Unified Total Synthesis, Stereostructural Elucidation, and Biological Evaluation of Sarcophytonolides

Article abstract of DOI:10.1021/acs.joc.8b01634

Sarcophytonolides are cembranolide diterpenes isolated from the soft corals of genus Sarcophyton. Unified total synthesis of sarcophytonolides C, E, F, G, H, and J and isosarcophytonolide D was achieved. The synthetic routes feature NaHMDS- or SmI₂-mediated fragment coupling, alkoxycarbonylallylation, macrolactonization, and transannular ring-closing metathesis. These total syntheses led to the absolute configurational confirmation of sarcophytonolide H, elucidation of sarcophytonolides C, E, F, and G, and revision of sarcophytonolide J and isosarcophytonolide D. We also evaluated the antifouling activity and toxicity of the synthetic sarcophytonolides H and J and their analogues as well as the cytotoxicity of the synthetic sarcophytonolides and the key synthetic intermediates.

Full text of DOI:10.1021/acs.joc.8b01634

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Article
Unified Total Synthesis, Stereostructural Elucidation,
and Biological Evaluation of Sarcophytonolides
Hiroyoshi Takamura, Takahiro Kikuchi, Kohei Iwamoto, Eiji Nakao, Naoki Harada, Taichi Otsu,
Noriyuki Endo, Yuji Fukuda, Osamu Ohno, Kiyotake Suenaga, Yue-Wei Guo, and Isao Kadota
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.8b01634 • Publication Date (Web): 09 Aug 2018
Downloaded from http://pubs.acs.org on August 9, 2018
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Unified Total Synthesis, Stereostructural Elucidation, and Biological Evaluation of
Sarcophytonolides
,
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Hiroyoshi Takamura,* Takahiro Kikuchi, Kohei Iwamoto, Eiji Nakao, Naoki Harada,
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Taichi Otsu, Noriyuki Endo, Yuji Fukuda, Osamu Ohno, Kiyotake Suenaga, YueꢀWei
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Guo, and Isao Kadota
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Department of Chemistry, Graduate School of Natural Science and Technology, Okayama
University, 3ꢀ1ꢀ1 Tsushimanaka, Kitaꢀku, Okayama 700ꢀ8530, Japan
2
Himeji EcoTech Co., Ltd., 841ꢀ49 Koh, Shirahamaꢀcho, Himeji 672ꢀ8023, Japan
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Department of Chemistry and Life Science, School of Advanced Engineering, Kogakuin
University, 2665ꢀ1 Nakano, Hachioji 192ꢀ0015, Japan
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Department of Chemistry, Faculty of Science and Technology, Keio University, 3ꢀ14ꢀ1
Hiyoshi, Kohokuꢀku, Yokohama 223ꢀ8522, Japan
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Shanghai Institute of Materia Medica, Chinese Academy of Sciences, 555 Zu Chong Zhi
Road, Zhangjiang HiꢀTech Park, Shanghai 201203, China
takamura@cc.okayamaꢀu.ac.jp
Table of Contents/Abstract Graphic
Abstract
Sarcophytonolides are cembranolide diterpenes isolated from the soft corals of genus
Sarcophyton. Unified total synthesis of sarcophytonolides C, E, F, G, H, and J and
isosarcophytonolide D was achieved. The synthetic routes feature NaHMDSꢀ or
2
SmI ꢀmediated fragment coupling, alkoxycarbonylallylation, macrolactonization, and
transannular ringꢀclosing metathesis. These total syntheses led to the absolute configurational
confirmation of sarcophytonolide H, elucidation of sarcophytonolides C, E, F, and G, and
revision of sarcophytonolide J and isosarcophytonolide D. We also evaluated the antifouling
activity and toxicity of the synthetic sarcophytonolides H and J and their analogues as well as
the cytotoxicity of the synthetic sarcophytonolides and the key synthetic intermediates.
Introduction
Corals are an important group of marine invertebrates and have proven to be a rich source of
1
secondary metabolites with a diversity of the chemical structure and biological activity.
2
Among natural products isolated from soft corals, cembranolide diterpenes exhibit a variety
3
4
5
6
7
of biological activities such as antibacterial, antifouling, antiviral, cytotoxic, ichthyotoxic,
8
and protein tyrosine phosphatase 1B inhibitory activities. In addition, it has been reported
that cembranoid diterpens are implicated in defense of soft corals against predators and
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a,9
competition between soft corals and hard corals.
Guo and coꢀworkers have isolated sarcophytonolides, cembranolide diterpenes, from the soft
8
,10
corals of the genus Sarcophyton collected at Hainan Province in China since 2005.
These
natural products have a 14ꢀmembered carbon skeleton and butenolide unit as common
structures as described in Figure 1. The gross structure of sarcophytonolide C (1) was
determined by the 2D NMR analysis and comparison of its NMR data with those of
brassicolide (5), of which the relative configuration was assigned by Xꢀray crystallographic
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6b,10a
analysis.
Although the relative stereochemistries at the C1 and C2 positions of 1 were
elucidated by NOE observations between Hꢀ2 and Hꢀ15, the configuration at the C8 position,
which is a chiral center remote from the C1 and C2 positions, could not be determined. The
relative configurations at the C6 and C8 positions of sarcophytonolide E (2) were assigned by
NOE correlations of Hꢀ6/H
3
ꢀ19 and analysis of coupling constants and splitting patterns of
H
2
ꢀ5 in the model wherein a hydrogen bond between the hydroxy and the carbonyl groups is
10b
formed. The relative configurations of sarcophytonolides F (3) and G (4), which have the
C7/C8 trisubstituted alkenes and the C6 stereoisomeric relationship, were revealed by NOE
10b
experiments and analysis of their coupling constants of H
2
ꢀ5. The relative stereochemistry
1
13
of sarcophytonolide H (6) was clarified by the similarity of H and C NMR data between 6
1
0b
and 3 and NOE observations such as those of Hꢀ2/Hꢀ14 and Hꢀ2/Hꢀ15.
The absolute
11
configuration of 6 was determined by applying the modified Mosher method. The relative
stereochemistry of sarcophytonolide D (7) possessing the αꢀoriented acetoxy group at the C14
1
0a,12
position was elucidated by NOE experiments.
The relative stereochemistries at the C1,
10c
10c
C2, and C14 positions of sarcophytonolides I (8), J (9), and isosarcophytonolide D
1
0d
1
13
(10)
were assigned by the analogy of their H and C NMR data to those of
sarcophytonolide D (7), whereas the C8 stereochemistry of 9 has remained to be clarified as
in the case of sarcophytonolide C (1). Among these sarcophytonolides, sarcophytonolides H
(
6) and J (9) display the antifouling activity against the cypris larvae of the barnacle Balanus
13
14
(
Amphibalanus) amphitrite with EC50 values of 5.98 and 7.50 ꢁg/mL, respectively.
Interestingly, there is also a report wherein sarcophytonolide J (9) has no inhibitory activity
13
against the larval settlement of the same barnacle. In 2013, as a preliminary communication,
we reported the total synthesis of two possible diastereomers of sarcophytonolide C (1),
15a,16
which resulted in the absolute configurational determination of this natural product.
In
2
016, we also reported the total synthesis of sarcophytonolide H (6) and isosarcophytonolide
D (10, proposed structure), which culminated in the absolute stereochemical confirmation of 6
15b
and revision of 10.
sarcophytonolides C, E, F, G, H, and J and isosarcophytonolide D by using NaHMDSꢀ or
SmI ꢀmediated fragment coupling, alkoxycarbonylallylation, macrolactonization, and
In this full paper, we disclose the unified total synthesis of
2
transannular ringꢀclosing metathesis (RCM) as key steps. These total syntheses culminated in
the absolute stereochemical confirmation of sarcophytonolide H, determination of
sarcophytonolides C, E, F, and G, and revision of sarcophytonolide J and isosarcophytonolide
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D. Furthermore, we report the cytotoxicity of the synthetic sarcophytonolides and the key
synthetic intermediates, and the antifouling activity and toxicity of the synthetic
sarcophytonolides H and J and their analogues.
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Figure 1. Structures of selected sarcophytonolides and brassicolide.
