Total synthesis and structural revision of viridicatumtoxin B

Article abstract of DOI:10.1002/anie.201304691

Will the real viridicatumtoxin B please stand up: The total synthesis of viridicatumtoxin B resulted in its structural revision and opens the way for analogue construction and biological evaluation of this complex tetracycline-like antibiotic. The highly convergent strategy employed allows for swift construction of the entire carbocyclic framework of the molecule. Copyright

Full text of DOI:10.1002/anie.201304691

Angewandte
Chemie
Communications
Total Synthesis
K. C. Nicolaou,* C. Nilewski,
C. R. H. Hale, H. A. Ioannidou,
A. ElMarrouni, L. G. Koch
&&&& — &&&&
Total Synthesis and Structural Revision of
Viridicatumtoxin B
Will the real viridicatumtoxin B please
stand up: The total synthesis of viridica-
tumtoxin B resulted in its structural revi-
sion and opens the way for analogue
construction and biological evaluation of
this complex tetracycline-like antibiotic.
The highly convergent strategy employed
allows for swift construction of the entire
carbocyclic framework of the molecule.
Angew. Chem. Int. Ed. 2013, 52, 1 – 7
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1
These are not the final page numbers!

.
Angewandte
Communications
DOI: 10.1002/anie.201304691
Total Synthesis
Total Synthesis and Structural Revision of Viridicatumtoxin B**
K. C. Nicolaou,* Christian Nilewski, Christopher R. H. Hale, Heraklidia A. Ioannidou,
Abdelatif ElMarrouni, and Lizanne G. Koch
The discovery and development of antibiotics in the first half
of the 20th century revolutionized medicine and led to
a dramatic enhancement of life expectancy.[1] However, the
emergence of drug-resistant bacterial strains caused by the
extensive use of antibiotics in both humans and livestock risks
rendering the worldꢀs arsenal of antibiotics ineffective.[2] The
development of novel antibacterial agents is, therefore, of
high priority.
The nefarious molecular architectures of tetracyclines
have challenged synthetic chemists for over six decades,[3]
with impressive results being reported most recently from the
Myers research group.[4] Structurally related to the tetracy-
cline antibiotics are the viridicatumtoxins (1–3, Figure 1),
which constitute an intriguing subclass of naturally occurring
potent antibacterial agents. Originally isolated in 1973 from
Figure 1. Molecular structures of viridicatumtoxin A (1), B (2), spiro-
hexaline (3), and tetracycline (4).
a Penicillium species,[5] viridicatumtoxin A (1) was structur-
ally elucidated through the aid of X-ray crystallographic
analysis in 1976.[6] In 2008, Kim and co-workers reported the
re-isolation of viridicatumtoxin A (1) as well as the isolation
of viridicatumtoxin B, whose structure was assigned as 2
(Figure 1).[7] Finally, spirohexaline (3) was reported in early
2013.[8] Viridicatumtoxins A (1) and B (2) were shown to
exhibit antibacterial activity against a variety of Gram-
positive bacterial strains, including methicillin-resistant
0.5 mgmLꢀ1, respectively; compare tetracycline (4, Figure 1)
and vancomycin: MIC = 8 and 0.25–1 mgmLꢀ1, respec-
tively].[7] Their mode of action was implied to involve
disruption of bacterial peptidoglycan biosynthesis through
inhibition of UPP synthase.[9] Despite their impressive anti-
bacterial activities, no synthetic approaches toward the
viridicatumtoxins have been reported to date.
Staphylococcus
aureus
(MRSA)
[MIC = 0.25
and
Inspired by the structural complexity, the incomplete and
curious structural assignment containing an epoxy hemiacetal
structural motif, and potent antibiotic properties of viridica-
tumtoxin B (2), we embarked on its total synthesis. Our
objectives were to fully elucidate the structure of viridica-
tumtoxin B (2) and establish the foundation for the synthesis
and biological evaluation of designed analogues within this
family of antibiotics. A number of distinct structural features
add significantly to the challenge of synthesizing viridicatum-
toxin B (2) compared to the tetracyclines (e.g. 4, Figure 1).
These features include the EF spirosystem of viridicatumtox-
in B, its hindered quaternary carbon center at C15, its highly
oxidized AB ring system, and the extensive substitution on
the ABCD tetracyclic core of the molecule. On the basis of
biosynthetic considerations[10] and the known structure of
viridicatumtoxin A (1),[6] we tentatively assigned the relative
configuration of viridicatumtoxin B (2) as shown in Figure 2.
