Regioselective silver-mediated Kondakov-Darzens olefin acylation

Article abstract of DOI:10.1002/chem.201102476

Enone construction: A silver-mediated olefin acylation reaction is described, in which five-, six-, and -seven-membered rings, tetrasubstituted olefins, bridged bicycles, spirocycles, and benzoxepinones are prepared. Highly selective intermolecular reactions are coupled to a Nazarov cyclization for the effective preparation of cyclopentenones, including the core of modhephene (see scheme). Copyright

Full text of DOI:10.1002/chem.201102476

DOI: 10.1002/chem.201102476
Regioselective Silver-Mediated Kondakov–Darzens Olefin Acylation
Nicholas T. Barczak and Elizabeth R. Jarvo*[a]
The Kondakov–Darzens acylation reaction dates back
over a century and provides a robust method for the prepa-
ration of enones.[1] It is distinguished from the more
common Friedel–Crafts acylation by the substitution of
arenes with olefinic nucleophiles.[2] The most commonly em-
ployed reagents for this transformation are superstoichio-
metric quantities of polyphosphoric acid or highly Lewis
acidic metal salts such as aluminumACTHNUTRGNEUNG(III) chloride, titan-
ACHTUNGTRENNUNGium(IV) chloride or tin(IV) chloride, in conjunction with
elevated temperatures.[3] Several reports describe use of
vinyl silanes to increase reaction rate and avoid harsh reac-
tion conditions, as well as provide the desired enones regio-
selectively.[4]
We anticipated that rapid and effective activation of car-
boxylic acids through their acyl chloride derivatives would
be promoted by the halophilic, commercially available, and
inexpensive anhydrous silver triflate (Scheme 1).[5] Tethering
Scheme 2. Intramolecular Kondakov–Darzens acylations: [a] Reaction
performed at 0.05m with 2.1 equiv of Cl(CO)2Cl, AgOTf and Et3N.
[b] Based on 1H NMR analysis of the crude reaction mixture with
PhSiMe3 as internal standard.
Scheme 1.
the olefin for intramolecular cyclization would provide con-
trol of regioselectivity in the absence of directing groups. In
this communication, we report the smooth transformation of
carboxylic acids to enones in a one-pot chlorination and
silver triflate-mediated Kondakov–Darzens acylation. The
proposed intermediate acylium ion is susceptible to attack
by pendant and exogenous olefins, allowing the synthesis of
a wide range of enones. Furthermore, by avoiding the use of
titanium, tin, or aluminum salts, olefin polymerization by-
products can be minimized.[3] We demonstrate the utility of
this synthetic method for assembly of bioactive compounds,
including benzoxepinones and terpenes.
We began our studies with substrates designed to provide
substituted cyclopentenones, cyclohexenones, and cyclohep-
tenones (Scheme 2). The facile preparation of a variety of
carboxylic acids bearing pendant olefins was accomplished
by treatment of the requisite carboxylic acid with two equiv-
alents of lithium diisopropylamide (LDA), followed by the
desired alkylating reagent. A one pot procedure for forma-
tion of the acid chloride and subsequent cyclization was em-
ployed, such that treatment of the carboxylic acid with
oxalyl chloride was followed by removal of the solvent and
addition of silver triflate and base. This procedure provided
five-, six-, and seven-membered rings (Scheme 2, 1a–1d) in
good yields. The presence of multiple olefins within a sub-
strate offers no interference to the desired reaction pathway
(1e). Trisubstituted olefins also underwent the desired reac-
tion to produce tetrasubstituted olefins (1 f and 1i).
[a] N. T. Barczak, Prof. E. R. Jarvo
Department of Chemistry, University of California
Irvine, Natural Sciences 1, 4114
Irvine CA 92697 (USA)
The presence of more than one carboxylic acid in a sub-
strate might introduce a convenient method for constructing,
in parallel, multiple new rings. Spirocycles are attractive tar-
gets because of their widespread presence in terpene natural
products.[6] Both 6–6 and 5–6 spirocycles 1g and 1h were
Fax : (+1)949-824-9920
E-mail: erjarvo@uci.edu
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201102476.
