(?)-Isoscopariusin A, a Naturally Occurring Immunosuppressive Meroditerpenoid: Structure Elucidation and Scalable Chemical Synthesis

Article abstract of DOI:10.1002/anie.202100288

(?)-Isoscopariusin A was isolated from the aerial parts of Isodon scoparius. Chemical synthesis and spectroscopic analysis established its structure as an unsymmetrical meroditerpenoid bearing a sterically congested 6/6/4 tricyclic carbon skeleton with se

Full text of DOI:10.1002/anie.202100288

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Total Synthesis
Hot Paper
(ꢀ)-Isoscopariusin A, a Naturally Occurring Immunosuppressive
Meroditerpenoid: Structure Elucidation and Scalable Chemical
Synthesis
Bing-Chao Yan+, Min Zhou+, Jian Li, Xiao-Nian Li, Shi-Jun He, Jian-Ping Zuo, Han-Dong Sun,
Ang Li,* and Pema-Tenzin Puno*
Abstract: (ꢀ)-Isoscopariusin A was isolated from the aerial
parts of Isodon scoparius. Chemical synthesis and spectro-
scopic analysis established its structure as an unsymmetrical
meroditerpenoid bearing a sterically congested 6/6/4 tricyclic
carbon skeleton with seven continuous stereocenters. A gram-
scale synthesis was achieved in 12 steps from commercially
available (+)-sclareolide. A cobalt catalyzed, hydrogen atom
transfer-based olefin isomerization was used to prepare
a trisubstituted alkene, which underwent stereoselective [2+2]
cycloaddition with a substituted keteniminium ion generated in
situ from the corresponding amide. The cyclobutanone prod-
uct was further elaborated into the fully substituted cyclo-
butane core through face-selective homologation, and the two
side chains were installed by using nickel-catalyzed cross-
electrophile coupling and carbodiimide-mediated esterifica-
tion, respectively. (ꢀ)-Isoscopariusin A displayed selective
inhibition of T-cell proliferation.
as treating autoimmune and inflammatory diseases.[1]
A
number of immunosuppressive drugs have been used clin-
ically, such as cyclosporine A, FK506, and rapamycin.[2]
However, their side effects including hepatoxicity and neph-
rotoxicity turn out to be a severe problem.[3] Natural products
have proven to be a rich source of immunosuppressive
agents.[4] Therefore, searching for naturally occurring novel
immunosuppressants is of remarkable medical interest. Since
the discovery of the fungal metabolite, cyclosporine A, in
1970, a series of natural products derived from microbes have
been reported to possess considerable immunosuppressive
activity.[4c,5] Moreover, natural immunosuppressive agents
derived from plants have aroused increasing attentions of
chemists.[6]
Isodon species have attracted considerable interest among
organic chemists because they are rich in structurally and
biologically interesting diterpenoids.[7] Isodon scoparius, a rare
herbage, has been used as an antipyretic agent by local
inhabitants in Shangrila City of Yunnan Province, P. R. China.
Our previous search for bioactive constituents from this
species has led to the discovery of three structurally unusual
meroditerpenoids, scopariusic acid[8] and scopariusicides A
and B.[9] These compounds feature a densely substituted
cyclobutane ring and exhibit immunosuppressive activities.
