Total synthesis of theopederin D

Article abstract of DOI:10.1002/anie.200802548

(Chemical Equation Presented) The transformation is complete! The total synthesis of theopederin D (see structure), a potent cytotoxin, has been achieved through oxidative cleavage of a carbon-carbon bond. Other key transformations include an acid-mediate

Full text of DOI:10.1002/anie.200802548

Angewandte
Chemie
DOI: 10.1002/anie.200802548
Natural Product Synthesis
Total Synthesis of Theopederin D**
Michael E. Green, Jason C. Rech, and Paul E. Floreancig*
Members of the mycalamide, theopederin, and onnamide
families of natural products,[1] which are exemplified by
mycalamide A (1) and theopederin D (2; Scheme 1), have
tionalization of a tetrahydrofuranol in the presence of a
tetrahydropyranol, and a glycal epoxide ring-opening.
We envisioned 2 as arising from subunits 3 and 4
(Scheme 2). This disconnection has proven to be effective
for coupling these types of fragments, even though stereo-
Scheme 1. Mycalamide A (1) and theopederin D (2).
inspired substantial synthetic studies[2] in response to their
intriguing structural features, labile functionality, as well as
their potent cytotoxic,[1,3] immunosuppressive,[4] and antivi-
ral[1] activities. For example theopederin D, isolated from a
marine sponge belonging to the Theonella genus found off the
coast of Japan, has ten stereocenters, an unusual amido
trioxadecalin unit, an acid-labile cyclic b,g-unsaturated acetal
fragment, a butyrolactone group, and has an IC50 value of
approximately 2 nm against murine P388 leukemia cells.[1c]
Most synthetic endeavors have been directed toward myca-
lamide A, but theopederin D, with its additional structural
complication of the butyrolactone group, has previously been
synthesized only once.[2d] Our efforts toward the synthesis of
this class of molecules stem from our studies on preparing
cyclic acyl aminals,[5] including amido trioxadecalins, through
electron transfer initiated cyclization (ETIC) reactions.[6] In
this process cyclic acetals are formed through formaldehyde
hemiacetal surrogates that add into acyliminium ions that are
generated by oxidation. Herein we report a total synthesis of
theopederin D in which we employ ETIC as the key amido
trioxadecalin construction step. Other notable transforma-
tions include an asymmetric aldehyde/acid chloride conden-
sation, a diastereoselective aldol reaction, a selective func-
Scheme 2. Retrosynthetic analysis of theopederin D. Bn=benzyl,
PG=protecting group.
control at C10 is often elusive. Rawal and co-workers,
however, reported[2f] that the amido trioxadecalin unit of
mycalamide A can be constructed with the correct stereo-
chemical orientation at C10 through the coupling of a
protected pederic acid unit (C1–C8) with an amino trioxade-
calin mediated by 1,3-dicyclohexylcarbodiimide (DCC). The
trioxadecalin group of 4 can be prepared through the ETIC
reaction of 5, in which the mixed acetal must contain a group
that departs as a highly stable carbocation. This acetal can be
derived from bis(hemiacetal) 6, which in turn can be prepared
from the known[7] keto alcohol 7.
Previous synthetic approaches to the pederic acid subunit
have relied upon chiral pool materials or chiral auxiliaries to
establish stereogenicity.[2,8] In considering that the pederic
acid unit is needed for the synthesis of all members of this
structural family, we felt that development of the first
approach to employ asymmetric catalysis as a means of
establishing absolute stereocontrol would prove to be of
general use. We initiated our route (Scheme 3) with an
aldehyde/acid chloride condensation using trimethylsilyl
[*] M. E. Green, J. C. Rech, Prof. P. E. Floreancig
Department of Chemistry
University of Pittsburgh
Pittsburgh, PA 15260 (USA)
Fax: (+1)412-624-8611
E-mail: florean@pitt.edu
[**] This work was supported by grants from the National Institutes of
Health (grant no. GM-62924) and the National Science Foundation
(grant no. CHE-0139851). M.E.G. isa recipient of a Novartis
Graduate Fellowship and J.C.R. was a recipient of a Roche Graduate
Excellence Award.