Our highly convergent strategy toward 2 was derived from the
retrosynthetic analysis shown in Figure 2. Thus, disconnection
of the indicated six carbon–carbon bonds through the
reactions shown revealed four key building blocks: allylic
[*] Prof. Dr. K. C. Nicolaou, Dr. C. Nilewski,[+] C. R. H. Hale,[+]
Dr. H. A. Ioannidou, Dr. A. ElMarrouni, L. G. Koch
Department of Chemistry and The Skaggs Institute for Chemical
Biology, The Scripps Research Institute
10550 North Torrey Pines Road, La Jolla, CA 92037 (USA)
and
Department of Chemistry and Biochemistry, University of California
San Diego 9500 Gilman Drive, La Jolla, CA 92093 (USA)
and
Department of Chemistry, BioScience Research Collaborative
Rice University
6100 Main Street, Houston, TX 77005 (USA)
E-mail: kcn@rice.edu
[+] These authors contributed equally to this work.
[**] Financial support was provided by the National Institutes of Health
(USA) (grant AI055475-11) and the Skaggs Institute of Research.
Fellowships to C.N. (Feodor Lynen Research Fellowship, Alexander
von Humboldt Foundation), C.R.H.H. (NSF Graduate Research
Fellowship), and A.E. (Fundaciꢀn Alfonso Martꢁn Escudero) are
gratefully appreciated. We thank Dr. R. K. Chadha for X-ray
crystallographic assistance and Dr. D. H. Huang and Dr. L.
Pasternack for NMR spectroscopic assistance. We thank Prof. W.-G.
Kim for graciously providing scanned NMR spectra of natural
viridicatumtoxin B.
bromide 5, cyclic anhydride 6, quinone monoketal
7
(known),[11] and isoxazole phenyl ester 8. Their construction
and assimilation into the targeted molecule would involve
three cyclizations (as indicated in Figure 2), the extrusion of
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201304691.
2
www.angewandte.org
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2013, 52, 1 – 7
These are not the final page numbers!

Angewandte
Chemie
Figure 2. Retrosynthetic analysis of viridicatumtoxin B (2). Bn=benzyl,
TBS=tert-butyldimethylsilyl, Teoc=2-(trimethylsilyl)ethoxycarbonyl.
carbon dioxide (from 6), and the rupture of the isoxazole ring
(8).
Scheme 1. a) Synthesis of allylic bromide 5; b) synthesis of cyclic
anhydride 6; and c) synthesis of isoxazole 8. Reagents and conditions:
a) a) H3PO4 (0.2 equiv), toluene, reflux, 90 min; b) MeI (3.9 equiv),
K2CO3 (2.0 equiv), acetone, 258C, 15 h; c) mCPBA (1.2 equiv), CH2Cl2,
0!258C, 3 h; d) NaOMe (1.5 equiv), MeOH, reflux, 17 h, 70% yield
over 4 steps; e) TBSCl (1.6 equiv), imidazole (2.0 equiv), CH2Cl2, 258C,
12 h; f) DIBAL-H (2.7 equiv), CH2Cl2, ꢀ78!08C, 70 min, 91% over
2 steps; g) Et3N (2.0 equiv), MsCl (1.7 equiv), CH2Cl2, ꢀ508C, 1 h;
then LiBr (3.5 equiv), THF, ꢀ50!ꢀ208C, 1 h, quant.; b) a) MeI
(4.0 equiv), K2CO3 (8.0 equiv), acetone, reflux, 15 h, 91%; b) NaH
(6.0 equiv), DEM (4.0 equiv), THF, 08C, 2.5 h; then LDA (1.0 equiv),
THF, 08C, 3.5 h, 65%; c) BBr3 (1.35 equiv), CH2Cl2, ꢀ78!258C,
30 min; d) BnBr (1.1 equiv), Ag2O (1.9 equiv), DMF, 258C, 15 h, 66%
over 2 steps; e) NaOH (27 equiv), H2O/EtOH 5:7, reflux, 15 h; f) Ac2O
(1.1 equiv), toluene, reflux, 1 h, 90% over 2 steps; c) a) MgCl2
(1.0 equiv), Et3N (2.0 equiv), AcCl (1.0 equiv), MeCN, 0!258C, 23 h,
96%; b) Me2SO4 (1.3 equiv), K2CO3 (1.3 equiv), DMF, 0!258C, 17 h,
54%; c) H2NOH·HCl (1.4 equiv), NaOMe (3.1 equiv), MeOH, 0!
258C, 24 h, 48%; d) BnBr (1.2 equiv), Ag2O (1.5 equiv), DMF, 258C,
18 h, 67%; e) NaOH (1.9 equiv), H2O/EtOH 3:10, 258C, 3 h, 99%;
f) PPh3 (1.05 equiv), PhOH (1.05 equiv), DIAD (1.05 equiv), THF,
reflux, 3 h, 78%; g) LiHMDS (2.2 equiv), THF, ꢀ788C, 30 min; then
TeocCl (2.2 equiv), ꢀ788C, 2 h, 86%. mCPBA=meta-chloroperoxyben-
zoic acid, DIBAL-H=diisobutylaluminum hydride, Ms=methanesul-
fonyl, DEM=diethylmalonate, LDA=lithium diisopropylamide,
DMF=dimethylformamide, DIAD=diisopropylazodicarboxylate,
PhOH=phenol, THF=tetrahydrofuran, LiHMDS=lithium hexamethyl-
disilazide.
