A Formal Approach to Xylosmin and Flacourtosides E and F: Chemoenzymatic Total Synthesis of the Hydroxylated Cyclohexenone Carboxylic Acid Moiety of Xylosmin

Article abstract of DOI:10.1021/acs.orglett.7b00194

The hydroxylated cyclohexenone carboxylic acid moiety of xylosmin was synthesized in eight steps from benzoic acid. The key steps in the synthesis involved the enzymatic dihydroxylation of benzoic acid by the whole cell fermentation with Ralstonia eutroph

Full text of DOI:10.1021/acs.orglett.7b00194

Letter
pubs.acs.org/OrgLett
A Formal Approach to Xylosmin and Flacourtosides E and F:
Chemoenzymatic Total Synthesis of the Hydroxylated
Cyclohexenone Carboxylic Acid Moiety of Xylosmin
Mukund Ghavre, Jordan Froese, Brennan Murphy, Razvan Simionescu, and Tomas Hudlicky*
Department of Chemistry and Centre for Biotechnology, Brock University, St. Catharines, ON L2S 3A1, Canada
S
* Supporting Information
ABSTRACT: The hydroxylated cyclohexenone carboxylic acid moiety of xylosmin was synthesized in eight steps from benzoic
acid. The key steps in the synthesis involved the enzymatic dihydroxylation of benzoic acid by the whole cell fermentation with
Ralstonia eutrophus B9, and Henbest epoxidation. Early attempts led to the synthesis of a C6 epimer of the methyl ester of the
hydroxylated cyclohexenone carboxylic acid moiety. The absolute stereochemistry of an advanced intermediate was confirmed by
X-ray crystallography. Complete characterization of the previously reported but not fully characterized hydroxylated
cyclohexenone carboxylic acid is provided.
In 2013, the cyclohexenone carboxylate portion of xylosmin
was reported as the product of base-catalyzed hydrolysis of a
compound related to xylosmin 1 (having a free hydroxymethyl
group on the monosaccharide residue, Figure 1).5 No physical
or spectral data, other than the optical rotation (reported as
+13.2 (c 0.07, MeOH) and +12.6 (c 0.06, MeOH)) were
provided for the free carboxylic acid. In this paper, we report
the chemoenzymatic synthesis, starting from benzoic acid, of
the hydroxylated cyclohexenone carboxylic acid fragment
common to xylosmin (1),6 flacourtoside E (2), and flacourto-
side F (3) (Figure 1).7
Our initial approach to the target cyclohexenone fragment
employed the enzymatic dihydroxylation of benzoic acid as
shown in Scheme 1. The use of arene dioxygenases for
desymmetrization of common aromatic compounds has gained
prominent utility in the synthesis of natural products. Toluene
and other arene dioxygenases that are overexpressed in E. coli
vectors are frequently employed in chemoenzymatic syntheses,
and this topic has been reviewed on several occasions.8 Less
common metabolites are the ipso-diols of type 5a and 5b9
produced by dihydroxylation of benzoic acids with benzoate
dioxygenase through the whole cell fermentation with Ralstonia
eutrophus B9.10 The ipso-diol 5a,10,11 derived from sodium
benzoate 4, was protected as its corresponding acetonide 6 and
carefully treated with bromine to provide the β-lactone 7, as
hikungunya and dengue viruses infect roughly 500 million
Cpeople per year, predominantly in the tropical zone of the
eastern hemisphere. Chikungunya and dengue fevers are
generally characterized by symptoms such as headaches, fever,
fatigue, vomiting, nausea, skin rashes, and blood-thinning and
can be fatal.1−3
No effective vaccines are available, but broad-spectrum
antivirals have had some effect in tackling the overarching virus,
although the greatest levels of success have been observed in
addressing the associated symptoms, usually through either
retroactive or preventative measures. Phenolic glycosides, found
in the flacourtia ramontchi plant of Madagascar, have shown
replicative inhibition against these arboviral diseases. Inhibitory
plant extracts containing glycosylated derivatives of 2-hydroxy-
methylhydroquinone as a common motif have been shown to
be effective against these viral diseases (Figure 1).4−7
Figure 1. Naturally occurring phenolglycosides found in the flacourtia
plant genus.7
Received: January 19, 2017
© XXXX American Chemical Society
A
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Unfortunately, the original plan for the double deprotection of
14 in the final step failed and the treatment of 14 under the
reported conditions [Ba(OH)2, or NaOMe/MeOH] resulted
only in decomposition to an aromatic product. Because of this
turn of events, the same sequence was executed again with a
silyl protecting group instead of the benzoate at C2, as shown
in Scheme 3.
