Total synthesis, assignment of the relative and absolute stereochemistry, and structural reassignment of phostriecin (aka Sultriecin).

Article abstract of DOI:10.1021/ja9097252

A total synthesis of phostriecin (2), previously known as sultriecin (1), its structural reassignment as a phosphate versus sulfate monoester, and the assignment of its relative and absolute stereochemistry are disclosed herein. Key elements of the work, which provided first the originally assigned sulfate monoester 1 and then the reassigned and renamed phosphate monoester 2, relied on diagnostic (1)H NMR spectroscopic properties of the natural product for the assignment of relative and absolute stereochemistry as well as the subsequent structural reassignment, and a convergent asymmetric total synthesis to provide the unequivocal authentic materials. Key steps of the synthetic approach include a Brown allylation for diastereoselective introduction of the C9 stereochemistry, an asymmetric CBS reduction to establish the lactone C5-stereochemistry, diastereoselective oxidative ring expansion of an alpha-hydroxyfuran to access the pyran lactone precursor, and single-step installation of the sensitive Z,Z,E-triene unit through a chelation-controlled cuprate addition with installation of the C11 stereochemistry. The approach allows ready access to analogues that can now be used to probe important structural features required for protein phosphatase 2A inhibition, the mechanism of action defined herein.

Full text of DOI:10.1021/ja9097252

Published on Web 01/28/2010
Total Synthesis, Assignment of the Relative and Absolute Stereochemistry,
and Structural Reassignment of Phostriecin (aka Sultriecin)
Christopher P. Burke, Nadia Haq, and Dale L. Boger*
Department of Chemistry and The Skaggs Institute for Chemical Biology, The Scripps Research Institute,
10550 North Torrey Pines Road, La Jolla, California 92037
Received November 16, 2009; E-mail: boger@scripps.edu
Sultriecin (1)1 was identified as an antitumor antibiotic isolated
from Streptomyces roseiscleroticus No. L827-7 and was an early
member of a growing family of related natural products2 that now
include fostriecin (3),3-5 cytostatin (4),6 phospholine (5, phoslac-
tomycin B),7 the leustroducsins (6),8 and the phoslactomycins (7)
(Figure 1).9 Sultriecin shares several features with other family
members, including the characteristic electrophilc R,ꢀ-unsaturated
lactone and hydrophobic Z,Z,E-triene capping the ends of an
extended structure that contains a central, functionalized 1,3-diol.
Unique to 1 and in contrast to other family members that contain
phosphate monoesters, sultriecin was assigned as a C9 sulfate ester
at the time of its disclosure in 1992.1
analogues but also could be adjusted to allow the preparation of
any diastereomer in the event that the initial stereochemical
assignment proved incorrect. The approach relies on a late-stage
single-step installation of the sensitive Z,Z,E-triene via chelation-
controlled addition of the cuprate derived from 9 to aldehyde 8. In
turn, the protected lactol of 8 was envisioned to arise from an
oxidative ring expansion of an R-hydroxyfuran that could be
accessed through the coupling of alkyne 10 with 2-furoyl chloride
followed by asymmetric (R)-CBS ketone reduction and stereose-
lective alkyne reduction (Figure 1).
In efforts on the synthesis and evaluation of members of this
class of antitumor agents that have since been shown to act as
protein phosphatase 2A (PP2A) inhibitors, we reported total
syntheses of 33c and 4,6c the establishment of their relative and
absolute configuration,4,6 and the preparation of a series of
analogues used to define structural features that are key to their
potent and unusually selective inhibition of PP2A.5,6 On the basis
of its functional biological activity and structural similarity to 3
and 4, we anticipated that 1 would also be a selective PP2A
inhibitor, albeit via a sulfate versus phosphate interaction with the
enzymatic bimetallic catalytic core. Herein, we report the first total
synthesis and stereochemical determination of 1, an unanticipated
and requisite structural reassignment of the natural product as
phosphate ester 2 (renamed phostriecin), and the establishment that
it does in fact represent an effective inhibitor of PP2A.
The structure of sultriecin was disclosed without a definition of
its relative or absolute stereochemistry, requiring its assignment
prior to initiating synthetic efforts (Figure 1). Earlier, we defined
the (5S,9S,11S)-stereochemistry for both fostriecin4 and cytostatin,6c
and this assignment was extended to sultriecin. As a result of an
intramolecular H-bond between the C9-phosphate and C11-OH
of fostriecin and cytostatin, the resulting cyclic structure adopts a
twist-boat conformation that gives rise to distinct 1H-1H coupling
constants between H11 and H10a (syn) or H10b (anti) (J ) 3.7 vs
9.6 Hz),4 with only the latter capable of being observed with
cytostatin (J ) 9.4 Hz)6 and similarly reported for sultriecin (J )
10.2 Hz).1 Thus, we reasoned that sultriecin, like cytostatin and
fostriecin, adopts a rigid H-bonded sulfate conformation exhibiting
a H11-H10 coupling constant diagnostic of the 10,11-anti con-
figuration, establishing the relative stereochemistry of the C10
methyl substituent and confirming the 9,11-anti configuration.
Additionally, the H4-H5 coupling constant reported for sultriecin
(J ) 2.5 Hz) is diagnostic of the cis-C4/C5 (J ) 2-3 Hz) versus
trans-C4/C5 (J ) 8-9 Hz) substitution on the half-chair conforma-
tion of the lactone10 and is identical to that observed with cytostatin
(H4-H5 J ) 2.7 Hz).6 Thus, the resulting (4S,5S,9S,10S,11S)-
diastereomer of 1 was targeted for synthesis. A convergent route
to sultriecin was designed that not only provides ready access to
Figure 1. (Top) Natural product structures. (Bottom) Assignment of relative
and absolute stereochemistry and key retrosynthetic disconnections for
sultriecin (1).
Synthesis of alkyne 10 was initiated with protection of methyl
(S)-3-hydroxy-2-methylpropionate (11) as a p-methoxybenzyl (PMB)
ether followed by reduction to alcohol 12 (PMBOC(dNH)CCl3,
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J. AM. CHEM. SOC. 2010, 132, 2157–2159 2157

