Solution phase synthetic approach to fellutamide B

Article abstract of DOI:10.1016/j.tetlet.2015.04.037

Abstract A convenient solution phase approach for the synthesis of fellutamide B with efficient purification techniques has been demonstrated on the molecule for the first time. The strategy involves the use of natural amino acids as starting materials and classical peptide coupling reactions. The synthesis has been achieved in 10 steps with overall yield of 26.7% making the synthesis facile.

Full text of DOI:10.1016/j.tetlet.2015.04.037

Tetrahedron Letters 56 (2015) 3999–4001
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Tetrahedron Letters
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Solution phase synthetic approach to fellutamide B
a
b
a,
Jhillu Singh Yadav a, , Soma Shekar Dachavaram , René Grée , Saibal Das
⇑
⇑
a Natural Products Chemistry Division, CSIR-Indian Institute of Chemical Technology, Tarnaka, Hyderabad 500 007, Andhra Pradesh, India
b Université de Rennes 1, Institut des Sciences Chimiques de Rennes, CNRS UMR 6226, Avenue du Général Leclerc, 35042 Rennes Cedex, France
a r t i c l e i n f o
a b s t r a c t
Article history:
A convenient solution phase approach for the synthesis of fellutamide B with efficient purification
techniques has been demonstrated on the molecule for the first time. The strategy involves the use of
natural amino acids as starting materials and classical peptide coupling reactions. The synthesis has been
achieved in 10 steps with overall yield of 26.7% making the synthesis facile.
Received 5 March 2015
Revised 6 April 2015
Accepted 8 April 2015
Available online 11 April 2015
Ó 2015 Elsevier Ltd. All rights reserved.
Keywords:
Fellutamide
Neurodegenerative diseases
Tuberculosis proteasome
Solution phase synthesis
Peptide coupling
In 1991, for the first time, Kobayashi and co-workers isolated
fellutamide B (Fig. 1) from the fungus Penicillium fellutanum, found
in the gastrointestine of the Candystripe Cardinal fish, Apogon
endekataenia, located off coast of Japan.1 Fellutamide B was origi-
nally identified for its cytotoxic properties, later on it was reported
to stimulate NGF synthesis and secretion, and also established as a
potent inhibitor of the Mycobacterium tuberculosis proteasome
with an inhibition constant (KI) of 6.8 nM against the M. tuberculo-
sis 20S core particle (CP) and human 20S CP with a KI of 11.5 nM.
Recently, it showed cytotoxicity against2 human epidermoid
for the development of new leads to TB drug, which may operate
via novel modes of action.
Owing to its interesting structural features and biological activ-
ities of this lipopeptide aldehyde, known as the fellutamide B,
herein we report the first flexible synthesis of fellutamide B start-
ing from natural amino acids.
Two efficient solid phase syntheses of fellutamide B have been
achieved, first by Crews and co-workers in 20065 by regioselective
reductive opening of a cyclic sulfate, and later coupling it to a solid
phase resin. Afterwards in 2012, Payne and co-workers demon-
strated the synthetic by using Weinreb amide-derived resin.6a
Later, in 2012, an abstract of Letter presented in ACS meeting
claims a greener approach in the solution phase6b by using
kinetic resolution and reagent less peptide coupling. Now herein,
we demonstrate a solution phase strategy using natural amino
acids along with the adaptable purification technique for the
target molecule.
Accordingly, the retrosynthetic approach is described
(Scheme 1) wherein the target compound was envisaged from
two key intermediates, fragment 3 and tri-peptide fragment 4.
And fragment 3 could be prepared via Evans aldol reaction of
decanal 5 in classical reaction conditions.
carcinoma KB, exhibiting IC50 value with 0.7 lg/mL, in addition
to NGF release from fibroblasts and glial-derived cells. These data
highlight its potential to aid in the treatment of neuronal injury
and neurodegenerative diseases.3 NGF and other neurotrophins
provide neuroprotective effects that may be important in the
development of treatments for neuronal injuries or neurodegener-
ation caused by stroke or central nervous system diseases, such as
Alzheimer’s and Parkinson’s diseases.
