Total Synthesis of the 7,10-Epimer of the Proposed Structure of Amphidinolide N, Part I: Synthesis of the C1-C13 Subunit

Article abstract of DOI:10.1002/chem.201504674

Amphidinolide N, the structure of which has been recently revised, is a 26-membered macrolide featuring allyl epoxide and tetrahydropyran moieties with 13 chiral centers. Due to its challenging structure and extraordinary potent cytotoxicity, amphidinolide N is a highly attractive target of total synthesis. During our total synthesis studies of the 7,10-epimer of the proposed structure of amphidinolide N, we have synthesized the C1-C13 subunit enantio- and diastereoselectively. Key reactions include an l-proline catalyzed enantioselective intramolecular aldol reaction, Evans aldol reaction, Sharpless asymmetric epoxidation and Tamao-Fleming oxidation. To aid late-stage manipulations, we also developed the 4-(N-benzyloxycarbonyl-N-methylamino)butyryl group as a novel ester protective group for the C9 alcohol.

Full text of DOI:10.1002/chem.201504674

DOI: 10.1002/chem.201504674
Communication
&
Natural Products
Total Synthesis of the 7,10-Epimer of the Proposed Structure of
Amphidinolide N, Part I: Synthesis of the C1–C13 Subunit
Koji Ochiai,[a] Sankar Kuppusamy,[a] Yusuke Yasui,[a] Tsubasa Okano,[a] Yasunobu Matsumoto,[a]
Nishant R. Gupta,[a] Yohei Takahashi,[b] Takaaki Kubota,[b, c] Jun’ichi Kobayashi,[b] and
Yujiro Hayashi*[a, d]
extensive NMR studies.[3] Amphidinolide N (3) is thus a unique
26-membered macrolide possessing allyl epoxide and tetrahy-
dropyran moieties with 13 chiral centers.
Abstract: Amphidinolide N, the structure of which has
been recently revised, is a 26-membered macrolide featur-
ing allyl epoxide and tetrahydropyran moieties with 13
chiral centers. Due to its challenging structure and extra-
ordinary potent cytotoxicity, amphidinolide N is a highly
attractive target of total synthesis. During our total syn-
thesis studies of the 7,10-epimer of the proposed struc-
ture of amphidinolide N, we have synthesized the C1–C13
subunit enantio- and diastereoselectively. Key reactions in-
clude an l-proline catalyzed enantioselective intramolecu-
lar aldol reaction, Evans aldol reaction, Sharpless asymmet-
ric epoxidation and Tamao–Fleming oxidation. To aid late-
stage manipulations, we also developed the 4-(N-benzy-
To date, no total synthesis of amphidinolide N has been re-
ported.[4] Due to the unique structure and the high potent cy-
totoxicity, amphidinolide N is an important target of the total
loxycarbonyl-N-methylamino)butyryl group as
a novel
ester protective group for the C9 alcohol.
The amphidinolides are a series of cytotoxic macrolides pos-
sessing unique structural features originating from laboratory
cultured, marine dinoflagellates Amphidinium sp. Specifically,
Kobayashi and co-workers showed amphidinolide N to possess
the most potent cytotoxic activity against murine lymphoma
L1210 and KB cells (IC50 =0.00005 and 0.00006 mgmLÀ1, respec-
tively) among all amphidinolides isolated so far.[1] The structure
of amphidinolide N was elucidated by NMR technique by Ko-
bayashi as shown 1 in 1994 (Figure 1). In 2012, Trost presented
the structure 2 in their paper during synthesis of the C13–C29
subunit.[2] In 2013, Kobayashi revised the structure to be 3 via
Figure 1. Structure of amphidinolide N and 7,10-epi-amphidinolide N (4).
synthesis, as well as other members of the amphidinolide
family.[5] Nicolaou synthesized the iso-epoxy-amphidinolide N
stereoisomer in 2006.[4a,b] Recently, Trost[2] and Sasaki[4c] report-
ed their independent stereoselective syntheses of the C13–C29
subunit.
[a] Dr. K. Ochiai, Dr. S. Kuppusamy, Y. Yasui, T. Okano, Y. Matsumoto,
Dr. N. R. Gupta, Prof. Dr. Y. Hayashi
Before July, 2012: Department of Industrial Chemistry
Faculty of Engineering, Tokyo University of Science
Kagurazaka, Shinjuku-ku, Tokyo 162-8601 (Japan)
When we initiated the total synthesis of amphidinolide N,
the relative stereochemistry was not completely elucidated.