C O M M U N I C A T I O N S
AgNO3, pyr, CH2Cl2, 25 °C, 15 min),17 and diastereoselective
reduction of ketone 20 (LiAlH4, Et2O, -60 °C, 2.5 h, 51-64% for
three steps). The stereochemistry of the resulting C4 alcohol was
necessarily inverted and directly protected as its pivalate ester using
the Mitsunobu reaction (diisopropyl azodicarboxylate (DIAD)/Ph3P,
pivalic acid, THF, 0 to 25 °C, 75%).18 Aldehyde 8 was obtained
following PMB removal (dichlorodicyanoquinone (DDQ), CH2Cl2/
H2O, 25 °C, 1 h, 74%) and oxidation of alcohol 23 (Dess-Martin
Periodinane (DMP), CH2Cl2, 25 °C, 1 h, 92%).19
Scheme 1. Synthesis of 8
Following an approach developed by Taylor and adopted in our
total synthesis of cytostatin,20 pyrilium tetrafluoroborate was treated
with n-pentyllithium (THF, -78 °C, 4 h) giving, after room-
temperature electrocyclic ring-opening of the adduct, the Z,E-
aldehyde 24 as a single isomer (eq 1). Aldehyde 24 was converted
to 9 via dibromoolefination (CBr4, PPh3, Et3N, CH2Cl2, 0 °C, 15
min, 85% for two steps)12 and selective E-bromide reduction
(Bu3SnH, Pd(PPh3)4, Et2O, 0 °C, 45 min, 78%).21
Incorporation of the sensitive Z,Z,E-triene tail commenced with
conversion of 9 to the corresponding cuprate (t-BuLi, Et2O, -78
°C, 1 h; CuI-PBu3, Et2O, -78 °C, 15 min) followed by slow
addition of aldehyde 8 (Et2O, -78 °C, 1 h, 85%, >5:1 dr), providing
26 derived from chelation-controlled addition to the aldehyde
(Scheme 2).6c,22 Removal of the pivalate ester (diisobutylaluminum
hydride, CH2Cl2, -78 °C, 2 h) followed by silylation of the
secondary alcohols of 27 (TBDPSCl, AgNO3, pyr/CH2Cl2, 25 °C,
16 h, 80%, two steps) afforded 28 that was treated with dilute HCl
(THF/H2O, 25 °C, 12 h, 71%) to simultaneously and selectively
remove the EE and TBS protecting groups. The resulting lactol
was selectively oxidized to give lactone 29 (Ag2CO3-Celite,
benzene, 80 °C, 1.5 h, 91%). Sulfate ester introduction (SO3-pyr,
THF, 25 °C, 80%) followed by desilylation (HF-pyr, pyr/THF, 25
°C, 60%) gave 1, which did not match the spectroscopic (1H NMR,
13C NMR, IR) or physical characteristics (TLC, [R]D, solubility,
stability to silica gel) reported for the natural product.
camphorsulfonic acid, CH2Cl2, 25 °C, 12 h; LiAlH4, Et2O, 0-25
°C, 12 h, 91%, two steps) (Scheme 1). After oxidation of 12 to the
corresponding aldehyde (dimethylsulfoxide, oxalyl chloride, Et3N,
CH2Cl2, -78 to 0 °C, 2 h), asymmetric allylboration (allyldi-
isopinocampheylborane, -100 °C, 4 h; NaOH, H2O2, 25 °C, 16 h,
14:1 dr)11 gave alcohol 13 that was protected as the ethoxyethyl
acetal (14) (ethyl vinyl ether, PPTS, CH2Cl2, 25 °C, 2 h). The
ethoxyethyl acetal (EE), despite the complicating diastereomeric
mixture it introduces, was chosen to direct a subsequent chelation-
controlled aldehyde addition and represents a uniquely effective
protecting group that is capable of selective removal under mild
acidic conditions in the presence of the labile triene and sensitive
allylic alcohol. Oxidative cleavage of olefin 14 (OsO4, NaIO4,
NMO, THF/H2O, 25 °C, 18 h, 73% for four steps) gave aldehyde
15 that was subjected to Corey-Fuchs homologation (CBr4, PPh3,
CH2Cl2, 0 °C, 10 min, 71%; n-BuLi, THF, -78 to 25 °C, 16 h,
93%)12 to give alkyne 10. Coupling of 10 with 2-furoyl chloride
(Pd(PPh3)2Cl2, CuI, Et3N, 25 °C, 24 h, 85%)13 provided ketone 17
that was subjected to asymmetric reduction (methyl-(R)-CBS-
oxazaborolidine,14 BH3-Me2S, THF, -40 °C, 3 h, 12.5:1 dr),
setting the C5 stereochemistry, followed by stereoselective reduc-
tion15 of the alkyne (LiAlH4, THF, 0 to 25 °C, 24 h, 84%, two
steps) to the trans olefin 18. The analogous asymmetric CBS
reduction of the corresponding R,ꢀ-unsaturated ketone with the trans
