Total synthesis of the ubiquitin-activating enzyme inhibitor (+)-panepophenanthrin

Article abstract of DOI:10.1002/anie.200351862

The [4+2] dimerization of epoxyquinol 1 is rendered irreversible by formation of a hemiacetal in the enantioselective total synthesis of the ubiquitin-activating enzyme inhibitor (+)-panepophenanthrin (2).

Full text of DOI:10.1002/anie.200351862

Angewandte
Chemie
Panepophenanthrin Total Synthesis
Total Synthesis of the Ubiquitin-Activating
Enzyme Inhibitor (+)-Panepophenanthrin**
Xiaoguang Lei, Richard P. Johnson, and
John A. Porco, Jr.*
The ubiquitin-proteasome pathway (UPP) regulates a variety
[
1]
of cellular processes by degradation of targeted proteins.
Ubiquitin (Ub) is first activated by ubiquitin-activating
enzyme (E1), and the activated ubiquitin is transferred by
transacylation to an active cysteine residue of ubiquitin-
conjugating enzymes (E2s). The E2 enzymes then cooperate
with ubiquitin ligases (E3s) to attach Ub to the amino group
of lysine residues on protein targets, leading ultimately to
protein degradation by the proteasome. It has been shown
that abnormal ubiquitination-mediated protein degradation
may be associated with human cancers, inflammation, and
[
2]
neurodegenerative disease. Small-molecule inhibitors of
ubiquitination may thus find clinical applications, provided
[
3]
that selective compounds may be uncovered. Recently, the
first naturally occurring inhibitor of ubiquitin-activating
enzyme, panepophenanthrin (1, Figure 1), was isolated from
[
4]
the mushroom strain Panus rudis Fr. IFO8994. This mole-
cule falls into a general class of epoxyquinoid natural products
[
5]
produced by Diels–Alder-type dimerization, including tor-
[
6]
[7]
reyanic acid (2) and epoxyquinols A (3) and B (4)
Figure 1). Herein, we report the first total synthesis of (+)-
(
panepophenanthrin, which utilizes a highly stereoselective
Diels–Alder dimerization of an epoxyquinol dienol mono-
mer, as well as initial experimental and computational studies
to probe the dimerization mechanism.
Our retrosynthetic route for panepophenanthrin is
[
28]
depicted in Figure 2a. Target molecule 1 may be derived
from hemiacetal formation of the hydroxy ketone 5. The
propensity for epoxyquinol derivatives to form both hydrates
and hemiacetals by reaction of water and alcohols with the
[
8]
electrophilic carbonyl has been documented. The X-ray
[*] Prof. Dr. J. A. Porco, Jr., X. Lei
Department of Chemistry and
Center for Chemical Methodology and Library Development
Boston University, 590 Commonwealth Avenue
Boston, MA 02215 (USA)
Fax: (+1)617-353-6466
E-mail: porco@chem.bu.edu
Prof. Dr. R. P. Johnson
Department of Chemistry, University of New Hampshire
Durham, NH 03824 (USA)
[**] We thank Chaomin Li for helpful discussions, Dr. Ryuichi Sekizawa
(Showa Pharmaceutical University) for providing authentic pane-
pophenanthrin, and Dr. Emil Lobkovsky (Cornell University) for X-
ray crystal structure analysis. Financial support from the American
Cancer Society (RSG-01-135-01, J.A.P, Jr.) and the NIH (P50
GM067041) is gratefully acknowledged. R.P.J. acknowledges the
Norman and Anna S. Waite Professorship for support.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Angew. Chem. Int. Ed. 2003, 42, 3913 –3917
DOI: 10.1002/anie.200351862
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Communications
epoxidation. Tartrate-mediated nucleophilic epoxidation
of 11 using NaHMDS as base cleanly produced epoxy
ketone 8 (80% yield, 95% ee). The absolute stereo-
chemistry of 8 was confirmed by X-ray crystal structure
[
27]
analysis
(Scheme 1) and is consistent with our pro-
posed transition-state model for tartrate-mediated nucle-
[
13a]
ophilic epoxidation.
Figure 1. Dimeric epoxyquinoid natural products 1–4.
Scheme 1. a) PhI(OAc) , MeOH, RT, 1 h, 96%; b) 1,3-propanediol,
2
BF ·Et O, DME, RT, 2 h, 75%; c) Ph COOH, NaHMDS (1m in THF),
3
2
3
l-DIPT, 4-Š molecular sieves, toluene, ꢀ558C, 48 h, 80%, 95% ee.
DIPT=diisopropyl tartrate, DME=dimethoxyethane, NaHMDS=
sodium bis(trimethylsilyl)amide, RT=room temperature.
