α-Aryl-substituted allenamides in an imino-nazarov cyclization cascade catalyzed by Au(I)

Article abstract of DOI:10.1021/ol302743k

An imino-Nazarov cyclization using α-aryl-substituted allenamides is described. This gold(I)-catalyzed cascade is efficient and regioselective in constructing a diverse array of synthetically useful aromatic-ring fused cyclopentenamides. The success in this transformation represents a solution to the challenge in establishing an imino-Nazarov cyclization process.

Full text of DOI:10.1021/ol302743k

ORGANIC
LETTERS
2012
Vol. 14, No. 22
5736–5739
r‑Aryl-Substituted Allenamides in an
Imino-Nazarov Cyclization Cascade
Catalyzed by Au(I)
Zhi-Xiong Ma, Shuzhong He, Wangze Song, and Richard P. Hsung*
Division of Pharmaceutical Sciences, School of Pharmacy, University of Wisconsin,
Madison, Wisconsin 53705, United States
rhsung@wisc.edu
Received October 4, 2012
ABSTRACT
An imino-Nazarov cyclization using R-aryl-substituted allenamides is described. This gold(I)-catalyzed cascade is efficient and regioselective in
constructing a diverse array of synthetically useful aromatic-ring fused cyclopentenamides. The success in this transformation represents a
solution to the challenge in establishing an imino-Nazarov cyclization process.
The Nazarov cyclization1 has captured an immense
amount of interest from the synthetic community in the
past three decades.2,3 This classical pericyclic process
Scheme 1. Nazarov Cyclizations: Challenge in an Imino Version
(1) Nazarov, I. N.; Torgov, I. B.; Terekhova, L. N. Izv. Akad. Nauk
SSSR, Otd. Khim. Nauk 1942, 200.
(2) For earlier reviews, see: (a) Habermas, K. L.; Denmark, S. E.;
Jones, T. K. In Organic Reactions; Paquette, L. A., Ed.; John Wiley & Sons:
New York, 1994; Vol. 45, pp 1ꢀ158. (b) Denmark, S. E. In Comprehensive
Organic Synthesis; Trost, B. M., Fleming, I., Eds.; Pergamon: Oxford, U.K.,
1991; Vol 5, pp 751ꢀ784. (c) Santelli-Rouvier, C.; Santelli, M. Synthesis
1983, 429.
(3) For recent reviews, see: (a) Vaidya, T.; Eisenberg, R.; Frontier,
A. J. ChemCatChem 2011, 3, 1531. (b) Shimada, N.; Stewart, C; Tius,
M. A. Tetrahedron 2011, 67, 5851. (c) Grant, T. N.; Rieder, C. J.; West,
F. G. Chem. Commun. 2009, 45, 5676. (d) Nakanishi, W.; West, F. Curr.
Opin. Drug Discovery Dev. 2009, 12, 732. (e) Frontier, A.; Collison, C.
Tetrahedron 2005, 61, 7577. (f) Tius, M. A. Eur. J. Org. Chem. 2005,
2193. (g) Pellissier, H. Tetrahedron 2005, 61, 6479. (h) Tius, M. A. Acc.
Chem. Res. 2003, 36, 284. (i) Harmata, M. Chemtracts 2004, 17, 416.
(4) Given the large volume, for some reports in 2012 on Nazarov
proceeds through a conrotatory 4π-electron electrocyclic
cyclizations, see: (a) Shimada, N.; Stewart, C.; Bow, W. F.; Jolit, A.;
Wong, K.; Zhou, Z.; Tius, M. A. Angew. Chem., Int. Ed. 2012, 51, 5727.
(b) Lebuf, D.; Gandon, V.; Ciesielski, J.; Frontier, A. J. J. Am. Chem.
Soc. 2012, 134, 6296. (c) Kerr, D. J.; Flynn, B. L. Org. Lett. 2012, 14,
1740. (d) Guo, F.; Wang, L.; Mao, S.; Zhang, C.; Yu, J.; Han, J.
ring closure of a pentadienyl cation via activation of
divinyl ketone 1 or its equivalent, leading to a key oxyallyl
cation 2 that can afford cyclopentenone 3 or substituted
cyclopentanones 4 through trapping [Scheme 1]. Despite
a diverse array of innovative structural designs that have
been developed,4ꢀ6 one aspect that has received very little
attention is an imino version of this cascade with the sole
exceptions of Tius’ seminal work on lithiated imino-Nazarov
ꢀ
Tetrahedron 2012, 68, 8367. (e) Alvarez, E.; Miguel, D.; Garcıa-Garcıa,
ꢀ
P.; Fernandez-Rodrıguez, M. A.; Rodrıguez, F.; Sanz, R. Synthesis
2012, 1874. (f) Raja, S.; Ieawsuwan, W.; Korotkov, V.; Rueping, M.
