Total synthesis of archazolid A

Article abstract of DOI:10.1021/ja071461o

The archazolids are complex polyketides isolated from the myxobacterium Archangium gephyra and are potent inhibitors of vacuolar type ATPases. Herein, we report the first total synthesis of archazolid A, which establishes unequivocally the relative and ab

Full text of DOI:10.1021/ja071461o

Published on Web 04/25/2007
Total Synthesis of Archazolid A
Dirk Menche,* Jorma Hassfeld,† Jun Li, and Sven Rudolph
Helmholtz-Zentrum fu¨r Infektionsforschung,‡ Medicinal Chemistry, Inhoffenstrasse 7, 38124 Braunschweig, Germany
Received March 1, 2007; E-mail: dirk.menche@helmholtz-hzi.de
Scheme 1 . Retrosynthetic Analysis of Archazolid A
The archazolids are structurally unique macrolides first isolated
by Ho¨fle et al. from the myxobacterium Archangium gephyra.1 They
display powerful growth-inhibitory activity against a number of
murine and human cancer cell lines at subnanomolar concentra-
tions,1 based on selective inhibition of vacuolar-type ATPases, in
vitro2 and in vivo.1 These multimeric proton translocating enzymes
present important targets from the perspective of medicinal
chemistry as their malfunction is associated with various diseases
such as cancer, osteoporosis, and renal acidosis.3 The archazolids
bind selectively to the membrane-bound Vo subunit c in a reversible,
noncovalent fashion,2 which adds to the attractiveness for further
development. Their unique structures comprise a polyunsaturated
24-membered macrolactone with 8 stereogenic centers and a
pendant thiazole side chain at C23. Recently, we proposed a full
stereochemical assignment, as indicated in 1 for archazolid A, the
most potent archazolid, (Scheme 1), by the use of extensive high-
field NMR experiments in combination with molecular modeling
and chemical derivatization.4 Herein, we disclose the first total
synthesis of archazolid A (1) and establish unequivocally its relative
and absolute configuration.
Scheme 2. Synthesis of the C3-C13 Subunit 3
As outlined retrosynthetically in Scheme 1, our synthetic
approach relies on assembly of three main building blocks of similar
complexity, that is, 2, 3, and 4. The 13E-alkene moiety was planned
to arise from an aldol condensation between methyl ketone 2 and
aldehyde 3, while a Heck cross-coupling of 2 with alkene 4 was
envisioned to deliver the 18E,20E-diene. In principle, this meth-
odology could be employed to close the macrocycle as an alternative
to a Horner-Wadsworth-Emmons macrocyclization or a more
conventional Yamaguchi reaction for ring closure, thus offering
considerable flexibility in the synthesis. Notably, the modular
synthetic approach employed is flexible, highly convergent, and
stereocontrolled, and thus offers the potential to provide useful
quantities of archazolid A as well as a range of structural derivatives
for SAR-studies.5
Scheme 3 . Preparation of the C14-C19 Subunit 2
As shown in Scheme 2, our synthesis of the C3-C11 subunit 3
utilizes a boron-mediated Paterson aldol reaction6 of lactate derived
ethyl-ketone 5 with readily available aldehyde 6 to give anti-aldol
7 with very high levels of diastereoselectivity and yield. After TBS
protection, aldehyde 8 was then generated by reduction and
periodate cleavage (85% from 7) and subsequently submitted to a
Still-Gennari modification of the HWE olefination with phospho-
nate 9.7 By employing KHMDS as the base in combination with
18-crown-6, Z-enone 10 was obtained in 88% yield as the only
detectable isomer. After conversion of 10 into enal 11 by DIBAl-H
reduction and allylic oxidation (MnO2), the required 11E-alkene
was installed by another Still-Gennari olefination, proceeding again
with very high stereoselectivity (ds >20:1) and yield (87%). The
synthesis of 3 was completed in two steps involving ester reduction
(DIBAL-H) and oxidation of the resulting primary alcohol with
Dess-Martin periodinane.
As shown in Scheme 3, construction of the C14-C19 subunit
2, starts with the E-vinyliodide 13, which was conveniently prepared
by a known method.8 After conversion to aldehyde 14 (85% yield),
the pivotal anti aldol coupling to install the centers at C16 and
C17 proceeded with excellent diastereoselectivity and yield (ds >
† Present address: Bayer Schering Pharma AG, Mu¨llerstrasse 178, D-13342
Berlin, Germany..
‡ Former name: Gesellschaft fu¨r Biotechnologische Forschung (GBF).
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J. AM. CHEM. SOC. 2007, 129, 6100-6101
