Manganese(III)-promoted tandem oxidation and cyclization of β-keto ester derivatives of terpenoids

Article abstract of DOI:10.1002/adsc.201100215

A new type of terpenoid cyclization directed by a β-keto ester moiety has been developed, which proceeded by manganese(III)-initiated oxidation of the β-keto ester, followed by an intramolecular hetero Diels-Alder reaction with the terpenoid chain. This reaction produces polycyclic dihydropyrans in high yields and stereoselectivities under mild conditions. Copyright

Full text of DOI:10.1002/adsc.201100215

COMMUNICATIONS
DOI: 10.1002/adsc.201100215
Manganese(III)-Promoted Tandem Oxidation and Cyclization of
N
b-Keto Ester Derivatives of Terpenoids
Zhilong Li,a,b Heejung Jung,b Mira Park,c Myoung Soo Lah,d and Sangho Kooa,b,*
a
School of Pharmacy, East China University of Science and Technology, Shanghai 20037, Peopleꢀs Republic of China
Department of Nano Science and Engineering, Department of Chemistry, Myong Ji University, Yongin, Kyunggi-Do
b
449-728, Korea
Fax : (+82)-31-335-7248; phone: (+82)-31-330-6185; e-mail: sangkoo@mju.ac.kr
Department of Chemistry and Applied Chemistry, Hanyang University, Ansan, Kyunggi-Do 426-791, Korea
Interdisciplinary School of Green Energy, Ulsan National Institute of Science and Technology, Ulsan 689-798, Korea
c
d
Received: March 25, 2011; Published online: August 10, 2011
The authors dedicate this article to Professor Eun Lee on the occasion of his 65th birthday and retirement after
devotion to education and research at Seoul National University.
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adcs.201100215.
Abstract: A new type of terpenoid cyclization di-
rected by a b-keto ester moiety has been developed,
which proceeded by manganeseACTHNUGTRNEUNG(III)-initiated oxida-
pected carbocyclic products (Scheme 1).[6] The struc-
ture of new crystalline product 2a (R=Me) was iden-
tified by an X-ray diffraction study.[7] We carefully in-
tion of the b-keto ester, followed by an intramolec-
ular hetero Diels–Alder reaction with the terpenoid
chain. This reaction produces polycyclic dihydropyr-
ans in high yields and stereoselectivities under mild
conditions.
vestigated this unusual MnACTHNGUTRENNU(G III)-promoted cyclization
of the terpenoid derivatives 1, and report herein the
mechanism and the structural requirements for this
novel cyclization reaction.
It was postulated, by comparison of the structures
of the starting material 1a and the product 2a, that in-
troduction of a hydroxy group and elimination of the
acetyl group should precede the cyclization. The in-
sertion of a hydroxy group at the radical carbon of di-
Keywords: cyclization; cycloaddition; manganese;
oxidation; radical reactions; terpenoids
alkyl malonate in the reaction with MnACHTUNRGTNEUNG(OAc)3·2H2O
has been reported, which underwent further oxidation
to a ketone.[8] The initial carbon radical generated by
Developing a regio- and stereo-controlled cyclization the reaction of 1a with MnACHTUNRTGNEUNG(OAc)3·2H2O does not un-
method of terpenoid chains is one of the most impor- dergo any type of cyclization, presumably due to the
tant and challenging research topics in the syntheses possible ring strain, but undergoes hydroxylation to
of polycyclic isoprenoid natural products.[1] The give 3a (R=Me). Further oxidation of the hydroxy
enzyme-catalyzed cascade cyclization of 2,3-oxido- group in 3a is accompanied by deacetylation to pro-
ACHTUNGTRENNUNGsqualene to lanosterol is a brilliant example in duce 4a (R=Me). We confirmed this MnACHTUNGTRENN(UGN III)-pro-
nature.[2] It is very difficult, however, to control the moted oxidation scenario in the reaction of ethyl 2-
mode of cyclization of terpenoids, which generally hexylacetoacetate (6), in which the afore-mentioned
leads to a mixture of regio- and stereoisomers, and two oxidation products 7 (46% yield) and 8 (38%
sometimes to polymerization products.[3] Radical reac- yield) were obtained (Scheme 1). It is obvious that 4a
tions may produce the regio- and stereo-controlled is the precursor to the cyclization product 2a.