Chem.;Asian J. 2012, 7, 2361. (g) Dateer, R. B.; Pati, K.; Liu, R.-S.
Chem. Commun. 2012, 48, 7200. (h) Gan, Z.; Wu, Y.; Gao, L.; Sun, X.;
Lei, J.; Song, Z.; Li, L. Tetrahedron 2012, 68, 6928. (i) Also see refs 11
and 12.
r
10.1021/ol302743k
Published on Web 11/02/2012
2012 American Chemical Society

R-aryl allenamides 7couldbeemployedinanimino-Nazarov
cyclization cascade through Brønsted acid activation.19
This electrophilic activation could give amido-pentadienyl
cation 8, which would undergo an electrocyclization
to afford 2-amido-allyl cation 9. It is noteworthy that a
distinct advantage in achieving a successful imino-Nazarov
cyclization is the synthetically useful enamide func-
tionality20 in the end product 10.
Scheme 2. An Approach to Imino-Nazarov Cyclization
We reasoned that unlike amino-pentadienyl cation
5, the electrocyclization of 7 could be more favored
given the reduced ability of an N-acyl nitrogen atom
to stabilize cations relative to an amino group. Such
rationalization would find prevalent support in the
related chemistry of vinyl N-acyl iminium ions21 and
aza-Prins cyclizations.22 However, after many attempts,
the Brønsted acid activation failed because we were
unable to control and overtake the hydrolysis pathway
reactions7a and, recently, with enamine-iminium ions.7b,c
There are examples in which stabilization of the pen-
tadienyl cation is assisted by the presence of an amino
or amide substituent, albeit not a formal imino-Nazarov
cyclization. Cha’s8 earlier work on cephalotaxine synthe-
sis and reports by Occhiato/Prandi,9 Frontier,10 and
Flynn11 employing ynamides, and recently by West12 in
a clever usage of allenamides, represent such elegant
cases. The challenge in developing a successful imino-
Nazarov cyclization is to overcome the propensity of the
2-amino-allyl cation 6 to ring open in favor of amino-
pentadienyl cation 5, which receives greater stabilization
from the amino nitrogen atom.13 We wish to commu-
nicate here our solution to this challenge through gold(I)
activation of allenamides.
(15) Also see: (a) Standen, P. E.; Kimber, M. C. Curr. Opin. Drug
Discovery Dev. 2010, 13, 645. (b) Deagostino, A.; Prandi, C.; Tabasso,
S.; Venturello, P. Molecules 2010, 15, 2667. (c) For general reviews on
allenes, see: Krause, N.; Hashmi, A. S. K. Modern Allene Chemistry;
Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, 2004; Vols. 1 and 2.
(16) Given the large volume, for reports on allenamide chemistry just
in 2012, see: (a) Reference 12. (b) Du, Y.; Krenske, E. H.; Antoline, J. E.;
Lohse, A. G.; Houk, K. N.; Hsung, R. P. J. Org. Chem. 2012, ASAP
Article (DOI:10.1021/jo3011792). (c) Singh, S.; Elsegood, M. R. J.;
Kimber, M. C. Synlett 2012, 23, 565. (d) Heffernan, S. J.; Carbery,
D. R. Tetrahedron Lett. 2012, 53, 5180. (e) Hayashi, R.; Ma, Z.-X.;
Hsung, R. P. Org. Lett. 2012, 14, 252. (f) X.-X.; Zhu, L.-L.; Zhou, W.;
Chen, Z. Org. Lett. 2012, 14, 436. (g) Husinec, S.; Petkovic, M.; Savic, V.;
ꢀ
ꢀ
Simic, M. Synthesis 2012, 399. (h) Suarez-Pantiga, S.; Hernandez-Dıaz,
ꢀ
C.; Piedrafita, M.; Rubio, E.; Gonzaleza, J. M. Adv. Synth. Catal. 2012,
354, 1651. (i) Faustino, H.; Paloma Bernal, P.; Luis Castedo, L.;
ꢀ
~
Fernando Lopez, F.; Mascarenas, J. L. Adv. Synth. Catal. 2012, 354,
1658. (j) Francos, J.; Grande-Carmona, F.; Faustino, H.; Iglesias-
Our long-standing interest in allenamides,14ꢀ16 and in par-
ticular their electrophilic activations,17,18 led us to explore
their potential use in this underdeveloped aspect of the
Nazarov cyclization. As shown in Scheme 2, R-vinyl or
€
ꢀ
Siguenza, J.; Dıez, E.; Alonso, I.; Fernandez, R.; Lassaletta, J. M.;
ꢀ
~
Lopez, F.; Mascarenas, J. L. J. Am. Chem. Soc. 2012, 134, 14322. (k)
Broggini, G.; Borsini, E.; Fasana, A.; Poli, G.; Liron, F. Eur. J. Org.
