Total synthesis of greensporone C

Article abstract of DOI:10.1016/j.tetlet.2017.07.074

The first total synthesis of greensporone C, a cytotoxic 14-membered resorcylic acid lactone, has been accomplished via a longest linear sequence of 16 steps in 3.3% overall yield. The key features of the synthesis include Mitsunobu esterification and ring-closing metathesis to construct the macrocycle and establish the (E)-olefin geometry, respectively. Our synthesis also confirmed the absolute stereochemistry of the natural product.

Full text of DOI:10.1016/j.tetlet.2017.07.074

Accepted Manuscript
Total synthesis of greensporone C
Kwanruthai Tadpetch, Laksamee Jeanmard, Vatcharin Rukachaisirikul
PII:
S0040-4039(17)30936-X
DOI:
http://dx.doi.org/10.1016/j.tetlet.2017.07.074
TETL 49156
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
13 June 2017
15 July 2017
21 July 2017
Please cite this article as: Tadpetch, K., Jeanmard, L., Rukachaisirikul, V., Total synthesis of greensporone C,
Tetrahedron Letters (2017), doi: http://dx.doi.org/10.1016/j.tetlet.2017.07.074
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Total synthesis of greensporone C
Kwanruthai Tadpetch*, Laksamee Jeanmard, Vatcharin Rukachaisirikul
Department of Chemistry and Center of Excellence for Innnovation in Chemistry, Faculty
of Science, Prince of Songkla University, Hat Yai, Songkhla 90112, Thailand
ABSTRACT
The first total synthesis of greensporone C, a cytotoxic 14-membered resorcylic acid
lactone, has been accomplished via a longest linear sequence of 16 steps in 3.3% overall
yield. The key features of the synthesis include Mitsunobu esterification and ring-closing
metathesis to construct the macrocycle and establish the (E)-olefin geometry,
respectively. Our synthesis also confirmed the absolute stereochemistry of the natural
product.
Keywords:
Greensporone C
Resorcylic acid lactone
Total synthesis
Ring-closing metathesis

* Corresponding author. Tel.: +66 74 288437; fax: +66 74 558841.
Email address: kwanruthai.t@psu.ac.th (K. Tadpetch).
Resorcylic macrolides are a group of fungal polyketide metabolites, consisting of
14-membered -resorcylic acid lactone (RAL) derivatives, that possess diverse
biological activities1 including cytotoxic,2 antifungal,3 and antimalarial2,4 properties as
well as inhibitory effects against ATPases and kinases.5 A subgroup of these RALs are
those containing a ketone functional group at the 10-position, which are radicicol6
derivatives. This group of metabolites has been shown to exhibit significant and
promising biological activities.7-10 Greensporone C (1) is a new 14-membered resorcylic
acid lactone which was isolated, along with 13 other RALs, from a culture of the
freshwater aquatic fungus Halenospora sp. by Oberlies and co-workers in 2014 (Fig. 1).11
This series of metabolites represent rare examples of RALs containing resorcylic acid
monomethyl ethers with non-chelated hydroxyl groups. Structurally, greensporone C
possesses only one stereogenic center at the 2 position. The absolute configuration of the
C2 stereogenic center of 1 and other co-metabolites in the series was proposed to be S
based on X-ray diffraction analysis of the bromobenzoyl derivative of one of the
analogues as well as Mosher’s ester analyses of those containing C5 alcohol stereogenic
centers. Additionally, greensporone C exhibited potent cytotoxicity against the MDA-
MB-435 (melanoma) and HT-29 (colon) cancer cell lines with IC50 values of 2.9 and 7.5
M, respectively. In order to prove the stereochemistry of the natural product and due to
potential biological activities of this group of macrolides, we set out a synthetic program
aimed at such compounds. Herein, we report the total synthesis of greensporone C.

