Mechanism-specified 5-exo termini differentiation of a C32-desmethyl C28-C34 aplyronine A analog segment

Article abstract of DOI:10.1016/j.tetlet.2015.04.098

A new mild, high yielding oxidative cleavage method allows access to a critical lactone aplyronine A analog precursor. Enhanced termini selectivity is achieved, granting access to a previously unavailable C28-C34 C-32-desmethyl actin binding tail fragment

Full text of DOI:10.1016/j.tetlet.2015.04.098

Tetrahedron Letters 56 (2015) 3868–3871
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Mechanism-specified 5-exo termini differentiation of a
C32-desmethyl C28–C34 aplyronine A analog segment
⇑
Thomas P. Bobinski, Philip L. Fuchs
Department of Chemistry, Purdue University, West Lafayette, IN 47907, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
A new mild, high yielding oxidative cleavage method allows access to a critical lactone aplyronine A
analog precursor. Enhanced termini selectivity is achieved, granting access to a previously unavailable
C28–C34 C-32-desmethyl actin binding tail fragment via vinyl sulfone polypropionate methodology.
Ó 2015 Elsevier Ltd. All rights reserved.
Received 14 April 2015
Accepted 23 April 2015
Available online 29 April 2015
Keywords:
Aplyronine A
Polyketide
Vinyl sulfone
Ozonolysis
Osmium catalysis
a-Hydroxy lactol
Introduction
A requires a concentration of 31 l
M.10–13 Aplyronine A increases
the lifespan of mice between 201% and 566% in 5 different tumor
models (566%: Lewis lung; 545%: P388 leukemia; 398% Ehrlich car-
cinoma; 255%: Colon 26 carcinoma; and 201%: B16 melanoma).9
These results provide convincing evidence that actin activity is
not the sole determinant of antineoplastic activity.
Recent seminal work by Kigoshi and co-workers has provided
definitive mechanistic insight into the antineoplastic properties of
aplyronine A (ApA). These results indicate that the potent cytotox-
icity of ApA is not solely due to its F-actin severing properties.
Photolabelling experiments revealed that the C7 trimethylserine
ester group binds to b-tubulin, leading to the formation of a
1:1:1 tubulin/aplyronine A/actin complex (Fig. 1) that exhibits its
antineoplastic activity at far lower concentrations than that of
the aplyronine A–actin complex. Evidence suggests that the
existence of this ternary complex in the interior of the cell
inhibits tubulin polymerization and enhances microtubule
depolymerization.
Aplyronine A (Fig. 1) is an exceptionally scarce (3 Â 10À7 wt %)
1
macrolide originally isolated from the sea hare Aplysia kurodai. It
2,3
has actin binding and depolymerizing properties,
as well as
potent in vivo antitumor activity.1 The initial inquiry into the
mechanism of aplyronine A’s antineoplastic properties strongly
focused upon its interaction with actin. Actin is the most abundant
protein in the eukaryotic cytoskeleton and is essential for the reg-
ulation of various cellular functions, such as muscle contraction,
cell division, and the migration of tumor cells. Various additional
small agents have been discovered that target actin show cytotox-
1
4
2
4
icity at concentrations above 100 nM. For example, ulapualides,
5
6
7
mycalolides, kabiramides, sphinxolides/reidispongiolides, swin-
holides,7 and bistramides8 are all actin-depolymerizing agents.
Complexes between these macrolides and actin are similar to the
aplyronine A–actin complex.
While aplyronine A’s ability to bind/depolymerize actin is sim-
ilar to other macrolides, its exceptional cytotoxicity is starkly
apparent. For example, jasplakinolide exhibits an IC50 of 100 nM
Results and discussion
9
,7
against HL-60 cells, Mycalolide B has an IC50 of 4.7 nM; but aply-
ronine A has an IC50 of 0.0029 nM after 72 h of incubation.
A primary goal of the Fuchs vinyl sulfone program is to access
biorelevant intermediates containing contiguous chiral carbon
7
Swinholide A depolymerizes actin at 4 nM–1
l
M while aplyronine
1
5
centers. While the vinyl sulfone strategy has successfully enabled
the construction of key intermediates of a number of natural prod-
⇑
Corresponding author. Tel.: +1 765 494 4292.
E-mail addresses: tbobinsk@purdue.edu (T.P. Bobinski), pfuchs@purdue.edu
P.L. Fuchs).
1
5
ucts, the aplyronine A approach has also revealed several limita-
tions. The design for synthesis of the aplyronine A core features
(
http://dx.doi.org/10.1016/j.tetlet.2015.04.098
040-4039/Ó 2015 Elsevier Ltd. All rights reserved.
0