As such, it can be envisaged that modifications to the structure
may result in higher affinity and selectivity for the M. tuberculosis
proteasome. Two potent irreversible oxathiazol-2-one inhibitors
of the M. tuberculosis proteasome were recently elucidated from
a high throughput screen of 20,000 compounds.4 These compounds
have shown to cross the cell wall and inhibit non-replicating
M. tuberculosis in vitro, which may therefore provide inspiration
Fragment
S-Leucinol
4
was taken up in the beginning, (Scheme 2)
6
was subjected to benzylchloroformate, i-Pr2NEt
mediated protection in CH2Cl2 furnishing the corresponding pro-
tected amino alcohol 7 in 95% yield. Further Swern oxidation of
amino alcohol7 gave the desired aldehyde 8 in 92% yield. In the
fellutamide family, the typical free aldehyde is the main key func-
tionality for its activity, hence it was kept protected as its acetal8 9
⇑
Corresponding authors. Tel.: +91 40 2719 3737; fax: +91 40 2716 0512.
E-mail address: yadavpub@gmail.com (J.S. Yadav).
http://dx.doi.org/10.1016/j.tetlet.2015.04.037
0040-4039/Ó 2015 Elsevier Ltd. All rights reserved.

4000
J. S. Yadav et al. / Tetrahedron Letters 56 (2015) 3999–4001
in order to regenerate the aldehyde group back, a reported protocol
O
O
was employed using PTSA, 4 N HCl and TMS iodide conditions in
THF, MeOH and DMF. Unfortunately, in our hands, success was
not achieved after repeated efforts. At this juncture, it was decided
to explore the possibility of using in 4 N HCl in 1,4-dioxane solvent
system at room temperature which at last furnished the desired
transformation smoothly.13 After the completion of reaction (mon-
itored by TLC) the reaction mixture was made basic by treatment
with NaHCO3 and the crude was obtained as a white solid which
was filtered and purified using MeOH/CHCl3 as eluent in column
chromatography, to furnish the target compound 1, fellutamide B
in 95% yield in pure form. The spectroscopic data and optical rota-
NH2
OH
O
O
H
N
N
H
N
H
O
H2N
O
Fellutamide-B (1)
Figure 1. Structure of fellutamide B.
tion obtained for
1 were in agreement with the reported
literature.1,15
by using triethoxy orthoformate and PTSA in EtOH in 82% yield.
Hydrogenation of 9 with 10% Pd/C in EtOAc produced the expected
deprotected compound 10 in 94% yield. After purification, it was
directly subjected to EDC, HOBT and i-Pr2NEt mediated peptide
Purification of such highly polar lipopeptides in the solution
phase is one of the crucial challenges towards the accomplishment
of such synthesis. Herein, we also report an easy purification pro-
cedure where least loss of desired materials was observed. The typ-
ical purification procedure followed for the synthesis of compound
12, 15 and 18 that undergo the coupling protocol has been
achieved successfully in excellent yields. The crude reaction mix-
ture was diluted with cold water (10–15 °C) which immediately
generates a white coloured froth. The white solid obtained in the
sintered funnel was washed thoroughly with water at 20–25 °C
at least three times. Finally, the solid was dissolved in MeOH, dried
over anhydrous Na2SO4 and concentrated. To conclude simple
coupling9 with Cbz-protected
L-glutamine 11 in DMF to obtain
the dipeptide 12 in 85% yield. Hydrogenation10 of 12 with 10%
Pd/C in MeOH gave compound 13 in 84% yield, opening the handle
for the next attachment. After purification, it was directly used for
second peptide coupling with Cbz-protected L-asparagine 14 in
DMF and furnished the awaited tripeptide 15 in 83% yield.
Subsequent hydrogenation of 15 using 10% Pd/C in MeOH pro-
duced the desired fragment 4 in 87% yield, which happened to be
the key fragment towards the final coupling with intermediate 3
to achieve the target.
