Tandem radical addition and cyclization of ε-substituted δ-Yne ketimines

Organic Letters

Volumn 7, Issue 1, 2005, Pages 11-14

  Fernández, Marta ^a   Alonso, Ricardo ^a  

^a UNIVERSITY OF SANTIAGO DE COMPOSTELA   (Spain)

Author keywords

[No Author keywords available]

Indexed keywords

ALLYLAMINE DERIVATIVE; IMINE;

ARTICLE; CYCLIZATION;

EID: 12344298742     PISSN: 15237060     EISSN: None     Source Type: Journal    
DOI: 10.1021/ol048104s     Document Type: Article

References (35)

1. Reviews: (a) Free Radical Cyclizations Involving Nitrogen. Fallis, A. G.; Brinza, I. M. Tetrahedron 1997, 53, 17543; includes a compilation of cyclization and ring-opening rate constants for some representative cases. (b) Martínez-Grau, A.; Marco-Contelles, J. Chem. Soc. Rev. 1998, 27, 155. (c) Friestad, G. K. Tetrahedron 2001, 57, 5461. (d) Kim, S.; Joon, J.-Y. Novel Radical Traps. In Radicals in Organic Synthesis, 1st ed.; Renaud, P., Sibi, M. P., Eds.; Wiley-VCH: Weinheim; New York, 2001; Vol. 2, Chapter 1. (e) Bertrand, M.; Feray, L.; Gastaldi, S. C. R. Chimie 2002, 5, 623. (f) Miyabe, H.; Ueda, M.; Naito, T. Synlett 2004, 7, 1140.
2. Reviews: (a) Free Radical Cyclizations Involving Nitrogen. Fallis, A. G.; Brinza, I. M. Tetrahedron 1997, 53, 17543; includes a compilation of cyclization and ring-opening rate constants for some representative cases. (b) Martínez-Grau, A.; Marco-Contelles, J. Chem. Soc. Rev. 1998, 27, 155. (c) Friestad, G. K. Tetrahedron 2001, 57, 5461. (d) Kim, S.; Joon, J.-Y. Novel Radical Traps. In Radicals in Organic Synthesis, 1st ed.; Renaud, P., Sibi, M. P., Eds.; Wiley-VCH: Weinheim; New York, 2001; Vol. 2, Chapter 1. (e) Bertrand, M.; Feray, L.; Gastaldi, S. C. R. Chimie 2002, 5, 623. (f) Miyabe, H.; Ueda, M.; Naito, T. Synlett 2004, 7, 1140.
3. Reviews: (a) Free Radical Cyclizations Involving Nitrogen. Fallis, A. G.; Brinza, I. M. Tetrahedron 1997, 53, 17543; includes a compilation of cyclization and ring-opening rate constants for some representative cases. (b) Martínez-Grau, A.; Marco-Contelles, J. Chem. Soc. Rev. 1998, 27, 155. (c) Friestad, G. K. Tetrahedron 2001, 57, 5461. (d) Kim, S.; Joon, J.-Y. Novel Radical Traps. In Radicals in Organic Synthesis, 1st ed.; Renaud, P., Sibi, M. P., Eds.; Wiley-VCH: Weinheim; New York, 2001; Vol. 2, Chapter 1. (e) Bertrand, M.; Feray, L.; Gastaldi, S. C. R. Chimie 2002, 5, 623. (f) Miyabe, H.; Ueda, M.; Naito, T. Synlett 2004, 7, 1140.
4. Reviews: (a) Free Radical Cyclizations Involving Nitrogen. Fallis, A. G.; Brinza, I. M. Tetrahedron 1997, 53, 17543; includes a compilation of cyclization and ring-opening rate constants for some representative cases. (b) Martínez-Grau, A.; Marco-Contelles, J. Chem. Soc. Rev. 1998, 27, 155. (c) Friestad, G. K. Tetrahedron 2001, 57, 5461. (d) Kim, S.; Joon, J.-Y. Novel Radical Traps. In Radicals in Organic Synthesis, 1st ed.; Renaud, P., Sibi, M. P., Eds.; Wiley-VCH: Weinheim; New York, 2001; Vol. 2, Chapter 1. (e) Bertrand, M.; Feray, L.; Gastaldi, S. C. R. Chimie 2002, 5, 623. (f) Miyabe, H.; Ueda, M.; Naito, T. Synlett 2004, 7, 1140.
