A total synthesis of sarcandralactone A: A general, concise, RCM enabled approach to lindenanolide sesquiterpenoids

Article abstract of DOI:10.1016/j.tetlet.2015.04.132

A total synthesis of lindenane-type sesquiterpenoid natural product sarcandralactone A and its close sibling 5-epi-shizukanolide has been accomplished through a concise strategy in which a one-pot γ-lactone annulation and a RCM reaction constitute pivotal

Full text of DOI:10.1016/j.tetlet.2015.04.132

Accepted Manuscript
A total synthesis of Sarcandralactone A: A general, concise, RCM enabled ap-
proach to lindenanolide sesquiterpenoids
Subburethinam Ramesh, Goverdhan Mehta
PII:
S0040-4039(15)00789-3
DOI:
http://dx.doi.org/10.1016/j.tetlet.2015.04.132
TETL 46272
Reference:
To appear in:
Tetrahedron Letters
Received Date:
Revised Date:
Accepted Date:
15 April 2015
28 April 2015
30 April 2015
Please cite this article as: Ramesh, S., Mehta, G., A total synthesis of Sarcandralactone A: A general, concise, RCM
enabled approach to lindenanolide sesquiterpenoids, Tetrahedron Letters (2015), doi: http://dx.doi.org/10.1016/
j.tetlet.2015.04.132
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Graphical Abstract
A total synthesis of Sarcandralactone A: A general, concise, Leave this area blank for abstract info.
RCM enabled approach to lindenanolide sesquiterpenoids
Subburethinam Ramesh and Goverdhan Mehta*

1
Tetrahedron Letters
journal homepage: www.els evier.com
A total synthesis of Sarcandralactone A: A general, concise, RCM enabled
approach to lindenanolide sesquiterpenoids
Subburethinam Ramesh and Goverdhan Mehta∗
School of Chemistry, University of Hyderabad, Hyderabad - 500 046, India
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
A total synthesis of lindenane-type sesquiterpenoid natural product sarcandralactone A and its
close sibling 5-epi-shizukanolide has been accomplished through a concise strategy in which a
one-pot γ-lactone annulation and a RCM reaction constitute pivotal steps.
2
009 Elsevier Ltd. All rights reserved.
Available online
Keywords:
Sesquiterpenoid synthesis
Lindenanolides
RCM
Simmons-Smith cyclopropanation
Julia-Kocienski reaction
First unraveled about half
a
century ago, lindenane
sesquiterpenoid family has been steadily growing and has
1
gathered considerable momentum (vide infra) in the recent past.
Numbering close to a hundred, lindenanes are predominantly
1
b,d
found in the plants of chloranthaceae family although they
have been sporadically encountered in other plant species
2
including marine sources. In structural terms, the lindenane 1
framework (1,3-cycloeudesmane) is a close sibling of the more
abundant eudesmane 2 (Figure 1) skeleton in which C1 and C3
are conjoined to form a cyclopropane ring. A distinctive feature
of lindenane sesquiterpenoids is that an oxidized C7-isopropyl
group is embedded in its framework as an annulated furan or a α-
butenolide moiety. Representative examples of resulting
furanolindenanes 3 - 4 and lindenanolides 5 - 8 are displayed in
Figure 2.
Figure 2. Representative examples of furanolindenane and lindenanolide.
Concurrently with their growing numbers, lindenanes, and in
particular, lindenanolides are beginning to attract increasing
attention from the practitioners of synthetic chemistry after a
surprisingly long period of dormancy. Indeed, the first synthesis
5a
of a lindenanolide was only reported in 2007 following which
5
there has been a spate of activity in the arena, largely stimulated
through the challenge posed by the intricacy of the structures
Figure 1. Lindenane and eudesmane skeleton.
