Synthesis of vinyliodides: Progress toward the total synthesis of a jatrophane diterpene

Article abstract of DOI:10.1016/j.tetlet.2013.05.024

A noticeable effort toward the total synthesis of a jatrophane diterpene has been made via the synthesis of various vinyliodides utilizing highly stereoselective Tanino-Miyashita olefination reaction; and utilizing the dithiane moiety as an acyl anion equ

Full text of DOI:10.1016/j.tetlet.2013.05.024

Tetrahedron Letters 54 (2013) 3919–3925
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Tetrahedron Letters
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Synthesis of vinyliodides: progress toward the total synthesis
of a jatrophane diterpene
⇑
Priya Mohan , Michael J. Fuertes
Department of Chemistry & Biochemistry, Texas Tech University, Lubbock, TX 79409, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
A noticeable effort toward the total synthesis of a jatrophane diterpene has been made via the synthesis
of various vinyliodides utilizing highly stereoselective Tanino–Miyashita olefination reaction; and utiliz-
ing the dithiane moiety as an acyl anion equivalent.
Received 8 March 2013
Revised 5 May 2013
Accepted 7 May 2013
Available online 16 May 2013
Ó 2013 Elsevier Ltd. All rights reserved.
Keywords:
Vinyl dibromide
Vinyl iodide
Wolkoff’s reagent
Tanino–Miyashita olefination
Parikh–Doering oxidation
Introduction
analogues, synthesis of compound 4 was targeted. C11–C12 double
bond is cleaved reterosynthetically using RCM where as C6–C7
The Euphorbia plant species contain the jatrophane family of
compounds, some of which inhibit permeability glycoprotein
(Pgp) effectively. Pgp is one of the major reasons for the develop-
ment of multi-drug resistance in malignant tumors and this often
times is responsible for the failure of chemotherapy. Hence over
the past two decades plants of this species have been widely inves-
tigated.1 Jatrophane diterpenes were isolated in 1970 by Kupchan
and co-workers from Euphorbia gossypiifolia2 and have shown po-
tential as Pgp inhibitor. Most of the jatrophane diterpenes feature
a characteristic bicyclic[10.3.0]pentadecane framework. The pres-
ence of C11–C12 double bond, C6–C17 exo-double bond along with
9-stereogenic centers explains the complexity of these molecules.
bond can be synthesized from vinyl iodide 6 and aldehyde 7 utiliz-
ing the Nozaki–Hiyama–Kishi coupling (Scheme 1).6
For the synthesis of bis-MOM vinyl iodide 6 we thought we
could utilize hydrozirconation,7 or hydrostannylation8 and hydro-
palladation on bis-MOM methylalkyne 16.
There were two possible routes for the synthesis of bis-MOM
methylalkyne: (1) installing the alkyne at an early stage, or (2)
installing the alkyne at a later stage. The synthetic challenges we
found when installing the alkyne at early stage will now be dis-
cussed (Scheme 2).
This synthetic scheme commenced with TBAF-mediated desily-
lation of 8 to provide mono-MOM primary alcohol 9, after which
Swern oxidation was employed to obtain aldehyde 10. Corey–
Fuchs dibromo olefination9 was employed on aldehyde to generate
vinyl dibromide 11 in a very moderate yield. This vinyl dibromide
was converted into alkyne 12 and later into internal alkyne 13
applying the usual protocol. To access bis-MOM vinyl iodide 6, oxi-
dative cleavage of double bond was performed on synthon 13 to
yield aldehyde 14 in 35% yield. A homoallylic alcohol 15 was gen-
erated using allylmagnesium bromide to afford 15 in 81% yield,
which was subsequently protected as MOM ether to furnish 16.
However, the synthesis of vinyl dibromide 11 and oxidative cleav-
age of olefin 13 remained a point of concern due to the very poor
yields of each step. Before moving forward, we opted to optimize
these steps.
Results and discussion
In our previous report,3 we adopted a retrosynthetic analysis
which involved the formation of C11–C12 and C5–C6 double bonds
using ring-closing metathesis4 (RCM) and Horner–Emmons–Wads-
worth olefination5 (HEW). However attempted HEW olefination
under various conditions went futile which led us to come up with
a new retrosynthetic pathway. We assume that a bulky base-pro-
moted hydrogen-abstraction can lead to a regioselective opening
of an epoxide to furnish an exo-double bond. In search of a capable
synthetic pathway to access jatrophane skeleton and its various
The synthesis of alkyne 12 from aldehyde 10 was one of the
hurdles in this synthetic strategy (Scheme 3). We tried to optimize
⇑
Corresponding author. Tel.: +1 806 588 0021; fax: +1 806 687 0998.
E-mail address: pmohan.eburonorganics@gmail.com (P. Mohan).
0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2013.05.024

