Stereocontrolled synthesis of a complex library via elaboration of angular epoxyquinol scaffolds

Article abstract of DOI:10.1021/jo050956y

We have accomplished the synthesis of a complex chemical library via elaboration of angular epoxyquinol scaffolds with distinct skeletal frameworks. The key strategy involves highly stereo-controlled [4 + 2] Diels-Alder cycloadditions of chiral, nonracemi

Full text of DOI:10.1021/jo050956y

Stereocontrolled Synthesis of a Complex Library via Elaboration
of Angular Epoxyquinol Scaffolds
Xiaoguang Lei,† Nava Zaarur,‡ Michael Y. Sherman,‡ and John A. Porco, Jr.*,†
Department of Chemistry and Center for Chemical Methodology and Library Development (CMLD-BU),
Boston University, 590 Commonwealth Avenue, Boston, Massachusetts 02215, and Department of
Biochemistry, Boston University School of Medicine, Boston, Massachusetts 02118
porco@chem.bu.edu
Received May 12, 2005
We have accomplished the synthesis of a complex chemical library via elaboration of angular
epoxyquinol scaffolds with distinct skeletal frameworks. The key strategy involves highly stereo-
controlled [4 + 2] Diels-Alder cycloadditions of chiral, nonracemic epoxyquinol dienes to generate
the scaffolds. Further scaffold diversification involves hydrogenation, epimerization, dehydration,
and condensation of the carbonyl group with alkoxyamine and carbazate building blocks. Further
elaboration of the scaffolds also provided new skeletal frameworks using hydroxyl-directed Diels-
Alder cycloaddition and reductive N-N bond cleavage. The overall process afforded 244 highly
complex and functionalized compounds. Preliminary biological screening of the library uncovered
six compounds which showed significant inhibition of Hsp 72 induction.
Introduction
thesis (DOS)2 provides access to such libraries, including
those based on natural product-like scaffolds.3
Due to the growing use of small-molecule libraries for
studies of protein function and discovery of novel drug
leads, the need for efficient generation of small-molecule
libraries with structural features of bioactive natural
products has grown dramatically.1 In this regard, ste-
reocontrolled synthesis of complex chemical libraries with
distinct skeletal frameworks via diversity-oriented syn-
As part of our overall interest in complex natural
product synthesis, we have accomplished syntheses of a
number of epoxyquinol-containing natural products.4 A
recent example is the enantioselective total synthesis of
the ubiquitin-activating enzyme inhibitor (+)-pan-
epophenanthrin 1 (Scheme 1).5 In this study, we success-
(2) (a) Schreiber, S. L. Science 2000, 287, 1964. (b) Liao, Y.; Hu, Y.;
Wu, J.; Zhu, Q.; Donovan, M.; Fathi, R.; Yang, Z. Curr. Med. Chem.
2003, 10, 2285. (c) Burke, M. D.; Schreiber, S. L. Angew. Chem., Int.
Ed. 2004, 43, 46. (d) Kim, Y.; Arai, M. A.; Arai, T.; Lamenzo, J. O.;
Dean, E. F., III; Patterson, N.; Clemons, P. A.; Schreiber, S. L. J. Am.
Chem. Soc. 2004, 126, 14740. (e) Burke, M. D.; Berger, E. M.; Schreiber,
S. L. J. Am. Chem. Soc. 2004, 126, 14095.
(3) For recent reviews, see: (a) Abel, U.; Koch, C.; Speitling, M.;
Hansske, F. G.. Curr. Opin. Chem. Biol. 2002, 6, 453. (b) Ortholand,
J. Y.; Ganesan, A. Curr. Opin. Chem. Biol. 2004, 8, 271. (c) Boldi, A.
M. Curr. Opin. Chem. Biol. 2004, 8, 281. (d) Tan, D. S. Comb. Chem.
High Through. Screen. 2004, 7, 631.
(4) (a) Li, C.; Lobkovsky, E.; Porco, J. A., Jr. J. Am. Chem. Soc. 2000,
122, 10484. (b) Li, C.; Pace, E. A.; Liang, M. C.; Lobkovsky, E.; Gilmore,
T. D.; Porco, J. A., Jr. J. Am. Chem. Soc. 2001, 123, 11308. (c) Li, C.;
Bardhan, S.; Pace, E. A.; Liang, M.-C.; Gilmore, T. D.; Porco, J. A., Jr.
Org. Lett. 2002, 4, 3267. (d) Li, C.; Johnson, R. P.; Porco, J. A., Jr. J.
Am. Chem. Soc. 2003, 125, 5095. (e) Li, C.; Porco, J. A., Jr. J. Am.
Chem. Soc. 2004, 124, 1310.
† Boston University.
‡ Boston University School of Medicine.
(1) For recent reviews, see: (a) Weber, L. Curr. Opin. Chem. Biol.
2000, 4, 295. (b) Wessjohann, L. A. Curr. Opin. Chem. Biol. 2000, 4,
303. (c) Hall, D. G.; Manku, S.; Wang, F. J. Comb. Chem. 2001, 3,
125. (d) Ganesan, A. Pure Appl. Chem. 2001, 73, 1033. (e) Breinbauer,
R.; Vetter, I. R.; Waldmann, H. Angew. Chem., Int. Ed. 2002, 41, 2878.
For selected examples, see: (f) Lee, D.; Sello, J. K.; Schreiber, S. L. J.
