Total synthesis of jiadifenolide

Article abstract of DOI:10.1002/anie.201404224

As a potent neurotrophic agent, the sesquiterpenoid jiadifenolide represents a valuable small-molecule lead for the potential therapeutic treatment of neurodegenerative diseases. A stereocontrolled total synthesis of this densely functionalized natural product is reported, central to which is an adventurous samarium-mediated cyclization reaction to establish the tricyclic core and the adjacent C5 and C6 quaternary stereocenters.

Full text of DOI:10.1002/anie.201404224

Angewandte
Chemie
DOI: 10.1002/anie.201404224
Natural Product Synthesis
Total Synthesis of Jiadifenolide**
Ian Paterson,* Mengyang Xuan, and Stephen M. Dalby*
Dedicated to Professor Richard J. K. Taylor on the occasion of his 65th birthday
Abstract: As a potent neurotrophic agent, the sesquiterpenoid
jiadifenolide represents a valuable small-molecule lead for the
potential therapeutic treatment of neurodegenerative diseases.
A stereocontrolled total synthesis of this densely functionalized
natural product is reported, central to which is an adventurous
samarium-mediated cyclization reaction to establish the tricy-
clic core and the adjacent C5 and C6 quaternary stereocenters.
the potential therapeutic treatment of neurodegenerative
conditions such as Alzheimerꢀs disease.[2] Its low natural
abundance (1.5 mgkgÀ1 plant material) makes total synthesis
of particular importance for the preparation of sufficient
quantities for further biological evaluation and to enable
access to a range of analogues for structure–activity relation-
ship (SAR) profiling.[3]
Our interest in jiadifenolide derives not only from its
potent bioactivity, but also from its complex seco-prezizaane
skeleton, placing it amongst the Illicium family of sesquiter-
penoids,[4] which has garnered substantial synthetic atten-
tion.[2a,5] Jiadifenolideꢀs intricate and densely functionalized
caged pentacyclic structure (1) comprises four rings emanat-
ing from a central, highly substituted B-ring cyclohexane. This
poses particular synthetic challenges, most notably the
controlled introduction of five contiguous quaternary stereo-
centers, which we aimed to address as part of a concise and
efficient total synthesis,[6] following that first reported by
Theodorakis in 2011 and most recently by Sorensen (2014).[7]
Our conceptually unique synthetic solution sought to
forge the central B-ring from a functionalized A,C-ring
precursor. As outlined retrosynthetically in Scheme 1, we
envisaged late-stage unveiling the D- and E-rings through
oxidation of the C13-pendant vinyl group of ABC-tricycle 2.
A critical transformation would thus be the reductive
cyclization of keto-butenolide 3 to establish the central B-
ring and the adjacent C5 and C6 quaternary stereocenters in
a single operation. Cyclization substrate 3 would be prepared
through aldol coupling of butenolide 5 with aldehyde 4.
Notably, a single C5 stereocenter would template for all
remaining stereochemistry under substrate-based control.
The preparation of aldehyde 4 commenced with Luche
reduction of cyclopentenone 6 (Scheme 2).[8] The ensuing
allylic alcohol then underwent hydroxy-directed epoxidation
with m-CPBA and TBS protection to deliver epoxide 7 in
65% yield over the three steps. The C2 methyl-bearing
stereocenter was then established through Lewis-acid medi-
ated rearrangement of 7 to selectively deliver the 2,5-syn-
configured cyclopentanone 8 (78%, 19:1 d.r.).[9,10] Horner–
Wadsworth–Emmons (HWE) homologation of 8 (73%,
> 19:1 E:Z) followed by reduction and acylation of 10 then
smoothly delivered allylic acetate 11 in readiness for an
Ireland–Claisen rearrangement to install the key C13 quater-
nary stereocenter.[11] In the event, heating the corresponding
TBS ketene acetal (LDA, TBSCl) in anhydrous benzene led
to the selective (10:1 d.r.) formation of a mixture of
rearranged products arising from preferential reaction at
the less hindered alkene p-face.[12] This material was subjected
directly to reduction with LiAlH4 to deliver alcohol 12 in 63%
J
iadifenolide (1, Scheme 1) is an architecturally complex
sesquiterpenoid first isolated from the pericarps of the
Chinese plant Illicium jiadifengpi by Fukuyama and co-
workers in 2009.[1] Preliminary biological investigation
revealed potent neurotrophic activity, promoting neurite
outgrowth in primary cultured rat cortical neurons at
concentrations as low as 10 nm. Given the important regu-
latory role of neurotrophins in the central nervous system,
jiadifenolide represents a valuable small-molecule lead for
Scheme 1. Retrosynthetic analysis of jiadifenolide (1).
