Total synthesis of anticancer agent EBC-23

Article abstract of DOI:10.1021/acs.orglett.6b01849

Total synthesis of spiroketal EBC-23 has been described by two divergent approaches from three simple building blocks. Gold-catalyzed cycloisomerization of alkynol and acid-mediated spirocyclization of diketalketone were successfully utilized to effect spiroketal formation. A Cu(I)-P(Cy)₃-catalyzed protocol for the highly regio- and stereocontrolled hydroboration of internal propargylic alcohol was effectively applied toward the β-hydroxy ketone via vinylboronates.

Full text of DOI:10.1021/acs.orglett.6b01849

Letter
pubs.acs.org/OrgLett
Total Synthesis of Anticancer Agent EBC-23
Dodda Vasudeva Reddy,†,‡ Gowravaram Sabitha,‡ Tadikamalla Prabhakar Rao,§
and Jhillu Singh Yadav*,†,‡
†Academy of Scientific and Innovative Research (AcSIR), New Delhi 110020, India
§
‡Division of Natural Products Chemistry and Centre for NMR & Structural Chemistry, CSIR-Indian Institute of Chemical
Technology, Hyderabad 500007, India
S
* Supporting Information
ABSTRACT: Total synthesis of spiroketal EBC-23 has been described by two divergent approaches from three simple building
blocks. Gold-catalyzed cycloisomerization of alkynol and acid-mediated spirocyclization of diketalketone were successfully
utilized to effect spiroketal formation. A Cu(I)−P(Cy)3-catalyzed protocol for the highly regio- and stereocontrolled
hydroboration of internal propargylic alcohol was effectively applied toward the β-hydroxy ketone via vinylboronates.
n 2007, Reddell and Gordon1 reported the isolation of a group
of novel spiroketals EBC-23, -24, -25, -72, -73, -75, and -76
Scheme 1. Retrosynthetic Analysis of EBC-23
I
(Figure 1) from the fruit of Cinnamomum laubatii (family
Figure 1. Spiroketals EBC-23, -24, -25, -72, -73, -75, and -76.
Lauraceae) found in the Australian tropical rainforests. Among
them, EBC-23 (1) exhibited in vitro anticancer activity against
several human cancer cell lines [i.e., melanoma (MM96L), breast
carcinoma (MCF7), prostate cancer (DU145)] and, more
importantly, inhibited the growth of a human prostate cancer
xenograft in mice with no observable side effects.2 This clearly
indicated its potential for the treatment of refractory solid tumors
in adults.1 The structural features of EBC-23 comprise six
stereocenters partly embedded in a unique fused α,β-unsaturated
lactone consisting of an acid-labile spiroketal unit. The structure
and absolute stereochemistry of EBC-23 was determined by the
Williams research group by 2D NMR studies2 and total synthesis
using Linchpin technology.3 The second synthesis was achieved
by Yamamoto et al.4 using super silyl chemistry.
Given the scarcity of 1 coupled with its anticancer activity and
our long-standing interest in the synthesis of 1,6-dioxaspiro[4.5]-
decane and 1,7-dioxaspiro[5.5]undecane moiety containing
molecules,5 we embarked on the total synthesis of 1 to enable
additional biological evaluation. Herein, we report the total
synthesis of EBC-23 (1) by two divergent approaches from three
simple building blocks. Retrosynthetically (Scheme 1), EBC-23
(1) can be obtained from 9 by Z-selective olefination followed by
lactonization. The spiro compound 9 could be prepared via two
divergent approaches, that is, paths A and B. In path A, 9 can be
derived via a one-pot acid-mediated deprotection/spirocycliza-
tion sequence of diketalketone 10, which in turn could be
prepared from propargylic alcohol 11 via a regioselective Cu(I)-
catalyzed hydroboration followed by oxidation of vinylboronates.
Compound 11 could be easily obtained by the coupling of
intermediates aldehyde 13 and terminal alkyne 12.
