Overcoming synthetic challenges of oridonin a-ring structural diversification: Regio- and stereoselective installation of azides and 1,2,3-triazoles at the c-1, c-2, or c-3 position

Article abstract of DOI:10.1021/ol4015865

Efficient and concise synthetic approaches have been developed for the rapid and diverse installation of azide functionalities at the C-1, C-2, or C-3 positions of oridonin (1) with highly controlled regio- and stereoselectivity, while keeping key reactive pharmacophores intact by utilizing unique preactivation strategies based on the common synthon 4. Further functionalization of these azides through click chemistry yielding triazole derivatives successfully provides access to an expanded natural scaffold-based compound library for potential anticancer agents.

Full text of DOI:10.1021/ol4015865

ORGANIC
LETTERS
2013
Vol. 15, No. 14
3718–3721
Overcoming Synthetic Challenges of
Oridonin A‑Ring Structural Diversification:
Regio- and Stereoselective Installation of
Azides and 1,2,3-Triazoles at the C‑1, C‑2,
or C‑3 Position
Chunyong Ding,† Yusong Zhang,‡ Haijun Chen,† Christopher Wild,† Tianzhi Wang,§
Mark A. White,§ Qiang Shen,‡ and Jia Zhou*,†
Chemical Biology Program, Department of Pharmacology and Toxicology and Sealy
Center for Structural Biology & Molecular Biophysics, University of Texas Medical
Branch, Galveston, Texas 77555, United States, and Department of Clinical Cancer
Prevention, Division of Cancer Prevention and Population Sciences, The University of
Texas M.D. Anderson Cancer Center, Houston, Texas 77030, United States
jizhou@utmb.edu
Received June 5, 2013
ABSTRACT
Efficient and concise synthetic approaches have been developed for the rapid and diverse installation of azide functionalities at the C-1, C-2, or C-3
positions of oridonin (1) with highly controlled regio- and stereoselectivity, while keeping key reactive pharmacophores intact by utilizing unique
preactivation strategies based on the common synthon 4. Further functionalization of these azides through click chemistry yielding triazole
derivatives successfully provides access to an expanded natural scaffold-based compound library for potential anticancer agents.
Oridonin (1), a highly oxygenated 7,20-epoxy-ent-kaur-
ane-type diterpenoid isolated from the traditional Chinese
herb isodon rubescens, is characterized by its densely
functionalized and stereochemistry-rich frameworks in-
cluding an exo-methylene cyclopentanone moiety in the
D-ring and a 6-hydroxyl-7-hemiketal group in the B-ring
(Figure 1).1 It has been attracting considerable attention
in recent years due to its remarkably unique and safe
anticancer pharmacological profile.1e,f In China, oridonin
injection is used alone or in combination with other drugs to
treat liver cancer and carcinoma of gastric cardia.2 Never-
theless, more extensive clinical applications of oridonin for
cancer therapy have been restricted, to a large degree, due to
its relatively moderate potency, limited aqueous solubility
and bioavailability.3 Consequently, modifications on the
scaffold of 1 have become an attractive strategy to create
better natural product-like compound libraries. Most pre-
vious approaches were attempted by coupling ester appen-
dages to hydroxyl groups of 1.1b,4 To date, little chemical
effort has been directed toward modifications of the
† Chemical Biology Program, Department of Pharmacology and Toxicology.
‡ The University of Texas M.D. Anderson Cancer Center.
§ Sealy Center for Structural Biology & Molecular Biophysics.
