Applications of the squarate ester cascade to the expeditious synthesis of hypnophilin, coriolin, and ceratopicanol

Article abstract of DOI:10.1021/ja020474t

The first applications of the squarate ester cascade to natural products synthesis have been realized. Only 10 laboratory steps mediate the conversion of diisopropyl squarate to (±)-hypnophilin (8). Key reactions include a combination of chlorination, reduction, dehydration, and oxidation maneuvers in the proper sequence. A penultimate precursor to 8 has previously been converted into coriolin (9), thereby allowing a formal synthesis of racemic 9 also to be claimed. A rather different strategy was employed to arrive at (±)-ceratopicanol (10). Of the seven steps involved, three consisted of the use of lithium in liquid ammonia. The three divergent synthetic objectives realized in these experiments involved (a) generation of an extended enolate anion and its regioselective C-methylation at the γ-carbon; (b) unprecedented reductive cleavage of a β-isopropoxy group in a 2,3-diisopropoxy-2-cyclopentenone setting; and (c) conventional conversion of an α-alkoxy ketone to the parent carbonyl system. Thus, the appreciable enhancement in structural complexity offered by the squarate cascade holds considerable potential for the concise synthesis of constitutionally intricate targets.

Full text of DOI:10.1021/ja020474t

Published on Web 07/12/2002
Applications of the Squarate Ester Cascade to the
Expeditious Synthesis of Hypnophilin, Coriolin, and
Ceratopicanol
Leo A. Paquette* and Feng Geng
Contribution from the EVans Chemical Laboratories, The Ohio State UniVersity,
Columbus, Ohio 43210
Received April 1, 2002
Abstract: The first applications of the squarate ester cascade to natural products synthesis have been
realized. Only 10 laboratory steps mediate the conversion of diisopropyl squarate to (()-hypnophilin (8).
Key reactions include a combination of chlorination, reduction, dehydration, and oxidation maneuvers in
the proper sequence. A penultimate precursor to 8 has previously been converted into coriolin (9), thereby
allowing a formal synthesis of racemic 9 also to be claimed. A rather different strategy was employed to
arrive at (()-ceratopicanol (10). Of the seven steps involved, three consisted of the use of lithium in liquid
ammonia. The three divergent synthetic objectives realized in these experiments involved (a) generation
of an extended enolate anion and its regioselective C-methylation at the γ-carbon; (b) unprecedented
reductive cleavage of a â-isopropoxy group in a 2,3-diisopropoxy-2-cyclopentenone setting; and (c)
conventional conversion of an R-alkoxy ketone to the parent carbonyl system. Thus, the appreciable
enhancement in structural complexity offered by the squarate cascade holds considerable potential for the
concise synthesis of constitutionally intricate targets.
Cascade reaction sequences, those processes that take place
via the sequential operation of several discrete chemical steps
following initiation, are inherently attractive for several reasons.
Scheme 1
1
For example, they often proceed with high levels of atom
2
economy and are generally accompanied by significant in-
creases in structural complexity. Their practical importance is
appreciably enhanced when the end product of the domino
3
process is amenable to synthetic modifications that deliver key
target compounds in particularly concise fashion. A decade ago,
the discovery was made in these laboratories that the sequential
1
,2-addition of a pair of alkenyl anions (either the same or
different) to a squarate ester sets into motion a sequence of
mechanistic events exemplified in Scheme 1.4 The second
nucleophilic attack, which can proceed in an anti or syn manner
5
to provide 1 and 2, respectively, is subject to steric or chelation
5
,6
control. When 1 is formed, there follows a charge-driven
*
Author for correspondence. E-mail: paquette.1@osu.edu.
(
(
(
1) Tietze, L. F. Chem. ReV. 1996, 96, 115.
2) Trost, B. M. Angew. Chem., Int. Ed. Engl. 1995, 34, 259.
3) (a) Paquette, L. A.; Wyvratt, M. J. J. Am. Chem. Soc. 1974, 96, 4671. (b)
Wyvratt, M. J.; Paquette, L. A. Tetrahedron Lett. 1974, 2433. (c) Paquette,
L. A., Wyvratt, M. J.; Berk, H. C.; Moerck, R. E. J. Am. Chem. Soc. 1978,
1
00, 5845.
(
(
4) Paquette, L. A. Eur. J. Org. Chem. 1998, 1709.
