Total Synthesis and Structural Revision of (+)-Muironolide A

Article abstract of DOI:10.1021/jacs.5b03531

Muironolide A is a fascinating tetrachlorinated marine polyketide isolated from the sponge of Phorbas sp. Only 90 μg had been isolated, and the structure was established by nanoscale NMR techniques. Herein we report the total synthesis of the substance with the assigned structure of muironolide A, propose a revised structure based on NMR data, and complete the enantioselective total synthesis of muironolide A.

Full text of DOI:10.1021/jacs.5b03531

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Communication
Total Synthesis and Structural Revision of (+)-Muironolide A
Qing Xiao, Kyle Young, and Armen Zakarian
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 30 Apr 2015
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Total Synthesis and Structural Revision of (+)-Muironolide A
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†
,‡
†
Qing Xiao, Kyle Young, and Armen Zakarian*
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Department of Chemistry and Biochemistry, University of California, Santa Barbara, California 93106, USA.
Supporting Information Placeholder
ment of stereochemistry at C21 and establishing the
absolute stereochemistry as the enantiomer of 3.
Although the initially assigned structure did not
ABSTRACT: Muironolide A is a fascinating tetra-
chlorinated marine polyketide isolated from the
sponge of Phorbas sp. Only 90 ꢀg had been isolat-
ed, and the structure was established by nanoscale
NMR techniques. Herein we report the total syn-
thesis of the substance with the assigned structure
of muironolide A, propose a revised structure based
on NMR data, and complete the enantioselective
total synthesis of muironolide A.
1
fully match the data for the natural product, incon-
sistencies appear to be confined to the fragment
produced by the degradation chemistry, thus un-
derscoring the power of the nanoscale NMR meth-
ods for determining the structure of this exceeding-
ly rare complex natural product.
In 2009, Molinski and co-workers reported the
structure of a remarkable marine natural product,
muironolide A (1), isolated from the sponge of the
Phorbas species, the same specimen that earlier
provided phorboxazoles A and B. With only 90 ꢀg
152 nmol) available, the complete structure of
1
2
(
muironolide A was determined using recently de-
veloped nanoscale NMR techniques as well as deg-
Figure 1. Original and revised structures of
muironolide A
3
radation chemistry with 30 ꢀg of the material. The
structure of muironolide A represents a new chemi-
cal entity, the main features of which include a 16-
membered diester lactone with an unprecedented
hexahydro-1H-isoindolone, a trichlorocarbinol es-
ter, and chlorocyclopropane subunits. Given the
extremely small amounts of this “nearly extinct”
natural product, only fragmented studies of its bio-
activity were possible, which, nevertheless, identi-
fied cytotoxic activity against the HCT-116 solid
colon tumor cell line (IC50 96.5 ꢀg/mL) and anti-
fungal activity against Cryptococcus neoformans
(MIC 16 ꢀg/mL).
The key features of our synthesis design are out-
4
lined in Scheme 1. The design calls for a late stage
macrolactone formation; the specific order of ester-
ification was intentionally left flexible for optimiza-
tion. A central transformation in this plan is the
exo-selective lanthanide-catalyzed intramolecular
Diels-Alder reaction for the construction of the
,
5 6
hexahydro-1H-isoindolone subunit. In this reac-
tion, the enol form of the γ,δ-unsaturated β-keto
amide precursor 3 serves as the dienophile, which
is further activated by raising the HOMO of the
diene after deprotonation in the presence of a lan-
thanide additive, giving rise to enolate chelate i.
Herein we report the enantioselective total syn-
thesis of compound 1 with the originally assigned
structure of muironolide
C21,C22,C23-diastereomer 2 (Figure 1). Based on
Scheme 1.
A
along with its
1
13
H and C NMR data from these two compounds,
none of which matched those of the natural sub-
stance, we made a projection for the corrected
structure. This structure was also accessed by total
synthesis, and characterization data for this com-
pound were found to be fully consistent with those
of the natural product, resulting in the reassign-
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NI,
zyloxymethylation (BOMCl, i-Pr
2
NEt, n-Bu
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CH Cl , 45 °C, 12 h, 92% yield), cross-metathesis
2
2
with methacrolein in the presence of 3 mol % of
Hoveyda-Grubbs II catalysts (HGII) afforded alde-
hyde 6 in 67% yield as a 10:1 mixture of E and Z
alkene isomers. The reaction was accompanied by
the recovery of ~20% of the starting material. Ole-
fination with dioxinone phosphonate 7 readily af-
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forded 8, which, upon thermolysis with amine 9
generated amide 10 in 93% yield. Cross-metathesis
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with methyl acrylate or functionalized acrylate
, ,
10 11 12
1
1
(7 mol % HGII, CH
2 2
Cl , 45 °C, 12 h) com-
pleted the synthesis of IMDA precursors 3a and
3
b.
