Total synthesis of pactamycin

Article abstract of DOI:10.1002/anie.201008079

Lest we forget: 50 years after pactamycin was first isolated from a fermentation broth of Streptomyces pactum var pactum, this highly functionalized aminocyclopentitol natural product has finally succumbed to total synthesis. The modular and stereocontrolled introduction of functional groups should lead to the synthesis of less toxic congeners that maintain the antibacterial and cytotoxic activities. Copyright

Full text of DOI:10.1002/anie.201008079

DOI: 10.1002/anie.201008079
Natural Product Synthesis
Total Synthesis of Pactamycin**
Stephen Hanessian,* Ramkrishna Reddy Vakiti, Stꢀphane Dorich, Shyamapada Banerjee,
Fabien Lecomte, Juan R. DelValle, Jianbin Zhang, and Benoꢁt DeschÞnes-Simard
Among the plethora of microbial secondary metabolites
produced by the soil bacterium of the Streptomyces family is
pactamycin, a structurally unique member of aminocyclopen-
titol-containing natural products (Scheme 1).
de-6-MSA-pactamycin and de-6-MSA-pactamycate, the nat-
ural congeners lacking the 6-methyl salicylic acid moiety.[5b–d]
A proposed structure of pactamycin was reported in 1970
by the Upjohn scientists as a result of seminal studies
involving chemical degradation.[6] It was subsequently cor-
rected in 1972 as a result of X-ray crystallographic studies, as
shown in Scheme 1.[7] To the best of our knowledge, pacta-
mycin is the most densely functionalized naturally occuring
aminocyclopentitol.[8] In spite of its unique architecture and
rich history in the realm of RNA structure and function,[3–5]
efforts toward the synthesis of pactamycin and its congeners
have been sparse. Knapp and Yu,[9] as well as Isobe and
coworkers[10] recently reported conceptually different
approaches toward the construction of the aminocyclopen-
tane core motif. Herein, we communicate the first total
synthesis of pactamycin and its naturally occurring congener,
pactamycate (Scheme 1).
Scheme 1. Structures of pactamycin and pactamycate.
Pactamycin was isolated in 1961 from a fermentation
broth of Streptomyces pactum var pactum by scientists at the
former Upjohn Company.[1] It exhibits activity against Gram-
positive and Gram-negative bacteria, in addition to potent
in vitro and in vivo cytotoxic effects.[2] Its further develop-
ment as a chemotherapeutic agent was curtailed owing to its
toxicity. The potent protein synthesis inhibitory activity of
pactamycin is attributed to the stage of translocation from the
A and P sites to the P and E sites during formation of certain
m-RNA-t-RNA complexes in prokaryotes as well as in
eukaryotes.[3] Pioneering X-ray crystallographic studies[4]
involving binding to the 30S site of Thermus thermophilus
show unique interactions, whereby pactamycin adopts a
spatial orientation so as to mimic an RNA nucleotide. The
two aromatic moieties stack against each other like consec-
utive RNA bases, while the core cyclopentane motif mimics
the RNA sugar-phosphate backbone, which results in an
intricate network of hydrogen-bonded interactions within the
30S site of the ribosome. Recent elegant studies on the
biosynthesis of pactamycin by Mahmud and coworkers[5a]
revealed a gene cluster which also produced pactamycate,
In considering a synthetic strategy, we were cognizant that
the densely functionalized cyclopentane core harboring three
contiguous tertiary centers would require a judicious choice
of well-orchestrated bond-forming sequences. Furthermore,
we wanted to adopt a modular approach for the introduction
of substituents and appendages to allow for diversification to
eventually prepare bioactive analogues while eliminating
toxicity.
Analysis of the structure of pactamycin led to the choice
of l-threonine as a partially hidden chiron, representing C1,
C2, C7, and C8, and ensuring the configuration of the
secondary hydroxy group in the hydroxyethyl appendage, as
well as the position of the amine group at C1 (Scheme 2). The
cyclopentenone core (C) would arise from a sequence of well-
documented reactions culminating with an intramolecular
aldol condensation. Systematic manipulation of the cyclo-
pentenone (C) would then eventually lead to pactamycin.
Straightforward as this plan may have been, its execution was
met with several unexpected roadblocks particularly involv-
ing the elaboration of the N,N-dimethylurea group at C1, and
the proximity of functional groups (see below).
