Synthetic Studies on Amphirionin-5: Stereochemical Assignment/Reassignment of the C1-C9 Portion through Stereodivergent Synthesis

Article abstract of DOI:10.1021/acs.orglett.5b03346

Synthesis of four diastereomers of the C1-C12 fragment of amphirionin-5 has been achieved in a convergent and stereodivergent manner. Detailed comparison of the ¹H and ¹³C NMR data of each compound with those reported for the natural product led to not only the stereochemical assignment of the relative configuration of the C4/C5 stereogenic centers but also reassignment of the proposed relative configuration at C9 of amphirionin-5.

Full text of DOI:10.1021/acs.orglett.5b03346

Letter
pubs.acs.org/OrgLett
Synthetic Studies on Amphirionin-5: Stereochemical Assignment/
Reassignment of the C1−C9 Portion through Stereodivergent
Synthesis
Moemi Kanto and Makoto Sasaki*
Graduate School of Life Sciences, Tohoku University, Katahira 2-1-1, Aoba-ku, Sendai 980-8577, Japan
S
* Supporting Information
ABSTRACT: Synthesis of four diastereomers of the C1−C12
fragment of amphirionin-5 has been achieved in a convergent and
1
stereodivergent manner. Detailed comparison of the H and 13C
NMR data of each compound with those reported for the natural
product led to not only the stereochemical assignment of the
relative configuration of the C4/C5 stereogenic centers but also
reassignment of the proposed relative configuration at C9 of
amphirionin-5.
inoflagellates of the genus Amphidinium are an enormously
rich source of structurally diverse secondary metabolites of
configuration has also remained undefined. These stereo-
chemical problems can only be addressed efficiently using a
synthetic approach.
Interestingly, this linear polyketide natural product was found
to exhibit potent proliferation-promoting activity on murine
bone marrow stromal ST-2 cells (282%) and murine osteoblastic
MC3T3-E1 cells (320%) at a dose of 10 ng/mL, whereas it did
not induce cellular differentiation or cellular morphological
changes at a dose range of 0.001−1000 ng/mL and also exhibited
no cytotoxicity at high doses (1−10 μg/mL).2
D
complex molecular architecture with potent biological activities.
In particular, a number of potent cytotoxic macrolides,
amphidinolides and iriomoteolides, have been isolated from
Amphidinium sp. to date.1 Recently, novel complex tetrahy-
drofuran-containing linear polyketide natural products with
intriguing biological activities have been isolated from
Amphidinium sp. by Tsuda and co-workers.2−4 Of these,
amphirionin-5 (1, Figure 1) was isolated in 2014 by Tsuda and
As part of our program toward the total synthesis and
complete stereochemical assignment of amphirionin-5, we
herein report the stereodivergent synthesis of four diastereo-
meric C1−C12 fragments and comparison of their NMR data
with those reported for the natural product. This has led both to
an assignment of the relative configuration of the C4/C5
stereogenic centers and a reassignment of the proposed relative
configuration at C9 of amphirionin-5.
Our stereochemical-determination strategy for establishing
the relative configuration of the C4/C5 stereogenic centers of
amphirionin-5 relied on the synthesis of two possible
diastereomers of the epoxide-containing C1−C12 fragments 2
and 3 (Figure 1). Comparison of their NMR spectroscopic data
with those of the natural product would assign the relative
configuration of C4/C5.6−8 Our retrosynthetic plan for the C1−
C12 fragments 2 and 3 is depicted in Scheme 1. We envisioned
that the 2,5-trans-substituted tetrahydrofuran ring of 2 and 3
would be constructed through a domino Sharpless asymmetric
dihydroxylation (SAD)9/stereospecific cyclization of mesylates 4
and 5, respectively.10 The two requisite diastereomeric epoxides
4 and 5 would be derived by branching from allylic alcohol 6 by
Katsuki−Sharpless asymmetric epoxidation11 using (+)- or
Figure 1. Structures of amphirionin-5 (1) and two possible
diastereomeric C1−C12 fragments 2 and 3.
