Total Synthesis and Structural Revision of Monocillin VII

Article abstract of DOI:10.1021/acs.orglett.9b02075

The first asymmetric total synthesis of macrolactone monocillin VII and its C-10′ epimer was achieved starting from a known chiral pure epoxide in 16 longest linear sequences. The present synthesis highlights the macrolactone formation involving an alkyne-dicobalt carbonyl complex under De Brabander's conditions followed by an unexpected regioselective hydration. The asymmetric total synthesis resulted in the revision of the configuration at C10′ and reassignment of the absolute configuration of the natural product.

Full text of DOI:10.1021/acs.orglett.9b02075

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Total Synthesis and Structural Revision of Monocillin VII
N. Arjunreddy Mallampudi,† Beduru Srinivas,† Jithender G. Reddy,‡ and Debendra K. Mohapatra*,†
†Department of Organic Synthesis and Process Chemistry and ‡Centre for NMR and Structural Chemistry, CSIR-Indian Institute of
Chemical Technology, Hyderabad 500007, India
S
* Supporting Information
ABSTRACT: The first asymmetric total synthesis of macrolactone monocillin
VII and its C-10′ epimer was achieved starting from a known chiral pure
epoxide in 16 longest linear sequences. The present synthesis highlights the
macrolactone formation involving an alkyne−dicobalt carbonyl complex under
De Brabander’s conditions followed by an unexpected regioselective hydration.
The asymmetric total synthesis resulted in the revision of the configuration at
C10′ and reassignment of the absolute configuration of the natural product.
he resorcylic acid lactones (RALs) are a class of natural
products derived from various organisms and are
consequence of their impressive biological activity profiles
coupled with structural similarity, considerable efforts were
devoted toward their synthetic and pharmacological studies.
Recently, Wei and co-workers4 have isolated eight new
resorcylic acid lactones, including monocillin VII (5), along
with nine known hypothemycin- and radicicol-type RALs from
the rice-grown cultures of Paecilomyces sp. SC0924. These
metabolites have shown antifungal activity against P. litchii and
Fusarium verticillioides and cytotoxicity against MCF-7, A549,
and HeLa tumor cell lines. In particular, monocillin VII
exhibits antifungal activity against P. litchii with an IC50 value
of 41.0 μM and cytotoxicity against MCF-7, A549, and HeLa
tumor cell lines with IC50 values of 4.9, 4.1, and 3.9 μM,
respectively.4 The key structural features of monocillin VII (5)
include the presence of a ketone moiety at the 2′ position, a
4′,8′-trans pyran ring with an unsaturation between 6′ and 7′,
and a methyl group at the 10′ position. In our group, we
developed a synthetic strategy for the synthesis of trans-2,6-
disubstituted dihydropyrans (DHPs) involving tandem isomer-
ization followed by C−O and C−C bond formation sequence
and have utilized the strategy for the synthesis of pyran-
containing natural products.5 In continuation of our interest in
the total synthesis of pyran containing natural products and
RALs,6 herein we disclose the total synthesis of macrolactone
monocillin VII (5) and the revision of the stereochemistry of
the natural product.
T
comprised of a β-resorcylate moiety annulated with 12- or
14-membered macrolactone (Figure 1).1 Although the
Initially, we planned the synthesis of 5 by transdisposition of
the isocouamrin to form the macrolactone. Formation of the
key DHP unit was envisaged by the tandem isomerization/C−
O/C−C bond-forming protocol5h developed by us, while
gold(I)-catalyzed 6-endo-dig cyclization strategy7 was antici-
pated for the synthesis of the isocoumarin fragment.
The key features for the synthesis of monocillin VII are
represented in Scheme 1. Initially (path A), we planned that
the keto-lactone functionalities could be achieved by the
hydrolysis of isocoumarin 8 followed by macrolactonization.
