Total synthesis of platencin

Article abstract of DOI:10.1002/anie.200800066

(Chemical Equation Presented) The asymmetric total synthesis of the newly discovered and potent antibiotic platencin has been achieved. The approach makes use of an asymmetric Diels-Alder reaction, a gold(I)-catalyzed cyclization, and a homoallyl radical rearrangement to forge the polycyclic architecture of this intriguing target (see scheme, SEM=2-(trimethylsilyl)ethoxymethyl).

Full text of DOI:10.1002/anie.200800066

Communications
DOI: 10.1002/anie.200800066
Natural Product Synthesis
Total Synthesis of Platencin**
K. C. Nicolaou,* G. Scott Tria, and David J. Edmonds
The evolution of bacteria with resistance towards clinically
used antibiotics requires constant efforts in the search for new
antibacterial compounds to target these menacing strains.[1]
Recent advances in screening technologies allowed a team of
scientists at Merck to develop and employ a target-based,
whole-cell,high-throughput screening assay for the detection
of inhibitors of the condensing enzymes FabH and FabF in the
biosynthesis of fatty acids in bacteria.[2,3] Their efforts were
S. aureus colony-forming units in an in vivo mouse model
compared to an untreated control.[3] Whereas several synthe-
ses of platensimycin (1) and its analogues have been
reported,[4] none for platencin (2) has yet appeared in the
literature. Herein we report an enantioselective total syn-
thesis of this newly discovered and highly promising anti-
biotic.
The molecular structure of platencin (2) resembles that of
platensimycin (1) in that they both contain an identical
domain (comprising C1–C9, C17, C1’–C8’–N8’) but they differ
significantly in their C8–C16 core domains (Scheme 1). As
such,a distinctly different approach from those employed to
synthesize platensimycin (1) is required to construct the
“right-hand” core structure of the molecule (enone 3),while
the rest of the pathway to platencin (2) should,in principle,
follow that already charted for platensimycin (1).[4a] Scheme 2
outlines,in retrosynthetic format,the devised strategy for the
synthesis of platencin (2),which defines enone 3 as the key
precursor to the target molecule and requires amide coupling
rewarded with the discovery of the much hailed antibiotics
[3]
platensimycin (1)[2] and platencin (2,Scheme 1).
Isolated
Scheme 1. Structures of platensimycin (1) and platencin (2).
from a new strain of Streptomyces platensis MA7339 found in
a soil sample collected in Spain,platencin ( 2) is a potent
inhibitor of Staphylococcus aureus fatty acid biosynthesis
(IC50 = 0.45 mm),SaFabH (IC 50 = 9.2 mm),and SaFabF (IC 50
=
4.6 mm). Platencin (2) exhibits broad-spectrum antibacterial
activity against many Gram-positive pathogens that show
resistance to current antibiotics (MIC50 ꢀ 0.06–4 mgmLꢁ1),
including S. aureus,methycillin-resistant S. aureus (MRSA),
macrolide-resistant MRSA,linezolid-resistant MRSA,van-
comycin intermediate S. aureus,vancomycin-resistant enter-
ococci,and Streptomyces pneumoniae.[3] Impressively,this
new antibiotic demonstrated a thousandfold reduction of
[*] Prof. Dr. K. C. Nicolaou, G. S. Tria, Dr. D. J. Edmonds
Department of Chemistry and
The Skaggs Institute for Chemical Biology
The Scripps Research Institute
10550 North Torrey Pines Road, La Jolla, CA 92037 (USA)
Fax: (+1)858-784-2469
E-mail: kcn@scripps.edu
and
Department of Chemistry and Biochemistry
University of California, San Diego
9500 Gilman Drive, La Jolla, CA 92093 (USA)
[**] We thank Dr. R. K. Chadha, Dr. D. H. Huang, and Dr. G. Siuzdak for
X-ray crystallographic, NMR spectroscopic, and mass spectrometric
assistance, respectively. We also gratefully acknowledge Dr. T. Lister
and J. Chen for helpful discussions. Financial support for this work
was provided by the National Institutes of Health (USA), the
National Science Foundation (under grant no. 0603217), The
Skaggs Institute of Chemical Biology, and Merck Sharp & Dohme
(postdoctoral fellowship to D.J.E.).
