Studies toward the total synthesis of cyclodidemniserinol trisulfate. Part I: 3,5,7-Trisubstituted 6,8-dioxabicyclo [3.2.1] octane core structure construction via a convergent and a linear stereoselective synthesis

Article abstract of DOI:10.1016/j.tetlet.2009.05.102

The core structure of the natural product cyclodidemniserinol trisulfate, a natural HIV-1 integrase inhibitor, was synthesized by employing intramolecular ketal formation strategy via a convergent synthesis and a linear synthesis approach, respectively. Both approaches relied on Shapless asymmetric dihydroxylation to introduce the chiral centers at 1- and 7-position, and the latter also utilized Sharpless asymmetric epoxidation to install the chiral center at 3-postion of the 3,5,7-trisubstituted-6,8-dioxabicyclo [3.2.1] octane. The established methodologies will be beneficial for further total synthesis and structural derivatization.

Full text of DOI:10.1016/j.tetlet.2009.05.102

Tetrahedron Letters 50 (2009) 4587–4591
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Tetrahedron Letters
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Studies toward the total synthesis of cyclodidemniserinol trisulfate.
Part I: 3,5,7-Trisubstituted 6,8-dioxabicyclo [3.2.1] octane core structure
construction via a convergent and a linear stereoselective synthesis
*
Jian-Hua Liu, Lai-Dong Song, Ya-Qiu Long
State Key Laboratory of Drug Research, Shanghai Institute of Materia Medica, Shanghai Institutes for Biological Sciences, Chinese Academy of Sciences, 555 Zuchongzhi Road,
Shanghai 201203, China
a r t i c l e i n f o
a b s t r a c t
Article history:
The core structure of the natural product cyclodidemniserinol trisulfate, a natural HIV-1 integrase inhib-
itor, was synthesized by employing intramolecular ketal formation strategy via a convergent synthesis
and a linear synthesis approach, respectively. Both approaches relied on Shapless asymmetric dihydroxy-
lation to introduce the chiral centers at 1- and 7-position, and the latter also utilized Sharpless asymmet-
ric epoxidation to install the chiral center at 3-postion of the 3,5,7-trisubstituted-6,8-dioxabicyclo [3.2.1]
octane. The established methodologies will be beneficial for further total synthesis and structural
derivatization.
Received 17 April 2009
Revised 17 May 2009
Accepted 27 May 2009
Available online 30 May 2009
Ó 2009 Elsevier Ltd. All rights reserved.
Cyclodidemniserinol trisulfate (Fig. 1) was isolated and charac-
terized from extracts of marine invertebrates, the Palauan ascidian
Didemnum guttatum by Faulkner and co-workers in 2000.1 This
natural marine product is structurally most closely related to
didemniserinolipid A from an Indonesian Didemnum sp.2 However,
the most notable difference is the presence of a 22-memberd addi-
tional ring containing a glycine unit and the presence of sulfate
groups. Moreover, cyclodidemniserinol trisulfate not only exhibits
cytotoxicity activity, but also inhibits the HIV-1 integrase with an
compound in a stereoselective manner can help the determination
of the absolute configuration. On the other hand, the low content of
the cyclodidemniserinol trisulfate in nature restricted its further
biological study. Furthermore, the total synthesis can afford gen-
eral methodology to derivatize the active structure. Natural prod-
uct-based combinatorial library is an efficient approach for the
innovative drug discovery.3 So, we initiated the project of total syn-
thesis of the natural HIV-1 integrase inhibitor, cyclodidemniserinol
trisulfate.
IC50 of 60
l
g/mL. Although the activity is not high, the new struc-
By retrosynthetic analysis of the target molecule, we were intri-
gued to start from synthesizing the 3,5,7-trisubstituted 6,8-dioxa-
bicyclo [3.2.1] octane. This bicyclic ketal-containing structure is
widely found in many natural products such as brevicomin,4
didemniserinolipid B, frontalin,5 multistriatin,6 attenol,7 and pin-
natoxin A,8 some of which display excellent cytotoxicity. Therefore,
the 6,8-dioxabicyclo [3.2.1] octane-based combinatorial library
afforded promising potent antitumor hits.9 Since it is the core
structure and possesses potential bioactivity, we decided to con-
struct the 3,5,7-trisubstituted-6,8-dioxabicyclo [3.2.1] octane as
the first stage of the total synthesis study.
