Synthesis of cruentaren A

Article abstract of DOI:10.1021/ol302999v

Cruentaren A, an antifungal benzolactone produced by the myxobacterium Byssovorax cruenta, is highly cytotoxic against various human cancer cell lines and a highly selective inhibitor of mitochondrial F-ATPase. A convergent and efficient synthesis of cruentaren A is reported, based upon a diastereoselective alkylation, a series of stereoselective aldol reactions utilizing Myers' pseudoephedrine propionamide, an acyl bromide mediated esterification, and a ring-closing metathesis (RCM) as the key steps. The RCM reaction was applied for the first time toward the total synthesis of cruentaren A, which led to a convergent and efficient synthesis of the natural product.

Full text of DOI:10.1021/ol302999v

ORGANIC
LETTERS
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Synthesis of Cruentaren A
Bhaskar Reddy Kusuma,† Gary E. L. Brandt,† and Brian S. J. Blagg*
Department of Medicinal Chemistry, The University of Kansas, 1251 Wescoe Hall Drive,
Malott 4070, Lawrence, Kansas 66045-7562, United States
bblagg@ku.edu
Received October 31, 2012
ABSTRACT
Cruentaren A, an antifungal benzolactone produced by the myxobacterium Byssovorax cruenta, is highly cytotoxic against various human cancer
cell lines and a highly selective inhibitor of mitochondrial F-ATPase. A convergent and efficient synthesis of cruentaren A is reported, based upon
a diastereoselective alkylation, a series of stereoselective aldol reactions utilizing Myers’ pseudoephedrine propionamide, an acyl bromide
mediated esterification, and a ring-closing metathesis (RCM) as the key steps. The RCM reaction was applied for the first time toward the total
synthesis of cruentaren A, which led to a convergent and efficient synthesis of the natural product.
Cruentaren A and its isomeric analogue, cruentaren B,
have been identified as the first novel structures isolated
from the fermentation broths of Byssovorax cruenta by
Hofle et al.1,2 Cruentaren A (1) was the major natural
product, which was isolated through bioassay guided
fractionation that focused upon the identification of novel
antifungal and cytotoxic agents. While cruentaren A de-
monstrated potentcytotoxic effects (IC50 of 8.3nMagainst
L929 mouse fibroblasts), its closely related derivative,
cruentaren B, was only marginally active.3
Cruentaren A is a member of the growing class of
benzolactone natural products that contains a resorcinol
derived 12-membered macrocyclic lactone and an N-acy-
Figure 1. Benzolactone natural product family.
lallylamine side chain (Figure 1).4 Despite structural simi-
larities to other natural products of this class, cruentaren
A possesses a unique mechanism of action. This natural
attention from the synthetic community. Four successful
total syntheses and numerous approaches to fragments
product induces cytotoxicity through selective inhibition
have been reported.5À8 In fact, all published syntheses rely
of F-ATPase but is devoid of inhibitory activity against
other ATPases.1À3
Due to the interesting architecture and a unique bio-
logical profile, cruentaren A has attracted significant
upon the ring-closing alkyne metathesis (RCAM) reaction
as a key step, followed by Lindlar reduction to introduce
(5) (a) Bindl, M.; Jean, L.; Hermann, J.; Muller, R.; Furstner, A.
Chem.;Eur. J. 2009, 15, 12310–12319. (b) Furstner, A.; Bindl, M.; Jean,
L. Angew. Chem., Int. Ed. 2007, 46, 9275–9278.
(6) (a) Vintonyak, V. V.; Cala, M.; Lay, F.; Kunze, B.; Sasse, F.;
Maier, M. E. Chem.;Eur. J. 2008, 14, 3709–3720. (b) Vintonyak, V. V.;
Maier, M. E. Org. Lett. 2007, 9, 655–658. (c) Vintonyak, V. V.; Maier,
M. E. Angew. Chem., Int. Ed. 2007, 46, 5209–5211.
(7) Fouche, M.; Rooney, L.; Barrett, A. G. M. J. Org. Chem. 2012,
77, 3060–3070.
† These authors contributed equally.
(1) Jundt, L.; Steinmetz, H.; Luger, P.; Weber, M.; Kunze, B.;
Reichenbach, H.; Hofle, G. Eur. J. Org. Chem. 2006, 22, 5036–5044.