(
Figures 2 & 3) and the prospect of enriching the chemical
In the preceding decade, an impressive number of complex,
polycyclic, functionally embellished and stereochemically
endowed, dimeric and oligomeric lindenanolides have surfaced in
the literature. Some prototypical examples 9-12 are captured in
Figure 3. In addition to their intricate molecular architecture,
dimeric lindenanolides along with some of their oligomeric
siblings exhibit remarkable range of bioactivity profile that
includes immunomodulatory, anti-inflammatory, anti-cancer, anti
diversity space around their scaffold to explore the bioactivity
potential. We were drawn in the fray as part of our interest in the
synthesis of eudesmane based sesquiterpenoids wherein a RCM
based strategy was effectively deployed to assemble the
6
framework in a short, diversity oriented sequence. From a
conceptual perspective, a contextual RCM based strategy seemed
very appropriate for the synthesis of lindenanes and
lindenanolides but did not seem to have been previously applied
3
,4
HIV-1, antifungal and neuroprotective to name a few.
—
∗
——
Corresponding author. Tel.: +91-40-23134848; fax: +91-40-23010785; e-mail: gmehta43@gmail.com

2
Tetrahedron Letters
OH
O
MeO2C
O
O
O
O
OH
CO Me
2
H
H
H
HO
O
O
OH
O
H
OH
H
9. shizukanol A
10. sarcanolide A
O
HO
H
O
O
O
O
O
HO
O
O
O
O
O
H
O
O
H
HO
O
OH O
1. chlorahololides B
H
OH
Scheme 1. Retrosynthetic analysis for lindenanolide prototype.
1
12. lindenanolide F
delivery of the three-membered ring on the β-face, syn to the
methyl group at the quaternary centre, to furnish 20 bearing the
basic tetracyclic framework of lindenanolides. The tetracycle 20
could then be subjected to either Wittig-type olefination and/or
additional functional group manipulations to eventuate in the
desired natural product target, Scheme 1.
Figure 3. Representative examples of dimeric lindenanolides.
5
to this family of natural products. It was therefore of interest to
develop a concise, generally applicable route to lindenanolides
wherein RCM was to serve as one of the pivotal steps.
Realization of this objective through a total synthesis of a
lindenanolide sesquiterpenoids sarcandralactone A 13 and 5-epi-
shizukanolide 14 (Figure 4) is disclosed in this letter.
HO
O
OEt
O
OEt
OEt
a
b
H
H
16
21
22
O
O
O
O
OH
3. sarcandralactone A
H
c
1
14. 5-epi-shizukanolide
O
O
OTMS
d
Figure 4. Lindenanolides: sarcandralactone A and 5-epi-shizukanolide
A
retrosynthetic perspective from
a
prototypical
H
H
lindenanolide 15 could be traced to
a
simple 1,3-
17
24
23
cyclohexanedione enol ether precursor 16 and is depicted in
Scheme 1. Evolution of the readily available 16 towards the
synthetic target required setting-up of the quaternary carbon
centre and installation of two terminal olefin bearing side arms
with requisite stereodisposition following kinetically controlled
allylation, sequential 1,2-addition, transposition and 1,4-vinyl
addition (16 →17). Further progress towards the objective
required one step γ-lactone annulation in 17 in a regioselective
manner and oxyfunctionalization of the allyl arm to deliver 18
which was now ready for the implementation of the RCM
protocol to furnish 19, Scheme 1. The intent in this plan was to
implement a stereoselective Simmons-Smith cyclopropanation in
o
Scheme 2. Reagents and conditions: (a) HMDS, n-BuLi, 0 C, HMPA, allyl
o
o
+
bromide, THF, -78 C – rt, 4 h, 82%; (b) MeLi, -40 C, THF, 1 h; (c) H O
95%; (d) CuBr.Me S, vinylmagnesium bromide, THF, TMSCl, -78 C, 1 h,
3
o
2
81%.
The retrosynthetic analysis presented in Scheme 1 unveiled
the contours of a concise strategy to access lindenanolides from a
readily available precursor 16 and set the stage for demonstrating
its viability. In the event, 16 was subjected to allylation under
conditions of kinetic control to regioselectively furnish 21,
Scheme 2. Addition of methyllithium to 21 led to enone 23
through the intermediate formation of tertiary alcohol 22 and
7
1
9
to
ensure
concomitant acid mediated transposition as described earlier.