5. Reviews: (a) Free Radical Cyclizations Involving Nitrogen. Fallis, A. G.; Brinza, I. M. Tetrahedron 1997, 53, 17543; includes a compilation of cyclization and ring-opening rate constants for some representative cases. (b) Martínez-Grau, A.; Marco-Contelles, J. Chem. Soc. Rev. 1998, 27, 155. (c) Friestad, G. K. Tetrahedron 2001, 57, 5461. (d) Kim, S.; Joon, J.-Y. Novel Radical Traps. In Radicals in Organic Synthesis, 1st ed.; Renaud, P., Sibi, M. P., Eds.; Wiley-VCH: Weinheim; New York, 2001; Vol. 2, Chapter 1. (e) Bertrand, M.; Feray, L.; Gastaldi, S. C. R. Chimie 2002, 5, 623. (f) Miyabe, H.; Ueda, M.; Naito, T. Synlett 2004, 7, 1140.
6. Reviews: (a) Free Radical Cyclizations Involving Nitrogen. Fallis, A. G.; Brinza, I. M. Tetrahedron 1997, 53, 17543; includes a compilation of cyclization and ring-opening rate constants for some representative cases. (b) Martínez-Grau, A.; Marco-Contelles, J. Chem. Soc. Rev. 1998, 27, 155. (c) Friestad, G. K. Tetrahedron 2001, 57, 5461. (d) Kim, S.; Joon, J.-Y. Novel Radical Traps. In Radicals in Organic Synthesis, 1st ed.; Renaud, P., Sibi, M. P., Eds.; Wiley-VCH: Weinheim; New York, 2001; Vol. 2, Chapter 1. (e) Bertrand, M.; Feray, L.; Gastaldi, S. C. R. Chimie 2002, 5, 623. (f) Miyabe, H.; Ueda, M.; Naito, T. Synlett 2004, 7, 1140.
7. Selected recent examples of the radical addition onto imine derivatives of aldehydes include the following. (i) Intramolecular: (a) Friestad, G. K.; Massari, S. E. J. Org. Chem. 2004, 69, 863. (ii) Intermolecular: (b) Halland, N.; Jorgensen, K. A. J. Chem. Soc., Perkin Trans. 1 2001, 11, 1290. (c) Friestad, G. K.; Shen, Y.; Ruggles, E. L. Angew. Chem. Int. Ed. 2003, 42, 5061. (d) Fernandez, M.; Alonso, R. Org. Lett. 2003, 5, 2461. (e) Singh, N.; Anand, R. D.; Trehan, S. Tetrahedron Lett. 2004, 45, 2911. (f) Yamada, K.; Yamamoto, Y.; Maekawa, M.; Tomioka, K. J. Org. Chem. 2004, 69, 1531.
8. Selected recent examples of the radical addition onto imine derivatives of aldehydes include the following. (i) Intramolecular: (a) Friestad, G. K.; Massari, S. E. J. Org. Chem. 2004, 69, 863. (ii) Intermolecular: (b) Halland, N.; Jorgensen, K. A. J. Chem. Soc., Perkin Trans. 1 2001, 11, 1290. (c) Friestad, G. K.; Shen, Y.; Ruggles, E. L. Angew. Chem. Int. Ed. 2003, 42, 5061. (d) Fernandez, M.; Alonso, R. Org. Lett. 2003, 5, 2461. (e) Singh, N.; Anand, R. D.; Trehan, S. Tetrahedron Lett. 2004, 45, 2911. (f) Yamada, K.; Yamamoto, Y.; Maekawa, M.; Tomioka, K. J. Org. Chem. 2004, 69, 1531.
