Total synthesis of (-)-scabronine G, an inducer of neurotrophic factor production

Article abstract of DOI:10.1021/ja055220x

The total synthesis of (-)-scabronine G has been achieved in a concise manner from the (-)-Wieland-Miescher ketone. Scabronine G and its more potent methyl ester (also prepared) display activity as nonpeptidyl inducers of nerve growth factor production. C

Full text of DOI:10.1021/ja055220x

Published on Web 09/09/2005
Total Synthesis of (-)-Scabronine G, an Inducer of Neurotrophic Factor
Production
Stephen P. Waters, Yuan Tian, Yue-Ming Li, and Samuel J. Danishefsky*
Laboratory for Bioorganic Chemistry and Laboratory of Biochemistry and Pharmacology, Sloan-Kettering Institute
for Cancer Research, 1275 York AVenue, New York, New York 10021, and Department of Chemistry, Columbia
UniVersity, HaVemeyer Hall, 3000 Broadway, New York, New York 10027
Received August 1, 2005; E-mail: s-danishefsky@mskcc.org
Scheme 1. Synthetic Strategy toward Scabronine G
The scabronines, metabolites from the bitter mushroom Sarcodon
scabrosus, are related to a broader class of angularly fused tricyclic
diterpenoids known as cyathanes.1 Our interest in scabronine G
(1) followed a report by Ohta which disclosed it to induce the
production and excretion of nerve growth factor (NGF) in 1321N1
human astroglial cells.2 Its methyl ester derivative (2) is even more
active in promoting excretion of NGF and an additional neurotro-
phin, interleukin 6 (IL-6). Consistent with these biochemical
markers, dramatic neuronal differentiation of rat pheochromocytoma
cells (PC-12) was also observed. Accordingly, compounds 1 and 2
fall in to a class of nonpeptidyl structures exhibiting neurotrophic
properties.3
Naturally occurring polypeptidyl neurotrophic factors play a
central role in mediating neuronal growth and survival.4 The study
of the mechanism of action of these factors (cf. NGF and BDNF)
is one of the central challenges to the neurosciences. The clinical
application of naturally occurring polypeptidyl neurotrophic factors
in reversal of neurodegenerative disorders (cf. Parkinson’s, Alzhe-
imer’s Diseases) has been investigated. However, unfavorable
pharmacokinetics require their direct infusion into appropriate
sectors of the brain, thus seriously complicating their progression
to medical application.5
Scheme 2. Attempted Synthesis of Cyclopentenone 6a
a Key: (a) Li/NH3, PhNTf2, THF, -78 °C, 73%; (b) allylSn(Bu)3,
Pd(PPh3)4, LiCl, THF, 55 °C, 96%; (c) 9-BBN, THF; NaOH, H2O2, 0 °C,
92%; (d) Jones reagent, acetone, 82%; (e) PPA, CH2Cl2, 75 °C, 60%.
One of the goals of our laboratory is that of identifying promising
small molecules with neurotrophic activity. Toward this end, we
are drawn to natural products which exhibit such activity and whose
structures invite new possibilities in chemical synthesis. Earlier in
our program, we reported total syntheses of the extensively oxidized
neurotrophic agents tricycloillicinone, merrilactone A, and jiade-
fenin.6 The scabronines struck us even more important in light of
the data reported above. Herein we describe the first total synthesis
of scabronine G.7
We operated from the pleasingly simple idea that scabronine G
can be Viewed as an annulated (ring A) one-carbon ring-expanded
(ring C) Version of the (-)-Wieland-Miescher ketone (3).8 Trapping
of the reductively generated trans BC fused kinetic enolate derived
from 4 would provide the as-yet undefined 5 (Scheme 1). The
condition placed on the Y and Z functions of 5 is that they be
integratable to afford 6. Anticipating conjugate attack of a cyano
nucleophile on the 4,9-enone,9 the remote C6-C9 backbone
relationship would be solved through sound stereoelectronic
principles rather than through ad hoc steric hindrance based
selectivities. In the concluding phases, sequential interpolation of
two C1 fragments, which emerge as C15 and C13, respectively,
would lead to 8 and thence to 1 and 2.
2). Chemoselective hydroboration of the terminal olefin and further
oxidation provided carboxylic acid 10. Interestingly, acid-mediated
Friedel-Crafts-type annulation of 10 provided 11 rather than the
expected 6.
We took this then disappointing outcome to presage potential
problems in fashioning the cyclopentenone moiety of 6 by cycliza-
tion of a three-carbon fragment based at C9 (see 5). Such a modality
would require an attack at C4, which is 1,3-diaxial to the angular
methyl group and ortho to the hindered C ring. The take-home
lesson for us, still keeping within the spirit of Scheme 1, was to
securely install the substitution at C4 via the Z group, leaving the
cyclization event to occur at C9. Reduction of 4 and acylation with
Mander’s reagent afforded the known ketoester 12 (Scheme 3).12
Conversion of the ketone to its enol triflate followed by hydride
reduction gave unsaturated ester 13.13 Transformation of the ester
in 13 to the corresponding Weinreb amide14 and subsequent addition