Preliminary investigations demonstrated the fea-
sibility of the planned IMDA reaction for the as-
Scheme 2.
sembly of the hexahydro-1H-isoindolone ring sys-
5
tem of muironolide A. When the substrate pos-
sesses the Z geometry at the C4–C5 double bond, as
in 3, the requisite relative stereochemistry of all
newly formed stereogenic centers is achieved. The
IMDA reaction can be performed directly by ther-
1
3
mal activation or under catalytic conditions. We
found that the thermal cycloaddition can be per-
formed simply by heating the substrate at 110 °C in
toluene, with no base additive required. It was also
characterized by a very high level of diastereocon-
trol favoring the exo-IMDA product (>30:1). To-
gether, these observations are indicative of a polar
concerted mechanism rather than a double Michael
addition pathway.
The reactive hydroxy diene intermediate is ac-
cessed from the β-keto amide precursor through
ketone-enol tautomerization (Scheme 3). We hy-
pothesize that the exo-IMDA pathway is strongly
favored due to hydrogen-bonding stabilization in
the transition structure TS1 (X = H). In the transi-
tion structure for the disfavored endo-IMDA path-
way the hydrogen bond is disrupted (TS2, X = H).
Furthermore, replacing the hydrogen bond in TS1
with a chelated metal enolate (TS1, X = M) was ex-
pected to increase electron density in the diene sys-
tem, raising its HOMO and activating the diene for
cycloaddition. As anticipated, such stabilized eno-
lates could be generated catalytically via soft enoli-
zation, and a screen of Lewis acid additives re-
vealed that lanthanide triflates or nitrates were ef-
To access the β-keto amide precursor of the
IMDA reaction, (+)-β-citronellene was first con-
verted to trichlorocarbinol 4 by selective ozonoly-
13
fective catalysts.
7
sis and reaction of the resultant aldehyde with
Scheme 3.
8
Me
3
SiCCl
3
in the presence of sodium formate. To
set the configuration at C17, an oxidation-reduction
sequence was required. Oxidation of 4 with
DMSO/(CF
tion of the trichloromethyl ketone with
Ru(cymene)Cl] and (R,R)-TsDPEN under trans-
3
CO)
2
O (80% yield) followed by reduc-
[
2
fer hydrogenation conditions delivered 5 in 97%
yield and 10:1 diastereoselectivity. After ben-
9
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Simultaneous removal of the tert-butyl ester and
benzyloxymethyl groups was achieved effectively by
exposure of 15 to trifluoroacetic acid in CH Cl at
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room temperature. Of various attempted methods
for macrolactonization of seco-acid 16, the Yama-
15
guchi reagent appeared most promising. Howev-
er, even under optimal conditions, hydroxyl acid 17
was isolated as a major product, apparently arising
from β-elimination in the chlorocyclopropyl acid
subunit. Other products resulted from higher order
macrolactonizations, and the amount of the intend-
ed product was low at best. Therefore, the alterna-
tive order of ester installations for the assembly of
the 16-membered macrolactone was tested.
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In the event, when 3b was treated with 10 mol %
of La(OTf) , 12 mol % of PYBOX ligand 12, and tri-
ethylamine in ethyl acetate at 45 °C, exo-IMDA
product 13 was obtained as an inseparable 3:1 mix-
ture with its C4,C5,C8,C11-diastereomer in 61%
Using the same condition for the IMDA reaction
3
of 3a (10 mol % La(OTf)
3
, 12 mol % ligand 12,
Et N, EtOAc, 45 °C, 24 h), isoindolone 18 was iso-
3
lated in 61% yield, again as a 3:1 mixture with its
diastereomer (Scheme 5). After reduction with so-
dium borohydride and purification by preparative
combined yield. Reduction of 13 with NaBH af-
4
forded a mixture of two isomeric products which
now could be readily separated by preparative
HPLC. The major product (14) afforded high quali-
ty single crystals suitable for X-ray crystallographic
analysis, which, critically, ascertained the configu-
ration of all stereogenic centers of the target natu-
ral product. The functionalization of the hexahy-
dro-1H-isoindolone ring system was concluded by
dehydration with DCC and CuCl upon heating in
16
HPLC, dehydration of 20 followed by removal of
the BOM group (trifluoroacetic acid, CH
2 2
Cl , 0 °C, 1
h) readily afforded trichlorocarbinol 22.