Results and Discussion
Retrosynthetic Analysis of 1a and 1b. Toward the absolute configurational determination
of natural sarcophytonolide C (1), we decided to synthesize 1a and 1b (Scheme 1), which are
two possible diastereomers of this natural product. In the retrosynthetic analysis, we designed
hydroxycarboxylic acids 11a and 11b as the key synthetic intermediates to construct the
1
7
cembranolide frameworks of 1a and its C8 epimer 1b by macrolactonization and subsequent
18
19
transannular RCM , respectively. We hypothesized that the macrolactonization precursors
11a,b could possibly supplied by the connection of sulfone 12, allylic bromides 13a and 13b,
and 2ꢀalkoxycarbonyl allylic metal reagent 14. The chiral pool synthesis starting from (S)ꢀ
and (R)ꢀcitronellols could provide the allylic bromides 13a,b, respectively. We envisioned that
this retrosynthetic bondꢀdisconnection could be also applied to the synthesis of C6
hydroxylated and/or C14 acetoxylated sarcophytonolides. Moreover, the use of geraniol and
nerol instead of citronellol as starting materials would potentially lead to the total synthesis of
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sarcophytonolides bearing the C7/C8 trisubstituted alkene moieties.
Scheme 1. Retrosynthetic Analysis of 1a and 1b
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Total Synthesis of Two Possible Diastereomers 1a and 1b of Sarcophytonolide C (1).
First, we investigated the enantioselective synthesis of sulfone 20 (Scheme 2). Monosilylation
of cisꢀ2ꢀbuteneꢀ1,4ꢀdiol with tertꢀbutyldimethylsilyl chloride (TBSCl) followed by Sharpless
20
asymmetric epoxidation with (+)ꢀdiethyl tartrate (DET) gave epoxy alcohol 15 in 86% yield.
1
The enantiomeric ratio of 17:1 was assigned by the H NMR spectra of (S)ꢀ and
(R)ꢀαꢀmethoxyꢀβꢀ(trifluoromethyl)phenylacetyl (MTPA) esters prepared from 15. The
21
epoxide 15 reacted with isopropenylmagnesium bromide in the presence of CuBrꢂSMe
2
to
22
provide 1,3ꢀdiol 16, regioselectively. Hydrogenation of the alkene 16 with (Ph
3
P)
P afforded sulfide 18.
After the alcohol 18 was protected as the TBS ether, sulfide 19 was oxidized to furnish the
3
RhCl and
2
3
2 3
subsequent selective thioetherification of diol 17 with (PhS) /nꢀBu
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sulfone 20.
Scheme 2. Synthesis of Sulfone 20
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Next, connection of the sulfone 20 and allylic bromides 21a and 21b, which were
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synthesized from (S)ꢀ and (R)ꢀcitronellols, was examined (Scheme 3). Thus, the anion
derived from 20 with sodium hexamethyldisilazide (NaHMDS) was treated with the optically
active 21a and 21b to produce the coupling products 22a and 22b in 88% and 91% yields,
2
6
respectively. Reductive removal of the sulfonyl groups of 22a,b under Birch conditions,
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wherein the pivaloyl (Piv) groups were partially removed, and protection of the resulting
alcohols afforded the corresponding pivalates. The primary TBS ethers were selectively
deprotected with camphorsulfonic acid (CSA) to give alcohols 23a and 23b. Oxidation of
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2
3a,b with 2,2,6,6ꢀtetramethylpiperidinyloxyl (TEMPO)/PhI(OAc)
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and subsequent Wittig
reaction yielded alkenes 24a and 24b. Reductive deprotection of the pivalates 24a,b was
carried out with diisobutylaluminum hydride (DIBALꢀH) to provide alcohols 25a and 25b.
Scheme 3. Synthesis of Alcohols 25a and 25b
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We next tried to synthesize the macrolactonization precursors 30a and 30b as shown in
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Scheme 4. The alcohols 25a,b were oxidized with TEMPO/PhI(OAc)
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and the resulting
aldehydes were treated with ethyl (2ꢀbromomethyl)acrylate (26)/Zn dust in THF/aqueous
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NH Cl to furnish the desired homoallylic alcohols 27a and 27b in 79% and 93% yields in
two steps as 1:1 diastereomeric mixtures, respectively. After the resulting hydroxy groups of
27a,b were protected as the methoxymethyl (MOM) ethers, the TBS moieties of 28a and 28b
were removed with tetrabutylammounium fluoride (TBAF) to give alcohols 29a and 29b.
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Alkaline hydrolysis of the ethyl esters 29a,b with LiOHꢂH O afforded hydroxycarboxylic
acids 30a and 30b.
Scheme 4. Synthesis of Hydroxycarboxylic Acids 30a and 30b
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Scheme 5. Completion of the Total Synthesis of 1a and 1b
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With the macrolactonization precursors 30a and 30b in hand, we next investigated the
construction of the cembranolide framework and completion of the total synthesis. Thus, the
hydroxycarboxylic acids 30a,b were subjected to the Shiina macrolactonization conditions
1
7,30
with 2ꢀmethylꢀ6ꢀnitrobenzoic anhydride (MNBA)
to give 15ꢀmembered macrolactones
3
1a and 31b in 85% and 78% yields, respectively (Scheme 5). After the MOM ethers 31a,b
31
18
were deprotected with BF ꢂOEt /Me S, transannular RCM of 32a and 32b was conducted
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by using the secondꢀgeneration Hoveyda–Grubbs catalyst (33) to produce butenolides 34a
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and 34b.
In these reactions, the prolonged reaction time caused the formation of
byproducts, therefore the starting materials 32a and 32b were recovered in 38% and 37%
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yields, respectively. Finally, TPAP (tetraꢀnꢀpropylammonium perruthenate) oxidation of
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4a,b provided the target molecules 1a and 1b in 27% (43% based on recovered 32a) and
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0% (48% based on recovered 32b) in two steps, respectively.
Absolute Configuration of Sarcophytonolide C (1). Having succeeded in the total
synthesis of 1a and 1b, we analyzed their 2D NMR spectra and assigned the signals in their
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H and C NMR spectra. Tables 1 and 2 depict the chemical shifts and their differences of
10a
1
13
natural sarcophytonolide C (1) and the synthetic products 1a and 1b in the H and C NMR
1
13
spectra, respectively. The H and C NMR data of 1a were in excellent agreement with those
1
13
of natural sarcophytonolide C (1), meanwhile the H and C NMR data of 1b were clearly
different from those of natural product 1. It was found that there were critical differences of
the chemical shifts between the natural product and the synthetic 1b at the C7, C8, and C9
1
13
positions in the H NMR data and at the C9, C10, and C11 positions in the C NMR data.
2
9
The sign of specific rotation of the synthesized 1a, [α]
D
= +92.2 (c = 0.19, CHCl
3
), was
20
10a,37
same as that of the data reported for the natural product, [α]
D
3
= +31.0 (c = 0.20, CHCl ).
Therefore, the absolute configuration of sarcophytonolide C (1) isolated from nature was
clarified to be 1S, 2S, and 8S as shown in 1a.
1
Table 1. H NMR Chemical Shifts and Their Deviations of Natural Sarcophytonolide C
a
(
1) and the Synthetic Products 1a and 1b
b
position
1
1a
1b
ꢃ(δ –δ )
ꢃ(δ –δ )
1
1a
1
1b
1
2
3
5
1.46
4.81
7.26
3.50
1.47
4.81
7.26
3.50
3.16
2.37
2.37
1.74
1.37
1.37
2.11
1.94
4.97
2.10
1.94
1.59
1.14
1.51
4.85
7.28
3.39
3.20
2.56
2.09
1.87
1.42
1.20
2.05
2.05
5.05
2.03
2.03
1.56
1.18
–0.01
0.00
–0.05
–0.04
–0.02
+0.11
–0.04
–0.19
+0.28
–0.13
–0.04
+0.16
+0.07
–0.10
–0.08
+0.07
–0.10
+0.02
–0.04
0.00
0.00
3
.16
2.37
.37
0.00
7
0.00
2
0.00
8
9
1.74
1.38
0.00
+0.01
–0.01
+0.01
+0.01
0.00
1
.36
2.12
.95
10
1
11
4.97
2.10
13
0.00
1
.93
1.58
.14
–0.01
–0.01
0.00
14
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2.08
0.99
1.00
0.93
1.60
2.10
1.00
1.01
0.93
1.61
2.05
1.03
1.04
0.92
1.59
–0.02
–0.01
–0.01
0.00
+0.03
–0.04
–0.04
+0.01
+0.01
1
1
1
6
7
9
0
2
–0.01
a
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
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4
4
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4
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4
4
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5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
NMR spectra of the natural product and the synthetic products were recorded at 400 MHz.
Chemical shifts are reported in ppm with reference to the internal residual solvent (CDCl
3
,
b
7.26 ppm). Data from reference 10a.