Notably, we accomplished the synthesis of scopariusicide A
Introduction
Immunosuppressive agents are used for protecting trans-
planted organs from a hostꢀs immune system response as well
[*] Dr. B.-C. Yan,[+] Dr. M. Zhou,[+] Dr. X.-N. Li, Prof. Dr. H.-D. Sun,
Prof. Dr. P.-T. Puno
ꢀ
by exploiting an amide directed C H arylation (Pd-catalyzed
State Key Laboratory of Phytochemistry and Plant Resources in West
China, and Yunnan Key Laboratory of Natural Medicinal Chemistry,
Kunming Institute of Botany, Chinese Academy of Sciences
Kunming 650201 (China)
3
ꢀ
sp C H bond b-arylation). Our continuing exploration for
the low-abundance constituents of I. scoparius resulted in the
discovery of (ꢀ)-isoscopariusin A (1, Figure 1), a novel
meroditerpenoid with an unprecedented 6/6/4 tricyclic carbon
skeleton bearing a fully substituted cyclobutane ring and
seven contiguous stereocenters. However, the confirmation of
the stereochemistry of 1, in particular, the congested cyclo-
butane core and the C3’’ stereocenter at the side chain, proved
to be a problem. Due to the low-abundance from natural
source and ambiguous structure, a chemical synthesis of
1 could not only confirm all the stereocenters indubitably but
also enable further biological study.[10]
E-mail: punopematenzin@mail.kib.ac.cn
Dr. S.-J. He, Prof. Dr. J.-P. Zuo
Laboratory of Immunopharmacology, State Key Laboratory of Drug
Research, Shanghai Institute of Materia Medica, Chinese Academy of
Sciences
Shanghai 201203 (China)
and
University of Chinese Academy of Sciences
Beijing 100049 (China)
Dr. B.-C. Yan,[+] Dr. J. Li, Prof. Dr. A. Li
State Key Laboratory of Bioorganic and Natural Products Chemistry,
Center for Excellence in Molecular Synthesis, Shanghai Institute of
Organic Chemistry, University of Chinese Academy of Sciences,
Chinese Academy of Sciences
345 Lingling Road, Shanghai 200032 (China)
E-mail: ali@sioc.ac.cn
Due to the fascinating architectures as well as remarkable
biological activities of the cyclobutane-containing natural
products, an increasing number of novel approaches targeting
these unique structures have emerged.[11] The assembly of an
unsymmetrical cyclobutane ring in a controlled and efficient
fashion in these unique structures has long been highly
desirable. [2+2] cycloadditions provide a direct and atom-
economical approach to access the cyclobutane scaffold with
up to four stereocenters in a single step.[12] While different
[+] These authors contributed equally to this work.
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.202100288.
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tion). Analysis of the 13C NMR and HSQC spectra revealed
37 carbon signals assigned to six methyls, eleven methylenes
(containing one sp2 methylene), twelve methines (containing
six sp2 methines and one oxygenated), and eight quaternary
carbons (including two carbonyls and three sp2 carbons).
Comprehensive analysis of 2D NMR spectra allowed the
establishment of the planar structure of 1 as a meroditerpe-
noid. Interestingly, the NMR data for 1 indicated that it can
be subdivided into three distinct structural fragments (Fig-
ure 2): a labdane-type diterpene nucleus (part A), a phenyl-
propanoid derivative (part B), and a C8 fatty alcohol (part C).
Part A was found to be similar to the diterpene nucleus of
labda-7,13E-dien-15-oic acid,[13] except that the C7-C8 double
bond was saturated in 1, which was proven by the HMBC
correlations from H-7 to C-5, C-8, C-9 and C-17. Part B was
deduced by the 1H-1H COSY correlations of H-7’/H-8’, H-2’/
H-3’, and H-5’/H-6’, together with the HMBC correlations
from H-2’, H-6’ to C-4’, C-7’, H-3’, H-5’ to C-1’, C-4’, H-7’ to C-
9’, and H-8’ to C-1’, C-9’. Part C was identified as 1-octene-3-
ol on the basis of 1H-1H COSY correlations (H2-1’’/H-2’’/H-3’’/
H2-4’’/H2-5’’/H2-6’’/H2-7’’/H3-8’’), together with the HMBC
correlations from H-1’’ to C-3’’, H-3’’ to C-5’’, H-4’’ to C-2’’,
H’’-7 to C-5’’, and H-8’’ to C-6’’. Parts A and B were directly
linked to form a four-membered ring via C-7/C-8/C-7’/C-8’
based on the 1H-1H COSY correlations (H2-6/H-7/H-8’/H-7’),
along with HMBC correlations from H3-17 to C-9, C-7’, H-7
to C-9’, and H-8’ to C-8. In addition, Parts B and C were
Figure 1. Structure of (ꢀ)-isoscopariusin A (1).
kinds of reactions including [2+2] photocycloaddition, oxida-
tive radical and acid-catalyzed [2+2] cycloaddition have
rendered a handful of cyclobutane-containing structures, it
remains uncertain whether or not the existing synthetic
methods and strategies can be used to access efficient
synthesis for the unprecedented cyclobutane-containing mer-
oditerpenoid 1. Herein, we report the structure elucidation
and scalable chemical synthesis of (ꢀ)-isoscopariusin A (1)
and its significant immunosuppressive selectivity on T lym-
phocytes. A gram-scale synthesis of 1 was achieved in 12 steps
from commercially available (+)-sclareolide.
