Total synthesis of cytotoxic macrolide amphidinolide B1 and the proposed structure of amphidinolide B2

Article abstract of DOI:10.1021/ja803012n

The first enantioselective total syntheses of cytotoxic macrolide amphidinolide B₁ and the proposed structure for amphidinolide B₂ have been accomplished. Key features of the syntheses include a diastereoselective aldol condensation, a spontaneous Wadsworth-Emmons macrocyclization and a directed epoxidation/elimination sequence. Copyright

Full text of DOI:10.1021/ja803012n

Published on Web 05/20/2008
Total Synthesis of Cytotoxic Macrolide Amphidinolide B1 and the Proposed
Structure of Amphidinolide B2
Liang Lu, Wei Zhang, and Rich G. Carter*
Department of Chemistry, Oregon State UniVersity, 153 Gilbert Hall, CorVallis, Oregon 97331
Received April 23, 2008; E-mail: rich.carter@oregonstate.edu
Scheme 1. Retrosynthetic Analysis of Amphidinolide B2
Amphidinolide B1 (1) and its C18 epimer amphidinolide B2 (2)
have generated significant attention from the synthetic community1,2
because of their potent cytotoxic activity against several cancer cell
lines. In fact, macrolide 1 is the most potent member of this family
with a reported IC50 value of 0.14 ng/mL against L1210 murine
leukemia cell line.3 Despite these efforts, their total syntheses remain
elusive targets. Macrolides 1 and 2 represent considerable synthetic
challenges because of their highly substituted C13-C15 diene
functionality,1c,4 the densely functionalized C21-C26 right-hand
portion, the labile C6-C9 epoxy alkene, and a 26-membered lactone.
Our retrosynthetic strategy starts by disconnection at C8,9 to reveal
the aldehydes 3 and 4 (Scheme 1). Further cleavage of the ester
linkage at C25 would lead to the alcohols 5 and 6 and the keto-
phosphonate 7. The C18-C19 bond should be available via a
diastereoselective aldol reaction with the aldehyde 8 and R-oxy
ketone 9. We have previously reported a chelation strategy for the
synthesis of the 18S stereochemistry present in amphidinolide
1b,c,5
B1
and this approach has been used by others in the field,6
including Fu¨rstner’s recent total syntheses of amphidinolide G and
H.6a Herein, we report a nonchelation strategy for construction of
the C18 stereochemistry and its application to the total syntheses of
cytotoxic macrolide amphidinolide B1 (1) and the proposed structure
of amphidinolide B2 (2).
Scheme 2. Synthesis of Western and Eastern Subunitsa
Our syntheses of the aldehyde and methyl ketone subunits
commenced with the previously reported intermediates 11 and 14
(Scheme 2).1a,c Conversion of the C9 nitrile 11 into the correspond-
ing acetate 12 followed by selective functionalization of the C18,19
alkene using AD mix ꢀ* provided the C18,19 diol as an inconse-
quential 6:1 mixture of diastereomers.1c Cleavage of the resultant
diol with sodium periodate provided the desired aldehyde 8. For
the synthesis of the Eastern subunit 9, Horner-Wadsworth-Emmons
olefination of aldehyde 14 followed by dihydroxylation yielded the
diol 16. Next, bis-silylation followed by selective C25 TES
deprotection yielded the free alcohol at C25. Finally, Mitsunobu
inversion of the alcohol followed by saponification and TMS
protection revealed the ketone 9.
a (i) DIBAL-H, PhMe; (ii) NaBH4, EtOH/CH2Cl2 (1:1), 85% (two steps);
(iii) Ac2O, pyr., CH2Cl2, 94%; (iv) AD Mix ꢀ*, t-BuOH, H2O, 88%, 6:1
dr; (v) NaIO4, THF, H2O, 99%; (vi) 15, NaH, DME, 76%; (vii)
K2OsO2 ·4H2O, (DHQ)2PHAL, K6Fe(CN)3, MeSO2NH2, K2CO3, t-BuOH,
H2O, 77%, 10:1 dr; (viii) TESOTf, 2,6-lut., CH2Cl2, -78 °C, 92%; (ix)
HOAc: THF: H2O (8:8:1), 0 °C, 89%; (x) DEAD, Ph3P, 4-NO2-C6H4CO2H,
THF, 82%; (xi) then Ba(OH)2 ·8H2O, MeOH, 0 °C, 72%; (xii) TMSOTf,
2,6-lutidine, CH2Cl2, -78 °C, 95%.
The key diastereoselective aldol coupling is shown in Scheme
3. Treatment of ketone 9 under our standard LDA, Et2O/THF
conditions that proved effective with the C21 OPMB series1c resulted
in low conversion and poor diastereoselectivity favoring the 18S
stereochemistry [approximately 1.5:1 dr (5:6)]. Interestingly, ad-
dition of TMEDA led to a dramatic rate acceleration and a reversal
of the selectivity [2:1 dr (6:5)]. Additional cooling of the reaction
to -100 °C led to improved diastereoselectivity [8:1 dr (6:5)] in
reasonable yield (65% overall). While we are still exploring the
nature of the diastereoselectivity, one possible explanation could
be a transition state which minimizes the dipoles of the C21 C-O
σ bond and the enolate.7 The alternate 18S diastereomer can be
obtained by performing the reaction at higher temperatures (-40 °C,
1.2:1 dr 5:6). Silyl protection of alcohols 5 and 6 separately using
TESCl/DMAP yielded the silyl ether compounds 17 and 18,
respectively.
With an effective strategy for the coupling of the Eastern and
Western subunits in hand, we turned our attention to the key
macrocyclization event (Scheme 4). We chose to initially explore
this sequence on the 18R compound 18. TMS deprotection of silyl
ether 18 at C25 and incorporation of the C1-C8 subunit provided
9
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J. AM. CHEM. SOC. 2008, 130, 7253–7255 7253

