Total synthesis of the protein phosphatase 2A inhibitor lactodehydrothyrsiferol

Article abstract of DOI:10.1002/anie.201007757

Cascading epoxides: The squalene-derived polyether lactodehydrothyrsiferol (1) has been prepared through a convergent sequence that features an epoxide-opening cascade to construct the tetrahydrofuran and tetrahydropyran subunits. Additional features include a stereodivergent diene diepoxidation, a monodeoxygenation of a triol, and complex fragment couplings through Suzuki and Nozaki-Hiyama-Kishi reactions. Copyright

Full text of DOI:10.1002/anie.201007757

Communications
DOI: 10.1002/anie.201007757
Natural Product Synthesis
Total Synthesis of the Protein Phosphatase 2A Inhibitor
Lactodehydrothyrsiferol**
Dane J. Clausen, Shuangyi Wan, and Paul E. Floreancig*
Red algae are a prolific source of squalene-derived poly-
ethers[1] that show moderate to high levels of cytotoxicity[2]
and very selective protein phosphatase 2A (PP2A) inhibi-
tion.[3] These unique structures (see Figure 1 for examples)
have inspired total syntheses of the family members venus-
We envisioned the construction of the polycyclic subunit
of 1 through a cascade reaction that would be initiated by
epoxide alkylation by an oxidatively generated oxocarbenium
ion.[12] The resulting epoxonium ion can be opened by an
appended nucleophile, such as an epoxide (to continue the
cascade) or an alkyl carbonate (to terminate the cascade).
Implementation of this strategy is complicated by the kinetic
regioselectivity of intramolecular nucleophilic epoxonium-
ion-opening reactions. We have shown[9c] that intramolecular
additions to bicyclo [3.1.0] epoxonium ions proceed prefer-
entially through an exo pathway to provide tetrahydrofurans,
while bicyclo [4.1.0] epoxonium ions react through an endo
pathway to yield oxepanes (Scheme 1). Our solution to the
Figure 1. Squalene-derived ethers from red algae.
atriol,[4] thyrsiferol and its esters,[4b,5] pseudodehydrothyrsi-
ferol,[6] and dioxepanedehydrothyrsiferol,[7] in addition to
subunit and analogue syntheses.[8] Our interest in this
molecule class arose from our work[9] on the construction of
cyclic ethers through epoxide-opening cascade reactions.[10]
Lactodehydrothyrsiferol (1), isolated from the red seaweed
Laurencia viridis found near the Canary Islands,[11] attracted
our attention because of its unique butyrolactone group and
the challenges associated with applying an epoxide-opening
cascade to construct the tetrahydropyran subunits. Herein, we
report the first total synthesis of 1. The sequence features an
oxidatively initiated cascade reaction, a stereodivergent diene
double epoxidation reaction, a diastereoselective fragment
coupling through a Nozaki–Hiyama–Kishi reaction, and a
selective monodeoxygenation of a triol.
Scheme 1. Synthesis of oxygen-containing heterocycles through epox-
onium-ion opening. DCE=1,2-dichloroethane, M.S.=molecular
sieves, NMQPF6 =N-methylquinolinium hexafluorophosphate.
tetrahydropyran synthesis was based on the work of McDon-
ald and co-workers, in which spirocyclization reactions are
used to dictate the regioselectivity of the epoxonium-ion
opening.[13] We tested this strategy under oxidative conditions
by irradiating (medium pressure mercury lamp, Pyrex filter)
homobenzylic ether 4 in the presence of the single-electron
oxidant N-methylquinolinium hexafluorophosphate and
air.[14] This led to the oxidative cleavage of the benzylic
carbon–carbon bond, thus forming an oxocarbenium ion that
reacts with the epoxide group to yield epoxonium ion 5.
Nucleophilic addition by the carbonate group with subse-
quent loss of the tert-butyl cation provided spirocycle 6 in
79% yield.[15]
[*] D. J. Clausen, Dr. S. Wan, Prof. Dr. P. E. Floreancig
Department of Chemistry, University of Pittsburgh
Pittsburgh, Pennsylvania 15260 (USA)
Fax: (+1)412-624-8611
E-mail: florean@pitt.edu
[**] We thank the National Institutes of Health, Institute of General
Medicine (GM062924) for generous support of this work. We thank
Prof. Josꢀ Fernꢁndez (Universidad de La Laguna) for copies of the
spectra for lactodehydrothyrsiferol.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201007757.
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Angew. Chem. Int. Ed. 2011, 50, 5178 –5181