A three-step sequence starting with l-threonine (1) led to
the oxazoline derivative 2 (Scheme 3).[11,12] Formation of the
enolate with LiHMDS, and condensation with O-TBDPS-2-
hydroxymethyl acrolein, and subsequent protection with
TESOTf afforded 3 as a single isomer. Reduction of the
benzyl ester to the aldehyde, treatment with MeMgBr, and
oxidation afforded the methyl ketone 4. Ozonolytic cleavage
of the exocyclic methylene group, followed by a highly
stereoselective Mukaiyama-type intramolecular aldol con-
densation afforded 5 as a crystalline intermediate.[13] Upon
treatment with trichloroacetyl chloride in pyridine, b-elimi-
nation took place to give the cyclopentenone 6.
[*] Prof. Dr. S. Hanessian, Dr. R. R. Vakiti, S. Dorich, Dr. S. Banerjee,
Dr. F. Lecomte, Dr. J. R. DelValle, J. Zhang, B. DeschÞnes-Simard
Department of Chemistry
Universitꢀ de Montrꢀal
Station Centre Ville, C.P. 6128, Montreal, Qc, H3C 3J7 (Canada)
Fax: (+1)514-434-5728
E-mail: stephen.hanessian@umontreal.ca
[**] We are grateful for financial support from NSERC and FQRNT
(Quꢀbec). We also thank warmly Prof. Taifo Mahmud for having
provided us with a precious sample of natural pactamycin.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201008079.
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Communications
From this point on, it was imperative to introduce the
epoxide and hydroxy group in that order, while securing the
desired configurations that would allow introduction of an
azide group with inversion at C2, and the aniline moiety
regioselectively at C3. In the event, base-induced epoxidation
of 6 afforded epoxide 7, which was stereoselectively reduced
under Luche conditions to give 8 (Scheme 4). The formation
Scheme 2. Strategic bond disconnections and key transformations
shown in their order of execution. A=l-threonine, B=oxygen-pro-
tected 2-hydroxymethyl acrolein, C=core cyclopentenone intermediate.
Scheme 4. Synthesis of the epoxide 14. Reagents and conditions:
a) H2O2 (30% w/w), 20% NaOH, MeOH/CH2Cl2 (7:1), 08C, 75%,
(88% bsmr); b) NaBH4, CeCl3·7H2O, MeOH/CH2Cl2 (1:1), 08C, 92%;
c) Tf2O, py, À788C to 08C, then Bu4NN3, toluene, RT, 87%; d) TFA/
MeCN/H2O (1:8:1), 08C to RT, 93%; e) DMP, CH2Cl2, 08C, 96%;
f) MeMgBr, THF, À788C; g) TBAF, THF, 08C, 86% (over 2 steps);
h) Zn(OTf)2, AcOH, 808C; i) K2CO3, MeOH, RT; j) TBDPSCl, TEA,
DMAP, RT, 85% (over 3 steps); k) Tf2O, py, CH2Cl2, À788C to 08C,
96%. DMAP=4-dimethylaminopyridine, DMP=Dess–Martin periodi-
nane, R=TBDPS, TBAF=tetra-n-butylammonium fluoride, TFA=tri-
fluoroacetic acid.
of the a-oriented epoxide and secondary alcohol as in 8 was
imperative, because the SN2 azide displacement of the
corresponding triflate in the diastereomeric b-epoxide in a
related series was unsuccessful. Although the azide group
could be easily introduced through the triflate ester of 8 to
give 9, it became necessary to “invert” the configuration of
the epoxide to contemplate the stereo- and regioselective
introduction of the aniline moiety by opening at C3. This
operation was postponed in favor of the obligatory carbon-
methyl branching at C5. Thus, selective cleavage of the TES
ether and oxidation to the ketone gave 10, which was treated
with MeMgBr to give 11 as a single diastereomer. An X-ray
structure of the phenyl oxazoline analogue of 10 confirmed its
structure and absolute configuration.[12,13] The “inversion” of
Scheme 3. Synthesis of the cyclopentenone intermediate 6. Reagents
and conditions: a) BnOH, PTSA, benzene, reflux, 67%; b) p-anisoyl
chloride, TEA, CH2Cl2, RT, 64%; c) SOCl2, MeCN, 08C, 85%;
d) LiHMDS, THF, À788C, O-TBDPS-2-hydroxymethylacrolein;
e) TESOTf, 2,6-lutidine, CH2Cl2, 08C to RT, 67% (over 2 steps);
f) DIBAL-H, CH2Cl2, À788C; g) MeMgBr, Et2O, 08C, 87% (over 2
steps); h) (COCl)2, DMSO, TEA, CH2Cl2, À788C to RT, 91%; i) O3,
CH2Cl2, À788C, then DMS, 84%; j) TiCl4, CH2Cl2, DIPEA, TMSCl, 08C,
85%; k) Cl3CCOCl, py, CH2Cl2, RT, 89%. Bn=benzyl, DIBAL-H=diiso-
butylaluminum hydride, DIPEA=N,N-diisopropylethylamine,
DMS=dimethyl sulfide, DMSO=dimethyl sulfoxide,
HMDS=1,1,1,3,3,3-hexamethyldisilazane, PMP=p-methoxyphenyl,
PTSA=toluene-p-sulfonic acid, py=pyridine, R=TBDPS=tert-butyldi-
phenylsilyl, TEA=triethylamine, TES=triethylsilyl, Tf=trifluorometha-
nesulfonyl, THF=tetrahydrofuran.