C O M M U N I C A T I O N S
Scheme 3. Key Diastereoselective Aldol Couplinga
Scheme 5. Synthesis of Proposed Structure of Amphidinolide B2
and its C8,9 Diastereomera
a (i) LDA, TMEDA, Et2O, THF, -100 °C then add 8, 65% (8:1 dr, 6:5);
(ii) LDA, TMEDA, Et2O, THF, -40 °C then add 8, 66% (1.2:1 dr, 5:6);
(iii) TESCl, DMAP, CH2Cl2, room temp.
Scheme 4. Closure of the Macrocyclea
a (i) HOAc/THF/H2O (8:8:1), -10°C; (ii) 5, Cl3C6H2COCl, Et3N then
DMAP, PhMe; (iii) Ba(OH)2 ·8H2O, MeOH, H2O, 89% for 19, 91% for
20; (iv) TPAP, CH2Cl2 then Ba(OH)2 ·H2O, THF, H2O, 0.01 M final
concentration, 51% of 21; (v) TPAP, CH2Cl2 then LiCl, i-Pr2NEt, MeCN,
0.01 M final concentration, 50% of 22.
a (i) (S)-CBS, BH3 ·DMS, THF, -20 °C, 3 h, >10:1 dr, 67%; (ii) Ti(Oi-
Pr)4, TBHP, CH2Cl2, -20 °C, 5 h, mol. sieves, 74%, 2:1 dr (24:25); (iii)
Bu3P, o-NO2C6H4SeCN, THF; (iv) TAS-F, DMF/THF/H2O (10/1/0.02),
0 °C; (v) TMSOOTMS, NaHCO3, CH2Cl2.
the keto phosphonate 20.8 Removal of the C9 acetate followed by
Ley’s TPAP oxidation revealed the target aldehyde 4. The corre-
sponding aldehyde 4 proved to be highly reactive as attempted
isolation led to considerable loss of material. Interestingly, signifi-
cant amounts of the aldehyde 4, formed during the TPAP oxidation,
appearedtoundergospontaneousintramolecularWadsworth-Emmons
olefination to provide the desired macrocycle 22. The conversion
could be driven to completion by the addition of LiCl and Hunig’s
base9 in 51% yield. A similar sequence was followed for construc-
tion of the 18S macrocycle 21. In this case, Ba(OH)2 proved more
effective for driving the macrocyclization to completion. Addition-
ally, we were pleased to observe that macrocycle 21 crystallized
upon standingsallowing us to confirm the stereochemistry in 21.10
With an efficient route into the macrocycles 21 and 22, the final
challenges were the incorporation of the C6-C9 allylic epoxide
moiety and deprotection. As before, we explored the initial
chemistry on the 18R macrolactone 22 (Scheme 5). Our previous
experiences with deprotection of late stage amphidinolide B
intermediates made us mindful of the difficulty of the final
deprotection sequence.1d Regioselective and stereoselective reduc-
tion of the C7 carbonyl functionality could be accomplished with
the (S)-CBS reagent.11 We next had intended to epoxidize the alkene
under Sharpless conditions;12 however, the presumed steric conges-
tion of the C7 alcohol thwarted this approach. Fortunately, use of
Ti(Oi-Pr)4 and TBHP in the absence of DIPT led to formation of
both the syn and anti diastereomerssfavoring the syn stereochem-
istry (2:1 dr).13 As the syn diastereomer 24 contained the proposed
stereochemistry in amphidinolide B2, we initially proceeded forward
with that diastereomer. Selenide incorporation14 and elimination
under our recently developed TPAP/NMO conditions1d,15 yielded
the fully functionalized macrocycle. Unfortunately, all attempts to
remove the silyl protecting groups under fluoride or acidic condi-
tions led to decomposition. Suspecting that the allylic epoxide might
be the culpable functionality, we next explored global deprotection
on the epoxy selenide 26. We were quite pleased to find that
treatment with TAS-F6a cleanly removed all silyl protecting groups
to provide the polyol 28. The final selenide oxidation/syn elimina-
tion proved problematic under standard (H2O2) conditions. This
issue was not completely surprising as we have previously
encountered this problem in our azaspiracid work15 as well as a