Received: June 25, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.6b01849
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Alkyne 12 could be obtained from known epoxide 16, whereas
aldehyde 13 could be derived from a monotosylated pyran 14.
Pyran 14 could be obtained from known 15 by employing Prins
cyclization.
PCy3−B2(pin)2 system in MeOH to give a diastereomeric
mixture of vinylboronates 24a and 24 in a 5:95 ratio (1H NMR).
The mixture was as such subjected to oxidation (NaBO3·
4H2O)16 to afford separable ketones 10a and 10 in good yield
(Scheme 4).
In path B, the spiro compound 9 could be generated via
gold(I)chloride-catalyzed spiroketalization of alkynol 17, which
in turn could be easily generated by the coupling of aldehyde 13
and alkyne 18. Alkyne 18 could be readily obtained from 12 by
simple deprotection/protection strategies.
Scheme 4. Synthesis of β-Hydroxy Keto Compound 10
Synthesis of EBC-23 commenced with a Prins reaction6 of
known monotosylated homoallyl alcohol 15,6b and tetradecanal
in the presence of TFA furnished trisubstituted tetrahydropyran
14 with high diastereoselectivity (Scheme 2). Compound 14 was
Scheme 2. Synthesis of Aldehyde 13
The β-hydroxy ketone 10 upon treatment with acid
(Amberlyst-15) in MeOH (Scheme 5) delivered as a chromato-
Scheme 5. Synthesis Spiro Compounds 25 and 26
subjected to Mitsunobu reaction7 using DIAD, TPP, and 4-
nitrobenzoic acid to obtain 19. Now, the tosyl group in 19 was
converted to the corresponding iodide 20 utilizing NaI.
Reductive ring opening of iodide 20 with Zn dust furnished8
compound 21, which upon ester hydrolysis followed by
isopropylidene protection afforded 22. The relative stereo-
chemistry of hydroxy groups in 22 was assigned by the
Rychnovsky acetonide method.9 Finally, oxidative cleavage of
the terminal double bond of 22 using Jin’s protocol (Lemieux−
Johnson oxidation)10 furnished aldehyde 13 in 26% yield over
seven steps from 15.
graphically (using silica gel) inseparable mixture of spiroketals,
1,7-dioxaspiro[5.5]undecane 25 and 1,6-dioxaspiro[4.5]decane
9 in a combined yield of 68%.17
The silylation allowed us to separate the mixture by a simple
chromatography to get 25 and 26 in a 3:2 ratio. The
stereochemistry of a newly formed spiro carbon in 25 and 9
was controlled by anomeric effects,13a,18 and the configuration of
other stereocenters resulted from the configuration of precursor
10. The complete structures of 25 and 26 were assigned by 1D
(1H, 13C, and DEPT135) and 2D (COSY, NOESY, HSQC,
HMBC, and TOCSY) NMR studies (see Supporting Informa-
tion).
Synthesis of alkyne 12 (Scheme 3) started with the opening of
known epoxide 1611 with the lithium acetylide−ethylene
Scheme 3. Synthesis of Alkyne 12
Since undesired isomer was obtained in the spiroketalization
due to the presence of two competitive acetal groups, to enhance
access to the desired spiroketal, another pathway was explored.
Accordingly, in path B, to differentiate hydroxy groups in alkyne
12, an isopropylidene group was cleaved to give diol 27, in which
the primary hydroxy group was selectively silylated as its TBDPS
ether to give 28 (Scheme 6). Subsequently, protection of the
secondary hydroxy group to its TES ether furnished alkyne 18 in
good yield.
Union of the two fragments by nucleophilic addition of
lithiated alkyne 18 to the aldehyde 13 (Scheme 7) afforded syn-
17 and anti-17 in a 2:3 ratio.13 The stereochemistry of the newly
generated stereogenic center at C11 in syn-17 was confirmed by
modified Mosher’s analysis.14
diamine complex to afford alkynol 23,12 which was converted
to its PMB ether 12 using PMBBr in 73% yield over two steps
from 16.