13
Table 2. C NMR Chemical Shifts and Their Deviations of Natural Sarcophytonolide C
a
(1) and the Synthetic Products 1a and 1b
b
position
1
1a
1b
ꢃ(δ –δ )
ꢃ(δ –δ )
1 1b
1
–0.1
+0.1
+0.2
0.0
1a
1
2
3
4
5
6
7
8
9
45.8
83.7
151.3
127.4
40.3
205.9
50.0
28.6
35.7
24.3
127.1
133.9
38.7
23.2
29.5
18.4
19.9
172.8
20.7
16.1
45.9
83.6
151.1
127.4
40.3
205.7
50.0
28.6
35.7
24.4
127.0
133.8
38.8
23.3
29.6
18.5
20.0
172.6
20.7
16.1
45.8
83.3
152.0
128.2
39.9
205.8
50.1
28.4
36.8
25.3
128.2
133.3
38.2
23.6
29.4
19.3
19.8
172.7
19.9
16.0
0.0
+0.4
–0.7
–0.8
+0.4
+0.1
–0.1
+0.2
–1.1
–1.0
–1.1
+0.6
+0.5
–0.4
+0.1
–0.9
+0.1
+0.1
+0.8
+0.1
0.0
+0.2
0.0
0.0
0.0
10
–0.1
+0.1
+0.1
–0.1
–0.1
–0.1
–0.1
–0.1
+0.2
0.0
11
1
1
2
3
14
15
16
17
18
19
20
0.0
a
NMR spectra of the natural product and the synthetic products were recorded at 100 MHz.
Chemical shifts are reported in ppm with reference to the internal residual solvent (CDCl
3
,
b
77.0 ppm). Data from reference 10a.
Calculations for 1a and 1b were carried out using density functional theory (DFT)
calculations at the ωB97XꢀD/6ꢀ31G* level. Conformational analysis and geometry
optimization of 1a and 1b were performed with the package SPARTAN’16 (Wavefunction
Inc., Irvine, CA). As described in Figure 2, in the calculated most stable conformer of 1a, the
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C8 carbon is located more inside the macrocycle structure in comparison with the case of the
most stable conformer of 1b, which seems to make the distances between C1/C2 and C8 in 1a
shorter than those in 1b. This conformational difference of 1a and 1b may have resulted in the
deviation in their NMR data in spite of the C8 stereocenter remote from the C1 and C2
positions.
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Figure 2. Calculated lowestꢀenergy conformers of 1a (above) and 1b (below).
Total Synthesis of Sarcophytonolide E (2). Next, we examined the stereoselective
synthesis of sarcophytonolide E (2) possessing the chiral alcohol moiety at the C6 position.
First, we surveyed the reaction conditions in asymmetric alkoxycarbonylallylation of the
aldehyde prepared from the alcohol 25a and it was proven that treatment of the aldehyde with
3
8
chiral allylic boronate 35 in toluene at room temperature provided the desired alcohol 36 in
22
a 4.4:1 diastereomeric ratio (Scheme 6). Further transformation from 36 to sarcophytonolide
E (2) was identical to that used for the total synthesis of 1a and 1b. Thus, MOM protection of
the resulting hydroxy group of 36 and removal of the TBS moiety afforded alcohol 37.
1
7,30
Alkaline hydrolysis of the ethyl ester 37 followed by Shiina macrolactonization
yielded
3
1
15ꢀmembered macrolactone 38. Deprotection of the MOM ether 38 with BF ꢂOEt /Me S
3
2
13
2
18
39
1
and subsequent transannular RCM produced sarcophytonolide E (2). The H and C NMR
data and the specific rotation of the synthesized 2 were in good accordance with those of the
10b,40
natural product,
which elucidated the absolute stereochemistry of natural
sarcophytonolide E to be that described in 2.
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Scheme 6. Total Synthesis of Sarcophytonolide E (2)
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
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0
Total Synthesis of Sarcophytonolides F (3) and G (4). Having completed the total
synthesis of 1a, 1b, and 2 bearing the chiral centers at the C8 positions, we next investigated
the stereoselective synthesis of sarcophytonolides F (3) and G (4) with the C7/C8
(E)ꢀtrisubstituted alkene portions. First, as shown in Scheme 7, the synthesis of allylic
bromide 41, which is a coupling partner of the sulfone 20, was carried out. Protection of
41
geraniol with PivCl gave pivalate 39. Treatment of 39 with SeO /TBHP/salicylic acid
2
provided the mixture of allylic alcohol 40 and the corresponding α,βꢀunsaturated aldehyde.
The mixture was treated with NaBH to afford the allylic alcohol 40 in 57% yield in two steps.
4
The structure of 40 was confirmed by NOE observations of H ꢀ1/Hꢀ3. The allylic alcohol 40
2
was converted to the allylic bromide 41 with CBr /PPh .
4
3
Scheme 7. Synthesis of Allylic Bromide 41
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6
We next tried the connection of the sulfone 20 and the allylic bromide 41, and
transformation to sarcophytonolides F (3) and G (4). Thus, the sulfone 20 was coupled with
the allylic bromide 41 in the presence of NaHMDS to afford the desired product 42 (Scheme
8
). After deprotection of the pivalate 42 with DIBALꢀH, reductive desulfonylation was
26
42
conducted under Birch conditions to give alcohol 43 in 79% yield in two steps. The
0
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alcohol 43 was converted to alkene 45 by the following sequence: 1) protection as the pivalate,
28
2
) selective removal of the primary TBS moiety, 3) TEMPO oxidation of alcohol 44, 4)
Wittig methylenation, and 5) deprotection of the pivalate. The aldehyde, synthesized from the
38
alcohol 45, was treated with the chiral allylic boronate 35 to result in the diastereoselective
22
formation of alcohol 46 in a 17:1 ratio. After protection of the alcohol 46 and subsequent
1
7,30
desilylation, hydrolysis of 47 and Shiina lactonization
were carried out to afford
18
macrolactone 48. Transannular RCM of the tetraene 48 proceeded smoothly in the presence
32
of the secondꢀgeneration Hoveyda–Grubbs catalyst (33), wherein two trisubstituted alkene
portions of 48 were inert to the reaction conditions, to produce the corresponding butenolide
in 76% yield. Finally, the MOM protecting group was cleaved with trimethylsilyl iodide
4
3,44
(TMSI)/HMDS
to furnish sarcophytonolide F (3). We next tried to transform
sarcophytonolide F (3) to sarcophytonolide G (4) by stereoinversion at the C6 position.
4
5
46
Mitsunobu reaction of 3 with pꢀnitrobenzoic acid/diethyl azodicarboxylate (DEAD)/Ph
followed by alkaline hydrolysis with Na CO in MeOH provided sarcophytonolides G (4) and
F (3) in 31% and 13% yields in two steps, respectively. The formation of 3 was caused by the
3
P
2
3
47
esterification with stereoretention in the first step. Changing reaction temperature and
45
carboxylic acid in Mitsunobu reaction could not improve the chemical yield of 4. Therefore,
we next examined the stereoselective reduction of the ketone. Treatment of the allylic alcohol
48
3
with Dess–Martin periodinane gave α,βꢀunsaturated ketone 49. Corey–Bakshi–Shibata
49
(
CBS) reduction using (R)ꢀMeꢀCBS/BH
3 2
ꢂSMe was applied to 49 to result in the formation
of 4 and 3 in a 1:1 diastereomeric ratio. After investigation of the reaction conditions,
50
2 2
fortunately, Luche reduction conditions in MeOH/CH Cl was found to be effective and
sarcophytonolide G (4) was produced in 72% yield as a sole product. Although the detailed
conformational analysis of 49 was not conducted, the stereochemical outcome in this Luche
reduction is understandable by the formation of chelation structure as described in Figure 3.
In this structure, the proton of methanol could coordinate to two carbonyl oxygens at the C6
and C18 positions and hydride could approach from the outside of the macrocyclic structure.
10b
Detailed comparison of the NMR data and specific rotations between the natural products
and the synthetic products in 3 and 4 revealed the absolute configurations of natural
4
0
sarcophytonolides F and G to be those shown in 3 and 4, respectively.
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Scheme 8. Total Synthesis of Sarcophytonolides F (3) and G (4)
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
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Figure 3. Plausible rationale for the configurational outcome in Luche reduction of ketone 49.