[9]
quinidine (TMSQ) and LiClO4 to form a b-lactone, which
was opened with the lithium enolate of tert-butyl acetate to
form 8 in 76% yield and greater than 99% ee. We converted 8
into the pederic acid derivative 9 in six steps through a slight
modification of Nakataꢀs[8d] stereoselective variant of the
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tion reactions in the syntheses of mycalamide A, even though
the terminal alkenes that are used as substrates react with
moderate selectivity. We chose to employ an aldol reaction to
introduce the C17–C20 unit and to set the stereocenter at C17
through remote induction from the C13 alkoxy group.[11] This
approach commenced by exposing 7 to MeOTf and 2,6-di-tert-
butylpyridine (DTBP), and the resulting methyl ether was
converted into a boron enolate and coupled with 4-pentenal
to provide 10. Attempts to use the diethylboron enolate
resulted in modest selectivity (d.r. 3:1). This result was
consistent with reports[12] that show methyl ethers to be less
effective at promoting 1,5-stereoinduction than sterically
more demanding alkyl ethers. Therefore we formed the
enolate with (+)-(Ipc)2BCl,[13] which improved the d.r. to 10:1
through a matching sense of induction between substrate and
reagent control, which has precedence in the synthesis of
leucascandolide A developed by Crimmins and Siliphai-
vanh.[14] Syn reduction of the b-hydroxy ketone[15] and cleav-
age of both alkenes under modified Johnson–Lemieux
reaction conditions[16] provided the highly polar bis(hemiace-
tal) 6. The completion of the sequence required that the
tetrahydrofuranyl alcohol and the tetrahydropyranyl alcohol
be distinguished from one another. We reasoned that the
tetrahydrofuranyl alcohol would undergo acid-mediated
solvolysis faster than the tetrahydropyranyl alcohol because
of its smaller difference in strain energy between the starting
material and the product. Indeed, tetrahydrofuranyl ethers
have been shown to be more labile than tetrahydropyranyl
ethers under acidic conditions.[17] Thus, the cyclic hemiacetal
groups were differentiated by selectively forming the tetra-
hydrofuranyl ether with MeOH and PPTS. The remaining
tetrahydropyranol was then dehydrated with TFAA and
iPr2NEt to yield 11. Oxygenation at C12 and installation of a
vinyl group at C11 with the requisite syn arrangement were
achieved by treating 11 with DMDO[18] and exposure of the
Scheme 3. Synthesis of benzoyl pederic acid 9. Reagentsand condi-
tions: a) TMSQ, LiClO4, iPr2NEt, Et2O; b) LDA, tBuOAc, THF, 76%
(over 2 steps). Bz=benzoyl, LDA=lithium diisopropylamide.
Meinwald approach.[8a] In our work we discovered that
[10]
cleavage of a methyl ester mediated by Me3SnOH
(to
form the C8 carboxylic acid) was more efficient than the
reported cleavage mediated by thiolate.
Our initial objective for the construction of the trioxade-
calin fragment was to develop a new and selective method for
creating the C17 stereocenter (Scheme 4). This stereocenter
has most commonly been set through Sharpless dihydroxyla-
resulting crude, labile glycal epoxide to trivinylalane.[19]
A
similar glycal epoxidation in the synthesis of mycalamide A
was reported by Nakata et al.[2b] The resulting C12 hydroxy
group was alkylated with benzyloxybutoxymethyl chloride[5c]
to introduce the precursor of the formaldehyde hemiacetal
surrogate. A nitrogen-containing unit was incorporated into
the structure through cleavage of the C11 vinyl group with O3
followed by conversion of the resulting unstable aldehyde
into sulfinyl imine 12 under standard reaction conditions.[20]
Homobenzylic amine 13 was subsequently constructed
through a sequence of BnMgCl addition and cleavage of the
sulfinyl group mediated by HCl. Nucleophilic addition gave
no stereocontrol at C10, which is evidence that the conforma-
tional bias of the substrate overwhelmed the directing effect
of the auxiliary. No change in selectivity was observed with
the diastereomeric sulfinyl imine, thus indicating that the lack
of control did not result from a mismatch between the
auxiliary and the substrate. The stereochemistry at this
position, however, is inconsequential since it will ultimately
be converted into a planar acyliminium ion. The preparation
of cyclization substrate 14 was completed by benzyl ether
hydrogenolysis, benzyl carbamate formation from the unpuri-
fied amino alcohol, and oxidative etherification[21] to form the
tetrahydrofuranyl ether.
Scheme 4. Synthesis of the cyclization substrate 14. Reagentsand
conditions: a) MeOTf, DTBP, CH2Cl2, 88%. b) (+)-Ipc2BCl, Et3N, Et2O;
then 5-pentenal, 65%, d.r. 10:1; c) Et2BOMe, THF; then NaBH4,
MeOH, 77%; d) OsO4, NaIO4, THF, H2O, 82%; e) PPTS, MeOH;
f) TFAA, iPr2NEt, CH2Cl2, 92% (over 2 steps); g) DMDO, acetone;
then trivinylalane, CH2Cl2, 100%; h) BBMCl, iPr2NEt, CH2Cl2, 77%;
i) O3, CH2Cl2, À788C; then (R)-tBuS(O)NH2, Ti(OiPr)4, CH2Cl2, 50%,
62% (based on recovered aldehyde); j) BnMgCl, THF, 65%; k) HCl,
MeOH, 80%; l) H2, Pd/C, MeOH; then CbzCl, Et3N, CH2Cl2, 70%;
m) PhI(OAc)2, hn, cyclohexane, 80%. BBM=benzyloxybutoxymethyl;
Cbz=benzyloxycarbonyl, DMDO=2,2-dimethyldioxirane,
Ipc2BCl=diisopinocamphenyl chloroborane, PPTS=pyridinium para-
toluenesulfonate, Tf=trifluoromethanesulfonyl, TFAA=trifluoroacetic
anhydride.