Am. Chem. Soc. 1999, 121, 10648. (g) Nicolaou, K. C.; Pfefferkorn, J.
A.; Roecker, A. J.; Cao, G. Q.; Barluenga, S.; Mitchell, H. J. J. Am.
Chem. Soc. 2000, 122, 9939. (h) Pelish, H.; Westwood, N.; Feng, Y.;
Kirchhausen, T.; Shair, M. D. J. Am. Chem. Soc. 2001, 123, 6740. (i)
Brohm, D.; Phillippe, N.; Mertzger, S.; Bhargava, A.; Muelle, O.; Lieb,
F.; Waldmann, H. J. Am. Chem. Soc. 2002, 124, 13171. (j) Couladouros,
E. A.; Strongilos, A. T. Angew. Chem., Int. Ed. 2002, 41, 3677. (k) Lo,
M. M.-C.; Neumann, C. S.; Nagayama, S.; Perlstein, E. O.; Schreiber,
S. L. J. Am. Soc. Chem. 2004, 126, 16077.
10.1021/jo050956y CCC: $30.25 © 2005 American Chemical Society
Published on Web 07/12/2005
6474
J. Org. Chem. 2005, 70, 6474-6483

Elaboration of Angular Epoxyquinol Scaffolds
SCHEME 1. Target and Diversity-Oriented Synthesis Using Epoxyketone Scaffolds
fully transformed chiral, nonracemic bromo epoxy ketone
2 to diene monomer 3, which smoothly underwent highly
selective exo-Diels-Alder cycloaddition to generate 1. We
also considered use of bromo epoxy ketone 2 for diversity-
oriented synthesis2 of natural product-like molecules.
Previous investigations have demonstrated the utility of
[4 + 2] cycloadditions in complex chemical library
construction.6 Diels-Alder cycloadditions of derived diene
4 with dienophiles such as maleimides may be used to
prepare angular compounds such as 5. Literature reports
have demonstrated the use of related cycloaddtions to
produce complex angular skeletons including studies by
Parker and co-workers employing an achiral diene 6 and
N-ethylmaleimide to prepare angular compound 7 (Scheme
1, inset A)7 and by Comins and co-workers using chiral
5-vinyldihydropyridones 8 and N-phenylmaleimide to
generate cycloadduct 9 (Scheme 1, inset B).8 We planned
to use solution-phase parallel synthesis methods (extrac-
tive workups, polymer reagent, and scavenger tech-
niques) for elaboration of scaffolds to prepare target
library structures.9
functionalized and stereochemically well-defined com-
pounds was completed. Further elaboration of the scaf-
folds and preliminary biological screening of library
members for inhibition of heat shock protein (HSP)
induction will also be described.
Results and Discussion
Synthesis of Epoxyquinol Scaffolds. Scaffold syn-
thesis was initiated by selective reduction of epoxyketone
25 with Super-Hydride to afford a syn-epoxyquinol as a
single diastereomer5,10 followed by silyl protection of the
secondary alcohol to provide compound 10 (Scheme 2).
Stille cross-coupling11 of 10 afforded chiral diene 11 in
good yield. Diels-Alder cycloaddition of 11 with N-
benzylmaleimide smoothly generated angular derivative
12 as a single diastereomer in quantitative yield. The
endo stereoselectivity of the cycloaddition was confirmed
by NOE analysis.12 Global deprotection of both the cyclic
ketal and the TBS ether with HF-CH3CN, followed by
quenching with methoxytrimethysilane (HF scavenger),13
afforded angular epoxyquinol 13 (95%). [4 + 2] cyclo-
Herein, we report the highly stereocontrolled solution-
phase synthesis of a complex chemical library via elabo-
ration of angular epoxyquinol scaffolds. Our approach
was initiated by parallel synthesis of angular epoxyquinol
scaffolds and further elaboration to afford distinct mo-
lecular frameworks. Subsequently, a library of 244 highly
(9) (a) Han, H.; Wolfe, M. M.; Brenner, S.; Janda, K. D. Proc. Natl.
Acad. Sci. U.S.A. 1995, 92, 6419. (b) Boger, D. L.; Tarby, C. M.; Myers,
P. L.; Caporale, L. H. J. Am. Chem. Soc. 1996, 118, 2109. (c) Cheng,
S.; Comer, D. D.; Williams, J. P.; Myers, P. L.; Boger, D. L. J. Am.
Chem. Soc. 1996, 118, 2567. For recent reviews on parallel solution-
phase synthesis using polymer reagents and scavengers, see: (d)
Parlow, J. J.; Devraj, R. V.; South, M. S. Curr. Opin Chem. Biol. 1999,
3, 320. (e) Flynn, D. L. Med. Res. Rev. 1999, 19, 408. (f) Brummer, O.;
Clapham, B.; Janda, K. D. Curr. Opin. Drug Discovery Dev. 2000, 3,
462. (g) Boger, D. L.; Desharnais, J.; Capps, K. Angew. Chem., Int.
Ed. 2003, 42, 4138. (h) Boger, D. L. Bioorg. Med. Chem. 2003, 11, 1607.
(10) Shimagaki, M.; Suzuki, A.; Nakata, T.; Oishi, T. Chem. Pharm.
Bull. 1988, 36, 3138.
(11) For a review of the Stille reaction, see: Farina, V.; Krishna-
murthy V.; Scott, W. J. Org. React. 1997, 50, 1.
(12) See the Supporting Information for further details.