9. Selected recent examples of the radical addition onto imine derivatives of aldehydes include the following. (i) Intramolecular: (a) Friestad, G. K.; Massari, S. E. J. Org. Chem. 2004, 69, 863. (ii) Intermolecular: (b) Halland, N.; Jorgensen, K. A. J. Chem. Soc., Perkin Trans. 1 2001, 11, 1290. (c) Friestad, G. K.; Shen, Y.; Ruggles, E. L. Angew. Chem. Int. Ed. 2003, 42, 5061. (d) Fernandez, M.; Alonso, R. Org. Lett. 2003, 5, 2461. (e) Singh, N.; Anand, R. D.; Trehan, S. Tetrahedron Lett. 2004, 45, 2911. (f) Yamada, K.; Yamamoto, Y.; Maekawa, M.; Tomioka, K. J. Org. Chem. 2004, 69, 1531.
10. Selected recent examples of the radical addition onto imine derivatives of aldehydes include the following. (i) Intramolecular: (a) Friestad, G. K.; Massari, S. E. J. Org. Chem. 2004, 69, 863. (ii) Intermolecular: (b) Halland, N.; Jorgensen, K. A. J. Chem. Soc., Perkin Trans. 1 2001, 11, 1290. (c) Friestad, G. K.; Shen, Y.; Ruggles, E. L. Angew. Chem. Int. Ed. 2003, 42, 5061. (d) Fernandez, M.; Alonso, R. Org. Lett. 2003, 5, 2461. (e) Singh, N.; Anand, R. D.; Trehan, S. Tetrahedron Lett. 2004, 45, 2911. (f) Yamada, K.; Yamamoto, Y.; Maekawa, M.; Tomioka, K. J. Org. Chem. 2004, 69, 1531.
11. Selected recent examples of the radical addition onto imine derivatives of aldehydes include the following. (i) Intramolecular: (a) Friestad, G. K.; Massari, S. E. J. Org. Chem. 2004, 69, 863. (ii) Intermolecular: (b) Halland, N.; Jorgensen, K. A. J. Chem. Soc., Perkin Trans. 1 2001, 11, 1290. (c) Friestad, G. K.; Shen, Y.; Ruggles, E. L. Angew. Chem. Int. Ed. 2003, 42, 5061. (d) Fernandez, M.; Alonso, R. Org. Lett. 2003, 5, 2461. (e) Singh, N.; Anand, R. D.; Trehan, S. Tetrahedron Lett. 2004, 45, 2911. (f) Yamada, K.; Yamamoto, Y.; Maekawa, M.; Tomioka, K. J. Org. Chem. 2004, 69, 1531.
12. Selected recent examples of the radical addition onto imine derivatives of aldehydes include the following. (i) Intramolecular: (a) Friestad, G. K.; Massari, S. E. J. Org. Chem. 2004, 69, 863. (ii) Intermolecular: (b) Halland, N.; Jorgensen, K. A. J. Chem. Soc., Perkin Trans. 1 2001, 11, 1290. (c) Friestad, G. K.; Shen, Y.; Ruggles, E. L. Angew. Chem. Int. Ed. 2003, 42, 5061. (d) Fernandez, M.; Alonso, R. Org. Lett. 2003, 5, 2461. (e) Singh, N.; Anand, R. D.; Trehan, S. Tetrahedron Lett. 2004, 45, 2911. (f) Yamada, K.; Yamamoto, Y.; Maekawa, M.; Tomioka, K. J. Org. Chem. 2004, 69, 1531.
13. Although most examples apply to the addition to the C atom of the C=N bond, intramolecular carbon-radical addition to the nitrogen of an azomethine is also known and synthetically useful; see: (a) Viswanathan, R.; Prabhakaran, E. N.; Plotkin, M. A.; Johnston, J. N. J. Am. Chem. Soc. 2003, 125, 163 and references therein.
14. Although more difficult and by far less developed than for aldimines. the radical addition onto ketimine derivatives is also useful: examples include the following. (i) Intramolecular: (a) Noya, B.; Alonso, R. Tetrahedron Lett. 1997, 38, 2745. (b) Noya, B.; Paredes, M. D.; Ozores, L.; Alonso, R. J. Org. Chem. 2000, 65, 5960 and references therein. (ii) Intermolecular: (c) Torrente, S.; Alonso, R. Org. Lett. 2001, 3, 1985 and references therein.
