Synthesis and biological evaluation of simplified mycothiazole analogues

Article abstract of DOI:10.1016/j.bmcl.2005.11.072

Thiazoline and oxazoline analogues of the natural product mycothiazole were synthesized from a common intermediate and evaluated in vitro against HCT-15 colon cancer cells and L₄ larvae of nematode Nippostrongylus brasiliensis. The nature of the heterocyclic moiety seems to modulate the cytotoxic or anthelmintic activity.

Full text of DOI:10.1016/j.bmcl.2005.11.072

Bioorganic & Medicinal Chemistry Letters 16 (2006) 1309–1311
Synthesis and biological evaluation of simplified
mycothiazole analogues
a
a
b
c
Graciela Mahler, Gloria Serra, Sylvia Dematteis, Jenny Salda n˜ a,
c
a,
*
Laura Dom ´ı nguez and Eduardo Manta
a
Departamento de Qu ı´ mica Org a´ nica, C a´ tedra de Qu ı´ mica Farmac e´ utica Facultad de Qu ı´ mica,
Universidad de la Rep u´ blica, Montevideo, Uruguay
b
C a´ tedra de Inmunolog ı´ a, Universidad de la Rep u´ blica, Montevideo, Uruguay
C a´ tedra de Farmacolog ı´ a, Facultad de Qu ı´ mica, Universidad de la Rep u´ blica, Montevideo, Uruguay
c
Received 20 October 2005; revised 15 November 2005; accepted 17 November 2005
Available online 27 December 2005
Abstract—Thiazoline and oxazoline analogues of the natural product mycothiazole were synthesized from a common intermediate
and evaluated in vitro against HCT-15 colon cancer cells and L larvae of nematode Nippostrongylus brasiliensis. The nature of the
heterocyclic moiety seems to modulate the cytotoxic or anthelmintic activity.
4
Ó 2005 Elsevier Ltd. All rights reserved.
The natural product mycothiazole (1) is a marine sec-
1
Within the context of synthesizing new bioactive com-
a
ondary metabolite isolated from Spongia mycofijiensis
1
pounds based on natural products, we were interested
in preparing stable and simplified analogues of myco-
thiazole in order to evaluate their biological activities
b
and Dactylospongia. This compound has in vitro
anthelmintic activity against Nippostrongylus brasilien-
1
a
5
sis as well as toxicity throughout the 60 NCI human
2
tumor cell line panel.
as cytotoxic and anthelmintic.
For the design of a series of first-generation analogues,
we considered to evaluate the absence of the conjugated
Z-diene moiety at the C side chain of mycothiazole in
18
16
14
4
OH
N
9
19
6
2
5
7
S
2
order to increase the stability and the substitution of
the thiazole ring by related heterocycles like thiazoline
11
13
NHCO2Me
(
-)-Mycothiazole (1) 20
6
and oxazoline (Scheme 1).
Mycothiazole was first synthesized by Shioiri and co-
3
a
workers who later presented the assignation of the chi-
ral carbon present at the molecule in 2003. They report-
ed that the synthetically pure material is rather labile as
Access to the analogues can be envisioned by the cou-
pling of aminoalcohols like 3 and alkenyl acid 2 via a
3
b
well as the natural product. Recently, a new formal
approach has been published by Cossy and co-
R
OH
N
3
c
2
NHCO Me
workers.
X
Analogues
The use of marine natural products as a target for the
development of new bioactive compounds is broadly
described in the literature.
X= O, S
4
NH2
O
OAc
R
+
NHCO2Me
HO
Keywords: Mycothiazole; Deoxo-Fluor; Thiazolines; Oxazolines;
Anthelmintic; HTC-15 colon cells.
OH
3a, R = H
2
3b, R= CH CH=CH2
2
*
Corresponding author. Tel.: +598 2 9241805; fax: +598 2 9241906;
e-mail: emanta@fq.edu.uy
Scheme 1. Retrosynthetic analogue approach.
0
960-894X/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2005.11.072