cyclization products for properly-designed terpen-
A
The reaction of the enol tautomer of 4a with Mn-
ACHTUNGTRNEN(UNG OAc)3·2H2O would produce the oxy-radical inter-
double bonds and the initiating carbon radical deter- mediate A, that might undergo the consecutive 9-
mine the mode of cyclization.[5] In a study of the endo and 5-endo cyclizations to give 2a directly. How-
MnACHTUNGTRENNUNG(III)-initiated radical cyclization of b-keto ester ever, this unprecedented oxy-radical cyclization is un-
derivatives 1 from poly-prenols, we found a new type likely based on the study of the steric/electronic re-
of regio- and stereo-controlled cyclization which led quirements of the terminal C=C bond for the cycliza-
to the heterocyclic compounds 2 instead of the ex- tion (vide infra). It is more plausible that the resonat-
Adv. Synth. Catal. 2011, 353, 1913 – 1917
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1913

Zhilong Li et al.
COMMUNICATIONS
Scheme 1. A new type of MnACTHNUGRTNEUNG(III)-promoted cyclization of the b-keto ester derivative 1 from terpenoid chains.
ing tertiary carbon radical B undergoes hydroxylation
to produce 5a (R=Me), which is prone to hetero-
Diels–Alder reaction to give 2a in a highly stereose-
lective manner (Scheme 1).[9] Extension of terpenoid
1a by the prenyl units does not change the mode of
cyclization. The terpenoid derivatives 1b and 1c which
were respectively prepared from farnesol and geranyl-
geraniol produced the same type of cyclization prod-
ucts 2b and 2c in fair yields. Diethyl malonate con-
taining an a-geranyl substituent also undergoes the
oxidation with decarboalkoxylation and intramolecu-
lar cycloaddition to give 2a (54% yield) but much
more slowly (12 h at 608C).
Table 1. Optimization of Mn
terpenoid 1a to produce the bicyclic dihydropyran 2a.
ACHTUNGTNER(NUNG III)-promoted cyclization of
Entry Mn
G
Solvent
T [8C] t [h] Yield
(equiv.) (equiv.)
[%]
1
2
3
4
5
6
7
8
1
2
3
4
3
3
3
3
3
3
3
3
3
1
3
3
3
1
1
1
1
0
3
1
1
0
1
3
0
1
0
1
3
1
EtOH[a] 25
EtOH[a] 25
EtOH[a] 25
EtOH[a] 25
EtOH[a] 25
EtOH[a] 25
6
4
4
3
4
4
8
3
3
3
4
6
19
32
60
60
47
59
29
5[c]
51
59
57
49
59
43
74
75
75
EtOH[a]
0
The above tandem MnACTHNUTRGNEU(GN III)-promoted oxidation
and intramolecular hetero-Diels–Alder reaction of
terpenoid derivative 1a has been optimized (Table 1).
EtOH[b] 25
9
MeCN
MeCN
MeCN
AcOH
AcOH
25
25
25
25
25
10
11
12
13
14
15
16
17
Three equivalents of Mn
the best yields (entries 1–4). Cu(II) is generally uti-
lized in Mn(III)-initiated radical cyclizations for the
ACHTUNGERTN(NUNG III) were required to give
ACHTUNGTRENNUNG
6
purpose of oxidation of the final primary or secon-
dary radicals to carbon-carbon double bonds.[10] We
cannot explicitly explain the role of Cu(II) in our re-
action, but it increases the yield of the reaction
AcOH[d] 25
EtOH[e] 25
EtOH[e] 25
EtOH[e] 55
0.5
24
24
3
A
[a]
[b]
99.9% EtOH was used.
(entries 3 vs. 5, 9 vs. 10, and 12 vs. 13). There is no fur-
ther effect of Cu(II) when more than one equivalent
are used (entries 6, 11, and 16). The reaction can be
carried out at room temperature under air for several
hours, but is sluggish at 08C to give a lower yield
even after a prolonged reaction time (entry 7).
Oxygen from the reaction medium or air seems to be
The solution in 99.9% EtOH was degassed by the freeze-
thaw technique, and the reaction was carried out under
an argon atmosphere.
Starting material was recovered in 61% yield.
Ultrasonic irradiation (130 W, 20 KHz) at 20% intensity
was applied.
[c]
[d]
[e]
94% EtOH (~6% water content) was used.