The coupling of lithium acetylide of alkyne 12 and the
aldehyde 13 was achieved to afford the isomers anti-11 and syn-
11 in a 2:3 ratio.13 The stereochemistry of a newly generated
stereogenic center at C11 in syn-11 was confirmed by modified
Mosher’s analysis14 of the 1H chemical shifts that revealed the S
absolute configuration at C11. The undesired isomer anti-11 was
converted to the required syn-11 by Mitsunobu inversion.7 To
prepare β-hydroxy ketone 10, syn-11 was subjected to high levels
of regio- and stereoselective hydroboration15 using a Cu(I)−
The syn-17 upon esterification with BzCl afforded benzoate
28. On the other hand, anti-17 was also converged to 28 under
B
DOI: 10.1021/acs.orglett.6b01849
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
The gold-catalyzed spiroketalization in methanol happens first to
give rise to 5-exo-dig cyclization over a possible 6-endo-dig
pathway. Solvolysis of the acetonide and protiodeauration of A
gives vinyl ether B, which upon addition of methanol gives rise to
ketal C, which on subsequent elimination of methanol forms
oxocarbenium ion D. Intramolecular participation of C13 alcohol
on C8 carbon of oxocarbenium ion forms the spiroketal 30.23
The complete structure of 30 was assigned by 1D (1H, 13C, and
DEPT135) and 2D (COSY, NOESY, HSQC, HMBC, and
TOCSY) NMR studies (see Supporting Information). Sap-
onification of the ester in 30 furnished 26 in 88% yield. The
spectroscopic data of spiro compound 26 obtained by path B
were found to match that synthesized by path A. Removal of the
silyl group in 26 using TBAF provided triol 9 in 86% yield. Now,
the stage was set for lactonization (Scheme 10).
Scheme 6. Synthesis of Alkyne 18
Scheme 7. Synthesis of Spiroketalization Precursor 29
Scheme 10. Synthesis of EBC-23
Mitsunobu conditions. Selective deprotection of TES ether19
provided spiroketalization precursor 29.
Treatment of 29 under Utimoto’s conditions (PdCl2, aqueous
CH3CN, reflux and Pd[CH3CN]2Cl2, CH3CN, reflux)20 failed to
give 30. Next, Trost’s precedent (AuCl, PPTS in THF) was
examined to effect spiroketalization.13a This method gave the
desired isomer with modest yield. Gratifyingly, changing the
solvent to methanol furnished the desired spiro compound 30 in
62% yield (Scheme 8).
Scheme 8. Synthesis of Spiroketal 30
Selective oxidation of the primary hydroxy group in triol 9
using a TEMPO−BAIB reagent system24 afforded aldehyde,
which was immediately treated with Still−Gennari reagent25 to
form the Z-olefinic ester 31. Finally, oxidative cleavage of PMB
ether with DDQ followed by lactonization in the presence of
PPTS furnished the target molecule, EBC-23 (1). The
spectroscopic properties (1H, 13C NMR, IR, and HRMS-ESI)
as well as specific rotation ([α]D) were in full accordance with the
natural product and synthetic products.1,3,4
In conclusion, the total synthesis of anticancer agent EBC-23
was accomplished by employing gold-catalyzed spiroketalization
of alkynol, wherein regioselectivity was achieved by selective
activation of alkyne based on electronic bias. This synthetic route
has 21 total and 16 linear steps and provides the natural product
in 4% yield over the longest linear sequence. For the synthesis of
β-hydroxy ketone, a Cu(I)−P(Cy)3-catalyzed protocol for the
highly regio- and stereoselective B2(pin)2 hydroboration of the
internal propargylic alcohol system in the presence of MeOH was
effectively utilized via vinylboronates. We believe that these
methods will be applicable for the preparation of a wide range of
natural products having spiroketals (such as 1,6-dioxaspiro[4.5]-
decane and 1,7-dioxaspiro[5.5]undecane systems) and their
analogues.