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Matsuda, M.; Ozaki, M. Chem. Pharm. Bull. 1981,29, 3208. (c) Yin, F.; Liang,
J. Y.; Liu, J. J. Chin. Pharm. Univ. 2003, 34, 302. (d) Sun, H. D.; Huang, S. X.;
Han, Q. B. Nat. Prod. Rep. 2006, 23, 673. (e)Abelson, P. H. Science 1990, 247,
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r
10.1021/ol4015865
Published on Web 07/08/2013
2013 American Chemical Society

oridonin A-ring to generate diversity, likely owing to the
synthetic challenges arising from its structural complexity
with multiple reactive functionalities. Recently, we re-
ported a concise synthesis of thiazole-fused oridonin deri-
vatives with enhanced activity and solubility,5 indicating
that the rational modifications on oridonin A-ring may
have great potential to generate better anticancer agents.
chemical electrophilic center susceptible to nucleophilic
attack (Michael addition) by various nucleophiles includ-
ing azide reagents under certain reaction conditions, lead-
ing to adducts at the β-position as evidenced in literature.9
Accordingly, a regioselective installation of the azide
functionality to the desired sites of the A-ring, but not
the enone moiety in the D-ring, is synthetically challenging
and essentially required, in developing efficient synthetic
protocols. With this goal in mind, we attempted to achieve
a differential reactivity of these two sites by activating the
functional group of the A-ring for a successfully controlled
selectivity.
Our synthesis commenced with 1, which is naturally
abundant and commercially available (Scheme 1). Protec-
tion of 7,14-dihydroxyl of 1 with 2,2-dimethoxypropane10
followed by selective activation of the 1-hydroxyl group
with MsCl solely provided mesylate 2 in 74% yield over
two steps, which was subsequently subjected to NaN3 in
DMF. However, the anticipated substitution reaction at
60 °C failed to give 1-azide 3 but resulted in high recovery
of 2. Increasing the reaction temperature to 120 °C
produced a complex mixture, presumably owing to the
Michael addition to the enone system in the D-ring by
azide anions. Therefore, a more reactive electrophilic
center deemed necessary to be created at the A-ring. To
introduce an allylic alcohol functional moiety to the
A-ring, mesylate 2 was chosen as a substrate to undergo
an elimination reaction in the presence of Li2CO3 at 110 °C
to provide 1-ene 4 in 84% yield, followed by an allylic
oxidation with selenium dioxide in refluxing 1,4-dioxane,
stereoselectively leading to the 3β-allylic hydroxyl 6 in
excellent yield. In this step, the 1-allylic seleninic acid
intermediate 5 was only formed from the β-face of the
A-ring because of a steric effect of the 7,20-epoxy ring,
eventually leading to an installation of the hydroxyl group
at the 3β-position in a stereoselective manner that was
unambiguously determined through X-ray crystallo-
graphic analysis. Initially, allylic alcohol 6 without any
preactivation was directly treated with diphenylphosphoryl
azide (DPPA)11 and DBU in THF at 0 °C followed by
warming up to 60 °C to install an azide group at C-3 for the
purpose of atom-economy. Unfortunately, only a diphenyl
phosphate intermediate 7, instead of anticipated 3-allylic
azide 8, was obtained in 80% yield. Mechanistically, the
reactive 3-allylic phosphate of intermediate 7 could not
undergo further substitution reaction with the azide anion
from the R-face due to the surrounding steric hindrance
of C-3R. Accordingly, the 3-allylic alcohol of 6 has to be
preactivated in the form of a better leaving group to install
the 3-azide with high regioselectivity.
Figure 1. Retrosynthetic analysis of azide- and 1,2,3-triazole-
substituted oridonin derivatives.
Click chemistry, a concept initiated by Sharpless,6 has
provided a number of nearly perfect “spring-loaded”
chemical reactions to elaborate an elegant and selective
modification on complex natural products. Particularly,
the Cu(I)-catalyzed azideÀalkyne cycloaddition (CuAAC)7
can efficiently afford 1,2,3-triazole scaffolds under mild
conditions even in the presence of chemically reactive
functionalities. Importantly, the resulting triazole deriva-
tives are fairly stable to metabolic degradation and capable
of actively participating in hydrogen bonding and dipoleÀ
dipole interactions, providing potential advantages includ-
ing target binding and cell permeability improvement.8
Herein, we, for the first time, disclose our effort for the
efficient synthesis of novel nitrogen-enriched oridonin deri-
vatives with azide and 1,2,3-triazole functionalizations at
the C-1, C-2 or C-3 position in a highly regio- and stereo-
specific manner.