Total Synthesis of Sarcophytonolide H (6). We next focused on the total synthesis of
sarcophytonolides possessing the acetoxy groups at their C14 positions. First, we tried to
synthesize sarcophytonolide H (6). We initially envisioned that the oxygenꢀfunctional group at
the C14 position could be introduced by oxidative desulfurization of the sulfone 42, which is
the synthetic intermediate of the total synthesis of sarcophytonolides F (3) and G (4). Actually,
deprotonation of 42 with nꢀBuLi or lithium diisopropylamide (LDA) and subsequent
5
1
52
treatment with Davis oxaziridine (50) or bis(trimethylsilyl)peroxide did not produce the
desired ketone 51 (Scheme 9). Therefore, we planned to introduce the C14 oxymethyne
moiety by other fragment couplings. Our synthesis of the C14 stereoisomers 53a and 53b,
53
2
wherein the SmI ꢀmediated reaction was used as the fragment coupling, is described in
Scheme 10. Selective acetylation of the diol 16 followed by TBS protection of the resulting
secondary alcohol gave the corresponding silyl ether. Reductive deprotection of the acetate
with DIBALꢀH afforded the alcohol, which was oxidized to aldehyde 52 with
28
2
TEMPO/PhI(OAc) . The aldehyde 52 was connected with the allylic bromide 41 by using
53,54
SmI
2
to furnish the desired αꢀadducts 53a and 53b in 53% and 40% yields, respectively. It
is noteworthy that the coupling was successful by using 1.2 equiv of the allylic bromide 41 to
5
5
the aldehyde 52 and the corresponding γꢀallylated product was not formed at all. The
observed NOEs of Hꢀ11/H ꢀ13 of 53a and 53b confirmed the geometries at their C11/C12
2
alkene portions, respectively. The stereochemistry at the C14 position of 53a was determined
11,40
by the modified Mosher method.
Scheme 9. Unsuccessful Attempt for Oxidative Desulfurization of Sulfone 42
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Scheme 10. Synthesis of Alcohols 53a and 53b
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Scheme 11. Total Synthesis of Sarcophytonolide H (6)
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6
Further transformation toward the total synthesis of sarcophytonolide H (6) is depicted in
Scheme 11. Thus, protection of the hydroxy group of 53a with MOMCl and subsequent
selective removal of the primary TBS moiety gave the corresponding alcohol. Introduction of
the terminal alkene portion followed by deprotection of the obtained pivalate 54 afforded
2
8
alcohol 55. TEMPO oxidation of 55 and reaction of the aldehyde with the chiral boronate
3
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35 provided the desired allylated product 56 in 88% yield in two steps in a diastereomeric
ratio of 13:1. After the C6 hydroxy group of 56 was protected as the pꢀmethoxybenzyl (PMB)
ether, the TBS ether underwent deprotection and hydrolysis of the ester to yield
17,30
hydroxycarboxylic acid 58. Shiina macrolactonization
and subsequent transannular
18
RCM were successfully performed to produce the corresponding butenolide. The obtained
5
6
MOM ether was selectively deprotected with HCl in iꢀPrOH to provide alcohol 60. Finally,
acetylation of the alcohol 60 and removal of the PMB moiety with
2,3ꢀdichloroꢀ5,6ꢀdicyanoꢀ1,4ꢀbenzoquinone (DDQ) took place to furnish sarcophytonolide H
(
6). The synthetic sarcophytonolide H (6) was in full agreement with the natural product in
1
13
10b,40
the H and C NMR data and the specific rotation.
Therefore, the absolute configuration
of natural sarcophytonolide H was confirmed as shown in 6.
Total Synthesis and Structural Revision of Isosarcophytonolide D. We next investigated
synthesis of the proposed structure 10 of isosarcophytonolide D with the 14R stereochemistry.
Thus, as shown in Scheme 12, the alcohol 53b was converted to 10 in 15 steps by a synthetic
57
route similar to that used toward the total synthesis of sarcophytonolide H (6). Having
synthesized the proposed structure 10 of isosarcophytonolide D, we analyzed the 2D NMR
1
13
data of the synthetic product 10. As a result, the significant differences in the H and
C
1
0d
40
NMR data between natural isosarcophytonolide D and the synthesized 10 were observed.
The detailed comparison revealed that the chemical shift deviations were especially critical
around the C14 position (Figure 4). We also considered that the stereochemistries at the C1
and C2 positions of isosarcophytonolide D would be same as those of other sarcophytonolides.
Therefore, we predicted the correct structure of isosarcophytonolide D isolated from nature to
be that drawn in 68, which is the C14 stereoisomer of 10 (Scheme 13). The predicted structure
6
8 was synthesized by oxidation of the synthetic sarcophytonolide H (6) with Dess–Martin
4
8
1
13
periodinane. As anticipated, the H and C NMR data and the specific rotation of the
10d,40
synthetic 68 matched those of the natural product.
Therefore, the absolute configuration
58
of natural isosarcophytonolide D was reassigned to be that shown in 68.
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Scheme 12. Total Synthesis of the Proposed Structure 10 of Isosarcophytonolide D
1
1
1
1
1
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3
Figure 4. Deviations of the C NMR chemical shifts between natural isosarcophytonolide D
and the synthetic product 10 (ꢃδ = δ – δ10 in ppm). N = natural product. The x and y axes
represent the carbon number and ꢃδ, respectively.
N
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6
Scheme 13. Synthesis of the Predicted Structure 68 of Isosarcophytonolide D
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Structural Prediction, Total Synthesis, and Structural Determination of
Sarcophytonolide J. As noted in Introduction, the relative configurations at the C1, C2, and
C14 positions of natural sarcophytonolides I (8), J (9), and isosarcophytonolide D (10) were
determined by the similarity of their NMR data to those of sarcophytonolide D (7). This way
of structural assignment and the stereochemical revision at the C14 position of
isosarcophytonolide D indicate that the C14 stereochemistries of sarcophytonolides D (7), I
(8), and J (9), which were originally assigned as 14R, should be also reexamined. Therefore,
we next decided to verify the stereostructure of sarcophytonolide J (9, Figure 5a), of which
the C8 configuration was not identified. First, in order to predict the C14 stereochemistry of
1
13
10c
sarcophytonolide J, we compared the H and C NMR data of natural sarcophytonolide J
with those of the synthetic products 68 (revised structure of isosarcophytonolide D) and 10
4
0
13
(C14 epimer of 68). Deviations of the C NMR chemical shifts at the C1, C2, C13, and C14
positions between natural sarcophytonolide J and the synthetic products 68 and 10 are
graphically depicted in Figure 5b. From these comparisons, it was found that the chemical
shift differences between natural sarcophytonolide J and 68 were smaller than those between
natural sarcophytonolide J and 10. In addition, for the prediction of the C8 stereochemistry of
1
13
10c
sarcophytonolide J, the H and C NMR data between natural sarcophytonolide J and the
40
synthetic products 1a (sarcophytonolide C) and 1b (C8 epimer of 1a) were compared. As a
result, it was elucidated that the chemical shift characteristics at the C7 to C10 positions of 1a
were more similar to those of 1b. Taken together, we could propose the predicted structure 69
of sarcophytonolide J (Figure 6), which bears the 8S and 14S absolute configurations same as
those of 1a and 68.
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8
9
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1
2
3
4
5
6
7
8
9
0
13
Figure 5. (a) Structures of 9, 68, 10, 1a, and 1b. (b) Deviations of the C NMR chemical
shifts between natural sarcophytonolide J and the synthetic products (ꢃδ = δ – δ in ppm). N
N
S
= natural product. S = synthetic product. The x and y axes represent the carbon number and
ꢃδ, respectively.
Figure 6. Predicted structure 69 of sarcophytonolide J.
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6
Scheme 14. Total Synthesis of the Predicted Structure 69 of Sarcophytonolide J
0
1
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8
9
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In order to confirm our stereostructural prediction of sarcophytonolide J as discussed above,
we commenced the total synthesis of the predicted structure 69. Sequence of the
transformation toward 69 was similar to that used toward the total synthesis of
2
sarcophytonolide H (6). Thus, SmI ꢀmediated reaction of the aldehyde 52 and the allylic
bromide 21a gave the desired product 70a and its C14 epimer 70b in 29% and 40% yields,
respectively (Scheme 14). The absolute stereochemistry at the C14 position of 70a was
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
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4
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4
4
4
5
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5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
40
verified by its derivatization and NOE experiments. Subsequently, the alcohol 70a was
transformed to our predicted structure 69 of sarcophytonolide J in overall 16 steps. As
1
13
expected, the synthetic 69 provided the H and C NMR data and the specific rotation which
1
0c,40
were identical to those reported for natural sarcophytonolide J.
These findings clearly
revealed that our stereochemical prediction of sarcophytonolide J is correct and this natural
product possesses the 8S and 14S absolute configurations as shown in 69.