Although many synthetic routes toward 6,8-dioxabicyclo [3.2.1]
octane and its derivatives have been reported, only a few involved
the 3,5,7-trisubstituted structure. The non-3-substituted analogs
can be conveniently synthesized through intramolecular ketal for-
mation reaction between a carbonyl group and a diol group,7 or
Diels–Alder reaction followed by intramolecular radical addition.10
However, for the 3-substituted structure, few synthesis was re-
ported, employing a different strategy to build the 6,8-dioxabicyclo
[3.2.1] octane, in which the ketal moiety was constructed before
ture as natural HIV-1 integrase inhibitor stimulated our interest to
synthesize it. On one hand, although 1H NMR, 13C NMR, COSY,
HMBC, HSQC–TOCSY, and MS were utilized to elucidate the struc-
ture, the absolute configuration of cyclodidemniserinol trisulfate
has not been determined and reported yet. Total synthesis of the
OR
O
NHR
O
1
NH
7
2
3
O
O
O 8
O 6
R=SO3Na
O
O
4
5
RO
Figure 1. The structure of cyclodidemniserinol trisulfate. The dashed circle
indicated the core structure of 3,5,7-trisubstituted-6,8-dioxabicyclo[3.2.1] octane.
* Corresponding author. Tel./fax: +86 021 50806876.
E-mail address: yqlong@mail.shcnc.ac.cn (Y.-Q. Long).
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.05.102

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J.-H. Liu et al. / Tetrahedron Letters 50 (2009) 4587–4591
the dioxabicyclic system was formed via an intramolecular alkyl-
ation.11 The main drawback of this approach was the restriction
of the 3-substituent structure type and lack of functionality
tolerance.
an (E)-alkene group. So the target structure was disconnected into
two fragments: a thioacetal containing fragment A, and an allylic
halide containing fragment B. The thioacetal group can achieve
the polar inversion of carbonyl group thus secure the alkylation.
In this strategy, the chiral centers could be introduced via Sharp-
less asymmetric dihydroxylation reaction.12
The synthetic approach is depicted in Scheme 2. Beginning with
the alkylation of the dianion of ethyl acetoacetate13, the alkyl chain
attached acetoacetate 1 was produced in high yield. After the ke-
tone group was protected with ethane-1,2-diol, the ester group
was reduced to the alcohol with lithiumaluminium hydride. Oxida-
tion of the primary alcohol 3 with pyridinium dichromate followed
by protecting the resultant aldehyde with 1,3-propandithiol fur-
nished the fragment A, thioacetal 5.
In the context of the total synthesis and structural derivatiza-
tion, the intramolecular ketal formation strategy would be optimal
for the synthesis of 3-substituted 6,8-dioxabicyclo [3.2.1] octane in
terms of the functional group tolerance and structural diversity.
Herein we thoroughly studied the general approach to synthesize
3,5,7-trisubstituted-6,8-dioxabicyclo [3.2.1] octane by employing
intramolecular ketal formation strategy. Two stereoselective syn-
thetic routes were developed, one via convergent synthesis, the
other via linear synthesis, to lead to the target molecule
successfully.
In the synthesis of fragment B, 1,5-pentandiol was used as the
starting material. The 1,5-pentandiol was mono-protected by
tert-butyldimethylchlorosilane (TBSCl), then the resulting alcohol
6 was oxidized via Swern Oxidation followed by condensation with
Strategy one: convergent stereoselective synthesis
On the basis of the retrosynthetic analysis of the bicyclic ketal
structure (Scheme 1), the convergent strategy was first chosen.
The ketal structure can be transformed into a ketone group and a
1,2-diol group, while the 1,2-diol structure can be transformed into
ylide reagent, ethyl phosphoranylidene acetate to give the
a,b-
unsaturated ester 7. The coupling constant (J) between H1 and H2
in the 1H NMR spectrum of 7 is 15.2 Hz, confirming the trans
geometry of the newly generated double bond. The unsaturated es-
ter was then reduced with DIBAL-H to yield alcohol 8 almost quan-
titatively, followed by iodination to afford allylic iodide 9 as
fragment B.
