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Total synthesis of (±)-debromoflustramine B via [4+1] cyclization of a bis(alkylthio)carbene and an indole isocyanate
Tetrahedron Letters 54 (2013) 4760–4762
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters journal homepage: www.elsevier.com/locate/tetlet
Total synthesis of (±)-debromoflustramine B via [4+1] cyclization of a bis(alkylthio)carbene and an indole isocyanate Saptarshi De ⇑, James H. Rigby Department of Chemistry, Wayne State University, Detroit, MI 48202, USA
a r t i c l e
i n f o
Article history: Received 3 June 2013 Revised 16 June 2013 Accepted 22 June 2013 Available online 30 June 2013
a b s t r a c t An efficient total synthesis of (±)-debromoflustramine B and (±)-debromoflustramide B has been achieved, through a novel [4+1] cyclization of an indole isocyanate and bis(alkylthio)carbene. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Japp–Klingermann Fisher indole synthesis Curtius rearrangement Bis(alkylthio)carbene [4+1] Cyclization
Alkaloids with the hexahydropyrrole[2.3-b]indole (HPI) core structure are abundant in nature and are known for a broad range of biological activities.1 Debromoflustramine B (1), isolated from the marine bryozoan Flustra foliacea, is one of the prominent members of the HPI family. It contains prenyl groups at C-3a and N-8 positions (Fig. 1).2 A recent study indicates debromoflustramine B (1) and its analogues are potent selective butyrylcholinesterase (BChE) inhibitors and hence promising therapeutic agents for Alzheimer’s disease.3 The major challenges that the construction of these natural products present are the sterically congested quaternary stereocenter at C-3a and the assembly of the pyrroloindoline core structure. The synthetically challenging structure along with interesting biological activity have prompted several total syntheses of 1.4 Herein is reported an efficient total syntheses of (±)-debromoflustramine B (1) and (±)-debromoflustramide B (2). The [4+1] cyclization of vinyl isocyanates5 with nucleophilic carbenes6 is of considerable interest because it affords a rapid entry into functionalized pyrrolidinone rings.7 These transformations are also well-suited to the challenges of preparing the quaternary carbon center present in the target molecule.7b,8d,10 While both bisalkoxy and bis(alkylthio)carbenes are suitable for ring formation, the latter offers more attractive opportunities for post-cyclization manipulation in the current context.8–10 The retro-synthetic analysis is outlined in Scheme 1. Based on previous observations,10 it was envisioned that 1 and 2 could be accessed from dithio-substituted intermediate 7, through simple functional group manipulation. The tricyclic intermediate 7 was ⇑ Corresponding author. Tel.: +1 903 217 8638. E-mail address: [email protected] (S. De). 0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.tetlet.2013.06.119
3a N
N H Me
X
X = H, H Debromoflustramine B (1) X = O, Debromoflustramide B (2) Figure 1. Structures of debromoflustramine B (1) and debromoflustramide B (2).
predicted to arise via the key [4+1] cyclization of indole isocyanate 5 and bis(propylthio)-carbene precursor 6. The isocyanate 5 would be readily available from the indole derivative 4, which in turn can be obtained using a Japp–Klingemann modified Fischer indole synthesis starting from aniline and ethyl 2-acetylhex-5-enoate (3).11a The synthesis commenced with the preparation of indole 4, which was previously synthesized by Murakami and co-workers using a Stille type coupling reaction.11b An inability to perform this reaction on larger scale led us to investigate an alternate pathway. Exploiting the Japp–Klingemann–Fischer indole synthesis method,10,12 4 could be obtained from aniline and the known compound 3, in satisfactory yield (Scheme 2). The requisite N-prenyl group was then installed by treating 4 with prenyl bromide in the presence of NaH. The terminal alkene group of 9 was successfully converted to the second required prenyl group through a cross metathesis reaction employing 2-methyl-2-butene in excellent yield.4e,j The resulting ester 10 was then hydrolyzed to the carboxylic acid 11.