A plausible mechanism for the Au(I)-catalyzed cycloisomeri-
zation of 4-alkynol 29 is shown in Scheme 9. This reaction is
initiated by the activation of the triple bond through the Au(I)
catalyst21 followed by addition of C5 alcohol to alkyne at C8 via a
5-exo-dig pathway to form an enol vinylgold22 intermediate A.
Scheme 9. Plausible Mechanism of the Cycloisomerization
Reaction
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.6b01849.
Experimental procedures, characterization details, 1H and
13C NMR spectra of all compounds (PDF)
C
DOI: 10.1021/acs.orglett.6b01849
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
2004, 104, 17. (d) Hoye, T. R.; Jeffrey, C. S.; Shao, F. Nat. Protoc. 2007,
2, 2451.
(15) (a) Kim, H. R.; Yun, J. Chem. Commun. 2011, 47, 2943. (b) Park, J.
AUTHOR INFORMATION
■
Corresponding Author
K.; Ondrusek, B. A.; Mcquade, D. T. Org. Lett. 2012, 14, 4790.
*E-mail: yadavpub@gmail.com.
Notes
́ ́
(c) Moure, A. L.; Gomez Arrayas, R.; Cardenas, D. J.; Alonso, I.;
Carretero, J. C. J. Am. Chem. Soc. 2012, 134, 7219. (d) Semba, K.;
Fujihara, T.; Terao, J.; Tsuji, Y. Chem. - Eur. J. 2012, 18, 4179. (e) Moure,
The authors declare no competing financial interest.
́ ́
A. L.; Mauleon, P.; Arrayas, R. G.; Carretero, J. C. Org. Lett. 2013, 15,
2054. (f) Barbeyron, R.; Benedetti, E.; Cossy, J.; Vasseur, J. J.;
Arseniyadis, S.; Smietana, M. Tetrahedron 2014, 70, 8431.
(16) (a) Jung, M. E.; Yoo, D. Org. Lett. 2011, 13, 2698. (b) Liu, P.;
Fukui, Y.; Tian, P.; He, Z.; Sun, C.; Wu, N.; Lin, G. J. Am. Chem. Soc.
2013, 135, 11700.
(17) Tursun, A.; Canet, I.; Aboab, B.; Sinibaldi, M. E. Tetrahedron Lett.
2005, 46, 2291.
(18) (a) Deslongchamps, P.; Rowan, D. D.; Pothier, N.; Sauve, G.;
Saunders, J. K. Can. J. Chem. 1981, 59, 1105. (b) Deslongchamps, P.
Stereoelectronic Effects in Organic Synthesis. In Organic Chemistry
Series; Baldwin, J., Ed.; Pergamon Press: Oxford, UK, 1983; Vol. 1, pp
5−53. (c) Perron, F.; Albizati, K. F. Chem. Rev. 1989, 89, 1617.
(d) Juaristi, E.; Cuevas, G. Tetrahedron 1992, 48, 5019. (e) Takaoka, L.
R.; Buckmelter, A. J.; LaCruz, T. E.; Rychnovsky, S. D. J. Am. Chem. Soc.
2005, 127, 528. (f) Aho, J. E.; Pihko, P. M.; Rissa, T. K. Chem. Rev. 2005,
105, 4406. (g) Trost, B. M.; O'Boyle, B. M. J. Am. Chem. Soc. 2008, 130,
16190.
(19) (a) Denmark, S. E.; Kobayashi, T.; Regens, C. S. Tetrahedron
2010, 66, 4745. (b) Brandt, D.; Dittoo, A.; Bellosta, V.; Cossy, J. Org.
Lett. 2015, 17, 816.
(20) (a) Utimoto, K. Pure Appl. Chem. 1983, 55, 1845. (b) Trost, B. M.;
Horne, D. B.; Woltering, M. J. Angew. Chem., Int. Ed. 2003, 42, 5987.