Results and Discussion
ꢀ
connected via a C O bond on the basis of the key HMBC
Structure Elucidation and Plausible Biogenetic Pathway of 1
correlation from H-3’’ to C-9’.
The relative configuration of 1 was partially assigned by
a ROESY spectrum (Figure 2). The ROESY correlations for
H-2’, H-6’ with H-7, H-8’, H3-17, and H-7/H3-17, H3-17/H3-20,
H-7/H-8’ indicated that H-7, H-8’ and Me-17 were cofacial,
which were arbitrarily assigned to be b-orientated. The
ROESY correlation of H-7’ with H-9 suggested that H-7’
adopted an a-orientation. However, the determination of the
relative configuration at C-3’’ was challenging due to its
location at the freely rotating side chain.
(ꢀ)-Isoscopariusin A (1) was obtained as colorless gum,
with the molecular formula C37H54O5, as established by a (+)-
HREIMS ion peak at m/z 578.3941 [M]+ (calcd 578.3971) and
eleven double bond equivalents (DBEs). The 1H NMR
spectrum displayed signals due to one 4-substituted phenyl,
one terminal olefin and one trisubstituted olefin, one oxy-
genated methine, four tertiary methyls, one secondary methyl
and one olefinic methyl (Table S1 in the Supporting Informa-
Figure 2. Key 1H-1H COSY (bold), HMBC correlations (blue arrows) and ROESY correlations (dashed arrows) of 1.
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Biogenetically, 1 might be formed via enzyme-dependent
intermolecular [2+2] cycloaddition between a labdane diter-
penoid (labda-7,13E-dien-15-oic acid) and an unusual ester 2
of 4-hydroxycinnamic acid and (R)-1-octene-3-ol (Scheme 1).
Therefore, based on comparisons with reported NMR data
for analogues[8,9] and a plausible biogenetic pathway, the
orientation of H-3’’ was arbitrarily deduced to be b-orien-
tated. However, we cannot confirm the absolute configura-
tion due to the sterically congested four-membered ring and
the C8 side chain, and attempts to obtain single crystals
suitable for X-ray diffraction experiments failed. Aiming to
provide unambiguous confirmation of the stereochemistry for
the naturally scarce meroditerpenoid 1 and further explore its
bioactivity, we engaged to develop an efficient strategy for the
synthesis of 1.
Figure 3. Retrosynthetic analysis of 1 based on a photochemical [2+2]
cycloaddition approach.
was smoothly prepared on the gram scale from the available 4
via a developed protocol (regioselective elimination of 5 to
terminal alkenes,[15] ozonolysis, regioselective formation of
vinyl triflate and Negishi coupling reactions) (Scheme 2).
With an ample amount of 7 in hand, we examined
different conditions for photochemical [2+2] cycloaddition
reactions (Table S2 in the Supporting Information). Direct
intermolecular [2+2] photocycloaddition was conducted with
diverse modified forms of 2 and additives (including aceto-
phenone and benzophenone). As a result, no desired product
was observed, and a mixture of cis and trans cinnamic acid
derivatives was isolated. It suggested that the intermolecular
Scheme 1. Hypothetical biogenetic pathway of 1.
Attempts to Construct the 6/6/4 Tricyclic Core of 1 Based on
a Photochemical [2+2] Cycloaddition Approach
Our first instinct was to apply intermolecular photo-
chemical [2+2] cycloaddition to biogenetically construct the
6/6/4 tricyclic carbon skeleton (Figure 3). The stereoselective
functionalization of poly-substituted cyclobutane derivatives
is a key issue for the synthesis. Photochemical [2+2] cyclo-
addition reactions involving compounds with an unsaturated
carbonyl, cycloalkenyl esters and olefins have proved to be
one of the most useful stereoselective approaches to assemble
cyclobutane derivatives.[14] Thus, we envisaged an intermo-
lecular photochemical [2+2] cycloaddition reaction for the
consecutive C7, C8, C7’ and C8’ stereocenters. In this strategy,
1 is expected to be constructed through photochemical [2+2]
cycloaddition between phenylpropanoid derivatives of 2 and
trisubstituted alkene 3, which can be tracked back to the
(+)-sclareolide (4) from commercial sources. The alkene 7
Scheme 2. Attempts to construct the 6/6/4 tricyclic core of 1 based on
a [2+2] photocycloaddition approach.