(2) Kunze, B.; Steinmetz, H.; Hofle, G.; Huss, M.; Wieczorek, H.;
Reichenbach, H. J. Antibiot. 2006, 59, 664–668.
(3) Kunze, B.; Sasse, F.; Wieczorek, H.; Huss, M. FEBS Lett. 2007,
581, 3523–3527.
(4) Boyd, M. R.; Farina, C.; Belfiore, P.; Gagliardi, S.; Kim, J. W.;
Hayakawa, Y.; Beutler, J. A.; McKee, T. C.; Bowman, B. J.; Bowman,
E. J. J. Pharmacol. Exp. Ther. 2001, 297, 114–120.
(8) Ramalinga, B.; Prasad, V.; Meshram, H. M. Tetrahedron: Asym-
metry 2010, 21, 1837–1844.
r
10.1021/ol302999v
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the cis-olefin that is present within the macrocyclic ring.
Due to the publication of prior existing routes, we pursued
an alternate method that does not rely upon the RCAM in
an effort to ultimately produce analogs that are not easily
obtained through other routes. Herein, we report the total
synthesis of cruentaren A that utilizes the RCM reaction
for construction of the desired cis-macrocyclic product.
under Myers’ optimized conditions11 to provide the dia-
stereomerically enriched methyl amide 10 (>20:1), which
upon subsequent lithium aluminum hydride reduction
gave the corresponding aldehyde, 11. Induction of stereo-
chemistry at C-16 and C-17 was accomplished through
use of a zirconium-mediated aldol reaction9 between the
(Z)-enolate of propionamide 12 and aldehyde 11 to give
diastereomerically enriched 13 (Scheme 1).
Carboxylic acid 14 was accessed by mild hydrolysis of
13 followed by conversion to the Weinreb amide 15 using
the (1-cyano-2-ethoxy-2-oxoethylidenaminooxy)dimethyl-
amino-morpholino-carbenium hexafluorophosphate
(COMU) coupling reagent. COMU is a third generation
uronium-type coupling reagent that is commonly used for
solution-phase peptide synthesis; the byproducts pro-
duced with COMU are water-soluble and easily removed.
Furthermore, COMU exhibits a less hazardous safety
profile than benzotriazole-based reagents and prevents
recemization during the coupling event.12 The secondary
alcohol of 15 was protected as the tert-butyldimethylsilyl
ether before subsequent reduction of the amide to give
aldehyde 17. Upon treatment of 17 with in situ generated
Soderquist’s allylborane intermediate that was generated
upon exposure of an allyl Grignard to 18,13 a highly
enantioenriched homoallylic alcohol 7 was produced with
excellent facial selectivity.
Figure 2. Retrosynthtic analysis of cruentaren A (1).
Scheme 1. Synthesis of Allylic Aclohol 7
Retrosynthetic analysis of cruentaren A (1), utilizing
the RCM reaction for construction of the 12-membered
lactone, required fragments 2 and 3 as relevant synthons.
Weinreb amide 5 could then be reacted with the benzylic
anion of 6 to provide ketone intermediate 4, which would
enable subsequent metathesis to form 2. Likewise, esterifi-
cation of secondary alcohol 7 would provide substrate 6
and, after RCM, the macrocycle. Staudinger ligation be-
tween the allylazide of 2 and acid 3 was envisioned to occur
before global deprotection to provide the natural product.
Successful stereochemical induction with Myers’ pseudo-
ephedrine chiral auxiliary would be required throughout this
synthetic strategy. Specifically, the synthesis of allyl alcohol
7 was envisioned to depend upon Myers’ psueodoephedrine
to construct all three stereocenters through sequential reac-
tions (Figure 2). Myers’ psueodoephedrines are readily
available from inexpensive, commercially available reagents
via a simple one-step process, and furthermore, these chiral
auxiliaries can be elaborated into a variety of useful entities
through well-described synthetic protocols.9
Stereochemical assignment of advanced intermediate 7
was accomplished by Mosher’s ester analysis,14 which
validated the desired syn stereochemistry of the 1,3-diol
relationship for 7. To determine the relative stereochemical
In the event, alkylation of (S,S)-pseudoephedrine pro-
pionamide 9 withcis-allyl bromide 810 proceeded smoothly
(9) Maier, M. E.; Ritschel, J. ARKIVOC 2008, xiv, 314–329.