1914
asc.wiley-vch.de
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2011, 353, 1913 – 1917

ManganeseACHTUNGTRENNUNG(III)-Promoted Tandem Oxidation and Cyclization of b-Keto Ester Derivatives
Table 2. Conditions and yields of the Mn
AHCTUNGTRENNcGUN lization of the homologous terpenoid 9.
ACHTUNGTREN(NUGN III)-promoted cy-
very important to insert a hydroxy group at the radi-
cal a-carbon of the b-keto ester. The reaction under a
carefully controlled argon atmosphere produced only
5% of the cyclization product 2a with the recovery of
the starting material (entry 8). The reaction works
well in a polar solvent such as EtOH, MeCN, and
AcOH. It is noteworthy that the use of lesser amount
of Mn
ACHTUNGTRENNUNG(III) is possible in AcOH using ultrasonic irra-
diation to provide the cyclization product 2a in 43%
yield (entry 14), where MnACTHNUTRGNEUGN(III) is reported to be re-
generated under the reaction conditions.[11] We found
that a small amount of water (~6%) in EtOH slowed
down the reaction rate, and thus increased reaction
time or temperature was required, in which improved
yields of the cyclization product were obtained (en-
tries 15–17). The best conditions are to use three equi-
valents of Mn
ACHTUNGTRENNUNG(OAc)3·2H2O and one equivalent of
Cu(OAc)2·H2O in 94% EtOH at 558C under air to
produce 2a in 75% isolated yield (entry 17).
ACHTUNGTRENNUNG
Entry[a] T [8C] t [h] Ratio[b] (10:11) Product (yield[c] [%])
In order to investigate more details of the above
cyclization mechanism, b-keto ester 9 derived from
the homologous polyprenol was prepared (see Sup-
1
2
3
4
25
25
25
55
5
3:1
5:6
1:4
10 (11), 11 (26)
11 (50)
11 (61)
12
48
3
porting Information). The MnACTHNUGRTNEUNG(III)-promoted reaction
1:20
11 (89)
of terpenoid derivative 9 produced d-hydroxy-b,g-un-
saturated-a-oxo ester 10 and hydroisochromene 11 in
variable yields depending on the reaction conditions
(Table 2). The cis-stereochemistry at the ring junction
of the major product 11 (a 9:1 mixture of diastereo-
[a]
[b]
[c]
Three equiv. of Mn
ACHTUGNRTEN(NUGN OAc)3·2H2O and one equiv. of
Cu(OAc)2·H2O were used for all the entries.
AHCTUNGTRENNUNG
1
Ratio was determined by analysis of the H NMR spectra
of the crude product.
The isolated yield of 10 was significantly lower than the
crude ratio due to the instability of 10.
1
mers) was deduced from the H NMR spin-spin cou-
4
pling constants: 3JH-4a,H-8a =4.8 Hz and JH-4,H-8a
=
1.6 Hz (long-range W-coupling). The stereochemistry
at C-5 is not clear, but the methyl group is believed to duced the oxidation products, d-hydroxy-b,g-unsatu-
be disposed to the b-phase to minimize the steric in- rated-a-oxo esters 13 and 15a–15c, respectively in 50–
teractions. It is noteworthy that d-hydroxy-b,g-unsatu- 61% yields at room temperature (Scheme 2). No fur-
rated-a-oxo ester 10 is mainly observed under the ther cyclization product has been obtained in a practi-
mild reaction conditions (entry 1), which may further cal yield even at higher temperatures. This result
undergo a hetero-Diels–Alder reaction to produce 11 matches well with the hetero-Diels–Alder mechanism
upon prolonged reaction time or elevated reaction of inverse electronic demand, which requires an elec-
temperature (entries 2–4). The formation of 10 and its tron-rich alkene. It speaks against the afore-men-
conversion to 11 strongly supports the above mecha-
nism including hetero-Diels–Alder reaction in
Scheme 1. The best yield of 11 (89%) was obtained
again with three equivalents of Mn
ACHTUNGTREN(UNNG OAc)3·2H2O and
one equivalent of Cu(OAc)2·H2O in EtOH (94%) at
AHCTUNGTRENNUNG
558C for 3 h. The stereochemistry of 11 can be easily
explained by the model drawn in Table 2.
The structural requirement for the MnACTHNUGRTENUNG(III)-promot-
ed oxidation and cyclization has been studied with the
terpenoid derivatives containing different substitution
patterns at the terminal carbon-carbon double bond.
