Total synthesis of the tremorgenic indole diterpene paspalinine

Article abstract of DOI:10.1002/anie.201206299

Succinct and stereoselective: A high-yielding two-step indole ring installation comprising the Stille cross-coupling and a Pd^II-mediated oxidative heterocyclization was exploited in a concise total synthesis of paspalinine. The trans-anti-trans CDE fused ring system of the heptacyclic natural product was established highly stereoselectively through hydroxy-directed cyclopropanation and allylic selenoxide rearrangement. Copyright

Full text of DOI:10.1002/anie.201206299

Angewandte
Chemie
DOI: 10.1002/anie.201206299
Natural Product Synthesis
Total Synthesis of the Tremorgenic Indole Diterpene Paspalinine**
Masaru Enomoto, Akira Morita, and Shigefumi Kuwahara*
Paspalinine (1) and paspalicine (2) produced by the ergot
fungus Claviceps paspali belong to a large family of natural
products, known as indole diterpenoids, that is characterized
by a unique hybrid molecular architecture comprising an
indole nucleus and a cyclic diterpenoid moiety of high
structural diversity (Scheme 1).[1–3] As observed in many
toward naturally occurring indole diterpenoids have, so far,
not yet come to completion.[13,14] In their total synthesis of
1 and 2,[12b–d] the Smith research group achieved highly
stereoselective construction of a trans-anti-trans CDE ring
intermediate containing the C3/C4 contiguous quaternary
stereocenters (one of the most challenging tasks for the total
synthesis of indole diterpenoids), but required a considerably
lengthy multistep sequence from a protected form of the
Wieland–Miescher ketone to obtain the tricyclic fused ring
system. We describe herein a novel total synthesis of
paspalinine (1) together with a formal synthesis of paspalicine
(2) that features not only a concise stereoselective formation
of the CDE ring system but also a high-yielding convenient
installation of the indole moiety and an efficient introduction
of the C13 tertiary hydroxy group through allylic selenoxide
rearrangement.
Scheme 1. Structures of paspalinine (1) and paspalicine (2).
Scheme 2 outlines our retrosynthetic analysis of 1 and 2.
Paspalinine (1) possessing the C13 hydroxy group would be
derived by oxidative migration of the C12/C13 double bond of
other indole diterpenoids, paspalinine (1), which bears
a hydroxy group at the C13 position, exhibits mammal
toxicity and often causes tremorgenic neurological disorders
called “paspalum staggers” in domestic animals when they
graze the pasture grass Paspalum dilatatum infected with
C. paspali;[3c,4,5] however, its congener 2, which lacks the
hydroxy function, induces no such symptoms.[1] In addition to
tremorgenicity, this family of natural products has a variety of
intriguing biological properties, such as insecticidal,[6] mito-
inhibitory,[7] and anti-MRSA activities.[8] Owing to their
impressive polycyclic ring system as well as the pharmaco-
logically and agriculturally important biological profiles,
indole diterpenoids have long become the subject of studies
from various aspects including biosynthesis,[9] structure–
activity relationship,[1,6] biosynthetic genes,[10] and action
mechanism.[11] The total synthesis of indole diterpenoids has
also been addressed by many research groups, and a series of
extensive studies by Smith and co-workers successfully led to
the total syntheses of as many as seven indole diterpenes
including 1 and 2,[12] while synthetic efforts by other groups
[*] Dr. A. Morita, Prof. Dr. S. Kuwahara
Laboratory of Applied Bioorganic Chemistry
Graduate School of Agricultural Science, Tohoku University
Tsutsumidori-Amamiyamachi, Sendai 981-8555 (Japan)
E-mail: skuwwahar@biochem.tohoku.ac.jp
Scheme 2. Retrosynthetic analysis of 1 and 2. Boc=tert-butoxycar-
bonyl, Tf=trifluoromethanesulfonyl.
Dr. M. Enomoto
Bioproduction Research Institute
National Institute of Advanced Industrial Science and Technology
Sapporo 062-8517 (Japan)
the b,g-unsaturated ketone 3 followed by removal of the N-
Boc protecting group on the indole ring, while paspalicine (2),
which is devoid of the hydroxy group, should also be obtained
from the common precursor 3 by N-Boc deprotection and
conjugation of the double bond with the C10 carbonyl. The
construction of the FG ring moiety of 3 would be achievable
through oxidation of the double bond on the side chain of 4
[**] We thank Prof. Hidetoshi Tokuyama (Tohoku University) for
valuable discussions. Financial support for this work was provided
by the Ministry of Education, Culture, Sports, Science, and
Technology (Japan) (no. 23780112)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201206299.