Cytotoxicity of the Synthetic Products. Having successfully completed the total synthesis
and established the stereostructures of sarcophytonolides, we next turned our attention to
assessment of the biological activity of the synthetic products. First, we evaluated the
growthꢀinhibitory activity by using the MTT assay with HL60 human leukemia cells. The
cells were treated in 96ꢀwell plates with various concentrations of the synthetic compounds
for 72 h. As described in Table 3, interestingly, the synthetic sarcophytonolide C (1a) was
inactive, whereas 8ꢀepiꢀsarcophytonolide C (1b) inhibited the growth of the cells with an IC50
value of 18.9 ꢁM, which indicates that the activity is affected by the C8 stereochemistries.
Compared to 1a, 1b was observed to be more soluble in water, which may also contribute to
the growthꢀinhibitory activity. Sarcophytonolide E (2), which possesses the C6 alcohol moiety,
exhibited the activity with an IC50 value of 33.0 ꢁM. Sarcophytonolide F (3) bearing the
C7/C8 trisubstituted alkene group showed the activity similar to that of 2 (IC50 = 35.9 ꢁM).
Introduction of the C14 acetoxy moiety was found to lower the inhibitory activity by
comparing IC50 values of sarcophytonolides F (3, 35.9 ꢁM) and H (6, 53.5 ꢁM). Since
8ꢀepiꢀsarcophytonolide C (1b) was the most active among five sarcophytonolides, the key
synthetic intermediates of 1b were next biologically evaluated. The cytotoxic activity of the
butenolide construction precursor 32b (IC50 = 13.7 ꢁM) was slightly improved in comparison
with that of 1b. The macrolactonization precursor 30b and the alcohol 25b retained the
activity, while their activities were decreased to IC50 values of 55.8 and 24.0 ꢁM, respectively.
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2
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5
6
a
Table 3. Growth-Inhibitory Activity against HL60 Human Leukemia Cells
0
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8
9
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5
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7
8
9
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9
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5
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7
8
9
0
compound
IC50
>100
18.9
33.0
35.9
53.5
13.7
55.8
24.0
1
1
2
3
6
3
a
b
2b
30b
2
5b
a
IC50 values are given in ꢁM
Antifouling Activity and Toxicity of the Synthetic Products. Since sarcophytonolides H
6) and J (69) which were isolated from nature are reported to be antifouling active against the
(
1
3,14
cypris larvae of barnacle Balanus (Amphibalanus) amphitrite,
we next evaluated the
59
antifouling activity and toxicity of the synthetic sarcophytonolides H (6) and J (69) and their
analogues against the cypris larvae of the same barnacle. The larvae were treated in 24ꢀwell
polystyrene plates with various concentrations of the synthetic compounds in the dark for 96 h
and the results are summarized in Table 4. The synthetic sarcophytonolide H (6) displayed the
antifouling activity with an EC50 value of 3.36 ꢁg/mL, which was in good agreement with that
13
60
of the natural product (5.98 ꢁg/mL). Diol 79 retained the antifoulant activity (EC50 = 3.08
ꢁ
g/mL), which indicates the acetyl group of 6 has little influence on this activity. The
6
0
antifouling activity of the butenolide construction precursor 80 was marginally increased in
comparison with those of 6 and 79 (EC50 = 1.61 ꢁg/mL). The hydroxycarboxylic acid 58
turned out to be antifouling active (EC50 = 3.27 ꢁg/mL), while the alkoxycarbonylallylation
precursor 55 exhibited no antifouling activity. Furthermore, the toxicity was also evaluated
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and it was found that 58 was weak toxic (LC50 = 19.4 ꢁg/mL) and other compounds 6, 79, 80,
and 55 had no toxicity. In addition to the sarcophytonolide H series, we next evaluated the
biological activity of the synthetic sarcophytonolide J (69) and its synthetic intermediates. The
synthetic sarcophytonolide J (69), the triene 76, and the hydroxyester 74 were antifouling
active with EC50 values of 0.95–2.36 ꢁg/mL without regard to difference of the molecular
framework. Interestingly, alcohol 81 also showed the antifoulant activity (EC50 = 1.74 ꢁg/mL)
in contrast to the allylic alcohol 55 in the sarcophytonolide H series. In the sarcophytonolide J
series, only compound 69 displayed the weak toxicity with a LC50 value of 34.5 ꢁg/mL and
other compounds 76, 74, and 81 were nonꢀtoxic. These obtained results of the biological
activity assessment shown in Table 4 denote that the triene 76, which exhibited the strongest
antifouling activity and no toxicity, is a promising candidate for the creation of
environmentally friendly antifouling agents.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Table 4. Antifouling Activity (EC50) and Toxicity (LC50) of the Synthetic
a
Sarcophytonolides H (6) and J (69) and Their Analogues
compound
EC50
3.36
3.08
1.61
3.27
>50
LC50
>50
>50
>50
19.4
>50
6
79
80
58
55
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69
1.50
0.95
2.36
1.74
34.5
>50
>50
>50
7
7
6
4
1
8
a
Against the cypris larvae of barnacle Balanus (Amphibalanus) amphitrite. EC50 and LC50
values are given in ꢁg/mL.
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9
0
Conclusion
Unified total synthesis of sarcophytonolides, cembranolide diterpenes isolated from the soft
corals of genus Sarcophyton, was accomplished. In their synthetic routes, NaHMDSꢀmediated
2
reaction of allylic bromide/sulfone in methylene series at the C14 position and SmI ꢀmediated
reaction of allylic bromide/aldehyde in acetoxymethyne series at the C14 position were
utilized as the fragment couplings, respectively. In addition, the total synthesis features
alkoxycarbonylallylation, macrolactonization, and transannular RCM. Because the chirality at
the C8 position of sarcophytonolide C (1) was not determined, the C8 stereoisomers 1a and
1
1
b were stereoselectively synthesized, which elucidated the absolute configuration of natural
to be that as drawn in 1a. Stereoselective total synthesis of sarcophytonolides E (2), F (3), G
(
4), and H (6) culminated in the absolute stereochemical determination of 2, 3, and 4 and
confirmation of 6. Furthermore, total synthesis of the proposed structure 10 and the predicted
structure 68 of isosarcophytonolide D revealed the correct structure of this natural product to
be that in 68. This stereochemical revision at the C14 position of isosarcophytonolide D and
the stereostructural elucidation of sarcophytonolide C led us to the stereochemical prediction
of sarcophytonolide J. Total synthesis of the predicted structure 69 of this natural product
verified the correct structure of natural sarcophytonolide J. After completing the total
synthesis of sarcophytonolides, we assayed the cytotoxicity of selected synthetic products
against HL60 cells and found that 1b, 2, 3, 6, 32b, 30b, and 25b showed the cytotoxicity,
while 1a was inactive. Moreover, we conducted evaluation of the antifouling activity and
toxicity of the synthetic sarcophytonolides H (6) and J (69) and their synthetic analogues
against the cypris larvae of barnacle Balanus (Amphibalanus) amphitrite. The obtained
findings suggest that 76, a synthetic intermediate of 69, is a good candidate for further
developing environmentally benign antifouling compounds. Further synthetic study of other
classes of cembranolide is currently underway.
Experimental Section
General Methods. Reagents were used as received from commercial suppliers unless
otherwise indicated. All reactions were carried out under an atmosphere of argon. Reaction
solvents were purchased as dehydrated solvents and stored with active molecular sieves 4Å
under argon prior to use for reactions. All solvents for workꢀup procedure were used as
received. Analytical thinꢀlayer chromatography (TLC) was performed with aluminium TLC
plates (Merck TLC silica gel 60F254). Column chromatography was performed with Fuji
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Silysia silica gel BWꢀ300 or Kanto Chemical silica gel 60N. Optical rotations were recorded
1
13
on a JASCO DIPꢀ1000. IR spectra were recorded on a JASCO FT/IRꢀ460 plus. H and C
NMR spectra were recorded on JEOL JNMꢀAL400 or Varian 400ꢀMR. Chemical shifts in the
1
NMR spectra are reported in ppm with reference to the internal residual solvent ( H NMR,
13
3 6 6 3 6 6
CDCl 7.26 ppm, C D 7.15 ppm; C NMR, CDCl 77.0 ppm, C D 128.0 ppm). The
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
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6
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1
2
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5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
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2
3
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8
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0
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2
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8
9
0
following abbreviations are used to designate the multiplicities: s = singlet, d = doublet, t =
triplet, q = quartet, m = multiplet, br = broad. Coupling constants (J) are in hertz. High
resolution mass spectra were recorded on a Micromass LCT (ESI–TOF–MS) spectrometer.