Fragment 3, on the other hand, required the preparation of
Crimmins chiral auxiliary 16, which was achieved easily using suit-
able acetic anhydride and commercially available chiral thiazolidi-
none auxiliary,11 along with pivaloyl chloride, Et3N and LiCl in THF
under inert atmosphere. In order to proceed towards the required
intermediate 3, commercial aldehyde 5 was subjected for auxil-
iary-base asymmetric aldol reaction12 with imide 16 and 1-decanol
5 in the presence of TiCl4 and DIPEA in CH2Cl2 (Scheme 3). The
desired adduct 17 was obtained in 72% yield as the major diastere-
omer (98% de) which was separable from its minor diastereomer
a
b
c
CbzHN
S-Leucinol
CbzHN
O
OH
7
6
8
e
d
O
O
HO2C
H2N
10
CbzHN
O
using
column
chromatography
(Hexane/EtOAc,
8:2).
O
O
O
CbzHN
9
Subsequently, adduct 17 was subjected to basic removal of the chi-
ral auxiliary under LiOHÁH2O in THF/H2O (4:1) to furnish the corre-
sponding acid 3 in 81% yield, (Scheme 4) ready to undergo the final
peptide coupling as planned.
NH2
11
O
Now, fragment 3 and 4 in hand were treated with EDC, HOBT
and i-Pr2NEt in DMF and compound 18 was obtained in 81% yield.
Now to construct the complete skeleton of the target molecule and
CbzHN
H2N
O
H2N
O
f
N
H
N
H
O
O
O
H2N
O
12
13
O
O
O
NH2
O
NH2
H
OBn O
O
g
h
H
N
O
N
N
H
CbzHN
1
N
H
N
H
8
CbzHN
O
O
O
O
14
NH
2
CO2H
O
2
H2N
O
15
H2N
O
O
O
NH2
OH
O
NH2
O
H
H
N
O
OH
N
O
H2N
N
H
8
H2N
N
H
O
O
3
O
O
4
H2N
O
H2N
O
4
1-Decanal
5
OH
H2N
Scheme 2. Reagents and conditions: (a) i-Pr2NEt, Cbz-chloride, CH2Cl2, 0 °C to rt,
4 h, 95%; (b) DMSO, oxalyl chloride, Et3N, CH2Cl2, À78 °C, 0.5 h, 92%; (c) triethyl
orthoformate, PTSA, EtOH, 0 °C–rt, 0.5 h, 82%; (d) H2, Pd/C, EtOAc, rt, 3 h, 94%; (e) 11,
EDC, HOBT, i-Pr2NEt, DMF, 0 °C–rt, 12 h, 85%; (f) H2, Pd/C, MeOH, rt, 3 h, 84%; (g) 14,
EDC, HOBT, i-Pr2NEt, DMF, 0 °C–rt, 12 h, 83%; (h) H2, Pd/C, MeOH, rt, 3 h, 87%.
S-Leucinol
6
Scheme 1. Retrosynthesis of fellutamide B.

J. S. Yadav et al. / Tetrahedron Letters 56 (2015) 3999–4001
4001
up a scope to take it further. Most likely, this would be also useful
for large scale synthesis, as well as generation of chemical libraries
towards more extensive biological studies.
O
S
OH
O
S
a
b
5
3
N
N
S
S
8
Bn
Bn
16
17
Acknowledgments
Scheme 3. Reagents and conditions: (a) TiCl4, i-Pr2NEt, CH2Cl2, 0 °C to À78 °C, 3 h,
72%; (b) LiOHÁH2O, THF/H2O (4:1), 0 °C–rt, 10 h, 81%.
This research has been performed as part of the Indo-French
‘Joint Laboratory for Sustainable Chemistry at Interfaces’. We thank
CSIR (12th Five year Plan, ORIGIN: CSC-0108), J.C. Bose & CSIR
Bhatnagar Fellowship and CNRS (University of Rennes 1) and
CEFIPRA/IFCPAR for their financial support of this research. S.S.D.
thanks CSIR, New Delhi for financial assistance.