The construction of building blocks 5, 6, and 8 is
summarized in Scheme 1. The synthetic route to allylic
bromide 5, the precursor for the EF spirocyclic ring system,
commenced with a modification of a known, four-step
sequence to convert geranic acid (9) into methyl ester allylic
alcohol 10 (Scheme 1a).[12] Silylation (TBSCl, imidazole) of
10 followed by ester reduction (DIBAL-H) produced allylic
alcohol 11 in 91% overall yield. Low-temperature mesylation
of the latter (MsCl, Et3N) followed by displacement of the
resulting mesylate moiety with LiBr produced the desired
fragment 5 in quantitative yield (Scheme 1a). Synthesis of the
cyclic anhydride 6 commenced from intermediate 13 [avail-
able in two steps from 4-chlororesorcinol (12) by a literature
procedure, Scheme 1b].[13] Selective mono-demethylation of
13 (directed by the nearby ester moiety on the aromatic ring)
was achieved through the use of BBr3. Reprotection of the
liberated phenolic group to give intermediate 14 was accom-
plished using Ag2O/BnBr (66% yield for 2 steps). Saponifi-
cation (aq NaOH) and anhydride formation (Ac2O) produced
the desired cyclic anhydride 6 (90% yield over 2 steps). The
synthesis of isoxazole fragment 8 commenced from known
Stork–Hagedorn isoxazole 16[3i,14] (Scheme 1c), which was
prepared by a modified literature process involving sequential
acylation of dimethylmalonate (AcCl, Et3N, MgCl2, 96%
yield),[15] methylation of the resulting enol (Me2SO4, K2CO3,
54% yield), cyclization (H2NOH·HCl, NaOMe, 48% yield),
and subsequent benzylation of the free hydroxy group (BnBr,
Ag2O, 67% yield). Saponification of the so-produced methyl
ester then furnished carboxylic acid 17 (aq NaOH, 99%
yield). Conventional esterification methods to prepare the
phenyl ester 18, such as through the intermediacy of the
corresponding chloride or the mixed anhydride, failed, as did
the use of peptide coupling reagents. The desired phenyl ester
formation, however, could be achieved under Mitsunobu
conditions (PPh3, PhOH, DIAD, 78% yield).[16] The need to
install an electron-withdrawing—and ideally easily remov-
able—group at the isoxazole methyl group was predicated on
AB ring model systems; such a group was found to be
required to facilitate a later conjugate addition. To this end,
deprotonation of the methyl group of phenyl ester 18
(LiHMDS) and quenching of the resulting anionic species
with TeocCl produced the desired fragment 8 (86% yield).
Quinone monoketal 7 was readily available in one step by
a literature procedure.[11]
With all four building blocks (5–8) in hand, the stage was
now set for their union, as shown in Scheme 2a. Annulation of
cyclic anhydride 6 with quinone monoketal 7 to afford
tricyclic compound 19 was achieved by using a one-step
Michael–Dieckmann/decarboxylation protocol (DBU, 658C,
Angew. Chem. Int. Ed. 2013, 52, 1 – 7
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
3
These are not the final page numbers!

.
Angewandte
Communications
proceeded under mild basic conditions (i.e. Na2CO3) to give
intermediate 20 (77% yield, d.r. ca. 1:1). Much to our delight,
ionization of the allylic TBS ether was readily achieved with
catalytic quantities of BF3·OEt2, presumably forming highly
stabilized but fleeting allylic cation 21, whose intramolecular
trapping by the nearby electron-rich arene led to spirocycle 22
as a single diastereomer (73% yield). The connectivity and
relative stereochemical relationships within pentacycle 22
were assigned by comparing its NMR spectroscopic data with
those of the related compound 23 (10-methoxy as opposed to
10-benzyloxy, Scheme 2b), whose structure was secured
through X-ray crystallographic analysis [m.p. = 114–1168C
(CHCl3/EtOAc), see ORTEP representation, Scheme 2b].[18]
To ready the newly acquired pentacycle 22 for cyclization
with isoxazole 8, and thus forge ring A of the growing
molecule, the former was treated with PhI(OAc)2 (PIDA,
phenolic oxidation) to produce B-ring enone 24, which,
however, did not undergo the desired Michael–Dieckmann
reaction with isoxazole 8. We hypothesized that this reluc-
tance was due to the adjacent electron-rich naphthalene ring,
which decreases the electrophilicity of the enone moiety.
Treatment of 24 with catalytic amounts of CSA (elimination
of methanol, 85% yield over 2 steps) and subsequent
exposure of the resulting C-ring quinomethide/B-ring p-
methoxyphenol to PhI(OAc)2 (PIDA) gave quinomethide 25
(90% yield), whose superior electrophilicity now allowed its
facile fusion with its intended nucleophilic partner. Thus, the
anticipated Michael–Dieckmann reaction between highly
reactive enone 25 and isoxazole 8 was achieved using
a slight excess of potassium tert-butoxide in toluene, which
afforded heptacycle 26 and its isomer 15-epi-26 [91% yield,
d.r. ca. 2:1 in favor of the natural C15 epimer (26), see below].