TBS Ether S1. To a solution of cisꢀ2ꢀbuteneꢀ1,4ꢀdiol (5.5 mL, 66.9 mmol) in DMF (67 mL)
were added imidazole (4.55 g, 66.9 mmol) and TBSCl (10.1 g, 66.9 mmol) at –30 °C. The
mixture was stirred at the same temperature for 2 h. The mixture was diluted with EtOAc,
2 2 4
washed with H O and brine, and then dried over Na SO . Concentration and column
chromatography (hexane, hexane/EtOAc = 6:1) gave monoꢀTBS ether S1 (6.31 g, 47%):
–1
1
colorless oil; R
f
= 0.36 (hexane/EtOAc = 4:1); IR (neat) 3367, 2953, 2929, 2858 cm ; H
NMR (400 MHz, CDCl
3
) δ 5.74–5.64 (m, 2 H), 4.26 (d, J = 5.1 Hz, 2 H), 4.20 (d, J = 5.9 Hz,
13
2 H), 2.01 (brs, 1 H), 0.91 (s, 9 H), 0.09 (s, 6 H); C NMR (100 MHz, CDCl
9.6, 58.9, 26.0, 18.4, –5.2; HRMS (ESI–TOF) calcd for C10
3
) δ 131.3, 130.0,
+
5
H
22
O
2
SiNa [M + Na] 225.1287,
found 225.1284.
Epoxy Alcohol 15. To a suspension of powdered MS4Å (3.20 g) in CH
added (+)ꢀDET (3.3 mL, 19.1 mmol), Ti(OiꢀPr) (5.7 mL, 19.1 mmol), and allylic alcohol S1
3.21 g, 15.9 mmol) in CH Cl (17 mL + 6.0 mL) at –25 °C. After the resulting mixture was
2 2
Cl (130 mL) were
4
(
2
2
stirred at the same temperature for 20 min, TBHP (ca. 5.0 M solution in
2,2,4ꢀtrimethylpentane, 6.4 mL, 32.0 mmol) was added to the mixture at the same temperature.
After the resulting mixture was stirred at the same temperature for 63 h, the reaction was
quenched with 3 M aqueous NaOH and the mixture was stirred at room temperature for 30
min. The mixture was filtered through a Celite pad and washed with EtOAc. The mixture was
2 2 4
washed with H O and brine, and then dried over Na SO . Concentration and column
chromatography (hexane/EtOAc = 4:1) gave epoxy alcohol 15 (2.98 g, 86%, enantiomeric
25
ratio = 17:1): colorless oil; R
f
= 0.34 (hexane/EtOAc = 2:1); [α]
D
3
–12.6 (c 1.00, CHCl ); IR
–
1 1
(neat) 3399, 2953, 2930, 2858 cm ; H NMR (400 MHz, CDCl
3
) δ 3.92 (dd, J = 11.7, 5.6 Hz,
1
0
H), 3.79–3.71 (m, 3 H), 3.25–3.17 (m, 2 H), 2.24 (brs, 1 H), 0.90 (s, 9 H), 0.10 (s, 3 H),
13
.09 (s, 3 H); C NMR (100 MHz, CDCl
3
) δ 61.6, 60.9, 56.4, 56.0, 25.9, 18.3, –5.2, –5.3;
+
HRMS (ESI–TOF) calcd for C10
H
22
O
3
SiNa [M + Na] 241.1236, found 241.1231.
Diol 16. To a suspension of CuBr·SMe
2
2
(294 mg, 1.43 mmol) in Et O (38 mL) was added
isopropenylmagnesium bromide (0.5 M in THF, 31.4 mL, 15.7 mmol) at –50 °C. The mixture
was gradually warmed up to –20 °C and stirred at the same temperature for 10 min. To the
resulting mixture was added epoxy alcohol 15 (1.04 g, 4.76 mmol) in Et
at –20 °C. The mixture was stirred at the same temperature for 21 h. The reaction was
quenched with saturated aqueous NH Cl. The mixture was diluted with Et O, washed with
2
O (5.2 mL + 2.0 mL)
4
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5
6
saturated aqueous NH
4
Cl, H
2
2 4
O, and brine, and then dried over Na SO . Concentration and
f
column chromatography (hexane/EtOAc = 3:1) gave diol 16 (1.09 g, 84%): colorless oil; R =
2
D
5
–1 1
0.43 (hexane/EtOAc = 2:1); [α]
–4.9 (c 1.00, CHCl
3
); IR (neat) 3399, 2929, 2857 cm ; H
NMR (400 MHz, CDCl
3
) δ 5.00 (t, J = 1.6 Hz, 1 H), 4.86 (s, 1 H), 3.81–3.60 (m, 5 H),
1
3
3
2.46–2.37 (m, 3 H), 1.80 (s, 3 H), 0.91 (s, 9 H), 0.09 (s, 6 H); C NMR (100 MHz, CDCl ) δ
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
143.4, 114.5, 71.3, 65.5, 62.2, 52.6, 25.9, 21.3, 18.3, –5.3, –5.3; HRMS (ESI–TOF) calcd for
+
C
13
H
28
O
3
SiNa [M + Na] 283.1705, found 283.1711.
Alkane 17. A mixture of alkene 16 (2.65 g, 10.2 mmol) and (Ph
mmol) in benzene (48 mL) and EtOH (16 mL) was stirred for 5 h under H
room temperature. Short column chromatography (hexane/EtOAc = 1:1), concentration, and
3
P)
3
RhCl (236 mg, 0.255
2
atmosphere at
column chromatography (hexane/EtOAc = 7:1) gave alkane 17 (2.48 g, 93%): colorless oil; R
f
26
D
=
0.50 (hexane/EtOAc = 2:1); [α]
+9.9 (c 1.00, CHCl
) δ 3.92–3.87 (m, 1 H), 3.80–3.63 (m, 4 H), 2.70 (brs,
H), 1.86–1.78 (m, 1 H), 1.64–1.58 (m, 1 H), 0.99 (d, J = 6.8 Hz, 3 H), 0.94 (d, J = 6.8 Hz, 3
3
); IR (neat) 3389, 2956, 2930, 2886,
–1 1
2855 cm ; H NMR (400 MHz, CDCl
3
1
13
H), 0.92 (s, 9 H), 0.11 (s, 6 H); C NMR (100 MHz, CDCl
3
) δ 72.8, 64.6, 60.9, 49.6, 26.2,
+
2
2
30 3
5.9, 21.7, 20.0, 18.3, –5.2, –5.3; HRMS (ESI–TOF) calcd for C13H O SiNa [M + Na]
85.1862, found 285.1862.
2
Sulfide 18. To a solution of diol 17 (1.74 g, 6.63 mmol) and (PhS) (4.34 g, 19.9 mmol) in
pyridine (33 mL) was added nꢀBu
was stirred at the same temperature for 1 h. The mixture was diluted with Et
O and brine, and then dried over Na SO . Concentration and column chromatography
hexane/EtOAc = 60:1) gave sulfide 18 (1.92 g, 82%): colorless oil; R = 0.51 (hexane/EtOAc
3
P (5.0 mL, 19.9 mmol) at room temperature. The mixture
2
O, washed with
H
2
2
4
(
f
27
D
–1 1
= 7:1); [α]
–16.0 (c 1.00, CHCl
) δ 7.36–7.33 (m, 2 H), 7.29–7.26 (m, 2 H), 7.17 (tt, J = 7.3, 1.5 Hz, 1 H),
.89–3.84 (m, 1 H), 3.75 (dd, J = 10.0, 3.4 Hz, 1 H), 3.60 (dd, J = 10.0, 8.3 Hz, 1 H), 2.99 (dd,
J = 12.8, 4.5 Hz, 1 H), 2.90 (dd, J = 12.8, 7.6 Hz, 1 H), 2.56 (brd, J = 2.9 Hz, 1 H), 2.17–2.08
m, 1 H), 1.67–1.61 (m, 1 H), 0.98 (d, J = 6.8 Hz, 3 H), 0.95 (d, J = 6.8 Hz, 3 H), 0.91 (s, 9 H),
3
); IR (neat) 3490, 2955, 2928, 2855 cm ; H NMR (400
MHz, CDCl
3
3
(
1
3
0.08 (s, 6 H); C NMR (100 MHz, CDCl
3
) δ 137.2, 129.2, 128.8, 125.9, 72.5, 65.9, 45.9, 31.7,
SSiNa [M +
2
7.5, 26.0, 21.3, 18.7, 18.3, –5.2, –5.2; HRMS (ESI–TOF) calcd for C19H O
34 2
+
Na] 377.1946, found 377.1950.