(13) (a) Hu, Y.; Porco, J. A., Jr. Tetrahedron Lett. 1998, 39, 2711.
(b) Kulkarni, B. A.; Roth, G. P.; Lobkovsky, E.; Porco, J. A., Jr. J. Comb.
Chem. 2002, 4, 56.
(5) Lei, X.; Johnson, R. P.; Porco, J. A., Jr. Angew. Chem., Int. Ed.
2003, 42, 3913.
(6) For select examples, see: (a) Boger, D. L. PCT Int. Appl. 1996,
WO 9603424. (b) Schurer, S. C.; Blechert, S. Synlett 1999, 12, 1879.
(c) Sun, S.; Murray, W. V. J. Org. Chem. 1999, 64, 5941. (d) Stavenger,
R. A.; Schreiber, S. L. Angew. Chem., Int. Ed. 2001, 40, 3417. (e)
Graven, A.; St. Hilaire, P. M.; Sanderson, S. J.; Mottram, J. C.; Coombs,
G. H.; Meldal, M. J. Comb. Chem. 2001, 3, 441. (f) Kwon, O.; Park, S.
B.; Schreiber, S. L. J. Am. Chem. Soc. 2002, 124, 13402.
(7) Parker, K. A.; Ruder, S. M. J. Am. Chem. Soc. 1989, 111, 5948.
(8) Kuethe, J. T.; Brooks, C. A.; Comins, D. L. Org. Lett. 2003, 5,
321.
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Lei et al.
SCHEME 2. Synthesis of Maleimide-Derived
Angular Epoxyquinol Scaffolds
SCHEME 4. Hydroxyl-Directed Diels-Alder
Cycloaddition
SCHEME 3. Synthesis of a Urazole-Containing,
Angular Epoxyquinol Scaffold
cycloadduct 20 as a single diastereomer. NOE analysis
confirmed that 20 was an endo cycloadduct in which the
facial selectivity of the maleimide cycloaddition occurred
syn to the secondary alcohol.12 The stereochemistry of 20
is consistent with a cycloaddition proceeding through the
chelated transition state shown in Scheme 4. In contrast
to previous results by Barriault and co-workers involving
use of a chiral diene bearing a tertiary alcohol, in the
present case we did not observe further ring closure of
addition of 11 with N-ethylmaleimide also produced
cycloadducts 14 and the derived epoxyquinols 15 in an
analogous manner. X-ray crystallographic analysis of
angular epoxyketone 15 unambiguously confirmed that
14 was an endo-selective cycloadduct in which the
addition of the maleimide occurred anti to both the
epoxide and silyl ether moieties.8
To alter the skeleton of angular [4 + 2] cycloadducts
such as 13, N-phenyltriazolinedione 16 was selected as
an alternative dienophile (Scheme 3). Cycloaddition of
diene 11 and 16 (toluene, 60 °C) afforded cycloadduct 17
as a single diastereomer in good yield. The stereochem-
istry of 17 was confirmed by X-ray crystallographic
analysis of a crystalline derivative (cf. Scheme 7). Global
deprotection (HF-CH3CN), followed by HF neutraliza-
tion with Me3SiOMe, afforded epoxyquinol 18, a scaffold
with a slightly altered framework relative to the male-
imide cycloadducts 13 and 15.
Hydroxyl-Directed Diels-Alder Cycloaddition.
Recently, Barriault and co-workers reported the highly
stereoselective hydroxyl-directed Diels-Alder reaction of
secondary and tertiary dienols with activated dieno-
philes.14 This interesting precedent prompted us to
evaluate use of a free secondary alcohol to direct Diels-
Alder cycloaddition to the proximal face of diene 19
(Scheme 4). Desilylation of 11 with TBAF (THF, rt)
afforded dienol 19 in good yield. Treatment of 19 with
N-benzylmaleimide under conditions described by
Barriault14a (MgBr2, Et3N, CH2Cl2, -15 °C) afforded
20 to an amido lactone as evidenced by IR analysis.12
A
control experiment of 19 with N-benzylmaleimide under
thermal conditions (80 °C, toluene) afforded a 2:1 mixture
of diastereomers in which the apparent hydroxyl-directed
isomer 20 was the minor product. Deprotection of 20
using polymer-supported arylsulfonic acid (MP-TsOH)
resin (1.45 mmol/g)15 afforded the angular epoxyquinol
21, which has a distinct skeleton in comparison to
cycloadducts such as 13.
Further Elaboration of Angular Scaffolds. With
a number of angular epoxyquinol scaffolds in hand, we
proceeded to explore further transformations to achieve
skeletal modifications. Initial efforts to develop selective
Michael addition or epoxide opening of substrates includ-
ing 13 and 18 using amines and thiols as nucleophiles
typically generated mixtures of products. Thus, we chose
to hydrogenate the enone of scaffolds such as 13. Com-
parison of nickel-,16 palladium-,17 rhodium-,18 and copper-
based19 catalysts showed no reactivity for reduction of
substrate 13. After considerable experimentation, we
found that Adams’ catalyst (PtO2) gave promising re-
(15) Flynn, D. L.; Crich, J. Z.; Devraj, R. V.; Hockerman, S. L.;
Parlow, J. J.; South, M. S.; Woodard, S. S. J. Am. Chem. Soc. 1997,
119, 4874.
(16) Jackman, L. M.; Webb, R. L.; Yick, H. C. J. Org. Chem. 1982,
47, 1824.