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P. Mohan, M. J. Fuertes / Tetrahedron Letters 54 (2013) 3919–3925
O
O
O
AcO
H
AcO
H
AcO
Epoxide
formation
Base
H
ONic
ONic
ONic
AcO
AcO
AcO
O
AcO
AcO
R2
AcO
R2
R2
AcO
2
3
1
OMOM
MOMO
HO
HO
I
12
MOMO
MOMO
11
metal-halide
exchange or,
NHK-coupling
RCM
6
6
7
+
O
O
4
O
H
5
7
Scheme 1. Modified retrosynthetic analysis for modal studies of jatrophane diterpenes.
MOMO
MOMO
TBAF
(COCl)2, DMSO, Et3N,
CH2Cl2, -78 oC, 82%
OTBDPS
OH
THF, rt, 88%
9
8
MOMO
MOMO
MOMO
n-BuLi
THF, -78 oC, 71%
CBr4, PPh3
CH2Cl2, 0 oC, 30%
Br
O
Br
12
10
11
O
MOMO
MOMO
OsO4, 2,6-lutidine
NaIO4
n-BuLi, MeI
THF, -78 oC, 90%
dioxane:H2O
(5:1), 35%
14
13
MOMO
MOMO
HO
MOMCl, DIPEA
TBAI
MOMO
MgBr
THF, 80 oC, 86%
THF, -78 oC, 81%
16
15
Scheme 2. Synthesis of alkyne 16.
the reaction under various conditions (Table 1). Initially, we tried a
two-step Corey–Fuchs protocol with overall yield of 21%. This
forced us to look for more efficient precedents. To this end, we
tried trimethylsilyldiazomethane10 and the Ohira reagent.11 Unfor-
tunately, both the Ohira reagent and TMS diazomethane gave poor
yield of the alkyne.
MOMO
MOMO
Conditions
O
10
12
O
O
P
Table 1
(OMe)2
CBr4/ PPh3
Si
N2
Different reagents attempted for alkyne synthesis
N2
Entry
Reagent
Temperature (°C)
Yield (alkyne) (%)
1
2
3
Ohira reagent
TMS diazomethane
Corey–Fuchs
rt
28
20
21
Corey-Fuchs
Ohira reagent
TMS diazomethane
À78
À78
Scheme 3. Conditions for alkyne synthesis from aldehyde.

P. Mohan, M. J. Fuertes / Tetrahedron Letters 54 (2013) 3919–3925
3921
H
CH2Cl2, H2O
CBr4
PPh3
P
Br
+
Br
CH3CN, 85%
17
MOMO
MOMO
Br
K2CO3, 17
O
THF, 0 oC, 80%
Br
10
11
Scheme 4. Synthesis of Wolkoff’s reagent and its application in vinyl dibromide synthesis.
MOMO
OMOM
Wolkoff's reagent
KOtBu
MOMO
MOMO
n-BuLi
THF, -78 o
88%
C
Br
THF, 0 oC, 80 o
74%
C
O
Br
19
18
MOMO
MOMO
MOMO
MOMO
n-BuLi, MeI
THF, -78 o
85%
C
16
20
Scheme 5. Synthetic scheme to install the alkyne in a later stage.
11 from aldehyde 10 (Scheme 4).12 Upon treatment of 11 with n-
BuLi (2.0 equiv) terminal alkyne 12 could be obtained in high yield.
Methylation of this terminal alkyne was achieved by treatment
with base following methyl iodide (Scheme 2).
Table 2
Attempted hydrozirconation in different reaction conditions
Solvent
Temperature (°C)
Time (h)
Results
The oxidative cleavage in the presence of an alkyne was low
yielding and we could not do anything different other than altering
the equivalents of OsO4 and NaIO4 with minimal significance on
the yield of the reaction. Therefore, we decided to install the alkyne
Benzene
Benzene
THF
rt
50
rt
24
36
36
No reaction
No reaction
No reaction
The Corey–Fuchs protocol consists of two steps: the first step is
vinyl dibromide synthesis and the second step is alkyne synthesis.
We were observing poor yield (30%) in the first step. So we looked
for another reagent that would achieve the synthesis of a vinyl
dibromide.
MOMO
MOMO
Cp2Zr(H)Cl, I2
I
x
THF, 60 o
C
Ultimately, the use of dibromomethyl triphenylphosphonium
bromide 17, which can be prepared according to the procedure
optimized by Wolkoff, resulted in an excellent yield of dibromide
13
21
Scheme 7. Efforts toward synthesis of vinyl iodide.
MOMO
MOMO
MOMO
MOMO
Cp2ZrHCl
THF, 60 oC, I2
x
I
16
6
Scheme 6. Efforts toward the synthesis of vinyl iodide 6.