2 2
TBS Ether 19. To a solution of alcohol 18 (1.29 g, 3.64 mmol) in CH Cl (36 mL) were
added 2,6ꢀlutidine (0.64 mL, 5.46 mmol) and TBSOTf (1.0 mL, 4.37 mmol) at 0 °C. The
mixture was stirred at the same temperature for 30 min. The mixture was diluted with Et O,
washed with H O and brine, and then dried over Na SO . Concentration and column
2
2
2
4
chromatography (hexane/EtOAc = 1:0, 70:1) gave TBS ether 19 (1.68 g, 98%): colorless oil;
28
D
R
f
= 0.66 (hexane/EtOAc = 20:1); [α]
3
–34.8 (c 1.00, CHCl ); IR (neat) 2955, 2929, 2862
–1 1
3
cm ; H NMR (400 MHz, CDCl ) δ 7.34–7.32 (m, 2 H), 7.27–7.24 (m, 2 H), 7.16–7.12 (m, 1
H), 4.14 (td, J = 6.6, 2.9 Hz, 1 H), 3.58 (dd, J = 6.6, 2.7 Hz, 2 H), 3.05 (dd, J = 12.7, 3.9 Hz, 1
H), 2.95 (dd, J = 12.7, 9.6 Hz, 1 H), 2.08–2.00 (m, 1 H), 1.75–1.70 (m, 1 H), 0.96 (d, J = 6.8
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8
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Hz, 3 H), 0.93 (d, J = 6.8 Hz, 3 H), 0.88 (s, 9 H), 0.85 (s, 9 H), 0.09 (s, 3 H), 0.08 (s, 3 H),
13
0
6
.03 (s, 3 H), 0.02 (s, 3 H); C NMR (100 MHz, CDCl
3
) δ 137.2, 129.2, 128.6, 125.5, 73.3,
5.0, 44.8, 31.0, 26.4, 26.0, 26.0, 22.6, 19.1, 18.3, 18.2, –4.1, –4.7, –5.2, –5.4; HRMS
+
(ESI–TOF) calcd for C25
H
48
O
2
SSi
2
Na [M + Na] 491.2811, found 491.2809.
Sulfone 20. To a solution of sulfide 19 (1.68 g, 3.58 mmol) in EtOH (36 mL) were added
30% aqueous H (3.6 mL, 35.8 mmol) and (NH Mo O (442 mg, 0.358 mmol) at
°C. The mixture was stirred at the same temperature for 2 h and at room temperature for 4 h.
The reaction was quenched with saturated aqueous Na at 0 °C. The mixture was diluted
with Et O, washed with H O and brine, and then dried over Na SO . Concentration and
column chromatography (hexane/EtOAc = 40:1) gave sulfone 20 (1.72 g, 96%): colorless oil;
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
O
)
6
7 2
O24·4H
2
2
4
0
2 2 3
S O
2
2
2
4
2
9
R
f
= 0.34 (hexane/EtOAc = 10:1); [α]
D
–17.8 (c 1.00, CHCl
) δ 7.92–7.90 (m, 2 H), 7.65–7.61 (m, 1 H), 7.56–7.52 (m, 2
H), 4.16–4.12 (m, 1 H), 3.57 (dd, J = 10.0, 5.7 Hz, 1 H), 3.50 (dd, J = 10.0, 8.1 Hz, 1 H), 3.36
dd, J = 14.8, 8.6 Hz, 1 H), 2.97 (dd, J = 14.8, 2.3 Hz, 1 H), 2.17–2.14 (m, 1 H), 2.09–2.01 (m,
H), 0.88 (s, 9 H), 0.87 (s, 9 H), 0.83 (d, J = 6.8 Hz, 3 H), 0.62 (d, J = 7.1 Hz, 3 H), 0.14 (s, 3
3
); IR (neat) 2955, 2929, 2855
–1
1
cm ; H NMR (400 MHz, CDCl
3
(
1
13
H), 0.08 (s, 3 H), 0.07 (s, 3 H), 0.04 (s, 3 H); C NMR (100 MHz, CDCl
3
) δ 139.6, 133.3,
1
29.0, 128.1, 73.4, 64.3, 52.5, 40.1, 25.9, 25.9, 25.6, 21.8, 18.4, 18.2, 18.1, –4.2, –4.7, –5.3,
+
48 4 2
–5.5; HRMS (ESI–TOF) calcd for C25H O SSi Na [M + Na] 523.2709, found 523.2709.
Sulfone 22a. To a solution of sulfone 20 (1.83 g, 3.65 mmol) in THF (28 mL) was added
NaHMDS (1.0 M in THF, 4.4 mL, 4.40 mmol) at –78 °C. The mixture was stirred at the same
temperature for 30 min. To the mixture was added allylic bromide 21a (1.40 g, 4.38 mmol) in
THF (4.0 mL + 2.0 mL + 2.0 mL) at –78 °C. The mixture was gradually warmed up to room
temperature for 2 h. The reaction was quenched with saturated aqueous NH
was diluted with Et O, washed with H O and brine, and then dried over Na
Concentration and column chromatography (hexane/EtOAc = 20:1) gave sulfone 22a (2.38 g,
4
Cl. The mixture
2
2
2
SO .
4
2
6
8
8%): colorless oil; R
f
= 0.14 (hexane/EtOAc = 20:1); [α]
D
+15.1 (c 1.00, CHCl
3
); IR (neat)
–1 1
2957, 2929, 2862, 1727 cm ; H NMR (400 MHz, CDCl
3
) δ 7.82–7.79 (m, 2 H), 7.58–7.54
(m, 1 H), 7.49–7.45 (m, 2 H), 5.11 (t, J = 6.7 Hz, 1 H), 4.33–4.30 (m, 1 H), 4.10–4.06 (m, 2
H), 3.73 (dd, J = 10.5, 4.4 Hz, 1 H), 3.67 (dd, J = 10.5, 6.8 Hz, 1 H), 3.48 (brd, J = 7.8 Hz, 1
H), 2.70–2.58 (m, 2 H), 2.36 (brd, J = 7.8 Hz, 1 H), 2.17–2.09 (m, 1 H), 1.82–1.60 (m, 3 H),
1
.53–1.35 (m, 2 H), 1.33–1.06 (m, 2 H), 1.20 (s, 9 H), 1.20 (s, 3 H), 0.97–0.89 (m, 9 H), 0.94
13
(s, 9 H), 0.87 (s, 9 H), 0.14 (s, 3 H), 0.13 (s, 9 H); C NMR (100 MHz, CDCl
3
) δ 178.5,
1
3
40.9, 132.8, 130.3, 128.8, 128.5, 127.9, 73.8, 68.4, 62.9, 61.9, 44.3, 38.8, 36.6, 35.7, 35.6,
5.5, 29.9, 27.3, 26.3, 26.2, 25.5, 23.2, 20.6, 19.6, 18.7, 18.5, 15.3, –3.2, –4.7, –5.1, –5.2;
+
HRMS (ESI–TOF) calcd for C40
H
74
O
6
SSi
2
Na [M + Na] 761.4642, found 761.4632.
Sulfone 22b. To a solution of sulfone 20 (1.52 g, 3.03 mmol) in THF (24 mL) was added
NaHMDS (1.0 M in THF, 3.6 mL, 3.60 mmol) at –78 °C. The mixture was stirred at the same
temperature for 30 min. To the mixture was added allylic bromide 21b (1.22 g, 3.82 mmol) in
THF (4.0 mL + 1.0 mL + 1.0 mL) at –78 °C. The mixture was gradually warmed up to room
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1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
temperature for 2 h. The reaction was quenched with saturated aqueous NH
4
Cl. The mixture
SO
Concentration and column chromatography (hexane/EtOAc = 20:1) gave sulfone 22b (2.03 g,
was diluted with Et
2
O, washed with H
2
O and brine, and then dried over Na
2
4
.