Advancement of 8 to the epoxyquinol monomer 6 is
shown in Scheme 2. Attempted chelation-controlled reduc-
[
17]
[18]
tion (with e.g. DIBAL-H
or Zn(BH4)2 ) led to poor
diastereoselectivity (1:1). In contrast, reduction of 8 with
Super-Hydride cleanly afforded syn-epoxy alcohol 12, which
Figure 2. a) Retrosynthetic analysis for panepophenanthrin (1), b) X-ray crystal
structure of 1, c) transition state structure for the dimerization of 6, d) pane-
poxydon (7).
crystal structure of 1 (Figure 2b) also shows the close
proximity of the tertiary hydroxy group and ketone, which
[
9]
should substantially favor formation of a hemiacetal bridge.
In principle, hemiacetal formation may precede intramolec-
[
10]
ular [4+2]cycloaddition.
The open-form precursor 5 may
[
11]
be derived from exo-Diels–Alder dimerization of epoxy-
quinol monomer 6 (Figure 2c), the conjugated diene isomer
of the natural product panepoxydon (7, Figure 2d). Recent
reports by Shotwell et al. have documented the facile
rearrangement of 7 to give conjugated isomers such as 6
[
12]
[
12c]
under mildly acidic conditions.
Epoxyquinol diene mono-
mer 6 may be derived from transformations of chiral,
nonracemic epoxy ketone 8, including a Heck-type coupling
to install the dienol. Compound 8 may be prepared as either
antipode by using tartrate-mediated asymmetric nucleophilic
[
13]
epoxidation of a quinone monoketal precursor.
The synthesis of epoxyquinol diene monomer 6 was
initiated by oxidation of the readily available monomethoxy
Scheme 2. a) LiEt BH, THF, ꢀ788C, 1 h, 98%; b) PPh , DIAD, 4-nitro-
3
3
benzoic acid, THF, ꢀ508C!RT, 1 h; NaOMe 1m, MeOH, RT, 30 m,
[
14]
[15]
hydroquinone 9 with PhI(OAc)2 to afford dimethoxyke-
tal 10 (Scheme 1). Transketalization of 10 with 1,3-propane-
80%; c) TBSCl, imidazole, DMF, RT, 10 h, 90%; d) Pd(OAc) , Ag CO ,
2 2 3
2
-methyl-3-buten-2-ol, DMF, 908C, 16 h, 80%; e) 10% HF, CH CN/
3
[
16]
diol under Pirrung's conditions afforded 1,3-dioxane 11,
CH Cl , RT, 1 h. DIAD=diisopropyl diazodicarboxylate, DMF=di-
2
2
which was found to be a suitable substrate for nucleophilic
methylformamide, TBS=tert-butyldimethylsilyl.
3
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was subsequently converted into the anti diastereomer 13 by a
estingly, Diels–Alder dimerization of 6 to give panepophen-
anthrin (1) also proceeded when the monomer was allowed to
[
19]
Mitsunobu protocol
(80%, two steps). Silylation of the
[
7d]
secondary hydroxy group afforded 14, which was subjected to
stand at 258C without solvent (24 h, 80%). The reaction
was conducted effectively in both standard laboratory glass-
ware and Teflon vials, which confirms that an ionic Diels–
Alder process is not strictly required for the [4+2]dimeriza-
tion. Subsequent experiments also revealed that syn-epox-
yquinol monomer 20 (Scheme 4) did not undergo Diels–
Alder dimerization under conditions reported for anti isomer
6 (neat, room temperature).
[
12c]
Heck-type coupling with 2-methyl-3-buten-2-ol
to afford
dienol 15 (80%). Double deprotection of 15 with aqueous HF
afforded the desired epoxyquinol monomer 6 (20%) and
panepophenanthrin (1, 40%). Synthetic 1 was confirmed to
1
be identical to natural panepophenanthrin by H and
1
3
C NMR data, mass spectrometry, and optical rotation, and
by TLC comparison in three solvent systems.
Our initial supposition was that the tandem deprotection/
Diels–Alder dimerization (15!1)may proceed through an
We next considered alternative mechanisms for the Diels–
Alder dimerization of monomer 6 (Scheme 5). In one case
(path a), the tertiary hydroxy group of one monomer adds to
the carbonyl group of another to generate a hemiacetal
intermediate. An intramolecular, inverse-
[
20]
ionic process (Scheme 3). Acid-catalyzed ionization of the
tertiary allylic alcohol of epoxyquinol 6 could in principle lead
demand Diels–Alder reaction then affords the
[
22]
dimeric natural product. An alternative mech-
anism (path b) may involve transition-state
[
23]
hydrogen bonding of the epoxyquinol mono-
[
24]
mers in a “pseudo-transannular,”
normal
Diels–Alder cycloaddition. Subsequent ring clo-
sure to form a five-membered ring hemiacetal
leads to 1.
To further probe the role of the tertiary
hydroxy group in the Diels–Alder dimerization
of 6, we prepared an epoxyquinol monomer
lacking this functionality (Scheme 6). Heck-type
coupling of 14 with 3,3-dimethylbutene afforded
2
2
1 (80%). Reaction of 21 with TBAF produced
2, which was treated with 0.2m HCl to effect
Scheme 3. Deprotection of 15 and subsequent Diels–Alder dimerization to give 1.
ketal hydrolysis forming monomer 23. Epoxy-
quinol 23 was cleanly dimerized to 24 (neat,
2
4 h). The regio- and stereochemistry of 24 was
to allylic cation 16, which may react with the carbonyl group
confirmed by X-ray structure analysis of bis-para-bromoben-
of another monomer to produce the tethered intermediate
zoate 25. Interestingly, 25 crystallized as a centrosymmetric
[
10,21]
[25]
¯
1
7.