O
OH
O
NH2
O
H
OH
a
N
O
H2N
N
H
Supplementary data
O
O
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2015.04.
037.
H2N
O
O
3
4
O
NH2
OH
O
H
References and notes
N
O
b
N
H
N
1
H
1. Shigemori, H.; Wakuri, S.; Yazawa, K.; Nakamura, T.; Sasaki, T.; Kobayashi, T.
Tetrahedron 1991, 47, 8529–8534.
O
O
2. (a) Hines, J.; Groll, M.; Fahnestock, M.; Crews, C. M. Chem. Biol. 2008, 15, 501–
512; (b) Carlson, E. E. ACS Chem. Biol. 2010, 5, 639–653; (c) Beck, J.; Guminski,
Y.; Long, C.; Marcourt, L.; Derguini, F.; Plisson, F.; Grondin, A.; Vandenberghe, I.;
Vispe, S.; Berl, V.; Aussagues, Y.; Ausseil, F.; Arimondo, P. B.; Massiot, G.; Sautel,
F.; Cantagrel, F. Bioorg. Med. Chem. 2012, 20, 819–831.
3. Yamaguchi, K.; Tsuji, T.; Wakuri, S.; Yazawa, K.; Kondo, K.; Shigemori, H.;
Kobayashi, J. Biosci., Biotechnol., Biochem. 1993, 57, 195–199.
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2010, 501, 214–220; (b) Lin, G.; Li, D.; De Carvalho, L. P. S.; Deng, H. T.; Tao, H.;
Vogt, G.; Wu, K. Y.; Schneider, J.; Chidawanyika, T.; Warren, J. D.; Li, H.; Nathan,
C. Nature 2009, 461, 621–626.
5. Schneekloth, J. S.; Sanders, J. L.; Hines, J.; Crews, C. M. Bioorg. Med. Chem. Lett.
2006, 16, 3855–3858.
6. (a) Giltrap, A. M.; Cergol, K. M.; Pang, A.; Britton, W. J.; Payne, R. J. Mar. Drugs
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243rd ACS National Meeting & Exposition, San Diego, CA, United States, March
25–29, 2012.
H2N
O
18
Scheme 4. Reagents and conditions: (a) EDC, HOBT, i-Pr2NEt, DMF, 0 °C–rt, 12 h,
81%; (b) 4 N HCl, 1,4-dioxane, rt, 2 h, 95%.
O
NH2
OBn O
8
O
H
N
1
N
H
N
H
O
O
2
H2N
O
7. Mirilashvili, S.; Chasid-Rubinstein, N.; Albeck, A. Eur. J. Org. Chem 2010, 4671–
4686.
Scheme 5. Unsuccessful route to form 1 from 2.
8. Fournial, A.; Ranaivondrambola, T.; Allainmat, M. M.; Robins, R. J.; Lebreton, J.
Eur. J. Org. Chem 2010, 152–156.
9. (a) Chandrasekhar, S.; Rao, C. L.; Seenaiah, M.; Naresh, P.; Jagadeesh, B.;
Manjeera, B.; Sarkar, A.; Bhadra, M. P. J. Org. Chem 2009, 74, 401–404; (b)
Chandrasekhar, S.; Pavankumarreddy, G.; Sathish, K. Tetrahedron Lett. 2009, 50,
6851–6854.
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Table 1
Conditions explored towards the removal of the benzyl group in the presence of
aldehyde
Entry
Condition
Solvent
Time (h)
11. Yadav, J. S.; Dachavaram, S. S.; Peddapuram, A.; Das, S. Tetrahedron Lett. 2014,
55, 1952–1955.