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and subsequent intramolecular bicyclic acetal formation. To
obtain 4, we planned to regioselectively introduce an
appropriate side chain fragment at the C12 position of
pentacyclic ring system 5. For the construction of the A–E
ring core structure 5, we envisaged a two-step protocol
consisting of the Stille coupling of triflate 6 with aniline
derivative 7 and oxidative cyclization of the resulting coupling
product to form the indole ring. The enol triflate 6, bearing
two contiguous methyl groups at the C3 and C4 quaternary
carbon atoms in trans fashion, was considered to be obtain-
able from enone 8, which would readily be prepared from
known bicyclic ketone 9.
of the cyclopropane ring of 14 with sodium naphthalenide in
THF and subsequent in situ trapping of the resulting enolate
intermediate with the Cominsꢀ reagent furnished the desired
triflate 6 with the C3 and C4 quaternary stereocenters
correctly installed.[17,18]
The installation of the indole ring portion was performed
by an efficient two-step sequence (Scheme 4). First, the enol
triflate 6 was subjected to the Stille coupling reaction with the
Our two-step preparation of the tricyclic enone 8 from 9
and its highly stereoselective elaboration into the CDE
segment 6 in four steps is delineated in Scheme 3. The
known starting material 9, prepared from the (+)-Wieland–
Scheme 4. Construction of A–E ring moiety 5. Reagents and condi-
tions: a) [Pd(PPh3)4], CuCl, LiCl, DMSO, 7, 508C, quant; b) Pd-
(OCOCF3)2, NaOAc, DMSO, 608C, 90%; c) aq HCl (2m), THF, 458C,
93%.
o-stannylated aniline derivative 7 under Coreyꢀs conditions to
give 15 quantitatively.[19–21] The o-alkenyl aniline derivative 15
was then treated with Pd(OCOCF3)2 in DMSO at 608C for
24 h in the presence of sodium acetate to successfully furnish
16 embedded with an indole ring in 90% yield.[22] This type of
PdII-mediated indole ring formation from o-alkenyl aniline
derivatives has sparsely been documented for preparing much
simpler indole derivatives with no substituent at the 2- and 3-
positions of the indole nucleus,[23] and has never been applied
to the total synthesis of complex natural products.[24–26] Finally,
selective removal of the acetal protecting group with hydro-
chloric acid accompanied by concomitant migration of the
Scheme 3. Preparation of CDE ring portion 6. Reagents and condi-
tions: a) LDA, 10, HMPA, THF, ꢀ788C!room temperature, then aq
HCl (1m), acetone, room temperature, 68%; b) Cs2CO3, THF, 508C,
77%; c) LiB(sBu)3 H, THF, ꢀ408C!room temperature, 99%; d) CH2I2,
Et2Zn, CH2Cl2, 08C; e) DMSO, (COCl)2, Et3N, CH2Cl2, ꢀ75 to 08C,
77% (2 steps); f) Na(C10H8), THF, ꢀ758C, then isoprene, Comins’
reagent, HMPA, THF, ꢀ75 to ꢀ108C, 42%. LDA=lithium diisopropyl-
amide, HMPA=hexamethylphosphoramide.
ꢀ
C13 C14 double bond gave the conjugated enone 5.
After having achieved the concise nine-step preparation
of the pentacyclic core intermediate 5 from the Wieland–
Miescher ketone derivative 9, we set about the construction of
the bicyclic FG ring moiety (Scheme 5). At first, an allyl
group was regioselectively installed at the C12 position of the
enone 5 by Tsujiꢀs palladium-catalyzed allylation protocol to
afford 18 via carbonate 17.[13d,27] Chain elongation of 18 by
cross-metathesis with 2-methyl-3-buten-2-ol proceeded
uneventfully, thus giving tertiary allylic alcohol 4. Based on
some successful precedents of highly enantioselective dihy-
droxylation of achiral tertiary allylic alcohols,[28] we antici-
pated that subjection of 4 to the asymmetric dihydroxylation
protocol would give rise to 19 with a high level of diastereo-
selectivity. Contrary to our expectation, however, the facial
selectivity of the dihydroxylation in this particular case
proved to be modest, thus providing an inseparable 62:38
mixture of the desired product 19 and its diastereomer 19ꢀ in
64% yield (86% based on recovered 4).[29] Treatment of the
mixture with 2,4,6-collidinium p-toluenesulfonate in metha-
nol afforded a 4:1 mixture of desired cyclic acetal 20 and the
Miescher ketone (> 99% ee) by slightly modifying Smithꢀs
procedure,[12b,d] was alkylated with bromide 10,[15] and the enol
ether moiety of the resulting product was chemoselectively
hydrolyzed in one pot to give 11 as an epimeric mixture.