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[2+2] photocycloaddition may be hindered for the large
trichloroacetyl chloride and active zinc to deliver the adduct
aromatic substitute, and that the Z/E isomerization of the
acyclic double bond in the precursors can occur. The visible
photoredox conditions developed by Yoon[16] was also tested,
but it proved to be fruitless in this particular case.
12 (Scheme 3). SmI2-mediated reduction efficiently provided
cyclobutanone 13. A mixture of 14 and its regioisomer (1:1.3)
was obtained in 61% yield involving vinyl triflate (KHMDS,
PhNTf2, ꢀ788C) and subsequent carbonylative coupling
[Pd(Ph3P)4, CO, 228C].[18] Compound 14 was obtained as
a single regioisomer by treatment of 13 with 2,4,6-triisopro-
pylbenzenesulfonyl hydrazide (Tris-NHNH2) and Na2SO4 as
a dehydrating reagent, followed by Shapiro coupling reaction
with careful handling with Tris-hydrazone (e.g., rather labile,
hygroscopic and irreproducible reaction results).[19] Arylation
of 14 with Grignard reagent 15 and CuBr·SMe2 afforded
a single product 16. Detailed analysis of its NMR spectrum
indicated that the H-7’ and H-8’ configurations were cofacial.
To obtain its stereoisomer 18, 16 was first treated with a base,
but it resulted in no conversion under thermodynamic (DBU,
toluene, 808C) and kinetic (LDA, THF, ꢀ788C) conditions,
respectively. To our delight, we managed to obtain the desired
adduct 18 in a two-steps approach[20] (KHMDS, PhSSPh; H2/
Raney Nickel) in 23% yield, along with its C8’ epimer 16
(51%) that can be recycled. Deprotection of 18 followed by
bromination furnished the bromide 19. The stereochemistry
of the thiophenyl derivative 17a and the bromide 19 was
corroborated by X-ray crystallographic analysis (Figure 5).
Following the “from south to north” strategy, we managed to
install the adjacent C7’ and C8’ stereocenters via Shapiro
coupling reaction, 1,4-addition and configuration inversion.
To improve the efficiency of the synthesis, we directed our
attention to the first a-arylation of cyclobutanone 13 followed
by installation of the chiral C’8 stereocenter (Scheme 4).
Attempts to proceed the Ni-catalyzed arylation of the a-
chloroketone 20 met with failure.[21] Under the conditions
reported by Britton and co-workers for the arylation of
cyclobutanones [(DtBPF)PdCl2, LiOtBu, 4-bromoanisole,
THF, 608C],[22] a-arylcyclobutanone 21 was obtained from
cyclobutanone 13 in 23% yield due to the poor diastereose-
lectivity.
First Route for the Synthesis of the Tricyclic Core of 1 through
Ketene-Based [2+2] Cycloaddition
Classical [2+2] cycloadditions of dichloroketene to olefins
can provide a dichlorocyclobutone adduct with highly regio-
and stereoselectivity.[17] We envisaged that the key intermedi-
ate 9 can be derived from [2+2] cycloaddition of trisubstituted
alkene 3 with dichloroketene 10, which can be in situ
generated from 11 (Figure 4).
To our delight, the alkene 7 underwent a smooth highly
regio- and stereoselective cycloaddition in the presence of
Figure 4. Retrosynthetic analysis of 1 through dichloroketene-based
[2+2] cycloaddition.
Scheme 3. First route for the synthesis of the tricyclic core of 1.
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Figure 5. ORTEP drawings of compounds 17a and 19.[34]
Scheme 4. Attempts to access a-arylcyclobutanone 21 via cross-cou-
pling reaction.
Figure 6. Retrosynthetic analysis of 1 through substituted ketene or
keteniminium salt based [2+2] cycloaddition.
be derived from 4 by reduction, elimination and bromination,
avoiding the spare manipulation of protection and depro-
tection.