15. Although more difficult and by far less developed than for aldimines. the radical addition onto ketimine derivatives is also useful: examples include the following. (i) Intramolecular: (a) Noya, B.; Alonso, R. Tetrahedron Lett. 1997, 38, 2745. (b) Noya, B.; Paredes, M. D.; Ozores, L.; Alonso, R. J. Org. Chem. 2000, 65, 5960 and references therein. (ii) Intermolecular: (c) Torrente, S.; Alonso, R. Org. Lett. 2001, 3, 1985 and references therein.
16. Although more difficult and by far less developed than for aldimines. the radical addition onto ketimine derivatives is also useful: examples include the following. (i) Intramolecular: (a) Noya, B.; Alonso, R. Tetrahedron Lett. 1997, 38, 2745. (b) Noya, B.; Paredes, M. D.; Ozores, L.; Alonso, R. J. Org. Chem. 2000, 65, 5960 and references therein. (ii) Intermolecular: (c) Torrente, S.; Alonso, R. Org. Lett. 2001, 3, 1985 and references therein.
17. For a review on Heteroatom Radical Addition-Cyclization and its Synthetic Application, see: Naito, T. Heterocycles 1999, 50, 505. For a recent example involving a C=N radical-cyclization step, see: Friestad, G. K.; Jiang, T.; Fioroni, G. M. Tetrahedron: Asymmetry 2003, 14, 2853.
18. For a review on Heteroatom Radical Addition-Cyclization and its Synthetic Application, see: Naito, T. Heterocycles 1999, 50, 505. For a recent example involving a C=N radical-cyclization step, see: Friestad, G. K.; Jiang, T.; Fioroni, G. M. Tetrahedron: Asymmetry 2003, 14, 2853.
19. 13C NMR, DEPT, and MS).
20. For synthetic studies on tetrodotoxin, see: (a) Torrente, S.; Noya, B.; Branchadell, V.; Alonso, R. J. Org. Chem. 2003, 68, 4772. (b) Ohyabu, N.; Nishikawa, T.; Isobe, M. J. Am. Chem. Soc. 2003, 125, 8798. (c) Hinman, A.; Du Bois, J. J. Am. Chem. Soc. 2003, 125, 11510. (d) Nishikawa, T.; Urabe, D.; Isobe, M. Angew. Chem., Int. Ed. 2004, 43, 4782 and references therein.
21. For synthetic studies on tetrodotoxin, see: (a) Torrente, S.; Noya, B.; Branchadell, V.; Alonso, R. J. Org. Chem. 2003, 68, 4772. (b) Ohyabu, N.; Nishikawa, T.; Isobe, M. J. Am. Chem. Soc. 2003, 125, 8798. (c) Hinman, A.; Du Bois, J. J. Am. Chem. Soc. 2003, 125, 11510. (d) Nishikawa, T.; Urabe, D.; Isobe, M. Angew. Chem., Int. Ed. 2004, 43, 4782 and references therein.
22. For synthetic studies on tetrodotoxin, see: (a) Torrente, S.; Noya, B.; Branchadell, V.; Alonso, R. J. Org. Chem. 2003, 68, 4772. (b) Ohyabu, N.; Nishikawa, T.; Isobe, M. J. Am. Chem. Soc. 2003, 125, 8798. (c) Hinman, A.; Du Bois, J. J. Am. Chem. Soc. 2003, 125, 11510. (d) Nishikawa, T.; Urabe, D.; Isobe, M. Angew. Chem., Int. Ed. 2004, 43, 4782 and references therein.
