Total synthesis of ripostatin A

Article abstract of DOI:10.1021/ol302219x

The first total synthesis of the bacterial RNA-polymerase inhibitor ripostatin A (1) was achieved. The route utilizes a cyclic methyl acetal intermediate and a sequence of a double Stille cross-coupling reaction followed by a ring-closing metathesis for t

Full text of DOI:10.1021/ol302219x

ORGANIC
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Total Synthesis of Ripostatin A
Wufeng Tang and Evgeny V. Prusov*
Helmholtz Centre for Infection Research, Inhoffenstr. 7, D-38124 Braunschweig,
Germany
Evgeny.Prusov@helmholtz-hzi.de
Received August 9, 2012
ABSTRACT
The first total synthesis of the bacterial RNA-polymerase inhibitor ripostatin A (1) was achieved. The route utilizes a cyclic methyl acetal
intermediate and a sequence of a double Stille cross-coupling reaction followed by a ring-closing metathesis for the construction of the
macrolactone ring. Additionally, an unprecedented formation of the 4-methoxy substituted tetrahydropyrans was observed during the acid
catalyzed acetalization of the β,δ-dihydroxyketone.
Overcoming the emerging resistance of pathogenic bac-
teria to the antibiotics currently in use requires a contin-
uous exploration of new cellular targets. In this regard, the
bacterial RNA-polymerase is considered as an under-
exploited target, with only one marketed drug existing at
present. Among several natural products with bacterial
RNA-polymerase inhibitory activity (Figure 1), ripostatins1
A (1) and B (2) attracted a considerable amount of synthetic
efforts from high profile research groups.2 Since the ripos-
tatins, along with corallo- and myxopyronin (4), bind to the
RNA-polymerase at a different binding site compared to
rifampicin (3),3 they were regarded as promising lead
structures for the development of new antibiotic agents.4
The chemical challenges in the synthesis of these mole-
cules arise from the sensitive nature of skipped polyene
motifs within the macrocycle (C2ꢀC9). Additionally, ripos-
tatin A is inherently unstable under even slightly basic
conditions and undergoes a ring-opening β-elimination lead-
ing to the inactive ripostatin C.1b Recently, three total syn-
theses of ripostatin B were independently accomplished.5 We
also reported the synthesis of 15-deoxyripostatin A, which
was used as a molecular probe to determine the structural
requirements for efficient binding to RNAP, and found it to
be inactive against several bacterial strains, thus underlining
the importance of the carbonyl or hydroxyl group in the C-15
position for the RNA-polymerase inhibitory activity. To
make our SAR studies of ripostatin analogs more compre-
hensive, an access to the ripostatin A and its modified
derivatives was required. Herein we report further develop-
ment of our synthetic strategy and its application in the total
synthesis of ripostatin A.
€
(1) Isolation: (a) Irschik, H.; Augustiniak, H.; Gerth, K.; Hofle, G.;
In our retrosynthetic analysis we opted to protect the
ketone functionality of ripostatin A as a cyclic methyl
acetal (Figure 2). Construction of the macrocyclic lactone
would utilize a sequence of a double Stille cross-coupling
Reichenbach, H. J. Antibiot. 1995, 48, 787–792. Structure elucidation:
€
(b) Augustiniak, H.; Hofle, G.; Irschik, H.; Reichenbach, H. Liebigs
Ann. 1996, 1657–1663.
(2) (a) Kujat, C.; Bock, M.; Kirschning, A. Synlett 2006, 419–422. (b)
Schleicher, K. D. PhD thesis, Massachusetts Institute of Technology, 2010.
(3) Mukhopadhyay, J.; Das, K.; Ismail, S.; Koppstein, D.; Jang, M.;
Hudson, B.; Sarafianos, S.; Tuske, S.; Patel, J.; Jansen, R.; Irschik, H.;
Arnold, E.; Ebright, R. H. Cell 2008, 135, 295–307.
(5) (a) Winter, P.; Hiller, W.; Christmann, M. Angew. Chem., Int. Ed.
2012, 51, 3396–3400. (b) Tang, W. F.; Prusov, E. V. Angew. Chem., Int.
Ed. 2012, 51, 3401–3404. (c) Glaus, F.; Altmann, K. H. Angew. Chem.,
Int. Ed. 2012, 51, 3405–3409.