R2
HO
R2
OH
O
The coupling of the fragment A with the fragment B via an SN2
reaction14 produced the alkene 10 in 50% yield. In the following
step, Shapless asymmetric dihydroxylation (AD) reaction was uti-
lized to install the chiral centers. When the AD reaction was carried
out according to the standard procedure12b at rt, the ee value of the
resulting diol was only about 55%. When the amount of chiral li-
gand, DHQD2PHAL, was increased to 0.1 equiv, the ee value of
the diol product 11 reached 87%, but the yield was just 31%. Finally,
the HCl/MeOH catalytic system was employed to catalyze the
deprotection and ring formation reactions simultaneously, afford-
ing the target structure 12 in yield of 58%. Meanwhile the tert-
butyldimethylsilyl (TBS) protective group was removed as well
for further structural extension.
OS
S
O
O
OH
O
R1
S
R3
R2
O
R3
OH
S
R3
O
S
H
SO
R3
Fragmet A
O
OS
S
R3
R2
X
R2
Fragmet B
Scheme 1. The retrosynthetic analysis of the 3,5,7-trisubstituted-6,8-dioxabicyclo
[3.2.1] octane with strategy one.
1) NaH,THF
2) n-BuLi
HO(CH2)2OH,
LAH, ether,
reflux
3) I(CH2)6OBn, 0oC
HC(OEt)3, 70-80oC
O
O
O
O
O
O
O
OBn
OBn
97%
EtO
HO
81%
EtO
95%
EtO
2
1
HS(CH2)3SH, DCM, 0oC
61%
PDC, DCM, rt.
64%
S
O
O
O
O
O
O
O
OBn
OBn
OBn
S
H
3
4
5
1) swern oxidation
2) Ph3P=CHCOOEt,
CH2Cl2, reflux
H2
1) NaH, THF, 0oC
2) TBSCl, rt.
DIBAL-H, hexane, -78oC
100%
TBSO
OEt
TBSO
OH
HO
OH
69%
71%
H1
O
6
7
O
O
5,
I2, Ph3P, imidazole,
CH3CN and Et2O, rt.
n-BuLi, HMPA, THF,
-78oC, then rt.
OBn
TBSO
OH
TBSO
OBn
I
S
OTBS
92%.
50%
OH
O
8
S
9
10
K2OsO4, K3Fe(CN)6,
K2CO3, DHQD2PHAL,
MeSO2NH2,tBuOH and
H2O, rt.
O
S
O
HCl-MeOH(1N), rt.
58%
O
HO
S
S
OBn
OTBS
S
31%, 87%e.e.
OH
11
12
Scheme 2. Convergent stereoselective synthesis of the 3,5,7-trisubstituted-6,8-dioxabicyclo [3.2.1] octane.

J.-H. Liu et al. / Tetrahedron Letters 50 (2009) 4587–4591
4589
As outlined in Scheme 4, the allylic alcohol 17 was conveniently
synthesized by employing the approaches described in the former
synthetic route. Sharpless asymmetric epoxidation (AE) reaction
was utilized to synthesize the epoxy alcohol 18 in a stereoselective
manner and introduced the 3-position chirality as desired. The ee
value of the product is 87%. Chemoselective reduction of 18 with
Red-Al afforded 1,3-diol 19. After protection and deprotection pro-
cedures, hydroxyl group at the 3-position of 19 was finally pro-
tected as Bn ether, resulting in primary alcohol 22. Probably
owing to the stereohindrance, the reaction of 3-hydroxyl protec-
tion proceeded slowly and in low yield. Oxidation of compound
22 into aldehyde followed by treatment with vinyl Grignard re-
agent gave terminal alkene 23. Following the Johnson–Claisen ap-
proach17, compound 23 was transformed into (E)-alkene 24 via
reaction with triethyl orthoacetate in xylene at refluxing tempera-
ture. Reduction of the ester 24 with lithiumaluminium hydride
produced the alcohol 25, then Sharpless AD reaction was again
utilized to construct two chiral centers to afford triol 26. The de va-
lue of the reaction cannot be calculated even with HPLC analysis,
Strategy two: linear stereoselective synthesis
After successful synthesis of the core structure by convergent
synthesis route, considering the flexible introduction of the chiral
center at 3-position, we tried another linear synthesis route.
