Synthesis of 14C-acanthoic acid

Article abstract of DOI:10.1002/jlcr.2914

¹⁴C-Acanthoic acid was prepared in four steps by a degradation/reconstitution strategy. ¹⁴C-Acanthoic acid was prepared in four steps by a degradation/reconstitution strategy. Copyright

Full text of DOI:10.1002/jlcr.2914

Technical Note
Received 2 February 2012,
Revised 5 March 2012,
Accepted 6 March 2012
Published online 28 March 2012 in Wiley Online Library
(wileyonlinelibrary.com) DOI: 10.1002/jlcr.2914
Synthesis of 14C-acanthoic acid
Sung-Whi Rhee,a Jeremiah P. Malerich,a Jung-Joon Lee,b and
a
*
Mary J. Tanga
14C-Acanthoic acid was prepared in four steps by a degradation/reconstitution strategy.
Keywords: diterpene; pimaradiene; olefination; regioselective
15-Oxo-16-norpimara-9(11)-en-19-oic acid (3)
Introduction
To a solution of diol 2 (468 mg, 1.38 mmol) and KH2PO4 (6.17 g,
45.3 mmol) in MeOH/H2O (170 mL, 4:1 ratio) at RT was added
NaIO4 (921 mg, 4.30 mmol). After 4 h, the reaction mixture was
diluted with ice (100 g) and extracted with CH2Cl2 (4 Â 80 mL).
The combined organic extracts were dried over MgSO4, filtered,
and concentrated to give aldehyde 3 (339 mg, 80%).
Acanthoic acid (1) is a pimaradiene diterpene that has been
implicated in several biological activities, notably suppression
of proinflammatory cytokines.1 Theodorakis et al. described the
enantioselective synthesis of acanthoic acid.2 Here, we describe
a four-step synthesis of the 14C-labeled acanthoic acid for use
in pharmacological studies. Our strategy involves degradation
of the vinyl group of unlabeled acanthoic acid followed by refor-
mation of the olefin to introduce the radiolabel. To this end, the
acanthoic acid was dihydroxylated with osmium tetroxide to
give vicinal diol 2 (Scheme 1A). Oxidative cleavage with sodium
periodate gave aldehyde 3, the substrate for radiolabeling. The
14C-methylene Wittig reagent (4) was prepared under standard
conditions and treated with aldehyde 3 to give 14C-acanthoic
acid (14C-1; Scheme 1B). This straightforward synthesis afforded
14C-1 in a regioselective manner and high specific activity
(52.7 mCi/mmol).
14C-Methylenetriphenylphosphorane (4)
Triphenylphosphine (322 mg, 1.2 mmol) was dried under vacuum
(1 torr) for 24 h and then dissolved in dry THF (5 mL), and the
solution was degassed (3 freeze/pump/thaw cycles). To this solution
was added 14CH3I (1.8 mmol, 100 mCi, 53 mCi/mmol; Amersham
CFQ9135, Piscataway, NJ). After 24 h at RT, the reaction mixture was
cooled to À78 ꢀC. KHMDS (5.0 mL, 0.5 M in PhMe, 2.5 mmol) was
added dropwise, and the cold bath was removed. After 3 h at RT,
the resulting solution of Wittig reagent 4 was used in the next step.
Experimental
14C-Acanthoic acid
Reactions were run under argon atmosphere. Thin layer chromato-
graphy (TLC) analyses were carried out on commercial pre-coated
silica gel 60F254 plates (E. Merck, Darmstadt, Germany; 5 Â 10 and
5 Â 20cm). The plates were scanned using a Bioscan System 200
Imaging Scanner (Bioscan, Washington, DC).
The solution of Wittig reagent 4 prepared previously was cooled to
À78 ꢀC. A solution of aldehyde 3 (121 mg, 0.4 mmol) in dry THF
(1.5 mL) was added dropwise. The mixture was allowed to warm
slowly to RT. After 18 h, the reaction mixture was cooled to À78 ꢀC
and diluted with HCl (20 mL, 1 M in H2O). The mixture was extracted
with Et2O (3Â 20 mL). The combined organic extracts were dried
over MgSO4, filtered, and concentrated. Two rounds of purification
by column chromatography (SiO2, 1cm d 10 cm column, 1:4
EtOAc : hexanes) gave 14C-1 (51mg, 8.9mCi, 52.7mCi/mmol, 97.8%
radiochemical purity by radio-TLC: 1:4 EtOAc : hexanes, Rf 0.45).
15,16-Dihydroxypimara-9(11)-en-19-oic acid (2)
To a solution of acanthoic acid (1; 479 mg, 1.5 mmol) and
N-methylmorpholine N-oxide (215 mg, 1.83 mmol) in t-BuOH/
THF/H2O (16 mL, 10:3:1 ratio) at room temperature (RT) was
added OsO4 (500 mL, 2.5 wt.% in t-BuOH, 0.033 mmol). After
140 h, the reaction mixture was cooled to 0 ꢀC, and an aqueous
slurry of Na2SO3 and talc was added. The mixture was allowed
to warm at RT. After 16 h, the mixture was filtered through Celite
(Johns Manville, Lompoc, CA), which was washed with CH2Cl2
(5 Â 20 mL). The filtrates were allowed to separate, and the
aqueous phase was extracted with CH2Cl2 (3 Â 100 mL). The
combined organic extracts were dried over MgSO4, filtered,
and concentrated to give 2 (468 mg, 87%) as a pale yellow solid,
which was used in the next step without further purification.
aBiosciences Division, SRI International, 333 Ravenswood Ave., Menlo Park, CA
94025-3135, USA
bKorea Research Institute of Bioscience and Biotechnology (KRIBB), 125
Gwahangno, Yuseong-gu, Daejon, Korea
*Correspondence to: Mary J. Tanga, Biosciences Division, SRI International, 333
Ravenswood Ave., Menlo Park, CA 94025–3135, USA.
E-mail: mary.tanga@sri.com
J. Label Compd. Radiopharm 2012, 55 186–187
Copyright © 2012 John Wiley & Sons, Ltd.

S.-W. Rhee et al.
Scheme 1. Synthesis of 14C-acanthoic acid. NMO = N-methylmorpholine N-oxide.
Acknowledgements
References
The authors are grateful for the financial support from the Korea
Research Institute of Bioscience and Biotechnology (KRIBB) and
the Ministry of Science and Technology of Korea.
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Immunol. 1996, 170(2), 212–221; b) M. Palladino, E. A. Theodorakis,
US Pat. 20030050338 A1.
[2] a) T. Ling, C. Chowdhury, B. A. Kramer, B. G. Vong, M. A. Palladino, E. A.
Theodorakis, J. Org. Chem. 2001, 66(26), 8843–8853; b) T. Ling, B. A.
Kramer, M. A. Palladino, E. A. Theodorakis, Org. Lett. 2000, 2(14),
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Conflict of Interest
The authors did not report any conflict of interest.
J. Label Compd. Radiopharm 2012, 55 186–187
Copyright © 2012 John Wiley & Sons, Ltd.
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