O
O
TiCl3
O
H
O
a
-MeOH
-OHTiCl3
OMe
O
O
H
OMe
OTiCl3
17
OMe
H
H
H
H
H
O
O
O
O
b
O
O
O
+
O
H
H
27
28
26
25
ORTEP of 27
c
4 3
Scheme 3. Reagents and conditions: (a) TiCl , Bu N, DCM, α,α-dimethoxyacetone, 17.5 h, 61%; (b) Grubbs' I (5 mol %), rt, 3 h, 89%; (c) Grubbs' I (5 mol %),
rt, 2 h, 87%.

3
Conjugate addition of divinylcopper to 23 set-up the quaternary
centre and delivered 17 via the intermediacy of the TMS-
enolether 24 and with predictable stereochemistry dictated by the
Towards this end, 19 was subjected to directed Simmons-
Smith cyclopropanation with CH in the presence of diethylzinc
to furnish 33 (X-ray structure displayed in Scheme 6) of desired
2 2
I
1
1
10,11
pre-existing allyl group, Scheme 2. Doubly armed 17 was
subjected to regioselective γ–lactone annulation following one
stereochemistry.
DMP oxidation in 33 was unexpectedly
eventful and furnished the tetracyclic ketone 34 in which a facile
and aberrant epimerization at the C-5 centre had occurred to
8
pot Ti(IV) mediated cross aldol protocol of Tanabe et al. using
1
1
α,α-dimethoxyacetone and involving several intermediates (see,
Scheme 3) to deliver bicylic diastereomeric lactones 25 and 26 in
deliver a cis-hydrindane moiety as in 34. The observed
epimerization of trans-hydrindane bearing framework to cis-
hydrindane, as revealed by NOESY spectrum (see supplementary
material), is not without precedence in such systems wherein the
angular methyl groups and the cyclopropane ring are cis-
disposed. Olefination of the carbonyl group in 34 following
Julia-Kocienski protocol was smooth but delivered 5-epi-
shizukanolide 14 as revealed by X-ray structure determination
1
1
a ratio of 3.4:1. Both the diastereomers 25 and 26 smoothly
underwent the contemplated RCM to furnish diastereomeric
1
1
tricycles 27 and 28, respectively, Scheme 3.
The
5
f,h
stereostructures of 27 and 28 were secured through single crystal
X-ray structure determination of the major isomer 27 (Scheme
1
0
3
). All the three stereogenic centers in 27 had the requisite
1
0
disposition for targeting lindenanolide natural products.
(Scheme 6) and not the expected natural product shizukanolide
11
5
.
However, when 14 was subjected to allylic oxidation with
2
, our targeted natural product sarcandralactone A 13 was
readily realized and its spectral characteristics ( H & C NMR)
The intent now was to stereoselectively oxy-functionalize the
allylic position of the cyclopentene ring in 27 to obtain 19, for a
directed Simmons – Smith cyclopropanation, either directly or
through the intermediacy of the enone 29, Scheme 4. Thus, 27
was exposed to several known allylic oxidation reagents like
SeO
1
13
5i, 9
were exactly identical with those reported in the literature for
the natural compound, Scheme 6.
H
PDC-TBHP, CrO
3
-3,5-dimethyl pyrazole, Mn(OAc)
3
, SeO
2
-
H
H
O
O
a
O
b
TBHP etc. to obtain either 19 or 29 but to no avail. This
unexpected resistance of 27 towards allylic oxidation forced us to
revert to the pre-RCM stage to attempt oxy-functionalization of
the allylic arm in diastereomer 25, Scheme 5.
O
O
O
H
O
H
H
HO
HO
19
33
34
H
H
H
O
O
O
O
O
O
c
H
H
H
O
HO
ORTEP of 33
19
27
29
H
H
O
d
O
Scheme 4. Attempted oxy-functionalization of allylic position in 27.