[*] Prof. Dr. I. Paterson, M. Xuan
University Chemical Laboratory, University of Cambridge
Lensfield Road, Cambridge, CB2 1EW (UK)
E-mail: ip100@cam.ac.uk
Homepage: http://www.paterson.ch.cam.ac.uk/
Dr. S. M. Dalby
Department of Process Chemistry, Merck & Co
PO Box 2000, Rahway, NJ 07065 (USA)
E-mail: stephen.dalby@merck.com
[**] We thank Clare College Cambridge (Fellowship to S.M.D.), the
EPSRC UK National Mass Spectrometry Facility at Swansea
University for mass spectra, Dr. J. E. Davies for crystallographic
analysis, Dr. J. M. Goodman for assistance with NMR prediction
studies, and Dr. M. Sidera for exploratory work.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201404224.
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Scheme 2. Preparation of A-ring keto-aldehyde 4. Reagents and con-
ditions: a) NaBH4, CeCl3, MeOH, À788C to 08C; b) m-CPBA, CH2Cl2,
08C; c) TBSCl, imid, CH2Cl2, 08C to RT, 65% over 3 steps; d) BF3·OEt2,
CH2Cl2, À208C, 78%, 19:1 d.r.; e) NaH, RT, 72 h, 73%, >19:1 (E):(Z);
f) LiAlH4, Et2O, 08C; g) Ac2O, py, DMAP, CH2Cl2, RT, 77% over 2
steps; h) LDA, TBSCl, THF, À788C to RT; PhH, reflux, 16 h, 10:1 d.r.;
i) LiAlH4, Et2O, 08C, 63% over 2 steps; j) 3n HCl, MeOH, (1:3), RT;
k) DMSO, (COCl)2, Et3N, CH2Cl2, À788C to RT, 74% over 2 steps.
m-CPBA=3-chloroperoxybenzoic acid, DMAP=N,N-dimethyl-4-amino-
pyridine, DMSO=dimethylsulfoxide, imid=imidazole, LDA=lithium
diisopropylamide, py=pyridine, TBS=tert-butyldimethylsilyl, THF=
tetrahydrofuran.
Scheme 3. Preparation of ABC-tricycle 2. Reagents and conditions:
a) Bu2BOTf, iPr2NEt, THF, À788C to À208C, 98%, 2:1 d.r.; b) DMP,
NaHCO3, CH2Cl2, 83%; c) TESCl, imid, DMF, RT, 15: 65%, 11-epi-15:
29%; d) SmI2, 658C, 2 h, 51%; e) PPTS, MeOH, CH2Cl2, RT, 88%;
f) PCC, NaOAc, SiO2, CH2Cl2, 2: 81%, 18: 11%. DMF=N,N-dimethyl-
formamide, DMP=Dess–Martin periodinane, PCC=pyridinium
chlorochromate, PPTS=pyridinium para-toluenesulfonate, TES=
triethylsilyl.
yield from 11. Finally, hydrolysis of the TBS ether and double
oxidation under Swern conditions gave the targeted aldehyde
4 in 74% yield (13% overall from 6).
Initial approaches at appending the C-ring butenolide
through addition of 3-metallated furyl derivatives to 4 all met
with failure, with optimally only trace amounts of adducts
observed.[13] By contrast (Scheme 3), it was found that
butenolide 5[14] and aldehyde 4 could be smoothly coupled
through a boron-mediated aldol reaction, which provided
adducts 13 as a 2:1 mixture of diastereomeric alcohols in
almost quantitative yield.[15] At this stage, with a view to
effecting the crucial reductive cyclization reaction to close the
central B-ring, alcohols 13 were oxidized to provide tricar-
bonyl 14. Disappointingly however, no productive cyclization
Figure 1. a) X-ray crystal structure of alcohol 17. b) Possible samarium-
chelate transition structure leading to 16.
generation of a single cycloadduct (51%), identified as 16 on
the basis of NMR and computational analysis.[10,18] More
rigorous structural proof was subsequently obtained through
X-ray crystallographic analysis of alcohol 17, formed upon
acid-mediated desilylation of 16 (Figure 1a).[19] Subsequent
oxidation with PCC then delivered the targeted ABC-ketone
2 (81%)[19] along with diol 18 (11%), containing the requisite
oxygenation at C10.[20] Attempts to encourage complete
oxidation to 18 were unsuccessful however.