3
0
9
1%): colorless oil; R
f
= 0.14 (hexane/EtOAc = 20:1); [α]
D
+17.7 (c 1.00, CHCl
3
); IR (neat)
–1 1
2956, 2929, 2858, 1728 cm ; H NMR (400 MHz, CDCl
3
) δ 7.82–7.79 (m, 2 H), 7.58–7.54
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
(m, 1 H), 7.49–7.45 (m, 2 H), 5.10 (t, J = 6.7 Hz, 1 H), 4.31–4.30 (m, 1 H), 4.11–4.05 (m, 2
H), 3.73 (dd, J = 10.5, 4.6 Hz, 1 H), 3.67 (dd, J = 10.5, 6.8 Hz, 1 H), 3.48 (brd, J = 8.3 Hz, 1
H), 2.71–2.58 (m, 2 H), 2.36 (brd, J = 6.8 Hz, 1 H), 2.17–2.09 (m, 1 H), 1.78–1.60 (m, 3 H),
1
.53–1.38 (m, 2 H), 1.31–1.08 (m, 2 H), 1.20 (s, 9 H), 1.20 (s, 3 H), 0.96 (d, J = 6.6 Hz, 3 H),
.94 (s, 9 H), 0.90 (d, J = 6.6 Hz, 3 H), 0.89 (d, J = 6.6 Hz, 3 H), 0.87 (s, 9 H), 0.14 (s, 3 H),
0
13
0.13 (s, 9 H); C NMR (100 MHz, CDCl
3
) δ 178.4, 141.0, 132.8, 130.3, 128.8, 128.5, 127.9,
73.8, 68.4, 62.9, 61.9, 44.3, 38.7, 36.6, 35.7, 35.6, 29.9, 27.3, 26.3, 26.2, 26.2, 25.5, 23.2, 20.6,
19.5, 18.7, 18.5, 15.3, –3.2, –4.7, –5.1, –5.2; HRMS (ESI–TOF) calcd for C40
H
74
O
6
SSi
2
Na
+
[
M + Na] 761.4642, found 761.4641.
3
Alcohol 23a. To a solution of lithium wire (2.23 g, 0.322 mol) in liquid NH (90 mL) was
added sulfone 22a (2.38 g, 3.22 mmol) in THF/tꢀBuOH (44 mL/22 mL) and THF (6.0 mL for
rinse) at –78 °C. The mixture was stirred at the same temperature for 5 min. The reaction was
quenched with a 1:1 solution of saturated aqueous NH
diluted with EtOAc, warmed up to room temperature, and stirred at the same temperature. The
mixture was washed with H O and brine, and then dried over Na SO . Concentration gave the
4
Cl and MeOH. The mixture was
2
2
4
mixture of the corresponding pivalate and alcohol, which was used for the next step without
further purification.
2 2
To a solution of the mixture obtained above in CH Cl (30 mL) were added pyridine (0.39
mL, 4.83 mmol) and PivCl (0.47 mL, 3.86 mmol) at room temperature. The mixture was
stirred at the same temperature for 11 h. The mixture was diluted with EtOAc, washed with
H
2
O and brine, and then dried over Na
2 4
SO . Concentration and short column chromatography
(hexane/EtOAc = 100:1) gave the corresponding pivalate (1.23 g), which was used for the
next step without further purification.
2 2
To a solution of the bisꢀTBS ether obtained above (1.23 g) in MeOH (10 mL) and CH Cl
(10 mL) was added CSA (142 mg, 0.615 mmol) at 0 °C. The mixture was stirred at the same
temperature for 1 h. The reaction was quenched with Et N. The mixture was diluted with Et O,
washed with H O and brine, and then dried over Na SO . Concentration and column
chromatography (hexane/EtOAc = 25:1) gave alcohol 23a (430 mg) and the bisꢀTBS ether
3
2
2
2
4
(617 mg).
2 2
To a solution of the bisꢀTBS ether recovered above (617 mg) in MeOH (5.0 mL) and CH Cl
(
5.0 mL) was added CSA (71.8 mg, 0.309 mmol) at 0 °C. The mixture was stirred at the same
N. The mixture was diluted with Et O,
O and brine, and then dried over Na SO . Concentration and column
temperature for 1 h. The reaction was quenched with Et
washed with H
3
2
2
2
4
chromatography (hexane/EtOAc = 25:1) gave alcohol 23a (310 mg, totally 741 mg, 47% in
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three steps): colorless oil; R
f
= 0.29 (hexane/EtOAc = 10:1); [α]
D
+3.8 (c 1.00, CHCl
3
); IR
–1
1
(
neat) 3520, 2956, 2930, 2862, 1731 cm ; H NMR (400 MHz, CDCl ) δ 5.09 (t, J = 6.6 Hz,
3
1
1
0
H), 4.14–4.04 (m, 2 H), 3.81–3.77 (m, 1 H), 3.61–3.53 (m, 2 H), 2.06–1.91 (m, 4 H),
.86–1.78 (m, 1 H), 1.71–1.53 (m, 3 H), 1.60 (s, 3 H), 1.48–1.15 (m, 6 H), 1.19 (s, 9 H),
13
3
.93–0.88 (m, 9 H), 0.91 (s, 9 H), 0.11 (s, 3 H), 0.09 (s, 3 H); C NMR (100 MHz, CDCl ) δ
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
178.5, 135.4, 124.4, 74.6, 64.3, 62.9, 47.2, 39.7, 38.8, 37.1, 35.6, 29.7, 27.9, 27.3, 26.0, 25.4,
2
4.7, 21.7, 19.5, 19.4, 18.2, 16.1, –4.2, –4.3; HRMS (ESI–TOF) calcd for C28
H
56
O
4
SiNa [M +
+
Na] 507.3846, found 507.3836.
3
Alcohol 23b. To a solution of lithium wire (1.91 g, 0.275 mol) in liquid NH (80 mL) was
added sulfone 22b (2.03 g, 2.75 mmol) in THF/tꢀBuOH (39 mL/19 mL) and THF (4.0 mL for
rinse) at –78 °C. The mixture was stirred at the same temperature for 10 min. The reaction
was quenched with a 1:1 solution of saturated aqueous NH
diluted with EtOAc, warmed up to room temperature, and stirred at the same temperature. The
mixture was washed with H O and brine, and then dried over Na SO . Concentration gave the
4
Cl and MeOH. The mixture was
2
2
4
mixture of the corresponding pivalate and alcohol, which was used for the next step without
further purification.
2 2
To a solution of the mixture obtained above in CH Cl (28 mL) were added pyridine (0.33
mL, 4.13 mmol) and PivCl (0.40 mL, 3.30 mmol) at room temperature. The mixture was
stirred at the same temperature for 11 h. The mixture was diluted with EtOAc, washed with
H
2
O and brine, and then dried over Na
2 4
SO . Concentration and short column chromatography
(
hexane/EtOAc = 100:1) gave the corresponding pivalate (1.37 g), which was used for the
next step without further purification.
To a solution of the bisꢀTBS ether obtained above (1.37 g) in MeOH (12 mL) and CH
2
Cl
2
(
12 mL) was added CSA (160 mg, 0.687 mmol) at 0 °C. The mixture was stirred at the same
temperature for 1 h. The reaction was quenched with Et
3
N. The mixture was diluted with Et
2
O,
washed with H
2
O and brine, and then dried over Na
2
SO
4
. Concentration and column
chromatography (hexane/EtOAc = 25:1) gave alcohol 23b (562 mg) and the bisꢀTBS ether
(366 mg).
To a solution of the bisꢀTBS ether recovered above (366 mg) in MeOH (3.0 mL) and CH
(3.0 mL) was added CSA (42.6 mg, 0.183 mmol) at 0 °C. The mixture was stirred at the same
temperature for 1 h. The reaction was quenched with Et N. The mixture was diluted with Et O,
washed with H O and brine, and then dried over Na SO . Concentration and column
2 2
Cl
3
2
2
2
4
chromatography (hexane/EtOAc = 25:1) gave alcohol 23b (169 mg, totally 731 mg, 55% in
29
D
three steps): colorless oil; R
f
= 0.29 (hexane/EtOAc = 10:1); [α]
3
+7.6 (c 1.20, CHCl ); IR
–1 1
(neat) 3475, 2956, 2929, 2858, 1731 cm ; H NMR (400 MHz, CDCl
3
) δ 5.09 (t, J = 7.1 Hz,
1
1
1
H), 4.14–4.04 (m, 2 H), 3.81–3.77 (m, 1 H), 3.61–3.53 (m, 2 H), 2.06–1.90 (m, 4 H),
.86–1.78 (m, 1 H), 1.71–1.51 (m, 2 H), 1.60 (s, 3 H), 1.48–1.26 (m, 6 H), 1.24–1.15 (m, 1 H),
13
.19 (s, 9 H), 0.93–0.88 (m, 9 H), 0.91 (s, 9 H), 0.11 (s, 3 H), 0.09 (s, 3 H); C NMR (100
MHz, CDCl
3
) δ 178.5, 135.4, 124.3, 74.6, 64.4, 62.9, 47.2, 39.7, 38.7, 37.1, 35.6, 29.7, 27.9,
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