Construction of a-Arylcyclobutanone 32 through Substituted
Ketene or Keteniminium Salt Based [2+2] Cycloaddition
Under optimized reaction conditions, diol derived from 4
underwent mesylation and elimination (Et3N, MsCl, toluene,
08C) in one pot, and subsequential bromination (LiBr, DMF,
408C) to afford an inseparable mixture of bromo alkenes
[terminal:internal (trisubstituted) = 1.8:1] on the decagram
scale. Using Shenviꢀs protocol for olefin isomerization,[23]
terminal olefin 30 was converted to trisubstituted olefin 25.
To avoid the undesired halogen exchange (Br ! Cl), we
In the first route for the synthesis of the tricyclic core of 1,
19 from 7 (Scheme 3), intermolecular [2+2] thermal cyclo-
addition between dichloroketene and trisubstituted alkene
was applied to assemble dichlorocyclobutanones. Although
Shapiro coupling reaction followed by 1,4-addition furnished
the key intermediate with a functionalized cyclobutane core
bearing two chiral centers, the mismatched C8’ stereochem-
istry and the subsequent operation of configuration inversion
significantly lowered the overall efficiency. Therefore, we
explored an alternative strategy in which a-arylcyclobuta-
nones were first established, followed by face-selective
homologation to access the key cyclobutane skeleton with
four chiral centers. With these thoughts in mind, we under-
took a retrosynthetic analysis of 1 (Figure 6). Disassembly of
the meroditerpenoid afforded the cross-coupling precursors
22 and 23. Acid 23 can be traced back to a-arylcyclobutanone
24 via stereo-controlled homologation. We envisaged that a-
arylcyclobutanone 24 could be rendered via substituted
ketene or keteniminium salt-based [2+2] cycloaddition of
alkene 25 in a “from North to South” strategy instead of the
initial establishment of the C8’ stereocenter. The alkene can
employed Co(SaltBu tBu)Br as a catalyst for olefin isomer-
,
ization instead of commonly used Co(SaltBu,tBu)Cl. With a large
quality of 25 in hand, we directed our attention to intermo-
lecular [2+2] cycloaddition to obtain a-arylcyclobutanone 32.
Brown and co-workers have developed a method for Lewis
acid-promoted [2+2] cycloaddition with aryl ketenes, which
provided a solution to the limit of [2+2] cycloaddition
reactions between poorly reactive and unstable monosubsti-
tuted aryl ketenes with alkenes.[24] To our delight, the desired
adduct 32 was obtained with high diastereoselectivity using
this protocol (Scheme 5), but the conversion rate was found to
be dissatisfactory (less than 10%), in spite of our attempts to
condition optimization, due to the instability of monosubsti-
tuted aryl ketenes and product inhibition.[25]
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Table 1: Optimization of the keteniminium salt based intermolecular
[2+2] cycloaddition reaction.
Entry[a]
Conditions
Yield [%][b]
Scheme 5. Construction of a-arylcyclobutanone 32 through ketene-
based [2+2] cycloaddition.
32
34
1
2
3
4
5
6
7
8
9
2,4,6-Collidine, 228C to 608C, DCE
2,4,6-Collidine, ꢀ408C to 08C, DCE
2-F-Pyridine, ꢀ408C to 08C, DCE
2-Cl-Pyridine, ꢀ408C to 08C, DCE
3-Cl-Pyridine, ꢀ408C to 08C, DCE
iPr2NEt, ꢀ408C to 08C, DCE
12
35
51
47
45
0
13
37
28
74
25
13
16
15
17
0
21
17
16
14
An alternative solution to the problem of cycloaddition
with unstable or unreactive ketenes and product inhibition is
the chemistry of keteniminium salts.[26] However, it still
remains a challenge to realize intermolecular [2+2] cyclo-
addition between a keteniminium salt and trisubstituted
alkene to form a-arylcyclobutanones. As illustrated previ-
ously, once the first step occurs, due to the stabilization of the
DTBMP, ꢀ408C to 08C, DCE
2-F-Pyridine, ꢀ408C to 08C, CH2Cl2
2-F-Pyridine, ꢀ408C to 08C, toluene
2-F-Pyridine, ꢀ408C to ꢀ108C, DCE
10
ꢀ
tertiary cation and the steric hindrance, the C N bond of the
[a] Reagents and conditions: 33 (1.2 equiv), base (1.2 equiv), Tf2O
(1.1 equiv), indicated solvent (0.12 M) at the indicated temperature for
12–30 h. [b] Isolated yield.