previous generation amphidinolide approach.1d We have attributed
this deleterious reactivity to the R-hydroxy ketone moiety. Although
our TPAP/NMO conditions15 are not compatible with the polyol
functionality of 28, we were gratified to find that bistrimethylsi-
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C O M M U N I C A T I O N S
a
Scheme 6. Total Synthesis of Amphidinolide B1
Acknowledgment. Financial support was provided by the
National Institutes of Health (NIH) (Grant GM63723) and Oregon
State University (OSU). The authors would like to thank Dr. Lev
N. Zakharov (OSU and University of Oregon) for X-ray crystal-
lographic analysis of compound 21, Professor Max Deinzer (OSU)
and Dr. Jeff Morre´ (OSU) for mass spectra data, Rodger Kohnert
(OSU) and Dr. Clemens Anklin (Bru¨ker Biospin) for NMR
assistance, and Dr. Roger Hanselmann (Rib-X Pharmaceuticals) for
his helpful discussions.
Supporting Information Available: Complete experimental pro-
cedures are provided, including 1H and 13C spectra, of all new
compounds. This material is available free of charge via the Internet
at http://pubs.acs.org.
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0 °C, 73%; (viii) TMSOOTMS, NaHCO3, CH2Cl2, 68%.
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lylperoxide (TMSOOTMS) cleanly facilitated selenide oxidation
with in situ syn elimination to reveal product 2. Surprisingly, this
compound 2 did not match with the spectra data provided for the
natural product amphidinolide B2.3 We followed a similar sequence
to the C8,9 epoxide diastereomer 30 which too did not correspond
with the reported data for amphidinolide B2. In both cases, the 1H
NMR shift for the H14 alkene was shifted significantly downfield
as compared to the natural product data. Careful inspection of the
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1
phidinolide B2 was based primarily on the differences in the H
NMR in the C17-C19 region of the natural product as compared to
amphidinolide B1 (1).3b It is important to note that Shimizu and
Clardy obtained X-ray crystallographic structure of natural product
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amphidinolide B1 and B2 are more complicated than initially
expected. On the basis of this information, we haVe concluded that
the proposed structure of amphidinolide B2 is incorrect.
Next, we shifted our focus to the total synthesis of amphidinolide
B1 (1) (Scheme 6). We applied an analogous strategy for the
synthesis of 1 as was described for the 18R series. It appears that
a slight reversal in selectivity in the epoxidation occurs with the
18S stereochemistrysnow with a modest preference for the
undesired C8,9 epoxide. Fortunately, these diastereomers are chro-
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selenide followed by TAS-F deprotection yielded the penultimate
intermediate. Finally, we were grateful to find that tandem selenide
oxidation/elimination using our bis-TMS peroxide conditions
yielded the natural product amphidinolide B1 (1). The synthesized
material 1 matched with the spectra data reported by Kobayashi
and co-workers for amphidinolide B1.3c
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In summary, the first total syntheses of amphidinolide B1 and
the proposed structure of amphidinolide B2 have been achieved.
The longest linear sequence is 25 steps from commercially available
lactic acid. Determination of the actual structure of amphidinolide
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