An efficient formal synthesis of the human telomerase inhibitor (±)-γ-rubromycin

Article abstract of DOI:10.1002/anie.200903316

Balancing act: The correct balance of electronic factors in the naphthazarin and isocoumarin fragments facilitates the acid-mediated spiroketalization step to afford the key densely functionalized spiroketal (see picture; EOM=ethoxymethyl) in the formal s

Full text of DOI:10.1002/anie.200903316

Communications
DOI: 10.1002/anie.200903316
Natural Product Synthesis
An Efficient Formal Synthesis of the Human Telomerase
Inhibitor (ꢀ )-g-Rubromycin**
Dominea C. K. Rathwell, Sung-Hyun Yang, Kit Y. Tsang, and
Margaret A. Brimble*
Angewandte
Chemie
7996
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Angew. Chem. Int. Ed. 2009, 48, 7996 –8000

Angewandte
Chemie
g-Rubromycin[1] (1, Scheme 1), the prototypical member of a
structurally related family of antibiotics known as the
rubromycins, consists of a densely oxygenated naphthazarin
ring and an isocoumarin moiety linked through a unique
aromatic 5,6-spiroketal ring system (Scheme 1). The rubro-
mycins exhibit a wide range of biological activity including
thalene[9–11] and isocoumarin[12] fragments together with
model doubly benzannelated[13–16] and naphthyl-benzanne-
lated[11,15] spiroketal systems have been reported. However,
only two successful total syntheses have been achieved to
date, namely, the aglycon of (ꢀ )-5 by Danishefsky and co-
workers,[17] in which spiroketalization of a hemiketal occurred
under Mitsunobu conditions, and (ꢀ )-1 by Kita et al.[18] who
used a double aromatic Pummerer-type reaction to install the
spiroketal.
In spite of these investigations, an efficient and flexible
strategy for the total synthesis of these natural products has
not been forthcoming; the most striking problem in the
synthesis is the acid-mediated spiroketalization to construct
the unique 5,6-bisbenzannelated spiroketal core. Difficulties
in the application of this seemingly straightforward approach
to the natural product have been reported by the research
groups of Danishefsky,[17] Kozlowski,[11] and Reißig[5,16] in
their respective synthetic efforts towards 5 and purpuromycin
4. Through comprehensive modeling studies, Kozlowski,
Reißig, and their respective co-workers concluded that the
presence of electron-withdrawing groups in the isocoumarin
unit markedly diminish the nucleophilicity of the phenolic
hydroxyl group, a key factor that prohibits the critical
spiroketalization step.[5,11,16]
In previous studies, our research group has demonstrated
that steric hindrance associated with the presence of methoxy
groups at C-9’ and C-10 (bis-ortho substituents) hamper the
spirocyclization step to result in the undesired formation of
the thermodynamically favored benzofuran.[19] We have also
reported a mild, efficient deprotection and spiroketalization
protocol that leads to the successful formation of the doubly
benzannulated spiroketal.[19] Inspired by Reißig and co-work-
ers,[5] who noted that with delicate electronic balancing of
aromatic units the acid-promoted spiroketalization should be
possible, we were convinced that use of a mild acid mediated
spiroketalization step that pays due attention to the use of an
appropriate isocoumarin ring precursor, should provide a
practical method for the synthesis of the rubromycins and
related analogues.
Herein, we report an efficient synthesis of Kitaꢀs penta-
cyclic spiroketal 6[18] in which isocoumarin ring formation is
delayed until after the key spiroketalization. This approach
thus constitutes a formal synthesis of (ꢀ )-g-rubromycin 1
(Scheme 2). Elimination of the electron-withdrawing meso-
meric effect on the eastern fragment of the advanced
spiroketal precursor 7 was expected to increase the nucleo-
philicity of the phenol, thus encouraging the key acid-
mediated cyclization to form the D ring (see the insert in
Scheme 2). Alkynol 8 was synthesized by the Sonogashira
acetylide coupling strategy[14] that we developed to construct
simpler aryl spiroketals, and was envisaged to be a flexible
building block from which several members of the rubromy-
cin family of antibiotics could be accessed.
Scheme 1. Selected natural products of the rubromycin class of anti-
biotics.
antimicrobial and anticancer properties.[1–6] b-Rubromycin (2)
and g-rubromycin display potent activity against human
telomerase (IC50 = 3 mm), the reverse transcriptase of HIV-1,
and the moloney murine leukemia virus.[4–7] a-Rubromycin
(3), the only member of the group to lack the aryl spiroketal
moiety, exhibits substantially decreased inhibitory potency
towards telomerase (IC50 > 200 mm), which suggests that the
spiroketal core is an essential pharmacophore for the
inhibition of telomerase.[4] Purpuromycin (4),[8] a potential