Scheme 3. Synthesis of the cascade cyclization substrate. Reagents
=
and conditions: a) Me3Al, Cp2ZrCl2, H2O, DCE, then I2, 94%; b) CH2
CHMgBr, [Pd(PPh3)4], PhMe, 91%; c) Oxalyl chloride, DMSO, CH2Cl2,
Et3N, À788C; d) Ph2CH2, nBuLi, THF, 82% (two steps); e) NaH, DMF,
then MeI, 96%; f) TBDPSCl, imidazole, DMF, 100%; g) nBuLi, THF,
then (CH2O)n, 94%; h) Bu3SnH, [Pd(PPh3)4], C6H6, then I2, CH2Cl2,
83%; i) (Boc)2O, N-methylimidazole, PhMe, 99%; j) 9-BBN dimer,
THF, then 16, [Pd(PtBu3)2], K3PO4, H2O, PhMe, 74%; k) Oxone, 18,
K2CO3, Bu4NHSO4, CH3CN, H2O, then 19, 82%. 9-BBN=9-borabicyclo
[3.3.1] nonane, Boc=tert-butyloxycarbonyl, DMF=N,N’-dimethylform-
amide, DMSO=dimethyl sulfoxide, TBDPS=tert-butyldiphenylsilyl,
THF=tetrahydrofuran.
Scheme 2. Retrosynthesis of 1. LG=leaving group, M=metal,
PG=protecting group.
This result allowed us to propose that the synthesis of 1
could proceed through 7 (Scheme 2). This precursor will be
derived from fragments 8 and 9. Fragment 8 can be accessed
through a cascade of epoxide-opening reactions involving
diepoxide 10, which can be prepared from a metal-mediated
coupling of alkenes 11 and 12. Subunit 9 can be prepared
through oxidative transformations on geranylpropyne 13.
The synthesis of the diepoxide precursor 20 for the
cyclization reaction is shown in Scheme 3. Methylalumination
and iodination of 4-pentyn-1-ol according to the Wipf
variant[16] of Negishiꢀs protocol[17] with subsequent palla-
dium-mediated coupling using vinylmagnesium bromide[18]
provided diene 14. While 14 could be prepared in a single
operation by coupling the vinylalane intermediate directly
with vinyl bromide, the two-step protocol proved to be
substantially more efficient. Oxidation, diphenylmethyl-
lithium addition, and methylation provided fragment 15.
The diphenylmethyl group was selected because of its
accessibility and high reactivity[19] in oxidative cleavage
reactions. Fragment 16 was prepared through a straightfor-
ward sequence from 4-pentyn-1-ol, and a palladium-mediated
hydrostannylation/iodination[20] served as the key step. Cou-
pling the fragments through a Suzuki reaction[21] proved to be
quite challenging. Hydroboration of 15 required the use of the
9-BBN dimer to form an alkylborane that showed reproduc-
ible reactivity. Catalyzing the coupling reaction with [Pd-
(PPh3)4] at room temperature resulted in a slow reaction, in
which 16 decomposed by the loss of the carbonate, and 15 was
regenerated, presumably through a b-hydride elimination.
Elevating the temperature caused alkene isomerization.
Success was finally achieved by using [Pd(PtBu3)2], a catalyst
that was shown by Fu and co-workers to be effective for
Suzuki reactions with aryl boronic acids,[22] to yield 17 in 74%
yield with no carbonate loss or b-hydride elimination. The
double epoxidation reaction of 17 was complicated by the
need to oxidize each alkene with opposite stereochemical
control. We solved this problem by exploiting the differential
reactivity of the two alkenes that arises from the inductive
deactivation by the allylic carbonate group. This allowed us to
use the less-reactive, first-generation, sorbose-derived Shi
catalyst 18[23] to effect the epoxidation of the more reactive
alkene. Upon completion of this reaction the more-reactive
pseudoenantiomeric fructose-derived second-generation Shi
catalyst 19[24] was added to promote the oxidation of the
allylic carbonate. This sequence resulted in the isolation of
diepoxide 20 in 82% yield. Although we were unable to
determine the diastereoselectivity of this reaction precisely,
we saw only two diastereomers by 13C NMR spectroscopy (no
attempt was made to control the stereochemical orientation
at the homobenzylic site). Through the synthesis of a
derivative[25] we showed that the product was a single
enantiomer, within the limits of NMR detection.
The key cascade cyclization of 20 (Scheme 4) proceeded
under the standard reaction conditions that were described in
Scheme 4. Completion of the left-hand fragment. Reagents and con-
ditions: a) hn, NMQPF6, O2, Na2S2O3, NaOAc, DCE, PhMe, 45% yield
of isolated product (75% based on recovered starting material);
b) mCPBA, Sc(OTf)3, CH2Cl2, 68%; c) IBX, DMSO, 93%. IBX=2-
iodoxybenzoic acid, mCPBA=meta-chloroperoxybenzoic acid, Tf=tri-
fluoromethanesulfonyl.
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Communications
Scheme 1 to provide 21 in 45% yield upon isolation (75%
yield based on recovered starting material). This reaction
stalled prior to the complete consumption of the starting
material and did not proceed further even after the addition
of more catalyst. The reason for this is not clear, but control
reactions show that benzophenone, a triplet-sensitizing prod-