5) (a) Negri, J. T.; Morwick, T.; Doyon, J.; Wilson, P. D.; Hickey, E. R.;
Paquette, L. A. J. Am. Chem. Soc. 1993, 115, 12189. (b) Paquette, L. A.;
Morwick, T. J. Am. Chem. Soc. 1995, 117, 1451. (c) Morwick, T.; Doyon,
J.; Paquette, L. A. Tetrahedron Lett. 1995, 36, 2369. (d) Morwick, T. M.;
Paquette, L. A. J. Org. Chem. 1997, 62, 627. (e) Paquette, L. A.; Morwick,
T. M.; Negri, J. T.; Rogers, R. D. Tetrahedron 1996, 52, 3075. (f) Tae, J.;
Paquette, L. A. Can. J. Chem. 2000, 78, 689.
(
6) (a) Paquette, L. A.; Doyon, J.; Kuo, L. H. Tetrahedron Lett. 1996, 37,
3
299. (b) Paquette, L. A.; Hamme, A. T., II; Kuo, L. H.; Doyon, J.;
Kreuzholz, R. J. Am. Chem. Soc. 1997, 119, 1242.
10.1021/ja020474t CCC: $22.00 © 2002 American Chemical Society
J. AM. CHEM. SOC. 2002, 124, 9199-9203
9
9199

A R T I C L E S
Paquette and Geng
Scheme 2
Scheme 3
a
Conditions: (a) Br2, CH2Cl2, 0 °C, 40 min; Et3N, 20 °C, 3 h (77%).
b) Ethylene glycol, Dowex 50X4-400, C6H6, reflux 3 days (80% br sm).
c) tert-Butyllithium, THF, -78 °C. (brsm ) based on recovered starting
(
(
material)
obtained via a 10-step sequence involving a combination of
chlorination, reduction, dehydration, and oxidation maneuvers
in the proper sequence. This degree of conciseness serves as
the basis for a formal total synthesis of coriolin (9) as well.1
An entirely different strategy has opened up a seven-step route
5,16
1
7,18
to the fungal metabolite ceratopicanol (10).
The latter
sequence is noteworthy in that three of the synthetic manipula-
tions involve the use of lithium in liquid ammonia for ac-
complishing different synthetic objectives.
Results and Discussion
(
()-Hypnophilin and (()-Coriolin. Central to the quest for
conrotatory opening of the cyclobutene ring with generation of
hypnophilin (8) is the seminal recognition by Steglich and co-
workers of its antitumor activity and ability to inhibit Gram-
positive and Gram-negative bacteria as well as diverse fungi
and yeasts. In addition, this linearly fused triquinane embodies
in its A ring an uncommon Michael acceptor arrangement that
is presumably responsible for its biological activity. These
considerations have prompted three successful syntheses of 8.
While the Little group made recourse to a 1,3-diyl trapping
the coiled 1,3,5,7-octatetraene 3. These intermediates are capable
6
4,5b
of rapid helical equilibration and regioselective cyclization
and have the ability to advance to product with asymmetric
13
7
induction. Arrival at 4 is realized somewhat more directly via
a dianionic oxy-Cope rearrangement when cis addition as in 2
8
operates. These mechanistic options can be distinguished when
9
additional stereochemical markers are present. The proper
positioning of an acetal group as in 4 allows for the operation
of an elimination reaction with the formation of 5. This step
guarantees that the subsequent transannular aldol ring closure
is completely regiodirected and that a highly functionalized
linear triquinane will be generated.10
1
4a
reaction as their key step, Curran deployed a tandem radical
cyclization, and Weinges completed an enantioselective route
by suitable structural modification of catalpol.
14b
1
4c
In 1969, the Umezawa group reported the isolation of coriolin
1
5
In this report, we detail for the first time the manner in which
select end products of these deep-seated cascade processes can
(9) and defined its structure. The more highly oxygenated
perimeter of this tricyclopentanoid system and its reputed
antitumor activity was sufficient cause for many investigators
to undertake its total synthesis. Since a key intermediate in
the routes devised by the Danishefsky, Ikegami, and Tatsuta
groups intersects our own abbreviated scheme, the present
undertaking also constitutes a stereocontrolled formal synthesis
of coriolin (9).
1
1
be utilized for the expeditious synthesis of natural products.
1
6
These goals demand that the signature part structure 7, invariably
generated from diisopropyl squarate, be amenable to reductive,
oxygenative, and alkylative modification (Scheme 2). Little
information is currently available concerning the chemical
17
18a
18b
1
2
alteration of this highly oxygenated cyclopentenone system.