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P. Mohan, M. J. Fuertes / Tetrahedron Letters 54 (2013) 3919–3925
MOMO
MOMO
(Scheme 6), hence we attempted to apply this on a relatively less
hindered alkyne 13 and were unsuccessful in obtaining iodide 22
(Scheme 7).
MOMO
MOMO
Bu3SnH / CuCN/I2
MeOH, -78 oC
After trying various attempts of hydrozirconation we turned
toward some other protocols. Hydrostannylation of an internal al-
kyne affords vinyl stannane which can be treated with iodine crys-
tals to yield vinyl iodide. The stannylcupration of alkyne 16 was
conducted using 2.0 equiv of the cyanocuprate Bu3Sn(Bu)CuCNLi
(Scheme 8). Unfortunately, most times we got a mixture of com-
pounds with traces of desired product 6 which was hard to purify
from an unknown impurity by chromatography.
Finally, synthesis of vinyl iodide 6 was accomplished by
Tanino–Miyashita’s approach with excellent Z-stereoselectivity
(Scheme 9).13 Although our target was to synthesize E-vinyl iodide,
but the fact that this tri-substituted double bond will eventually be
converted into epoxide to yield exo-double bond under the influ-
ence of a bulky base makes things upright.
After synthesizing vinyl iodide 6 we wanted to validate our
strategy by testing the NHK coupling reaction with aldehyde 26,
which is structurally similar to 7. The synthesis of aldehyde 26
was accomplished by sequential hydrogenation of enolate ester
22, LiAlH4 reduction, and Swern oxidation (Scheme 10).
Although, our attempt to combine aldehyde 25 and vinyl iodide
6 using the NHK coupling reaction under various conditions failed
(Scheme 11).
I
16
6
Scheme 8. Efforts toward synthesis of vinyl iodide 6.
MOMO
MOMO
CuI / MeLi
Br
I2, -78 o
C
I
Br
11
21
MOMO
MOMO
MOMO
MOMO
CuI / MeLi
I2, -78 o
Z:E (23:1)
Br
C
Br
I
19
6
Scheme 9. Efforts toward the synthesis of vinyl iodide.
at a later stage by pursuing the synthesis of alkyne from aldehyde
18 instead of aldehyde 10 (Scheme 5).
As a last effort to combine vinyl iodide 21 and aldehyde 25, we
attempted lithiation of 21 by treatment with two equivalents of n-
BuLi. Lithiated 21 was then treated with aldehyde 25 to furnish
coupled product 27 as inseparable 1:1 mixture of diastereomers
in 49% yield. Alcohol 27 was then protected as a silylether using
TBDPSCl (Scheme 12).
However, we did not pursue this compound any further judging
the difficulty of a regioselective oxidative cleavage of double bond.
The success of this reaction indicates the possibility of coupling
two fragments with less sterically hindered vinyl iodide. It is note-
worthy here that this reaction could not work with bis-MOM vinyl
iodide 6.
Vinyl dibromide 19 was obtained in 74% yield by treating alde-
hyde 18 with Wolkoff’s reagent and potassium tert-butoxide. Vinyl
dibromide 19 was converted to alkyne 20 in 88% yield, which was
subsequently treated with n-BuLi and methyl iodide to yield an
internal alkyne 21.
Initially, we attempted hydrozirconation by Schwartz’s reagent
under various conditions on bis-MOM alkyne 16 as a means to syn-
thesize vinyl iodide 6 (Table 2).
Repeated failures of hydrozirconation–iodination were
indicated toward steric congestion of bis-MOM methyl alkyne 16
O
O
LiAlH4
H2, Pd/C
EtOAC, 60 o
6h, 96%
C
THF, rt, 3h, 98%
EtO
OTBS
EtO
OTBS
22
23
O
(COCl)2, DMSO, Et3N,
CH2Cl2, -78 oC, 81%
HO
OTBS
H
OTBS
24
25
Scheme 10. Synthesis of aldehyde 25.
MOMO
MOMO
OMOM
OTBS
O
MOMO
NiCl2, CrCl2
x
H
OTBS
+
DMF
I
25
6
HO
26
Scheme 11. Attempted NHK-coupling reaction.