uct that is formed in the oxidative cleavage reaction, does not
inhibit the reaction. However, the unreacted diepoxide can be
resubjected to the reaction conditions to access gram
quantities of 21. The synthesis of 8 was completed by a one-
pot lactone formation/silyl ether cleavage using mCPBA and
Sc(OTf)3,[26] with subsequent alcohol oxidation using IBX.[27]
No diastereomers were isolated in this sequence, thus
indicating that diastereocontrol in the diepoxidation reaction
was high.
The right-hand fragment of 1 was prepared (Scheme 5)
through the addition of lithiated trimethylsilylpropyne[28] to
geranyl chloride with a subsequent desilylative work-up to
yield 13. Sharpless asymmetric dihydroxylation[29] of the
Scheme 6. Completion of the synthesis. Reagents and conditions:
a) CrCl2, NiCl2·DMP, 25, Proton Sponge, Mn, Cp2ZrCl2, LiCl, CH3CN,
84%, d.r.=8:1; b) K2CO3, MeOH, 91%; c) TsCl, Et3N, Bu2SnO, CH2Cl2,
=
92%; d) NaBH4, HMPA, 508C, 76%; e) Me3P C(H)CN, C6H6, 808C,
40%; f) Bu4NF, THF, 77%. DMP=2,9-dimethylphenanthroline,
HMPA=hexamethylphosphoramide.
Tsunoda dehydration reagent (Me3P C(H)CN),[34] a step that
=
is analogous to the endgame of the synthesis of pseudodehy-
drothyrsiferol reported by Hioki et al.[6] This led to the
formation of 28 in 40% yield. Silyl ether cleavage by Bu4NF
resulted in the isolation of 1. All spectral data for synthetic 1
matched the values that were reported for the natural
product.[11]
We have reported the first total synthesis of lactodehy-
drothyrsiferol, the longest linear sequence of which is a 16
step route. This is the shortest route that has yet been
reported for any member of this molecule class. The route
featured an epoxide-opening cascade cyclization to prepare
the tetrahydrofuran subunit and one tetrahydropyran ring.
Other notable transformations include a Suzuki coupling that
employed an iodinated allylic carbonate, a diepoxidation
reaction that exploited the differential reactivities of the
alkenes and two pseudoenantiomeric catalysts to achieve the
desired stereochemical outcome, an efficient and mild one-
pot transformation of an alkyne to a vinyl iodide through
Scheme 5. Synthesis of the right-hand fragment. Reagents and con-
ditions: a) 1-Trimethylsilylpropyne, nBuLi, THF, À788C, then Bu4NF,
88%; b) AD-Mix b, CH3SO2NH2, tBuOH, H2O, 53%; c) 18, Oxone,
K2CO3, Bu4NHSO4, CH3CN, H2O, then Py·CSA, 83%, d.r.=13:1;
d) TESCl, imidazole, DMAP, DMF, 89%; e) Et3SiH, [CpRu-
(NCCH3)3]PF6, CH2Cl2, then I2, 2,6-lutidine, 82%. Cp=cyclopenta-
dienyl, DMAP=4-dimethylaminopyridine, Py·CSA=pyridinium cam-
phorsulfonate, TES=triethylsilyl.
dimethyl-substituted alkene was highly enantioselective[25]
and moderately regioselective, and provided 22 in 53%
yield. A Shi epoxidation using catalyst 18 and subsequent
treatment with pyridinium camphorsulfonate provided tetra-
hydrofuran 23 as a 13:1 mixture of diastereomers.[25] The
diastereomers were readily separated by MPLC methods and
the minor stereoisomer was shown to arise from imperfect
stereocontrol in the epoxidation step.[25] Silyl ether formation
proceeded under standard reaction conditions, and then
hydrosilylation under Trostꢀs protocol[30] with subsequent
iodination resulted in the formation of vinyl iodide 24 in 82%
yield for the one-pot process.
The completion of the synthesis (Scheme 6) commenced
with the union of 8 and 24 through a reagent-controlled
diastereoselective Nozaki–Hiyama–Kishi coupling[31] using
ligand 25, to form allylic alcohol 26 in 84% yield as an 8:1
mixture of diastereomers.[25] The cyclic carbonate was con-
verted into diol 27 through methanolysis, selective tosylation
of the primary alcohol of the resulting triol by the Lilly
protocol[32] (at which point a single diastereomer could be
isolated), and reduction with NaBH4 in warm HMPA.[33] The
final ring closure was conducted by exposing 27 to the
hydrosilylation chemistry,
a diastereoselective Nozaki–
Hiyama–Kishi reaction for complex fragment coupling, and
a selective sequence for the deoxygenation of a single hydroxy
group from a triol. The modular nature of the synthesis and
the reliance upon reagent control to establish the stereocen-
ters makes this sequence well suited for the construction of
analogues that can be used to test hypotheses regarding the
structure–activity relationships of this interesting class of
PP2A inhibitors.
Received: December 9, 2010
Revised: March 15, 2011
Published online: April 21, 2011
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Angew. Chem. Int. Ed. 2011, 50, 5178 –5181

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Keywords: domino reactions · natural products · oxidation ·
oxygen heterocycles · total synthesis
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