We selected the structure 4 as our target molecule.[6] Accord-
ing to the latest revised structure of amphidinolide N (3) in
2013,[3] 4 is epimeric at the C7 and C10 positions. Herein, and
in the following paper, we describe our efforts that have culmi-
nated in the synthesis of 7,10-epimer of the proposed struc-
ture of amphidinolide N (4). Clearly, a synthetic strategy
toward 4 would be beneficial for the total synthesis of amphi-
dinolide N.
[b] Dr. Y. Takahashi, Prof. Dr. T. Kubota, Prof. Dr. J. Kobayashi
Graduate School of Pharmaceutical Sciences, Hokkaido University, Sapporo
060-0812 (Japan)
[c] Prof. Dr. T. Kubota
Present address: Showa Pharmaceutical University, 3-3165 Higashi-Tamaga-
wagakuen, Machida, Tokyo 194-8543 (Japan)
[d] Prof. Dr. Y. Hayashi
Present address: Department of Chemistry, Graduate School of Science,
Tohoku University, Aoba-ku, Sendai 980-8578 (Japan)
Fax: (+81)22-795-6566
Our retrosynthetic analysis of 4 is shown in Scheme 1. We
considered the allyl epoxy moiety to be relatively labile and
thus should be constructed under mild and neutral conditions
E-mail: yhayashi@m.tohoku.ac.jp
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201504674.
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Scheme 1. Retrosynthesis of C7,C10-epi-amphidinolide N (4), PMB=p-methoxybenzyl, TES=triethylsilyl, TIPS=triisopropylsilyl, TBS=tert-butyldimethylsilyl.
Scheme 2. Retrosynthesis of the C1–C13 Subunit 7. PMB=p-methoxybenzyl, MMTr=(p-methoxyphenyl)diphenylmethyl, TES=triethylsilyl, TIPS=triisopropyl-
silyl, Tr =trityl.
at a late stage of the synthesis. Moreover, the ketone at C9
would be constructed at a late stage to suppress possible de-
hydration and epimerization at C10, in which the selection of
the protecting group at the C9 hydroxy group would be criti-
cal. Therefore, 4 would be synthesized from 5 via Grieco–Nishi-
zawa elimination[7] and oxidation of alcohol at C9. Compound
5 would be constructed by macrolactonization;[8] 6 would be
constructed with three subunits, C1–C13 (7), C14–C16 (8),[4a,b]
and C17–C29 (9). All carbon frameworks would be coupled by
a double Enders RAMP hydrazone diastereoselective alkyla-
tion,[4a,b,9] which has been employed by Nicolaou and co-work-
ers.[4a,b] Herein, we report the synthesis of the C1–C13 subunit
7 of 7,10-epimer of the proposed structure of amphidinolide N
(4) with 8 chiral centers, among which six are contiguous.
Retrosynthesis of 7 is illustrated in Scheme 2. The stereogen-
ic centers at C2 and C3 of 7 could be constructed by Evans
syn-aldol reaction.[10] The stereogenic centers at C9 and C10 of
10 would also be controlled by Evans syn-aldol reaction.[10]
Sharpless asymmetric epoxidation,[11] differentiation of two pri-
mary hydroxyl groups, followed by Tamao–Fleming oxida-
tion[12] will lead to chiral epoxide 11. The stereocenters at C6
and C7 of 12 would be constructed by proline-catalyzed intra-
molecular aldol reaction of 13.[13] Dialdehyde 13 could be de-
rived from commercially available 14.
Scheme 3. Synthesis of 12 via proline catalyzed intramolecular aldol reac-
tion. a) Homoallylmagnesium bromide/THF, RT, 98%; b) O3/DCM, À788C
then Ph3P/MeOH, À788C to RT, quant.; c) l-proline (10 mol%)/DCM, RT;
d) Ph3P=CHCOOEt/toluene, RT, 54% (2 steps, anti/syn=10: 1, 99% ee for
anti).
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Scheme 4. Synthesis of 11. a) DIBAL-H/DCM, À308C, 82%; b) TBHP, (+)-DET, Ti(OiPr)4, MS4Š/DCM, À508C to À308C, 98%; c) p-NO2-BzCl, Et3N, DMAP/DCM, RT,
quant.; d) Hg(CF3COO)2, Et3N/DCM, À308C then NaBH4/MeOH, EtOAc, À508C; e) mCPBA, KHF2/MeOH, EtOAc, RT, 57% (2 steps); f) 1-adamantanecarbonyl chlo-
ride, pyridine/DCM, À788C, 63%; g) PMBOC(=NH)CCl3, TfOH/Et2O, RT; h) K2CO3/MeOH, THF, 08C, 67% (2 steps); i) TIPSOTf, 2,6-lutidine/DCM, 08C; j) DIBAL-H/
DCM, À788C; k) SO3-pyridine, Et3N, DMSO/DCM, RT, 47% (3 steps). DIBAL-H=diisobutylaluminium hydride, TBHP=tert-butyl hydroperoxide, DET=diethyl tar-
trate, Bz=benzoyl, MS=molecular sieves, DMAP=4-dimethylaminopyridine, mCPBA=m-chloroperbenzoic acid, PMB=p-methoxybenzyl, DMSO=dimethyl-
sulfoxide.