Considerable progress has now been made to offset this
deficiency. Thus, the sesquiterpene hypnophilin (8)13,14 has been
Retrosynthetically, we envisioned the lithiated acetal 11 to
be a properly delineated reactant. This building block was
conveniently generated by bromination-dehydrobromination of
the known 5,5-dimethyl-2-cyclopentenone, standard acetal-
ization, and halogen-metal exchange in the presence of tert-
butyllithium, as outlined in Scheme 3.
(
(
(
7) Paquette, L. A.; Tae, J. J. Org. Chem. 1998, 63, 2022.
1
9
8) Paquette, L. A.; Morwick, T. M. J. Am. Chem. Soc. 1997, 119, 1230.
9) (a) Paquette, L. A.; Kuo, L. H.; Hamme, A. T., II; Kreuzholz, R.; Doyon,
J. J. Org. Chem. 1997, 62, 1730. (b) Paquette, L. A.; Kuo, L. H.; Doyon,
J. Tetrahedron 1996, 52, 11625. (c) Paquette, L. A.; Kuo, L. H.; Doyon,
J. J. Am. Chem. Soc. 1997, 119, 3038. (d) Paquette, L. A.; Kuo, L. H.;
Tae, J. J. Org. Chem. 1998, 63, 2010.
(
10) (a) Paquette, L. A.; Doyon, J. J. Am. Chem. Soc. 1995, 117, 6799. (b)
Paquette, L. A.; Doyon, J. J. Org. Chem. 1997, 62, 1723.
11) Preliminary communication of a portion of this investigation: Geng, F.;
Liu, J.; Paquette, L. A. Org. Lett. 2002, 4, 71.
(15) Isolation: (a) Takeuchi, T.; Iinuma, H.; Iwanaga, J.; Takahashi, S.; Takita,
T.; Umezawa, H. J. Antibiot. 1969, 22, 215. (b) Takahashi, S.; Naganawa,
H.; Iinuma, H.; Takita, T.; Maeda, K.; Umezawa, H. Tetrahedron Lett.
1971, 1955. (c) Nakamura, H.; Takita, T.; Umezawa, H.; Kunishita, M.;
Nakayama, Y.; Iitaka, Y. J. Antibiot. 1974, 27, 301.
(16) Synthesis: (a) Reference 14a. (b) Reference 14b. (c) Weinges, K.; Braun,
R.; Huber-Patz, U.; Irngartinger, H. Liebigs Ann. Chem. 1993, 1133. (d)
For a more extensive compilation, see footnote 5 of ref 14a.
(
(
12) Morwick, T. M.; Paquette, L. A. J. Org. Chem. 1996, 61, 146.
13) Isolation: (a) Kupka, J.; Anke, T.; Giannetti, B. M.; Steglich, W. Arch.
Microbiol. 1981, 130, 223. (b) Giannetti, B. M.; Steffan, B.; Steglich, W.;
Kupka, J.; Anke, T. Tetrahedron 1986, 42, 3587. (c) Steglich, W. Pure
Appl. Chem. 1981, 53, 1233.
14) Prior synthetic studies: (a) Van Hijfte, L.; Little, R. D.; Petersen, J. L.;
Moeller, K. D. J. Org. Chem. 1987, 52, 4647. (b) Fevig, T. L.; Elliott, R.
L.; Curran, D. P. J. Am. Chem. Soc. 1988, 110, 5064. (c) Weinges, K.;
Iatridou, H.; Dietz, U. Liebigs Ann. Chem. 1991, 893. (d) See also:
Weinges, K.; Dietz, U.; Oeser, T.; Irngartinger, H. Anew. Chem., Int. Ed.
Engl. 1990, 29, 680.
(
(17) (a) Danishefsky, S.; Zamboni, R.; Kahn, M.; Etheredge, S. J. J. Am. Chem.
Soc. 1980, 102, 2097. (b) Danishefsky, S.; Zamboni, R.; Kahn, M.;
Etheredge, S. J. J. Am. Chem. Soc. 1981, 103, 3460.
(
(18) (a) Iseki, K.; Yamazaki, M.; Shibasaki, M.; Ikegami, S. Tetrahedron 1981,
37, 4411. (b) Tatsuta, K.; Akimoto, K.; Kinoshita, M. J. Antibiot. 1980,
33, 100.