(17) Brown, M.; Piszkiewicz, L. W. J. Org. Chem. 1967, 32, 2018.
(18) Berkowitz, W. F.; Choudhry, S. C.; Hrabie, J. A. J. Org. Chem.
1982, 47, 824.
(14) (a) Barriault, L.; Thomas, J. D. O.; Clement, R. J. Org. Chem.
2003, 68, 2317. (b) Barriault, L.; Ang, P. J. A.; Lavigne, R. M. A. Org.
Lett. 2004, 6, 1317. For selected other examples, see: (c) Trost, B. M.;
Lee, D. C. J. Org. Chem. 1989, 54, 2271. (d) Sanyal, A.; Snyder, J. K.
Org. Lett. 2000, 2, 2527. (e) Atherton, J. C. C.; Jones, S. Tetrahedron
Lett. 2001, 42, 8239.
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Elaboration of Angular Epoxyquinol Scaffolds
SCHEME 5. Elaboration of an Angular
Epoxyquinol Scaffold
To expand the skeletal diversity of angular scaffolds,
hydrogenation using Adams’ catalyst (PtO2) was con-
ducted on substrate 20. In the event, hydrogenation of
20 cleanly afforded a single cis isomer (J1,2 ) 6.8 Hz)
possessing an intact epoxide, identified as the dearoma-
tized compound 25 by both 1H NMR and HRMS analysis
(Scheme 6).12 Literature reports have demonstrated
examples of catalytic hydrogenation of the benzene
nucleus using Adam’s catalyst.24 The high selectivity
observed in the hydrogenation of 20 may be explained
by the cup-shaped conformation of 20 (see Scheme 6,
ChemBats3D structure) in which case hydrogenation of
20 occurs from the less hindered, convex face. In contrast
to this result, attempted hydrogenation of the TBS-
protected [4 + 2] cycloadduct 12 under the same condi-
tions led to no reaction. The low reactivity of 12 toward
hydrogenation is likely due to strong steric hindrance,
in which the bulky TBS group blocks the â face and the
benzyl group blocks the R face. Deprotection of 25 using
MP-TsOH resin in acetone/water provided epoxyquinol
26 in good yield. 1H NMR analysis revealed that 26 was
a decalin-like cis isomer (J1,2 ) 5.6 Hz).
Angular urazole cycloadduct 18 was also subjected to
hydrogenation using Adams’s catalyst. This reaction was
not clean and showed low selectivity for cis versus trans
isomers. Interestingly we found that the ketal-protected
compound 17 could be hydrogenated using 15 wt. % of
Adams’ catalyst to generate a single diastereomer 27 in
which case the epoxide moiety was intact (Scheme 7).
Analysis of a molecular model of 17 shows that the
hydrogenation has proceeded from the convex (â) face of
angular scaffold 17. Further deprotection of 27 afforded
the angular epoxyquinol 28. X-ray (Figure 1) and 1H
NMR analysis unambiguously revealed that hydroge-
nated product 28 was a cis decalin-like isomer. In an
effort to epimerize cis isomer 28 to a trans ring system,
the compound was treated with 1.0 equiv of anhydrous
HCl which afforded only starting material. Analysis of
the X-ray structure of 28 also shows that this cis isomer
has a O1-C1-C2-H2 dihedral angle of -47°, which is not
suitable for σ-C-H/π*-CdO overlap to facilitate enoliza-
sults.20 Treatment of 13 with 5 wt % PtO2 under a
hydrogen atmosphere in EtOAc afforded the hydro-
genated cis diol 22 as a major product (Scheme 5). The
epoxyketone moiety also underwent hydrogenolysis un-
der the reaction conditions.21 The ratio of cis and trans
isomers 22 and 23 was determined to be 9:1 by 1H NMR
analysis. The relative stereochemistries of 22 and 23
were determined by evaluation of 1H NMR coupling
constants. In decalin ring systems, the trans protons
generally have larger coupling constants than the cor-
responding cis protons.22 Interestingly, we found that by
treatment with 1.0 equiv of anhydrous HCl, cis isomer
22 could be cleanly transformed to the thermodynami-
cally favored trans isomer 23 in good yield.23 Further
evaluation of acid catalysts led to the serendipitous
finding that trans isomer 23 could be cleanly dehydrated
to enone 24 using 0.2 equiv of a macroporous p-toluene-
sulfonic acid resin (MP-TsOH). Thus, hydrogenation,
epimerization, and dehydration processes may be em-
ployed to alter the angular epoxyquinol scaffold to
generate a number of advanced scaffolds with distinct
molecular frameworks bearing stereochemical and func-
tional group diversity.2
SCHEME 6. Hydrogenation of Cycloadduct 20
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Lei et al.
SCHEME 7. Hydrogenation and Attempted
Epimerization of the Angular Urazole Scaffold
SCHEME 8. Attempted Reductive N-N Bond
Cleavage
SCHEME 9. Scaffold Modification Using Oxime
Formation
tion/deprotonation to the corresponding trans isomer of
28 (Figure 1).25
FIGURE 1. Proton alignment for enolization.