Chem. 2012, 3617.
(17) For an account, see: Lohse, A. G.; Hsung, R. P. Chem.;Eur. J.
2011, 17, 3812.
(18) (a) Via oxidation: Xiong, H.; Hsung., R. P.; Berry, C. R.;
Rameshkumar, C. J. Am. Chem. Soc. 2001, 123, 7174. (b) Via Brønsted
acid: Berry, C. R.; Hsung, R. P.; Antoline, J. E.; Petersen, M. E.;
Rameshkumar, C.; Nielson, J. A. J. Org. Chem. 2005, 70, 4038. (c) Via
halogen: Hayashi, R.; Walton, M. C.; Hsung, R. P.; Schwab, J.; Yu, X.
Org. Lett. 2010, 12, 5768.
(19) For a Brønsted acid activation of allenol ethers, see: (a) Wu,
Y.-K.; West, F. G. J. Org. Chem. 2010, 75, 5410. For an oxidative
activation, see: (b) Spencer, W. T., III; Levin, M. D.; Frontier, A. J. Org.
Lett. 2011, 13, 414. (c) Malona, J. A.; Cariou, K.; Frontier, A. J. J. Am.
Chem. Soc. 2009, 131, 7560. For a gold activation, see: (d) Zhang, L.;
Wang, S. J. Am. Chem. Soc. 2006, 128, 1442.
(20) For a leading review on enamides, see: (a) Carbery, D. R. Org.
Biomol. Chem. 2008, 9, 3455. Also see: (b) Rappoport, Z. The Chemistry
of Functional Groups; John Wiley and Sons: New York, 1994. (c) (b)Tracey,
M. R.; Hsung, R. P.; Antoline, J.; Kurtz, K. C. M.; Shen, L.; Slafer, B. W.;
Zhang, Y. In Science of Synthesis, Houben-Weyl Methods of Molecular
Transformations; Weinreb, S. M., Ed.; Georg Thieme Verlag KG: Chapter
21.4, 2005.
(21) For reviews, see: (a) Maryanoff, B. E.; Zhang, H.-C.; Cohen,
J. H.; Turchi, I. J.; Maryanoff, C. Chem. Rev. 2004, 104, 1431. (b) Royer,
J.; Bonin, M.; Micouin, L. Chem. Rev. 2004, 104, 2311.
(5) For leading references on arrested-Nazarov, see: (a) Scadeng, O.;
Ferguson, M. J.; West, F. G. Org. Lett. 2011, 13, 114. (b) Rostami, A.;
Wang, Y.; Arif, A. M.; McDonald, R.; West, F. G. Org. Lett. 2007, 9,
703. (c) Song, D.; Rostami, A.; West, F. G. J. Am. Chem. Soc. 2007, 129,
12019. (d) White, T. D.; West, F. G. Tetrahedron Lett. 2005, 46, 5629.
(e) Giese, S.; Kastrup, L.; Stiens, D.; West, F. G. Angew. Chem., Int. Ed.
2000, 39, 1970. (f) Bender, J. A.; Arif, A. M.; West, F. G. J. Am. Chem.
Soc. 1999, 121, 7443.
(6) Also see: (a) Dhoro, F.; Tius, M. A. J. Am. Chem. Soc. 2005, 127,
12472. (b) Dhoro, F.; Kristensen, T. E.; Stockmann, V.; Yap, G. P. A.;
Tius, M. A. J. Am. Chem. Soc. 2007, 129, 7256. (c) Nair, V.; Bindu, S.;
Sreekumar, V.; Chiaroni, A. Org. Lett. 2002, 4, 2821.
(7) (a) Tius, M. A.; Chu, C. C.; Nieves-Colberg, R. Tetrahedron Lett.