(19) Padwa, A.; Curtis, E. A.; Sandanayaka, V. P. J. Org. Chem. 1996, 61, 73.
9200 J. AM. CHEM. SOC.
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VOL. 124, NO. 31, 2002

Synthesis of Hypnophilin, Coriolin, and Ceratopicanol
A R T I C L E S
Scheme 4
a
Conditions: (a) 11, THF, -78 °C, 5 min; CH2dCHLi, 0 °C, 2 h; rt 16 h; degassed NH4Cl solution; 36 h. (b) 10% H2SO4, overnight, rt (24% from 12).
c) CH3SO2Cl, Et3N, (DMAP), CH2Cl2, rt, overnight (72%). (d) Li, NH3, THF, -78 °C, 1.5 h; Li benzoate, -78 °C (72%). (e) LiAlH4, THF, -10 °C, 5 min;
(
1
3
N HCl (76%). (f) CH3OCH2Cl, (i-Pr)2NEt, CH2Cl2, rt, overnight (quant). (g) LDA, HMPA, THF, CH3I, rt, overnight (95%). (h) CH3Li, ether, 0 °C, 2 h;
0% H2SO4, THF, reflux 10 h (85%). (i) LDA, DMPU, THF; TMSCl; Pd(OAc)2, CH3CN, rt, overnight (60%). (j) K2CO3, 30% H2O2, H2O, CH2Cl2, rt, 3
h (80% br sm). (brsm ) based on recovered starting material)
The addition of 2 equiv of 11 to diisopropyl squarate (12)20
at -78 °C was followed by the introduction of excess vinyl-
lithium (Scheme 4). Irrespective of the stereoselectivity adopted
during the 1,2-addition of the second of these organometallic
reagents, our expectation was that both pathways would
eventuate in the formation of 4. Spontaneous â-elimination
alkylation of the saturated alkoxide was likely to be kinetically
dominant, and this routing was therefore not pursued.
10
within 4 to generate 5 is precedented in our earlier pilot studies
and was anticipated to advance via 5 into a fully regiocontrolled
transannular aldol cyclization to give 6. The cis, anti-fused
tricycle 13, which constitutes the doubly protonated form of 6,
was indeed isolated and purified chromatographically. However,
convenience dictated that mild acid hydrolysis to deliver 14 be
accomplished prior to workup. In this way, the diketone was
isolated in 24% yield.
The cis, anti, cis intra-ring stereorelationship present in 15
and 16 was conveniently corroborated by NOESY studies, the
implementation of which also confirmed the R-orientation of
the hydroxyl group, as required of hypnophilin (see formulas).
Dehydrative 1,3-carbonyl transposition within 16 was now
mandated, and success was realized by treatment with lithium
aluminum hydride followed by workup with dilute aqueous acid.
The choice of protecting group proved not to be crucial, and
we therefore settled on generation of the MOM ether 18. When
methyl iodide was added to the lithium enolate of 18, methy-
lation was achieved with excellent regioselectivity (95% yield
of 19) and retention of the cis ring fusion (for the usual
A comparable level of success was enjoyed during the
subsequent conversion of 14 into chloride 15 (72%). Since the
exposure to thionyl chloride and triethylamine proceeded with
1
2
retention of configuration, an SNi process is likely operative.
Attempts to directly replace the Cl atom in 15 by a methyl group
were next accorded attention, but to no avail. For example, no
reaction was observed in the presence of trimethylaluminum
under various conditions. In a similar vein, when the possible
dehydrochlorination of 15 failed, the planned conjugate addition
of several methylcopper reagents could not be evaluated.
21
thermodynamic reasons). At this juncture, treatment of 19 with
methyllithium and subsequent heating with 30% sulfuric acid
served to introduce the final carbon atom, effect dehydration,
hydrolyze the enol ether, and bring about removal of the MOM
group to furnish 20 (85%). This tactic set the stage for facile
introduction of a second conjugated double bond, now neces-
sarily endocyclic. This second site of unsaturation was generated
by oxidation of the silyl enol ether of 20 with palladium
These results led us to explore a more streamlined alternative
protocol consisting of dissolving metal reduction with lithium
liquid ammonia. Under these conditions, dechlorination and
stereocontrolled reduction of the ring C ketone carbonyl
occurred concurrently, without any deleterious consequences
in the highly oxygenated A ring. The inertness exhibited by
the latter subunit stems from transient conversion of the
chlorinated enone typified by 22 into the extended enolate 23.
