Synthesis Toward and Stereochemical Assignment of Clathsterol: Exploring Diverse Strategies to Polyoxygenated Sterols

Article abstract of DOI:10.1021/acs.orglett.6b01029

Herein we describe a synthesis of the trisulfate derivative of clathsterol (1), a marine sterol endowed with impressive structural features and moderate inhibitory activity against HIV-1 reverse transcriptase. By synthesizing two possible isomers of the side chain, the stereochemistry of 1 is assigned. In creating chiral side chains from steroidal lactone, our strategies, including an addition/reduction procedure to give C22R-OH, an epoxide-opening reaction, and a [3.3]-rearrangement to induce the generation of C24S-Et and C24R-Et respectively, are highly flexible and complementary to each other.

Full text of DOI:10.1021/acs.orglett.6b01029

Letter
pubs.acs.org/OrgLett
Synthesis Toward and Stereochemical Assignment of Clathsterol:
Exploring Diverse Strategies to Polyoxygenated Sterols
Tao Zhou, Feng Feng, Yong Shi,* and Wei-Sheng Tian*
Key Laboratory of Synthetic Chemistry of Natural Substances, Shanghai Institute of Organic Chemistry, Chinese Academy of
Sciences, 345 Lingling Road, Shanghai 200032, China
S
* Supporting Information
ABSTRACT: Herein we describe a synthesis of the trisulfate derivative of
clathsterol (1), a marine sterol endowed with impressive structural features and
moderate inhibitory activity against HIV-1 reverse transcriptase. By synthesizing two
possible isomers of the side chain, the stereochemistry of 1 is assigned. In creating
chiral side chains from steroidal lactone, our strategies, including an addition/
reduction procedure to give C22R−OH, an epoxide-opening reaction, and a [3.3]-
rearrangement to induce the generation of C24S-Et and C24R-Et respectively, are
highly flexible and complementary to each other.
olyoxygenated steroids are widely isolated from marine and
Pterrestrial organisms and possess a variety of pharmaceuti-
cally attractive bioactivities.1 Their diverse and complex
structures have attracted extensive and fruitful synthetic
endeavors.2 For targets with similar structural features,
developing robust and flexible synthetic strategies are
encouraged. Herein we report our efforts on the synthesis of
clathsterol (1, Figure 1), the strategies of which might be
applicable to many natural sterols.
them as targets for two reasons. First, in natural sterols, most of
the 22,23-diols, if any, are cis- and 22R,23R-configured.4 Second,
the configuration of C22 in 2 was assigned as S, we believed that
the configuration of C22 in 1 was inverted during the ring-closing
reaction although certain unambiguity of the mechanism might
exist.
Our retrosynthetic analysis is depicted in Scheme 1. We
considered allylic alcohol 5 as a common precursor. The syn,syn-
unit of 3 was to be constructed through an epoxidation/epoxide-
opening process, while the syn,anti-unit of 4 was to be established
through a Claisen-type rearrangement followed by a dihydrox-
ylation. Alcohol 5 could be stereoselectively prepared by an
Scheme 1. Retrosynthetic Analysis
Figure 1. Structures of clathsterol.
Clathsterol (1) is a sterol sulfate isolated by Kashman and co-
workers from the Red Sea sponge Clathria sp. in 2001.3 It inhibits
human deficiency virus type 1 (HIV-1) reverse transcriptase at a
concentration of 10 μM. Kashman assigned the structure of 1
mainly by interpretation of spectral data of 1 and its derivative 2,
but left three chiral centers (C22, C23, and C24) unassigned due
to the conformational lability of the side chain, thus arising eight
possible isomers. Possessing three sets of vicinal diols that
present differentially as 15α-acetate, 22,23-dibutylate, and 2β,3α-
disulfate salt, along with its unassigned side-chain configuration,
1 was a challenging and intriguing target for us.
We narrowed the eight isomers of 1 down to 22R,23R,24S-3
(syn, syn) and 22R,23R,24R-4 (syn, anti) (Scheme 2) and set
Received: April 9, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.6b01029
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Letter
support further exploration. Therefore, 7 was reduced with
LiAlH4 and the primary C22-OH of the resultant diol was
selectively protected as TBS ether to give 8. The exposed
secondary C16-OH in 8 was then converted into xanthate, which
underwent Chugaev elimination to install the required C15−
C16 double bond. Desilylation and Jones oxidation provided acid
10 in 89−93% yield from 8 on the 30 g scale. With this six-step
procedure we accumulated 10 more rapidly (only one
purification was needed).