T. P. Bobinski, P. L. Fuchs / Tetrahedron Letters 56 (2015) 3868–3871
3869
Figure 1. Actin–aplyronine A analogs–tubulin ternary complex.
three key polypropionate arrays: C7–C10, C23–C26 and C29–C32.
Successful assembly of the C7–C10 and C23–C26 and assembly of
the C1–C23 macrolactone has been achieved by El-Awa, Noshi,
The new scheme (Scheme 2) returned to 9 the enantiopure
stereodiad precursor of blue fragment 1, thus avoiding preparation
1
9
of 4-ent altogether. Epoxidation of diene 9, with dimethyldioxi-
1
5
Ò
and Hong (Fig. 1).
rane (DMDO) formed in situ from Oxone and acetone provides
1
8,19
Synthesis of enantiopure vinyl sulfones 1, 4, and 4-ent featured
catalytic Jacobsen epoxidation of achiral 2-phenylsulfonyl-1,3-cy-
cloheptadiene, which is prepared from cycloheptanone in a single
operation on the kilogram scale (Scheme 1).16 Transformation of
vinyl sulfones 1 and 4 to lactones 2 and 5 and further on to ter-
enantiopure epoxide diastereomer 10.
Nucleophilic addition
of sodium borohydride effects reduction of 10 to provide alcohol
11 in 82% yield. 2-Azido-2-methylpropanoic acid was prepared
from 2-bromo-2-propionic acid and sodium azide using a simplifi-
2
3
cation of the existing literature protocol. 2-Azido-2-methyl-
propanoic acid forms the acid chloride in situ with oxalyl
chloride, dimethylformamide (DMF), dimethylaminopyridine
(DMAP), and triethylamine (TEA), which suffers reaction with alco-
hol 11 to smoothly afford azapivalate ester 12. Cleavage of the sily-
lether of 12 with camphorsulfonic acid or cerium chloride yields
C31 alcohol 13, thus setting the stage for oxidative cleavage of
the vinylsulfone moiety of C32 desmethyl C28–C34 triad 13.
Ozonolysis of 13 generates the Criegee intermediate, thus test-
ing the competitive reactivity of the C34 acyl sulfone and the C28
oxonium carbon with the C31 hydroxyl oxygen. In the event, the
free hydroxyl, which is equidistant from both functions, suffers
regiospecific addition to the C28 carbonyl oxide (pathway a) rather
than the C34 acylsulfone (pathway b) yielding lactol 14 (Fig. 2). The
1
7,18
mini-differentiated segments 3 and 6 was previously reported.
Unfortunately, base-mediated vinyl sulfone chemistry, established
in several isomeric substrates, was uniformly unsuccessful for the
preparation of allylic sulfone 7 from 4-ent.1
9,20
Faced with a looming program deadline and encouraged by in
silico modeling by Hirst,21 which had previously calculated F-actin
depolymerization ability, predicted pIC50 values of 4.40, 4.08, and
4
.05 for aplyronine A, aplyronine A C32 epi-methyl, and aplyronine
A C32 desmethyl respectively. Authentic aplyronine A has a pIC50
value of 4.51 which correlates well with the calculated value of
2
2
4
.40. Based on these calculations, it was decided that synthesis
of C32 desmethyl analog would provide a more expeditious
approach to the desired target.
Scheme 1. Initial aplyronine A vinyl sulfone polyketide segment strategy.