enamine tends to twist and the cation is rearranged or
undergoes b-elimination instead of the four-membered ring
formation.[27] In spite of this, we envisaged that some
optimized conditions that can stabilize the tertiary cation
may work on the issue. Interestingly, under Ghosezꢀs con-
ditions (entry 1 in Table 1),[27d,e] the rearranged and elimina-
tion adduct 34 was obtained in 25% yield, and a small ratio
for the desired product 32 was also observed. Encouraged by
the result, we continued to investigate other reaction con-
ditions. Given the instability of the intermediate 34a, a lower
temperature (ꢀ408C to 08C) was adopted, which led to an
improved yield for 32. Further study on the influences of
different bases demonstrated that 2-fluoropyridine behaved
in a more superior way compared to 2-chloropyridine, 3-
chloropyridine, and so on. Solvent effects on the reaction
were also studied and 1,2-dichloroethane (DCE) showed the
best performance. Under the optimized conditions (entry 10),
the [2+2] cycloaddition proceeded smoothly on a three-gram
scale to deliver 32 (74% yield) with the desired stereoselec-
tivity. This finding sheds a new light on the chemistry of
keteniminium salts in the intermolecular [2+2] cycloaddition
of poly-substituted alkenes.
(17:1 dr at C8’). Treatment of 35 with PhI(OAc)2 and
a catalytic amount of ABNO (36) afforded carboxylic acid
37 in 89% yield.[29] In situ TBDPS protection of carboxylic
acid followed by nickel-catalyzed cross-coupling with (E)-3-
bromobut-2-en-1-ol 21 under the Weixꢀs conditions[30] fur-
nished the desired E-product 39 in moderate yield, which
overcame the cross-coupling reactionꢀs limited compatibility
with carboxylic acid. Oxidation of allylic alcohol to aldehyde,
followed by condensation with the chiral fatty alcohol 40,[31]
furnished the ester 41 with good overall efficiency. Lindgren-
Krauss-Pinnick oxidation and TBAF removal of the TBDPS
protective group in one pot rendered 1 in 79% yield. Over 1 g
of synthetic 1 was prepared through this route, every reaction
in which can be reliably carried out on a gram scale. The NMR
data, specific rotation, and CD spectrum of 1 were in full
agreement with those obtained for the authentic natural
sample.
In a similar protocol, 42, the stereoisomer at C3’’ of 1, was
also synthesized from 39 (see the Supporting Information),
whose NMR spectrum indicated obvious differences at C3’’
compared with 1, which provided an unambiguous evidence
for the stereochemical assignment of (ꢀ)-isoscopariusin A.
Thus, the absolute configuration of the natural (ꢀ)-isoscopar-
Completion of the Gram-scale Synthesis and Stereochemical
Assignment of (ꢀ)-Isoscopariusin A
As illustrated in Scheme 6, carbonyl methylenation of 32
using the Jamison modification of Taikai-Utimoto protocol,[28]
followed by selective hydroboration-oxidation using Sia2BH/
H2O2 conditions, afforded alcohol 35 with superior selectivity
iusin A
was
determined
to
be
5S,7S,8R,9R,10S,13E,7’S,8’R,3’’R. Given that some natural
producs may not be enantiopure,[32] or have similar chemical
structures from two different sources,[33] the chemical syn-
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Scheme 6. Completion of the synthesis of 1 on a gram scale.
thesis of (ꢀ)-isoscopariusin A unequivocally resolves the
stereochemistry and reveals the enantiopure nature of the
natural isoscopariusin A.
exhibiting potent and selective inhibition of T-cell prolifer-
ation. A concise and efficient synthesis of 1 was achieved from
commercially available (+)-sclareolide in 12 steps on a gram
scale, thereby clarifying the complete assignment of its
relative and absolute stereochemistry. The synthesis exhibited
the following features: (a) trisubstituted alkenes, the precur-
sors for the [2+2] cycloaddition reaction, prepared through
two approaches featuring Negishi coupling and catalytic
olefin isomerization, respectively, (b) intermolecular [2+2]
cycloaddition of keteniminium salt with trisubstituted alkenes
for the construction of a-arylcyclobutanones, and face-
selective homologation, which were developed to install four
consecutive stereocenters for the key intermediate 35, and (c)
nickel-catalyzed cross-electrophile coupling of vinyl bromide
with alkyl bromide, which was applied to deliver the late
intermediate 39. These endeavors pave the way for further
structural modification and biological studies of this potential
and structurally fascinating class of immunosuppressive
agents.