Scheme 2. Retrosynthetic analysis of platencin (2). TMSE=2-(trime-
thylsilyl)ethyl, SEM=2-(trimethylsilyl)ethoxymethyl, TIPS=triisopropyl-
silyl.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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with aniline derivative 4 (which traces back to nitroarene 5[5])
and final deprotection. In our previous work on platensimycin
(1),we employed an aniline derivative bearing a methyl ester
Scheme 2). A retro gold-catalyzed cyclization[7] revealed
acetylenic TIPS enol ether 11 as a possible precursor. The
synthesis of 11 from enone 12 was envisioned,with 12
potentially being available though an asymmetric Diels–
Alder reaction.
group and methoxymethyl (MOM) ethers to mask its
[4a]
carboxylic acid and phenolic moieties,respectively.
The
MOM groups dictated the use of acidic hydrolysis conditions
for their removal in the final step. In this case,to ensure the
survival of the molecule under the deprotection conditions,
we chose the TMSE ester as the obligatory protecting group
for the carboxylic acid and decided to leave the phenolic
groups unprotected. This modification has the extra advant-
age of shortening the sequence by a number of steps,as
compared to the route used for the total synthesis of
platensimycin (1),as a result of easier access to the aniline
fragment and the avoidance of any deprotection of the
phenolic groups. Enone 3 was envisioned to be derived
through an aldol condensation of the corresponding ketoal-
The construction of enone 3 (Scheme 3) started from the
known diene 14 (available from ketone 13 by literature
procedures)[8] and the rather volatile dienophile 16,prepared
from acetylenic alcohol 15 in a one-pot procedure involving
oxidation (SO3·Py,DMSO,Et N,0 8C) followed by addition of
3
Eschenmoserꢀs salt[9] (53% yield). A Rawal asymmetric
Diels–Adler reaction[8,10] between 14 and 16 in the presence
of CrIII-salen catalyst 17[11] furnished cyclohexene 18 in 92%
yield. Reduction of 18 with lithium aluminum hydride
followed by exposure to aqueous HCl led to hydroxy enone
12 in 63% yield and 93% ee.[12] Formation of the SEM ether
19 (SEMCl,Et 3N,THF,70 8C,94% yield) and its subsequent
dehyde,which could,in turn,be obtained from diene
6 by
exposure to TIPSOTf (Et3N,CH Cl2, ꢁ78!08C,97% yield)
2
Wacker oxidation. A homoallyl radical rearrangement[6] was
then envisioned as a means to connect bicyclo[2.2.2] diene 6
to bicyclo[3.2.1] ketone 10 (through radical species 8 and 7,
gave acetylenic diene 11. Treatment of 11 with the catalyst
derived from [AuCl(PPh3)] (2 mol%) and AgBF4 (2 mol%)
induced the desired cyclization,[7] thereby generating the
Scheme 3. Asymmetric synthesis of the core enone (3) of platencin. Reagents and conditions: a) N-benzyl methyl carbamate (1.0 equiv), 13
(2.0 equiv), TsOH (0.05 equiv), CHCl3, reflux, 24 h, 89%; b) TBSOTf (1.1 equiv), Et3N (3.0 equiv), Et2O, ꢁ78!08C, 1 h, 87%; c) SO3·Py
(2.0 equiv), DMSO (5.0 equiv), Et3N (5.0 equiv), CH2Cl2, 258C, 2 h; then Me2NCH2Cl (1.5 equiv), 258C, 12 h, 53%; d) 16 (1.0 equiv), 14
(1.7 equiv), 17 (0.05 equiv), 4-Š M.S., CH2Cl2, ꢁ608C, 60 h, 92%; e) LiAlH4 (1.5 equiv), Et2O, ꢁ78!ꢁ408C, 2 h; then HCl (2m in MeOH,
10 equiv), 258C, 16 h, 63%; f) SEMCl (1.2 equiv), Et3N (4.0 equiv), DMAP (0.1 equiv), THF, reflux, 16 h, 94%; g) TIPSOTf (1.5 equiv), Et3N
(3.0 equiv), ꢁ78!08C, 1 h, 97%; h) [AuCl(PPh3)] (0.02 equiv), AgBF4 (0.02 equiv), toluene/MeOH (10:1), 258C, 30 min, 94%; i) allylmagnesium
