C-H-functionalization logic guides the synthesis of a carbacyclopamine analog

Article abstract of DOI:10.3762/bjoc.10.161

The chemical synthesis of carbacyclopamine analog 2, a cyclopamine analog with an all-carbon E-ring, is reported. The use of C-H-functionalization logic and further metal-catalyzed transformations allows for a concise entry to this new class of acid-stable cyclopamine analogs.

Full text of DOI:10.3762/bjoc.10.161

C–H-Functionalization logic guides the synthesis of a
carbacyclopamine analog
Sebastian Rabe1, Johann Moschner1, Marina Bantzi1, Philipp Heretsch*2
and Athanassios Giannis*1
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2014, 10, 1564–1569.
1Universität Leipzig, Institut für Organische Chemie, Johannisallee 29,
D-04103 Leipzig, Germany and 2Rice University, Department of
Chemistry, BioScience Research Collaborative, 6100 Main St.,
Houston, Texas 77005, United States
doi:10.3762/bjoc.10.161
Received: 16 April 2014
Accepted: 23 June 2014
Published: 09 July 2014
Email:
Philipp Heretsch* - heretsch@rice.edu; Athanassios Giannis* -
giannis@uni-leipzig.de
This article is part of the Thematic Series "Natural products in synthesis
and biosynthesis".
* Corresponding author
Guest Editor: J. S. Dickschat
Keywords:
© 2014 Rabe et al; licensee Beilstein-Institut.
License and terms: see end of document.
cyclopamine; C–H-functionalization; hedgehog signaling pathway;
natural products; steroidal alkaloids
Abstract
The chemical synthesis of carbacyclopamine analog 2, a cyclopamine analog with an all-carbon E-ring, is reported. The use of
C–H-functionalization logic and further metal-catalyzed transformations allows for a concise entry to this new class of acid-stable
cyclopamine analogs.
Introduction
The isolation of cyclopamine (1, Figure 1) nearly 50 years ago employing C–H-functionalization logic [11]. In addition, the
followed by the determination of its biological target and phar- rational design and chemical synthesis of several analogs and
macological profile has drawn considerable interest from derivatives by this [12,13] and other groups [14-17] have been
research groups in biology, chemistry and medicine [1-4]. As disclosed. Here, we report the synthesis of a carbacyclopamine
the first identified hedgehog signaling inhibitor, cyclopamine analog (2, Figure 1), an analog of the natural product with an
exerts its anticancer activity through a novel mechanism of all-carbon E-ring and a pyridine F-ring.
action, which manifests itself in its remarkable activity against
several types of human cancer, including medulloblastoma, Cyclopamine inhibits the 7-transmembrane protein smoothened
basal cell carcinoma, and rhabdomyosarcoma [5-10]. The from converting into its active form [18,19]. Without active
impressive biological activity combined with a unique molec- smoothened, protein kinases like PKA, GSK3β, and CK1ε
ular architecture and the synthetic challenge it poses already let phosphorylate the transcription factors Gli2 and Gli3, thereby
to the first synthesis of cyclopamine by this laboratory creating binding sites for the adapter protein β-TrCP [20]. The
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cyclopamine analog that still exhibits similar inhibitory activity
on hedgehog-signaling. For the sake of brevity of the overall
synthetic sequence we defined carbacyclopamine analog 2 (see
Figure 1) as our primary target.
Results and Discussion
A retrosynthetic analysis identified diazo compound 3 as a key
intermediate in the synthesis of 2 (see Scheme 1). We envi-
sioned a rhodium-catalyzed C–H-insertion into the C17–H bond
to occur with a high degree of selectivity (both regio- and
stereoselectivity) to form the all-carbon E-ring (for its structure
see 11, Scheme 2). Furthermore, a Wagner–Meerwein
Figure 1: Structures of cyclopamine (1) and carbacyclopamine analog
2.
Gli family of transcription factors acts as an effector of the rearrangement was thought to establish the C-nor-D-homo
hedgehog signaling pathway and thus, is associated with a wide steroid system, and a gold-catalyzed amination/annulation/
array of physiological effects, including cell fate determination, aromatization sequence was planned to install the pyridine
proliferation and patterning. The so-formed Gli/β-TrCP com- F-ring. Diazo compound 3 originates from diene 4 through stan-
plex becomes subject to ubiquitinylation, mediated by the Cul1- dard transformations, the latter being accessible from commer-
based E3 ligase. Eventually, this results in partial proteosomal cially available and inexpensive dehydroepiandrosterone (5) by
degradation to form Gli3-R [21], or in the case of Gli2 [22,23], the means of a copper-mediated C–H hydroxylation in the
in complete degradation. In addition, the Gli3-R factor acts as a 12-position and a palladium-catalyzed coupling of methyl acry-
transcription inhibitor of hedgehog-response genes [10].
late to an activated enol ether in the 17-position.
As part of our continuing work on acid-stable cyclopamine In synthetic direction (Scheme 2), dehydroepiandrosterone (5)
analogs [12,13] we have now focused on the role of the allylic was protected as its tetrahydropyranyl ether (3,4-dihydro-2H-