P. Mohan, M. J. Fuertes / Tetrahedron Letters 54 (2013) 3919–3925
3923
MOMO
MOMO
n-BuLi, 25
THF, -78 o
49%
C
I
HO
21
27
OTBS
MOMO
TBDPSCl, Et3N, DMAP
CH2Cl2, rt, 71%
TBDPSO
28
OTBS
Scheme 12. Reaction between vinyl iodide 22 and aldehyde 26.
Our inability to conduct the regioselective oxidative cleavage on
28 led us to find an acyl anion equivalent to serve the purpose. 1,3-
Dithiane14 was supposed to serve two purposes for us, first to
introduce an acyl anion equivalent, second to give high diastere-
oselectivity. In addition to this, we can also utilize umplong chem-
istry for the allyl addition as well. A modified retrosynthetic
scheme was developed which can give us the flexibility of allyla-
tion on 29 at a later stage to furnish the triene 5. Compound 29
might be synthesized using NHK coupling reaction or, metal-halo-
gen exchange reaction (Scheme 13).
Even a mild oxidation method such as TPAP/NMO15 was not
suitable as we were observing some elimination product with only
30% of the desired aldehyde 35.
Freshly prepared Dess–Martin periodinane16 was utilized as it
was backed by ample precedence showing its utility for the oxida-
tion of alcohols in thio-compounds, though most of our attempts
could afford only 45% yield of aldehyde 35. The best optimization
of this step was achieved using the Parikh–Doering oxidation17 uti-
lizing reagents (SO3ÁPy, DMSO) which could afford dithiane alde-
hyde in 60% yield (Table 3).
For the synthesis of vinyl iodide 30 cyclopentanone TBDPS ether
31 was treated with the anion of 1,3-dithiane to produce dithiane
alcohol 32 stereoselectively supported by NOE experiment. After
the protection of tertiary alcohol 32 as the MOM ether, the TBDPS
ether was cleaved with TBAF to generate primary alcohol 34. Oxi-
dation of primary alcohol 34 was capricious and inconsistent as we
had sulfur in our substrate (Scheme 14).
After having reproducible access to dithiane aldehyde 35 we
wanted to move forward toward the synthesis of dithiane vinyl io-
dide 30. Two synthetic pathways were possible to reach our syn-
thetic destination. Although our initial attempts of installing the
vinyl iodide group utilizing Swartz’s reagent were not successful
with compound 16, we tried this methodology on dithiane meth-
ylalkyne 38 which was synthesized from dithiane alkyne 37.
HO
HO
MOMO
MOMO
12
RCM
allylation
11
5
6
4
O
O
5
S
S
MOMO
I
S
S
NHK coupling
MOMO
O
30
metal-halogen
exchange
+
O
29
H
7
Scheme 13. Modified retrosynthetic scheme utilizing dithiane chemistry.