€
(4) Habich, D.; Nussbaum, F. Angew. Chem., Int. Ed. 2009, 48, 3397–
3400.
r
10.1021/ol302219x
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spectra revealed the presence of a second methoxy group in
the C-13 position of the tetrahydropyrane ring (Scheme 1,
ripostatin A numbering). Except for the original report on
ripostatin A isolation,1a we found no evidence in the
literature regarding such concomitant formation of the
methyl ether during the acetalization of β,δ-dihydroxyke-
tones. We speculate that the ether 8 forms via the acid
catalyzed water elimination followed by the addition of
methanol. In a control experiment, exposure of the sepa-
rately prepared R,β-unsaturated ketone 9 to the reaction
conditions resulted in formation of 8 in 53% yield.7
Scheme 1. CSA-Catalyzed Deprotection/Acetalizationa
Figure 1. Natural products with bacterial RNA-polymerase
inhibitory activity.
reaction and ring-closing metathesis. The required double
vinyliodide precursor can be obtained via esterification of
the β-idodoacrylic acid with a 4-hydroxy-THP-intermediate,
which, in turn, should be easily accessible from the corre-
sponding polyol fragment.
a CSA = D,L-10-camphorsulfonic acid, DCC = dicyclohexylcarbodiimide.
Being unable to achieve a direct synthesis of methyl
acetal, we decided to employ a two-step protocol. Treat-
ment of the aldol adduct 7 with TBAF in DMF8 led to a
clean conversion into the corresponding β,δ-dihydroxyke-
tone, which upon exposure to PPTS in a mixture of MeOH
and HC(OMe)3 undergoes clean cyclization into the de-
sired intermediate 10 (Scheme 2). We next examined the
use of a modifiedYamaguchi protocol9 for esterification of
the iodoacrylic acid 11 as it was done in the synthesis of
ripostatin B. However, a direct application of the earlier
established conditions afforded the target ester 12 with
yields varying between 45% and 51%, along with several
byproducts which arise from HI elimination (13) and
subsequent water addition (14).
Seeking to improve this step, we studied the acylation of
sodium alkoxide10 derived from the alcohol 10 with the
corresponding acid chloride but obtained a significantly
lower yield of the desired ester.
Figure 2. Retrosythetic analysis of ripostatin A.
During our synthesis of 15-deoxyripostatin A we estab-
lished that Mitsunobu reaction11 provides superior results
Taking advantage of the correctly positioned carbonyl
group inthe Patersonaldol6 adduct7,5b a directconversion
to the corresponding cyclic acetal was attempted. Whereas
no reaction occurred with PPTS as a catalyst, exposure
of the hydroxyketone 7 to CSA in methanol resulted in
complete consumption of the starting material and forma-
tion of a new product. However, analysis of its NMR
(7) Complete details of this investigation will be published in a
separate communication.
ꢀ ꢀ
(8) Henryon, V.; Liu, L. W.; Lopez, R.; Prunet, J.; Ferezou, J.-P.
Synthesis 2001, 2401–2414.
(9) Inanaga, J.; Hirata, K.; Saeki, H.; Katsuki, T.; Yamaguchi, M.
Bull. Chem. Soc. Jpn. 1979, 52, 1989–1993.
ꢀ
(10) Zhu,W.;Jimenez, M.; Jung, W.-H.; Camarco, D. P.; Balachandran,
R.; Vogt, A.; Day, B. W.; Curran, D. P. J. Am. Chem. Soc. 2010, 132, 9175–
9187.
(6) (a) Paterson, I.; Goodman, J. M.; Lister, M. A.; Schumann, R. C.;
McClure, C. K.; Norcross, R. D. Tetrahedron 1990, 46, 4663–4684.
(b) Paterson, I.; Coster, M. J.; Chen, D. Y.-K.; Gibson, K. R.; Wallace,
D. J. Org. Biomol. Chem. 2005, 3, 2410–2419. (c) Cowden, C. J.;
Paterson, I. Org. React. 1997, 51, 1–200.
(11) For a review, see: (a) Swamy, K. C.; Kumar, N. N.; Balaraman,
E.; Kumar, K. V. Chem. Rev. 2009, 109, 2551–2651. For examples of use
in the total synthesis, see: (b) Custar, D. W.; Zabawa, T. P.; Hines, J.;
Crews, C. M.; Scheidt, K. A. J. Am. Chem. Soc. 2009, 131, 12406–12414.
(c) Martin, S. F.; Dodge, J. A. Tetrahedron Lett. 1991, 32, 3017–3020.
B
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the methyl acetals 10 and 16 were separately converted to
the ester 12 via Yamaguchi or Mitsunobu esterification
methods, respectively.
Scheme 2. Esterification of the Cyclic Acetal Core
According to our plan, two terminal allylic groups were
simultaneously introduced via Stille cross-coupling13 with
allyltributylstannane and the resulted diene 17 was sub-
jected to the ring-closing metathesis reaction14 under
standard conditions to give the macrocyclic lactone 18 in
71% yield as a single geometrical isomer (Scheme 4).
Deprotection of the primary TBS group using the
HF•Et3N complex in THF followed by implementation
of the one-pot Dess-Martin oxidation/Pinnick oxidation
protocol reported by Altmann5c provided us with the
earlier described ripostatin A methyl acetal. The last chal-
lenge was to achieve a mild hydrolysis of the cyclic acetal 19
in the presence of a rather sensitive skipped polyene motif.