2001, 42, 2419. (b) Bow, W. F.; Basak, A. K.; Jolit, A.; Vicic, D. A.; Tius,
M. A. Org. lett 2010, 12, 440. (c) Shimada, N.; Ashburn, B. O.; Basak,
A. K.; Bow, W. F.; Vicic, D. A.; Tius, M. A. Chem. Commun. 2010, 46,
3774. For an example postulating an imino-Nazarov reaction pathway,
ꢀ
see: (d) Suarez-Pantiga, S.; Rubio, E.; Alvarez-Rua, C.; Gonzalez, J. M.
Org. Lett. 2009, 11, 13.
(8) Kim, S. H.; Cha, J. K. Synthesis 2000, 2113.
(9) (a) Larini, P.; Guarna, A.; Occhiato, E. G. Org. Lett. 2006, 8, 781.
(b) Occhiato, E. G.; Prandi, C.; Ferrali, A.; Guarna, A. J. Org. Chem.
2005, 70, 4542.
(10) (a) Bitar, A. Y.; Frontier, A. J. Org. Lett. 2009, 11, 49. (b)
Malona, J. A.; Colbourne, J. M.; Frontier, A. J. Org. Lett. 2006, 8, 5661.
(11) Kerr, D. J.; Miletic, M.; Chaplin, J. H.; White, J. M.; Flynn, B. L.
Org. Lett. 2012, 14, 1732.
(12) Wu, Y.-K.; Niuz, T.; West, F. G. Chem. Commun. 2012, 48,
9186.
(13) Smith, D. A.; Ulmer, C. W., II. J. Org. Chem. 1997, 62, 5110.
(14) For a leading review on allenamide chemistry, see: Hsung, R. P.;
Wei, L.-L.; Xiong, H. Acc. Chem. Res. 2003, 36, 773.
(22) For reviews, see: Hiemstra, H.; Speckamp, W. N. In Compre-
hensive Organic Synthesis; Brossi, A., Ed.; Academic Press: San Diego,
1988; Vol. 2, pp 271ꢀ339. (b) Hanessian, S.; Tremblay, M.; Marzi, M.;
Del Valle, J. R. J. Org. Chem. 2005, 70, 5070.
(23) For reviews, see: (a) Boorman, T. C.; Larrosa, I. Chem. Soc. Rev.
2011, 40, 1910. (b) Bandini, M. Chem. Soc. Rev. 2011, 40, 1358.
€
(c) Krause, N.; Winter, C. Chem. Rev. 2011, 111, 1994. (d) Furstner,
A. Chem. Soc. Rev. 2009, 38, 3208. (e) Gorin, D. J.; Sherry, B. D.; Toste,
€
F. D. Chem. Rev. 2008, 108, 3351. (f) Furstner, A.; Davies, P. W. Angew.
Chem., Int. Ed. 2007, 46, 3410. (g) Hashmi, A. S. K. Chem. Rev. 2007,
107, 3180.
Org. Lett., Vol. 14, No. 22, 2012
5737

Table 1. Screening for a Suitable Metal Catalyst
Table 2. A Au(I)-Catalyzed Imino-Nazarov Cyclization
14 þ
15d
time
(h)
conversion
(%)b
13a
entrya
catalyst
AuCl3
(%)c
(%)c
1
2
3
4
5
6
30
1
>95
>95
48
55g
>95
75g
ꢀ
55
45
36
36
48
24
AuCl3/AgSbF6
AuCl3/AgSbF6
AuCl3/AgSbF6
AuCl/AgSbF6
AuCl(PPh3)/
AgSbF6
ꢀ
a Unless noted, all reactions were carried out at 0.1 mmol scale in
2 mL of CH2Cl2 (concn = 0.05 M) at rt with the addition of 5 mol %
catalyst. b Isolated yields. c The reaction was run at 50 °CinDCE.d No reaction.
e
f
22
40
5
ꢀ
ꢀ
ꢀ
60
50h
7
JohnphosAuCl/
AgSbF6
5
>95
91
ꢀ
8
IPrAuCl/AgSbF6
IPrAuCl
1
>95
<5
97
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
9
21
20
24
12
ꢀ
reaction had taken place possibly postdimerization
through the intermediacy of 140 [addition of 12a to acti-
vated-12a] Gratifyingly with AuCl(PPh3)/AgSbF6, 13a
was obtained for the first time in 50% yield along with
the 14/15 mixture in 24% yield. Further optimizations
revealed that the best yield was reached when using
IPrAuCl/AgSbF6, leading to 13a in 97% yield (entry 8)
with JohnphosAuCl/AgSbF6 affording 13a in 91% yield.