Scheme 2. Synthesis of 22,23,24-syn,syn Isomer
Both the C2−C3 and the C15−C16 double bonds in 10 were
epoxidized with mCPBA, and the diepoxide was treated with 2 N
H2SO4 to open the epoxides intra- and intermolecularly, giving
lactone 11 after protecting all the hydroxyl groups in 6 as
triethylsilyl (TES) ethers. Then 11 was treated with isopentynyl
lithium and the resultant hemiketal was reduced with NaBH4 to
afford diol 13 in 79% yield on the multigram scale as a single
isomer. Although the mechanism of the excellent stereocontrol
observed herein was not fully understood, this lactone addition/
reduction process provides a rapid access to C22R-configured
sterols,7 which are difficult to obtain with high stereoselectivity
from 22-aldehydes.8 Partial hydrogenation of the triple bond of
13 to cis-double bond through Rosenmund reduction gave 5, and
a highly stereoselective epoxidation of 5 directed by its C22R−
OH9 provided 14 in 93% yield.
To install the correct configuration of the side chain of
brassinolide, Mori and co-workers developed an effective
procedure for opening a similar epoxide with AlMe3.8b Rather
smoothly a similar reaction proceeded by treating 14 with a
premixed solution of 10 equiv of Et3Al and 0.6 equiv of n-BuLi in
cyclohexane at ambient temperature for 24 h, providing the
desired product 15 in 48% yield, along with 16, the intra-
molecular epoxide opening product, in 36% yield. Clearly, under
the activation of Lewis acid AlEt3, the epoxide in 14 was attacked
by the adjacent C16-OH, hence generating 16.
Before optimizing this epoxide-opening reaction, two butyrate
esters at C22 and C23 were installed by treating 15 with butyrate
anhydride in pyridine for 2 days in 84% yield. We found that both
1
the C22−H and the C23−H displayed as single peaks in H
NMR, which were different from those of natural products (d for
C22−H and dd for C23−H). Further verification was needed.
Acid hydrolysis of the butyrate, as reported by Kashman and co-
workers, followed by acetylation of the crude product with Ac2O
in pyridine provided triacetate 17 in 35% yield. The 2D NOESY
analysis assigned the configuration of C22 in 17 to be S (cross
peaks of C22−H and C16−H/C21−Me), which suggested that
the ring-closing reaction is a simple SN2 process without
neighbor group participation involved, and thus, C22 of
1
clathsterol is R-configured. However, although the H NMR
data of 17 highly resemble those of 2, the 13C NMR signals are
apparently different at C18−C29 area (see Supporting
Information for a detailed list). The correct configuration of 1
is not 22R,23R,24S (syn, syn). Focus was then turned to the
22R,23R,24R-isomer 4.
To synthesize the syn,anti-product, we planned to transfer the
stereochemistry from C22 to C24 through a Claisen-type
rearrangement (Scheme 3).8f,g,10 As in 14, the unprotected
C16−OH participated in the reaction again. Heating 5 with
excess triethyl orthoacetate in the presence a catalytic amount of
propionic acid, the typical Johnson−Claisen rearrangement
conditions, produced trace or no desired product 18. An
intramolecular substitution occurred, forming a THF ring. We
found that it was propionic acid that caused the side reaction.
Refluxing a solution of 5 in triethyl orthoacetate without
addition of isopentynyl lithium to lactone 6 followed by NaBH4
reduction and Rosenmund reduction. Two trans-diol units in
lactone 6 were to be introduced via epoxide-opening reactions of
a diepoxide derived from lactone 7. To gain more isomers for the
situation of clathsterol (1) being neither 3 nor 4, we could change
the configuration of C22 in 5 from R to S, C23−C24 double
bond from cis to trans, and diol-forming processes on steroidal
core from the epoxide-opening reaction to dihydroxylation.
We have reported a concise synthesis of 6.5 Therein lactone 7
was prepared from tigogenin in three steps and on the 50 g scale
with an overall yield of 75%,6 and diene acid 10 was prepared
through a three-step procedure featuring a Chugaev elimination
of xanthate derived from 16-hydroxyl-22-carboxylate dianion.