Several conditions were investigated in order to liberate the
ketone group of ripostatin A, and it was found that simple
exposure of 19 to the aqueous dioxane resulted in a clean
conversion to the target compound.
for the ester formation in the case of the 4-hydroxytetra-
hydropyran system. Since Mitsunobu esterification pro-
ceeds with the inversion of configuration at the alcohol
position, preparation of the diastereomeric β-hydroxyketone
utilizing an enantiomeric enolization agent in the Paterson
aldol reaction was investigated (Scheme 3). Unexpectedly, a
nearly 1:1 mixture of diastereomeric adducts was obtained,
which implies the existence of a strong substrate facial
selectivity bias in this particular reaction.12 The aldol adducts
were carried through two following steps and separated at
the stage of cyclic methyl acetals 10 and 16.
Scheme 4. Endgame of the Synthesis
Scheme 3. Alternative Route to the Cyclic Acetal Core
From our previous experience with ripostatin B and
some other natural products we learned that the appear-
ance of 1H and 13C NMR spectra of these compounds can
be greatly affected by exact isolation and purification
procedures. In order to perform a correct comparison of
spectroscopic data of our material with those reported in
the literature, a predominantly keto-form of ripostatin A
was obtained by slow evaporation of its waterꢀmethanol
Here, several attempts were made to perform the isomer-
ization of 10 to 16 by ester formation with p-nitrobenzoic or
acetic acid and following ester cleavage. However, the
overall yield of these sequences was rather low. Therefore,
^
(13) (a) Abarbria, M.; Parrain, J.-L.; Duchene, A. Tetrahedron Lett.
1995, 36, 2469–2472. (b) Rivkin, A.; Njardarson, J. T.; Biswas, K.; Chou,
T.-C.; Danishefsky, S. J. J. Org. Chem. 2002, 67, 7737–7740.
€
(12) This result not only contradicts the expected outcome of the
Paterson aldol reaction, but also, according to the polar Cornforth
model, an 1,3-anti aldol adduct should be favorable in the case of
β-silyloxy aldehydes. At present, we have no explanation for this
phenomenon.
(14) For reviews on ring-closing metathesis, see: Furstner, A. Angew.
Chem., Int. Ed. 2000, 39, 3012–3043. (b) Nicolaou, K. C.; Bulger, P. G.;
Sarlah, D. Angew. Chem., Int. Ed. 2005, 44, 4490–4527. Angew. Chem.
2005, 117, 4564–4601. (c) Gradillas, A.; Perez-Castells, J. Angew. Chem.,
Int. Ed. 2006, 45, 6086–6101. Angew. Chem. 2006, 118, 6232–6247.
Org. Lett., Vol. XX, No. XX, XXXX
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solution. The resulted amorphous solid was dried in vacuo,
1
cross-coupling reaction, and ring-closing metathesis.
The success of this synthesis confirmed the applicability
of our strategy for the preparation of macrocyclic skipped
trienes. Additionally, a rather interesting and earlier
undescribed formation of methyl ethers during acid cat-
alyzed acetalization was discovered. Further applications
of this general approach to the synthesis of other stabi-
lized analogs of ripostatin A and B are currently under-
way in our laboratory and will be reported in due course.
and both H and 13C NMR spectra were recorded im-
mediately after sample preparation. The measurements
were repeated after 48 h and revealed, as expected, the
presence of a 4:3 equilibrium mixture of keto- and cyclic
acetal forms.15
In summary, the first total synthesis of the bacterial
RNA-polymerase inhibitor ripostatin A was accom-
plished. The longest linear sequence consist of 14 steps
(starting from S-epichlorohydrine) and proceeds with 5%
overall yield. The key steps in the synthesis are a Paterson
aldol reaction, Mitsunobu esterification, double Stille
Acknowledgment. Financial support from DFG (Grant
PR1328-1/1 to E.V.P.) is gratefully acknowledged.
(15) The spectroscopic properties of synthetic 1 were in good agree-
ment with the literature data for ripostatin A (1H and 13C NMR,
HRMS, UV, and optical rotation).1b The 1H NMR spectrum of our
material corresponds exactly with that of the natural product (a graphi-
cal comparison is provided in the Supporting Information). Deviation of
some 13C-resonances can be explained by concentration effects and
residual amounts of buffer components present in the original specimen.
[R]2D0 = þ15.2 (c = 0.5 in MeOH); lit. [R]2D0 = 15.1 (c = 1 in MeOH).
Supporting Information Available. Experimental pro-
cedures and full spectroscopic data for all new com-
pounds. This material is available free of charge via the
Internet at http://pubs.acs.org.
The authors declare no competing financial interest.
D
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