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the epoxide in 11 was achieved by treatment of the
corresponding primary alcohol with Zn(OTf)2 in AcOH to
give the triol 13 with C4 inversion. Presumably, this arose
from the spiroepoxide 12 which underwent solvolysis to
afford the primary acetate as in 13. A two-step sequence
restored the robust TBDPS ether group, and the resulting
triol was converted in situ into the epoxide 14 through the
secondary triflate (70% overall yield from 11). An X-ray
crystal structure validated the suggested sequence of inver-
sions in going from 11 to 14.[13]
Highly stereoselective epoxide opening at C3 with the
aniline derivative 15 in the presence of Yb(OTf)3[14] afforded
the core structure 16 as the sole regioisomer (Scheme 5).
Cleavage of the oxazoline moiety with aqueous HCl[15] led to
the p-methoxybenzoyl ester, which was transformed into the
acetonide 18 in straightforward manner.
Formation of an intermediate isocyanate in the presence
of diphosgene,[16] then treatment with dimethylamine gave the
urea 19 in excellent yield. Treatment of the ester with
DIBAL-H, subsequent oxidative cleavage of the olefin to the
methyl ketone, and hydrolysis of the acetonide function led to
20. Esterification with the cyanomethyl ester 21,[17] then
reduction of the azide group in the presence of Lindlarꢀs
catalyst, afforded pactamycin which was purified by chroma-
tography on silica gel. Synthetic pactamycin exhibited spec-
troscopic and chiroptical properties identical to the originally
1
published data.[6,12] Furthermore, H NMR data at 700 MHz,
13C NMR data at 100 MHz, HPLC data, as well as single
crystal X-ray structures of several intermediates provide
hitherto unavailable characterization features for future
synthetic endeavors.[12] Pactamycin is reported to be unstable
in solution as evidenced by a change in optical rotation in
different solvents, thus losing some of its biological activity
with time.[6]
Scheme 5. Synthesis of pactamycin from the epoxide 14. Reagents and
conditions: a) 3-(prop-1-en-2-yl)aniline (15), Yb(OTf)3, toluene, 808C,
81%, (91% brsm); b) 2n HCl, THF, RT, 63%, (83% after two cycles);
c) TASF, DMF, 08C to RT, 95%; d) 2,2-DMP/CH2Cl2 (1:5), CSA, 08C to
RT, 86%; e) Cl3COCOCl, activated charcoal, TEA, THF, À468C, then
HNMe2, À468C to RT, 86%; f) DIBAL-H, CH2Cl2, À788C, 90%; g) cat.
OsO4, THF/acetone/H2O (5:5:1), NMO, then NaIO4, THF/H2O (1:1),
RT, 80%; h) TFA/MeCN/H2O (5:1:1), 08C to RT, 85%; i) 21, K2CO3,
DMA, RT, 96%; j) Lindlar’s cat., H2, MeOH/EtOH (1:1), 85%.
brsm=based on recovered starting material, CSA=10-camphorsul-
fonic acid, DMA=N,N-dimethylacetamide, DMF=N,N-dimethylforma-
mide, 2,2-DMP=2,2-dimethoxypropane, NMO=4-methylmorpholine
N-oxide, R=TBDPS, PMBz=p-methoxybenzoyl, TASF=tris(dimethyla-
mino)sulfonium difluorotrimethylsilicate.