chloride (4.0 equiv), CuBr·Me2S (2.0 equiv), THF, ꢁ788C, 1.5 h, 74%; j) NaBH4 (2.5 equiv), MeOH, ꢁ58C!258C, 1 h, 97%; k) CS2 (10 equiv),
KHMDS (5.0 equiv), MeI (5.0 equiv), THF, ꢁ788C!258C, 1.5 h, 100%; l) nBu3SnH (2.0 equiv), AIBN (0.08 equiv), toluene, 1008C, 20 min;
m) PdCl2 (0.25 equiv), CuCl (1.5 equiv), O2 (balloon), DMF/H2O (6.6:1), 258C, 24 h, 50% (2 steps); n) TASF (10 equiv), DMPU, 858C, 1.5 h, 80%
(based on recovered starting material); o) TPAP (0.03 equiv), NMO (6.5 equiv), CH2Cl2, 258C, 4 h, 54%; p) NaOH (6.0 equiv), EtOH, 258C, 19 h,
99%. Bn=benzyl, Ts=p-toluene sulfonyl, TBS=tert-butyldimethylsilyl, Tf=trifluoromethanesulfonyl, Py=pyridine, DMSO=dimethyl sulfoxide,
M.S.=molecular sieves, DMAP=4-dimethylaminopyridine, THF=tetrahydrofuran, HMDS=hexamethyldisilazide, AIBN=2,2’-azobis(isobutyroni-
trile), DMF=N,N-dimethylformamide, TASF=tris(dimethylamino)sulfonium difluorotrimethylsilicate, DMPU=1,3-dimethyl-3,4,5,6-tetrahydro-
2(1H)-pyrimidinone, TPAP=tetra-n-propylammonium perruthenate, NMO=4-methylmorpholine N-oxide.
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bicyclic enone 20 in 94% yield. Reaction of substrate 20 with
Enone 3 was converted into the required carboxylic acid
28 by a sequence similar to that developed by our research
group for the corresponding platensimycin carboxylic acid.[4a]
allylmagnesium chloride/CuBr·Me2S at low temperature
resulted in conjugate addition to the enone moiety to give
cyclohexanone derivative 10 in 74% yield. This compound
was reduced with NaBH4 and the resulting mixture of
diastereoisomeric alcohols (ca. 2:1) was converted into xan-
thate 9 by reaction with KHMDS,CS 2,and MeI (ca. 2:1
Thus,as shown in Scheme 4,sequential alkylation of
3
[13]
mixture of diastereoisomers,97% yield for the two steps).
Treatment of the mixture of xanthates 9 with nBu3SnH in the
presence of catalytic amounts of AIBN at 1008C[6] then
furnished the desired rearranged product 6 contaminated
with about 15% of a by-product,presumed to arise from 5-
exo-trig cyclization onto the pendant allyl group. The
selectivity of this reaction for the desired rearrangement
product was dependent on the protecting group residing on
the primary alcohol of the substrate,with small unbranched
groups favoring the required pathway. Rearrangement of
each diastereomerically pure xanthate 9 gave essentially
identical results. Wacker oxidation[14] (PdCl2 cat.,CuCl,O ) of
2
diene 6 produced methyl ketone 21 (50% yield from 9).
Cleavage of the SEM group in 21 to give the hydroxy ketone
was best achieved with TASF in DMPU at 858C in 80% yield
based on 70% conversion. Interestingly,the hydroxy ketone
product presumed to be formed initially in this reaction was
found to exist as hemiacetal 22,which was isolated as a single
diastereoisomer. Hemiacetal 22 crystallized from CDCl3/
CH2Cl2 (m.p. 115–1178C) as colorless needles. An X-ray
crystallographic analysis of 22 revealed its [2.2.2] bicyclic
structural motif (see ORTEP drawing,Figure 1),thus con-
firming that the radical rearrangement had taken place as
anticipated,and that this series of compounds had the
required relative configuration.[15] Oxidation of hemiacetal
22 was achieved with NMO in the presence of catalytic
amounts of TPAP,[16] and led to keto aldehyde 23 (54% yield),
whose exposure to ethanolic NaOH furnished the targeted
enone 3 in 99% yield through the expected aldol condensa-
tion.