O
O
In this endeavor better luck awaited us and after some
exploratory experiments, it was possible to devise conditions
OH
H
14
1
3
(
SeO
2
-excess TBHP) under which 25 delivered a mixture of
sarcandralactone A
ORTEP of 14
5-epi-shizukanolide
readily separable oxy-functionalized products 18, 30, 31 in
Scheme 6. Reagents and conditions: (a) CH
2
I
2
, Et
2
Zn, ZnI , CH Cl , 0 - 10
2
2
2
preoperatively useful yield and in a ratio of 1.6:1.6:1, Scheme
o
o
2 2
C, 18 h, 84%; (b) DMP, CH Cl , 0 C- rt, 3 h, 86%; (c) 1-(tert-butyl)-5-
1
1
5
.
Stereo-differentiation between the structures of 18 and 30
o
(
methylsulfonyl)-1H-tetrazole, NaHMDS, THF, -78 C, 2.1 h, 72%; (d) SeO
2
,
was arrived at through single crystal X-ray structure
a
o
dioxane, 80 C, 20 min, 80%.
10
determination on 30 (Scheme 5). Quite pleasingly, all the three
oxy-functionalized products 18, 30 & 31 underwent smooth
RCM to furnish tricyclic products 19, 32 & 29 respectively,
For the sake of completeness of this effort, we carried
forward the epimeric allylic alcohol 32 towards the lindenanolide
framework. Consequently, it was subjected to hydroxyl directed
11
Scheme 5.
Tricyclic allylic alcohol 19 with desired
stereochemistry attributes was now poised for elaboration to the
natural products sarcandralactone A 13 and shizukanolide 5.
cyclopropanation with CH I in the presence of diethylzinc to
2 2
11
furnish tetracyclic product 35, Scheme 7. Further oxidation of
35 with DMP to 36 was in this case quite facile and uneventful.
Interestingly, in the case of 36, no epimerization of the trans-
hydrindane moiety to the cis-isomer was observed during the
25
a
1
1
DMP oxidation protocol (cf. 34). This observation clearly
indicated that subtle interplay of stereoelectronic factors
profoundly affect the relative cis vs trans thermodynamic
stability of the embedded hydrindane systems.
H
H
H
O
O
O
O
O
O
H
H
H
HO
18
HO
30
O
31
H
H
H
O
O
b
O
a
O
O
O
c
d
b
H
H
H
O
HO
HO
3
2
35
36
c
ORTEP of 30
H
H
H
O
O
O
H
O
O
O
O
O
H
HO
H
H
O
HO
1
9
32
29
H
ORTEP of 37
o
Scheme 5. Reagents and conditions: (a) SeO
h, 1.6:1.6:1, 92% (BRSM); (b) Grubbs' I (5 mol %), CH
c) Grubbs' I (5 mol %), rt, 2.5 h, 83%; (d) Grubbs' I (5 mol %), rt, 6 h, 82%.
2
/ TBHP, CH
3
CN/ DCM, 60 C,
37
o
2
2
Cl
2
, rt, 2.5 h, 78%;
Scheme 7. Reagents and conditions: (a) Et Zn, ZnI , CH I , CH Cl , 0-10 C,
2
2
2
2
2
2
o
(
2 2
16 h, 81%; (b) DMP, CH Cl , 0 C- rt, 2h, 84%; (c) 1-(tert-butyl)-5-
o
(
methylsulfonyl)-1H-tetrazole, NaHMDS, THF, -78 C, 2.1 h, 75%.

4
Tetrahedron
Lastly, Julia-Kocienski olefination on the cyclopentanone
carbonyl of 36 led to 37, a diastereomer of natural product
L.; Liu, B. J. Am. Chem. Soc. 2013, 135, 9291-9294; (k) Zhang,
H.; Nan, F. Chin. J. Chem. 2013, 31, 84-92; (l) Du, B.; Yuan, C.;
Yu, T.; Yang, L.; Yang, Y.; Liu, B.; Qin, S. Chem. Eur. J. 2014,
20, 2613–2622; (m) Yang, L.; Yue, G.; Yuan, C.; Du, B.; Deng,
H.; Liu, B. Synlett 2014, 2471–2474.