Electron-rich dimethyl substitution at the terminal sp2
carbon of 1a and 9 seems to be a prerequisite for the
intramolecular Diels–Alder reaction of the hetero-
diene with inverse electronic demand to give rise to
2a and 11, respectively. Other terpenoid derivatives
12 and 14a–14c with less electron-rich, mono-alkyl or
Scheme 2. The effect of the substitution pattern at the termi-
nal carbon-carbon double bond on the Mn(III)-promoted
AHCTUNGTRENNUNG
no substitution at the terminal sp2 carbon only pro- cyclization.
Adv. Synth. Catal. 2011, 353, 1913 – 1917
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
1915

Zhilong Li et al.
COMMUNICATIONS
Table 3. MnACHTUNGTRENNUNG
tives.[a]
Entry
Substrate
Product
1
65% [1ACTHNURGTNE(NUG 8:1):1.2]
2
3
73% [15
74% [22
ACHTUNGTRENNUNG(6:1):1]
ACHTUNGTRENNUNG(11:1):1]
4
81% [1ACTHNGUTERNN(UG 6:1):3]
[a]
Three equiv of Mn
G
ACHTUNGTRNEN(UNG OAc)2·H2O were used in EtOH (94%) at 558C for 4–5 h.
[b]
tioned oxy-radical mechanism, in which the steric (6:1), and 22 (11:1), which presumably originated
congestion at the reaction center generally retards the from the tertiary alcoholic centers. The fused 6–5–6
cyclization.
tricyclic ring 23 was composed of an easily separable
b-Keto esters 16–19 of the modified terpenoids 1:3 diastereomeric mixture. The stereoisomerism may
with dialkyl substitution at the terminal sp2 carbon have originated from the carbocyclic B–C junction,
have been synthesized, and the generality of the which can be deduced from the high trans selectivity
tandem sequence of oxidation and cyclization has observed at the heterocyclic A–B junction (the 6–5
been tested (Table 3). All the syntheses of the sub- fused ring junction). The less polar, minor isomer also
strates were based on our sulfone-mediated chain-ex- contained a small amount of a diastereomer (6:1) ori-
tension strategy for acyclic terpenoids (see Supporting ginated presumably from the tertiary alcoholic center.
Information for detail).[3f] The Mn
tandem oxidation and intramolecular hetero-Diels– tion protocol of the MnAHCTNUGTRENNNUG
A
In summary, we developed a completely new reac-
(III)-promoted tandem oxida-
Alder reaction of the terpenoid chains is quite gener- tion and cyclization of terpenoids containing a b-keto
al, and polycyclic dihydropyrans 20–23 have been effi- ester unit. The carbon radical center of b-keto ester
ciently synthesized in 65–81% yields under the above undergoes oxidation and subsequent intramolecular
optimized conditions. The reaction is generally stereo- hetero-Diels–Alder reaction with the terpenoid chain
selective, and an exclusive trans diastereoselectivity to give bicyclic dihydropyrans 2 and 11. The dialkyl-
(>15:1) was observed at the newly formed heterocy- substitution pattern at the terminal sp2 carbon of the
clic junction for the 6–5 fused ring cases (entries 2–4). terpenoids is a structural requirement for the intramo-
However, a low level of cis selectivity (1.2:1) was ob- lecular hetero-Diels–Alder reaction of inverse elec-
served for the oxygen-tethering terpenoid 16, which tronic demand. Only the oxidation product, d-hy-
led to the 6–6 fused bicyclic system (entry 1).[12] It is droxy-b,g-unsaturated-a-oxo esters 13 and 15a–15c
straightforward to distinguish the stereoisomers at the prevails for the terpenoid chains with mono-alkyl or
ring junction: H-4 from the cis isomer has an extra no substitution at the terminal sp2 carbon. This
1
long-range W-coupling in H NMR (e.g., see 11 in tandem sequence is highly effective, and various poly-
Table 2). Other diastereomers were also observed for cyclic dihydropyrans 20–23 are synthesized in high
the isomers with the trans ring junction: 20 (8:1), 21 yields and stereoselectivities by a single reaction from
1916
asc.wiley-vch.de
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2011, 353, 1913 – 1917

ManganeseACHTUNGTRENNUNG(III)-Promoted Tandem Oxidation and Cyclization of b-Keto Ester Derivatives
T. Jones, R. A. Lerner, K. D. Janda, D. W. Christianson,
Angew. Chem. 1999, 111, 1859–1864; Angew. Chem.