The desired stereochemical outcome of the SmI2-medi-
ated reductive cyclization reaction of 15 is consistent with
a chelated boat-type transition structure such as 19 (Fig-
ure 1b), in which the C11 substituent is equatorially dis-
posed.[21] This may explain the failure of the epimeric C11
TES ether (natural configuration, pseudo-axially disposed) to
undergo analogous cyclization, attesting, along with failed
of 14 could be induced under a range of conditions utilizing
[16]
either SmI2
or alternative reagents,[17] returning only
starting material or, under more forcing conditions, decom-
position products. A similar situation was observed for
alcohols 13. The apparent instability of 14 with respect to
potential reagents for cyclization led to the examination of
TES ethers 15 and 11-epi-15 as alternative substrates, which
were chromatographically separable. Now, gratifyingly, it was
found that addition of 15 to a freshly prepared solution of
SmI2 (ca. 6 equiv) in THF and heating to 658C, led to the
2
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Keywords: cyclization · neurological agents · samarium ·
.
terpenoids · total synthesis
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Scheme 4. Total synthesis of jiadifenolide (1). Reagents and condi-
tions: a) TMSOTf, Et3N, THF, À788C; b) OsO4, NMO, tBuOH/H2O,
RT; 99% over 2 steps; c) Me4NBH(OAc)3, AcOH, MeCN, À208C,
87%, >19:1 d.r.; d) TESCl, imid, DMF, RT, 96%; e) OsO4, NMO, py,
tBuOH/H2O, RT; f) TPAP, NMO, 4 ꢀ MS, CH2Cl2, RT, 84% over 2
steps; g) HF·py, THF, RT, 86%. NMO=N-methylmorpholine-N-oxide,
TMS=trimethylsilyl, TPAP=tetrapropylammonium perruthenate.
substrates 13 and 14, to the challenge of this adventurous
transformation.
With the requisite carbon skeleton of jiadifenolide
secured, completion of the total synthesis was accomplished
through controlled oxygenation at C10, C11, C14, and C15,
with attendant cyclization to form the D,E-rings (Scheme 4).
Accordingly, chemoselective dihydroxylation (OsO4, NMO)
of the TMS-enol ether of 2 (TMSOTf, Et3N), cleanly provided
the C10 tertiary alcohol (18, 99%). This was then utilized to
set the adjacent C11 alcohol stereocenter through hydroxy-
directed reduction with Me4NBH(OAc)3 (87%, > 19:1
d.r.).[22] TES protection proved necessary to facilitate the
subsequent unveiling of the D,E-rings (20, 96%). In the event,
pyridine-accelerated dihydroxylation of the C13-pendant
alkene of 20 provided a diol which then underwent smooth
oxidative lactonization upon treatment with TPAP, NMO to
form the E-ring (84%).[23] Finally, desilylation occurred with
concomitant hemiacetalization to form the D-ring and
provided synthetic (Æ)-jiadifenolide (1, 86%), which exhib-
ited spectral characteristics (1H/13C NMR, IR, HRMS) iden-
tical in all respects to that reported for the natural sample.[1]
In summary, we have completed the total synthesis of the
neurotrophic agent jiadifenolide in 2.3% yield over 23 steps,
showcasing a pivotal SmI2-mediated reductive cyclization
reaction to establish the tricyclic core. Notably, the synthesis
demonstrates a high degree of solely substrate-based stereo-
control to establish this densely functionalized structure,
wherein the full relative configuration is templated for by
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scaffold for further biological evaluation and analogue
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Received: April 11, 2014
Published online: && &&, &&&&
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4
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Angewandte
Chemie
Communications
Natural Product Synthesis
I. Paterson,* M. Xuan,
S. M. Dalby*
&&&& — &&&&
Total Synthesis of Jiadifenolide
Synthetic adventure: The total synthesis
of the pentacyclic sesquiterpenoid jiadi-
fenolide, a potent neurite outgrowth
promoter, is reported. The synthesis
showcases an adventurous samarium-
mediated cyclization reaction to establish
the ABC-tricyclic core and adjacent qua-
ternary stereocenters. Late-stage oxida-
tion of a pendant vinyl group completes
the highly stereocontrolled assembly of
this intricate structure.
Angew. Chem. Int. Ed. 2014, 53, 1 – 5
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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