1
1
shizukanolide 5. A single crystal X-ray crystal structure
determination of 37 is shown in Scheme 7 and secured its
structure.
1
0
6
7
.
.
(a) Mehta, G.; Kumaran, R. S. Tetrahedron Lett. 2003, 44, 7055–
In conclusion, we have outlined a concise approach of
general applicability to lindenanolide sesquiterpenoids, from a
readily accessible building block. The effort has culminated in
the total synthesis of natural product sarcandralactone A and 5-
epi-shizukanolide and paves the way for the adaptation of the
approach for the syntheses of other members of this class.
7
1
1
059; (b) Kumaran, R. S.; Mehta, G. Tetrahedron 2015, 71, 1547–
554; (c) Kumaran, R. S.; Mehta, G. Tetrahedron 2015, 71, 1718–
731.
(a) Mehta, G.; Bera, M. K. Tetrahedron Lett. 2009, 50, 3519-
3
522; (b) Mehta, G.; Dhanabal, T.; Bera, M. K. Tetrahedron Lett.
010, 51, 5302-5305; (c) Sow, B.; Bellavance, G.; Barabé, F.;
2
Acknowledgments
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Commun. 2002, 2542-2543.
8
9
1
.
.
SR wishes to thank the University Grants Commission (UGC,
India) for the award of Dr. D. S. Kothari post-doctoral
fellowship. GM acknowledges the research support from Eli
Lilly and Jubilant-Bhartia Foundations.
He, X. -F.; Yin, S.; Ji, Y. -C.; Su, Z. -S.; Geng, M. -Y.; Yue, J. -
M. J. Nat. Prod. 2010, 73, 45–50.
0. Single crystal X-ray data for 27 and 33 was collected on a Bruker
AXS SMART APEX CCD diffractometer at 291 K using graphite
References and notes
α
monochromated MoK radiation (λ = 0.7107 Å). The data were
1
.
(a) Cao, C. -M.; Peng, Y.; Shi, Q. -W.; Xiao, P. -G. Chem.
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reduced by SAINTPLUS; an empirical absorption correction was
applied using the package SADABS and XPREP was used to
determine the space group. The crystal structure was solved by
direct methods using SIR92 and refined by the full-matrix least-
2
squares method on F using SHELXL97. Crystal data for 27:
(
CCDC 1046034), C13
16 2
H O , M = 204.26, monoclinic, P2(1)/n, a
=
6.5954(15) Å, b = 16.390(4) Å, c = 10.327(2) Å, V = 1091.9(4)
3
3
Å , Z = 4, ρcalcd = 1.242 g/cm , 6604 reflections measured, 2147
9
0-100; (g) Fraga, B. M. Nat. Prod. Rep. 2013, 30, 1226–1264;
unique (R(int) = 0.0303), R1 = 0.0540 and wR2 = 0. 1873. Crystal
(
h) Yan, H.; Li, X -H.; Zheng, X. -F; Sun, C. -L; Liu, H. -Y. Helv.
data for 33: (CCDC 1046037), C14
18 3
H O , M = 234.28, monoclinic,
Chim. Acta. 2013, 96, 1386–1391.
P2(1)/n, a = 11.204(6) Å, b = 10.011(5) Å, c = 11.797(6) Å, V =
2
3
.
.
(a) Zhang, W.; Guo, Y. -W.; Mollo, E.; Cimino, G. Chin. J. Nat.
Med. 2005, 3, 280-283; (b) Kao, S. -Y.; Su, J. -H.; Hwang, T. -L.;
Sheu, J. -H.; Wen, Z. -H.; Wu, Y. -C.; Sung, P. -J. Mar. Drugs
3
3
1
220.4(11) Å , Z = 4, ρcalcd = 1.270 g/cm , 11993 reflections
measured, 2413 unique (R(int) = 0.0254), R1 = 0.0510 and wR2 =
.1513.; Single crystal X-ray data for 30, 14, 37 was collected on
Oxford CCD X-ray diffractometer (Yarnton, Oxford, UK)
equipped with Cu-K radiation (k = 1.54 Å) source. The data were
0
2
011, 9, 1534–1542; (c) Almeida, M. T. R.; Moritz, M. I. G.;
Capel, K. C. C.; Pérez, C. D.; Schenkel, E. P.