12912
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2011, 17, 12912 – 12916

COMMUNICATION
formed with overall yields of 67 and 51%, respectively, by
the cyclization of simple malonic acid derivatives.
lating enone 2c. Cuparenone can be converted to the natu-
ral product cuparene in one step through a Reetz dimethyla-
tion.[14,15]
We postulated that this intramolecular cyclization would
also enable the rapid construction of benzoxepinones
(Scheme 4). This class of salicylic acid derivatives have been
The commercial availability of many different arylacetic
acids allows the preparation of a diverse array of enones
through this method. Current methods for the installation of
unsaturation in a-monosubstituted ketones frequently yield
mixtures of olefin regioisomers.[7] In contrast, disconnection
by intramolecular acylation provides a clear selective proto-
col for introduction of a,b-unsaturation. For example, the
less conjugated enone 1j was obtained exclusively from cyc-
lization of the substituted arylacetic acid.[8]
Although a,a-disubstitution may bolster yields by
a
Thorpe–Ingold or reactive rotamer effect, successful cycliza-
tion of citronellic acid to pulegone (1i) indicates that a,a-
disubsitution is not crucial.[9] Additionally, although Beak
and others have noted that tertiary chloroformates can be
decarbonylated to produce tertiary carbocations, products
arising from such a process have not been observed under
these reaction conditions.[10] The ability to utilize such a
transformation for the synthesis of a’,a’-disubstituted
enones provides a means to circumvent the challenges asso-
ciated with the direct arylation[11] or alkylation of cyclic
enones.[12]
Having identified suitable conditions under which to per-
form this cyclization, we selected the terpene (Æ)-cuparene
as a viable natural product target. Commercial available tol-
ylpropionic acid (2a) provides a convenient starting point
for a formal synthesis (Scheme 3). C-Allylation provided the
Scheme 4. Synthesis of benzoxepinones.[a] [a] 1.1 equiv Cl(CO)2Cl, then
1.1 equiv AgOTf, CH2Cl2 (0.1m), 228C. [b] Reaction performed at 08C
and 0.05m.
identified as core structures in natural products forbaglin A
and B, isolated from the plant genus Algaia.[16 Furthermore,
they have been postulated as intermediates on the biosyn-
thetic pathway of the algaforbesins, rocaglamides, and silves-
trol.[17 Benzoxepinones have also been utilized in the syn-
thesis of ephedrine-like anorexigenics.[18 Simple phenolic O-
allylation of substituted salicylic acids[19 followed by intra-
molecular Kondakov–Darzens acylation provides the de-
sired benzoxepinones in good yields (Scheme 4). Notably,
this route provides the desired compounds in half of the
number of steps previously reported.[20] Additionally, we
were pleased to note that benzazepinone 3e was also suc-
cessfully prepared by cyclization of the requisite allylic ani-
line.
We reasoned that in the substrates containing more than
one olefin, a combination of ring size and olefin substitution
pattern would dictate the selective formation of one prod-
uct. We prepared a series of substrates with differentially
substituted olefins to investigate the relative rates of com-
peting cyclizations and determine the selectivity of the reac-
tion (Scheme 5). Indeed, cyclization of compound 4a, which
could undergo cyclization to provide two different cyclopen-
tenones, was regioselective for the more substituted olefin
and provided enone 4b. We observed that diene 4c cyclizes
to produce cyclohexenone 4d[21] with complete selectivity;
the alternative cyclopentenone is not formed. This could be
due to eclipsing interactions present in an envelope transi-
tion state, which destabilize the transition state leading to
five-membered ring formation.[22] However, increasing olefin
substitution can influence the course of the reaction and
override a preference for six-membered ring formation. For
Scheme 3. Formal synthesis of (Æ)-cuparene. Reaction conditions:
a) LDA (2.2 equiv), C3H5Br (1.5 equiv), THF, À50 to 508C; b) Cl(CO)2Cl
(1.1 equiv), then AgOTf (1.1 equiv), CH2Cl2; c) [PdACTHNUGTRNEUG(N PPh3)4] (1 mol%),
ZnCl2 (10 mol%), Ph2SiH2 (1.2 equiv).