As illustrated in Figure 1, our general synthetic strategy
to achieve these molecules involves a key and challenging
step of azide installation at each of three sites of the A-ring
with controlled regio- and stereoselectivity, while not
destroying other featured functionalities. It is well docu-
mented that the presence of the R,β-unsaturated ketone
(enone) system in the D-ring of 1 is the main structural
determinant for its anticancer activity and destruction of
this enone system could counteract its bioactivity.1a,b,d
On the other hand, this bioactive enone system is also a
(5) Ding, C.; Zhang, Y.; Chen, H.; Yang, Z.; Wild, C.; Chu, L.; Liu,
H.; Shen, Q.; Zhou, J. J. Med. Chem. 2013, 56, 5048.
(6) Kolb, H. C.; Finn, M. G.; Sharpless, K. B. Angew. Chem. 2001,
113, 2056. Angew. Chem., Int. Ed. 2001, 40, 2004.
(7) Rostovtsev, V. V.; Green, L. G.; Fokin, V. V.; Sharpless, K. B.
Angew. Chem. 2002, 114, 2708. Angew. Chem., Int. Ed. 2002, 41, 2596.
(8) (a) Dalvie, D. K.; Kalgutkar, A. S.; Khojasteh-Bakht, S. C.;
Obach, R. S.; O’Donnell, J. P. Chem. Res. Toxicol. 2002, 15, 269. (b)
Horne, W. S.; Yadav, M. K.; Stout, C. D.; Ghadiri, M. R. J. Am. Chem.
Soc. 2004, 126, 15366. (c) Agalave, S. G.; Maujan, S. R.; Pore, V. S.
Chem.;Asian J. 2011, 6, 2696.
(9) (a) Angelini, T.; Lanari, D.; Maggi, R.; Pizzo, F.; Sartori, G.;
Vaccaro, L. Adv. Synth. Catal. 2012, 354, 908. (b) Castrica, L.; Fringuelli,
F.; Gregoli, L.; Pizzo, F.; Vaccaro, L. J. Org. Chem. 2006, 71, 9536. (c)
Taylor, M. S.; Zalatan, D. N.; Lerchner, A. M.; Jacobsen, E. N. J. Am.
Chem. Soc. 2005, 127, 1313. (d) Guerin, D. J.; Horstmann, T. E.; Miller,
S. J. Org. Lett. 1999, 1, 1107.
(10) Zhou, W.; Cheng, Y. Acta Chim. Sinica 1990, 48, 1185.
(11) (a) Kuliszewska, E.; Hanbauer, M.; Hammerschmidt, F.
Chem.;Eur. J. 2008, 14, 8603. (b) Kuliszewska, E.; Hanbauer, M.;
Hammerschmidt, F. J. Org. Chem. 2003, 68, 3546.
Org. Lett., Vol. 15, No. 14, 2013
3719

1R-azide 11 would be initially produced and then undergo
[3,3]-sigmatropic shift to provide 3R-azide 8. Further evi-
dence for this isomerization was also achieved by heating 1R-
azide 11 in refluxing CH3CN to solely give 3R-azide 8. This
surprisingly high 3R-allylic azide preference is probably as-
cribed to its less crowded steric environment. As depicted in
Scheme 2, the 3-azide group is far away from the bridgehead
20-methylene group and possesses a more favorable steric
surrounding relative to the 1-azide group, leading to this
selective thermal isomerization of 1- to 3-type allylic azide.