Figure 1. Representative structures of 14-membered resorcylic acid
lactones (RALs).
structures of most of these compounds are similar to each
other, they display varied biological activities such as
antimalarial, antifungal, cytotoxic, antiplasmodial, estrogenic,
and nematicidal properties, and several members of these
RALs acts as potent kinase and ATPase inhibitors.2 For
example, radicicol (1) showed potent HSP90 inhibitory
activity,3b−d while some other RALs exhibited mitogen-
activated protein kinase inhibition irreversibly and cytotoxicity
against human cells through CDK/GSK-3 inhibition.3a As a
Received: June 17, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b02075
Org. Lett. XXXX, XXX, XXX−XXX

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Letter
furnished the corresponding epoxide in 95% yield. The epoxide
was treated with Super-Hydride in THF to afford the required
alcohol 13 along with its minor diastereomer 14 in 91%
combined yield, which was separated through silica gel column
chromatography. The stereochemistry of both of the hydroxyl
groups in compounds 13 and 14 were assigned individually by
following the modified Mosher’s method (Figures S1 and S2).9
Esterification of the major alcohol 13 with both (S)- and (R)-
methoxy(trifluoromethyl)phenylacetic acid (MTPA) revealed
a positive chemical shift difference [Δδ = (δS− δR) × 103] for
protons on C10 (Figure S1), while protons on C8 through C1
showed negative chemical shift differences, which is consistent
with C-9 bearing an R-configuration. The hydroxyl group
present in 13 was protected as its TBDPS ether using
TBDPSCl and imidazole in CH2Cl2 to obtain 15 with 95%
yield. The PMB group was oxidatively removed under DDQ
conditions10 to furnish the primary alcohol 16 in 93% yield.
The primary alcohol 16 was oxidized under Dess−Martin
periodinane conditions to afford the corresponding aldehyde,
which was immediately subjected to one carbon homologation
using Ohira−Bestmann reagent11 to afford alkyne 10 in 82%
yield.
Scheme 1. Retrosynthetic Plan Featuring Gold-Catalyzed 6-
Endo-Dig Cyclization
Synthesis of the aryl triflate 9, required for the Sonogashira
coupling, was synthesized from commercially available 2,4,6-
trihydroxybenzoic acid (17) (Scheme 3). Reaction of 17 with
The required aryl triflate 9 could be accessed from
commercially available 2,4,6-trihydroxybenzoic acid (17).
We initiated our study with the synthesis of alkyne fragment
10 required for the key coupling reaction (Scheme 2).
Scheme 3. Synthesis of Aryl Triflate 9
Scheme 2. Synthesis of Alkyne fragment 10
SOCl2, DMAP, and acetone gave the acetonide 18 in 82%
yield. The hydroxyl group in 18 was selectively protected as its
benzyl ether under Mitsunobu conditions12 to obtain 19 in
84% yield. Finally, the free hydroxyl group in 19 was converted
to triflate 9 using Tf2O and Et3N in CH2Cl2 with 90% yield.7
Having both the coupling partners in hand, we planned to
perform the key Sonogashira coupling reaction. Initial attempts
under standard Sonogashira conditions13 either failed or
produced a lower amount (<10%) of required product 20,
and the homocoupled product 21 was obtained as the major
product. To circumvent this problem, we optimized the
coupling reaction conditions using various palladium catalysts
by changing the ligands and bases (Table 1). Finally,
Pd(dppf)Cl2·CH2Cl2, CataCxium A, and K2CO3 conditions
using acetonitrile as the solvent at 60 °C furnished the
heterocoupled product 20 in 76% yield (Table 1).
Synthesis of dihydropyran 12 was carried out by following a
tandem isomerization/C−O/C−C bond-forming reaction of
known aldehyde 115 with 86% yield. The trans geometry of the
pyran ring was confirmed through 2D-NOE studies. The
terminal double bond was subjected to Sharpless asymmetric
dihydroxylation in the presence of AD-mix-α8 to afford (S)-
diol with its minor diastereomer in 89% combined yield.