Rev. Bras. Farmacogn. 2014, 24, 446–467.
α
reduced by SAINTPLUS; an empirical absorption correction was
applied using the package SADABS and XPREP was used to
determine the space group. The crystal structure was solved by
direct methods using SIR92 and refined by the full-matrix least-
For examples of biological activities of oligomeric lindenanolides,
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4
56; (b) Zhang, M.; Wang, J.-S.; Oyama, M.; Luo, J.; Guo, C.; Ito,
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S. A.; Palermo, J. A.; Sánchez, M. J. Nat. Prod. 2014, 77, 1579–
585.
2
squares method on F using SHELXL97. Crystal data for 30:
7
(
8
2
20 3
CCDC 1046036), C15H O , M = 248.31, triclinic, P-1, a =
3
.574(2) Å, b = 9.656(3) Å, c = 10.069(3) Å, V = 704.7(4) Å , Z =
1
3
, ρcalcd = 1.170 g/cm , 4946 reflections measured, 4104 unique
4
.
For examples of biological activities of dimeric lindenanolides,
see (a) Xu, Y. -J.; Tang, C. -P.; Ke, C. -Q.; Zhang, J. -B.; Weiss,
H. -C.; Gesing, E. -R.; Ye, Y. J. Nat. Prod. 2007, 70, 1987-1990;
(
R(int) = 0.0459), R1 = 0.0874 and wR2 = 0.2526; Crystal data for
4: (CCDC 1046040), 230.30, monoclinic,
P121/c1, a = 15.0093(7) Å, b = 6.48902(19) Å, c = 13.0772(5) Å,
1
15 18 2
C H O , M =
(
b) Yang, S. -P.; Gao, Z. -B.; Wu, Y.; Hu, G. -Y.; Yue, J. -M.
3
3
V = 1209.67(8) Å , Z = 4, ρcalcd = 1.253 g/cm , 4350 reflections
Tetrahedron 2008, 64, 2027–2034; (c) Fang, P. -L.; Cao, Y. -L.;
Yan, H.; Pan, L. -L.; Liu, S. -C.; Gong, N. -B.; Lü, Y.; Chen, C. -
X.; Zhong, H. -M.; Guo, Y.; Liu, H. -Y. J. Nat. Prod. 2011, 74,
measured, 2298 unique (R(int) = 0.0148), R1 = 0.0557 and wR2 =
0
2
1
2
0
.1659; Crystal data for 37: (CCDC 1046041), C15
30.30, monoclinic, P2 /c, a = 8.3521(3) Å, b = 13.7763(5) Å, c =
0.9918(3) Å, V = 1262.23(7) Å , Z = 4, ρcalcd = 1.212 g/cm
18 2
H O , M =
1
1
408–1413; (d) Ni, G.; Zhang, H.; Liu, H. -C.; Yang, S. -P.; Geng,
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J.; Xiong, J.; Liu, S. -T.; Liu, X. -H.; Hu, J. -F. Chem.
3
3
,
435 reflections measured, 2369 unique (R(int) = 0.0213), R1 =
.0621 and wR2 = 0.1987.
-
Biodiversity 2014, 11, 919-928; (f) Dembitsky, V. M. J. Mol.
Genet. Med. 2015, 9, 1-8.
1
1. All compounds reported here are racemic and were fully
1
13
characterized on the basis of IR, H NMR, C NMR and HRMS
spectral data. Spectral data of selected compounds are given in the
Supplementary material.
5
.
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Adlington, R. M.; Baldwin, J. E. Synlett 2007, 2679–2682; (b)
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Lu, Y. -S.; Peng, X. -S. Org. Lett. 2011, 13, 2940–2943; (d) Qian,
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