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The key cyclization proceeded by irradiation of 14
(medium-pressure mercury lamp, Pyrex filtration) in
the presence of 6 mol% of N-methylquinolinium
hexafluorophosphate (NMQPF6) and O2[22] to provide
trioxadecalin 17 in 76% yield as a 2:1 mixture of
diastereomers at C10 (Scheme 5). By using these
remarkably selective non-acidic oxidative fragmenta-
tion conditions, the requisite acyliminium ion 15 was
formed in the presence of two highly acid-labile
tetrahydrofuranyl ethers. Acetal addition provides
oxonium ion 16, which loses the tetrahydrofuranyl
cation to yield 17. The stereochemical outcome of this
Scheme 6. Completion of the synthesis. DMAP=4-dimethylaminopyridine,
py=pyridine.
of the aldehyde decomposition product was isolated, but
variable amounts of the diastereomer at C7, resulting from
ketene formation in the acylation, were observed. These
results are consistent with previous studies[2] showing that the
intermediate amino trioxadecalin unit is configurationally
labile and that the stereochemical outcome of the acylation is
quite difficult to control. Studies on related structures in the
psymberin/pederin series of natural products have also shown
that remote structural differences can cause significant
reactivity differences at C10.[23] The synthesis was completed
through cleavage of the benzoyl group of 19 under standard
reaction conditions[2d] to yield theopederin D in 66% yield.
Spectroscopic data for the synthetic material[2d] matched
those reported for the natural product[1c]
.
We have reported a brief total synthesis of the immuno-
suppressant and cytotoxic agent theopederin D. The longest
linear sequence from 7, which is available from commercially
available material in two steps, requires 16 steps that need
purification (or 18 steps by including solvent changes) and in
an overall yield of 0.8%. This sequence compares favorably
with the most efficient approaches to any member of this
structural class. For comparison, the elegant synthesis of
mycalamide A by Rawal and co-workers[2f] was accomplished
from diethyl tartrate in 20 steps that require purification (or
23 steps by including solvent changes), and the landmark
synthesis of theopederin D by Kocienski et al.[2d] proceeded in
33 steps from ethyl isobutyrate. Our sequence highlights the
capacity of oxidative carbon–carbon bond activation (medi-
ated by electron transfer) to effect highly chemoselective
transformations in densely functionalized structures—as
indicated by the formation of an acyliminium ion in the
presence of two acid-labile tetrahydrofuranyl ethers. The use
Scheme 5. Synthesis of the amido trioxadecalin 18. Reagentsand
conditions: a) hn, NMQPF6, O2, NaOAc, Na2S2O3, touluene, DCE,
76%; b) Jonesreagent, acetone, 64%. DCE =1,2-dichloroethane.
reaction, in which the orientation at C10 in the major product
is opposite to that of the natural product, is inconsequential
for the completion of the synthesis. Hong and Kishi have
reported[2a] that the amino trioxadecalin which forms from
cleavage of the Cbz group is configurationally labile under
acidic, basic, or neutral conditions. Treatment of 17 with Jones
reagent produced lactone 18 in 64% yield.
We initially attempted the notoriously difficult fragment
coupling by hydrogenolytic cleavage of the Cbz group of 18
and exposing the crude amine 9 to the DCC/DMAP
conditions reported by Rawal and co-workers.[2f] However,
these reaction conditions resulted in a very low yield of the
desired amide 19: the undesired C10 diastereomer 20 was the
dominant coupling product, while the aldehyde decomposi-
tion product that resulted from amino trioxadecalin opening
and b-alkoxy group elimination was a major impurity. We
reasoned that the decomposition pathway could be sup-
pressed by using a more reactive acylating agent, therefore we
treated 9 with SOCl2 and pyridine to form the acid chloride[8b]
and subsequently mixed it with the crude amine in the
presence of DMAP. This sequence provided amides 19 and 20
in a combined 40% yield as a 1:1 mixture (Scheme 6). None
À
of the stereoselective aldol reaction to form the C16 C17
bond, while providing a new method for the generation of the
C17 stereocenter, makes this route applicable to numerous
members of the theopederin and mycalamide families of
natural products. The catalytic asymmetric approach to
pederic acid will improve the accessibility to this ubiquitous
subunit, thereby facilitating subsequent analogue studies.
Received: May 31, 2008
Published online: August 7, 2008
À
Keywords: C C activation · chemoselectivity · cyclization ·
radical ions · total synthesis
.
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