Reaction of diol with 1-tosylimidazole in the presence of NaH
After having the coupled product, the TBDPS group was
deprotected with NH4F in MeOH at 50 °C to afford the
alkyne 7 in 85% yield. Compound 7 was subjected to
PPh3AuCl and AgSbF6 under 1,4-dioxane conditions to furnish
B
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Letter
Co2(CO)8 in CH2Cl2 at 0 °C to afford the Co complex 23 in
almost quantitative yield (Scheme 4). Most notably the
presence of cobalt complex was evident in the NMR spectra
of the sample while the mass spectrum established the
presence of cobalt complex formation in compound 23. As
this complex was not very stable, we immediately used it in the
macrocyclization reaction. Consistent with the above hypoth-
esis, when compound 23 was treated with NaH in a mixture of
THF and DMF (9:1) at 50 °C, the only isolable product of the
reaction was compound 6 in 52% yield accompanied by the
recovery of 35% of the alcohol 7.16 Now the stage was set to
execute the vital gold-catalyzed regioselective hydration
reaction15 for the introduction of ketone. Delightfully,
treatment of lactone 6 with PPh3AuCl, AgOTf, and H2O in
1,4-dioxane furnished the hydration product exclusively at the
nonbenzylic position of the alkyne with good yield. The
observed high regioselectivity is presumably attributed to the
intramolecular assistance of ester group for the hydration
reaction. To the best of our knowledge, such a regioselective
gold-catalyzed hydration on alkynyl lactones has not been
reported in the syntheses of RALs. With keto lactone 24 in
hand, we proceeded to the completion of the synthesis of
monocillin VII. Debenzylation of keto lactone 24 was carried
out with excess TiCl4 in CH2Cl2 at 0 °C to afford monocillin
VII (5) with 89% yield.19
Table 1. Optimization of the Sonogashira Coupling of
Aryltriflate 9 and Alkyne 10
yield
(%)
a
b
entry
conditions
1
2
3
4
5
6
7
(PPh3)2PdCl2, Et3N, CuI, DMF, rt
(PPh3)4Pd, Et3N, CuI, DMF, rt
Pd(dba)2, Et3N, CuI, DMF, rt
(PPh3)2PdCl2, Et3N, CuI, CH3CN, rt
Pd(dppf)2·CH2Cl2, K2CO3, DMF, rt
Pd(dppf)2·CH2Cl2, K2CO3, CH3CN, rt
Pd(dppf)2·CH2Cl2, K2CO3 CataXCium A, CH3CN, 60
°C
10
5
0
15
10
20
c
76
c
8
9
Pd(dppf)2·CH2Cl2, K2CO3, Xantphos, CH3CN, 60 °C
Pd(dppf)2·CH2Cl2, K2CO3, nBu3P, CH3CN, 60 °C
30
5
c
However, the 1H NMR and 13C NMR data for the synthetic
compound 5 showed deviations from the data reported for the
natural product,4 strongly suggesting a structural misassign-
ment during the isolation of the natural product. Moreover, the
c
10
Pd(dppf)2·CH2Cl2, Cs2CO3, CataXCium A, CH3CN,
60 °C
35
a
The reactions were performed with 10 (1 mmol), 9 (1.2 mmol),
catalysts (5 mol %), and base (1.5 mmol) in solvent (2 mL) at
20
b
specific rotation for the synthetic compound 5 {([α]D −7.3
specified temperatures under argon atmosphere. Yield of isolated
c
product after column chromatography. 5 mol % of phosphine ligand.
(c 0.26, MeOH)} showed the same sign but varied in
20
In all cases, formation of homocoupled product 21 was also observed.
magnitude with that of natural product {lit. ([α]D −31.2 (c
0.25, MeOH)}. As some other members of this series isolated
from Paecilomyces sp. SC0924 having the S-configured methyl
group at the 10′ position,4 we thought that the natural product
may be the diastereomer of the proposed structure. Thus, we
undertook the total synthesis of 10′(S)-isomer (5a) of the
proposed structure. Accordingly, (4′R,8′S,10′S)-monocillin VII
(5a) was synthesized from compound 14 by a synthetic route
similar to that described for 5 (Scheme 5), where alcohol 14
was prepared from compound 12 using AD-mix-β. Delight-
fully, the spectral data (1H and 13C NMR) of 5a were in good
agreement with those reported for the natural product (see the
comparison of the NMR data of natural monocillin VII,
synthetic 5 and 5a in Table S1). The specific rotation for
the isocoumarin derivative 8 in 88% yield.7 To obtain the keto
acid derivative, standard ester hydrolysis conditions were
attempted (LiOH or KOH in THF/H2O).14 However, all
attempts to hydrolyze the isocoumarin under different reaction
conditions were unsuccessful and led to an intractable mixture
of products. Disappointed yet undaunted by the failure to
afford the keto acid intermediate, we proceeded to revise our
synthetic protocol (path B). The pivotal macrolactone
formation was envisaged by an intramolecular macrolactoniza-
tion keeping an alkyne group in the carbon chain in the 14-
membered ring. Our hypothesis for this route stems from the
observation that the regioselectivity in hydration of alkynes can
be achieved by neighboring group assistance,15 so that the keto
group present at 2′ could be accessed from the alkyne via
intramolecular oxygen-assisted regioselective hydration reac-
tion.