desired intermediate 2b. The corresponding acyl chloride
was efficiently cyclized to enone 2c, which was subsequently
reduced to cuparenone. Alternatively, we found that cycliza-
tion followed by in situ reduction with catalytic Pd0/ZnCl2,
and 1.2 equivalents of diphenylsilane[13] resulted in expedi-
ent formation of (Æ)-cuparenone, without the need for iso-
Chem. Eur. J. 2011, 17, 12912 – 12916
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
12913

E. R. Jarvo and N. T. Barczak
Scheme 6. Intermolecular diene acylation.
Involving this acylation in a tandem sequence could pro-
vide a strategy to rapidly build structural complexity. Acy-
lating olefins with a,b-unsaturated acyl chlorides gives divin-
yl ketones, which are Nazarov cyclization substrates.[30] The
use of silver triflate to mediate acylation results in the pro-
duction of one equivalent of triflic acid that could serve as
an acid catalyst for the Nazarov cyclization. Acylation of cy-
clopentene with 3,3-dimethylacryoyl chloride (Scheme 7) re-
Scheme 5. Intramolecular competition acylation reactions. Reaction con-
ditions: 1.1 equiv Cl(CO)2Cl, then 1.1 equiv AgOTf, CH2Cl2 (0.1m),
228C.
Scheme 7. Intermolecular acylation–Nazarov.
instance, acid 4e cyclizes to produce b-methylcyclopente-
none 4 f exclusively, without the formation of the alternative
cyclohexenone. This result is consistent with the greater ki-
netic reactivity of the more highly substituted olefin.[23]
Strategically embedding the pendant reactive olefin
within a ring presents opportunities for the preparation of
bridged bicyclic structures and at the same time, provides
competition between formation of alternative ring sizes. For
example, cyclization of acid 4g produced selective closure to
the five-membered ring, resulting in the [3.1.2] bicycle 4h[24]
instead of the [2.2.2] bicyclooctene 4i[25] in a 14:1 ratio. The
origin of this selectivity is unclear at present, and seemingly
contradicts the selectivity observed in the cyclization of acid
4c. Cyclization of carboxylic acid 4j results in preferential
closure to provide the six-membered ring, not the seven-
membered ring, generating a 4:1 mixture of [3.1.3][26] and
[2.3.2][27] bicyclononenes (4k and 4l, respectively).
sulted in the smooth and rapid preparation of divinyl ketone
8. The Nazarov cyclization was affected by adding toluene
and heating the mixture to reflux overnight. Notably, neither
purification of the intermediate divinylketone nor the use of
stoichiometric tin(IV) chloride was necessary.[31]
We reasoned that combining use of diene mixture 6 in
with an a,b-unsaturated carboxylic acid could provide a
means to access the core structure of triquinane natural
product modhephene[32] through the Nazorov cyclization of
sterically congested divinyl ketone 10 (Scheme 8). Our
silver-mediated acylation of 6 with 3,3-dimethylacryoyl chlo-
ride, followed by application of rigorous heating conditions
The high regioselectivity observed in our competition ex-
periments in the intramolecular reaction led us to investi-
gate the analogous intermolecular bond construction. We
chose diene 6[28] as a model substrate because it contains a
trisubstituted and monosubstituted olefin, which would
allow for direct evaluation of the selectivity of intermolecu-
lar acylations (Scheme 5) and could serve as a building
block for terpene synthesis (see below). Diene 6 can be pre-
pared rapidly by copper-catalyzed elimination[29] of tertiary
alcohol 5. Gratifyingly, acylation with isovaleroyl chloride
produced only enone 7, through attack of the major isomer
of diene 6 (Scheme 6).
