Concise total synthesis of phidianidine A and B

Article abstract of DOI:10.1016/j.tetlet.2013.08.063

The shortest total synthesis of cytotoxic indole alkaloids phidianidine A and B is described. Rapid assembly of the 1,2,4-oxadiazole core from a novel N-hydroxyguanidine and the corresponding indole-3-acetic acid chloride led to formal syntheses of phidianidine A and B in only three steps from known compounds. Deprotection under standard conditions provided the trifluoroacetate salts of phidianidine A and B in quantitative yield.

Full text of DOI:10.1016/j.tetlet.2013.08.063

Tetrahedron Letters 54 (2013) 6002–6004
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Concise total synthesis of phidianidine A and B
⇑
Jacob C. Buchanan , Brandon P. Petersen , Stephen Chamberland
Department of Chemistry, Central Washington University, 400 East University Way, Ellensburg, WA 98926-7539, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 3 July 2013
Revised 13 August 2013
Accepted 19 August 2013
Available online 27 August 2013
The shortest total synthesis of cytotoxic indole alkaloids phidianidine A and B is described. Rapid assem-
bly of the 1,2,4-oxadiazole core from a novel N-hydroxyguanidine and the corresponding indole-3-acetic
acid chloride led to formal syntheses of phidianidine A and B in only three steps from known compounds.
Deprotection under standard conditions provided the trifluoroacetate salts of phidianidine A and B in
quantitative yield.
Ó 2013 Elsevier Ltd. All rights reserved.
Keywords:
Total synthesis
Marine natural product
1,2,4-Oxadiazole
Heterocycles
Guanidinylation
Phidianidine A (1) and B (2) are cytotoxic indole alkaloids iso-
lated from the marine ophisthobranch mollusk Phidiana militaris
in 2011 and are believed to be the first natural products to contain
a 1,2,4-oxadiazole ring.1 Although compounds 1 and 2 are not toxic
to normal human epithelial kidney (HEK293) cells, they are potent
growth inhibitors of aggressive HeLa human epithelial cervical
cancer and C6 rat glioma tumor cell lines (IC50 from 0.4 to
O
N
NH
N
N
H
H2N
N
H
R
N
H
1, R = Br, Phidianidine A
2, R = H, Phidianidine B
O
OH
N
HO
NBoc
1.5
blasts and 3T3-L1 murine embryonic fibroblasts (IC50 from 0.1 to
5.4
M).1,2 Remarkably, phidianidine A and B also exhibit selective
binding within two disparate classes of central nervous system
(CNS) targets, such as the -opioid receptor (vs the d- and -opioid
lM), as well as non-tumor H9c2 rat embryonic cardiac myo-
NH2
+
N
H
N
H
R
BocHN
N
H
l
3
4, R = Br
5, R = H
l
j
receptors) and the dopamine transporter (vs the norepinephrine
and serotonin transporters).2 Selective inhibition of key CNS tar-
gets may also enable 1 and 2 to be used as analgesics for pain man-
agement and to treat certain CNS pathologies. Our laboratory was
drawn to these natural products due to their intriguing architec-
ture and activity toward diverse biological targets.
The novel structure and pharmacological profile of phidianidine
A and B also inspired others to pursue their total synthesis, but
drawbacks of these pioneering efforts prompted us to explore an
alternative strategy.2,3 For example, although 1,2,4-oxadiazole
assembly was the key step in each synthesis, construction of the
linear portion included extraneous nitrogen protecting group
manipulations.2,3 One approach enlisted an azide as an amine pro-
tecting group, but the preparation of low-molecular weight azides
Scheme 1. Retrosynthetic analysis of phidianidine A and B.
represents a significant safety concern when working with under-
graduates.3,4 In tandem with our safety concerns and our desire to
achieve a rapid synthesis of phidianidine A and B, we seek to rem-
edy discrepancies we found in the reported spectroscopic data.
Despite the success of other synthetic efforts, we believed a
shorter and safer route was possible. Specifically, our goal was to
develop a synthetic strategy to prepare phidianidine A and B in a
straightforward, convergent fashion in which the guanidine was
installed first, eliminating unnecessary protecting group steps.
We report a streamlined synthesis of compounds 1 and 2 that is
two steps shorter than any known route, clarifies uncertainties in
the assignment of 1H and 13C NMR peaks, addresses important
safety issues, and was performed by undergraduates.
⇑
Corresponding author. Tel.: +1 (509) 963 1126; fax: +1 (509) 963 1050.
Our retrosynthesis was inspired by Gavagnin, Guo, and co-
workers’ proposed biosynthetic hypothesis and began with the
preparation of novel N-hydroxyguanidine 3, which contains the
E-mail address: chambers@gwmail.cwu.edu (S. Chamberland).
Undergraduate research participant. Each student contributed equally to this
work.
0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2013.08.063