3
870
T. P. Bobinski, P. L. Fuchs / Tetrahedron Letters 56 (2015) 3868–3871
SO
2
Ph
SO Ph
SO
34
2
Ph
SO
34
2
Ph
SO
34
2
Ph
2
1
0
9
O
O
28
1
0
5
5
28
N3
28
N3
1
1
a
O
11
b
c
d
9
6
6
29 32
29 32
29 32
HO
O
O
OTES
OTES
OTES
OTES
OH
9
10
11
12 13
4
O, 0 °C, 87%; (b) NaBH , MeOH, 82%; (c) 2-azido-2-methylpropanoic acid,
Ò
Scheme 2. Generation of vinylsulfone 13. Reagents and conditions: (a) Oxone , NaHCO
3
, acetone–H
2
(
COCl)
2 3
, DMF, DMAP, TEA, 75%; (d) (1S)-(+)-10-CSA, MeOH 78% or CeCl , MeOH 100%.
O
O
Me S
2
S
HO
O
O
5-exo
O
a
O
O
O
O
3
, MeOH
S
b
O C28-OH
N
3
O
28
34
O
N3
28
33
3
9
4
32
31
NaHCO
O
OH
3
28
30
34
32 33
O
2
0
31
O
N3
O
29
31
3
O
14
OH
1
3
Lactol Methylester
Criegee Zwitterion
Acylsulfone
Figure 2. Ozonolysis of C32 desmethyl cyclic vinylsulfone 13.
Ph
N
O
N
N
N
S
TESO
a series R1 = H
O
O
CH
2
R1
2
C
25
27
O
b series R2 = Me
17
a, b , c
N
3
OMe
O
OMe
34
HO
O
O
O
O
3
4
N
3
2
8
O
N3
28 HO
O
O
OH
31
3
4
O
O
O
3
1
R
3
1
2
8
O
14
16 a, b
d, e
15
2
R CH
2
3
PPh I
18
Scheme 3. Attempted in situ olefination of lactol 14. Reagents and conditions: LiHMDS (1.2 equiv), THF/HMPA (5:1), À78?0 °C, 30 min; bLiHMDS (1.2 equiv), THF/DMF/
a
c
d
e
HMPA (3:9:1), À78?0 °C, 30 min; NaH (50 equiv), THF, À78?0 °C, 30 min; n-BuLi, THF, À42 °C?25 °C, 20 h; DBU, THF, À40 °C?25 °C, 30 min.
O
O
N
3
O
S
OH
HO 28
O
HO 34
28
N
3
28
34
3
3
4
O
O
31
2
9
b
2
9
a
O
1
O
28
O
O
O
31
O 34
N
3
O 29
29
8
3%
N
3
100%
OH
31
OH
O
1
3
O
19
20
transient
acyloin
Scheme 4. Osmylation and oxidative cleavage of stereotetrad 14. Reagents and conditions: (a) N-methylmorpholine-N-oxide (NMO) (2.5 equiv), citric acid (3.0 equiv),
OsO (0.10 equiv), MeCN/H O (v/v 4:1), rt, 24 h; (b) Pb(OAc) (2.5 equiv), MeOH, rt, 5 min.
K
2
4
2
4
complete recovery of trans–anti-trans lactol 14 suggests this sub-
to the C34 ketone. Finally, Criegee oxidation of lactol 19 smoothly
affords target containing C34 lactone terminus C28 lactone-alde-
hyde 20, thereby achieving regiospecific termini-differentiation
unattainable by ozonolysis (Scheme 4).
strate is incapable of providing significant equilibrium populations
of aldehyde 15 to furnish adduct 16 from nucleophiles 17,2
4,25
and
1
8 under the olefination conditions of Scheme 3.
Since the ozonolysis method was incapable of generating a
reactive termini-differentiated lactol, an alternative oxidative
cleavage was developed. The vinyl sulfone polypropionate strategy
has recently been augmented by a new mild, citric acid assisted
catalytic dihydroxylation reaction.26 In Scheme 4 the transient acy-
loin formed from the dihydroxylation of vinyl sulfone 13 forms
Conclusion
This investigation solves an initial limitation of the vinyl sulfone
polyketide strategy. The scope of the polypropionate is also
increased. A previously unattainable positioning of the polyketide
bridged bicyclic a-hydroxy lactol 19 via addition of the C31 alcohol

T. P. Bobinski, P. L. Fuchs / Tetrahedron Letters 56 (2015) 3868–3871
3871
on the carbon backbone furnishes a novel isomer. A mechanism-
specified 5-exo termini-differentiation oxidative cleavage of the
cyclic vinyl sulfone intermediate 13 has been achieved. The com-
plementarity of ozonolysis and osmylation has been elucidated.
Access to the C32 desmethyl, actin-binding tail, lactone precursor
5. Suenaga, K.; Kimura, T.; Kuroda, T.; Matsui, K.; Miya, S.; Kuribayashi, S.;
Sakakura, A.; Kigoshi, H. J. Am. Chem. Soc. 1999, 121, 5605–5606.
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Paterson, I.; Ashton, K.; Britton, R.; Cecere, G.; Chouraqui, G.; Florence, G. J.;
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Kobayashi, M.; Kawazoe, K.; Okamoto, T.; Sasaki, T. Chem. Pharm. Bull. 1994, 42,
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(20) of the of the aplyronine A analog has been established.
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1
1
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We thank Arlene Rothwell and Dr. Karl Wood from Purdue
University for MS data and Purdue University for support of this
research.
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Supplementary data
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Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2015.04.
16. Park, T.; Torres, E.; Fuchs, P. L. Synthesis 2004, 1895.
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2
2
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