According to the retrosynthetic analysis as shown in Scheme 3,
the 3-position chiral center was installed before the ring-formation
step, and kept intact through the whole retrosynthetic disconnec-
tion. Similar to strategy one, the bicyclic ketal structure was dis-
connected into a ketone and a diol group, then the diol group
was further transformed into the alkene group. The alkene can
be generated from 1,3-diol. The 1,3-diol moiety could be easily
constructed by chemoselective ring opening of 2,3-epoxy alcohol
with Red-Al.15 And the 2,3-epoxy alcohol could be further trans-
formed into the allylic alcohol, which could be easily prepared
following the procedures described in strategy one. In this
synthesis, the chiral centers could be introduced via Sharpless
asymmetric dihydroxylation and Sharpless asymmetric epoxida-
tion,16 respectively.
R2
HO
R2
R1
O
O
O
O
O
OH
O
R1
O
R1
R3
R2
R3
O
R3
R1
O
O
O
O
O
O
R3
OH
R3
OH
O
O
O
O
R3
OH
R3
OH
Scheme 3. The retrosynthetic analysis of the 3,5,7-trisubstituted-6,8-dioxabicyclo [3.2.1] octane with strategy two.
1) NaH, n-BuLi, THF
HO(CH2)2OH,
2) I(CH2)2OBn, 0oC then rt.
HC(OEt)3, 70-80oC
O
O
O
O
O
LAH, ether, reflux
100%
O
O
O
O
OBn
OBn
EtO
OBn
EtO
O
66%
88%
EtO
HO
13
14
15
tBuOOH, (-)DIPT, Ti(O iPr)4,
O
1) swern oxidation
2) Ph3P=CHCOOEt,
CH2Cl2, reflux
O
H
3 O
DIBALH, hexane/CH2Cl2, -78oC
85%
4AMS, CH2Cl2, -40oC then -20oC
O
O
OBn
OBn
EtO
HO
86%
H2
16
17
Red-Al, THF, 0 oC then rt.
OH
TBSCl, imidazole, DMF, rt.
69%
OH
O
O
O
O
O
O
O
OBn
OBn
OBn
83% (two steps)
TBSO
HO
HO
18
19
20
1) swern oxidation,
2) CH2=CHMgBr,
THF, -78oC to 0oC
1) NaH,
BnO
BnO
TBAF, THF, rt.
94%
BnO
O
O
O
O
2) BrBn, Bu4NI, THF, rt.
O
O
OBn
OBn
OBn
O
HO
HO
TBSO
42%
21
22
23
β,
CH3C(OEt)3, propionic
acid, xylene, reflux
ADmix MeSO2NH2
LAH,
tBuOH-H2O, 0oC
BnO
BnO
O
ether, reflux
O
O
O
OBn
OBn
76% (two steps)
HO
100%
EtO
O
87%
24
25
OH
BnO
HO
OH
.
O
BF3 Et2O-Et2O, 0oC
85%
OBn
O
O
HO
BnO
OBn
26
27
Scheme 4. Linear stereoselective synthesis of the 3,5,7-trisubstituted-6,8-dioxabicyclo [3.2.1] octane.

4590
J.-H. Liu et al. / Tetrahedron Letters 50 (2009) 4587–4591
probably due to the similar polarity of compound 26 and its diaste-
reoisomer, which just differed in the configuration at 1-and 7-po-
sition. However, the product was produced almost quantitatively
in this reaction.
When HCl was used to catalyze the intramolecular ketal forma-
tion reaction in MeOH, which was a successful procedure in the
former synthetic route, the exchange of benzyloxyl group to meth-
oxyl group at 3-position was observed, leading to the side product
28. When the solvent was changed to ether or dioxane, the epimer-
ization at 3-position occurred. We proposed the mechanism of the
side reaction as follows (Scheme 5): the reactant was easily elimi-
accomplished under the condition of BF3ÁEt2O in Et2O at 0 °C,
affording the alcohol 27 as the major product in yield of 85%. The
configurations of 27 and its 3-epimer epi-27 were determined with
NOESY (Scheme 6). The existence of NOE effect between Ha and Hb
revealed the S configuration of 3-position in compound 27, while
such NOE effect was not observed in the diastereoisomer epi-27.
Considering the natural product cyclodidemniserinol trisulfate
possessed both antitumor and HIV-1 integrase inhibitory activity,
we chose tumor cell line (MDA-MB435) and HIV-1 integrase to
evaluate the bioactivity of compound 27 and its 3-epimer epi-27.