Remarkably, neutral IPrAuCl itself turned out to be
completely inactive (entry 9), while AgSbF6 alone gave
14 and 15, albeit sluggishly (entry 10). Lastly, Pt catalysts
were also examined and gave very poor results in this
process (entries 11 and 12).
Having established the optimum catalytic system, the
scope of the imino-Nazarov process was explored [Table 2].
Notably, for allenamides equipped with more bulky sub-
stituents on the nitrogen (n-Bu in 12b and Bn in 12c),
reactions were slower, although yields were still excellent
(entries 1 and 2). The structure of 13b was unambiguously
assigned through its X-ray structure [left in Figure 1]. The
cyclization of 12d, bearing a 4-methoxybenzenesulfonyl
[MBS] substituent, also worked but at elevated temperature.
Lastly, allenamides with different substituents on the aryl
i
10
11
12
AgSbF6
64
24
ꢀ
PtCl2
<5
i
PtCl4
>95
82j
a Unless noted, all reactions were carried out at 0.1 mmol scale in
2 mL of CH2Cl2 (concn = 0.05 M) at rt with the addition of 5 mol %
catalyst. b Conversion determined by 1H NMR. c Isolated yields. d 14 and
15 could not be separated by column chromatography. e 20 mL of CH2Cl2
were used (concn = 0.005 M). f 35 mg of 4 A MS were added. g Based on
recovered 12a. h Inseparable mixture with 12a. i 10 mol % of catalyst was
used. j An inseparable mixture of 14, 15, and unidentified products.
that led to amido ketones 11 through a facile trapping
of the initial enone.
Inspired by recent gold catalysis23 particularly in the area
of a Nazarov-type process24 with the report by Zhang,19d we
turned our attention to gold catalysts. As shown in Table 1, a
trivalent gold catalyst was first investigated. Reactions of 12a
proceeded smoothly in the presence of a catalytic amount of
AuCl3 or cationic AuCl3/AgSbF6 (entries 1 and 2) but gave
an inseparable mixture of dimers 14 and 1525 with no desired
Nazarov cyclization product 13a. Attempts to suppress the
dimerization by lowering the concentration (entry 3) or addi-
tion of 4 A MS (entry 4) proved to be fruitless.
Nevertheless, we were encouraged by dimers 14 and
15 because they imply that the desired imino-Nazarov
(24) For examples of Nazarov-like processes through activations of
vinyl allenes, see: (a) Chen, B.; Fan, W.; Chai, G.; Ma, S. Org. Lett. 2012,
ꢁ
14, 3616. (b) Lemiere, G.; Gandon, V.; Cariou, K.; Hour, A. J. Am.
Chem. Soc. 2009, 131, 2993. (c) Oh., C. H.; Karmakar, S. J. Org. Chem.
2009, 74, 370. (d) Bhunia, S.; Liu, R.-S. J. Am. Chem. Soc. 2008, 130,
16488. (e) Shi, F.-Q.; Li, X.; Xia, Y.; Zhang, L.; Yu, Z.-X. J. Am. Chem.
Soc. 2007, 129, 15503. (f) Lee., J. H.; Toste, F. D. Angew. Chem., Int. Ed.
2007, 46, 912. (g) Funami, H.; Kusama, H.; Iwasawa, N. Angew. Chem.,
ꢀ
ꢀ
Int. Ed. 2007, 46, 909. (h) Marion, N.; Dıez-Gonzalez, S.; Fremont, P.;
de; Noble, A. R.; Nolan, S. P. Angew. Chem., Int. Ed. 2006, 45, 3647.
(i) Shi, X.; Gorin, D. J.; Toste, F. D. J. Am. Chem. Soc. 2005, 127, 5802.
(25) See Supporting Information.
Figure 1. X-ray structures of 13b and 20.
Org. Lett., Vol. 14, No. 22, 2012
5738

Scheme 3. Regioselective Imino-Nazarov Cyclizations
Scheme 5. Synthesis of Indole-Fused Cyclopentenamides
Intrigued by this competition, another reaction with po-
tentially competing imino-Nazarov 4π-electron and 6π-
electron electrocyclization was examined using allenamide
22 [Scheme 4]. The reaction of 22 led to 25 and 26 as the
onlydiscernibleproductswith thelatterlikelycomingfrom
hydrolysis of the initial Nazarov cyclization product 25.