1310
G. Mahler et al. / Bioorg. Med. Chem. Lett. 16 (2006) 1309–1311
O
O
Li
OTBS
a, b
c
d
EtO
EtO
4
5
O
OAc
O
OAc
OTBS
e, f, g
NHCO2Me
EtO
h
EtO
6
7
O
OAc
O
OAc
NHCO2Me i, j
NHCO2Me
EtO
HO
8
2
Scheme 2. Synthesis of acid 2. Reagents and conditions: (a) Br
BF ÆEt O, Py, 1 h, 65%; (e) TBAF 1 M, 2 h, 96%; (f) CCl
O, THF, À80 °C; Ac
5%; (h) H , Lindlar, AcOEt, rt, 1 h, 70%; (i) LiOH, THF/MeOH/H
2
, AcOH, 55 °C, 22 h, 88%; (b) NaBH
4 2 3
, MeOH, 2 h; (c) K CO , 2 h, 82%; (d)
3
2
2
4
, Ph
3
P, PhH, reflux, 6 h, 97%; (g) KOCN, MeOH, DMF, 100 °C, 1 h,
2
6
2
2
O, rt, 1 h; (j)Ac
O, Py, rt, 12 h, 70%.
O
OAc
cyclodehydration and oxidation sequence. This method-
ology allows the preparation of different types of hetero-
cycles from the same intermediates.
NH2
+
OH
NHCO2Me
HO
O
3b
2
OAc
H
a
NHCO2Me b, c
Aminoalcohol 3b was synthesized using the methodolo-
gy developed in our group by a Wittig coupling of
homoallyltriphenylphosphonium bromide with the
N
9
OH
OR
5
a
homologated Garner’s aldehyde.
N
c
NHCO2Me
11, R = H
S
Acid 2 was prepared as depicted in Scheme 2. The se-
quence started with the bromination of ethyl 2,2-dime-
1
0, R = Ac
thylacetoacetate 4 using Br in acetic acid at 55 °C to
2
Scheme 3. Synthesis of thiazoline analogues 10 and 11. Reagents and
conditions: (a) EDCl, CH Cl N,
, 0 °C to rt, 6 h, 60%; (b) TBSCl, Et
CH Cl 100%; (c) Lawesson’s reagent, PhH, reflux, 40%; (d)
LiOHÆH O, THF/H O/MeOH, 100%.
afford the corresponding bromoketone (88%). This
intermediate was converted into the epoxide 5 via a
bromohydrine using NaBH4 in basic media (82%).
Alkylation of epoxide 5 was carried out using lithio–
propargylic–OTBS and BF ÆOEt as a catalyst. A solu-
2
2
3
2
2
,
2
2
3
2
tion of lithio acetylide (1 equiv) was added dropwise to
a
OAc
9
N
a solution of epoxide 5 (1 equiv) and BF ÆOEt (1 equiv)
3
2
2
NHCO Me
in THF at À80 °C. Using this procedure, we obtained
only the desired alkynyl compound 6 (65% yield). Inter-
estingly, the protocol described by Yamaguchi and Hir-
O
12
Scheme 4. Synthesis of oxazoline 12. Reagents and conditions: (a)
Deoxo-Fluor, CH Cl
, À20 °C, 1 h, 60%.
7
ao (1.5 equiv of the acetylide and BF ÆOEt ) provided
2
2
3
2
compound 6 (50%) along with the product resulting
from dialkylation at the epoxide and at the ester group
can be explained by spontaneous cyclodehydration of
the b-hydroxythioamide. This type of reaction was not
expected, because usually b-hydroxythioamide cyclode-
hydration needs an activating reagent to give the thiaz-
oline, even though we found one precedent in the
(
10%). The synthesis of carbamate 7 was achieved by
conversion of the OTBS-group into chloride followed
by reaction with potassium cyanate in MeOH. This al-
lows the direct substitution of the chloride by the carba-
9
mate group via an isocyanate to afford compound 7
8
literature. Deprotection of compound 10 with LiOH
led to the analogue 11 (100% yield).
(
65% yield). Hydrogenation of alkynyl compound 7
using Lindlar catalyst afforded the corresponding Z-al-
kene 8 (70% yield). Acid 2 was obtained by hydrolysis
of compound 8 with LiOH followed by acetylation of
the secondary alcohol (70% yield).
The first oxazoline analogue, 12, was prepared from
amide 9, using Deoxo-Fluor cyclodehydrative reagent,
in a clean fashion (60%), see Scheme 4.
1
0
With the two fragments in hand, the target compounds
were assembled using our planned strategy: segment
condensation and cyclodehydration to form the hetero-
cycle core. The thiazoline analogue was synthesized
using the sequence shown in Scheme 3. The coupling
of acid 2 and aminoalcohol 3b with EDCI (1-(3-dimeth-
ylamino-propyl)-3-ethyl-carbodiimide hydrochloride)
provided the amide 9 in 60% yield. Protection of com-
pound 9 followed by treatment with Lawesson’s reagent
provided thiazoline 10 (40% yield). The formation of 10
Analogue 14 contains an oxazoline moiety, while the
17–C19 side chain present in mycothiazole is absent.
The product was prepared using the same strategy used
C
for compound 12 (Scheme 5). Starting with 2-amino-
1
1
pent-4-en-1-ol (3a), oxazoline 14 was obtained as a
diasteromeric mixture in 70% yield.
Attempts to aromatize the compounds 10, 12, and 14,
using MnO
strategy has not granted the access to analogues of
or NiO , were unsuccessful, and the current
2
2