co-workers from cultivated algal cells of the benthic dinoflagellate
Amphidinium sp. (KCA09053 strain) collected off the coast of
Iriomote Island, Okinawa Prefecture, Japan.2 The gross structure
and partial stereochemical assignment of amphirionin-5 was
elucidated on the basis of 2D-NMR data and J-based configura-
tional analysis5 and found to consist of a linear polyketide
skeleton containing two tetrahydrofuran rings, a trans-epoxide,
and 11 stereogenic centers. However, despite extensive NMR
studies, the relative configurations of the C4/C5 stereogenic
centers and the stereochemistry of the two isolated stereogenic
centers at C12 and C26 could not be resolved, and the absolute
Received: November 20, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.5b03346
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Letter
20:1). The p-methoxyphenylmethyl (MPM) group of 13 was
oxidatively removed with DDQ (87%), and the resultant primary
alcohol was oxidized to the corresponding aldehyde. Treatment
of the aldehyde with lithiated dimethyl methylphosphonate,
followed by oxidation with tetra-n-propylammonium perruthen-
ate (TPAP)/N-methylmorpholine N-oxide (NMO),18 provided
β-keto phosphonate 14 (66% yield for the three steps). HWE
reaction of 14 with isobutyraldehyde (8) under Masamune−
Roush conditions (i-Pr2NEt, LiCl, MeCN)19 led to (E)-enone 7
in 99% yield as a single stereoisomer (E/Z > 20:1). Finally, CBS
reduction12 of enone 7 using (R)-2-methyl-CBS-oxazaborolidine
15 provided the desired allylic alcohol 6 in 93% yield with an 11:1
diastereomer ratio.20 The absolute configuration of the C521
stereogenic center was unambiguously established by a modified
Mosher analysis.22
Scheme 1. Retrosynthesis of Two Diastereomeric C1−C12
Fragments 2 and 3
Katsuki−Sharpless asymmetric epoxidation11 of allylic alcohol
6 using (+)-diisopropyl tartrate (DIPT) as a chiral ligand
delivered epoxy alcohol 1623 in 90% yield with high
diastereoselectivity (dr ca. 23:1) (Scheme 3). Alcohol 16 was
converted to the corresponding mesylate 4 (MsCl, Et3N), which
was then subjected to SAD using AD-mix-β.9 Diastereoselective
dihydroxylation and concomitant stereospecific cyclization
proceeded smoothly to form a tetrahydrofuran ring, and the
desired C1−C12 fragment 2 was obtained in 90% yield for the
two steps. The 2,5-trans configuration of the tetrahydrofuran ring
in 2 was confirmed by means of HMBC spectra and NOE data,
and the absolute configuration of the C9 stereogenic center was
unambiguously established by a modified Mosher analysis.22,23
The diastereomeric fragment 3 was prepared in a similar
fashion from allylic alcohol 6 via epoxy alcohol 17 (Scheme 3). In
this case, Katsuki−Sharpless asymmetric epoxidation of 624 with
(−)-DIPT is a typical “mismatched” pair11b,c and thus resulted in
a 1.4:1 mixture of epoxides 17 and 16, which were readily
separated by flash column chromatography on silica gel. Epoxide
17 was advanced by employing the sequence described in the
conversion of 16 to 2 to furnish 3 (86% yield, two steps).23
The 1H and 13C NMR chemical shifts in the C1−C9 region of
the two diastereomeric C1−C12 fragments 2 and 3 were
compared in detail with those of the corresponding moiety of the
natural product.25 As shown in Figure 2, the 13C NMR chemical
shifts in the C1−C6 region of fragment 2 matched with those
reported for the natural product within 0.7 ppm, while the
diastereomer 3 displayed obviously different chemical shifts. In
particular, the observed 13C NMR chemical shifts for C5 and C6
of 3 distinctively deviated from those of the natural product (Δδ
> 1.0 ppm). These results strongly suggested that natural
amphirionin-5 has the (3S*,4S*,5R*)-configuration shown for
structure 2. In contrast, there were large and similar discrepancies
in the 13C NMR chemical shifts for C7, C9, and C30 in the right-
hand region of both compounds 2 and 3. From these significant
deviations in the NMR data between the synthetic fragments 2/3
and the natural product, we inferred that the C9 stereogenic
center of amphirionin-5 might be misassigned and that the most
likely configuration of the C1−C9 portion of amphirionin-5 is
represented by the revised structure 18 (see Scheme 4).
(−)-tartrate ester. Compound 6 would be accessed by means of
Corey−Bakshi−Shibata (CBS) reduction12 of enone 7, which in
turn would be assembled from three fragments, isobutyraldehyde
(8), aldehyde 9, and sulfone 10, through Julia−Kocienski
olefination13 and Horner−Wadsworth−Emmons (HWE) re-
action, in a convergent manner.