10.1021/ja071461o CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
Scheme 4 . Assembly of the C20-C1′′ Subunit 4
(∼1.5:1). Gratifyingly, after evaluating different catalysts, additives,
and solvents, preparatively useful selectivity (6:1) was obtained,
by performing the reaction at 80 °C in the presence of TBACl and
H2O. Notably, this conversion constitutes one of the first examples
of controlling the E-to Z-ratio in a Heck coupling on such an
elaborate substrate.13 After attachment of phosphonate 26 by use
of BOP, oxidative removal of the PMB group, and Swern oxidation,
the resulting keto-phosphonate 27 was successfully cyclized by
employing NaH as base. For the required reduction of the C15-
ketone, best results in terms of diastereoselectivity and yield were
obtained by use of oxazaborolidine-assisted borane reduction (ds
>20:1, 73%).14 Finally, deprotection with HF/pyridine in THF gave
archazolid A (1) in 80% yield. The spectroscopic data (1H NMR,
13C NMR) and specific rotation of our synthetic material were in
agreement with those published for an authentic sample of
archazolid A,1 thus allowing confident assignment of the relative
and absolute configuration of archazolid A and validating our earlier
proposal.4
Scheme 5 . Completion of the Synthesis
In conclusion, this expedient first total synthesis of archazolid
A proceeds in 20 steps and 4% overall yield (longest linear
sequence) and establishes unequivocally the relative and absolute
configuration. Notable features include highly enantio- and dias-
tereoselective anti aldol reactions, an aldol condensation for
construction of the delicate (Z,Z,E)-triene-system, an advantageous
E-selective Heck-coupling on a highly elaborate substrate and a
subsequent HWE macrocyclization. Importantly, this modular,
convergent synthesis should be amenable to designed analogues
of this novel V-ATPase inhibitor, thus enabling extensive explora-
tion of its biological potential.
Acknowledgment. We thank the “Fonds der Chemischen
Industrie”, the “VW-Stiftung” and the DFG for generous funding
and Antje Ritter and Henning Sto¨ckmann for technical support.
Supporting Information Available: Experimental procedures,
1
characterization data, and H and 13C NMR spectra. This material is
available free of charge via the Internet at http://pubs.acs.org.
References
20:1, 96%) by use of Masamune’s chiral ephedrine-derived ethyl
ketone.9 After methylation of the 17-OH of 16 with Ag2O/MeI,
removal of the ephedrine auxiliary was best performed reductively
(LiAlH4) to give 17, which was converted to fragment 2 by DMP-
oxidation, addition of MeMgBr and DMP-oxidation in 80% yield
over three steps.
Our preparation of the C11-C1′′ subunit 4, as shown in Scheme
4, starts with readily available R-hydroxyacid 18,10 which was
converted to thioamide 19 in four-steps and 58% yield, by amide
formation, treatment with TBSCl and the Lawesson reagent. After
cyclization with 20 and liberation of the 1′-hydroxyl with TBAF
(76%), the carbamate was introduced in two steps on thiazol 2111
by the use of carbonyldiimidazole and trapping of the activated
carbamate with methylamine. After DIBAl-H reduction of ester 22
to aldehyde 23, the desired alcohol 4 was prepared with excellent
diastereoselectivity and useful yield (65%) through Brown’s asym-
metric crotylation protocol.12
In a rationale to install the presumably4 labile (2,5)-enoate unit
of archazolid A (Scheme 1) in succeeding reactions, our strategy
for fragment union relied on first combining 2 and 3 (Scheme 5).
This was accomplished by employing a boron-mediated aldol
reaction followed by a two-step elimination to give 24 in 94% yield.
Subsequent Heck reaction of 24 with 4 under more conventional
conditions, however, gave 25 with only poor E/Z-diastereoselectivity
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(11) On large scale, minor amounts of epimerization at C1′ occurred during
cyclization. This can be readily remedied by an oxidation-reduction
sequence (90%) (see Supporting Information).
(12) Brown, H. C.; Bhat, K. S.; Randad, R. S. J. Org. Chem. 1989, 54, 1570.
(13) Depending on the conditions also some isomerization along the labile
(9Z,11Z,13E) triene system, that is, the 11Z-double bond, may occur. This
minor diastereomer can be removed after macrocyclization. Notably, after
cyclization the triene system is more stable toward isomerization.
(14) Corey, E. J.; Helal, C. J. Angew. Chem., Int. Ed. 1998, 37, 1986.
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