Neither compound showed inhibition against the HIV-1 integrase,
indicating the important effect of the 22-memberd additional ring
and the sulfate groups on the HIV-1 integrase inhibition. However,
the compound 27 exhibited an IC50 value of 7.8 lM to inhibit the
growth of the MDA-MB 435 cell, whereas the epi-27 displayed
no antiproliferative effect against the tumor cell line at the concen-
tration of 20 lM, suggesting the importance of the 3-chirality on
the antitumor activity. This interesting result provided useful clue
for further structural elaboration and medicinal chemistry study of
3,5,7-trisubstituted 6,8-dioxabicyclo [3.2.1] octane-based focused
library.
In conclusion, by employing either a convergent synthesis or a
linear synthesis approach, we accomplished the construction of
core structure 3,5,7-trisubstituted-6,8-dioxabicyclo [3.2.1] octane.
In the context of further total synthesis, the two strategies were
evaluated. In general, convergent synthesis strategy was made up
of 13 reactions, with the longest route consisting of eight steps.
The final trisubstituted bicycle structure was obtained in 0.9%
overall yield with unsolved chirality at 3-position. However, the
linear synthesis strategy encompassed 19 steps according to the
longest route, with 5.3% overall yield and an unambiguous chirality
at 3-postion.
Obviously, the lower yield of strategy one will hamper its appli-
cation in total synthesis. Although the synthesis route was
relatively long, strategy two was preferred for utilization in the to-
tal synthesis, because of its higher yield and the installed unambig-
uous chiral center at 3-postion. Furthermore, the preliminary
biological assay provided interesting results of the 3,5,7-trisubsti-
tuted-6,8-dioxadicyclo [3.2.1] octane structure and its 3-eipmer
in inhibiting the growth of the tumor cells. Our developed syn-
thetic methodology will be beneficial for the further medicinal
chemistry study and structural elaboration.
nated to yield the a,b-unsaturated system with the catalysis of HCl.
When MeOH was used as solvent, the methoxyl group attacked the
unsaturated system via Michael-addition reaction. As a result, ben-
zyloxy-methoxyl exchanged by-product generated. When ether or
dioxane was used as solvent, there was no other nucleophile spe-
cies present in the reaction system except for the benzyloxyl
group, which was just eliminated from the reactant. So the benzyl-
oxyl group attached again via 1,4-addition non-stereoselectively
and caused epimerization. In order to prove the proposed mecha-
nism, a drop of MeOH was added to the reaction system of catalytic
amount of HCl in Et2O, TLC revealed that there appeared benzyl-
oxy-methoxyl exchanged by-product.
Since the proton acid HCl was the causing factor for the side
reaction, we decided to try non-proton Lewis acid in the non-pro-
ton solvent such as Et2O, to secure the desired dioxabicycle struc-
ture formation. As expected, the final step was successfully
BnO
HO
O
O
OBn
OBn
OBn
HO
HO
26
OH
H+
BnO
O
HO
OH
OH
OBn
O
O
BnO
H+
27
O
OH
HO
HO
OH
Et2O or
dioxane
MeOH
(MeO-)
(BnO-)
Acknowledgement
O
O
OH OBn
OH OMe
OBn
OBn
This work was financially supported by the National Natural
Science Foundation of China (No. 20372068 and 30672528).
HO
OH
OH
Supplementary data
OH
OBn
OH
OBn
O
O
O
O
Synthetic procedure, NMR data and HRMS data of all new
compounds; and 1H and 13CNMR spectra of important interme-
diates, including the gCOSY and NOESY spectrum of 27 and
epi-27, are provided. Supplementary data associated with this
article can be found, in the online version, at doi:10.1016/
j.tetlet.2009.05.102.
MeO
BnO
28
Scheme 5. Proposed mechanism of side reaction in intramolecular ketal formation
step.
References and notes
O
O
Ha
BnO
Ha H
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OBn
OH
BnO
O
O
Hb
b
NOE Effect
No NOE Effect
27
epi-27
Scheme 6. The NOE analysis of compound 27 and epi-27.

J.-H. Liu et al. / Tetrahedron Letters 50 (2009) 4587–4591
4591
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