(10) Sreedhar, E.; Venkanna, A.; Chandramouli, N.; Babu, K. S.;
Rao, J. M. Eur. J. Org. Chem. 2011, 6, 1078–1083.
(11) (a) Myers, A. G.; Yang, B. H.; Chen, H.; McKinstry, L.;
Kopecky, D. J.; Gleason, J. L. J. Am. Chem. Soc. 1997, 119, 6496–
6511. (b) Rodriguez, M.; Vicario, J. L.; Badia, D.; Carrillo, L. Org.
Biomol. Chem. 2005, 3, 2026–2030.
(12) (a) El-Faham, A.; Funosas, R. S.; Prohens, R.; Albericio, F.
Chem.;Eur. J. 2009, 15, 9404–9416. (b) Fahama, E. L.; Albericioa, F.
J. Pept. Sci. 2010, 16, 6–9.
(13) Canales, E. K.; Prasad, G.; Soderquist, J. A. J. Am. Chem. Soc.
2005, 127, 11572–11573.
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relationship of the C-16 methyl group, acid-mediated TBS
hydrolysis and subsequent acetonide protection of the
resultant 1,3-diol enabled the formation of 19, which was
shown to exhibit the correct stereochemical relationship as
deterimined by Rynchnovsky’s method (Scheme 2).15
of time and effort, synthesis of ester 6 was finally
achieved upon treatment of acid 2 with oxalyl bromide
(COBr)2, DIPEA, and catalytic DMF in DCM at 0 °C
for 30 min, followed by the addition of alcohol 3 and
4-(dimethylamino)pyridine (DMAP). Complete conver-
sion to the desired ester 6 occurred within 10 min in high
yield. Alkylation of 6 with Weinreb amide 5 proceeded by
generation of the benzylic anion of 6 to produce advanced
intermediate 4 in a reasonable yield (Scheme 4).
Scheme 2. Stereochemistry Confirmation at C-16 Carbon
Prior to RCM of the terminal olefins, stereoselective
reduction of ketone 4 was considered. Attempts to affect
stereoselective reduction of ketone 4 under various condi-
tions enlisting the CBS reagent were unsuccessful as 4 was
highly resistant to reduction under these constraints. How-
ever, reduction of ketone 4 utilizing Noyori’s asymmetric
transfer hydrogenation conditions proved successful.17,18
In the event, ketone 4 was treated with sodium formate
and catalytic RuCl[(S,S)-Tsdpen](p-cymene) in DMF and
water to generate alcohol 23 in quantitative yield and
reasonable diastereomeric selectivity (∼5:1). Unfortu-
nately, the epimeric product was not separable via column
chromatography at this stage (Scheme 4).
The synthetic route to provide Weinreb amide fragment
5 was straightforward and provided a rapid means to
generate significant quantities of this intermediate for the
optimization of latter steps. Alkylation of propionamide
12 with allyl bromide under Myers’ conditions generated
R-methyl amide, 20 (dr >20:1). Hydrolysis and sub-
sequent COMU-mediated coupling produced Weinreb
amide 5 without loss of enantioenrichment on multigram
scale. A similar zirconium-mediated aldol reaction be-
tween propionamide 9 and butanal as described above to
provide amide 22, followed by hydrolysis, gave carboxylic
acid 3 without diastereomeric loss. The successful COMU-
mediated coupling reaction16 suggested that the hydroxyl
group of compound 3 did not require protection. There-
fore, the free alcohol of 3 was left unprotected (Scheme 3).
Scheme 4. Synthesis of Key Macrolactone 25
Scheme 3. Synthesis of the Weinreb Amide 5 and Side Chain
Acid 3
Construction of the macrocyclic ring involved esterifica-
tion of allylic alcohol 7 with commercially available ben-
zoic acid 22. Synthesis of ester 6 proved to be challenging,
and unsuccessful results were obtained following the
methods reported by Furstner and Vintonyak for the
related alkynyl derivative.4,5 After a significant investment
Consequently, a mixture of diastereomeric alcohols (23)
was subjected to RCM catalysis, and gratifyingly, the cis-
olefin containing macrocycle was obtained in diastereo-
merically pure form, and the alcohol epimers were readily
separable via column chromatography. Unfortunately, an
unexpected side reaction did occur, in which cis to trans
isomerization of the allylic ether side chain was observed.