Scheme 4. Completion of the total synthesis of platencin (2). Reagents and
conditions: a) KHMDS (1.1 equiv), MeI (8.0 equiv), THF/HMPA (4:1),
ꢁ78!08C, 2 h, 68%; b) KHMDS (4.0 equiv), allyl iodide (8.0 equiv), THF/
HMPA (4:1), ꢁ78!08C, 3 h, 86%; c) 25 (5.0 equiv), Hoveyda–Grubbs II
cat. (0.1 equiv), benzoquinone (0.1 equiv), benzene, 708C, 1 h; d) Me3NO
(5.0 equiv), THF, 708C, 1 h; e) NaClO2 (3.0 equiv), NaH2PO4 (5.0 equiv),
2-methyl-2-butene (10 equiv), tBuOH/H2O (1:1), 258C, 20 min, 39%
(3 steps); f) 4 (3.2 equiv), HATU (3.2 equiv), Et3N (4.2 equiv), DMF, 258C,
14 h, 61%; TASF (2.0 equiv), 408C, 40 min, 93%. HMPA=hexamethylphos-
phoramide, HATU=O-(7-azabenzotriazol-1-yl)-N,N,N’,N’-tetramethyluro-
nium hexafluorophosphate.
[a) KHMDS,MeI,68% yield; b) KHMDS,allyl iodide,
86% yield] gave triene 24,which underwent regioselective
cross-metathesis with vinyl boronate 25[17] in the presence of
the second-generation Hoveyda–Grubbs catalyst[18] and ben-
zoquinone[19] to afford boronate 26 (E/Z ca. 3:1). Oxidation of
the so-obtained mixture of boronates with Me3NO (THF,
708C) followed by Pinnick oxidation (NaClO2) then led to the
desired carboxylic acid 28,via aldehyde 27,in 39% overall
yield from 24.
The requisite aniline fragment 4 was conveniently pre-
pared from Giannisꢀ nitro ester 5[5] through ester exchange
[20]
(TMSEOH, nBu2SnO,61% yield)
and catalytic hydro-
genation (10% Pd/C,100% yield),as shown in Scheme 5.
Finally,coupling of the carboxylic acid fragment 28 with
aniline fragment 4 was achieved through the action of HATU
(Et3N,DMF,25 8C,61% yield,unoptimized) to furnish
platencin derivative 29,from which platencin ( 2) was
liberated by treatment with TASF (DMF,40 8C,93% yield).
Synthetic platencin (2) exhibited spectral data consistent with
its structure and in accord with those reported[3] for the
natural product.[21]
Figure 1. ORTEP drawing of 22 derived from X-ray crystallographic
analysis (non-hydrogen atoms are shown as 30% ellipsoids).
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Scheme 5. Preparation of aniline fragment 4. Reagents and conditions:
a) TMSEOH (14.8 equiv), nBu2SnO (1.5 equiv), 708C, 3 h, 61%; b) H2
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The described synthetic strategy allows access to platencin
(2) in its naturally occurring form and opens the way to the
construction of analogues of this newly discovered and
valuable lead for drug discovery in the area of infectious
diseases. Such studies are anticipated,as are other synthetic
routes to the natural substance.
Received: January 7,2008
Keywords: antibiotics · gold catalysis · natural products ·
.
radical reactions · total synthesis
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[21] Synthetic platencin (2) exhibited [a]3D5 = ꢁ12.5 degcm3 gꢁ1 dmꢁ1
[3b]
(c = 0.39 g
per
100 cm3,MeOH)
[Lit.
[a]2D3
=
ꢁ7.0 degcm3 gꢁ1 dmꢁ1 (c = 0.85 g per 100 cm3,MeOH)].
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