Although the direct methylation of 23 was entertained, O-
2
2
acetate. Arrival at hypnophilin (8) was now accomplished by
(
21) Paquette, L. A.; Doherty, A. M. Polyquinane Chemistry; Springer-Verlag:
Berlin, 1987.
(22) (a) Roberts, M. R.; Schlessinger, R. H. J. Am.Chem. Soc. 1979, 101, 7626.
(b) Trost, B. M.; Nishimura, Y.; Yamamoto, K.; McElvain, S. S. J. Am.
Chem. Soc. 1979, 101, 1328. (c) Wender, P. A.; Lechleiter, J. C. J. Am.
Chem. Soc. 1980, 102, 6340.
(20) Liu, H.; Tomooka, C. S.; Moore, H. W. Synth. Commun. 1997, 27, 2177.
J. AM. CHEM. SOC.
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VOL. 124, NO. 31, 2002 9201

A R T I C L E S
Paquette and Geng
Scheme 5
a
Conditions: (a) KOt-Bu, DME, -78 °C f rt; 10% aq HOAc. (b) MCPBA, CH2Cl2; DBU, C6H6 (48% from 21 brsm). (c) Ac2O, DMAP, py (98%). (d)
LiOH, THF; 30% H2O2 (12%).
Scheme 6
a
Conditions: (a) CH3SO2Cl, DMAP, py (73%). (b) Li, NH3; CH3I, HMPA (74%). (c) Li, NH3 (75%). (d) H2, 5% Pd/C (91%). (e) Li, NH3 (77%). (f)
NaBH4, MeOH (75%).
regioselective epoxidation through the agency of hydrogen
peroxide and potassium carbonate. The spectral properties of
synthetic 8 were identical in all respects to those of the natural
product as supplied to us by Prof. D. Little and Dr. H. Schick.
these organometallics was available by alane reduction25 of
2-bromo-5,5-dimethylcyclopentenone, subsequent O-methyla-
tion, and lithium-halogen exchange as before. The second
nucleophile had its customary effect on heightening the overall
5
b,8
yield of the cascade,
in this case to the 44% level. The
As alluded to above, our abbreviated route to 21 also
represents a formal total synthesis of (()-coriolin (9) (Scheme
spontaneous loss of methanol that follows ring closure to the
doubly enolic 1,3,5-cycloctatriene (see 4) once again neatly sets
a double bond into position. In this example, the unsaturated
linkage is not additionally functionalized. The chloro ketone
5
). This key intermediate has previously been taken on to its
â,γ-unsaturated isomer 24, monoepoxidized, and subjected to
1
8a
â-elimination. For characterization purposes, Ikegami pre-
pared the diacetate 26, independent synthesis of which had been
2
9 was obtained using exactly the same chemistry as in the
accomplished earlier by Tatsuta.1 The conversion of 26 into
8b
synthesis of 15. Reduction of this compound with lithium in
liquid ammonia afforded an extended enolate anion (see 23)
that underwent alkylation with methyl iodide only at the
γ-position to furnish 30 (74% isolated). Despite our prior
concerns over possible steric inhibition, this protocol proved to
be an exceptionally direct means for introducing the second
quaternary center.
coriolin was then achieved conventionally.1
8b,19
(()-Ceratopicanol. The discovery by Hanssen and Abraham
of ceratopicanol (10) in agar cultures of the ascomycete
Ceratosystis piceae proved noteworthy in that it constituted
powerful indirect evidence for the biosynthetic origins of the
entire class of those sesquiterpenoids associated with the
23
When 30 was subjected to a second dissolving metal
reduction, the result was efficient reductive cleavage of the
â-isopropoxy substituent to give 31. This transformation is
recognized to diminish the level of oxygenation 50% of the way.
To set the stage for removal of the second isopropoxy group,
humulene cascade. This previous missing link features the five
contiguous chiral centers, including two vicinal bridgehead
quaternary carbons demanded of its origin from the protoilludyl
cation. Three total syntheses of ceratopicanol have been
2
4
reported. These approaches have varied widely in their
efficiency, the range of synthetic steps spanning from a low of
3
1 was catalytically hydrogenated to furnish the tetrahydro
derivative 32, which proved well suited to a third reduction
maneuver and generation of saturated ketone 33. Borohydride
7
to a high of 21. In developing our own approach to 10, we
were intrigued by the likelihood of implementing a squarate-
based strategy that would rival the shortest known route while
developing new chemistry in support of this pathway. The details
of this concise synthesis of ceratopicanol follow.