Scheme 1. Synthesis of Enone 10 from Sodium Benzoate
Scheme 3. Failed Attempts toward the Synthesis of Enone 15
shown in Scheme 1. A similar protocol was reported by Banwell
in a formal approach to the C-ring of vinblastine.12
Treatment of 7 with sodium methoxide in methanol
generated methyl ester 8 from lactone 7 and provided, through
the intramolecular displacement of the bromide by the
intermediate alkoxide, epoxide 8. Cyclohexenyl epoxide 8 was
considered as a putative precursor to the conduritol carboxylate
unit found in compounds 1−3.
A mild epoxide opening was achieved with catalytic Bi(OTf)3
in acetonitrile and water,13,14 leading to opening of the
cyclohexenyl epoxide at the allylic position without the loss
of either the ester or acetonide protecting groups. The selective
oxidation of the allylic alcohol in diol 9 proved to be more
difficult than initially expected. Surprisingly, oxidation with
manganese dioxide always led to the 1,2-diketone product and
not to the desired hydroxyenone 10. Other oxidants (DDQ,
IBX, PIFA) either proved ineffective or led to complex
mixtures. Ultimately, when IBX was dissolved in the ionic
liquid [bmim]Cl15 and water, the selective oxidation of the
allylic alcohol to the desired enone 10 was achieved. The
expected epimerization of the C6 hydroxyl failed under both
Mitsunobu conditions and base catalysis. The reported
conditions for hydrolysis of the phenolglycosides (Ba(OH)2,
80 °C)5 failed as well and led only to extensive decomposition.
To approach the synthesis of the natural stereoisomer a new
strategy was devised that utilized ipso-diol 5b, which was
converted to methyl ester 11.11,16 Protection of the distal
alcohol (with reference to the carboxylate) in 11 as a p-
bromobenzoate was followed by the hydroxyl-directed
Donohoe osmylation17 to furnish triol 13 in a stereo- and
regioselective fashion, Scheme 2.
The secondary silyl ether was synthesized by a previously
published sequence18 and subjected to Donohoe dihydrox-
ylation. Unfortunately, this dihydroxylation afforded the
trihydroxy regioisomer 18, as opposed to the regioisomer of
type 17 as was previously observed. In order to confirm the
assignment of structure 18, the allylic alcohol was oxidized to
1
afford enone 19. The H NMR spectrum of 19 did not show
the expected ABX pattern of the olefinic signals and the proton
at C2, but rather the presence of two clean doublets (δ 6.90 and
6.19, J = 9.9 Hz) corresponding to the olefinic hydrogens of the
enone and therefore confirming the assignment of regiochem-
istry as that shown in 19. The relative stereochemistry of the
alcohol moieties of compounds 18 and 19 was not further
investigated, but it may be assumed that these would be anti- to
the TBS group, with the oxidation proceeding through a classic
Upjohn mechanism, instead of the hydrogen bond-controlled
Donohoe pathway.
In light of these observations, we decided to return to an
approach based on the use of benzoyl protecting groups. To
this end, the p-nitrobenzoyl ester 20 was produced from ester
11, as shown in Scheme 4. Benzoyl ester 20 was then subjected
to a Henbest epoxidation to afford epoxide 21,19 which was
treated with Bi(OTf)3 as previously described to effect the
epoxide opening.
Oxidation of 13 with IBX in ionic liquid failed, in contrast to
the successful oxidation of 9; however, the allylic alcohol in 13
was cleanly oxidized to the desired enone 14 with DDQ.