Accordingly, macrolactonization formation of seco-acid
derivative 7 was performed under De Brabander’s conditions16
(NaH, THF, 0 °C) to afford the lactone 6, albeit in poor yield
(15%), with the recovery of starting material (∼76%) and
nonreproducible results. Most likely, conformational con-
straints on the backbone imposed by the linear structural
nature of the alkyne unit restrict the cyclization leading to very
poor yield. It is well known that reaction of dicobalt
octacarbonyl with alkynes can lead to stable complexes
wherein the geometry of the cobalt-complexed alkyne is
distorted to approximately 140°.17 To resolve the problem of
macrolactonization with very poor yield, it was envisioned that
Co-complexed intermediate 23 may deliver compound 6 with
good yield.18 Accordingly, compound 7 was treated with
20
synthetic 5a ([α]D −19.7 (c 0.21, MeOH) also showed the
20
same sign but deviated in magnitude {(lit. [α]D −31.2 (c
0.25, MeOH)}.
Extensive NMR studies on synthetic compounds 5 and 5a
were carried out in C5D5N using 1D (1H and 13C) and 2D
(DQF-COSY, TOCSY, NOESY, HSQC and HMBC) NMR
experiments. In comparison to the reported natural product,
1
the H and 13C NMR spectral data of compound 5 showed
major discrepancies, whereas the data from compound 5a are
in full agreement (Table S1). However, the presence of
medium-range NOE correlations between H-1′β/H-4′, H-3′β/
H-4′, H-9′β/H-4′, and H-9′α/H-8′ in both compound 5 and
5a suggested that the configurations at H-4′ and H-8′ are in α-
and β-orientations, respectively. The characteristic difference
in compound 5 is the presence of NOE between H-8′/CH3-
11′ supporting an “R” configuration, whereas in compound 5a
it is between H-8′/H-10′ supporting an “S” configuration at
C10′. This is further confirmed by the scalar coupling (J)
C
DOI: 10.1021/acs.orglett.9b02075
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Letter
Scheme 4. Synthesis of Monocillin VII (5)
and the dicobalt carbonyl complex of alkyne to execute smooth
macrocyclization under De Brabander’s conditions followed by
unexpected regioselective hydration and to introduce the
functionality relevant to the synthesis of RALs and related
natural products. The total synthesis of monocillin VII and its
Scheme 5. Synthesis of Monocillin VII (5a)
1
C10′-epimer followed by comparison of their H and 13C
NMR data with those of the natural product led us to correct
the previously proposed structure to that of (4′R,8′S,10′S)-
monocillin VII (5a).
between H-9′β/H-10′ which is 2.0 and 10.4 Hz in compound
5 and 5a (10.8 Hz as observed in natural product),
respectively. Overall, chemical shifts and characteristic scalar
couplings of compound 5a are in full agreement with that of
natural product (Table S1), whereas the stereocenter at C10′
carbon shows an “S” configuration in compound 5a. Complete
NMR studies of both compounds adequately supported that
the compound 5 has a (4′R,8′S,10′R) configuration (Figure
S4) and compound 5a has a (4′R,8′S,10′S)-configuration
(Figure S6). Given that, the complete NMR spectral data were
reassigned the absolute stereochemistry of the natural product
to compound 5a with a 4′R,8′S,10′S configuration.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b02075.
Detailed experimental procedures and spectral data (1H,
13C and 2D-NMR) for all new compounds (PDF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: mohapatra@iict.res.in.
ORCID
In conclusion, an efficient convergent synthetic route has
been developed for an asymmetric first total synthesis of
monocillin VII in 16 longest linear sequences starting from a
known chiral pure epoxide which was obtained by Jacobsen’s
hydrolytic kinetic resolution. The 2,6-trans-disubstituted
tetrahydropyran ring present in monocillin VII was achieved
by using the tandem isomerization followed by a C−O/C−C
bond-forming reaction developed by our group. The work
reported herein highlights the construction of the macro-
lactone ring system following a convergent union of the
elaborated subunits through modified Sonogashira coupling
Debendra K. Mohapatra: 0000-0002-9515-826X
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors gratefully acknowledge the financial support
received from Department of Science and Technology-Science
and Engineering Research Board (DST-SERB), New Delhi,
D
DOI: 10.1021/acs.orglett.9b02075
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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DOI: 10.1021/acs.orglett.9b02075
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