4761
S. De, J. H. Rigby / Tetrahedron Letters 54 (2013) 4760–4762
SPr
1 and 2
SPr
SPr N
N H Me
SPr N
O
N
O
8
+ N H
CO2H
N
CON3
7
[4+1] cyclization
NCO
N
CO2Et
12
11
SPr SPr
SPr SPr
N
O
N
6, PhH, reflux
Et3N, DPPA
N
N
N
SPr SPr
LiAlH4, THF reflux
N
O
N H R
O
6
4
7
5
13, R = H
NaH, MeI THF
8, R = Me O
NH2
O O
SPr SPr O
N N
+ 3
4 steps, only one column 52% from 11
6
heat Scheme 1. Retrosynthetic analysis for 1 and 2.
SPr SPr SPr
SPr NH2
N
NaNO2, aq HCl, 0 oC KOH, 3
N C O
N
N
7
O
then EtOH/HCl, reflux N H
CO2Et
5
4 58% (> 5 gm scale)
Br
Scheme 3. Synthesis of the tricyclic core structure 8 and proposed mechanism of the [4+1] cyclization.
Grubbs 2nd generation catalyst (5 mol%)
NaH, DMF 95%
N
CO2Et
DCM, reflux 93%
N
SPr
CO2R
SPr N
N H
9
10, R = Et 11, R = H
LiOH, EtOH/H2O Reflux, 86%
O
Raney Ni EtOH, rt 83%
N
N
X
H
8 X = O, debromoflustramide B (2)
Scheme 2. Synthesis of carboxylic acid 11.
LiAlH4 THF, reflux quantitative
X = H, H debromoflustramine B (1)
With important intermediate 11 in hand, attention turned toward the synthesis of the pyrrolidinone core structure of the natural product. Carboxylic acid 11 was converted into the acyl azide 12 by treatment with diphenyl phosphoryl azide (DPPA) in the presence of triethylamine (Et3N). The resultant acyl azide 12 was then refluxed in dry benzene to generate the required isocyanate 5 in situ via Curtius rearrangement (Scheme 3). Excess dithiooxadiazoline 6 (bis(propylthio)carbene precursor) was then added portion wise to the reaction mixture while still at reflux to form the desired adduct. The resultant crude imine tricycle 7 was immediately reduced with LiAlH4 to intermediate 13. Labile intermediate 13 was immediately N-methylated by treatment with methyl iodide in the presence of NaH to give 8 in 52% yield over four steps starting from carboxylic acid 11. It is noteworthy that the reaction proceeded efficiently to afford the sterically congested C-3a quaternary center.9,10 Only a single column purification was required for the entire sequence from 11 to 8.
Scheme 4. Total synthesis of (±)-debromoflustramine B (1) and (±)-debromoflustramide B (2).
After successfully constructing the tricyclic core of the target molecule, attention was turned to the synthetic end game (Scheme 4). The thiopropyl groups in intermediate 8 underwent smooth reductive desulfurization with Raney-Ni to provide the required methylene group in (±)-debromoflustramide B (2) and, finally the lactam 2 was reduced to afford the natural product (±)-debromoflustramine B (1) by refluxing with LiAlH4. The synthesis was achieved in 10 linear steps from aniline. The spectral data of 1 matched with those reported in the literature.4j–l In conclusion, the total syntheses of (±)-debromoflustramide B (2) and (±)-debromoflustramine B (1) have been achieved without employing any nitrogen protecting group, compared to the other reported syntheses. The key steps were the Japp–Klingermann
4762
S. De, J. H. Rigby / Tetrahedron Letters 54 (2013) 4760–4762
Fisher indole synthesis to form indole carboxylate 4 and a [4+1] cyclization of the corresponding indole isocyanate and bis(propylthio)carbene to access the tricyclic core containing the critical sterically crowded C-3a all carbon quaternary stereocenter. Acknowledgment The authors wish to thank the Wayne State University for support of the research. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.tetlet.2013.06. 119.
5. 6.
7.
References and notes 8. 1. Ruiz-Sanchis, P.; Savina, S. A.; Albericio, F.; A´lvarez, M. Chem. Eur. J. 2011, 17, 1388. 2. (a) Christophersen, C. Acta Chem. Scand. 1985, B39, 517; (b) Holst, P. B.; Anthoni, U.; Christophersen, C.; Nielsen, P. H. J. Nat. Prod. 1994, 57, 997. 3. (a) Rivera-Becerril, E.; Joseph-Nathan, P.; Pe´rez-A´lvarez, V. M.; Morales-Ríos, M. S. J. Med. Chem. 2008, 51, 5271; (b) Mitchell, M. O.; Figliozzi, R. W.; Guzel, M. Med. Chem. 2010, 6, 141. 4. For some notable synthesis of debromoflustramine B (a) Bruncko, M.; Crich, D.; Samy, R. J. Org. Chem. 1994, 59, 5543; (b) Morales-Ríos, M. S.; Suárez-Castillo, O.