topical agent for vaginal infections,[2] and heliquinomycin (5),
a selective inhibitor of DNA helicase are structurally related
to the rubromycins.[6] Thus, 56 years since the isolation of 1,
the rubromycins still attract considerable synthetic efforts
because of their unprecedented structures and intriguing
pharmacological profile.[5] Numerous syntheses of the naph-
[*] D. C. K. Rathwell, S.-H. Yang, K. Y. Tsang, Prof. Dr. M. A. Brimble
Department of Chemistry, The University of Auckland
23 Symonds Street, Auckland (New Zealand)
Fax: (+64)9-373-7422
http://www.che.auckland.ac.nz/staffsites/BrimbleM/
E-mail: m.brimble@auckland.ac.nz
Our initial attention focused on the development of an
efficient route to the naphthazarin fragment 9 that has already
challenged several research groups.[5,9–11] In the process, a
novel allyloxylation/Claisen rearrangement strategy that
facilitated regioselective ipso and ortho functionalization of
1,4-naphthoquinones was developed.
[**] We thank the New Zealand Foundation for Research, Science and
Technology for the award of a Top Achiever Doctoral Scholarship to
K.Y.T. and D.C.K.R. We are also grateful to Profs. Y. Kita and Fujioka
for providing an authentic sample of 6.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200903316.
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Communications
Scheme 2. Retrosynthesis of 1.
The synthesis of naphthalene fragment 9 hinged on the
Claisen rearrangement of allyloxynaphthoquinone 11a that
started from the known bromoquinone 10[10,20] (Scheme 3).
Surprisingly, direct C-2 allyloxylation of bromoquinone 10
that would allow a subsequent Claisen rearrangement proved
problematic. In preliminary studies, allyloxylation of bromo-
quinone 10 by formation of the sodium or potassium
allyloxide was accompanied by degradation, and afforded
11a in poor yields (20–30%). Eventually, the facile allylox-
ylation of bromoquinone 10 using excess allyl alcohol and
cesium carbonate took place, but disappointingly afforded a
mixture of regioisomers 11a and 11b.
Normal substitution proceeds in an ipso fashion by a
nucleophilic attack on the carbon bonded to the halogen (C-
2) with subsequent expulsion of halide to give 11a, whilst
similar nucleophilic attack on the adjacent carbon (C-3) gives
an intermediate adduct which upon dehydrohalogenation
affords the cine-substitution product 11b. The regiochemistry
of 11b was determined by its conversion into the crystalline
Claisen product 14b. A single-crystal X-ray structure was
obtained for 14b, which confirmed the regiochemistry of the
cine product 11b obtained from the allyloxylation.[21] After
many unproductive attempts to improve the regiochemistry
by activating the double bond of the quinone with iodine in
the presence of transition metal salts (CeCl3 ,[22] Cu(OAc)2 ,[23]
or HgO[24]), we decided to investigate the effect of the leaving
group (X) on the regiochemical outcome of the key allylox-
ylation step.
Scheme 3. Top: X-ray crystal structure of 14b. Thermal ellipsoids are
set at 50% probability. Center: Leaving-group effect on the outcome of
step a. Bottom: Synthesis of naphthazarin 9: a) Cs2CO3, allyl alcohol
(excess), toluene, RT, 1 h, then filtration; b) microwave, 1408C, tolu-
ene, 1 h, 97%; c) NaN3, MeCN, RT, 3 days, then filtration and
evaporation; d) microwave, 1408C, toluene, 0.5 h, then evaporation;
e) chloromethylethyl ether (EOMCl), iPr2NEt, CH2Cl2, 08C!RT, 70%
from bromoquinone 10; f) 1) Na2S2O4, nBu4NBr, THF/H2O (1:1),
0.5 h; 2) MeI, Cs2CO3, Na2S2O4 (cat.), H2, DMF, 608C, 2 h, 92%; g)
2,6-lutidine, OsO4 (cat.), NaIO4, dioxane/water (3:1), 90%.
this reaction. Azidoquinone 12 underwent clean conversion
to the desired allyl ether 11a (normal substitution) as a single
product, whilst iodoquinone 13 reacted cleanly to afford only
the cine substitution product 11b. The high regioselectivity
observed can be attributed to a combination of steric and
ꢁ
bond-dipole effects. Thus, decreasing the steric bulk at the C
X bond (I > Br@ N3) facilitates the desired ipso substitution
at C-2. Also, if one considers the azide fragment as a
pseudohalogen (absolute electronegativity: I 6.7, Br 7.5, N3
7.7 eV),[25] increased C X bond polarization may also lead to
ꢁ
To this end, bromoquinone 10 was converted to azidoqui-
none 12 by using sodium azide, whilst iodoquinone 13 was
prepared in a similar manner to bromoquinone 10 (leaving-
group effect, Scheme 3). Pleasingly, the nature of the leaving
group dramatically affected the regiochemical outcome of
ipso attack being favored over cine substitution. While a
limited number of examples of the use of azide as a leaving
group have been reported,[26] its application to the synthesis of
complex molecules remains relatively unexplored. In the
present work, the highly substituted nature of the target