Scheme 5. Synthesis of intermediate 10. a) (R)-4-benzyl-3-propionyl-2-oxazolidinone, nBu2BOTf, Et3N/DCM, À788C then 08C; b) TESOTf, 2,6-lutidine/DCM,
À788C, 80% (2 steps); c) LiBH4/THF, MeOH, 08C, 95%; d) SO3-pyridine, Et3N, DMSO/DCM, RT, 86%; e) Ph3P=C(Me)COOEt/toluene, reflux, 90%; f) DIBAL-H/Et2O,
À788C, 93%; g) PPTS/DCM, MeOH, 08C, 95%; h) MMTrCl, DMAP/pyridine, RT; i) 4-(N-benzyloxycarbonyl-N-methylamino)butyric acid, 2,4,6-trichlorobenzoyl
chloride, Et3N/toluene, RT then DMAP, RT; j) NH4F/MeOH, reflux, 82% (3 steps); k) SO3-pyridine, Et3N, DMSO/DCM, RT, 98%. TIPS=triisopropylsilyl, PMB=p-me-
thoxybenzyl, Bn=benzyl, TES=triethylsilyl, DMSO=dimethylsulfoxide, DIBAL-H=diisobutylaluminium hydride, PPTS=pyridinium p-toluenesulfonate,
Cbz=benzyloxycarbonyl, DMAP=4-dimethylaminopyridine, MMTr=(p-methoxyphenyl)diphenylmethyl.
Our synthesis starts from 14 (Scheme 3), which was treated
with homoallyl Grignard reagent to give bis-homoallyl silane
15. Ozonolysis of 15 afforded bis-aldehyde 13 in quantitative
yield. Organocatalysis is a powerful tool for asymmetric synthe-
sis. In particular, proline is inexpensive, stable to air and water,
and an environmentally friendly catalyst.[14] In this case, we uti-
lized the l-proline-catalyzed intramolecular aldol reaction as
developed by List[13] for the construction of the C6 and C7 ste-
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reocenters. The reaction afforded 12 in good yield with excel-
lent diastereo- and enatio-selectivity (anti/syn 10:1, 99% ee for
anti isomer, diastereomer ratio on Si=1:1) after Wittig reaction,
and this reaction is appropriate for the large scale prepara-
tion.[15]
a five-step sequence consisting of PMB protection,[16] removal
of the two p-nitrobenzoyl groups, TIPS protection, DIBAL-H re-
duction, and Parikh–Doering oxidation.[17]
Synthesis of intermediate 10 is described in Scheme 5. Evans
aldol reaction[10] of 11 proceeded in a highly diastereoselective
manner, followed by TES protection to afford 27 in good yield.
Reductive removal of the chiral auxiliary was accomplished by
treatment of LiBH4 to furnish 28, which was converted into 32
in a four-step sequence consisting of Parikh–Doering oxida-
tion,[17] Wittig reaction, reduction of ester, followed by removal
of TES group with PPTS. We protected the hydroxyl group at
the C9 position after protection of the allyl alcohol. We first se-
lected the acetyl group as a protective group at C9 but we
could not remove it under various conditions without affecting
other functionality before and after macrolactonization (see
Supporting Information). Presumably, steric hindrance at the
C9 position suppressed its deprotection. The protecting group
at C9 had to be deprotected at the late stage of the synthesis
under neutral conditions among other protecting groups.
Thus, the novel alcohol protective group, that is, the 4-(N-ben-
zyloxycarbonyl-N-methylamino)butyryl group, was developed.
The ester formed would thus be readily removed under neutral
hydrogenation conditions, followed by intramolecular nucleo-
philic substitution (Scheme 6).[18] The new protective group
was installed by Yamaguchi esterification.[8] TIPS protecting
group of the primary alchol was selectively removed with NH4F
in MeOH[19] to afford alchol 33, which was converted into alde-
hyde 10 via Parikh-Doering oxidation.[17]
Synthesis of the epoxy aldehyde intermediate 11 is shown in
Scheme 4. DIBAL-H reduction of 16 gave allyl alcohol 17.