Figure 1. Structure of greensporone C (1).
The retrosynthetic approach towards greensporone C is illustrated in Scheme 1.
Compound 1 would be derived from macrocycle 2 via oxidation. Ring-closing metathesis
(RCM) was envisioned as the key macrocyclization strategy to establish the (E)-geometry
of the C8–C9 olefin. The RCM diene precursor 3 would be united by Mitsunobu
esterification of the two key fragments: benzoic acid derivative 4 and (R)-non-8-en-2-ol
(5). The requisite benzoic acid 4 could be elaborated from known phenol 6.
Scheme 1. Retrosynthesis of greensporone C (1).
The synthesis of benzoic acid derivative 4 commenced with selective
monoprotection of phenol 612 with the ethoxymethyl (EOM) group, followed by
methylation of the remaining hydroxyl group using iodomethane and K2CO3 in DMF
(Scheme 2). Reduction of both the aldehyde and ester functional groups in 7 was
accomplished using excess NaBH4 in MeOH to give diol 8. It should be noted that the

reduction strategy to manipulate the oxidation states of the two carbonyl groups was
chosen in order to circumvent facile lactonization during the course of the synthesis.
Selective acetylation of the presumably less sterically hindered hydroxyl group of 8 using
acetic anhydride (1 equiv.) resulted in the formation of monoacetate 9 in 72% yield based
on the recovered diol. Protection of the other hydroxyl group with t-butyldimethylsilyl
chloride (TBSCl) furnished silyl ether 10 in 97% yield. Conversion of 10 to the
corresponding aldehyde 11 was achieved in two high-yielding steps: removal of the
acetate group by methanolysis (K2CO3, MeOH) to reveal the 1° alcohol, followed by IBX
oxidation. Addition of vinylmagnesium bromide to aldehyde 11 gave racemic allylic
alcohol 12 in moderate yield (68% based on recovered aldehyde). The stereoselectivity of
this step was inconsequential since the newly generated alcohol chiral center would
eventually be oxidized to a ketone. Attempts to reduce the number of synthetic steps by
removal of the TBS group and simultaneously oxidizing both alcohols failed and resulted
in formation of the undesired lactol, possibly due to the higher reactivity of the benzylic
alcohol compared to allylic alcohol in this substrate. Thus, we proceeded to protect the
allylic alcohol with the EOM group using a large excess of EOMCl and DIPEA in the
presence of catalytic tetrabutylammonium iodide (TBAI) to give the corresponding EOM
ether in 98% yield.13 Subsequent removal of the TBS protecting group using TBAF
smoothly delivered benzylic alcohol 13 in 96% yield. Finally, IBX oxidation of 13 gave
the benzaldehyde derivative 14 in 87% yield. Further oxidation of 14 under Pinnick
oxidation conditions afforded the desired benzoic acid 4 in 81% yield after careful acidic
workup to avoid undesired deprotection of the EOM groups.

Scheme 2. Synthesis of benzoic acid 4.
Synthesis of the requisite chiral alcohol fragment, (R)-non-8-en-2-ol (5), is outlined
in Scheme 3. Regioselective opening of commercially available, optically pure (R)-
propylene oxide with the acetylide generated in situ by deprotonation of 5-
(benzyloxy)pentyne (15) with n-BuLi in the presence of BF3·OEt2 gave (R)-propargylic
alcohol 16 in good yield and
>99% ee.14 The newly generated alcohol was then
masked as the t-butyldiphenylsilyl (TBDPS) ether 17. Alkyne reduction and concomitant
removal of the benzyl protection group of 17 via catalytic hydrogenation in EtOAc
provided alcohol 18 in 72% yield. To install the last carbon of this fragment, the 1°
alcohol was oxidized using iodobenzene diacetate in the presence of catalytic TEMPO to
deliver aldehyde 19 in 75% yield. Wittig olefination of 19 using
methyltriphenylphosphonium bromide and KOt-Bu gave the desired alkene, which after
TBDPS deprotection using TBAF furnished (R)-non-8-en-2-ol (5) in 60% yield. The
NMR spectroscopic data and specific rotation of 5 matched those previously described in
the literature.15