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naphthazarin necessitated extension of our synthetic reper-
toire to the use of an azide leaving group.
Subsequently, microwave irradiation of allyl ether 11a
was used to effect the desired Claisen rearrangement and
delivered naphthol 14a, which was subsequently protected as
an ethoxymethyl (EOM) ether 15. Significantly, the conver-
sion of bromoquinone 10 to 15 only required one purification
step and proceeded in 70% yield over four steps, thus
facilitating an efficient large-scale synthesis. Reductive dime-
thylation of 15 required the careful use of a reductive
environment (hydrogen atmosphere and sodium dithionite)
using cesium carbonate as base to overcome the considerable
steric constraints imposed when introducing additional
alkoxy groups onto the heavily substituted naphthalene
ring. One-pot oxidative cleavage[27] of the terminal olefin 16
subsequently provided aldehyde 9.
Scheme 5. Attempted lithiated acetylide coupling: a) LiHMDS, THF,
ꢁ788C, 1 h, 41%.
The synthesis of isocoumarin precursor 17 commenced
with commercially available guaiacol (18, Scheme 4). Para-
selective bromination, followed by protection as an EOM
Scheme 4. Syntheses of isocoumarin precursors, iodide 23, and acety-
lene intermediate 17: a) NBS, DMF, 08C, 0.5 h, 83%; b) EOMCl,
iPr2NEt, CH2Cl2, 08C!RT, 89%; c) LDA, THF, 08C, 0.5 h, then
dimethyl malonate, 08C, 0.5 h, then 19, THF, 08C, 71%; d) NaHSO4/
SiO2, CH2Cl2, RT, 16 h, 92%; e) NaHCO3, tetramethylammonium
dichloroiodate, CH2Cl2, RT, 2 h, 51%; f) EOMCl, iPr2NEt, CH2Cl2,
08C!RT, 93%; g) trimethylsilylacetylene, Et3N, CuI (15 mol%), PPh3
(25 mol%), Pd(OAc)2 (10 mol%), DMF, RT, 16 h, 98%; h) K2CO3,
THF/MeOH (1:1), RT, 1.5 h, 91%.
Scheme 6. Completion of advanced spiroketal intermediate 6: a) eth-
ynyl magnesium bromide (0.5m in THF), THF, 08C, 1 h, 90%; b) [Pd-
(PPh3)4] (10 mol%), CuI (15 mol%), Cs2CO3, DMF, 6 h, 8 (91%), 26
(trace); c) Pd/C, H2, NaHCO3, EtOAc, 3 h, 99%; d) IBX, DMSO, 608C,
0.5 h, 95%; e) NaHSO4/SiO2, CH2Cl2, RT, 4 h, 80%; f) 10% KOH,
MeOH, 08C!RT, 16 h; g) cerium(IV) ammonium nitrate, MeCN/H2O
(5:1), 08C, 0.5 h (58% over two steps).
ether provided bromophenol 19, which was then converted to
diester 20 by slow dropwise addition of LDA to dimethyl
malonate followed by addition of bromide 19.[18,28] After
removal of the EOM group, iodination of the resulting
naphthol 21 with tetramethylammonium dichloroiodate
afforded iodide 22. After reprotection as an EOM ether
(23), Sonogashira coupling with trimethylsilylacetylene fol-
lowed by desilylation under mild basic conditions provided
the key acetylene coupling partner 17.
With the key naphthazarin 9 and isocoumarin precursor
17 fragments in hand, their union by acetylide addition was
explored (Scheme 5). To our surprise, attempts to generate
the lithiated acetylide of isocoumarin precursor 17 afforded
adduct 24, which resulted from the aldol condensation of
aldehyde 9 with the ester enolate derived from 17.
acetylene 25 using a Sonogashira reaction. Aldehyde 9 was
converted to the key propargyl alcohol fragment 25 by
Grignard addition with ethynyl magnesium bromide. Pleas-
ingly, Sonogashira reaction of key alkyne 25 with iodide 23
proceeded in an excellent yield using a non-amine base
(cesium carbonate) and degassed solvents, in order to
minimize formation of the homocoupled product 26. Hydro-
genation of the resulting alkyne 8 in ethyl acetate buffered
with sodium bicarbonate over 10% Pd/C then provided the
saturated secondary alcohol in quantitative yield. Oxidation
Accordingly, the coupling strategy for the naphthalene
and isocoumarin fragments was revised (Scheme 6) so that
iodide 23 (prepared as depicted in Scheme 4) was coupled to
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Communications
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agreement with those recorded for an authentic sample kindly
provided by Professor Kita (see the Supporting Information).
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synthesis of (ꢀ )-g-rubromycin (1) in 17 steps from 1,2,4-
trimethoxybenzene. The synthesis is significantly shorter than
Kitaꢀs approach[18] and produces 1 in a much better overall
yield. The present synthesis demonstrates the previously
unrealized application of acid-mediated spiroketalization for
the synthesis of the challenging rubromycin family of natural
products. The synthetic route also makes the use of a novel
regioselective allyloxylation of a 2-azido-1,4-naphthoquinone
to facilitate a subsequent Claisen rearrangement. Further
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Received: June 19, 2009
Published online: July 30, 2009
Keywords: antibiotics · azides · natural product synthesis ·
.
spiroketals · total synthesis
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