Attempted Reductive N-N Bond Cleavage. We
were also interested to further modify urazole cycload-
ducts such as 28. Recently, Lee and co-workers reported
a new approach to macrocyclic amides via ring expansion
induced by reductive N-N bond cleavage of a bicyclic
hydrazine derivative using Na/NH3.26 We applied this
protocol to the elaboration of the urazole scaffold with
the hope to generate a nine membered-ring dilactam. Use
of milder reductive conditions (e.g., SmI2, DMPU)27 led
to epoxide opening and dehydration to afford enone 29
in 88% yield (Scheme 8). In an effort to selectively reduce
the N-N bond, we employed protected compound 30,
prepared in analogy to 27, in a dissolving metal (Na/NH3)
reduction. In the event, ring-opened compound 31 was
isolated in 75% yield. NMR studies, including HMBC
analysis12 revealed that 31 was a primary formamide
produced via N-N bond reduction and selective reductive
ring opening.28
Further Scaffold Modification Using Oxime For-
mation. Scaffolds such as 22, 23, and 28 each contain
carbonyl groups which may be subjected to further
diversification. We first chose alkoxyamines for diversi-
fication of the ketone moiety.29 Oxime formation with
(19) (a) Lipshutz, B. H.; Chrisman, W.; Noson, K.; Papa, P.; Sclafani,
J. A.; Vivian, R. W.; Keith, J. M. Tetrahedron 2000, 56, 2779. (b)
Moritani, Y.; Appella, D. H.; Jurkauskas, V.; Buchwald, S. L. J. Am.
Chem. Soc. 2000, 122, 6797.
(20) Horwell, D. C.; Timms, G. H. Synth. Commun. 1979, 9, 223.
(21) Schiehser, G. A.; White, J. D. J. Org. Chem. 1980, 45, 1864.
(22) Hagiwara, K.; Kon-no, M.; Nakano, T.; Uda, H. J. Chem. Soc.,
Perkin Trans. 1 1994, 2417.
(23) Yang, Z.; Shannon, D.; Truong, V. L.; Deslongchamps, P. Org.
Lett. 2002, 4, 4693.
(24) (a) Smith, H. A.; Alderman, D. M., Jr.; Shacklett, C. D.; Welch,
C. M. J. Am. Chem. Soc. 1949, 71, 276. (b) Smith, H. A.; Alderman, D.
M., Jr.; Shacklett, C. D.; Welch, C. M. J. Am. Chem. Soc. 1949, 71,
3772. (c) Germain, J. E.; Bourgeois, Y. Bull. Soc. Chim. Fr. 1960, 2127.
(25) (a) Corey, E. J.; Sneen, R. A. J. Am. Chem. Soc. 1956, 78, 6269.
(b) For a review of stereoelectronic control in the reactions of ketones
and their enolates, see: Pollack, R. M. Tetrahedron 1989, 45, 4913.
(26) Kim, Y. J.; Lee, D. Org. Lett. 2004, 6, 4351.
(28) For an example of amide bond cleavage by hydride reduction,
see: Wang, J. J.; Hu, W. P.; Chung, H. W.; Wang, L. F.; Hsu, M. H.
Tetrahedron 1998, 54, 13149.
(27) Enders, D.; Funabiki, K. Org. Lett. 2001, 3, 1575.
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Elaboration of Angular Epoxyquinol Scaffolds
SCHEME 10. Library Design
scaffolds, we initiated efforts toward library synthesis.
Our library synthesis strategy is depicted in Scheme 10.
Condensation of the angular ketone-bearing scaffolds
with different nucleophiles at a late stage may be used
to incorporate a final diversification point. Three struc-
turally distinct ketone scaffolds prepared by hydrogena-
tion and epimerization of angular epoxyquinol scaffolds
were selected for library synthesis (vide infra), in which
case topological diversity was achieved through stereo-
chemistry. A key step in our library synthesis was the
highly diastereoselective [4 + 2] Diels-Alder cycloaddi-
tion of epoxyquinol diene 11 with maleimides and tri-
azolinediones to generate highly funtionalized and com-
plex angular polycyclic angular scaffolds.6a Due to dif-
ficulty in using MgBr2 in a parallel synthesis setting,
we did not include the hydroxyl-directed Diels-Alder
cycloadducts (cf. 21, Scheme 4) in our final library
synthesis. Both skeletal and functional diversity are
thus achieved by choice of the dienophile component.
Diene 11 was prepared rapidly from the chiral non-
racemic bromo-epoxyketone 2 in good yield (Scheme
2).
Parallel Synthesis of Angular Epoxyquinol Scaf-
folds. We employed parallel solution-phase synthesis9
approaches to produce angular epoxyquinol scaffolds
using [4 + 2] cycloaddition with maleimide and tri-
azolinedione dienophiles. Treatment of diene 11 with 10
diverse maleimides31 (1.2 equiv, inset A, Scheme 11) in
toluene at 80 °C for 6 h afforded the Diels-Alder
cycloadducts A′1-A′10. Subsequently, polymer-sup-
ported anthracene dienophile scavenger resin 3732 (1.12
mmol/gram, 2.0 equivalents) was added to the reaction
mixture to sequester excess maleimide. After 12 h, the
reaction mixtures were filtered, concentrated, and treated
with 5% HF/CH3CN, followed by HF scavenging using
the liquid scavenger TMSOMe13 to afford 10 angular
epoxyquinol scaffolds A1-10 in good yield and purity.