ether oxygen in the acid-mediated E-ring cleavage and decom- pyran, cat. pyridinium para-toluenesulfonate, CH2Cl2, 25 °C,
position of cyclopamine [24]. We envisioned that its replace- quantiative, inconsequential mixture of diastereoisomers), and
ment by a methylene group would create an acid-stable the C17 carbonyl group was transformed into its 2-picolylimine
Scheme 1: Retrosynthetic analysis of carbacyclopamine analog 2.
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Scheme 2: Synthesis of carbacyclopamine analog 2.
(2-picolylamine, cat. para-toluenesulfonic acid, toluene, The remaining homoallylic alcohol was then masked as
111 °C, 92% yield). Subjecting the latter to modified an i-steroid [29,30] (para-toluenesulfonyl chloride, cat.
Schönecker conditions ([Cu(MeCN)4]PF6, O2, acetone, 25 °C) 4-dimethylaminopyridine, pyridine, 25 °C; then KOAc, MeOH,
[25-27], gave, after acidic work-up (AcOH/MeOH, 1:1, 90 °C), 64 °C, 85% yield for the two steps) to give 8. Generation of an
diol 6 in 46% yield as the only isolable product. Protection of enol triflate from the carbonyl group in 8 (potassium hexam-
the newly installed 12β-hydroxy group as a triethylsilyl ether ethyldisilazide, phenyl bis-triflimide, THF, −20 → −10 °C, 85%
via the bis-protected intermediate (not shown, triethylsilyl tri- yield) set stage for a Heck-reaction with methyl acrylate (cat.
fluoromethanesulfonate, 2,6-lutidine, CH2Cl2, 0 °C; then Pd(OAc)2, cat. PPh3, Et3N, DMF, 70 °C, 64% yield). The
HF·pyridine, THF, 0 °C, 67% yield for the two steps) [28] gave hydrogenation of the so-obtained diene 4 (for its structure see
hydroxy ketone 7.
Scheme 1) proceeded smoothly (H2, cat. Pd/C, MeOH, 25 °C,
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82% yield) under concomitant removal of the triethylsilyl ether To determine the ability of carbacyclopamine analog 2 to
[31] in 12-position to give methyl ester 9 as a single diastereo- inhibit Gli1-dependent luciferase expression, we employed Shh-
isomer. At this point, extensive experimentation suggested a LIGHTII cells, a clonal mouse fibroblast cell line which stably
change of protecting groups to enable the pending C–H inser- incorporates a Gli-dependent firefly luciferase reporter and a
tion reaction at C17.
constitutive Renilla luciferase reporter [18]. The analog was
tested in a concentration range from 0.01 μM to 100 μM but
Therefore, the i-steroid was reverted to the homoallylic alcohol showed no activity (data not shown).
(cat. para-toluenesulfonic acid, 1,4-dioxane/H2O, 10:1, 65 °C),
the methyl ester was hydrolyzed under basic conditions (LiOH, Conclusion
THF/H2O, 1:1, 68% yield for the two steps), and the alcohol We have reported the synthesis of a new, acid stable
moieties were protected as formyl esters (formic acid, 50 °C, cyclopamine analog. The implementation of C–H-functionaliza-
85% yield) to give key intermediate 10.
tion logic in the synthetic planning and further metal-catalyzed
transformations allows for a fast access to carbocyclopamine
Employing the formyl protecting groups [32], diazoketone 3 analog 2. At the same time, these investigations strongly
(for its structure see Scheme 1) was readily obtained from acid suggest the role the ether oxygen plays in the acid-catalyzed
10 via the corresponding acid chloride (oxalyl chloride, decomposition of cyclopamine. This study may find further
CH2Cl2, 25 °C; then diazomethane, THF, 25 °C, 85% yield for application in the rational design of new hedgehog inhibitors
the two steps) [33]. The anticipated C–H insertion then based on lead structure 2. Future work will focus on the syn-
proceeded uneventfully under oxygen-free conditions (cat. thesis of carbacyclopamine analogs with a piperidine F-ring and
Rh2(OAc)4, CH2Cl2, 41 °C) [34,35] to give cyclopentanone 11 their biological investigation.
in 52% yield as a single isomer. Removal of the formyl ester
protecting groups (LiOH, THF/H2O, 1:1, 30 °C, 94% yield) and
Supporting Information
selective protection of the 3-hydroxy group as a tert-butyl-
dimethylsilyl ether (tert-butyldimethylsilyl chloride, imidazole,
Supporting Information File 1
DMF, 25 °C, 95% yield) furnished alcohol 12.
Experimental procedures, characterization data, and copies
of 1H and 13C NMR spectra for new compounds.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-10-161-S1.pdf]
Treatment of the latter under our previously reported condi-
tions [36] (Comins’ reagent, 4-dimethylaminopyridine, toluene,
111 °C) gave an inseparable mixture of the rearranged olefin
products, with the desired endo-product 13 as the major
constituent (89% combined yield, endo-product 13:exo-product
14, 4:1). Isolation of desired product 13 by chromatography was
then achieved after selective hydrogenation of only the
exocyclic olefin in 14 (H2, cat. Rh/C, EtOAc, 25 °C, structure
of hydrogenation product not shown) employing the mixture of
the isomers 13 and 14 from the previous step.
Acknowledgements
We thank Dr. Lothar Hennig, Universität Leipzig, for his help
with recording and interpreting the 2D NMR spectra.
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