Studies directed towards the synthesis of stevastelins - A macrolactonization approach to stevastelin B

Article abstract of DOI:10.1016/S0040-4039(01)00893-0

A synthetic approach towards the cyclic depsipeptide immunosuppressant stevastelin B, based on the stereoselective synthesis of its propionate-derived fatty acid segment 6 that was coupled with the tripeptide 7 leading to the advanced stage acyclic intermediate 5, is described. In spite of our best efforts, the crucial macrolactonization reaction was not successful.

Full text of DOI:10.1016/S0040-4039(01)00893-0

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 5085–5088
Studies directed towards the synthesis of stevastelins—a
macrolactonization approach to stevastelin B
Tushar K. Chakraborty,* Subhash Ghosh and Shantanu Dutta
Indian Institute of Chemical Technology, Hyderabad 500 007, India
Received 20 April 2001; accepted 24 May 2001
Abstract—A synthetic approach towards the cyclic depsipeptide immunosuppressant stevastelin B, based on the stereoselective
synthesis of its propionate-derived fatty acid segment 6 that was coupled with the tripeptide 7 leading to the advanced stage
acyclic intermediate 5, is described. In spite of our best efforts, the crucial macrolactonization reaction was not successful. © 2001
Elsevier Science Ltd. All rights reserved.
Isolated from a culture of a Penicillium sp. NK374186,
the stevastelins A (1), B (2), B3 (3) and C3 (4) represent
a family of novel cyclic depsipeptide immunosuppre-
sants that inhibit the dual specificity phosphatase,
VHR.1–3 Three more stevastelin congeners, named A3,
D3 and E3, were subsequently isolated from the same
source.4 While the sulfated derivative stevastelin A (1)
exhibits potent inhibitory activities against VHR in
extracellular enzyme preparations, it has little effect in
cellular preparations.5,6 The converse is true for stev-
astelin B (2). The free threonyl hydroxyl group in 2
enables it to penetrate cells much better than its sul-
fated version 1. Once inside, it gets converted to an
active form by intracellular sulfation or phosphoryla-
tion of its hydroxyl function. The pronounced biologi-
cal activities of stevastelin molecules made us interested
in undertaking their total synthesis. Herein, we report
an attempt to synthesize stevastelin B following an
approach that could also lead to some other members
of the family.
Me Me
Me Me
3
5
Me(CH2)12
O
5
3
Me(CH2)12
O
Me
Me
Me
Me
OH HN
O
OH
O
HN
O
O
O
O
O
O
NH
NH
N
N
H
Me
H
OR2
OR2
OR1
Me
OR1
1: stevastelin A: R1 = SO3H, R2 = Ac
2: stevastelin B: R1 = H, R2 = Ac
3: stevastelin B3: R1 = H, R2 = Ac
4: stevastelin C3: R1 = H, R2 = H
Me Me
Me(CH2)12
O
Me
Me
Me
OBn
OH
O HN
Me Me
O
Me
O
PMB
O
H
N
HO2C
Me(CH2)12
CO2H + BocHN
N
H
CO2Me
NH
N
H
Me
OAc OPMB
O
Me
BnO
OBn
OBn
6
7
5
* Corresponding author. Fax: +91-40-7173387, 7173757; e-mail: chakraborty@iict.ap.nic.in
0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)00893-0

5086
T. K. Chakraborty et al. / Tetrahedron Letters 42 (2001) 5085–5088
overall yield—silyl protection, LiBH4 reduction, fol-
lowed by Swern oxidation. Addition of the titanium
enolate derived from the acyloxazolidinethione 98 to the
aldehyde 8 gave the non-Evans syn aldol product 10 as
the only isolable diastereomer in 70% yield.9 The stereo-
chemistry of the product was assigned at a later stage.
Reduction of 10 with sodium borohydride in ethanol
gave the diol 11 in 80% yield. Next, a p-methoxyben-
zylidene protection of the 1,3-diol moiety was followed
by selective opening of the benzylidine ring using
DIBAL-H to give the intermediate 12 in 78% yield in
two steps. Swern oxidation of 12 gave an aldehyde, in
90% yield, which on treatment with CH3(CH2)12MgBr
in THF furnished the expected adduct 13 in 40% yield
along with the undesired isomer 14.3 Compound 13 was
then converted into a known fragment 15, obtained
during the degradation studies on stevastelins,3 in four
steps—PMB deprotection, acetonide protection of the
resulting 1,3-diol, desilylation, followed by benzoyl pro-
tection of the primary hydroxyl group. The NMR data
Retrosynthetically, stevastelin B (2) can be obtained by
macrolactonization of the acyclic hydroxy acid 5 which,
in turn, could be prepared by coupling the two key
intermediates 6 and 7, the former with the long chain
fatty acid moiety and the latter possessing the tripeptide
component. The salient feature of our synthesis is the
stereoselective construction of the C1–C18 fragment 6
with four consecutive stereocenters following two key
steps: (i) a Ti(IV)-mediated diastereoselective non-
Evans syn aldol reaction7 using a 2-oxazolidinethione
based chiral auxiliary,8 which was expected to fix the C3
and C4 stereocenters; and (ii) the nucleophilic addition
of a Grignard reagent derived from a long-chain halide
to the C5-aldehyde to instill the requisite stereochem-
istry at C5.
Scheme 1 outlines in detail the stereoselective synthesis
of 6. The starting aldehyde 8 was prepared from methyl
(S)-3-hydroxy-2-methylpropionate in three steps in 75%
NaBH4, EtOH,
Bn
TiCl4, DIPEA,
Bn
0 oC
o
Me
Me Me
CH2Cl2, -78 C
Me
N
O
N
O
OTBDPS
+
OTBDPS
OHC
HO
80%
O
70%
O
OH
S
9
S
8
10
1. (COCl)2, DMSO,
Et3N, CH2Cl2, -78 oC
MeO
1.
CH(OMe)2
Me Me
Me Me
CSA (cat.), CH2Cl2
HO
OTBDPS
OTBDPS
90%
OPMB
2. DIBAL-H, CH2Cl2, -78 oC
to 0 oC
OH
2. CH3(CH2)12MgBr,
THF, 0 oC to rt
12
11
78%
Me Me
Me Me
+
OTBDPS
CH3(CH2)12
OTBDPS
CH3(CH2)12
OH OPMB
14
OH OPMB
Me Me
OBz
13 (40%)
CH3(CH2)12
4 steps
1. Ac2O, Et3N
13
O
O
DMAP (cat.), CH2Cl2
2. TBAF, THF
85%
15
EDCI, HOBt, CH2Cl2, 0 oC, then
Me Me
Jones Oxidation
70%
6
CH3(CH2)12
OH
Me
Me
OBn
O
OAc OPMB
H
N
TFA,
16
TFA.H2N
N
H
CO2Me
+ DIPEA
CH2Cl2, 0 oC
7
O
BnO
Me
Me Me
17
Me(CH2)12
O
65%
Me
Me
OAc O HN
PMB
5
MeO2C
O
O
NH
N
H
OBn
Me
OBn
18
Scheme 1.