Int. Ed. 1999, 38, 1743–1747; c) J. Hasserodt, K. D.
Janda, R. A. Lerner, J. Am. Chem. Soc. 2000, 122, 40–
45. For other enzymatic cyclization of terpenoid chains,
see: d) B. Greenhagen, J. Chappell, Proc. Natl. Acad.
Sci. USA 2001, 98, 13479–13481; e) M. J. Rynkiewicz,
D. E. Cane, D. W. Christianson, Proc. Natl. Acad. Sci.
USA 2001, 98, 13534–13548; f) T. Toyomasu, M. Tsuka-
hara, A. Kaneko, R. Niida, W. Mitsuhashi, T. Dairi, N.
Kato, T. Sassa, Proc. Natl. Acad. Sci. USA 2007, 104,
3084–3088.
b-keto esters of terpenoids 16–19. Studies on the
scope and synthetic applications of this new terpenoid
cyclization are currently underway and will be report-
ed in due course.
Experimental Section
(4aS,5R,7aR)-Ethyl 5-Hydroxy-1,1,5-trimethyl-
1,4a,5,6,7,7a-hexahydrocyclopenta[c]pyran-3-
carboxylate (2a)
[3] a) P. F. Vlad, Pure Appl. Chem. 1993, 65, 1329–1336;
b) J. D. White, R. W. Skeean, G. L. Trammell, J. Org.
Chem. 1985, 50, 1939–1948; c) N. K. N. Yee, R. M.
Coates, J. Org. Chem. 1992, 57, 4598–4608; d) J.-F.
Zhao, Y.-J. Zhao, T.-P. Loh, Chem. Commun. 2008,
1353–1355; e) C. Tsangarakis, C. Raptis, E. Arkoudis,
M. Stratakis, Adv. Synth. Catal. 2008, 350, 1587–1600;
f) J. Kuk, B. S. Kim, H. Jung, S. Choi, J.-Y. Park, S.
Koo, J. Org. Chem. 2008, 73, 1991–1994.
[4] a) B. B. Snider, Chem. Rev. 1996, 96, 339–363; b) J. Jus-
ticia, A. Rosales, E. BuÇuel, J. L. Oller-Lꢂpez, M. Val-
divia, A. Haꢃdour, J. E. Oltra, A. F. Barrero, D. J.
Cꢄrdenas, J. M. Cuerva, Chem. Eur. J. 2004, 10, 1778–
1788; c) S. Handa, P. S. Nair, G. Pattenden, Helv. Chim.
Acta 2000, 83, 2629–2643; d) C. Heinemann, X. Xing,
K.-D. Warzecha, P. Ritterskamp, H. Gçrner, M.
Demuth, Pure Appl. Chem. 1988, 70, 2167–2176.
[5] a) J. Justicia, J. L. Oller-Lꢂpez, A. G. CampaÇa, J. E.
Oltra, J. M. Cuerva, E. BuÇuel, D. J. Cꢄrdenas, J. Am.
Chem. Soc. 2005, 127, 14911–14921; b) A. F. Barrero,
J. F. Q. del Moral, M. M. Herrador, I. Loayza, E. M.
Sꢄnchez, J. F. Arteaga, Tetrahedron 2006, 62, 5215–
5222.
To a stirred solution of (E)-ethyl 2-acetyl-5,9-dimethyldeca-
4,8-dienoate (1a) (0.513 g, 1.93 mmol) in 99.9% EtOH
(30 mL) were added Mn
ACHTUGNRTEN(NUGN OAc)3·2H2O (1.597 g, 5.78 mmol)
and Cu(OAc)2·H2O (0.396 g, 1.93 mmol). The reaction flask
ACHTUNGTRENNUNG
was opened to the air, and the mixture was stirred at room
temperature for 3.5 h. The resulting mixture was treated
with 1M HCl, extracted with CH2Cl2, dried over anhydrous
Na2SO4, filtered, and concentrated under reduced pressure.