In order to confirm the absolute stereochemistry of triol 22,
this compound was subjected to benzoylation with p-
bromobenzoyl chloride to afford the tetraester 25. This
compound was analyzed by X-ray crystallography. The
structure (Figure 2) confirmed the absolute stereochemistry
of 22 as shown in Scheme 4. Triol 22 was then subjected to
selective oxidation with DDQ to furnish enone 23. In contrast
to previous observations involving the failure to hydrolyze p-
bromobenzoate 14, the p-nitrobenzoate ester in enone 23 was
smoothly hydrolyzed with potassium carbonate (0.5 equiv) in
methanol over 45 min to afford 24, the methyl ester of the
hydroxylated cyclohexenone carboxylic acid. Further hydrolysis
(K2CO3, 0.5 equiv, H2O, rt, 2 h) provided the desired
hydroxylated cyclohexenone carboxylic acid 15 as a solid [mp =
135−137 °C (methanol/pentane, 1:3); [α]2D2 −49.4 (c = 0.5,
Scheme 2. Synthesis of Diester 14 from Diol 5b
B
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directed epoxidation of p-nitrobenzoate ester 20 and
subsequent manipulations that included mild conditions for
the hydrolysis of the ester moieties. Carboxylic acid 15 is not a
very stable compound (degradation occurs at room temper-
ature with notable decomposition within 12 h or so;
degradation proceeds at a slower rate even at −80 °C). In
the original report,5 no data were provided for the free
carboxylic acid except for the optical rotation of +13.2 (c 0.07,
MeOH) and +12.6 (c 0.06, MeOH). The hydrolysis of
compound 26 (Figure 3), closely related to xylosmin and
Scheme 4. Synthesis of 15, the Hydroxylated Cyclohexenone
Carboxylic Acid Moiety of Xylosmin
Figure 3. Free hydroxyl derivative of xylosmin 26.5
methanol)]. This compound was not fully characterized in the
original report,5 and therefore this work represents the first
synthesis of the hydroxylated cyclohexenone carboxylic acid
moiety of xylosmin.
The previous attempts at the base-catalyzed isomerization of
the C6 stereogenic center in enone 10 proved unsuccessful.
Nevertheless, in order to ascertain that the stereochemical
integrity of the C6 stereogenic center remained unchanged
during the base hydrolysis of 24, the freshly prepared free acid
15 was converted back to the methyl ester 24 with
diazomethane. This ester was found to be identical (vide
NMR and optical rotation) with the material used in the final
hydrolysis.
Acid 15, the hydroxylated cyclohexenone carboxylic acid
fragment of xylosmin, was synthesized, and the absolute
stereochemistry of ester 25 was determined to be as reported
for the natural product (the structure of xylosmin was
confirmed by X-ray crystallography6). Several approaches
were investigated to prepare the hydroxylated cyclohexenone
carboxylic acid 15, and the successful one was based on the
having a levorotatory optical rotation, under acidic conditions
was reported5 to give D-glucose as confirmed by the optical
rotation value ([α]2D5 +50.5 (c 0.10, H2O) of the sugar obtained.
The basic hydrolysis of 26, as reported in the literature,5
provided material, assumed to be the hydroxylated cyclo-
hexenone carboxylic acid fragment, 15, with a dextrorotatory
optical rotation. Our experience with the hydrolysis of esters
14, 23, and 24 raises some questions whether the hydroxylated
cyclohexenone carboxylic acid fragment was ever obtained
under the harsh basic hydrolysis conditions reported. In
addition, as the sign of the optical rotation of the carboxylic
acid 15 obtained by synthesis is opposite to that reported for
the material obtained by isolation, we may conclude that the
material reported5 from the basic hydrolysis of 26 was simply
contaminated with the free sugar. The authors of the original
report confirmed by correspondence that they obtained no
other analytical or spectral data for the free carboxylic acid.
Based on the X-ray confirmation of structure 25 as shown,
and given the fact that epimerization of the hydroxyl group at
the α-position in ketones of type 14, 23, and 24 was not
Figure 2. Crystal structure of ester 25.
C
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detected, we are reasonably certain that 15 was attained with
the correct absolute stereochemistry, as that determined for the
natural product.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.7b00194.
■
S
Experimental and spectral data are provided for the
compounds 5b, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 18,
19, 20, 21, 22, 23, and 24 (PDF)
X-ray data is provided for 25 (CIF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: thudlicky@brocku.ca.
ORCID
Tomas Hudlicky: 0000-0003-3239-6815
Notes
The authors declare no competing financial interest.
(18) Palframan, M. J.; Kociok-Kohn, G.; Lewis, S. E. Org. Lett. 2011,
13, 3150.
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(b) Prileschajew, N. Ber. Dtsch. Chem. Ges. 1909, 42, 4811.
ACKNOWLEDGMENTS
■
The authors are grateful to the following agencies for financial
support of this work: Natural Sciences and Engineering
Research Council of Canada (NSERC) (Idea to Innovation
and Discovery Grants), Canada Research Chair Program,
Canada Foundation for Innovation (CFI), TDC Research, Inc.,
TDC Research Foundation, the Ontario Partnership for
Innovation and Commercialization (OPIC), and The Advanced
Biomanufacturing Centre (Brock University). The skillful
assistance of Helen Endoma (Brock University) in the
fermentations is gratefully appreciated. Hesham Shamshoum’s
(Brock University) help in the preparation of starting materials
is appreciated. We also thank Professor Marko Mihovilovic
(Vienna University of Technology) for providing a fresh culture
of Ralstonia eutrophus B9.
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