Sharpless asymmetric epoxidation[11] of 17 using (+)-diethyl
tartrate as a ligand afforded the epoxide 18 in excellent yield
and diastereoselectivity. Two hydroxyl groups of 18 were pro-
tected with p-nitrobenzoyl chloride to afford bis-p-nitroben-
zoate 19. This protecting group was selected because it has to
be discriminated from other acyl protecting groups afterwards
(22!23). Tamao-Fleming oxidation[12] of 19 gave diol 20 in
moderate yield. Selective protection of the two primary hy-
droxyl groups of 20 was troublesome, but treatment of 20
with bulky 1-adamantanecarbonyl chloride at low temperature
afforded 21 in 63% yield, importantly with recovery of the
starting material (30%). 21 was converted into aldehyde 11 in
Scheme 6. Concept for novel protective group, Cbz=benzyloxycarbonyl.
Scheme 7. Synthesis of the C1–C13 subunit 7. a) (R)-4-benzyl-3-propionyl-2-oxazolidinone, nBu2BOTf, Et3N/DCM, À788C then 08C, 84%; b) lithium n-octane-
thiolate/THF, À408C, 91%; c) TAS-F, H2O/THF, DMF, RT, 84%; d) PPTS/MeOH, DCM, 08C then RT, 96%; e) TESOTf, 2,6-lutidine/DCM, À788C, 98%; f) NH4F/MeOH,
DCM, 08C to RT, 85%; g) MsCl, Et3N/THF, 08C then LiBr, RT, 98%. Cbz=benzyloxycarbonyl, TIPS=triisopropylsilyl, PMB=p-methoxybenzyl, Bn=benzyl,
MMTr=(p-methoxyphenyl)diphenylmethyl, TES=triethylsilyl, DMSO=dimethylsulfoxide, TAS-F=tris(dimethylamino)sulfonium difluorotrimethylsilicate,
PPTS=pyridinium p-toluenesulfonate.
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[5] For review, see: a) J. Kobayashi, J. Antibiot. 2008, 61, 271; b) J. Kobaya-
shi, T. Kubota, J. Nat. Prod. 2007, 70, 451; c) J. Kobayashi, M. Tsuda, Nat.
Prod. Rep. 2004, 21, 77; d) J. Kobayashi, K. Shimbo, T. Kubota, M. Tsuda,
Pure Appl. Chem. 2003, 75, 337; e) M. Ishibashi, J. Kobayashi, Heterocy-
cles 1997, 44, 543.
Synthesis of the C1–C13 subunit 7 from 10 is shown in
Scheme 7. Evans aldol reaction[10] afforded 34 with highly dia-
stereoselectivity. Removal of the chiral auxiliary was accom-
plished by the treatment of 34 with lithium n-octanethiolate[20]
to give thioester 35. As the TIPS protecting group was difficult
to be deprotected at a later stage of the synthesis, without af-
fecting labile moieties such as allyl epoxide, it was replaced by
TES. Removal of the TIPS group was troublesome. When TBAF
was employed, the yield was low largely due to dehydration to
afford a,b-unsaturated thioester. On the other hand, treatment
with TAS-F in the presence of water afforded 36 in good
yield.[21] Addition of water and TAS-F were essential for sup-
pressing the dehydration.[22] Deprotection of MMTr group with
PPTS gave the triol 37. After TES protection of all free hydroxyl
groups of 37, the treatment of 38 with NH4F in MeOH[19] result-
ed in the selective deprotection of the allyl silyl ether to give
39. Finally, bromination of allyl alcohol 39 afforded the C1–C13
subunit 7.
In summary, the synthesis of the C1–C13 subunit 7 of 7,10-
epimer of the proposed structure of amphidinolide N (4) was
accomplished. The stereogenic centers at C6 and C7 were con-
trolled by a large-scale, l-proline mediated intramolecular aldol
reaction in a highly practical and diastereo/enantioselective
manner. The C2, C3, C9 and C10 stereogenic centers were con-
structed by Evans syn-aldol reaction, while those at C4 and C5
were controlled by Sharpless epoxidation. We also developed
the novel protective group 4-(N-benzyloxycarbonyl-N-methyla-
mino)butyryl group for ester protection of the C9 alcohol. Ste-
reoselective synthesis of the C1–C13 subunit culminated in the
total synthesis of 7,10-epimer of the proposed structure of am-
phidinolide N (4). These efforts are disclosed in the following
paper.[23]
[6] J. Kobayashi, Personal communication.
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Keywords: aldol reaction
epoxidation · Tamao-Fleming oxidation · total synthesis
·
organocatalyst
·
Sharpless
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T. M. Francis, Org. Biomol. Chem. 2006, 4, 2119; b) K. C. Nicolaou, P. G.
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Hayashi, Chem. Eur. J. 2016, 22, DOI: 10.1002/chem.20154675.
Received: November 20, 2015
Published online on February 4, 2016
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