The Immunosuppressive Activity of (ꢀ)-Isoscopariusin A (1)
With an adequate amount of the synthetic (ꢀ)-isoscopar-
iusin A in hand, we evaluated the immunosuppressive activity
of 1 against the proliferation of T and B lymphocytes in vitro
with cyclosporin A (CsA) as the positive control (Table 2).
1 showed significantly inhibitory activity against ConA-
induced T-cell proliferation (IC50 value of 0.68 mM) and
moderate activity against LPS-induced B-cell proliferation
(IC50 value of 13.81 mM), which suggested immunosuppres-
sive selectivity on T lymphocytes. As far as we know, 1 is a T
lymphocyte-selective immunosuppressive meroditerpenoid
possessing an unprecedented 6/6/4 tricyclic core, which
implies a possible different mechanism of action.
Table 2: Immunosuppressive effect of 1 on murine lymphocyte prolifer-
ation induced by ConA (5 mgmLꢀ1) or LPS (10 mgmLꢀ1).[a]
Acknowledgements
cmpd
CC50
[mM]
ConA-induced T-cell
proliferation
LPS-induced B-cell
proliferation
Financial support for this work was provided by the Second
Tibetan Plateau Scientific Expedition and Research (STEP)
program (2019QZKK0502), the NSFC-Joint Foundation of
Yunnan Province (U2002221), the National Natural Science
Foundation of China (81673329, 21931014 and 22007089),
Chinese Academy of Sciences (Strategic Priority Research
Program XDB20000000 and Key Research Program of
Frontier Sciences QYZDB-SSW-SLH040), the CAS “Light
of West China” Program and CAS Interdisciplinary Innova-
tion Team (Pema-Tenzin Puno), the Yunnan Science Fund for
Distinguished Young Scholars (2019FJ002). Authors are
grateful to Prof. Jun Deng for discussions, and Xiao-Li Bao
and Ling-Ling Li from the Instrumental Analysis Center of
Shanghai Jiao Tong University for X-ray crystallographic
analysis.
IC50 [mM]
SI
IC50 [mM]
SI
1
CsA
19.65
4.09
0.68
0.02
28.90
204.50
13.81
0.70
1.42
5.84
[a] CC50, the cytotoxic concentration of the compound that reduces cell
viability by 50%; IC50, the inhibitory concentration of the compound that
reduces cell proliferation by 50%; Selective Index (SI)=CC50/IC50.
Conclusion
In conclusion, an unprecedented meroditerpenoid, (ꢀ)-
isoscopariusin A (1), was obtained in low abundance from I.
scoparius, featuring a 6/6/4 tricyclic carbon skeleton and
&&&&
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(for 19) contain(s) the supplementary crystallographic data for
this paper. These data are provided free of charge by the joint
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Accepted manuscript online: February 23, 2021
Version of record online: && &&
,
&&&&
&&&&
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Research Articles
Total Synthesis
B.-C. Yan, M. Zhou, J. Li, X.-N. Li, S.-J. He,
J.-P. Zuo, H.-D. Sun, A. Li,*
P.-T. Puno*
&&&& — &&&&
(ꢀ)-Isoscopariusin A, a Naturally
Occurring Immunosuppressive
Meroditerpenoid: Structure Elucidation
and Scalable Chemical Synthesis
Reported herein is the isolation, structure
determination of (ꢀ)-isoscopariusin A by
spectroscopic analysis and concise
chemical synthesis. The unprecedented
meroditerpenoid bears a 6/6/4 tricyclic
carbon skeleton with seven continuous
stereocenters, and exhibits selective
inhibition of T-cell proliferation. A gram-
scale synthesis was achieved in 12 steps
from (+)-sclareolide.
Angew. Chem. Int. Ed. 2021, 60, 2 – 11
ꢀ 2021 Wiley-VCH GmbH
www.angewandte.org
&&&&
These are not the final page numbers!