First total synthesis and structure confirmation of diacetylenic polyol (+)-oploxyne B

Article abstract of DOI:10.1016/j.tetlet.2013.08.020

The first total synthesis of the natural product (+)-oploxyne B is achieved. The synthesis has led to the confirmation of absolute stereochemistry of the natural product. The natural product displayed cytotoxic activity with IC₅₀ values varying

Full text of DOI:10.1016/j.tetlet.2013.08.020

Tetrahedron Letters 54 (2013) 5616–5618
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First total synthesis and structure confirmation of diacetylenic polyol
(+)-oploxyne B
a
a
b
b,
P. Srihari a, , A. Sathish Reddy , J. S. Yadav , D. Yedlapudi , Shasi V. Kalivendi
⇑
⇑
a Division of Natural Products Chemistry, CSIR-Indian Institute of Chemical Technology, Hyderabad 500007, India
b Chemical Biology, CSIR-Indian Institute of Chemical Technology, Hyderabad 500007, India
a r t i c l e i n f o
a b s t r a c t
Article history:
The first total synthesis of the natural product (+)-oploxyne B is achieved. The synthesis has led to the
confirmation of absolute stereochemistry of the natural product. The natural product displayed cytotoxic
activity with IC50 values varying from 16 to 53 lM in four cancer cell lines tested.
Received 19 June 2013
Revised 29 July 2013
Accepted 1 August 2013
Available online 12 August 2013
Ó 2013 Elsevier Ltd. All rights reserved.
Keywords:
Cadiot–Chodkiewicz coupling
Bromo acetylene
Polyacetylene
Ohira–Bestmann
Acetonide
Recently synthetic organic chemists have become interested in
the total synthesis of polyacetylenic alcohols because of their wide
spectrum of structural diversity and broad array of biological prop-
erties.1 The polyacetylene alcohols are isolated from natural
sources and include a wide variety of plant species, cultures of fun-
gi, marine sponges, corals, etc.2 Yang et al. have recently isolated
two new diynes namely oploxyne A and oploxyne B (Fig. 1) during
their investigations on the inhibitors for the formation of NO and
prostaglandin E2 (PGE2) from the CH2Cl2 extract of the stem of
Oplopanax elatus.3 Though, initially the structures were established
based on NMR spectroscopy through chiral derivatization, the total
synthesis4 of originally proposed structures for oploxyne A and
oploxyne B from our group had reconfirmed the absolute structure
of oploxyne A while revising the structure of natural oploxyne B as
its enantiomer based on the outcome of the analytical data and
optical rotation of the synthetic product. It was also found that
the synthetic compound (ꢀ)-oploxyne B was found to be effective
of natural oploxyne
B
starting from the readily available
D
(ꢀ)-diethyl -tartrate and (ꢀ)-ribose in a convergent fashion.
D
Retrosynthetically, we envisaged the target compound to be
synthesized based on a convergent approach utilizing two key
intermediates, terminal alkyne 4 and the brominated alkyne 5.
Accordingly, these two fragments could be coupled under
Cadiot–Chodkiewicz conditions and subjected to deprotection of
PMB and acetonide moieties to get the desired target molecule.
The key fragment 4 can be in turn synthesized from olefin 6 by oxi-
dative degradation followed by one step alkyne formation with
Ohira–Bestmann reagent. Compound 6 can be obtained from epox-
ide 7 through a ring opening reaction of epoxide 7 by a Grignard
reaction followed by protection of the generated secondary alcohol
as the corresponding methyl ether. Epoxide 7 in turn was easily
accessible from commercially available D-ribose. Simultaneously,
against neuroblastoma cell line with an IC50 value of 12
M for reference drug doxorubicin whereas in other cell lines like
lung, prostate and breast cancer the IC50 values were found 25, 24
ad 28 M. The reference doxorubicin showed IC50 values of 11.5,
11 and 12 M. These results have prompted us to take up the total
lM against
OH
OMe OH
9
l
5
O
5
OH
l
OH
(-)-Oploxyne B 2
(Proposed structure)
(+)-Oploxyne A 1
OH
l
synthesis of natural oploxyne B to enable further screening. In con-
tinuation of our programme on the total synthesis of biologically
active natural products,5 we herein report the first total synthesis
OMe OH
5
OH
(+)-Oploxyne B 3
(Revised structure)
OH
⇑
Corresponding authors. Tel.: +91 4027191815; fax: +91 4027160512.