Exposure of the mixture to intramolecular Horner–Wads-
worth–Emmons olefination conditions brought about
a stereoconvergent cyclization to afford 8 as a single diaster-
eomer. Reduction of 8 with l-Selectride proceeded highly
stereoselectively, thereby giving 12 in an excellent yield of
99%; the stereochemistry of 12 was established by observing
diagnostic NOE correlations (see the Supporting Informa-
tion). The allylic alcohol 12 was subjected to hydroxy-directed
Simmons–Smith cyclopropanation to afford 13 as a single
diastereomer,[16] which was then oxidized to cyclopropyl
ketone 14 in 77% yield for the two steps. Reductive cleavage
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a more efficient route for the conversion of the intermediate 3
into 1, since the oxidative migration of the double bond of 21
to give 1 by Smith and co-workers required two steps,
resulting in a modest yield of 31% (see Scheme 5). To our
delight, treatment of 3 with KHMDS in THF and subsequent
enolate trapping with PhSeCl gave an a-selenenylated ketone
intermediate, which, upon in situ exposure to hydrogen
peroxide, underwent oxidation to the corresponding selen-
oxide followed by concomitant [2,3]-sigmatropic rearrange-
ment to furnish 22 in an acceptable yield of 57%
(Scheme 6).[31] Finally, removal of the Boc protecting group
in 67% yield completed our efficient total synthesis of
paspalinine (1), the spectral data of which showed good
agreement with those reported by Smith and co-workers.[12d]
Scheme 6. Completion of the total synthesis of paspalinine (1).
Reagents and coditions: a) KHMDS, PhSeCl, THF, ꢀ70 to ꢀ158C,
then aq H2O2, NaHCO3, room temperature, 57%; b) SiO2, ca. 133 Pa,
90–1108C, 67%. KHMDS=potassium hexamethyldisilazide.
In conclusion, the concise total synthesis of paspalinine
(1), which required only two protecting groups, was accom-
plished from the known Wieland–Miescher ketone derivative
9 by the seventeen-step sequence (0.67% overall yield)[32] that
features the efficient introduction of the C3 quaternary
methyl group trans to the C4 methyl group through the
hydroxy-directed cyclopropanation of the allylic alcohol 12,
the palladium-mediated two-step indole ring formation lead-
ing to the A–E ring moiety 5 in high yield, and the one-pot
installation of the C13 tertiary hydroxy group through allylic
selenoxide [2,3]-sigmatropic rearrangement. The ready access
to 5 (14% overall yield from 9 in nine steps), which is
a common structural motif in all paspalane-type indole
diterpenoids, would facilitate synthetic endeavors toward
other natural products of the paspalane family with complex
molecular architectures as well as important biological
activities.
Scheme 5. Formal synthesis of paspalicine (2) and paspalinine (1).
Reagents and conditions: a) tBuOK, Alloc-Cl, THF, room temperature;
b) [Pd(PPh3)4], Ph3P, DME, room temperature, then DBU, room
=
temperature, 72% (2 steps); c) CH2 CHC(OH)Me2, Grubbs’ 2nd gen-
eration catalyst, CH2Cl2, reflux, 78%; d) K2OsO2(OH)4, (DHQD)2PHAL,
K3Fe(CN)6, K2CO3, NaHCO3, tBuOH/H2O, 158C, 64% (62:38 mixture
of 19 and 19’, 86% based on recovered starting material); e) CPTS,
MeOH, 58C, 49% (4:1 mixture of 20 and 20’); f) DMP, NaHCO3,
CH2Cl2, room temperature, 71%; g) SiO2, ca. 133 Pa, 90–1008C, 71%.
Alloc=allyloxycarbonyl, DME=1,2-dimethoxyethane, DBU=1,8-
diazabicyclo[5.4.0]undec-7ene,
(DHQD)2PHAL=bis(dihydroquinidino)phthalazine, CPTS=2,4,6-colli-
dinum p-toluenesulfonate, DMP=Dess–Martin periodinane.
corresponding enol ether form 20ꢀ (49% yield). Interestingly,
this bicyclic ring formation took place preferentially for 19,
and a substantial amount of its diastereomer 19’ was
recovered unchanged. Oxidation of the mixture of 20 and
20’ with the Dess–Martin periodinane afforded ketone 3 in
71% yield after chromatographic purification. In view of the
potential acid lability of the allylic acetal unit in the FG ring of
3, removal of the N-Boc protecting group was conducted by
heating 3 adsorbed on SiO2 in vacuo,[30] thereby furnishing 21
Received: August 6, 2012
Revised: October 10, 2012
Published online: November 7, 2012
Keywords: alkaloids · natural products · palladium ·
.
paspalinine · total synthesis
1
in 71% yield, the H and 13C NMR spectral data of which
were identical with those reported by Smith and co-work-
ers.[12d] Since the b,g-unsaturated ketone 21 has previously
been transformed into paspalicine (2) and paspalinine (1;
Scheme 5),[12d] the present synthesis of 21 constitutes a formal
total synthesis of 1 and 2.
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