However, the xanthate was obtained in unstable yields (30−
50%) and was difficult to purify. We needed a robust method to
B
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Scheme 3. Synthesis of 19 by Claisen-Type Rearrangements
Scheme 4. Synthesis of 29
SO3·py in DMF at 90 °C (24 h, or microwave, 5 min) and
treatment with NaHCO3 (ion exchange) directly formed the
trisulfate sodium salt 29 in high yield. Lowering temperature and
employing other sulfation reagents15 gave complex reaction
mixture, suggesting that the reactivities of three hydroxyl groups
toward these sulfation reagents are too close to be effectively
differentiated, thus giving mixtures of random sulfated products
or fully sulfated product. Because these sulfate sodium salts were
unable to isolate from a mixture unless the reaction was clean, we
did not obtain a sample pure enough for collecting satisfactory
spectral data of clathsterol (1) although it was generated as minor
product in some cases. Selective sulfation method for substitutes
with many hydroxyl groups, such as carbohydrates, whose sulfate
derivatives often have biological activities, is valuable and needs
further investigation.
propionic acid for 2 h successfully delivered 18 in good yield,
which was reduced with LiBHEt3 to give 19. Likewise, other
[3.3]-rearrangements that could proceed under neutral con-
ditions also gave good results. Employing a modified Claisen
rearrangement of allylic vinyl ethers,11 19 was prepared in 65%
overall yield through a sequence involving a NaH/KH promoted
Michael-type addition of 5 to aryl vinyl sulfoxide 20 at room
temperature, a concurrent thermal elimination and [3.3]-
rearrangement, and a NaBH4 reduction of the resulting aldehyde
21. Allylic alcohol 5 also underwent Eschenmoser−Claisen
rearrangement12 to furnish amide 23, which was easily reduced to
19 with LiBHEt3. Finally, the C29−OH of 19 was removed
through mesylation and reduction with LiAlH4, giving 24 in
excellent yield.
In summary, based on a flexible synthetic strategy we assigned
the clathsterol (1) as 22R,23R,24R by comparing the NMR data
of the synthetic samples with those of natural samples. The
synthesis of trisulfate sodium salt 29 was accomplished in 21
steps and 11% overall yield starting from 7. Noteworthy
transformations include a convenient and scalable nine-step
procedure for lactone 11, an addition of alkynyl lithium to
lactone followed by NaBH4 reduction to stereoselectively give
C22R−OH, a selective epoxidation/epoxide-opening procedure
to install the 22R,23R,24S-configured side chain and Claisen-
type rearrangements under nonacidic conditions to transfer the
stereochemistry from C22 to C24. Our endeavors allow quick
access to C22R−OH compounds from steroidal lactone and
C23R-, C24R/S-stereochemistry from C22R−OH and therefore
are valuable for establishing chiral side chains that are frequently
associated with the unusual structural features of a great number
of natural sterols.
The C15−OH of 24 was selectively exposed by removing the
TES group with a catalytic amount of PPTS in MeOH/DCM
and acetylated with Ac2O to provide 25 in excellent yield
(Scheme 4). Dihydroxylation and esterification of the hydroxyl
groups were then investigated. Dihydroxylation of 25 gave the
products in excellent yield and moderate selectivity (2/1 by 1H
NMR), favoring the desired 26.13,14 The newly generated C22-
OH and C23-OH in 26 were acylated with butyrate anhydride in
pyridine and the TES ethers at ring A were removed with HCl,
giving 27. The NMR data of D ring and side chain of 27 were in
good agreement with those of 1 since they have the same side
chain, which was remote from the variation on the steroidal
skeletons. To further verify the structure of 1, we also tried to
convert 27 to 2. Acid hydrolysis of 27 (6 N HCl, dioxane, reflux, 6
h) unexpectedly removed its C23-butylate, giving not 2 but 28
after treating with Ac2O in pyridine. The 2D NOESY analysis
assigned the configuration of C22 in 28 to be S, thus verifing the
stereochemistry of 26. Pleasingly, we found that the NMR data of
28 were in good agreement with those of 2. We assigned the
correct configuration of 1 as 22R,23R,24R (syn,anti).
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.6b01029.
Then we reached the endgame stage of this synthesis. The
C16−OH is usually unreactive toward many reagents due to the
steric hindrance. Therefore, we considered the selective sulfation
of 27 to be feasible but found it quite challenging. Reaction with
Experimental details, spectral data, and 1H and 13C NMR
spectra of all the new compounds (PDF)
C
DOI: 10.1021/acs.orglett.6b01029
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AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: shiong81@sioc.ac.cn.
*E-mail: wstian@sioc.ac.cn.
Notes
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We thank the National Natural Science Foundation of China
(21272258, 21572248) for financial support.
■
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