This study suggests that while the cationic imino-Nazarov
intermediate could exist in two equilibrating conforma-
tions 23 and 24 differing only at the ‘s’-marked CꢀC bond
[or being trans-and cis-1-aza-dienes, respectively, shown in
green], 6π-electron electrocyclization of 3-aza-triene 2426,27
was also not competitive in this case.
Lastly, while cyclopentenamides are synthetically useful,
we recognized that our imino-Nazarov cyclization could pro-
vide a concise approach to indole-fused cyclopentenamides
[or cyclopenta[b]indoles],24a thereby representing an opportu-
nity to develop a new strategy for natural product synthesis.28
Consequently, as shown in Scheme 5, under the optimized
conditions, cyclizations of R-indolyl-substituted allenamides
28a and 28b proceeded smoothly to afford indole-fused
cyclopentenamides 29a and 29b, respectively, in good yields.
We have showcased here an imino-Nazarov cyclization
using R-aryl-substituted allenamides. This Au(I)-catalyzed
cascade represents a successful solution to overcome
the challenge in designing an imino-Nazarov cyclization
cascade. It is a highly efficient and regioselective transfor-
mation in constructing a diverse array of aromatic-ring
fused cyclopentenamides. Further mechanistic studies and
applications of this cyclization process in total synthesis
are currently underway.
ring (entries 4ꢀ9) also proved to be quite effective overall
with exceptions of 2-MeO and 4-Br substituents (entries 5
and 9). In particular, while the reaction of 12f is very tardy,
the cyclization of 12j was completely attenuated. We are not
clear of the precise rationale at this point. However, it is
noteworthy that the high level of regioselectivity found in
cases of 12e and 12k was remarkable. While these two cycliza-
tions appear to favor the less hindered C6-position with no
detectable cyclization at the C2-position, this preference is
likely due to direct donation from the C3ꢀOR group.
To further investigate the regioselectivity issue, allen-
amides 16 and 19 were subjected to the cyclization condi-
tions [Scheme 3]. For allenamide 16withanR-(2-naphthyl)
substituent, cyclization took place selectively at C1 [see 170],
affording 17 exclusively in 97% yield. This regioselec-
tivity is also likely an electronic preference given that
cyclization atC1wouldinvolve the cyclohexadiene portion
of the naphthyl ring, thereby leading to greater cation
stabilization: Benzylic stabilization in 170 vs pseudo-
benzylic stabilization in 180 through cyclization from
C3. On the other hand, for allenamide 19 with an R-(1-
naphthyl) group, a competing 6π-electron cyclization of
the reaction intermediate [see 210] could deliver 1H-phe-
nalene 21. However, the reaction exclusively proceeded
through the imino-Nazarov cyclization pathway via 200,
leading to 20 in excellent yield. The structure of 20 was also
confirmed by its single crystal X-ray structure [Figure 1].
It is noteworthy that 17 and 20 are structurally almost
identical with the difference residing in the position for the
respective enamide motif.
Acknowledgment. We thank the NIH [GM066055] for
funding. We thank Dr. Vic Young of the University of
Minnesota for providing X-ray structural analysis.
Supporting Information Available. Experimental pro-
cedures, NMR spectra, X-ray structural files, and character-
izations for all new compounds. This material is available free
of charge via the Internet at http://pubs.acs.org.
Scheme 4. 4π- vs 6π-Electron Electrocyclization
(26) Okamura, W. H.; de Lera, A. R. In Comprehensive Organic
Synthesis; Trost, B. M., Fleming, I., Paquette, L. A., Eds.; Pergamon Press:
New York, 1991; Vol. 5, pp 699ꢀ750.
(27) For leading references, see: (a) Tanaka, K.; Katsumura, S.
J. Am. Chem. Soc. 2002, 124, 9660. (b) Sklenicka, H. M.; Hsung,
R. P.; McLaughlin, M. J.; Wei, L.-L.; Gerasyuto, A. I.; Brennessel,
W. W. J. Am. Chem. Soc. 2002, 124, 10435.
(28) For some examples, see: (a) Feng, T.; Li, Y.; Wang, Y.-Y.; Cai,
X.-H.; Liu, Y.-P.; Luo, X.-D. J. Nat. Prod. 2010, 73, 1075. (b) He, Y.-L.;
Chen, W.-M.; Feng, X.-Z. Phytochemistry 1994, 37, 1055.
The authors declare no competing financial interest.
Org. Lett., Vol. 14, No. 22, 2012
5739