9. 10. 11. 12.
R.; Joseph-Nathan, P. J. Org. Chem. 1999, 64, 1086; (c) Cardoso, A. S.; Srinivasan, N.; Lobo, A. M.; Prabhakar, S. Tetrahedron Lett. 2001, 42, 6663; (d) Tan, G. H.; Zhu, X.; Ganesan, A. Org. Lett. 1801, 2003, 5; (e) Austin, J. F.; Kim, S. G.; Sinz, C. J.; Xiao, W. J.; MacMillan, D. W. C. Proc. Natl. Acad. Sci. U.S.A. 2004, 101, 5482; (f) Morales-Ríos, M. S.; Rivera-Becerril, E.; Joseph-Nathan, P. Tetrahedron: Asymmetry 2005, 16, 2493; (g) López-Alvarado, P.; Caballero, E.; Avendaño, C.; Menéndez, J. C. Org. Lett. 2006, 8, 4303; (h) Miyamoto, H.; Okawa, Y.; Nakazaki, A.; Kobayashi, S. Tetrahedron Lett. 2007, 48, 1805; (i) Cardoso, A. S. P.; Marques, M. M. B.; Srinivasan, N.; Prabhakar, S.; Lobo, A. M. Tetrahedron 2007, 63, 10211; (j) Schammel, A. W.; Boal, B. W.; Zu, L.; Mesganaw, T.; Garg, N. K. Tetrahedron 2010, 66, 4687; (k) Trost, B. M.; Zhang, Y. Chem. Eur. J. 2011, 17, 2916; (l) Zhou, Y.; Xi, Y.; Zhao, J.; Sheng, X.; Zhang, S.; Zhang, H. Org. Lett. 2012, 14, 3116; (m) Zhang, Z.; Antilla, J. C. Angew. Chem., Int. Ed. 2012, 51, 11778; (n) Ozawa, T.; Kanematsu, M.; Yokoe, H.; Yoshida, M.; Shishido, K. J. Org. Chem. 2012, 77, 9240. For a review of vinyl isocyanate chemistry, see: Rigby, J. H. Synlett 2000. For an overview of nucleophilic carbene chemistry, see: Warkentin, J. In Advances in Carbene Chemistry; Brinker, U. H., Ed.; ; JAI: Greenwich, 1998; Vol. 2, p 245. (a) Rigby, J. H.; Cavezza, A.; Ahmed, G. J. Am. Chem. Soc. 1996, 118, 12848; (b) Rigby, J. H.; Cavezza, A.; Heeg, M. J. J. Am. Chem. Soc. 1998, 120, 3664; (c) Rigby, J. H.; Laurent, S.; Cavezza, A.; Heeg, M. J. J. Org. Chem. 1998, 63, 5587; (d) Rigby, J. H.; Cavezza, A.; Heeg, M. J. Tetrahedron Lett. 1999, 40, 2473. (a) Rigby, J. H.; Danca, M. D. Tetrahedron Lett. 1999, 40, 6891; (b) Rigby, J. H.; Laurent, S. J. Org. Chem. 1999, 64, 1766; (c) Rigby, J. H.; Laurent, S.; Dong, W.; Danca, M. D. Tetrahedron 2000, 56, 10101; (d) Rigby, J. H.; Dong, W. Org. Lett. 2000, 2, 1673. Rigby, J. H.; Burke, P. J. Heterocycles 2006, 67, 643. Rigby, J. H.; Sidique, S. Org. Lett. 2007, 9, 1219. (a) Williams, R. M.; Lee, B. H. J. Am. Chem. Soc. 1986, 108, 6431; (b) Yokoyama, Y.; Ito, S.; Takahashi, Y.; Murakami, Y. Tetrahedron Lett. 1985, 26, 6457. (a) Heath-Brown, B.; Philpott, P. G. J. Chem. Soc. 1965, 7185; (b) Zhao, S.; Liao, X.; Wang, T.; Flippen-Anderson, J.; Cook, J. M. J. Org. Chem. 2003, 68, 6279.