For urazole-containing scaffolds (Scheme 12), a hydro-
genation step was incorporated after the [4 + 2] cycload-
ditions and scavenging. Final deprotection led to four
additional epoxyquinol scaffolds S21-24. Figure 2 shows
the angular epoxyquinol scaffolds prepared using the
Diels-Alder cycloaddition, dienophile scavenging, hy-
drogenation, and silyl deprotection sequence.
ketones 22 and 23 was conducted in CH2Cl2 using 1.0
equiv of o-benzylhydroxyamine (Scheme 9). Cis compound
22 was found to afford a 1:1 mixture of E/Z isomers 32.
The E/Z isomers were not separable by reversed-phase
HPLC. Treatment of the E/Z mixture with 1.0 equivalent
of anhydrous HCl afforded a 5:1 mixture of E/Z isomers.
The two isomers were assigned in the 1H NMR based on
the difference of chemical shifts of the methylene pro-
tons.30 Due to deshielding effects, the methylene protons
of E oxime isomer experience a greater downfield (δ1,2
)
2.97 ppm) than the corresponding Z isomer protons (δ1,2
) 2.70 ppm). In this acid-catalyzed oxime isomerization
process, epimerization of cis decalin-like ring to the trans
structure was not observed. Interestingly, treatment of
trans compound 23 with o-benzylhydroxyamine produced
the E isomer 33 as the major product. For the relatively
labile epoxyketone scaffold 28, 1.0 equiv of acetic acid
was needed to catalyze the oxime formation, and ethyl
acetate was found to be an optimal solvent. A mixture of
1
E/Z isomers 34 (>5:1, based on H NMR analysis) was
isolated in good yield. In contrast, attempted oxime
formation of epoxyketone 13 and enone 24 did not afford
clean reactions.
Library Design. After successful methodology devel-
opment including elaboration of angular epoxyquinol
SCHEME 11. Parallel Synthesis of Maleimide-Derived, Angular Epoxyquinol Scaffolds
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Lei et al.
SCHEME 12. Synthesis of Urazole-Containing Scaffolds
SCHEME 13. Synthesis of Advanced Scaffolds
Synthesis of Advanced Scaffolds. Hydrogenation of
the angular epoxyquinol scaffolds A1-10 with 5 wt %
Adams’ catalyst (PtO2) generated the cis isomers S1-10
as major products (Scheme 13). Purification of the cis
isomers by silica gel chromatography provided the pure
cis scaffolds S1-10 in 35-50% yields. As previously
described (cf. Scheme 5), the mixtures of cis and trans
isomers were subjected to epimerization using a slight
excess of anhydrous HCl to generate the pure trans
scaffolds S11-20 in 30-44% yields based on A1-10.
Dehydration of S11-20 was further conducted using MP-
TsOH resin to afford enones D1-10.
Library Synthesis. Final carbonyl diversification
reactions of advanced scaffolds were conducted in the
solution phase using a resin-scavenging protocol (Scheme
14). Twenty angular maleimide scaffolds S1-20 were
incubated with nine different nucleophiles including six
alkoxyamines33 and three carbazates (1.05 equiv, rt, 30
h) in anhydrous CH2Cl2; four angular urazole scaffolds
S21-24 were treated with five alkoxyamines (1.00 equiv,
rt, 12 h) and acetic acid (1.0 equiv) in anhydrous EtOAc.
After condensation, reaction mixtures were treated with
polymer-supported methylisatoic anhydride resin34 (2.65
(29) For a review of oxime formation, see: (a) A˜ bele, E.; Lukevics,
E. Org. Prep. Proced. Int. 2000, 32, 235-264. For recent examples of
library syntheses employing alkoxyamines as diversity reagents, see:
(b) Maly, D. J.; Choong, I. C.; Ellman, J. A. Proc. Natl. Acad. Sci. U.S.A.
2000, 97, 2419. (c) Ishikawa, T.; Kamiyama, K.; Matsunaga, N.;
Tawada, H.; Iizawa, Y.; Okonogi, K.; Miyake, A. J. Antibiot. 2000, 53,
1071. (d) Pelish, H. E.; Westwood, N. J.; Feng, Y.; Kirchhausen, T.;
Shair, M. D. J. Am. Chem. Soc. 2001, 123, 6740. (e) Armstrong, J. I.;
Ge, X.; Verdugo, D. E.; Winans, K. A.; Leary, J. A.; Bertozzi, C. R.
Org. Lett. 2001, 3, 2657. (f) Wipf, P.; Reeves, J. T.; Balachandran, R.;
Day, B. W. J. Med. Chem. 2002, 45, 1901.
(30) (a) Karabatsos, G. J.; Taller, R. A. J. Am. Chem. Soc. 1964, 86,
4373. (b) Karabatsos, G. J.; Taller, R. A. Tetrahedron 1968, 24, 3347.
(31) For details on the syntheses of dienophiles M5 and T4, see the
Supporting Information.
(32) For the synthesis and application of dienophile scavenger resin
37, see: Lei, X.; Porco, J. A., Jr. Org. Lett. 2004, 6, 795.
(33) (a) B1, B2, B6, B7, and B8 are commercially available. (b) For
the syntheses of B3, B4, and B5, see: Supporting Information. (c) For
synthesis of B9, see: Renaudet, O.; Dumy, P. Tetrahedron Lett. 2001,
42, 7575.
FIGURE 2. Angular epoxyquinol scaffolds (yield (%) and
purity (%) shown in parentheses). Purity was measured by
HPLC/ELSD.