E-mail address: srihari@iict.res.in (P. Srihari).
Figure 1. Structures of (+)-oploxyne A, (ꢀ)-oploxyne B and (+)-oploxyne B.
0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2013.08.020

P. Srihari et al. / Tetrahedron Letters 54 (2013) 5616–5618
5617
OMe OH
5
OH
3
(+)-OPLOXYNE-B
OH
OMe
OMe
Br
+
5
O
5
O
OPMB
OPMB
O
O
8
6
7
5
4
O
O
OH
D-Diethyl tartrate
D-Ribose
O
9
O
O
Scheme 1. Retrosynthesis for (+)-oploxyne B 3.
Scheme 2. Synthesis of key intermediate alkyne 4.
the bromo alkyne 5 can be synthesized from the olefin 8 through
oxidative degradation, followed by conversion of aldehyde to mon-
osubstituted acetylene with Ohira–Bestmann reagent and further
treatment with NBS in the presence of AgNO3. Olefin 8 in turn
IBX, THF:DMSO (1:1)
O
rt, 2 h
Ref 8
OBn
(-)-Diethyl D-tartrate
HO
CH3P(Ph)3Br,
NaHMDS, THF
-10 °C- rt, 3 h
3 steps
O
63% overal yield
was obtained from commercially available (ꢀ)-diethyl
D-tartrate
70% (over two steps)
(Scheme 1).
O
O
Pd(OH)2
I2, imidazole, PPh3
C6H6, 1.5 h, 70%
OBn
OH
Thus, the synthesis began with D(ꢀ)-ribose which was con-
verted to alcohol 10 in three steps (acetonide protection, TBDPS
protection, followed by one carbon Wittig reaction). The free sec-
ondary alcohol in 10 was mesylated with MsCl in the presence of
N-methyl imidazole. TBDPS deprotection with TBAF followed by
DBU treatment afforded epoxide 7.6 Compound 7 on ring opening
reaction under Grignard conditions (epoxide opening with in situ
generated n-hexylmagnesium bromide) afforded chiral secondary
alcohol 11, which was further protected as the corresponding
methyl ether 6 since this motif (–OMe) was present in the natural
product. The terminal alkene was converted to alkyne through oxi-
dative degradation, that is, ozonolysis followed by exposure of the
resulting aldehyde 12 to Ohira–Bestmann reagent7 to afford the
key intermediate alkyne 4 (Scheme 2).
H2 , ethyl acetate
24 h, rt, 90%
O
O
9
14
O3, PPh3, CH2Cl2
-78 °C, 5 min.
O
1) nBuLi, THF, -78 °C, 3 h
I
2) NaH, BrCH2C6H4OCH3
THF, 30 h, 0 °C-rt,
70% (over two steps)
X, MeOH, K2CO3, 14 h,
0 °C-rt, 70%
O
OPMB
15
8
Br
NBS, AgNO3
acetone, 1 h,
O
O
P
OMe
OMe
X =
OPMB
OPMB
16
5
0 °C, 80%
N2
Scheme 3. Synthesis of bromo alkyne 5.
OMe
OMeO
CuCl, nBuNH2
NH2OH.HCl
Et2O, 0 oC - rt,
30 min, 90%
The other key fragment bromo alkyne 5 was synthesized start-
Br
+
ing from (ꢀ)-diethyl
D-tartrate (see Scheme 3). Commercially avail-
able (ꢀ)-diethyl -tartrate was converted to alcohol 13 by
5
5
O
O
O
OPMB
D
17
5
4
following known procedures in three steps (acetonide protection,
LiAlH4 mediated diester reduction and selective mono benzyla-
tion).8 The free alcohol 13 was oxidized with 2-iodoxybenzoic acid
(IBX)9 to afford the aldehyde which was immediately treated with
methyltriphenylphosphorane that was generated from meth-
yltriphenylphosphoniumbromide and NaHMDS to get olefin 14.