Scheme 3. Synthesis of (R)-non-8-en-2-ol (5).
With the key benzoic acid derivative 4 and chiral alcohol 5 in hand, we proceeded
to complete the synthesis of greensporone C (Scheme 4). Union of the two key fragments
4 and 5 under Mitsunobu esterification conditions using diisopropyl azodicarboxylate
(DIAD) and PPh3 in toluene smoothly gave the ester RCM diene precursor 3 in 69%
yield. This step should also provide the correct stereochemistry of the stereogenic center
at the 2-position. The stage was then set for the key RCM to assemble the macrocyclic
core of 1. Thirupathi and Mohapatra recently reported a similar RCM protocol in the
synthesis of cryptosporiopsin A and, to the best of our knowledge, their synthesis is the
only example of RCM to assemble the C8–C9 olefin for this subgroup of RALs
containing a ketone group at the 10-position.16 The RCM precursor in their synthesis was
slightly different from ours by the protecting group choices. Their key RCM proceeded
smoothly to give the desired (E)-macrocycle in 74% yield using the second-generation
Grubbs catalyst. Gratifyingly, RCM of diene 3 using the second-generation Grubbs
catalyst (10 mol%) in CH2Cl2 at reflux with high dilution (5 mM) proceeded to
completion within 4.5 h and furnished the RCM products in 90% combined yield with the
(E)-olefin isomers identified as the major product. Although we could not separate all
four diastereomers of the RCM products, we were able to separate the major
diastereomers of macrolactones (E)-(2) (56% yield) and (Z)-(20) (14% yield) in an

approximately 4:1 ratio; their structures were confirmed by 1D, 2D NMR spectroscopic
and HRMS data. The inseparable minor diastereomers of the (E)- and (Z)-macrocyclic
adducts 21 (20% yield) were confirmed by HRMS data. No attempts were made to assign
the absolute configuration of the alcohol stereogenic center of each macrocylic product
because it would be oxidized to the ketone functional group in subsequent steps. Removal
of both EOM protecting groups of 2 using HCl in THF at ambient temperature furnished
diol 22 in 89% yield. Attempts to oxidize the resultant allylic alcohol of 22 with Dess-
Martin periodinane was unsuccessful and unreacted diol was recovered. Gratifyingly, the
use of pyridinium chlorochromate (PCC) in CH2Cl2 at room temperature resulted in the
formation of greensporone C (1) in 30% yield. The 1H and 13C NMR spectroscopic data
as well as HRMS data of synthetic greensporone C were nearly identical to those reported
for the natural product. Additionally, the specific rotation of synthetic 1 ([ ]D27 +53.9, c
0.56, MeOH) was in accordance with and had the same sign as the natural product 1
([ ]D20 +112, c 0.33, MeOH). The CD spectrum of synthetic 1 showed a negative Cotton
effect at 234 nm, which indicated the S absolute configuration of C2 analogous to that of
E-dehydrocurvularin reported by Marsaiolia and co-workers.17 Thus, our synthesis
confirmed the absolute configuration of the natural product greensporone C as proposed
by Oberlies and co-workers.

Scheme 4. Completion of the synthesis of greensporone C (1). The absolute
stereochemistry of the * stereogenic center was not determined.
Conclusion
In summary, the first total synthesis of greensporone C (1) has been accomplished
via a longest linear sequence of 16 steps in 3.3% overall yield from phenol 6. The key
features of the synthesis include Mitsunobu esterification and ring-closing metathesis to
construct the macrocycle and establish the (E)-olefin geometry, respectively. Our
synthesis also confirmed the absolute stereochemistry of the natural product.
Acknowledgements
This work was financially supported by The Thailand Research Fund (Grant No.
TRG5880272) and Prince of Songkla University. Center of Excellence for Innovation in
Chemistry (PERCH-CIC) is acknowledged for partial support. Additional support is

generously provided by the Development and Promotion of Science and Technology
Talents Project (DPST) for L. Jeanmard.
Supplementary data
Supplementary data associated with this article can be found, in the online version,
at
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Highlights
 The first total synthesis of greensporone C has been achieved.
 The key steps include Mitsunobu esterification and ring-closing metathesis.
 The synthesis confirmed the absolute stereochemistry of the natural product.

Graphical Abstract
Leave this area blank for abstract info.
Total synthesis of greensporone C
Kwanruthai Tadpetch, Laksamee Jeanmard, Vatcharin Rukachaisirikul

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