Scheme 1. Attempts to Synthesize 1-Azide and 3-Azide Derivatives
3 and 8
Scheme 2. Synthesis of 3-Azide-, 1-Azide-, 1,2,3-Triazole-sub-
stituted Oridonin Derivatives 8, 11, 14, 15, 18 and 19
In pursuance of this goal, we intended to convert the
3-allylic alcohol into the 3-allylic chloride for installation
ofthe3-azide group(Scheme 2). Interestingly, chlorination
of6 withSOCl2 indryetheratrtexclusivelyled to1β-allylic
chloride 10 (73%),12 the regio- and stereostructure of
which was determined by HMBC and NOESY experi-
ments (Figure S1 in Supporting Information). In this step,
owing to the intrinsic structural feature of the substrate,
the in situ generated 3-allylic mesylate 9 underwent an SNi’
reaction with the internal attackof chloride on C-1, instead
of the allylic site C-3, leading to the shift of the double
bond, in which a stable hexacyclic ring transition state was
likely formed from the β-face.13 Subsequent nucleophilic
substitution reaction of 10 with NaN3 in dry DMF at rt
neatly afforded 1-allylic azide 11 (71%) with inversion of
configuration despite steric crowding.14 The high propensity
of allylic azides to undergo [3,3]-sigmatropic rearrangement15
suggested a facile route potentially available to 3R-allylic
azide 8. Gratifyingly, performing this substitution reaction
at 70 °C smoothly redirected the reaction to 3R-azide 8
(74%) with a total control of regio- and stereoselectivity as
determined by HMBC and NOESY experiments (Figure S2
in Supporting Information). These results implied that
With key intermediates 11 and 8 in hand, the subsequent
click reaction with phenylacetylene catalyzed by CuI in dry
CH3CN at rt followed by removal of the acetonide groups
with 5% HCl (aq) achieved the corresponding 1,2,3-tria-
zole derivatives 14 and 15 in excellent yields, the structures
of which were also confirmed by 1D and 2D NMR experi-
ments (Figures S3 and S4 in Supporting Information). The
stereochemistry of 14 was further unambiguously secured
by X-ray crystallographic analysis. As additional proof for
selective isomerization of the 1- to 3-type allylic azide, the
click reaction of 1-azide 11 in refluxing CH3CN solely
provided 3-triazole 13 (70%) instead of 1-triazole 12. To
further explore their versatility, triazole derivatives 18 and 19
have also been synthesized in a same fashion with good yields.
Regioselective installation of an azide group at the C-2
of the A-ring also utilized the preactivation strategy to
(12) Formation of mesylate followed by treatment with LiCl was also
attempted, but failed to give regio- and stereoselective product 10, while
resulting in a mixture of several isomers instead.
ꢀ
(13) (a) Lovchik, M. A.; Goeke, A.; Frater, G. J. Org. Chem. 2007,
72, 2427. (b) Gurdeep, R. Organic Name Reactions, Reagents and
Molecular Rearrangements; KRISHNA Prakashan Media: Meerut, India,
2008; pp A19ÀA20.
(14) Indeed, 11 was not very stable and a minor amount of 8 was
identified when purified with silica gel column.
(15) (a) Gagneaux, A.; Winstein, S.; Young, W. G. J. Am. Chem. Soc.
1960, 82, 5956. (b) Vanderwerf, C. A.; Heasley, V. L. J. Org. Chem. 1966, 31,
3534. (c) Trost, B. M.; Pulley, S. R. Tetrahedron Lett. 1995, 36, 8737. (d)
Podeschwa, M. A. L.; Plettenburg, O.; ltenbach, H.-J. Org. Biomol. Chem.
2003,1, 1919. (e) Chang, Y.-K.; Lo, H.-J.; Yan, T.-H. Org. Lett. 2009,11,4278.
3720
Org. Lett., Vol. 15, No. 14, 2013

Table 1. Antiproliferative effects of representative triazole-
substituted analogues against human breast cancer cell lines
Scheme 3. Synthesis of 2-Azide-, 1,2,3-Triazole-substituted
Oridonin Derivatives 21, 24 and 25
IC50 (μM)a
compounds
MCF-7
MDA-MB-231
Oridonin (1)
6.6
0.38
2.1
2.3
2.0
1.9
29.4
0.48
1.8
12
14
15
23
25
1.8
1.1
3.5
a Breast cancer cell lines: MCF-7 and MDA-MB-231. Software:
MasterPlex ReaderFit 2010, MiraiBio, Inc. Values are mean ( SE of
three independent experiments.
exhibited significantly improved antibreast cancer activity
in comparison with oridonin.