Scheme 8. Formal synthesis of (Æ)-modhephene.
12914
www.chemeurj.org
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2011, 17, 12912 – 12916

Regioselective Olefin Acylation
COMMUNICATION
resulted in construction of Oppolzerꢁs intermediate 11[33] in
good yield. Oppolzer has converted 11 to modhephene
through a stereospecific ene cyclization to Dreidingꢁs pro-
pellane intermediate (12).[34]
[4] a) Y. Hatanaka; I. Kuwajima, Tetrahedron Lett. 1986, 27, 719–722;
I. Kuwajima, Tetrahedron Lett. 1986, 27, 719–722; b) S. E. Den-
mark; J. P. Germanas, Tetrahedron Lett. 1984, 25, 1231–1234; J. P.
Germanas, Tetrahedron Lett. 1984, 25, 1231–1234; c) K. Fukuzaki,
E. Nakamura, I. Kuwajima, Tetrahedron Lett. 1984, 25, 3591–3594;
d) K. Mikami, N. Kishi, T. Nakai, Tetrahedron Lett. 1983, 24, 795–
798; e) E. Nakamura, J.-I. Shimada, I. Kuwajim, J. Chem. Soc.
Chem. Commun. 1983, 499–501; f) S. D. Burke, C. W. Murtiashaw,
M. S. Dike, S. M. S. Strickland, J. O. Saunders, J. Org. Chem. 1981,
46, 2400–2402.
[5] a) For an early report of activation of acylhalides using silver tetra-
fluoroborate and subsequent Kondakov–Daarzens reaction, see:
W. A. Smit, A. V. Semonovsky, V. F. Kucherov, T. N. Chernova,
M. Z. Krimer, O. V. Lubinskaya, Tetrahedron Lett. 1971, 12, 3101–
3106; b) J.-M. Weibel, A. Blanc, P. Pale in Silver in Organic Chemis-
try (Ed.: M. Harmata), Wiley, New York, 2010, Chapter 10.
[6] For a review of spirocycle synthesis in terpene natural products see:
M. Sannigrahi, Tetrahedron 1999, 55, 9007–9071.
In conclusion, we report the intramolecular and intermo-
lecular Kondakov–Darzens reactions in which mild activa-
tion of the intermediate acyl halide is achieved using silver
triflate. This transformation provides a rapid assembly of
enones, including benzoxepinones. Competition experiments
are consistent with cyclization via formation of the more
stable carbocation intermediate, followed by a slight prefer-
ence for formation of six-membered rings. Tandem acylation
and Nazarov cyclization are also demonstrated. Application
of this method in the formal syntheses of cuparene and
modhephene demonstrates the utility of this method.
[7] a) K. C. Nicolaou, T. Montagnon, P. S. Baran, Y.-L. Zhong, J. Am.
Chem. Soc. 2002, 124, 2245–2258; b) J. M. Sayer, H. Yagi, J. V. Sil-
verton, S. L. Friedman, D. L. Whalen, D. M. Jerina, J. Am. Chem.
Soc. 1982, 104, 1972–1978.
Experimental Section
[8] This strategy was employed recently for the synthesis of 3-chlorocy-
clopentenones; see Reference [3f].
[9] For recent discussions of the gem-dimethyl effect see: a) L. Ringer,
D. H. Magers, J. Org. Chem. 2007, 72, 2533–2537; b) S. M. Bachrach,
J. Org. Chem. 2008, 73, 2466–2468.
[10] a) P. Beak, Acc. Chem. Res. 1976, 9, 230–236; b) U. Biermann,
M. O. Juergen, J. Am. Chem. Soc. 2004, 126, 10319–10330; c) P. Eil-
bracht, Sci. Synth. 2009, 48, 393–438.