(21) Teles, J. H.; Brode, S.; Chabanas, M. Angew. Chem. 1998, 110,
1475; Angew. Chem., Int. Ed. 1998, 37, 1415.
ACKNOWLEDGMENTS
■
D.V.R. thanks the Council of Scientific and Industrial Research
(CSIR), India, for financial assistance in the form of a fellowship,
and D.V.R. and G.S. thank CSIR, India, for financial support as
part of XII five-year programme under project ORIGIN (CSC-
0108). J.S.Y. thanks CSIR and DST, India, for Bhatnagar and J.C.
Bose Fellowships, respectively.
REFERENCES
■
(1) Reddell, P. W.; Gordon, V. A. PCT Int. Appl. WO 2007070984A1,
2007.
(2) (a) Dong, L.; Schill, H.; Grange, R. L.; Porzelle, A.; Johns, J. P.;
Parsons, P. G.; Gordon, V. A.; Reddell, P. W.; Williams, C. M. Chem. -
Eur. J. 2009, 15, 11307. (b) Williams, C. M. Sleepless Nights: The Total
Synthesis of the Anticancer Australian Rainforest Polyketide EBC-23. In
Strategies and Tactics in Organic Synthesis; Harmata, M., Ed.; Academic
Press: Oxford, UK, 2014; Vol. 10, pp 249−270.
(3) Dong, L.; Gordon, V. A.; Grange, R. L.; Johns, J.; Parsons, P. G.;
Porzelle, A.; Reddell, P.; Schill, H.; Williams, C. M. J. Am. Chem. Soc.
2008, 130, 15262.
(4) Albert, B. J.; Yamaoka, Y.; Yamamoto, H. Angew. Chem., Int. Ed.
2011, 50, 2610.
(22) Dube,
therein.
(23) (a) Antoniotti, S.; Genin, E.; Michelet, V.; Genet
́
P.; Toste, F. D. J. Am. Chem. Soc. 2006, 128, 12062 and refs
(5) (a) Yadav, J. S.; Chetia, L. Org. Lett. 2007, 9, 4587. (b) Yadav, J. S.;
Rajender, V.; Rao, Y. G. Org. Lett. 2010, 12, 348. (c) Sabitha, G.; Rao, A.
S.; Yadav, J. S. Org. Biomol. Chem. 2013, 11, 7218. (d) Yadav, J. S.;
Reddy, P. A. N.; Reddy, Y. J.; Meraj, S.; Prasad, A. R. Eur. J. Org. Chem.
2013, 2013, 6317.
(6) (a) Arundale, E.; Mikeska, L. A. Chem. Rev. 1952, 51, 505.
(b) Klunder, J. M.; Onami, T.; Sharpless, K. B. J. Org. Chem. 1989, 54,
1295. (c) Yadav, J. S.; Reddy, Y. J.; Reddy, P. A. N.; Reddy, B. V. S. Org.
Lett. 2013, 15, 546. (d) Brun, E.; Bellosta, V.; Cossy, J. Chem. Commun.
2014, 50, 6718. (e) Brun, E.; Bellosta, V.; Cossy, J. J. Org. Chem. 2015,
80, 8668.
(7) (a) Mitsunobu, O.; Yamada, M. Bull. Chem. Soc. Jpn. 1967, 40, 935.
(b) Mitsunobu, O.; Wada, M.; Sano, T. J. Am. Chem. Soc. 1972, 94, 679.
(c) Yadav, J. S.; Kumar, N. N.; Reddy, M. S.; Prasad, A. R. Tetrahedron
2007, 63, 2689.
(8) Yadav, J. S.; Reddy, P. A. N.; Kumar, A. S.; Prasad, A. R.; Reddy, B.
V. S.; Al Ghamdi, A. A. Tetrahedron Lett. 2014, 55, 1395.
(9) Rychnovsky, S. D.; Richardson, T. I.; Rogers, B. N. J. Org. Chem.