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters journal homepage: www.elsevier.com/locate/tetlet
Total synthesis of (±)-debromoflustramine B via [4+1] cyclization of a bis(alkylthio)carbene and an indole isocyanate Saptarshi De ⇑, James H. Rigby Department of Chemistry, Wayne State University, Detroit, MI 48202, USA
a r t i c l e
i n f o
Article history: Received 3 June 2013 Revised 16 June 2013 Accepted 22 June 2013 Available online 30 June 2013
a b s t r a c t An efficient total synthesis of (±)-debromoflustramine B and (±)-debromoflustramide B has been achieved, through a novel [4+1] cyclization of an indole isocyanate and bis(alkylthio)carbene. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Japp–Klingermann Fisher indole synthesis Curtius rearrangement Bis(alkylthio)carbene [4+1] Cyclization
Alkaloids with the hexahydropyrrole[2.3-b]indole (HPI) core structure are abundant in nature and are known for a broad range of biological activities.1 Debromoflustramine B (1), isolated from the marine bryozoan Flustra foliacea, is one of the prominent members of the HPI family. It contains prenyl groups at C-3a and N-8 positions (Fig. 1).2 A recent study indicates debromoflustramine B (1) and its analogues are potent selective butyrylcholinesterase (BChE) inhibitors and hence promising therapeutic agents for Alzheimer’s disease.3 The major challenges that the construction of these natural products present are the sterically congested quaternary stereocenter at C-3a and the assembly of the pyrroloindoline core structure. The synthetically challenging structure along with interesting biological activity have prompted several total syntheses of 1.4 Herein is reported an efficient total syntheses of (±)-debromoflustramine B (1) and (±)-debromoflustramide B (2). The [4+1] cyclization of vinyl isocyanates5 with nucleophilic carbenes6 is of considerable interest because it affords a rapid entry into functionalized pyrrolidinone rings.7 These transformations are also well-suited to the challenges of preparing the quaternary carbon center present in the target molecule.7b,8d,10 While both bisalkoxy and bis(alkylthio)carbenes are suitable for ring formation, the latter offers more attractive opportunities for post-cyclization manipulation in the current context.8–10 The retro-synthetic analysis is outlined in Scheme 1. Based on previous observations,10 it was envisioned that 1 and 2 could be accessed from dithio-substituted intermediate 7, through simple functional group manipulation. The tricyclic intermediate 7 was ⇑ Corresponding author. Tel.: +1 903 217 8638. E-mail address: [email protected] (S. De). 0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.tetlet.2013.06.119
3a N
N H Me
X
X = H, H Debromoflustramine B (1) X = O, Debromoflustramide B (2) Figure 1. Structures of debromoflustramine B (1) and debromoflustramide B (2).
predicted to arise via the key [4+1] cyclization of indole isocyanate 5 and bis(propylthio)-carbene precursor 6. The isocyanate 5 would be readily available from the indole derivative 4, which in turn can be obtained using a Japp–Klingemann modified Fischer indole synthesis starting from aniline and ethyl 2-acetylhex-5-enoate (3).11a The synthesis commenced with the preparation of indole 4, which was previously synthesized by Murakami and co-workers using a Stille type coupling reaction.11b An inability to perform this reaction on larger scale led us to investigate an alternate pathway. Exploiting the Japp–Klingemann–Fischer indole synthesis method,10,12 4 could be obtained from aniline and the known compound 3, in satisfactory yield (Scheme 2). The requisite N-prenyl group was then installed by treating 4 with prenyl bromide in the presence of NaH. The terminal alkene group of 9 was successfully converted to the second required prenyl group through a cross metathesis reaction employing 2-methyl-2-butene in excellent yield.4e,j The resulting ester 10 was then hydrolyzed to the carboxylic acid 11.
4761
S. De, J. H. Rigby / Tetrahedron Letters 54 (2013) 4760–4762
SPr
1 and 2
SPr
SPr N
N H Me
SPr N
O
N
O
8
+ N H
CO2H
N
CON3
7
[4+1] cyclization
NCO
N
CO2Et
12
11
SPr SPr
SPr SPr
N
O
N
6, PhH, reflux
Et3N, DPPA
N
N
N
SPr SPr
LiAlH4, THF reflux
N
O
N H R
O
6
4
7
5
13, R = H
NaH, MeI THF
8, R = Me O
NH2
O O
SPr SPr O
N N
+ 3
4 steps, only one column 52% from 11
6
heat Scheme 1. Retrosynthetic analysis for 1 and 2.