One pot debenzylation and hydrogenation of the double bond
was achieved with Pd(OH)2 to yield the saturated alcohol 9 which
upon treatment with I2, triphenylphosphin10 and imidazole affor-
ded the corresponding iodide 15. Our initial efforts to treat iodide
15 with excess base to get the chiral propargyl alcohol did not suc-
ceed and prompted us to proceed via chiral allyl alcohol. Towards
this, iodide 15 was treated with n-BuLi to undergo elimination
affording chiral allyl alcohol,11 which was immediately protected
as the corresponding PMB ether 8 with PMBBr and NaH. The olefin
was subjected to ozonolysis and the resulting aldehyde was con-
verted to alkyne 16 following Ohira–Bestmann procedure and
was then brominated with NBS the in presence of AgNO3 to afford
the key intermediate brominated alkyne 5.12
OPMB
OMe OH
TFA, CH2Cl2
0 oC - rt, 30 h, 70%
5
OH
(+)-Oploxyne B 3
OH
Scheme 4. Synthesis of natural product (+)-oploxyne B 3.
rable with the natural product. The optical rotation of the final
product was found to be
+12.5 (c 0.04, MeOH); lit.2
+11.7 (c 0.06, MeOH).
The natural product along with the intermediate compound 4
½ ꢁ
a 2D5
½ ꢁ
a 2D5
and its acetonide deprotected product 4a (from Scheme 2) when
evaluated for their in vitro cytotoxicity employing MTT assay15 in
four different human cancer cell lines displayed IC50 values vary-
ing from 16 to 53
(+)-oploxyne B displayed lowest IC50 of 16
line (A549) and 25 M, 31 M and 54 l
l
M (Table 1). Although the natural product
M in lung cancer cell
M in neuroblastoma
l
l
l
(SK-N-SH), prostate cancer (DU145) and breast cancer (MCF-7)
respectively, the intermediate compound 4 and its acetonide
deprotected product 4a did not show any remarkable cytotoxicity.
In conclusion, a facile convergent strategy for the total synthesis
With the two intermediates in hand, the stage was set to couple
them and proceed further for the total synthesis. The key reaction
Cadiot–Chodkiewicz coupling13 of alkyne 4 with brominated al-
kyne 5 resulted in the formation of masked oploxyne B 17
(Scheme 4). Finally, deprotection of isopropylidene and PMB moi-
ety in 17 with TFA afforded the desired target compound oploxyne
B. The spectroscopic data14 was analysed and found to be compa-
of oploxyne B starting from commercially available
(ꢀ)-diethyl -tartrate has been achieved. The natural product
was synthesized with an overall yield of 16% in 12 steps starting
D-ribose and
D

5618
P. Srihari et al. / Tetrahedron Letters 54 (2013) 5616–5618
Table 1
In vitro antiproliferative activity of the compounds 3, 4 and 4aa
IC50 values in
lM
A549 (lung cancer)
MCF-7 (breast cancer)
27
SK-N-SH (neuroblastoma)
DU-145 (prostrate cancer)
86
Oploxyne Ab
NT
1
7
1
2
(+)-Oploxyne B 3
4
4a
16.29 4.67
96.55 12.35
No effect
6.13 0.54
53.78 6.24
>100
No effect
15.12 2.96
25.0 4.39
99.8 21.7
No effect
31.34 9.39
>100
No effect
14.19 0.84
Doxorubicin (Reference)
7.22 1.16
a
Cell lines were treated with different concentrations of compounds (1
lM, 10 lM, 25 lM and 50 lM) for 48 h. (see Supplementary data). Cell viability was measured
employing MTT assay. IC50 values are indicated as the mean SD of three independent measurements.
b
The values for oploxyne A are from the previous results wherein the reference doxorubicin showed IC50 of 11.5, 12, 9 and 11.5
respectively.4
lM for MCF-7, SK-N-SH and DU-145
7. (a) Ohira, S. Synth. Commun. 1989, 19, 561–564; (b) Muller, S.; Liepold, B.; Roth,
G. J.; Bestmann, H. J. Synlett 1996, 521–522.
8. (a) Feit, P. W. J. Med. Chem. 1964, 7, 14–17; (b) Toda, F.; Tanaka, K. Tetrahedron.