1997, 62, 2925.
̂
, J. P. J. Am.
Chem. Soc. 2005, 127, 9976. (b) Liu, B.; De Brabander, J. K. Org. Lett.
2006, 8, 4907. (c) Li, Y.; Zhou, F.; Forsyth, C. J. Angew. Chem., Int. Ed.
2007, 46, 279. (d) Fang, C.; Pang, Y.; Forsyth, C. J. Org. Lett. 2010, 12,
4528. (e) Tlais, S. F.; Dudley, G. B. Beilstein J. Org. Chem. 2011, 7, 570.
(f) Benson, S.; Collin, M. P.; Arlt, A.; Gabor, B.; Goddard, R.; Furstner,
̈
A. Angew. Chem., Int. Ed. 2011, 50, 8739. (g) Wang, X.; Paxton, T. J.; Li,
N.; Smith, A. B., III. Org. Lett. 2012, 14, 3998.
(24) (a) De Mico, A.; Margarita, R.; Parlanti, L.; Vescovi, A.;
Piancatelli, G. J. Org. Chem. 1997, 62, 6974. (b) Paterson, I.; Florence, G.
J.; Gerlach, K.; Scott, J. P.; Sereinig, N. J. Am. Chem. Soc. 2001, 123, 9535.
(25) (a) Still, W. C.; Gennari, C. Tetrahedron Lett. 1983, 24, 4405.
(b) Ando, K. Tetrahedron Lett. 1995, 36, 4105. (c) Pihko, P. M.; Salo, T.
M. Tetrahedron Lett. 2003, 44, 4361. (d) Reddy, D. V.; Sabitha, G.;
Yadav, J. S. Tetrahedron Lett. 2015, 56, 4112.
(10) (a) Pappo, R.; Allen, D. S., Jr.; Lemieux, R. U.; Johnson, W. S. J.
Org. Chem. 1956, 21, 478. (b) Yu, W.; Mei, Y.; Hua, Z.; Jin, Z. Org. Lett.
2004, 6, 3217. (c) Chakraborty, T. K.; Chattopadhyay, A. K. J. Org.
Chem. 2008, 73, 3578.
(11) (a) Abushanab, E.; Bessodes, M.; Antonakis, K. Tetrahedron Lett.
1984, 25, 3841. (b) Abushanab, E.; Vemishetti, P.; Leiby, R. W.; Singh,
H. K.; Mikkilineni, A. B.; Wu, D. C.; Saibaba, R.; Panzica, R. P. J. Org.
Chem. 1988, 53, 2598. (c) Krishna, P. R.; Lavanya, B.; Sharma, G. V. M
Tetrahedron: Asymmetry 2003, 14, 419. (d) Jung, M. E.; Yoo, D. Org.
Lett. 2007, 9, 3543.
(12) (a) Inanaga, J.; Kawanami, Y.; Yamaguchi, M. Bull. Chem. Soc. Jpn.
1986, 59, 1521. (b) Lee, J.; Parker, K. A. Org. Lett. 2012, 14, 2682.
(13) (a) Trost, B. M.; O'Boyle, B. M.; Hund, D. J. Am. Chem. Soc. 2009,
131, 15061. (b) Sabitha, G.; Rao, A. S.; Sandeep, A.; Yadav, J. S. Eur. J.
Org. Chem. 2014, 2014, 455.
(14) (a) Dale, J. A.; Mosher, H. S. J. Am. Chem. Soc. 1973, 95, 512.
(b) Ohtani, I.; Kusumi, T.; Kashman, Y.; Kakisawa, H. J. Am. Chem. Soc.
́
1991, 113, 4092. (c) Seco, J. M.; Quinoa, E.; Riguera, R. Chem. Rev.
̃
D
DOI: 10.1021/acs.orglett.6b01849
Org. Lett. XXXX, XXX, XXX−XXX