[11] a’-Arylation of enones has not, to our knowledge, been reported;
aryl nucleophile conjugate addition is facile with transition-metal
catalysis. For reviews of catalytic conjugate additions, see: a) N.
Krause, A. Hoffmann-Rçder, Synthesis 2001, 0171–0196; b) T. Hay-
ashi, K. Yamasaki, Chem. Rev. 2003, 103, 2829–2844; c) A. Gutnov,
Eur. J. Org. Chem. 2008, 4547–4554.
Freshly distilled oxalyl chloride (94 mL, 1.1 mmol, 1.1 equiv) was added
to a stirred solution of 2,2-diphenylpent-4-enoic acid (250 mg, 1.0 mmol,
1.0 equiv) and one drop of DMF in dry dichloromethane (5 mL) at room
temperature, under N2. After 15 min, the solvent was removed in vacuo.
Fresh CH2Cl2 (5 mL) was introduced, and the solution was added,
through a syringe, to stirring silver triflate (280 mg, 1.1 mmol, 1.1 equiv)
under nitrogen. The reaction mixture was stirred for two hours, after
which time triethylamine (150 mL, 1.1 mmol, 1.1 equiv) was added. The
mixture was stirred for 15 min and was then purified by flash chromatog-
raphy on silica gel, eluting with 30% Et2O/pentane (Rf =0.74), to afford
1a as a white solid (203 mg, 87%). M.p. 86–878C; 1H NMR (500 MHz,
CDCl3): d=7.85 (dt, J=1H), 7.31–7.20 (m, 10H), 6.27 (dt, J=5.6,
2.6 Hz, 1H), 3.51 ppm (d, J=2.4 Hz, 2H); 13C NMR (125 MHz, CDCl3):
d=209.0, 162.5, 143.4, 133.0, 128.7, 128.2, 126.9, 60.1, 48.0 ppm; IR (KBr
pellet): v˜ =3054, 2987, 2306, 1706, 1492, 1444, 1421, 1265 cmÀ1; HRMS
(ESI): m/z: calcd for C17H14ONa: 257.0942 [M+Na]+; found: 257.0942.
[12] Treatment of enones with base can result in competing a’- and g-de-
protonation. For discussions, see: a) J. E. Bartmess, J. P. Kiplinger, J.
Org. Chem. 1986, 51, 2173–2176; b) J. A. Amupitan, E. Huq, M.
Mellor, E. G. Scovell, J. K. Sutherland, J. Chem. Soc. Perkin Trans. 1
1983, 747–749.
[13] E. Keinan, N. Greenspoon, J. Am. Chem. Soc. 1986, 108, 7314–
7325.
[14] M. T. Reetz, J. Westermann, S.-H. Kyung, Chem. Ber. 1985, 118,
1050–1057.
[15] For strategies for enantioselective synthesis of 2b, see: a) Use of the
Boeckman auxiliary: R. K. Boeckman, Jr., J. E. Pero, D. J. Boehm-
ler, J. Am. Chem. Soc. 2006, 128, 11032–11033; b) Resolution of
propionic acid 2b: S. K. Shah, N. Chen, R. N. Ravindra, S. G. Mills,
L. Malkowitz, M. S. Springer, S. L. Gould, J. A. DeMartino, A. Care-
lla, G. Carver, K. Holmes, W. A. Schleif, R. Danzeisen, D. Hazuda,
J. Kessler, J. Lineberger, M. Miller, E. A. Emini, M. MacCoss,
Bioorg. Med. Chem. Lett. 2005, 15, 977–982.
Acknowledgements
This work was supported by an NSF CAREER Award (CHE-0847273).
We thank Dr. John Greaves for mass spectrometric analyses.