of vinylmagnesium bromide provided the divinyl ketone, 14. Lewis
acid-mediated Nazarov cyclization provided the requisite cyclo-
pentenone 6 as a single olefin isomer.15 Indeed, conjugate addition
of Nagata’s reagent9 to the enone in 6 and trapping of its derived
aluminum enolate with TMSCl gave a silyl enol ether which was
converted to 15 as shown.16 Installation of the isopropyl group via
Negishi coupling (notably, to a secondary sp3 center) afforded 16.17
The orchestration of stereocontrolled Nagata addition with enolate
trapping and cross-coupling has apparently not been widely
We first describe an initiative which, while unsuccessful from
the perspective of our proposed total synthesis, provided a valuable
teaching in structuring our later work. From 4,10 kinetically
controlled enol triflation11 followed by Stille cross-coupling gave
diene 9 in the expected stereo- and regiocontrolled manner (Scheme
9
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10.1021/ja055220x CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
Scheme 3. Synthesis of Intermediate 7a
a Key: (a) Li/NH3, t-BuOH, THF, -78 °C; NCCO2Me, 72%; (b) NaH,
PhNTf2, DME, 98%; (c) Pd(PPh)3, Bu3SnH, LiCl, THF, 55 °C, 91%; (d)
LHMDS, Me(OMe)NH‚HCl, THF, -10 °C, 79%; (e) vinylMgBr, THF,
-10 °C, 84%; (f) FeCl3, CH2Cl2, 72%; (g) Et2AlCN, THF, Et3N, TMSCl;
(h) t-BuOK, THF, -78 °C, N-(5-chloro-2-pyridyl)triflimide, 86% over two
steps; (i) i-PrMgCl, ZnCl2, LiCl, (dppf)PdCl2, THF, 55 °C, 75%; (j) DIBAL-
H, CH2Cl2, -78 °C, 88%; (k) NaClO2, NaH2PO4, t-BuOH/H2O; (l) MeI,
K2CO3, DMF; (m) THF, HCl/H2O; 92% over three steps.
Figure 1. Images of differentiation and neurite outgrowth of PC-12 cells
after treatment with the 1321N1 cell culture medium conditioned by: (A)
DMSO (negative control), (B) scabronine G methyl ester (2, 30 µM), and
(C) compound 8 (30 µM), and graphical evaluation of neurite outgrowth
of PC-12 cells (*P < 0.001 relative to DMSO control).
Scheme 4. Synthesis of Scabronine Ga
Acknowledgment. Support for this work was provided by the
National Institutes of Health (Grant HL 25848). We thank Professor
Tomihisa Ohta (Kanazawa University, Japan) for kindly providing
the NMR spectra of scabronine G and its methyl ester.
a Key: (a) NaH, HCO2Me, DME, 97%; (b) n-PrSH, TsOH, PhH, 50
°C, 93%; (c) n-BuLi/MeOCH2SPh, THF, -78 °C; (d) HgCl2, HCl/MeCN,
80 °C, 86% over two steps; (e) DBU, benzene, 75 °C, quant; (f) TsOH,
HO(CH2)2OH, PhH, 89%; (g) aq. NaOH, MeOH, 55 °C, then HCl, 87%.
Supporting Information Available: Experimental procedures,
characterization data, assay protocols, and a further discussion on
cyclopentenone intermediate 11. This material is available free of charge
via the Internet at http://pubs.acs.org.
practiced. Conversion of the nitrile to the corresponding methyl
ester and deketalization provided cyclohexanone 7.
The stage was now set for further elaboration to scabronine G.
Ketone 7 was converted to thiopropylmethylidene intermediate 17
in two steps (Scheme 4). Addition of lithiated methoxymethyl
phenyl sulfide afforded diastereomeric alcohols 18, which, upon
treatment with HgCl2 in acidic medium, underwent ring expansion
to afford the cross-conjugated cycloheptenone 8.18 Thermodynami-
cally favored isomerization of the olefin in 8 (cf. ref 7b) afforded
scabronine G methyl ester (2), whose spectral data were identical
to those derived from natural sources. The natural product itself
(1) was accessed, after chemoselective protection of the aldehyde
function, by saponification of the ester and subsequent hydrolysis
of the dioxolane moiety.
Fully synthetic scabronine G methyl ester (2) effectively
enhanced the biosynthesis and secretion of neurotrophic factors from
1321N1 human glial cells. In turn, significant neurite outgrowth
of PC-12 cells was observed after treatment with the conditioned
1321N1 cell culture medium (Figure 1). To our delight, synthetic
intermediate 8, a cross-conjugated analogue of 2, displayed greater
activity as evidenced by increased neurite length. These effects were
comparable to those produced by direct exposure of PC-12 cells to
purified NGF (50 ng/mL, see graph). Very small differences in
neurite outgrowth may enable otherwise failed synapses to be
fruitful. The ability of 2 and, particularly 8, to extend the length of
pre-existing neurites invites the study of their applicability in
neurodegenerative disorders.
References
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In summary, the first total synthesis of scabronine G, in optically
pure form, has been achieved in a high-yielding sequence from
readily available materials. We emphasize that this sequence, which
borrows primarily from the more classical canons of organic
synthesis, lends itself to both multigram scale-up and to molecular
modification. Moreover, it was demonstrated that an olefin isomer
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currently underway.
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