The relative stereochemical assignment of C4 and C4a in 26
3
was based on the observed J4,4a = 10.0 Hz coupling constant,
which is indicative of an H4,H4a anti relationship. From this
point on, the two C15 diastereomers were carried forward as
a mixture through several additional steps until separation
became convenient. The use of a phenyl ester for the
Michael–Dieckmann reaction was critical for success in the
union of enone 25 and isoxazole 8, in agreement with previous
observations by White et al.[19] and Myers and co-workers.[4d]
Indeed, the methyl ester counterpart of isoxazole fragment 8
failed to enter this cyclization, leading instead to the initially
formed conjugate addition adduct whose subsequently
attempted ring closure under a variety of basic conditions
failed.
The next task, that of removing the Teoc group with
concomitant decarboxylation, proved problematic, initially
leading to decomposition[20a,b] or low yields of the desired
product (i.e. 27)[20c] under various conditions. After consid-
erable experimentation it was found that using a freshly
prepared solution of TBAF buffered with NH4F[21] in
degassed THF achieved the desired goal of removing the
Teoc group in 86% yield. We hypothesized that these
buffered conditions served to keep the C1,C12 b-diketone
moiety of 26 in its fully protonated state, thereby allowing for
smooth anion formation during the decarboxylation step. This
transformation marked the successful completion of the
entire carbon framework of viridicatumtoxin B (2).
Scheme 2. a) Synthesis of heptacycle 27 and b) ORTEP representation
of spirocycle 23 (thermal ellipsoids at 30% probability; gray=C,
red=O, green=H). Reagents and conditions: a) 7 (3.0 equiv), NaH
(3.0 equiv), THF, 08C, 45 min; then 258C, 1 h; b) DBU (5.0 equiv),
toluene, 658C, 4.5 h, 54% over 2 steps; c) CSA (0.02 equiv), CH2Cl2,
258C, 30 min, 99%; d) 5 (1.1 equiv), Na2CO3 (10 equiv), DMF, 258C,
1 h, 77%, d.r. ca. 1:1; e) BF3·OEt2 (0.15 equiv), CH2Cl2, 08C, 20 min,
73%; f) PIDA (1.2 equiv), MeOH/CH2Cl2 1:1, 0!258C, 1 h; g) CSA
(0.07 equiv), CH2Cl2, 08C, 5 min, 85% over 2 steps; h) PIDA
(1.2 equiv), MeOH/CH2Cl2 10:1, 258C, 1.5 h, 90%; i) 8 (1.1 equiv),
tBuOK (1.2 equiv), toluene, 258C, 15 min, 91%, d.r. ca. 2:1; j) TBAF
(10 equiv), NH4F (20 equiv), degassed THF, 258C, 5 min, 86%.
DBU=1,8-diazabicyclo[5.4.0]undec-7-ene, CSA=(ꢁ)-camphor-10-sul-
fonic acid, PIDA=iodobenzene diacetate, TBAF=tetra-n-butylammo-
nium fluoride.
41% yield). However, a two-step protocol involving exposure
of 6 and 7 to NaH to induce anion generation and cyclo-
addition followed by work-up and subsequent treatment with
DBU to promote decarboxylation was preferred, as it proved
more efficient in producing 19 (54% yield overall).[17]
Reaction of ketal 19 with catalytic amounts of CSA caused
extrusion of methanol, thereby furnishing the corresponding
anthrone (99% yield), whose alkylation with allylic bromide 5
4
www.angewandte.org
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2013, 52, 1 – 7
These are not the final page numbers!