T. K. Chakraborty et al. / Tetrahedron Letters 42 (2001) 5085–5088
5087
1. TBAF, THF
1. SO3-Py, DIPEA,
DMSO, CH2Cl2, 0 oC
2. TBSOTf, 2,6-lutidine,
CH2Cl2, 0 oC
Me Me
Me Me
CH3(CH2)12
TBSO
CH3(CH2)12
OH
CO2H
OPMB
13
TBSO
OPMB
2. NaClO2, NaH2PO4,
2-methyl-2-butene, tBuOH, rt
95%
3. CSA (cat.), MeOH-CH2Cl2
0 oC
20
19
68%
Me Me
Me Me
Me(CH2)12
O
Me(CH2)12
O
Me
Me
Me
Me
Coupling with 17
CSA (cat.), MeOH,
TBSO
O
HN
OH
O
HN
(same method as described
in Scheme 1: 6 to 18)
0 oC
PMB
O
PMB
O
MeO2C
MeO2C
O
O
NH
NH
N
N
75%
70%
H
H
OBn
OBn
Me
OBn
Me
OBn
LiOH, THF-MeOH-H2O
0 oC
21
22
Macrolactonisation
5
96%
Scheme 2.
of 15 were identical with those reported for the
degraded product.3 The remaining part of the synthesis
was then carried out by acetate protection of the sec-
ondary alcohol in 13 followed by silyl deprotection to
give the primary alcohol 16 in 85% yield from 13.
Finally, Jones oxidation of 16 furnished the desired
fatty acid segment 6 in 70% yield.
the C5-silyloxy acid 20 in 95% yield. The acid 20 and
the peptide segment 17 were then coupled to give
compound 21 in 70% yield. TBS-deprotection of 21
gave the intermediate 22 that was subjected to ester
hydrolysis to furnish the desired hydroxy acid 5.
The stage was now set to carry out the crucial macro-
lactonization reaction, but, unfortunately, even after
trying several methods, for example the Yamaguchi
method,10 dipyridyl disulfide,11 DCC-DMAP (cat.),12
etc. we were unable to prepare the desired cyclised
product from 5. Whether the above failure should be
attributed to an inherent structural feature of the
cyclization intermediate 5, which possibly could not
attain the required conformation for ester bond forma-
tion, or to steric hindrance due to the presence of long
chain, is not yet clear. Efforts are now going on to form
the ester bond first and then carry out a macrolac-
tamization reaction in order to complete the targeted
total synthesis of stevastelin B. These studies will be
reported in due course.
The peptide segment 7 was synthesized from commer-
cially available protected amino acids by standard solu-
tion phase peptide synthesis conditions using
1-ethyl-3-(3-(dimethylamino)propyl)carbodiimide hydro-
chloride (EDCI) and 1-hydroxybenzotriazole (HOBt) as
coupling agents and dry CH2Cl2 as solvent.
Both the halves of the molecule were now ready to be
coupled. The tripeptide 7 was accordingly treated with
50% TFA in CH2Cl2 at 0°C to effect Boc deprotection,
leading to the free-NH2 containing tripeptide 17, which
was coupled with the acid 6 following the peptide
coupling method described above to obtain the coupled
product 18 in 65% yield. At this stage, we thought that
both the ester and the acetate groups would be
hydrolyzed upon treatment with base. Unfortunately,
the proton NMR spectrum of the hydrolyzed product
revealed that only the ester function had been
hydrolyzed, while the acetate remained intact. Attempts
to deprotect the acetate first, before the hydrolysis of
the ester function, using various methods did not
succeed.
Acknowledgements
The authors wish to thank Drs. A. C. Kunwar and M.
Vairamani for NMR and mass spectroscopic assistance,
respectively, and the CSIR, New Delhi, for research
fellowships to S.G. and S.D.
To overcome this setback, it was decided to use silyl
protection for the C5–OH, instead of an acetate group.
This alternative approach is shown in Scheme 2. Desily-
lation of 13 and then di-TBS protection was followed
by selective primary TBS-deprotection to give the com-
pound 19 in 68% yield. A two-step oxidation of 19 gave
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