Keywords: acylation · cuparene · enones · modhephene ·
regioselectivity · tandem acylation–Nazarov
[16] V. Dumontet, O. Thoison, O. R. Omobuwajo, M.-T. Martin, G. Per-
romat, A. Chiaroni, C. Riche, M. Paꢂs, T. Sꢃvenet, A. H. A. Hadi,
Tetrahedron 1996, 52, 6931–6942.
[1] a) J. Kondakov, J. Russ. Phys. Chem. Soc. 1894, 26, 5; b) G. Darzens,
Compt. Rend. 1910, 150, 707.
[2] J. G. Sharefkin, J. Chem. Educ. 1962, 39, 206–207.
[17] a) M. Lu King, C. C. Chang, H. C. Ling, E. Fugita, M. Ochiai, A. T.
McPhail, J. Chem. Soc. Chem. Commun. 1982, 1150–1151; b) B. Y.
Hwang, B.-N. Su, H. Chai, Q. Mi, L. B. S. Kardono, J. J. Afriastini, S.
Riswan, B. D. Santarsiero, A. D. Mesecar, R. Wild, C. R. Fairchild,
G. D. Vite, W. C. Rose, N. R. Farnsworth, G. A. Cordell, J. M. Pezzu-
to, S. M. Swanson, A. D. Kinghorn, J. Org. Chem. 2004, 69, 3350–
3358; c) S. K. Lee, B. Cui, R. R. Mehta, A. D. Kinghorn, J. M. Pezzu-
to, Chem.-Biol. Interact. 1998, 115, 215–228.
[18] V. K. Tandon, K. A. Singh, A. K. Awasthi, J. M. Khanna, B. Lal, N.
Anand, Bioorg. Med. Chem. Lett. 2004, 14, 2867–2870.
[19] J. Pandey, M. Mishra, S. S. Bisht, A. Sharma, R. T. Tripathi, Tetrahe-
dron Lett. 2008, 49, 695–698.
[3] a) H. O. House, Modern Synthetic Reactions, 2nd ed., W. A Benja-
min, New York, 1972, pp. 786–816; b) J. K. Groves, Chem. Soc. Rev.
1972, 1, 73–97; c) M. V. Luzgin, V. N. Romannikov, A. G. Stepanov,
J. Am. Chem. Soc. 1996, 118, 10890–10891; d) S. H. Prasad, G. Na-
gendrappa, Ind. J. Chem. Sec. B: Org. Chem. Incl. Med. Chem. 1997,
36, 691–694; e) E. Armengol, A. Corma, L. Fernandez, H. Garcia,
J. Primo, Appl. Catal. A 1997, 158, 323–335; f) Y. Xu, M. McLaugh-
lin, C.-Y. Chen, R. A. Reamer, P. G. Dormer, I. W. Davies, J. Org.
Chem. 2009, 74, 5100–5103; g) M. Mushfiq, A. R. Khan, A.
Shamim, R. Rehman, Synth. Commun. 2010, 40, 1508–1515; h) J.-I.
Matsuo, T. Hoshikawa, S. Sasaki, H. Ishibashi, Chem. Pharm. Bull.
2010, 58, 591–592.
Chem. Eur. J. 2011, 17, 12912 – 12916
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
12915

E. R. Jarvo and N. T. Barczak
[20] S.-R. Li, H.-M. Chen, L.-Y. Chen, J.-C. Tsai, P.-Y. Chen, S. C.-N.
Hsu, E.-C. Wang, ARKIVOC 2008, 2, 172–182.
[21] a) J. T. Mohr, D. C. Behenna, A. M. Harned, B. M. Stoltz, Angew.
Chem. 2005, 117, 7084–7087; Angew. Chem. Int. Ed. 2005, 44, 6924–
6927.
[22] For a discussion of torsional effects in transition states, see: a) K. N.
Houk, M. N. Paddon-Row, N. G. Rondan, Y. D. Wu, F. K. Brown,
D. C. Spellmeyer, J. T. Metz, Y. Li, R. J. Loncharich, Science 1986,
231, 1108–1117; b) R. Bucourt in Topics in Stereochemistry,
Volume 8 (Eds.: E. L. Eliel and N. L. Allinger), Wiley, New York,
2007.