The synthesis of allylic alcohol 6 started with the known imide
11.14 Reductive removal of the chiral auxiliary in 11 with LiBH4
(MeOH, THF, 0 °C)15 afforded alcohol 12 in 92% yield
(Scheme 2). Parikh−Doering oxidation16 of 12 provided
aldehyde 9, which was then coupled with the known sulfone
1017 through Julia−Kocienski olefination (KHMDS, DME, −55
°C)13 to give (E)-alkene 13 in 64% yield from alcohol 12 (E/Z >
Scheme 2. Synthesis of Allylic Alcohol 6
Thus, inversion of the C9 hydroxy group of 2 and 3 was
performed using modified Mitsunobu conditions (p-
NO2C6H4CO2H, Ph3P, diethyl azodicarboxylate (DEAD),
THF)26 followed by methanolysis (K2CO3, MeOH) to afford
alcohols 18 and 19, respectively, as shown in Scheme 4. Their
NMR data were again compared with those of the natural
product (Figure 3). Clearly, the 13C NMR chemical shifts in the
C1−C9 region of diastereomer 18 were virtually identical to
B
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Scheme 3. Synthesis of the Diastereomeric C1−C12 Fragments 2 and 3
Figure 2. Differences in 13C NMR chemical shifts between amphirionin-
5 (125 MHz) and synthetic fragments 2 and 3 (150 MHz). Δδ = δ
(natural product) − δ (synthetic fragment) in ppm (CDCl3).
Figure 3. Differences in 13C NMR chemical shifts between amphirionin-
5 (125 MHz) and synthetic fragments 18 and 19 (150 MHz). Δδ = δ
(natural product) − δ (synthetic fragment) in ppm (CDCl3).
Scheme 4. Synthesis of Diastereomers 18 and 19
In conclusion, four diastereomers C1−C12 fragments of
amphirionin-5 have been synthesized in a stereodivergent
manner. The key features of the synthesis route include (1)
convergent synthesis of the common intermediary allylic alcohol
by employing Julia−Kocienski olefination, Horner−Wads-
worth−Emmons reaction, and Corey−Bakshi−Shibata reduc-
tion and (2) efficient construction of the 2,5-trans-substituted
tetrahydrofuran ring by a domino Sharpless asymmetric
dihydroxylation/stereospecific cyclization. Comparison of the
NMR data of the four diastereomers with those of the natural
product allowed not only assignment of the relative configuration
of the C4/C5 stereogenic centers but also reassignment of the
proposed configuration at C9 of amphirionin-5. Further studies
aimed at the complete stereochemical assignment and total
synthesis of amphirionin-5 are underway and will be reported in
due course.
those reported for the natural product.25 In contrast, as with the
case of compound 3, distinct differences were observed in the 13C
NMR chemical shifts of the diastereomer 19 and the natural
ASSOCIATED CONTENT
* Supporting Information
■
S
3
product in the C3−C6 region. Furthermore, JH,H data of the
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.5b03346.
C1−C9 portion of 18 correspond well to the data of
amphirionin-5.25 These results convincingly defined the relative
configuration of the C1−C9 portion of amphirionin-5 as that
represented by structure 18 with the (3S*,4S*,5R*,9S*)-
stereochemistry.
Experimental procedures, characterization data for all new
compounds, stereochemical assignments for selected
compounds, comparison of the NMR data of compounds
C
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2, 3, 18, and 19 with the data of amphirionin-5, and 1H and
stereogenic centers has to be developed. For related examples, see:
(a) Boyle, C. D.; Harmange, J.-C.; Kishi, Y. J. Am. Chem. Soc. 1994, 116,
4995−4996. (b) Harmange, J.-C.; Boyle, C. D.; Kishi, Y. Tetrahedron
Lett. 1994, 35, 6819−6822. (c) Yajima, A.; Qin, Y.; Zhou, X.; Kawanishi,
N.; Xiao, X.; Wang, J.; Zhang, D.; Wu, Y.; Nukada, T.; Yabuta, G.; Qi, J.;
Asano, T.; Sakagami, Y. Nat. Chem. Biol. 2008, 4, 235−237. (d) Kim, H.-
J.; Kishi, Y. J. Am. Chem. Soc. 2008, 130, 1842−1844. See also ref 7g.
(9) Kolb, H. C.; VanNieuwenhze, M. S.; Sharpless, K. B. Chem. Rev.
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(10) For related domino cyclizations, see: (a) Marshall, J. A.; Sabatini,
J. J. Org. Lett. 2005, 7, 4819−4822. (b) Murthy, A. S.; Mahipal, B.;
Chandrasekhar, S. Eur. J. Org. Chem. 2012, 2012, 6959−6966.
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13C NMR spectra for all new compounds (PDF)
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: masasaki@m.tohoku.ac.jp.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank Professor Masashi Tsuda (Kochi University) for
providing us with the NMR spectra of amphirionin-5 and
Professor Haruhiko Fuwa (Tohoku University) for helpful
discussions. This work was financially supported in part by a
Grant-in-Aid from Scientific Research (B) (No. 25282228) and
Scientific Research on Innovative Areas “Chemical Biology of
Natural Products” (No. 23102016)27 from the Japan Society for
the Promotion of Science (JSPS).
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D
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Org. Lett. XXXX, XXX, XXX−XXX