SPr SPr SPr
SPr NH2
N
NaNO2, aq HCl, 0 oC KOH, 3
N C O
N
N
7
O
then EtOH/HCl, reflux N H
CO2Et
5
4 58% (> 5 gm scale)
Br
Scheme 3. Synthesis of the tricyclic core structure 8 and proposed mechanism of the [4+1] cyclization.
Grubbs 2nd generation catalyst (5 mol%)
NaH, DMF 95%
N
CO2Et
DCM, reflux 93%
N
SPr
CO2R
SPr N
N H
9
10, R = Et 11, R = H
LiOH, EtOH/H2O Reflux, 86%
O
Raney Ni EtOH, rt 83%
N
N
X
H
8 X = O, debromoflustramide B (2)
Scheme 2. Synthesis of carboxylic acid 11.
LiAlH4 THF, reflux quantitative
X = H, H debromoflustramine B (1)
With important intermediate 11 in hand, attention turned toward the synthesis of the pyrrolidinone core structure of the natural product. Carboxylic acid 11 was converted into the acyl azide 12 by treatment with diphenyl phosphoryl azide (DPPA) in the presence of triethylamine (Et3N). The resultant acyl azide 12 was then refluxed in dry benzene to generate the required isocyanate 5 in situ via Curtius rearrangement (Scheme 3). Excess dithiooxadiazoline 6 (bis(propylthio)carbene precursor) was then added portion wise to the reaction mixture while still at reflux to form the desired adduct. The resultant crude imine tricycle 7 was immediately reduced with LiAlH4 to intermediate 13. Labile intermediate 13 was immediately N-methylated by treatment with methyl iodide in the presence of NaH to give 8 in 52% yield over four steps starting from carboxylic acid 11. It is noteworthy that the reaction proceeded efficiently to afford the sterically congested C-3a quaternary center.9,10 Only a single column purification was required for the entire sequence from 11 to 8.
Scheme 4. Total synthesis of (±)-debromoflustramine B (1) and (±)-debromoflustramide B (2).
After successfully constructing the tricyclic core of the target molecule, attention was turned to the synthetic end game (Scheme 4). The thiopropyl groups in intermediate 8 underwent smooth reductive desulfurization with Raney-Ni to provide the required methylene group in (±)-debromoflustramide B (2) and, finally the lactam 2 was reduced to afford the natural product (±)-debromoflustramine B (1) by refluxing with LiAlH4. The synthesis was achieved in 10 linear steps from aniline. The spectral data of 1 matched with those reported in the literature.4j–l In conclusion, the total syntheses of (±)-debromoflustramide B (2) and (±)-debromoflustramine B (1) have been achieved without employing any nitrogen protecting group, compared to the other reported syntheses. The key steps were the Japp–Klingermann
4762
S. De, J. H. Rigby / Tetrahedron Letters 54 (2013) 4760–4762
Fisher indole synthesis to form indole carboxylate 4 and a [4+1] cyclization of the corresponding indole isocyanate and bis(propylthio)carbene to access the tricyclic core containing the critical sterically crowded C-3a all carbon quaternary stereocenter. Acknowledgment The authors wish to thank the Wayne State University for support of the research. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.tetlet.2013.06. 119.
5. 6.
7.
References and notes 8. 1. Ruiz-Sanchis, P.; Savina, S. A.; Albericio, F.; A´lvarez, M. Chem. Eur. J. 2011, 17, 1388. 2. (a) Christophersen, C. Acta Chem. Scand. 1985, B39, 517; (b) Holst, P. B.; Anthoni, U.; Christophersen, C.; Nielsen, P. H. J. Nat. Prod. 1994, 57, 997. 3. (a) Rivera-Becerril, E.; Joseph-Nathan, P.; Pe´rez-A´lvarez, V. M.; Morales-Ríos, M. S. J. Med. Chem. 2008, 51, 5271; (b) Mitchell, M. O.; Figliozzi, R. W.; Guzel, M. Med. Chem. 2010, 6, 141. 4. For some notable synthesis of debromoflustramine B (a) Bruncko, M.; Crich, D.; Samy, R. J. Org. Chem. 1994, 59, 5543; (b) Morales-Ríos, M. S.; Suárez-Castillo, O.