In summary, efficient and concise synthetic approaches
have been developed for the rapid and diverse installation
of azide functionalities at the C-1, C-2 or C-3 position of
the oridonin A-ring, while keeping key reactive pharma-
cophores intact by utilizing unique preactivation strategies
based on the common synthon 4. Selective chlorination
of 3-allylic alcohol 6 followed by substitution reaction
with NaN3 provided 1-azide 11, which further underwent
a selective [3,3]-sigmatropic rearrangement to solely afford
3-azide 8; while selective Prilezhaev epoxidation of 1-ene 4
followed by epoxide ring-opening with azide exclusively
led to 2-azide 21. By harnessing the intrinsic structural
features of the oridonin scaffold, a high degree of regio-
and stereoselective control was achieved throughout the
whole sequence. Further functionalizations of these azides
through click chemistry yielding 1,2,3-triazole derivatives
readily provides access to an expanded natural scaffold-
basedcompoundlibrary. Intriguingly, thesenew molecules
have demonstrated superior antiproliferative effects against
breast cancer cells to oridonin. Our success in the efficient
construction of oridonin-based azides or triazoles opens
new synthetic avenues to the development of novel poten-
tial natural product-like anticancer agents.
achieve differential reactivity with respect to other reactive
functionalities (Scheme 3). After considering various pos-
sibilities, the key synthon 4 prepared in Scheme 1 was
subjected to epoxidation through m-CPBA-mediated Pri-
lezhaev reaction.16 To our delight, this reaction not only
occurred preferentially at the 1-ene rather than the 16-exo-
methylene but also formed 1,2-epoxide ring stereoselec-
tively from the less sterically hindered β-face to exclusively
give 1S,2R-epoxide 20 (86%). Ring-openings of unsym-
metrical epoxides by nucleophiles usually lead to nonre-
gioselctive products.17 In our case, azidolysis of epoxide 20
with NaN3 occurred preferentially with attack of azide
anion on C-2 due to its less sterically crowded surrounding
relative to C-1. Conducting the reaction in EtOH/H2O
(1:1) at 85 °C using NH4Cl as a coordinating salt for 24 h
solely afforded 2-azide 21 (51%),18 the structure of which
was also unequivocally determined by X-ray crystallo-
graphic analysis. In this step, different solvents such as
MeOH/H2O, CH3CN/H2O, acetone/H2O or DMF resulted
in lower yields than that in aqueous ethanol; extending
reaction time decreased the yield; after screening other
coordinating salts, LiClO4 proved to be optimal leading to
a better yield (60%) than NH4Cl (Table S1 in Supporting
Information). The subsequent click reactions of 21 with
phenyacetylene or 4-tert-butylphenylacetylene followed
by treatment with 5% HCl (aq) successfully generated
2-triazole compounds 24 and 25 in 58 and 80% yields (two
steps), respectively.
Acknowledgment. This work was supported by grants
P50 CA097007, P30 DA028821, R21 MH093844 from the
National Institutes of Health, R. A. Welch Foundation
Chemistry and Biology Collaborative Grant from Gulf Coast
Consortia (GCC) for Chemical Genomics, Sealy and Smith
Foundation grant (to the Sealy Center for Structural Biology
and Molecular Biophysics), and John Sealy Memorial En-
dowment Fund.
The antiproliferative effects of representative 1,2,3-triazole-
substituted analogues against breast cancer cell lines in-
cluding MCF-7 (ER-positive) and MDA-MB-231 (triple-
negative) are summarized in Table 1, indicating that these
new compounds with 1,2,3-triazole installed in the A-ring
Supporting Information Available. Experimental pro-
cedures, characterization data, copies of NMR spectra
for new compounds, and X-ray CIF files for compounds
6, 14 and 21. This material is available free of charge via
the Internet at http://pubs.acs.org.
(16) McMurry, J. E.; Isser, S. J. J. Am. Chem. Soc. 1972, 94, 7132.
(17) Scriven, E. F. V.; Turnbull, K. Chem. Rev. 1988, 88, 297.
(18) This ring opening reaction required harsh condition and was still
incomplete probably due to 1,3-diaxial interactions in the resulting
product 21.
The authors declare no competing financial interest.
Org. Lett., Vol. 15, No. 14, 2013
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