Several solvents, including THF, benzene, and methanol,
(14) (a) Rieser, M. J.; Hui, Y. H.; Rupprecht, J. K.; Kozlowski, J. F.;
Wood, K. V.; McLaughlin, J. L.; Hanson, P. R.; Zhuang, Z.; Hoye, T. R.
J. Am. Chem. Soc. 1992, 114, 10203–10213. (b) Hoye, T. R.; Jeffrey,
C. S.; Shao, F. Nat. Protoc. 2007, 2, 2451–2458.
(15) Rychnovsky, S. D.; Richardson, T. I. J. Org. Chem. 1997, 62,
2925–2934.
(17) Gladiali, S.; Alberico, E. Chem. Soc. Rev. 2006, 36, 226–236.
(18) Noyori, R.; Hashiguchi, S. Acc. Chem. Res. 1997, 30, 97–102.
(16) Samarasimhareddy, M.; Hemantha, H. P.; Ananda, K.;
Sureshbabu, V. V. Protein Pept. Lett. 2012, 19, 406–410.
Org. Lett., Vol. XX, No. XX, XXXX
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were screened; however, the most minimal isomerization
occurred in DCM. The optimized reaction conditions
included the addition of 5 mol % of catalyst at 0 °C to
a 0.5 mM solution of alcohol 23 in DCM, followed by
warming to 20 °C. RCM was complete after 3.5 h but
required treatment with saturated aqueous potassium
carbonate (K2CO3) for catalyst deactivation in order to
prevent additional isomerization during workup. These
reaction conditions furnished the epimerically pure cis-
macrocyclic product, 24, in 79% yield as a 4:1 (cis/trans)
mixture of the olefinic side chain isomers. Compound 24
was then sequentially TBS-protected at C-8 to generate 25,
followed by PMB-removal to give allyl alcohol 26.
Scheme 5. Conversion of 25 to Cruentaren A (1)
Conversion of 26 to cruentaren A and completion of
the total synthesis involved four additional manipula-
tions (Scheme 5). The allylic alcohol of 26 was converted
to the corresponding allyl-azide (2) upon treatment
with Zn(N3)2(pyridine)2,19 diisopropyl azodicarboxylate,
and triphenylphosphine. One-pot azide reduction and
COMU-mediated amide formation was achieved via
Staudinger ligation conditions to provide the protected
variant of cruentaren A (27) as a single diastereomer.
Cleavage of the C-3 methyl ether was achieved by subject-
ing compound 27 to boron trichloride in DCM at low
temperature, followed by removal of the corresponding
bis(TBS-ether) with HF-pyridine in acetonitrile to furnish
cruentaren A (1). The spectroscopic data for synthetic
cruentaren A (1) were in agreement with data reported
for the natural product.1
Additional studies for cruentaren A and analogues are
currently under investigation with the goal of more
thoroughly understanding the role F-ATPase plays in the
maturation of nascent polypeptides.20À22 The results from
such studies will be disclosed in due course.
In summary, a convergent and efficient total synthesis
of cruentaren A has been described with a longest linear
sequence of 18 steps. The synthesis is dependent upon the
use of Myers’ diastereoselective alkylation, a series of
stereoselective aldol reactions utilizing pseudoephedrine
propionamide, a Soderquist allyaltion, an acyl bromide
mediated esterification, and RCM as the key steps.
Acknowledgment. The authors gratefully acknowledge
the support of this project by the NIH (CA109265).
G.E.L.B. acknowledges the Madison and Lila Self
Graduate Fellowship and the American Foundation for
Pharmaceutical Education for financial support.
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.
(19) Viaud, M. C.; Rollin, P. Synthesis 1990, 130–132.
(20) Papathanassiu, A. E.; MacDonald, N. J.; Bencsura, A.; Vu,
H. A. F. Biochem. Biophys. Res. Commun. 2006, 345, 419–429.
(21) Papathanassiu, A. E.; MacDonald, N. J.; Emlet, D. R.; Vu,
H. A. Cell Stress Chaperones 2011, 16, 181–193.
(22) Francis, B. R.; Thorsness, P. E. Mitochondrion 2011, 11, 587–
600.
The authors declare no competing financial interest.
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