Angewandte
Chemie
Soon after arriving at compound 27, we realized that its
conversion into the more advanced intermediates of obliga-
tory higher oxygenation (at C4a and C12a) would present
challenges. Among the most serious issues were the multitude
of functional groups present in the substrate, a problem
exacerbated by the scarcity of protons around the AB ring
system, which made spectroscopic analysis rather tedious, as
well as the practical problems arising from the insoluble
nature of the intermediates. After extensive experimentation,
and as shown in Scheme 3a, the hydroxylation of 27 at C12a
were separated chromatographically, with the natural diaste-
reomer taken through the remaining steps. The assignment of
the structural configurations of 28 and 15-epi-28 were based
on comparisons of the NMR spectroscopic data of the two
isomers with those of the 10-methoxy counterpart 29
(Scheme 3b), whose structure had been unambiguously
proven by X-ray crystallographic analysis [m.p. = 213–2158C-
(decomp) (EtOAc), see ORTEP representation, Sche-
me 3b].[18]
From intermediate 28, a number of strategies and tactics
were envisioned for the installment of the C4a hydroxy group
(see Scheme 3a). Our initial efforts were directed at the
generation of enol ether 30 and its hydroxy-directed a-
epoxidation, followed by hydrolysis of the resulting epoxide
to obtain compound 31. However, all attempts to remove
a molecule of methanol from ketal 28 under a variety of both
Lewis and Brønsted acidic conditions did not lead to any
detectable enol ether product (30).[24] Faced with this
predicament, we proceeded to hydrolyze the dimethyl ketal
moiety of 28 under acidic conditions (aq HCl, quant.) with the
intention of converting the resulting triketone (32) into the
desired product (31) through oxygenation of its enolate (at
C4a). All attempts, however, to achieve this conversion were
met with failure (see 32 to 31, Scheme 3a). The reluctance of
these substrates to react as projected may be attributed to
unwanted rearrangement pathways, including b-elimination/
aromatization or ring-expansion/lactone formation involving
the C12a hydroxy group.[4a,25]
Speculating that a C4a,C5 diol could be introduced
through dihydroxylation, we next attempted to reduce the
C5 ketone within 32 (see Scheme 3a) to its corresponding
alcohol by employing 1,3-directed reduction reagents[26] such
as NaBH(OAc)3, expecting that we might eventually be able
to dehydrate the latter to the desired C4a,C5 olefinic bond
within ring B. Surprisingly, however, treatment of triketone 32
with a slight excess of NaBH(OAc)3 in EtOAc/acetone (1:1)
at 408C produced the 1,2-directed reduction product 33 (47%
yield) instead, as evident from diagnostic HMBC correlations.
This result seemingly stands in contrast to the recently
reported 1,3-reduction of a similar system by Tatsuta et al.,[27]
but is most likely a result of the electron-withdrawing effect
exerted by the adjacent isoxazole system which renders the
C1 ketone more electrophilic. Interestingly, the use of
solvents other than EtOAc/acetone (1:1) resulted in intract-
able mixtures of reduction products. Recognizing the poten-
tial implications of this unexpected reduction, and building on
the intelligence we had accumulated thus far on the reactivity
of our system, we reasoned that a substrate such as 33 could
be less prone to the suspected undesired reactions such as b-
elimination followed by a thermodynamically downhill aro-
matization of ring A.
Scheme 3. a) Unsuccessful attempts to install the C4a hydroxy group
and synthesis of C1 hydroxy compound 33 and b) ORTEP representa-
tion of heptacycle 29 (thermal ellipsoids at 30% probability; gray=C,
red=O, blue=N, green=H). Reagents and conditions: a) [Ni(acac)2]
(0.2 equiv), DMDO (5.1 equiv), CH2Cl2, ꢀ78!ꢀ608C, 6.5 h, 36%,
60% brsm, 50% after one recycle; b) NaCNBH3 (10 equiv), THF,
ꢀ78!ꢀ608C, 90 min, 39% for 28, 19% for 15-epi-28, chromatograph-
ically separated; c) 2N aq HCl/THF 1:10, 258C, 5 h, quant.; d) NaBH-
(OAc)3 (1.2 equiv), EtOAc/acetone 1:1, 408C, 105 min, 47%; acac=
acetylacetonate, DMDO=dimethyldioxirane.
was finally achieved with dimethyldioxirane (DMDO) in the
presence of catalytic amounts of [Ni(acac)2][22] at ꢀ788C in
dichloromethane,[23] thereby leading to the desired hydroxy-
lated product in 50% yield after one recycle of the 40%
recovered starting material. Reduction of the quinomethide
moiety of the latter with NaCNBH3, as a soft hydride source,
was then carried out to afford naphthalene derivative 28
(+ 15-epi-28) in 58% combined yield for the two diastereo-
mers (28/15-epi-28 ca. 2:1). At this stage, the two isomers
Following this reasoning, and to exploit the fortuitous
formation of 33, the C1 hydroxy group of the latter was
silylated to afford TBS ether 34 (TBSOTf, 2,6-lutidine, 61%
yield, Scheme 4). Much to our delight, the desired C4a
hydroxylation was successfully performed by generating the
trianion (35) of 34 with excess KHMDS at ꢀ788C and
quenching it with the Davis oxaziridine reagent,[28] thereby
leading to C4a-hydroxylated compound 36 (20% yield
Angew. Chem. Int. Ed. 2013, 52, 1 – 7
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

.