double bond already installed to provide 18 directly was much less
diastereoselective (ca. 2.5:1 dr). Intermediate 21 was obtained as a
mixture of anomers following oxidative ring expansion of 18 (N-
bromosuccinimide, NaHCO3/NaOAc, THF/H2O, 0 °C, 1 h),16 tert-
butyldimethylsilyl (TBS) protection of resultant lactol 19 (TBSCl,
Scheme 2. Synthesis of Sultriecin (1)
Although several possibilities for this non-correlation with the
natural product could be envisioned, including the accuracy of our
stereochemical assignments as well as spectroscopic perturbations
derived from the protonation state or salt form of the sulfate, the
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only real distinctive difference observed in the 1H NMR of synthetic
1 and the natural product was the chemical shift (CD3OD, δ 4.82
vs 4.64) and multiplicity (ddd, J ) 8.4, 6.0, 1.8 Hz vs dddd, J )
9.6, 7.8, 7.2, 1.8 Hz) of C9-H adjacent to the putative sulfate ester.
Diagnostic of what proved to be a required structural reassignment,
the C9-H of the natural product exhibited an additional long-range
coupling (JP-H9 ) 7.8 Hz) characteristic of a phosphate (monoiso-
topic mass ) 492.1889) versus sulfate ester (monoisotopic mass
) 492.1794).23 Consequently, phosphate ester 2 was targeted
for synthesis (Scheme 3). Alcohol 29 was phosphorylated
(i-Pr2NP(OFm)2, tetrazole, CH2Cl2/CH3CN, 25 °C, 1 h; H2O2, 15
min, 96%)6 to give 31 that was desilylated (HF-pyr, pyr/THF, 25
°C, 4 d). Removal of the fluorenylmethyl groups in 32 (Et3N,
CH3CN, 25 °C, 16 h; Dowex Na+, 63% for two steps) unmasked
the phosphate, giving 2 (phostriecin) that was found to possess
properties identical to those reported for 1 as well as a sample24 of
natural “sultriecin” (1H NMR, 31P NMR, [R]D, TLC, HPLC,
HRMS), the latter of which displayed a 31P NMR signal like that
found with synthetic 2 (δ 3.4, CD3OD).
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Scheme 3. Synthesis of Phostriecin (2)
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(23) It also explains the observation6c that the corresponding sulfate of cytostatin
(sulfocytostatin) was found to be inactive against PP2A.
Thus, the total syntheses of 1 and 2 led to an unequivocal
reassignment of the structural composition and established the
relative and absolute stereochemical configuration of the natural
product (renamed phostriecin) heretofore known as sultriecin. Key
steps include a Brown allylation with controlled introduction of
the C9 stereochemistry, a CBS reduction to establish the lactone
C5-stereochemistry, diastereoselective oxidative ring expansion of
an R-hydroxyfuran to access the pyran lactone precursor, and single-
step installation of the sensitive triene unit through a chelation-
controlled cuprate addition with installation of the C11 stereo-
chemistry. This approach also allows ready access to analogues
that can now be used to probe important structural features required
for PP2A inhibition, the mechanism of action defined herein.25
These and related studies will be reported in due course.
Acknowledgment. We gratefully acknowledge financial support
from the National Institutes of Health (CA042056). We thank
Danielle R. Soenen for initiating the studies on sultriecin, Dr. Ernest
Lacey (Bioaustralis) for helpful discussions, and Prof. Richard E.
Honkanen for the PP2A inhibition studies. N.H. was a Skaggs
Fellow.
Supporting Information Available: Full experimental details. This
material is available free of charge via the Internet at http://pubs.acs.org.
(24) Commercially available from Bioaustralis.
(25) PP2A inhibition (IC50): 1, >100 µM; 2, 0.72 µM.
References
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