10
Esterification of 22 with carboxylic acid 23 to
ester 24 was best achieved using the Yamaguchi
conditions (97% yield) (Scheme 6). Both the methyl
ester and the triethylsilyl ether in 24 could be
cleaved in one step under nucleophilic conditions
with microwave irradiation. In contrast to interme-
diate 16, the resulting seco acid 25 underwent a
rather clean macrolactonization upon mild heating
in toluene with 2,4,6-trichlorobenzoyl chloride,
DMAP, and pyridine, affording 26 in 55% yield.
After careful optimization, we found that the final
amide deprotection is ideally performed under oxi-
dative conditions upon heating at 100 °C with DDQ
in dioxane with controlled water content of 0.09%
1
4
toluene, giving 15 in high yield. Of note, other
methods (MsCl, Et N, then DBU; Burgess or Mar-
3
tin reagents) afforded the deconjugated C9-C10
alkene as the sole product.
Scheme 4.
17
1
13
(
90% yield). The H and C NMR data of the re-
sulting compound did not match those reported for
the natural product.
Scheme 5.
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Because carboxylic acid 23 has its origin in an
enzymatic resolution process, its enantiomer ent-
3 was also readily available. Replicating the same
1
0
2
sequence of reactions that delivered 1 with ent-23
afforded another isomer of muironolide A, com-
pound 2. Its NMR data also did not match the nat-
ural substance; however, chemical shift analysis of
1
13
H and C NMR spectra revealed a markedly closer
correlation (Figure 2). As can be seen from the
1
3
analysis of C NMR spectra, the major differences
are confined to the C21-C26 region, i.e. mostly to
the trans-chlorocyclopropyl ketide (CCK) unit.
Originally, the configuration of this section of the
muironolide A could not be determined spectro-
scopically and required an audacious effort that
combined degradation chemistry with 30 ꢀg of the
natural sample, chemical correlation, and Mosher
ester analysis. In our work, we ascribed a much
closer correlation of NMR data between the natural
product and 2, vs. 1, to the inverted configuration
at C21, which we presumed to have a larger impact
on the macrocycle conformation vis-a-vis the exo-
cyclic CCK unit. On the basis of these conjectures,
as well as an alternative interpretation of HPLC
correlation data in the original report, we hypothe-
sized that muironolide A is the C21 epimer of 1.
This hypothesis was put to test when 22 had been
10
converted to the macrolactone using CCK acid 24
and the same four-step sequence of reactions
Scheme 7). The physical data for the resulting
(
compound were in full agreement with those re-
ported for natural muironolide A; however, circular
dichroism (CD) spectroscopy demonstrated that it
Figure 2. Differences in the 13C NMR chemical shifts
between (a) 1, (b) 2, and (c) 3 (synthetic) and natural
muironolide A.
1
0
is enantiomeric to the natural sample, thus estab-
lishing its absolute configuration as opposite to that
of 3.
Scheme 6.
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Scheme 7.
Journal of the American Chemical Society
Graduate Fellowship (DGE-1144085). Dr. Guan Wu is
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thanked for assistance with X-ray crystallography.
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muironolide A resulted in the adjustment of con-
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configuration of the natural product. Delivering 25
mg of the natural product in the first unoptimized
effort, over 250 times the amount isolated from the
natural sources, it paves a way for a more systemat-
ic evaluation of the biological profile of muironolide
A. Perhaps more importantly, this work is a high-
light of the powerful combination of nanoscale
NMR analysis with total synthesis for uncovering
the full potential of “nearly extinct”, exceedingly
rare components of natural product extracts.
0
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‡
Boehringer Ingelheim Shanghai Pharmaceuticals Co.,
Ltd., No. 1010 Longdong Avenue, Zhangjiang, Shang-
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Author Contributions
Young, K.; Xiao, Q.; Zakarian, A. Eur. J. Org. Chem. 2015,
2337.
†
authors contributed equally.
14
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Supporting Information
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Detailed experimental procedures, copies of
NMR spectra, CD measurements. This material is
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The structure of 20 was confirmed by chemical correlation
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ACKNOWLEDGMENT
We thank Professor Molinski for insightful discus-
sions and for performing the CD measurements of 3.
This work is supported by the NSF CHE-1463819,
Amgen, and Eli Lilly. K.Y. is supported by an NSF
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