Angewandte
Communications
the presence of the C5 signal at d = 194.1 ppm in the 13C NMR
spectrum of (ꢁ)-38 and the absence of the reported[7] C5
signal at d = 116.4 ppm.[30] No spectroscopic evidence for the
existence of an epoxy hemiacetal structural motif was
observed in the 13C NMR spectrum of (ꢁ)-38, thus compelling
us to revise the structure of natural viridicatumtoxin B to that
of 38. Pleasantly, synthetic viridicatumtoxin B (38) crystal-
lized in crystals suitable for X-ray analysis from CH2Cl2/
EtOH [m.p. = 245–2478C(decomp), see ORTEP representa-
Scheme 4. C4a oxidation and completion of the synthesis. Reagents
and conditions: a) TBSOTf (40 equiv), 2,6-lutidine (60 equiv), CH2Cl2,
0!258C, 1 h, 61%; b) KHMDS (3.4 equiv), THF, ꢀ788C, 1 h; then
Davis ox. (3.9 equiv), ꢀ788C, 1.7 h, 20%+45% recovered 34;
c) HF·py (excess), MeCN, 0!508C, 25 h, 61%; d) DMP (3.0 equiv),
DCE, 0!508C, 7.5 h, 66%; e) H2, Pd black (4.9 equiv), 1,4-dioxane/
MeOH 1:1, 258C, 8 min, 98%; TBSOTf=tert-butyldimethylsilyl trifluoro-
methanesulfonate, KHMDS=potassium hexamethyldisilazide, Davis
ox.=(ꢁ)-trans-2-(phenylsulfonyl)-3-phenyloxaziridine, py=pyridine,
DMP=Dess–Martin periodinane, DCE=1,2-dichloroethane.
Figure 3. ORTEP representation of synthetic viridicatumtoxin B (38;
thermal ellipsoids at 30% probability; gray=C, red=O, blue=N,
green=H).
tion, Figure 3], which provided unambiguous proof of its
structure.[18]
In conclusion, the total synthesis of racemic viridicatum-
toxin B has been achieved through a highly convergent
synthetic strategy from four easily accessible building
blocks. This accomplishment led to the revision of the
originally assigned epoxy hemiacetal structure of viridica-
tumtoxin B (2, Figure 1) to its hydroxy ketone form (38,
Scheme 4). Efforts currently in progress aim to achieve an
enantioselective total synthesis of viridicatumtoxin B (38) and
confirm its absolute configuration. The developed chemistry
sets the stage for further advances to occur in the field,
including the design, synthesis, and biological evaluation of
analogues of the viridicatumtoxins as potential leads for drug
discovery to combat bacterial infections.
+ 45% recovered starting material, 36% brsm). Key to the
success of this reaction were the free phenol at C11, an
unprotected alcohol at C12a (which allows alkoxide forma-
tion, thereby preventing b-elimination), and protection of the
secondary alcohol at C1. Structural deviations from this
substrate resulted in no conversion, rearrangement pathways,
or b-elimination of the C12a oxygen atom. The stereochem-
ical outcome of this hydroxylation was revealed by NOESY
correlations of product 36 (see the Supporting Information),
and was in agreement with our expectations based on steric
considerations (see structure 35, Scheme 4).
Removal of the TBS group from compound 36 with HF·py
then led to the expected hydroxy compound 37 (61% yield).
From NMR studies, we surmised that compound 37 exists in
equilibrium with its cyclic hemiacetal isomer 37’, with the two
isomers interconverting slowly as evidenced from their
unusually broad 1H NMR signals at ambient temperature.
In support of this notion was the fact that the 1H NMR
spectrum of this mixture acquired at ꢀ408C displayed two
sets of signals. Finally, oxidation of 37/37’ with DMP[29]
furnished the corresponding C1 ketone (66% yield), which
was subjected to hydrogenolysis[4d] (two benzyl ethers and an
Received: May 30, 2013
Published online: && &&, &&&&
Keywords: antibiotics · natural products · structural revision ·
.
tetracyclines · total synthesis
[1] a) F. von Nussbaum, M. Brands, B. Hinzen, S. Weigand, D.
Hꢁbich, Angew. Chem. 2006, 118, 5194 – 5254; Angew. Chem. Int.
Ed. 2006, 45, 5072 – 5129; b) K. C. Nicolaou, J. S. Chen, D. J.
Edmonds, A. A. Estrada, Angew. Chem. 2009, 121, 670 – 732;
Angew. Chem. Int. Ed. 2009, 48, 660 – 719.
ꢀ
N O bond; H2, Pd black, 98% yield) to afford compound
(ꢁ)-38 as shown in Scheme 4. The physical properties of (ꢁ)-
38 matched those reported for viridicatumtoxin B[7] except for
6
www.angewandte.org
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2013, 52, 1 – 7
These are not the final page numbers!

Angewandte
Chemie
[2] a) C. Walsh, Antibiotics, Actions, Origins, Resistance, 1st ed.,
ASM, Washington, DC, 2003; b) S. Rachakonda, L. Cartee, Curr.
Med. Chem. 2004, 11, 775 – 793; c) S. B. Levy, B. Marshall, Nat.
Med. 2004, 10, S122 – S129; d) C. Nathan, Nature 2004, 431, 899 –
902.
[12] I. Kadꢂs, G. ꢃrvai, G. Horvꢂth, Org. Prep. Proced. Int. 1998, 30,
79 – 85.
[13] W. E. Bauta, D. P. Lovett, W. R. Cantrell, Jr., B. D. Burke, J.
Org. Chem. 2003, 68, 5967 – 5973.