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P. Mohan, M. J. Fuertes / Tetrahedron Letters 54 (2013) 3919–3925
O
S
S
S
S
HO
MOMCl, DIPEA, TBAI
THF, 80 oC, 75%
MOMO
1,3-dithiane, n-BuLi
THF, -78 oC, 77%
OTBDPS
OTBDPS
OTBDPS
31
32
33
TBAF
THF, rt, 98%
S
MOMO
S
OH
34
Scheme 14. Application of dithiane as an acyl anion equivalent.
Following another pathway we converted aldehyde 35 into dib-
romoalkene 36 applying Wolkoff’s reagent. Me2CuLi mediated vi-
nyl iodide synthesis under Tanino–Miyashita olefination protocol
was employed on 1,1-dibromoalkene 36 and to our great relief,
we finally synthesized dithiane vinyl iodide 30 albeit in modest
yield. However, in the key, NHK coupling reaction with aldehyde
7, all attempts to deliver the coupling product 40 were unsuccess-
ful. Even attempts of metal-halide exchange of vinyl iodide 30 fol-
lowed by aldehyde addition were also unsuccessful and this led to
an end to any further synthetic adventures utilizing dithiane
chemistry (Scheme 16).
Two significant retrosynthetic pathways were pursued. Bis-
MOM vinyl iodide was synthesized and was tried for coupling reac-
tion under NHK reaction condition to yield an advanced organic
synthon toward the synthetic target. However, the failure led us
to try similar set of reaction on less congested mono-MOM vinyl
iodide. The product obtained was not pursued any further. In the
end, we introduced dithiane to serve our purpose of providing an
acyl anion equivalent. Various new compounds have emerged dur-
ing synthesis. Various vinyl iodide analogues were prepared from
Table 3
Attempted oxidation of 34 under different reaction conditions
S
S
oxidation
MOMO
MOMO
S
S
OH
O
35
34
Oxidation
Time (h)
Results (%)
Swern
PDC
TPAP/NMO
DMP
SO3ÁPy, DMSO
2
8
3
3
2
Decomposition
20
30
45
60
Unfortunately, the hydrozirconation–iodination protocol on this
substance proved to be unfruitful as most times we recovered
the starting material (Scheme 15).
S
S
S
Wolkoff's reagent
KOt Bu
S
S
MOMO
CuI / MeLi;
MOMO
MOMO
S
I2, -78 oC, 50%
E:Z (4:1)
Br
THF, 0 oC, 71%
O
I
Br
30
35
36
X
Cp2Zr(H)Cl
I2, THF
n-BuLi
THF, -78 oC,
81%
S
S
S
S
MOMO
MOMO
n-BuLi, MeI
THF, -78 o
82%
C
38
37
Scheme 15. Application of dithiane as an acyl anion equivalent.

P. Mohan, M. J. Fuertes / Tetrahedron Letters 54 (2013) 3919–3925
3925
S
S
NiCl2, CrCl2
MOMO
O
S
DMSO, reflux
S
or,
MOMO
+
x
H
t-BuLi, THF, -78 o
C
HO
I
7
30
39
Scheme 16. Attempts to couple vinyl iodide 30 and aldehyde 7.
5. (a) Wang, Y.; West, F. G. Synthesis 2002, 99–103; (b) Schmidt, U.; Langner, J.;
Kirschbaum, B.; Brau, C. Synthesis 1994, 1138.
6. (a) Takai, K.; Tagashira, M.; Kuroda, T.; Oshima, K.; Utimoto, K.; Nozaki, H. J. Am.
Chem. Soc. 1986, 108, 6048; (b) Takai, K.; Kimura, K.; Kuroda, T.; Hiyama, T.;
Nozaki, H. Tetrahedron Lett. 1983, 24, 5281; (c) Lee, J. Y.; Miller, J. J.; Hamilton, S.
S.; Sigman, M. S. Org. Lett. 2005, 7, 1837–1839.
vinyl dibromide. However, the failure of NHK coupling to couple
the vinyl iodide with aldehyde led to end this synthetic pursuit.
Acknowledgment
7. (a) Hart, D. W.; Schwartz, J. J. Am. Chem. Soc. 1974, 96, 8115; (b) Panek, J. S.; Hu,
T. J. Org. Chem. 1997, 62, 4912.
8. Betzer, J. F.; Delaloge, F.; Muller, B.; Pancrazi, A.; Prunet, J. J. Org. Chem. 1997, 62,
7768.
We would like to thank the Texas Tech University for providing
financial support to this research.
9. (a) Corey, E. J.; Fuchs, P. L. Tetrahedron Lett. 1972, 36, 3769; (b) Gibtner, T.;
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Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2013.
05.024.
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