[23] H. Mayr, B. Kempf, A. R. Ofial, Acc. Chem. Res. 2003, 36, 66–77.
[24] D. M. Hudyma, J. B. Stothers, C. T. Tan, Org. Magn. Reson. 1974, 6,
614–616.
[25] J. Doyon, W. He, L. A. Paquette, J. Org. Chem. 1994, 59, 2033–
2042.
[26] E. N. Marvell, R. S. Knutson, T. McEwen, D. Sturmer, W. Federici,
K. Salisbury, J. Org. Chem. 1970, 35, 391–396.
[27] L. A. Paquette, F. Pierre, C. E. Cottrell, J. Am. Chem. Soc. 1987,
109, 5731–5740.
[28] T. Nishiyama, J. F. Woodhall, E. N. Lawson, W. Kitching, J. Org.
Chem. 1989, 54, 2183–2189.
Vol. 5 (Eds.: B. M. Trost, I. Fleming, L. A. Paquette), Pergamon
Press, Oxford, 1991, pp. 751–784; c) K. L. Habermas, S. E. Den-
mark, Org. React. 1994, 45, 1–158; d) H. Pellissier, Tetrahedron
2005, 61, 6479–6517; e) M. Tius, Eur. J. Org. Chem. 2005, 2193–
2206; f) A. J. Frontier, C. Collison, Tetrahedron 2005, 61, 7577–7606.
[31] L. A. Paquette, W. E. Fristad, D. S. Dime, T. R. Bailey, J. Org.
Chem. 1980, 45, 3017–3028.
[32] For total syntheses of modhephene, see: a) A. B. Smith III, P. J.
Jerris, J. Am. Chem. Soc. 1981, 103, 194–195; b) A. B. Smith III, P. J.
Jerris, J. Org. Chem. 1982, 47, 1845–1855; c) P. A. Wender, G. B.
Dreyer, J. Am. Chem. Soc. 1982, 104, 5805–5807; d) D. Wilkening,
B. P. Mundy, Tetrahedron Lett. 1984, 25, 4619–4622; e) B. P. Mundy,
D. Wilkening, K. B. Lipkowitz, J. Org. Chem. 1985, 50, 5727–5731;
f) C. P. Jasperse, D. P. Curran, J. Am. Chem. Soc. 1990, 112, 5601–
5609; g) C. A. Dvorak, V. H. Rawal, Chem. Commun. 1997, 2381–
2382; h) B. Mudryk, T. Cohen, J. Org. Chem. 1991, 56, 5760–5761;
i) H.-Y. Lee, D. K. Moon, J. S. Bahn, Tetrahedron Lett. 2005, 46,
1455–1458; j) G. Stork, R. J. Mook, Tetrahedron Lett. 1986, 27,
4529–4532.
[33] a) W. Oppolzer, F. Marazza, Helv. Chim. Acta 1981, 64, 1575–1578;
b) W. Oppolzer, K. Battig, Helv. Chim. Acta 1981, 64, 2489–2491.
[34] a) M. Karpf, A . S. Dreiding, Tetrahedron Lett. 1980, 21, 4569–4570;
b) M. Karpf, A. S. Dreiding, Helv. Chim. Acta 1981, 64, 1123–133.
[29] K. Laali, R. J. Gerzina, C. M. Flajnik, C. M. Geric, A. M. Dombros-
ki, Helv. Chim. Acta 1987, 70, 607–611.
[30] a) W. Nakanishi, F. G. West, Curr. Opin. Drug Discovery Dev. 2009,
12, 732–751; b) S. E. Denmark in Comprehensive Organic Synthesis,
Received: August 9, 2011
Published online: October 17, 2011
12916
www.chemeurj.org
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2011, 17, 12912 – 12916