9. 10. 11. 12.
R.; Joseph-Nathan, P. J. Org. Chem. 1999, 64, 1086; (c) Cardoso, A. S.; Srinivasan, N.; Lobo, A. M.; Prabhakar, S. Tetrahedron Lett. 2001, 42, 6663; (d) Tan, G. H.; Zhu, X.; Ganesan, A. Org. Lett. 1801, 2003, 5; (e) Austin, J. F.; Kim, S. G.; Sinz, C. J.; Xiao, W. J.; MacMillan, D. W. C. Proc. Natl. Acad. Sci. U.S.A. 2004, 101, 5482; (f) Morales-Ríos, M. S.; Rivera-Becerril, E.; Joseph-Nathan, P. Tetrahedron: Asymmetry 2005, 16, 2493; (g) López-Alvarado, P.; Caballero, E.; Avendaño, C.; Menéndez, J. C. Org. Lett. 2006, 8, 4303; (h) Miyamoto, H.; Okawa, Y.; Nakazaki, A.; Kobayashi, S. Tetrahedron Lett. 2007, 48, 1805; (i) Cardoso, A. S. P.; Marques, M. M. B.; Srinivasan, N.; Prabhakar, S.; Lobo, A. M. Tetrahedron 2007, 63, 10211; (j) Schammel, A. W.; Boal, B. W.; Zu, L.; Mesganaw, T.; Garg, N. K. Tetrahedron 2010, 66, 4687; (k) Trost, B. M.; Zhang, Y. Chem. Eur. J. 2011, 17, 2916; (l) Zhou, Y.; Xi, Y.; Zhao, J.; Sheng, X.; Zhang, S.; Zhang, H. Org. Lett. 2012, 14, 3116; (m) Zhang, Z.; Antilla, J. C. Angew. Chem., Int. Ed. 2012, 51, 11778; (n) Ozawa, T.; Kanematsu, M.; Yokoe, H.; Yoshida, M.; Shishido, K. J. Org. Chem. 2012, 77, 9240. For a review of vinyl isocyanate chemistry, see: Rigby, J. H. Synlett 2000. For an overview of nucleophilic carbene chemistry, see: Warkentin, J. In Advances in Carbene Chemistry; Brinker, U. H., Ed.; ; JAI: Greenwich, 1998; Vol. 2, p 245. (a) Rigby, J. H.; Cavezza, A.; Ahmed, G. J. Am. Chem. Soc. 1996, 118, 12848; (b) Rigby, J. H.; Cavezza, A.; Heeg, M. J. J. Am. Chem. Soc. 1998, 120, 3664; (c) Rigby, J. H.; Laurent, S.; Cavezza, A.; Heeg, M. J. J. Org. Chem. 1998, 63, 5587; (d) Rigby, J. H.; Cavezza, A.; Heeg, M. J. Tetrahedron Lett. 1999, 40, 2473. (a) Rigby, J. H.; Danca, M. D. Tetrahedron Lett. 1999, 40, 6891; (b) Rigby, J. H.; Laurent, S. J. Org. Chem. 1999, 64, 1766; (c) Rigby, J. H.; Laurent, S.; Dong, W.; Danca, M. D. Tetrahedron 2000, 56, 10101; (d) Rigby, J. H.; Dong, W. Org. Lett. 2000, 2, 1673. Rigby, J. H.; Burke, P. J. Heterocycles 2006, 67, 643. Rigby, J. H.; Sidique, S. Org. Lett. 2007, 9, 1219. (a) Williams, R. M.; Lee, B. H. J. Am. Chem. Soc. 1986, 108, 6431; (b) Yokoyama, Y.; Ito, S.; Takahashi, Y.; Murakami, Y. Tetrahedron Lett. 1985, 26, 6457. (a) Heath-Brown, B.; Philpott, P. G. J. Chem. Soc. 1965, 7185; (b) Zhao, S.; Liao, X.; Wang, T.; Flippen-Anderson, J.; Cook, J. M. J. Org. Chem. 2003, 68, 6279.