Intramolecular ionic Diels–Alder reaction of 17 to
racemate (P1 space group).
Production of dimer 24
give 18 followed by addition of water anti to the epoxide
would afford 1 directly. To further judge the validity of an
ionic Diels–Alder process, we revised our synthetic approach
in order to prepare 6 for alternative dimerization experiments
confirms that that tertiary hydroxy group of monomer 6 and
hydrogen-bond organization is not essential for successful
Diels–Alder dimerization. Acetylation of monomer 22 led to
26, which was hydrolyzed to give monomer 27. This com-
(
Scheme 4). Desilylation of 15 with TBAF cleanly produced
epoxy alcohol 19 (90%). Gratifyingly, treatment of 19 with
.2m HCl in CH CN/CH Cl (1:1) cleanly effected ketal
0
3
2
2
hydrolysis to afford epoxyquinol monomer 6 (95%). Inter-
Scheme 4. a) TBAF 1m in THF, THF, RT, 3 h, 90%; b) 0.2m HCl,
CH CN/CH Cl (1:1), RT, 2 h, 95%; c) no solvent, RT, 24 h, 80%.
3
2
2
TBAF=tetrabutylammonium fluoride.
Scheme 5. Possible mechanisms for the Diels–Alder dimerization.
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contrast, direct intermolecular cycloaddition of two epoxy-
ketones proceeds through a reactive dienophile and thus a
ꢀ1
much lower intrinsic barrier of 24.8 kcalmol . Figure 4 shows
the predicted transition-state structure. The initial stage of
Figure 4. B3LYP/6-31G*-optimized transition-state structure for the
formation of panepophenanthrin (1).
intermolecular cycloaddition is slightly endothermic, and
presumably reversible, but hemiacetal formation “locks” the
structure and thus renders panepophenanthrin isolable.
Thermolysis experiments on panepophenanthrin are consis-
tent with these general energetic considerations (Scheme 7).
Scheme 6. Top: a) Pd(OAc) , Ag CO , 3,3-dimethylbutene, DMF, 908C, 24 h,
2
2
3
When dimer 1 was heated (CD OD, 608C, 24 h), starting
sealed tube, 80%; b) TBAF 1m in THF, THF, RT, 3 h, 90%; c) AcOH, DIC,
DMAP, CH Cl , RT, 30 m, 91%; d) 0.2m HCl, CH CN/CH Cl (1:1), RT, 2 h,
3
material was recovered; however, thermolysis of untethered
2
2
3
2
2
9
5%; e) neat, RT, 24 h, 90%. DIC=1,3-diisopropylcarbodiimide, DMAP=4-
dimer 24 (CD OD, 508C, 12 h) led to quantitative production
3
1
dimethylaminopyridine. Bottom: X-ray structure of 25.[27]
(
H NMR) of monomer 23 in solution.
pound also dimerized to afford 28, which was identical to
material obtained by acetylation of 24. These studies support
the normal [4+2]pathway (Scheme 5, path b), in which
Diels–Alder dimerization is followed by hemiacetal forma-
tion.
To further understand the timing of the [4+2]dimeriza-
tion and hemiacetal formation, we performed computational
studies by disconnecting panepophenanthrin in a retro-[4+2]
fashion and optimizing each stage at the B3LYP/6-31G* level
[
26]
of theory. Our results are summarized in Figure 3. Initial
formation of the hemiacetal link provides the advantage of an
intramolecular [4+2]reaction, but renders the dienophile
much less reactive because it is no longer an enone. The
ꢀ1
predicted overall free-energy barrier is 39.2 kcalmol . In
Scheme 7. Thermolysis of hemiacetal-bridged (1) and nonbridged (24)
dimers.
In summary, the first enantioselective total synthesis of
the ubiquitin-activating enzyme inhibitor (+)-panepophenan-
thrin (1) has been achieved employing tartrate-mediated
asymmetric nucleophilic epoxidation and stereoselective
Diels–Alder dimerization of an epoxyquinol dienol mono-
mer. Modification of the epoxyquinol monomer leading to
panepophenanthrin by replacing a tertiary hydroxy group
with a methyl group gave mechanistic insight into the critical
ꢀ1
[
4+2]dimerization. Additional experimental and computa-
Figure 3. Relative energies (in kcalmol , B3LYP/6-31G*) for the inter-
mediates in the two paths for Diels–Alder dimerization.
tional studies related to panepophenanthrin and applications
3
916
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Keywords: cycloaddition · epoxyquinoids · natural products ·
.
[
total synthesis
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Angew. Chem. Int. Ed. 2003, 42, 3913 –3917
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