G. Mahler et al. / Bioorg. Med. Chem. Lett. 16 (2006) 1309–1311
1311
O
OAc
for further design of structurally modified and potential-
ly bioselective new analogues.
NH2
OH
NHCO2Me
HO
O
+
3a
2
OAc
a
b
H
Acknowledgments
N
NHCO2Me
This research was supported by a grant from PEDECI-
BA (project URU/84/002). The cytotoxic assays were
supported by NIH/FIRCA. We appreciate greatly
PhD Silvina Garc ´ı a valuable suggestions and correc-
tions to this paper.
1
3
OH
OAc
N
NHCO2Me
O
14
Scheme 5. Synthesis of oxazoline 14. Reagents and conditions: (a)
EDCl, CH Cl Cl
, 0 °C to rt, 6 h, 50%; (b) Deoxo-Fluor, CH
À20 °C, 1 h, 70%.
2
2
2
2
,
Supplementary data
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/
j.bmcl.2005.11.072.
Table 1. Cytotoxicity and anthelmintic assay results for compounds
10, 11, 12, and 14
Entry
Compound
N. brasiliensis
Cytotoxicity
LC50 (lM)
HCT-15 IG50 (lM)
References and notes
1
2
3
4
5
6
7
Albendazole
Mytomycin C
0.34
—
—
1.6
1
2
. (a) Crews, P.; Kakou, Y.; Qui n˜ o a` , E. J. Am. Chem. Soc.
988, 110, 4365; (b) Cutignano, A.; Bruno, I.; Bifulco, G.;
Casapullo, A.; Debitus, C.; G o´ mez-Paloma, L.; Riccio, R.
Eur. J. Org. Chem. 2001, 66, 775.
. NCI-USA results can be consulted on Internet:
www.dtp.nci.nih.gov//NSC 647640.;
a
nt
10
11
>300
a
nt
1
13
150
430
12
14
40
40
b
c
44
Mycothiazole
120
a
b
c
nt = not tested.
EC100 reported by Crews et al.
3. (a) Sugiyama, H.; Yokokawa, F.; Shioiri, T. Org. Lett.
2000, 2, 2149; (b) Sugiyama, H.; Yokokawa, F.; Shioiri, T.
Tetrahedron 2003, 59, 6579; (c) Le Flohic, A.; Meyer, C.;
Cossy, J. Org. Lett. 2005, 7, 339.
1a
2
NCI database (NSC 647640).
4
. Simmons, L.; Andrianasolo, E.; McPhail, K.; Flatt, P.;
Gerwick, W. H. Mol. Cancer Ther. 2005, 4, 333.
mycothiazole containing an aromatic heterocyclic
1
moiety.
5
. For developed synthetic strategies toward developed
precursor of mycothiazole analogues in the group, see:
2
(
2
a) Mahler, G.; Serra, G.; Manta, E. Synth. Commun.
005, 35, 1481; (b) Mahler, G.; Serra, G.; Antonow, D.;
The analogues were evaluated against HCT-15 cells (co-
lon cancer) using sulfurhodamine B (SRB)-assay and
Manta, E. Tetrahedron Lett. 2001, 42, 8143; (c) Serra, G.;
Mahler, G.; Manta, E. Heterocycles 1998, 48, 2035.
1
3
mytomycin C as standard. The anthelmintic assay
was performed on the L larvae of the nematode N. bra-
6. Oxazolines are isoelectronic substitutes for thiazolines.
For an example, see: Wipf, P.; Reeves, J.; Balachadran,
R.; Day, B. J. Med. Chem. 2002, 45, 1901.
. Yamaguchi, M.; Hirao, I. Tetrahedron Lett. 1983, 24, 391.
. Argabrigth, P. A.; Rider, H. D.; Sieck, R. J. Org. Chem.
1965, 50, 3317.
. Aitken, R. A.; Armstrong, D.; Galt, R.; Mesher, S.
J. Chem. Soc., Perkin Trans. 1 1997, 935.
0. Phillips, A.; Wipf, P.; Uto, Y.; Williams, D. Org. Lett.
2000, 2, 1165.
1. (RS) 2-Amino-pent-4-en-1-ol was synthesized by reduc-
4
siliensis using albendazole as a standard according with
1
the described protocol.
4
7
8
While the overall activities are moderate, there are some
differences among the assayed compounds (Table 1).
Thiazoline analogue 11 is the most active on HCT-15 cells
showing the same cytotoxicity as mycothiazole. The acet-
ylated thiazoline 10 shows poor anthelmintic activity.
9
1
1
Oxazoline containing compounds 12 and 14 show similar
behaviors, both being better as anthelmintic than cyto-
toxic on a HCT-15 cell line. The anthelmintic activity
of oxazoline 12 is better than that of its parent thiazoline
10. The nature of the heterocyclic moiety appears to play
a role in determining the activity of these compounds.
tion of allylglycine with LiAlH
0%.
2. For references of inactivated thiazoline oxidation using
NiO , see: Minster, D.; Jordis, U.; Evans, D.; Hecht, S.
J. Am. Chem. Soc. 1977, 99, 8078, For references of
oxidation using MnO , see: Ref 5.
3. Skehan, P. V. Cytotoxicity and Cell Growth Assays, in
Cell Biology. A Laboratory Handbook, 2nd ed., 1998,
Vol. I.
4
in THF at reflux,
7
1
2
2
1
In conclusion, we prepared the first series of simplified
and stable thiazoline and oxazoline analogues to myco-
thiazole. The different cytotoxic properties resulting
from their biological evaluation will serve as the basis
14. Gordon, S.; Costa, L.; Incerti, M.; Manta, E.; Salda n˜ a, J.;
Dom ´ı nguez, L.; Mariezcurrena, R.; Suescum, L. Il Far-
maco 1997, 52, 603.