1
2
3
4
Pd/C, H2
MeOH
MeOH
EtOH
THF
3
4
2
2
Pd(OH)2, H2
Raney Ni, H2
TiCl4
12. (a) Yadav, J. S.; Rajender, G. Eur. J. Org. Chem. 2011, 6781–6788; (b) Yadav, J. S.;
Rajender, G.; Ganganna, B.; Srihari, P. Tetrahedron Lett. 2010, 51, 2154–2156;
(c) Crimmins, M. T.; Slade, D. J. Org. Lett. 2006, 10, 2191–2194.
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14. (a) Noda, A.; Aoyagi, S.; Machinga, N.; Kibayashi, C. Tetrahedron Lett. 1994, 44,
8237–8240; (b) Saaki, K.; Minowa, N.; Kuzuhara, H.; Nishiyama, S. Bioorg. Med.
Chem. 2005, 13, 4900–4911.
silica gel column chromatography (MeOH/CHCl3) furnished the
respective coupled peptide in 80–85% isolated yields.
It is noteworthy to also mention that, when the secondary alco-
hol in compound 3 was benzyl-protected at the pre-final stage,
(Scheme 5) its removal after the final coupling from compound 2
was unsuccessful in the presence of terminal aldehyde under sev-
eral reductive conditions14 (Table 1).
In all the entries (1–4), removal of the benzyl group along with
simultaneous reduction of aldehyde to alcohol was observed. Even
by using <1 equiv or <1–5 mol % of catalysts, the desired selectivity
was not achieved successfully in our hands.
In summary, we have developed a convenient solution phase
approach to fellutamide B in 10 simple reaction steps with good
yields. A typical workup procedure has been also demonstrated
towards the purification of polar and lipophilic peptides. The strat-
egy described herein towards synthesizing fellutamide B may
allow an easy access and availability for further study and opens
15. Fellutamide-B (1): To a stirred solution of 18 (0.3 g, 0.0005 mmol) in 2 mL 1,4-
dioxane at room temperature was added 3 mL 4 N HCl, stirring was continued
for about 2 h, at 0 °C the reaction mixture slowly neutralized with solid
NaHCO3 then white solid was generated in solvent system which was filtered
through the sintered funnel and washed with water (3 Â 5 mL). Collected solid
washed with 20 mL MeOH which was dried over Na2SO4, and concentrated will
give crude peptide which was purified by chromatography (CHCl3/MeOH) to
gave 1 (0.251 g, 95%), as off white solid; IR (KBr): 3432, 2925, 1628, 1143 cm1;
[a
]
D
32: À25 (c 0.3, MeOH); 1H NMR (500 MHz, DMSO-d6): d = 9.37 (s, 1H), 8.28
(d, J = 7.36 Hz, 1H), 8.09–8.16 (m, 2H), 7.48 (s, 1H), 7.26 (s, 1H), 6.95 (d,
J = 7.36 Hz, 1H), 6.88 (m, 1H), 6.77 (s, 1H), 4.59 (m, 1H), 4.20 (m, 1H), 4.04 (m,
1H), 3.79 (br s, 1H), 2.56 (m, 1H), 2.46 (m, 1H), 2.18–2.22 (m, 2H), 2.02–2.11
(m, 1H), 1.85–1.97 (m, 1H), 1.76 (m, 1H), 1.65 (m, 1H), 1.49 (m, 2H), 1.17–1.42
(m, 16H), 0.76–0.93 (m, 9H); 13C NMR (300 MHz, DMSO-d6): d = 201.6, 173.8,
171.8, 171.7, 171.2, 171.0, 67.4, 56.6, 52.4, 52.3, 49.8, 43.4, 36.9, 36.8, 31.3,
31.2, 29.0, 28.9, 28.7, 28.3, 23.9, 23.0, 22.0, 21.3, 13.9; MS (ESI) (m/z): 556
[M+H]+, 578 [M+Na]+, 610 [M+MeOH+Na]+; HRMS (ESI): Calcd for C27H50N5O7
[M+H]+: 556.3705. Found: 556.3739, Calcd for C28H53N5O8Na [M+MeOH+Na]+:
610.3786. Found: 610.3822.