J. C. Buchanan et al. / Tetrahedron Letters 54 (2013) 6002–6004
6003
that solvent.9 We also normalized 1H and 13C NMR chemical shift
data in CD3OD and DMSO-d6 for natural and synthetic phidianidine
A and B to tetramethylsilane to correct for variation in both the
spectral reference peak and chemical shift value chosen by each
author.1–3,10
NBoc
NBoc
a
NH2
N
H
BocHN
N
H
NHCN
BocHN
72%
6
7
88% from 1,5-pentanediamine
ref. 5a
In conclusion, we have completed the shortest total synthesis of
phidianidine A (1) and B (2) to date (4 steps, longest linear se-
quence from known amine 6, 6% and 7% overall yield, respectively).
Overall yields are 5.4% and 5.7%, respectively, if the synthesis be-
gins with commercially available 1,5-pentanediamine (cadaver-
ine). Our convergent strategy featured direct introduction of an
N,N0-di-Boc guanidine that was carried through the synthesis. This
simplified approach circumvented the cumbersome protected
amine-to-amine-to-guanidine sequence characteristic of all prior
syntheses of compounds 1 and 2, and should enable the rapid prep-
aration of analogs for biological evaluation. By avoiding the prepa-
ration of low-molecular weight azides and diazides, we provide a
safer and more direct route to terminal-guanidine containing com-
pounds, which is a primary concern of undergraduate researchers
amid the changing safety culture in academia. We plan to apply
our direct guanidinylation approach to the preparation of other
bioactive, guanidine-containing natural products, and will report
our results in due course.
OH
N
NBoc
b
N
H
NH2
BocHN
N
H
61%
3
Scheme 2. Reagents and conditions: (a) BrCN (1.2 equiv), NaHCO3 (6.0 equiv), H2O/
CH2Cl2, 0 °C, 0.5 h, then, rt, 1 h; (b) NH2OHꢀHCl (1.2 equiv), K2CO3 (3.0 equiv), EtOH,
rt, 3 h.
terminal guanidine in a protected form (Scheme 1).1 Exposure of
known N,N0-di-Boc guanidine5 6 to cyanogen bromide under bi-
phasic conditions provided cyanamide 7, which was converted to
N-hydroxyguanidine
3
using hydroxylamine hydrochloride
(Scheme 2).6 To our knowledge, a similarly functionalized N-
hydroxyguanidine has not been prepared previously.
With N-hydroxyguanidine 3 in hand, the 1,2,4-oxadiazole core
of phidianidine A and B was then constructed by joining the acid
chloride derived from the known 6-bromoindole-3-acetic acid7
(4) or indole-3-acetic acid (5), respectively, with compound 3
(Scheme 3). Acid chlorides 8 and 9 were generated from their
respective indole-3-acetic acids by treatment with oxalyl chloride
at 0 °C. Each acid chloride was then condensed with N-hydroxy-
guanidine 3 to afford the respective 1,2,4-oxadiazoles 10 and 11,
which constituted formal total synthesis of phidianidine A and
B.3a,8,9 Completion of each total synthesis was achieved by depro-
tection with TFA under standard conditions. The 1H and 13C NMR
spectra in CD3OD and DMSO-d6 of synthetic phidianidine A (1)
and B (2) are identical to those reported for the natural products.1
Although our 1H and 13C NMR spectra matched the reported
data,1 we discovered that some specific peak assignments dis-
agreed with our 2D-NMR data. After close examination of our
2D-NMR data (COSY, HSQC, and HMBC) in CD3OD, we suggest reas-
signment of certain 1H and 13C NMR peaks in compounds 1 and 2 in
Acknowledgments
We dedicate this Letter to Colorado State University Distin-
guished Professor Robert M. Williams on the occasion of his 60th
birthday. We thank Professor Gil Belofsky (CWU) for assistance
with 2D-NMR spectroscopy. We also thank Dr. John Greaves and
Ms. Shirin Sorooshian of the University of California, Irvine Mass
Spectrometry Facility for mass spectrometric data. Partial funding