The synthesis of crystalline pactamycate,[6] a naturally
occurring congener,[5a] is shown in Scheme 6. Treatment of 17
with DIBAL-H, then diphosgene,[16] resulted in the formation
of the corresponding cyclic carbamate. Oxidative cleavage of
the exo-methylene group in the latter afforded 23, which was
converted in two steps into the 2-azido precursor 24. Hydro-
genation in presence of Lindlarꢀs catalyst gave crystalline
pactamycate. An X-ray crystal structure confirmed the
structure of pactamycin and the original assignment of its
absolute configuration for the first time (Scheme 6).[7,12,13]
The successful and seemingly straightforward total syn-
thesis of pactamycin described here underscores the impor-
tance (and frustrations) of many unwanted transformations
caused by the proximity of reactive functional groups
anchored on the cyclopentane core. For example, having
introduced the primary amino group at C1 in intermediate 17
by acidic hydrolysis of the oxazoline 16, there only remained
to convert it into the N,N-dimethylurea, bringing us within a
few steps from the intended target, pactamycin. In practice,
various attempts at reaction of 17 with N,N-dimethylcarba-
moyl chloride resulted in the formation of the precursor
oxazoline 16 (Scheme 7A). In fact, triethylamine and DMAP
alone effected the same transformation into 16. Attempted
formation of the desired N,N-dimethylurea by treatment of 17
with diphosgene to give an intermediate isocyanate, then
quenching with dimethylamine, formed the six-membered
cyclic carbamate 25 in 91% yield even at À468C (Sche-
me 7B)![13] This remarkably facile reaction, spanning a
tertiary alcohol and a highly hindered amine, demonstrates
the importance and unexpected consequences of spatial
proximity in a confined architecture such as that found in 17.
The total synthesis of pactamycin and pactamycate by the
route described here was achieved in 29 linear steps and 3.0%
overall yield starting with the known oxazoline 2 readily
available from l-threonine.[11] The modular introduction of
functional groups allows for a great deal of flexibility in the
quest for the synthesis of less toxic congeners that maintain
their antibacterial and cytotoxic activities.[18] Efforts toward
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Communications
[3] See for example: a) G. Dinos, D. N. Wilson, Y. Teraoka,
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Scheme 6. Synthesis of pactamycate. Reagents and conditions: a) DIBAL-H,
CH2Cl2, À788C, 89%; b) Cl3COCOCl, activated charcoal, TEA, THF, À468C,
86%; c) cat. OsO4, THF/acetone/H2O (5:5:1), NMO, then NaIO4, THF/H2O
(1:1), RT, 82%; d) TASF, DMF, 08C to RT, 93%; e) 21, K2CO3, DMA, RT, 98%;
f) Lindlar’s cat., H2, MeOH/EtOH (1:1), 83%. R=TBDPS.
[8] For the synthesis of hydroxy aminocyclopentitols such as
trehazolamine containing a tertiary alcohol group, see: a) S.
Knapp, A. Purandare, K. Rupitz, S. G. Withers, J. Am. Chem.
Soc. 1994, 116, 7461 – 7462; b) B. E. Ledford, E. M. Carreira, J.
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[12] See the Supporting Information.
[13] CCDC 805348 (5), 805349 (10), 805350 (14), 805351 (pactamy-
cate), and 811840 (25) contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
Scheme 7. Unexpected results arising from the closeness of functional
groups on the polysubstituted cyclopentane core structure. Reagents
and conditions: a) N,N-dimethylcarbamoyl chloride, TEA, CH2Cl2, RT,
96%; b) Cl3COCOCl, activated charcoal, TEA, THF, À468C, 91%.
R=TBDPS.
[14] P. Serrano, A. Llebaria, A. Delgado, J. Org. Chem. 2002, 67,
7165 – 7167.
[15] a) G. Cook, P. S. Shanker, S. L. Peterson, Org. Lett. 1999, 1, 615 –
617; b) A. I. Meyers, G. P. Roth, D. Hoyer, B. A. Barner, D.
Laucher, J. Am. Chem. Soc. 1988, 110, 4611 – 4624; c) R.
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8096 – 8097.
[16] N. Alouane, A. Boutier, C. Baron, E. Vrancken, P. Mangeney,
Synthesis 2006, 885 – 889. To the best of our knowledge, this
method has not been used for the preparation of isocyanates
from primary amines.
[17] R. Shen, C. T. Lin, E. J. Bowman, B. J. Bowman, J. A. Porco, Jr.,
Org. Lett. 2002, 4, 3103 – 3106, and references herein.
[18] For a recent report describing the antiprotozoal activity of
pactamycin, see: K. Otoguro, M. Iwatsuki, A. Ishiyama, M.
Namatame, A. Nishihara-Tukashima, S. Shibahara, S. Kondo, H.
Yamada, S. Omura, J. Antibiot. 2010, 63, 381 – 384.
these goals are presently being actively pursued in our
laboratory.
Received: December 21, 2010
Published online: March 2, 2011
Keywords: natural products · pactamycate · pactamycin ·
.
polysubstituted cyclopentanes · total synthesis
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Chemother. 1962, 191 – 198.
[2] B. K. Bhuyan, A. Dietz, C. G. Smith, Antimicrob. Agents
Chemother. 1962, 184 – 190.
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