The crude product was purified by SiO2 flash chromatogra-
phy to give 2a; yield: 0.291 g (1.14 mmol, 59%). 1H NMR
(CDCl3): d=1.16 (s, 3H), 1.20–1.27 (m, 1H), 1.32 (t, J=
7.2 Hz, 3H), 1.43 (s, 3H), 1.44 (s, 3H), 1.78 (dddd, J=12.4,
6.4, 6.4, 6.4 Hz, 1H), 1.91 (dd, J=12.4, 2.0 Hz, 1H), 1.91–
1.96 (m, 2H), 2.17 (ddd, J=12.4, 12.4, 6.8 Hz, 1H), 4.25 (m,
2H), 6.02 (d, J=2.0 Hz, 1H); 13C NMR (CDCl3): d=14.4,
19.8, 23.6, 27.2, 28.4, 40.9, 47.3, 47.6, 61.3, 77.5, 80.6, 106.3,
145.7, 163.5; IR (KBr): n=3503, 2976, 2939, 2907, 2874,
1732, 1718, 1626, 1466, 1447, 1373, 1269, 1101 cmÀ1; HR-MS
(CI+): m/z=255.1595, calcd. for C14H23O4: 255.1996.
The reaction of 1a (0.25 g, 0.94 mmol) with Mn-
ACHTUNGTRENNUNG(OAc)3·2H2O (0.78 g, 2.82 mmol) and CuCAHUTNGTREN(NUGN OAc)2·H2O
(0.19 g, 0.94 mmol) in 94% EtOH (10 mL) at 50–608C for
3 h produced 2a in 75% isolated yield (0.18 g, 0.71 mmol).
[6] a) D. Yang, M. Xu, Org. Lett. 2001, 3, 1785–1788; for
review on manganeseACTHNUGRTENUNG(III) acetate, see: b) A. S. Demir,
M. Emrullahoglu, Current Org. Synth. 2007, 4, 321–350.
[7] CCDC 767268 contains the supplementary crystallo-
graphic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
[8] T. Linker, U. Linker, Angew. Chem. 2000, 112, 934–
936; Angew. Chem. Int. Ed. 2000, 39, 902–904.
[9] Reviews on the hetero-Diels–Alder reaction, see:
a) K. A. Jørgensen, Angew. Chem. 2000, 112, 3702–
3733; Angew. Chem. Int. Ed. 2000, 39, 3558–3588;
b) M. A. Rizzacasa, A. Pollex, Org. Biomol. Chem.
2009, 7, 1053–1059; c) K. A. Jørgensen, Eur. J. Org.
Chem. 2004, 2093–2102; d) H. Pellissier, Tetrahedron
2009, 65, 2839–2877.
Acknowledgements
This research was supported by Basic Science Research Pro-
gram (No. 2009-0072552) and by Priority Research Centers
Program (No. 2010-0028300) both through the National Re-
search Foundation of Korea funded by the Ministry of Edu-
cation, Science and Technology (MEST).
References
[1] a) R. A. Yoder, J. N. Johnston, Chem. Rev. 2005, 105,
4730–4756; b) E. Brunoldi, M. Luparia, A. Porta, G.
Zanomi, G. Vidari, Current Org. Chem. 2006, 10, 2259–
2282; c) A. Sakakura, A. Ukai, K. Ishihara, Nature
2007, 445, 900–903; d) S. R. Harring, T. Livinghouse, J.
Chem. Soc. Chem. Commun. 1992, 503–505; e) E. D.
Edstrom, T. Livinghouse, J. Org. Chem. 1987, 52, 951–
953; f) T. R. Hoye, A. J. Caruso, M. J. Kurth, J. Org.
Chem. 1981, 46, 3550–3552.
[10] a) E. I. Heiba, R. M. Dessau, J. Am. Chem. Soc. 1971,
93, 524–527; b) E. I. Heiba, R. M. Dessau, J. Am.
Chem. Soc. 1972, 94, 2888–2889; c) J. K. Kochi, Acc.
Chem. Res. 1974, 7, 351–360.
[11] M. Allegretti, A. D’Annibale, C. Trogolo, Tetrahedron
1993, 49, 10705–10714.
[12] See Experimental Section in the Supporting Informa-
tion for the stereochemical assignment of each product
in Table 3.
[2] a) D. E. Cane, in: Comprehensive Natural Products
Chemistry, Vol. 2, (Eds: D. Barton, K. Nakanishi),
Elsevier, Oxford, 1999; b) C. M. Paschall, J. Hasserodt,
Adv. Synth. Catal. 2011, 353, 1913 – 1917
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
1917