institute of Science and
Technology) for X-ray structure determination, and Mr.
Daisuke Tanaka (Keio University) for early contributions to
this work. This work was partially supported by the Korea
Institute of Science and Technology (KIST) institutional
program (Project No. 2Z05610) and by the Nano Con-
vergence Industrial Strategic Technology Development Pro-
gram (No. 20000105) funded by the Ministry of Trade,
Industry & Energy (MOTIE, Korea).
(16) Johnson, C. R.; Adams, J. P.; Braun, M. P.; Senanayake, C. B.
́
W.; Wovkulich, P. M.; Uskokovlc, M. R. Tetrahedron Lett. 1992, 33,
917−918.
REFERENCES
■
(17) Barros, M. T.; Maycock, C. D.; Ventura, M. R. Tetrahedron Lett.
2002, 43, 4329−4331.
(1) Argoudelis, A. D.; Jahnke, H. K.; Fox, J. A. Antimicrob. Agents
Chemother. 1962, 1961, 191−197.
(18) Since the C5-epimer of 13 derived from 8 turned out to be
unsuitable for further conversion, 8 was epimerized to 9 in two steps;
for the detailed experimental procedure and the results, see the
Supporting Information.
(2) (a) Hara, T.; Niida, T.; Sato, K.; Kondo, S.; Noguchi, T.;
Kohmoto, K. J. Antibiot. 1964, 17, 266. (b) Rinehart, K. L., Jr.; Weller,
D. D.; Pearce, C. J. J. Nat. Prod. 1980, 43, 1−20. (c) Dobashi, K.;
Isshiki, K.; Sawa, T.; Obata, T.; Hamada, M.; Naganawa, H.; Takita,
T.; Takeuchi, T.; Umezawa, H.; Bei, H.; Zhu, B.; Tong, C.; Xu, W. J.
Antibiot. 1986, 39, 1779−1783. (d) Hurley, T. R.; Smitka, T. A.;
Wilton, J. H.; Bunge, R. H.; Hokanson, G. C.; French, J. C. J. Antibiot.
1986, 39, 1086−1091. (e) Elshahawi, S. I.; Shaaban, K. A.; Kharel, M.
K.; Thorson, J. S. Chem. Soc. Rev. 2015, 44, 7591−7697.
(19) Venkat Narsaiah, A.; Reddy, B. V. S.; Premalatha, K.; Reddy, S.
S.; Yadav, J. S. Catal. Lett. 2009, 131, 480−484.
̆
(20) Kroutil, J.; Trnka, T.; Cerny, M. Synthesis 2004, 446−450.
(21) For a comparison of the LCMS chromatogram and 1H and 13
C
NMR spectra of natural and synthetic 2, see the Supporting
Information.
(3) Iwatsuki, M.; Nishihara-Tsukashima, A.; Ishiyama, A.;
Namatame, M.; Watanabe, Y.; Handasah, S.; Pranamuda, H.;
Marwoto, B.; Matsumoto, A.; Takahashi, Y.; Otoguro, K.; Omura,
S. J. Antibiot. 2012, 65, 169−171.
(4) (a) Bhuyan, B. K.; Dietz, A.; Smith, C. G. Antimicrob. Agents
Chemother. 1962, 1961, 184−190. (b) Cohen, L. B.; Goldberg, I. H.;
D
DOI: 10.1021/acs.orglett.9b00905
Org. Lett. XXXX, XXX, XXX−XXX