Lett. 1988, 29, 551–554; (c) Roulland, E. Angew. Chem., Int. Ed. 2008, 47, 3762–
3765; (d) Takahashi, S.; Ogawa, N.; Koshino, H.; Nakata, T. Org. Lett. 2005, 7,
2783–2786.
9. Frigerio, M.; Santagostino, M.; Sputore, S. J. Org. Chem. 1999, 64, 4537–4538.
10. Garegg, J.; Samuelsson, B. J. Chem. Soc. Perkin I 1980, 2866–2869.
11. The allyl alcohol can also be prepared from iodide through Boord reaction
conditions wherein Zinc is used to get the reductive elimination Schneider, C.;
Kazmaier, U. Synthesis 1998, 1314–1320.
from D-ribose using Cadiot–Chodkiewicz coupling reaction as the
key step. Also, we have confirmed the true structure of the natural
product correcting the initial mis-assignment and this result is also
in favour towards supporting the revised structure. The cytotoxic
properties exhibited by the natural product (+)-oploxyne B were
similar to those of (ꢀ)-oploxyne B and warrants detailed structure
function studies for improved efficacy.
Acknowledgments
12. The procedure for bromination was followed from Yun, H.; Danishefsky, S. J. J.
Org. Chem. 2003, 68, 4519–4522.
13. Cadiot, P.; Chodkiewicz, W. In Chemistry of Acetylenes; Viehe, H. G., Ed.; Marcel
Dekker: New York, 1969; pp 597–647.
P.S.H. and S.V.K. thank CSIR, New Delhi for financial support as
part of XII Five Year plan programme under title ORIGIN (CSC-
0108) and SMiLE (CSC-111). A.S.R. thanks CSIR, New Delhi for
financial assistance in the form of fellowship.
14. Spectroscopic data for selected products 4, colorless oil. ½a D25
ꢀ48.00 (c 3.0,
ꢁ
CHCl3). Rf = 0.7 (10% EtOAc–Hexane). IR [neat]: 3309, 2985, 2929, 2856, 1735,
1461, 1340, 1106, 865, 497 cmꢀ1 1H NMR (300 MHz, CDCl3): d 4.65 (dd, J = 2.1,
.
5.2 Hz, 1H), 4.03 (dd, J = 5.3, 8.5 Hz, 1H), 3.51 (s, 3H), 3.53–3.50 (m, 1H), 2.51
(d, J = 2.1 Hz, 1H), 1.58 (s, 3H), 1.50–1.40 (m, 2H), 1.37 (s, 3H), 1.35–1.22 (m,
13
10H), 0.88 (t, J = 6.9 Hz, 3H) ppm.
C NMR (75 MHz, CDCl3): d 110.7, 81.2,
Supplementary data
80.6, 80.2, 75.2, 61.2, 58.6, 31.7, 30.9, 29.5, 29.1,27.7, 26.2, 24.7, 22.6, 14.0 ppm.
MS(ESI): m/z 291 [M+Na]+. HRMS(ESI) m/z calculated for C16H28O3Na
291.19307, found 293.19324. Compound 5, yellow oil. Rf = 0.65 (5% EtOAc-
Supplementary data (experimental procedures and analytical
data for all the new compounds) associated with this article can
be found, in the online version, at http://dx.doi.org/10.1016/
j.tetlet.2013.08.020.