[14] G. Stork, A. A. Hagedorn III, J. Am. Chem. Soc. 1978, 100,
3609 – 3611.
[3] a) L. H. Conover, K. Butler, J. D. Johnston, J. J. Korst, R. B.
Woodward, J. Am. Chem. Soc. 1962, 84, 3222 – 3224; b) H.
Muxfeldt, W. Rogalski, J. Am. Chem. Soc. 1965, 87, 933 – 934;
c) A. I. Gurevich, M. G. Karapetyan, M. N. Kolosov, V. G.
Korobko, V. V. Onoprienko, S. A. Popravko, M. M. Shemyakin,
Tetrahedron Lett. 1967, 8, 131 – 134; d) J. J. Korst, J. D. Johnston,
K. Butler, E. J. Bianco, L. H. Conover, R. B. Woodward, J. Am.
Chem. Soc. 1968, 90, 439 – 457; e) H. Muxfeldt, G. Hardtmann, F.
Kathawala, E. Vedejs, J. B. Mooberry, J. Am. Chem. Soc. 1968,
90, 6534 – 6536; f) D. H. R. Barton, P. D. Magnus, T. Hase, J.
Chem. Soc. C 1971, 2215 – 2225; g) H. Muxfeldt, G. Haas, G.
Hardtmann, F. Kathawala, J. B. Mooberry, E. Vedejs, J. Am.
Chem. Soc. 1979, 101, 689 – 701; h) H. H. Wasserman, T.-J. Lu,
A. I. Scott, J. Am. Chem. Soc. 1986, 108, 4237 – 4238; i) G. Stork,
J. J. La Clair, P. Spargo, R. P. Nargund, N. Totah, J. Am. Chem.
Soc. 1996, 118, 5304 – 5305; j) K. Tatsuta, T. Yoshimoto, H.
Gunji, Y. Okado, M. Takahashi, Chem. Lett. 2000, 646 – 647;
k) J. S. Wzorek, T. F. Knçpfel, I. Sapountzis, D. A. Evans, Org.
Lett. 2012, 14, 5840 – 5843.
[4] a) M. G. Charest, D. R. Siegel, A. G. Myers, J. Am. Chem. Soc.
2005, 127, 8292 – 8293; b) M. G. Charest, C. D. Lerner, J. D.
Brubaker, D. R. Siegel, A. G. Myers, Science 2005, 308, 395 –
398; c) J. D. Brubaker, A. G. Myers, Org. Lett. 2007, 9, 3523 –
3525; d) C. Sun, Q. Wang, J. D. Brubaker, P. M. Wright, C. D.
Lerner, K. Noson, M. Charest, D. R. Siegel, Y.-M. Wang, A. G.
Myers, J. Am. Chem. Soc. 2008, 130, 17913 – 17927; e) P. M.
Wright, A. G. Myers, Tetrahedron 2011, 67, 9853 – 9869; f) D. A.
Kummer, D. Li, A. Dion, A. G. Myers, Chem. Sci. 2011, 2, 1710 –
1718.
[5] a) R. D. Hutchison, P. S. Steyn, S. J. Van Rensburg, Toxicol.
Appl. Pharmacol. 1973, 24, 507 – 509. We propose that viridica-
tumtoxin be renamed “viridicatumtoxin A.” It has been sug-
gested that viridicatumtoxin A was isolated from Penicillium
aethiopicum and not the originally reported Penicillium viridi-
catum; see b) J. C. Frisvad, O. Filtenborg, Mycologia 1989, 81,
837 – 861.
[6] a) C. Kabuto, J. V. Silverton, T. Akiyama, U. Sankawa, R. D.
Hutchison, P. S. Steyn, R. Vleggaar, J. Chem. Soc. Chem.
Commun. 1976, 728 – 729; b) J. V. Silverton, C. Kabuto, T.
Akiyama, Acta Crystallogr. Sect. B 1982, 38, 3032 – 3037.
[7] C.-J. Zheng, H.-E. Yu, E.-H. Kim, W.-G. Kim, J. Antibiot. 2008,
61, 633 – 637.
[15] M. W. Rathke, P. J. Cowan, J. Org. Chem. 1985, 50, 2622 – 2624.
[16] V. P. Fitzjarrald, R. Pongdee, Tetrahedron Lett. 2007, 48, 3553 –
3557.
[17] Mechanistic studies of the one-pot procedure involving the use
of DBU on related systems suggested a stepwise pathway. The
use of a strong base (i.e. NaH or LDA) with homophthalic
anhydrides and quinones (Tamura Diels–Alder reaction) is
typically depicted as a concerted [4+2] cycloaddition, but the
synchronicity of the reaction has been questioned: a) C. D. Cox,
T. Siu, S. J. Danishefsky, Angew. Chem. 2003, 115, 5783 – 5787;
Angew. Chem. Int. Ed. 2003, 42, 5625 – 5629; b) M. Gonzꢂlez-
Lꢄpez, J. T. Shaw, Chem. Rev. 2009, 109, 164 – 189.