6480 J. Org. Chem., Vol. 70, No. 16, 2005

Elaboration of Angular Epoxyquinol Scaffolds
SCHEME 14. Library Synthesis
mmol/g, 2.0 equiv) in CH2Cl2 to afford the desired
condensation products after filtration of the resin and
concentration in vacuo in good yield and purity. Applica-
tion of this general protocol led to the preparation of 200
highly functionalized and complex compounds. The li-
brary was fully characterized by LC/MS.12 The yield
range was 73-95%, and the purity range was 85-99%
(ELSD).35
Analysis of Library Members. We included all
scaffolds, enones, oximes and alkylidenecarbazates in the
final library (total ) 244 compounds). As shown in Figure
3, analysis of library members reveals that there are 10
distinct frameworks possessing different stereochemistry
and functional groups. Three (6-6-5) fused rings com-
prise the basic skeleton with five to seven continuous
stereogenic centers. Representative natural products with
6-6-5 ring systems are shown in Figure 4.36 Interest-
ingly, these steroid architectures display functional group
(e.g., enone, diol, and glycoside) and stereochemical
diversity which is analogous to that found in library
members (Figure 3). Evaluation of two important chemi-
cal properties (molecular weight and cLogP) of our final
library members indicates that the molecular weight
range is 263-766 (average molecular weight ) 467) and
the cLogP range is -0.73-5.83 (average cLogP ) 2.21)
(Figure 3).37
Preliminary Biological Evaluation. The 244-mem-
bered library was screened to search for inhibitors of
induction of heat shock proteins (HSPs). HSPs are stress-
inducible proteins that serve as molecular chaperones in
protein folding and degradation. HSPs also protect cells
from a variety of stressful treatments, including anti-
cancer drugs.38 Targeting activity of a major heat shock
protein Hsp90 has been suggested as a novel approach
to cancer therapy.39 In fact, specific inhibitors of Hsp90,
including radicicol and geldanamycin and its derivatives,
have been developed and tested in clinical trials.40 In the
treatment of prostate and some other cancers, radiation
and other therapies can be combined with hyperthermia
in order to facilitate killing of cancer cells, especially
under hypoxic conditions of solid tumors. Hyperthermia
can cause rapid and strong induction of a cohort of
HSPs,41 which may counteract its antitumor activity.
Furthermore, development of resistance to hyperthermia
was associated with overproduction of certain HSPs,
including Hsp72 and Hsp27.42 Inhibitors of the heat
shock response have been identified, including querce-
(34) Coppola, G. M. Tetrahedron Lett. 1998, 39, 8233.
(35) HPLC purity values refer to the sum of the HPLC (ELSD) area
% for oxime E/Z isomers.
(37) Lipinski, C. A.; Lombardo, F.; Dominy, B. W.; Feeney, P. J. Adv.
Drug Delivery Rev. 1997, 23, 3.
(38) For reviews of HSPs, see: (a) Benjamin, I. J.; McMillan, D. R.
Circ. Res. 1998, 83, 117. (b) Gabai, V. L.; Sherman, M. Y. J. Appl.
Physiol. 2002, 92, 1743.
(36) Amurensoside D: (a) Riccio, R.; Iorizzi, M.; Minale, L.; Oshima,
Y.; Yasumoto, T.. Starfish saponins. J. Chem. Soc., Perkin Trans. 1
1988, 6, 1337. Withaferin A: (b) Kupchan, S. M.; Anderson, W. K.;
Bollinger, P.; Doskotch, R. W.; Smith, R. M.; Renauld, J. A.; Schnoes,
H. K.; Burlingame, A. L.; Smith, D. H. J. Org. Chem. 1969, 34, 3858.
Hyocholsaeure: (c) Lehmann, G.; Tepper, H. J.; Hilgetag, G. Biochem.
Z. 1964, 340, 75. Haplosamate A: (d) Qureshi, A.; Faulkner, D. John
Tetrahedron 1999, 55, 8323.
(39) Goetz, M. P.; Toft, D. O.; Ames, M. M.; Erlichman, C. Ann.
Oncol. 2003, 14, 1169.
(40) Banerji, U.; Judson, I.; Workman, P. Curr. Cancer Drug Targets
2003, 3, 385.
(41) (a) Roti, J. L. Int. J. Hyperthermia 2004, 20, 109. (b) Jolly, C.;
Morimoto, R. I. J. Natl. Cancer Inst. 2000, 92, 1564.
J. Org. Chem, Vol. 70, No. 16, 2005 6481

Lei et al.
FIGURE 3. Representative library members.
erance and sensitized cells to heat shock. Furthermore,
quercetin was demonstrated to enhance cancer regression
after hyperthermia treatments in mice xenografts.45 All
of these compounds, however, have relatively low potency
and low specificity and exhibit significant toxicity that
is unrelated to their activity as inhibitors of the heat
shock response. Therefore, we seek to identify small
molecules that will potently inhibit induction of HSPs
to overcome resistance and promote antitumor activity
of hyperthermia.