Hexane). ½a 2D5
ꢁ
+93.22 (c 0.3, CHCl3). IR [neat]: 2983, 2933, 2877, 1632, 1458,
1238, 1032, 997, 510 cmꢀ1
.
1H NMR (500 MHz, CDCl3): d 7.29–7.26 (m, 2H),
6.89–6.86 (m, 2H), 4.71 (d, J = 11.4 Hz, 1H), 4.42 (d, J = 11.4 Hz, 1H), 4.00 (t,
J = 6.4 Hz, 1H), 3.80 (s, 3H), 1.78–1.70 (m, 2H), 0.98 (t, J = 7.4 Hz, 3H) ppm. 13
C
NMR (125 MHz, CDCl3): d 159.0, 129.6, 129.4 (2C), 113.6 (2C), 79.3, 70.3, 70.1,
55.0, 45.0, 28.6, 9.5 ppm. Compound 17, colorless oil. Rf = 0.5 (10% EtOAc-
References and notes
Hexane). ½a 2D5
ꢁ
+43.20 (c 1.5, CHCl3). IR [neat]: 2926, 2854, 2235, 1721, 1609,
1461, 1376, 1251, 1061, 863, 759, 511 cmꢀ1. (1H NMR, 500 MHz, CDCl3): d
7.31–7.25 (m, 2H), 6.90–6.86 (m, 2H), 4.79–4.68 (m, 2H), 4.43 (d, J = 11.3 Hz, 1
H), 4.05 (dd, J = 8.5, 5.8 Hz, 2 H), 3.81 (s, 3 H), 3.53 (s, 3H), 3.52–3.49 (m, 1H),
1. Gung, B. W. C. R. Chim. 2009, 12, 489–505; For review see (b) Annabelle, L. K.;
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1.82–1.72 (m, 2H), 1.59 (s, 3H), 1.58–1.41 (m, 3H), 1.38 (s, 3H), 1.35–1.24 (m, 9
13
H), 1.00 (t, J = 7.5 Hz, 3 H), 0.87 (t, J = 6.7 Hz, 3H) ppm.
C NMR (75 MHz,
CDCl3): d 159.3, 129.6, 113.8, 111.6, 111.0, 81.5, 80.3, 79.4, 75.0, 71.2, 70.5,
69.8, 69.6, 67.8, 58.8, 55.2, 31.8, 31.0, 29.5, 29.2, 28.7, 27.8, 26.1, 24.7, 22.6,
14.1, 9.7 ppm. MS(ESI): m/z 493 [M+Na]+. HRMS(ESI) m/z calculated for
C
29H42O5Na 493.29245, found 293.29388. Compound 3, colorless oil. Rf = 0.25
(40% EtOAc-Hexane). ½a D25
ꢁ
+12.5 (c 0.04, MeOH); lit.1½a D25
ꢁ
+11.7 (c 0.06, MeOH).
IR [neat]: 3389, 2928, 2856, 1725, 1632, 1461, 1378, 1278, 1086, 1045, 969,
865, 757, 581 cmꢀ1 1H NMR (300 MHz, CDCl3): d 4.55–4.50 (m, 1H), 4.39 (t,
.
J = 6.2 Hz, 1H), 3.69-3.61 (m, 1H), 3.61–3.55 (m, 1H), 3.44 (s, 3H), 3.42 (s, 1H),
2.79–2.63 (br s, 1H), 2.10–2.01 (br s, 1H), 1.80–1.71 (m, 2H), 1.68–1.57 (m, 2H),
1.37–1.23 (m, 10H), 1.02 (t, J = 7.3 Hz, 3H), 0.89 (t, J = 6.7 Hz, 3H) ppm.13 C NMR
(75 MHz, CDCl3): d 81.4, 80.5, 77.6, 73.0, 70.5, 68.8, 65.4, 64.0, 57.4, 31.8, 30.5,
29.8, 29.2(2C), 25.0, 22.6, 14.1, 9.3 ppm. MS(ESI): m/z 333 [M+Na]+. HRMS (ESI)
m/z calculated for C18H30O4Na 333.20363, found 333.20386.
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