[18] CCDC 941202, 941203, and 945867 contain the supplementary
crystallographic data for compounds 23, 29, and 38, respectively
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. Compounds 23 and 29 were pre-
pared by similar routes to those described herein and will be
reported in the full account of this work.
[19] a) J. D. White, E. G. Nolen, Jr., C. H. Miller, J. Org. Chem. 1986,
51, 1150 – 1152; b) J. D. White, F. W. J. Demnitz, Q. Xu, W. H. C.
Martin, Org. Lett. 2008, 10, 2833 – 2836.
[20] a) C. Gioeli, N. Balgobin, S. Josephson, J. B. Chattopadhyaya,
Tetrahedron Lett. 1981, 22, 969 – 972; b) K. C. Nicolaou, K. R.
Reddy, G. Skokotas, F. Sato, X. Y. Xiao, C. K. Hwang, J. Am.
Chem. Soc. 1993, 115, 3558 – 3575; c) S. E. Denmark, T. Kobaya-
shi, C. S. Regens, Tetrahedron 2010, 66, 4745 – 4759.
[21] A. Fꢅrstner, H. Weintritt, J. Am. Chem. Soc. 1998, 120, 2817 –
2825.
[22] W. Adam, A. K. Smerz, Tetrahedron 1996, 52, 5799 – 5804.
[23] M. Gibert, M. Ferrer, F. Sꢂnchez-Baeza, A. Messeguer, Tetrahe-
dron 1997, 53, 8643 – 8650.
[24] Electron-rich a-methoxystyrenes are known to be very labile,
even at near-neutral pH values; see, for example: J. P. Richard,
K. B. Williams, J. Am. Chem. Soc. 2007, 129, 6952 – 6961.
[25] A. I. Scott, E. Yamaguchi, S.-K. Chung, Tetrahedron Lett. 1975,
16, 1369 – 1372.
[26] D. A. Evans, K. T. Chapman, E. M. Carreira, J. Am. Chem. Soc.
1988, 110, 3560 – 3578.
[27] K. Tatsuta, T. Fukuda, T. Ishimori, R. Yachi, S. Yoshida, H.
Hashimoto, S. Hosokawa, Tetrahedron Lett. 2012, 53, 422 – 425.
[28] F. A. Davis, O. D. Stringer, J. Org. Chem. 1982, 47, 1774 – 1775.
[29] D. B. Dess, J. C. Martin, J. Org. Chem. 1983, 48, 4155 – 4156.
[30] We thank Professor W.-G. Kim for kindly providing us with the
1H and 13C NMR spectra of natural viridicatumtoxin B obtained
at 300 MHz and 226 MHz, respectively, from a small sample of
natural material. These spectroscopic data were in accord with
those obtained by us at 600 MHz and 151 MHz for synthetic
material, thus confirming the identity of the two samples (except
for the racemic nature of ours). Furthermore, we noted certain
discrepancies in the reported chemical shift values in Ref. [7]
compared to those actually found in the provided spectra of the
natural product. In particular, the originally unreported carbonyl
resonance near d = 194 ppm which we assigned as C5 was indeed
present in the scanned authentic spectrum of the natural
product. For the spectra of the natural and synthetic materials
and further details, see the Supporting Information.
[8] J. Inokoshi, Y. Nakamura, Z. Hongbin, R. Uchida, K. Nonaka,
R. Masuma, H. Tomoda, J. Antibiot. 2013, 66, 37 – 41.
[9] N. Koyama, J. Inokoshi, H. Tomoda, Molecules 2013, 18, 204 –
224.
[10] a) A. E. de Jesus, W. E. Hull, P. S. Steyn, F. R. van Heerden, R.
Vleggaar, J. Chem. Soc. Chem. Commun. 1982, 902 – 904;
b) R. M. Horak, V. J. Maharaj, S. F. Marais, F. R. van Heerden,
R. Vleggaar, J. Chem. Soc. Chem. Commun. 1988, 1562 – 1564;
c) H. Zhou, Y. Li, Y. Tang, Nat. Prod. Rep. 2010, 27, 839 – 868;
d) Y.-H. Chooi, R. Cacho, Y. Tang, Chem. Biol. 2010, 17, 483 –
494; e) Y. Li, Y.-H. Chooi, Y. Sheng, J. S. Valentine, Y. Tang, J.
Am. Chem. Soc. 2011, 133, 15773 – 15785; f) Y.-H. Chooi, P.
Wang, J. Fang, Y. Li, K. Wu, P. Wang, Y. Tang, J. Am. Chem. Soc.
2012, 134, 9428 – 9437.
[11] Prepared in one step from 4-methoxyphenol: A. Pelter, S.
Elgendy, Tetrahedron Lett. 1988, 29, 677 – 680.
Angew. Chem. Int. Ed. 2013, 52, 1 – 7
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
7
These are not the final page numbers!