To identify inhibitors of induction of HSPs, we devel-
oped a two-stage screen of chemical libraries. The first
stage involved a cell-based screen for general HSP-
mediated refolding of heat-denatured luciferase. In a pilot
screen, we evaluated 244 compounds from the chemical
library; 22 compounds showed significant inhibitory
effects. The hits obtained at the first stage of the screen
were further evaluated in the second stage involving a
direct testing for inhibition of induction of HSPs by
immunoblotting. As a readout, we used induction of
Hsp72 in cells exposed to mild heat shock. In this assay,
Chinese hamster ovary CHO cells were incubated with
various concentrations of the chemical compounds for 16
h at 37 °C before treatment. Control cells were kept
without compounds. To induce HSPs, cells were exposed
to mild heat shock by immersing plates in a 45 °C water
bath for 10 min. Cells were kept for 6 h at 37 °C to allow
for induction of HSPs. Finally, cells were lysed and the
levels of Hsp72 were monitored by immunoblotting with
anti-Hsp72 antibody. Six compounds (scaffolds A1, A3,
A6, A7, A8, and A9) showed significant activities in this
assay (Figure 5). IC50’s for inhibition of Hsp 72 induction
for A1, A3, A6, A7, A8, and A9 were found to be 0.75,
1.5, 0.75, 3.0, 1.5, and 1.5 µM, respectively.12 These initial
results suggest that the epoxyketone and enone moieties
FIGURE 4. Representative natural products with 6-6-5 ring
systems.
tin,43 KNK437,44 and stresgenin B.45 These compounds
have been shown to repress development of thermotol-
(42) (a) Calderwood, S. K.; Asea, A. Int. J. Hyperthermia 2002, 18,
597. (b) Khoei, S.; Goliaei, B.; Neshasteh, R. A.; Deizadji, A. FEBS
Lett. 2004, 561, 144. (c) Coss, R. A.; Storck, C. W.; Wachsberger, P. R.;
Reilly, J.; Leeper, D. B.; Berd, D.; Wahl, M. L. Int. J. Hypertherm. 2004,
20, 93.
(43) Asea, A.; Ara, G.; Teicher, B. A.; Stevenson, M. A.; Calderwood,
S. K. Int. J. Hypertherm. 2001, 17, 347.
(44) (a) Yokota, S.; Kitahara, M.; Nagata, K. Cancer Res. 2000, 60,
2942. (b) Ohnishi, K.; Takahashi, A.; Yokota, s.; Ohnishi, T. Int. J.
Radiat. Biol. 2004, 80, 607.
(45) Akagawa, H.; Takano, Y.; Ishii, A.; Mizuno, S.; Izui, R.;
Sameshima, T.; Kawamura, N.; Dobashi, K.; Yoshioka, T. J. Antibiot.
1999, 52, 960.
6482 J. Org. Chem., Vol. 70, No. 16, 2005

Elaboration of Angular Epoxyquinol Scaffolds
control and with cells incubated with the compounds.
This result reinforces that compounds 18 and A1 do not
have general inhibitory effects on protein synthesis, but
rather their effects on induction of HSPs are specific.12
Compounds identified in this pilot screen will be further
developed to obtain a potent, nontoxic inhibitor of induc-
tion of HSPs. These preliminary biological screening
results indicate that the angular epoxyquinol frameworks
described herein should provide access to novel probe
molecules for biological research.
Conclusion
We have developed an efficient approach to a highly
functionalized, complex and natural product-like chemi-
cal library. The key strategies involve highly stereo-
controlled [4 + 2] Diels-Alder cycloaddition of a chiral
nonracemic epoxyquinol diene with reactive dienophiles
to generate angular epoxyquinol scaffolds and their
further diversification using hydrogenation, epimeriza-
tion, and carbonyl condensation with different building
blocks. Related studies toward the generation of different
skeletal epoxyquinol scaffolds using hydroxyl-directed
Diels-Alder cycloadditions, and reductive N-N bond
cleavage have also been addressed. As a result, a 244-
membered library with 10 distinct skeletons was ef-
ficiently synthesized. Preliminary biological screening of
this chemical library revealed that six compounds showed
significant inhibition of the induction of Hsp 72 with
IC50’s in the low micromolar range. Further biological
evaluation of the epoxyquinol-derived library is in progress
and will be reported in due course.
FIGURE 5. Inhibition of induction of Hsp72 by angular
compounds A1, A3, A6, A7, A8, and A9 (3.0 µM concentration).
may be required for the inhibition of HSPs. Therefore,
angular, urazole-containing compound 18 (Scheme 3) was
also tested for the inhibition of Hsp72 induction in the
direct assay and was found to inhibit induction (IC50
)
1.5 µM).12 To test for nonspecific effects of the selected
compounds, we studied whether they inhibit induction
of a reporter luciferase under the control of tetracycline-
regulated promoter. No inhibition of the induction of
luciferase by tetracycline was found for any of the
compounds. These data indicate that the compounds do
not have general effects on inhibition of transcription or
translation or enhancement of protein degradation. Com-
pounds 18 and A1 were evaluated in an additional control
experiment. Cells were infected with a retrovirus encod-
ing the green fluorescent protein (GFP), and after 16 h
the compounds were added to the cells. The level of the
GFP was measured by immunoblotting. After 16 h of
infection, the cells did not express detectable levels of
GFP; after 40 h, similar levels of the GFP were found in
Acknowledgment. Financial support from the
NIGMS CMLD initiative (P50 GM067041) is gratefully
acknowledged. We thank Prof. John Snyder, Prof. Scott
Schaus, Dr. Aaron Beeler, and Dr. Ruichao Shen
(Boston University) for helpful discussions. We also
thank Mr. Joshua Giguere for providing alkoxyamine
B5 and Metter-Toledo AutoChem (Millersville, MD) for
assistance with instrumentation.
Supporting Information Available: Experimental pro-
cedures and characterization data for new compounds. This
material is available free of charge via the Internet at
http://pubs.acs.org.
JO050956Y
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