1
reduction of 33 furnished racemic ceratopicanol (10), the H
1
3
and C NMR spectra of which compared very well with the
2
3,24
reported data.
Arrival at the first tricyclic intermediate 28 was brought about
by treating diisopropyl squarate (12) in turn with the cyclopen-
tenyllithium 27 and 2-lithiopropene (Scheme 6). The first of
Summary
The demonstration that electrocyclic rearrangements of
squarate ester derivatives are a valuable synthetic tool has been
independently explored in extensive work carried out in the
(
23) Hanssen, H.-P.; Abraham, W.-R. Tetrahedron 1988, 44, 2175.
(
24) (a) Mehta, G.; Karra, S. R. J. Chem. Soc., Chem. Commun. 1991, 1367.
(b) Clive, D. L. J.; Magnuson, S. R. Tetrahedron Lett. 1995, 36, 15. (c)
Baralotto, C.; Chanon, M.; Julliard, M. J. Org. Chem. 1996, 61, 3576.
(25) Jorgenson, M. J. Tetrahedron Lett. 1962, 559.
9
202 J. AM. CHEM. SOC. VOL. 124, NO. 31, 2002
9

Synthesis of Hypnophilin, Coriolin, and Ceratopicanol
A R T I C L E S
research groups headed by Liebeskind26 and by Moore.27 In the
present variant, functionalized triquinane ring systems are
generated from easily prepared starting materials in a single
reaction, forming four new C-C bonds. Moreover, the cascade
sequence furnishes the triquinane nucleus with sufficient
functionality to allow for a myriad of further chemical trans-
formations. The present study constitutes the first applications
of this methodology to complex molecule synthesis. The
squarate ester cascade is among those reactions that dramatically
and efficiently change molecular topography. The efficient
application of this reaction methodology to the total synthesis
of hypnohilin and ceratopicanol as well as a formal synthesis
of coriolin clearly demonstrates its potential for influencing a
broad range of synthesis activities.
(26) (a) Liebeskind, L. S.; Fengl, R. W.; Wirtz, K. R.; Shawe, T. T. J. Org.
Chem. 1988, 53, 2482. (b) Liebeskind, L. S.; Wirtz, K. R. J. Org. Chem.
1990, 55, 5350. (c) Liebeskind, L. S.; Fengl, R. W. J. Org. Chem. 1990,
55, 5359. (d) Mingo, P.; Zhang, S.; Liebeskind, L. S. J. Org. Chem. 1999,
64, 2145. (e) Shi, X.; Liebeskind, L. S. J. Org. Chem. 2000, 65, 1665. (f)
Acknowledgment. We gratefully acknowledge the generous
financial support of the National Science Foundation, and thank
Jian Liu for some early experiments.
Shi, X.; Amin, R.; Liebeskind, L. S. J. Org. Chem. 2000, 65, 1650.
27) (a) Santora, V. J.; Moore, H. W. J. Am. Chem. Soc. 1995, 117, 8486. (b)
Moore, H. W.; Yerxa, B. R. AdV. Strain Org. Chem. 1995, 4, 81. (c) Santora,
V. J.; Moore, H. W. J. Org. Chem. 1995, 117, 886. (d) Winters, M. P.;
Stranberg, M.; Moore, H. W. J. Org. Chem. 1994, 59, 7572. (e) Taing,
M.; Moore, H. W. J. Org. Chem. 1996, 61, 329. (f) Onofrey, T. J.; Gomez,
D.; Winters, M. P.; Moore, H. W. J. Org. Chem. 1997, 2, 5658. (g)
MacDougall, J. M.; Verma, S. K.; Santora, V. J.; Turnbull, P.; Hernandez,
C.; Moore, H. W. J. Org. Chem. 1998, 63, 6905. (h) Erguden, J.; Moore,
H. W. Org. Lett. 1999, 1, 375. (i) Verma, S. K.; Nguyen, Q. H.; McDougall,
J. M.; Fleischer, E. B.; Moore, H. W. J. Org. Chem. 2000, 65, 3379.
(
Supporting Information Available: Experimental details and
characterization data for all compounds (PDF). This information
is available free of charge via the Internet at http://pubs.acs.org.
JA020474T
J. AM. CHEM. SOC.
9
VOL. 124, NO. 31, 2002 9203