was provided by a CWU College of the Sciences (COTS) Undergrad-
uate Research Grant (COTS-URG) for J.C.B. and CWU Office of
Undergraduate Research Undergraduate Research Fellowships for
J.C.B. and B.P.P.
Supplementary data
Supplementary data (complete experimental procedures, copies
of all spectral data, tables of normalized 1H and 13C NMR peak data,
2D NMR data to support corrected peak assignments, and full char-
acterization) associated with this article can be found, in the online
version, at http://dx.doi.org/10.1016/j.tetlet.2013.08.063.
O
O
Cl
HO
a
b
N
H
N
H
R
R
References and notes
8, R = Br
9, R = H
4, R = Br
5
, R = H
1. Carbone, M.; Li, Y.; Irace, C.; Mollo, E.; Castelluccio, F.; Di Pascale, A.; Cimino, G.;
Santamaria, R.; Guo, Y.-W.; Gavagnin, M. Org. Lett. 2011, 13, 2516–2519.
2. Brogan, J. T.; Stoops, S. L.; Lindsley, C. W. ACS Chem. Neurosci. 2012, 3, 658–664.
3. (a) Lin, H.-Y.; Snider, B. B. J. Org. Chem. 2012, 77, 4832–4836; (b) Manzo, E.;
Pagano, D.; Carbone, M.; Ciavatta, M. L.; Gavagnin, M. ARKIVOC 2012, ix, 220–
228.
4. (a) González-Bobes, F.; Kopp, N.; Li, L.; Deerberg, J.; Sharma, P.; Leung, S.;
Davies, M.; Bush, J.; Hamm, J.; Hrytsak, M. Org. Process Res. Dev. 2012, 16, 2051–
2057; (b) Bräse, S.; Gil, C.; Knepper, K.; Zimmermann, V. Angew. Chem., Int. Ed.
2005, 44, 5188–5240. and references cited therein.
O
N
NBoc
c
N
N
H
N
BocHN
H
R
N
H
10, R = Br, 14% from 4
11, R = H, 15% from 5
5. (a) Castagnolo, D.; Raffi, F.; Giorgi, G.; Botta, M. Eur. J. Org. Chem. 2009, 334–
337; (b) Baker, T. J.; Tomioka, M.; Goodman, M. Org. Synth. 2000, 78, 91–98; (c)
Feichtinger, K.; Zapf, C.; Sings, H. L.; Goodman, M. J. Org. Chem. 1998, 63, 3804–
3805.
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2008, 51, 924–931; (b) Martin, N. I.; Woodward, J. J.; Marletta, M. A. Org. Lett.
2006, 8, 4035–4038; (c) Xian, M.; Fujiwara, N.; Wen, Z.; Cai, T.; Kazuma, S.;
Janczuk, A. J.; Tang, X.; Telyatnikov, V. V.; Zhang, Y.; Chen, X.; Miyamoto, Y.;
Taniguchi, N.; Wang, P. G. Bioorg. Med. Chem. Lett. 2002, 10, 3049–3055.
7. Baran, P. S.; Shenvi, R. A. J. Am. Chem. Soc. 2006, 128, 14028–14029.
8. The change in solvents from CH2Cl2 to ClCH2CH2Cl was necessary because acid
CF3CO2
O
N
NH2
N
N
H
N
H
H2N
R
N
H
1, R = Br, Phidianidine A, 99%
2, R = H, Phidianidine B, 99%
chlorides
8 and 9, as well as N-hydroxyguanidine 3, are insoluble in
Scheme 3. Reagents and conditions: (a) (COCl)2 (3.0 equiv), DMF (cat.), CH2Cl2, 0 °C,
1.5 h; (b) (i) 3 (1.1 equiv), CH2Cl2, rt, 3 h; (ii) ClCH2CH2Cl, 83 °C, 2 h; (c) TFA/CH2Cl2
(1:10 v/v), rt, 8 h.
ClCH2CH2Cl; however, heating above 80 °C was required for 1,2,4-oxadiazole
formation.

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J. C. Buchanan et al. / Tetrahedron Letters 54 (2013) 6002–6004
9. After workup, we obtained quantitative mass balance for each 1,2,4-
oxadiazole-forming reaction; however, the isolated yield of pure compounds
10 and 11 was low. Compounds 10 and 11 were stable during silica-gel column
chromatography (3:2 hexanes/ethyl acetate), which was confirmed by 2D-TLC.
Snider and Lin (Ref. 3a) also prepared compounds 10 and 11 from acid
chlorides 8 and 9, respectively, confirming their stability to silica gel. Methanol
quenching experiments indicated that acid chlorides 8 and 9 were formed
cleanly, suggesting that the 1,2,4-oxadiazole formation step was problematic in
each case. We deemed it unnecessary to determine the structures of polar
reaction products obtained from column chromatography, so at this time, we
cannot speculate as to the cause of the reduced yield of our 1,2,4-oxadiazole-
forming reaction.
10. See Supplementary data.