23. For synthetic studies on tetrodotoxin, see: (a) Torrente, S.; Noya, B.; Branchadell, V.; Alonso, R. J. Org. Chem. 2003, 68, 4772. (b) Ohyabu, N.; Nishikawa, T.; Isobe, M. J. Am. Chem. Soc. 2003, 125, 8798. (c) Hinman, A.; Du Bois, J. J. Am. Chem. Soc. 2003, 125, 11510. (d) Nishikawa, T.; Urabe, D.; Isobe, M. Angew. Chem., Int. Ed. 2004, 43, 4782 and references therein.
24. Other targets, besides tetrodotoxin (ref 7), having nitrogenated quaternary centers include the following. (-)-Huperzine A: (a) Yamada, F.; Kozikowski, A. P.; Reddy, E. R.; Pang, Y.; Miller, J. H.; McKinney, M. J. Am. Chem. Soc. 1991, 113, 4695. (-)-Cephalotaxine: (b) Isono, N.; Mori, M. J. Org. Chem. 1995, 60, 115. Immunosuppressant FR901483; (c) Snider, B. B.; Lin, H.; Foxman, B. M. J. Org. Chem. 1998, 63, 6442. (+)-Harichlorine: (d) Trauner, D.; Schwarz, J. B. Danishefsky, S. J. Angew. Chem., Int. Ed. 1999, 38, 3542. (e) Wright, D. L.; Schulte J. P., II; Page, M. A. Org. Lett. 2000, 13, 1847. Lactacystin: (i) Donohoe, T. J.; Sintim, H. O.; Sisangia, L.; Harling, J. D. Angew. Chem., Int. Ed. 2004, 43, 2293. Myriocin: (j) Lee, K.-Y.; Oh, C.-Y.; Kim, Y.-H.; Joo, J.-E.; Ham, W.-H. Tetrahedron Lett. 2002, 43, 9361.
25. Other targets, besides tetrodotoxin (ref 7), having nitrogenated quaternary centers include the following. (-)-Huperzine A: (a) Yamada, F.; Kozikowski, A. P.; Reddy, E. R.; Pang, Y.; Miller, J. H.; McKinney, M. J. Am. Chem. Soc. 1991, 113, 4695. (-)-Cephalotaxine: (b) Isono, N.; Mori, M. J. Org. Chem. 1995, 60, 115. Immunosuppressant FR901483; (c) Snider, B. B.; Lin, H.; Foxman, B. M. J. Org. Chem. 1998, 63, 6442. (+)-Harichlorine: (d) Trauner, D.; Schwarz, J. B. Danishefsky, S. J. Angew. Chem., Int. Ed. 1999, 38, 3542. (e) Wright, D. L.; Schulte J. P., II; Page, M. A. Org. Lett. 2000, 13, 1847. Lactacystin: (i) Donohoe, T. J.; Sintim, H. O.; Sisangia, L.; Harling, J. D. Angew. Chem., Int. Ed. 2004, 43, 2293. Myriocin: (j) Lee, K.-Y.; Oh, C.-Y.; Kim, Y.-H.; Joo, J.-E.; Ham, W.-H. Tetrahedron Lett. 2002, 43, 9361.
26. Other targets, besides tetrodotoxin (ref 7), having nitrogenated quaternary centers include the following. (-)-Huperzine A: (a) Yamada, F.; Kozikowski, A. P.; Reddy, E. R.; Pang, Y.; Miller, J. H.; McKinney, M. J. Am. Chem. Soc. 1991, 113, 4695. (-)-Cephalotaxine: (b) Isono, N.; Mori, M. J. Org. Chem. 1995, 60, 115. Immunosuppressant FR901483; (c) Snider, B. B.; Lin, H.; Foxman, B. M. J. Org. Chem. 1998, 63, 6442. (+)-Harichlorine: (d) Trauner, D.; Schwarz, J. B. Danishefsky, S. J. Angew. Chem., Int. Ed. 1999, 38, 3542. (e) Wright, D. L.; Schulte J. P., II; Page, M. A. Org. Lett. 2000, 13, 1847. Lactacystin: (i) Donohoe, T. J.; Sintim, H. O.; Sisangia, L.; Harling, J. D. Angew. Chem., Int. Ed. 2004, 43, 2293. Myriocin: (j) Lee, K.-Y.; Oh, C.-Y.; Kim, Y.-H.; Joo, J.-E.; Ham, W.-H. Tetrahedron Lett. 2002, 43, 9361.
27. Other targets, besides tetrodotoxin (ref 7), having nitrogenated quaternary centers include the following. (-)-Huperzine A: (a) Yamada, F.; Kozikowski, A. P.; Reddy, E. R.; Pang, Y.; Miller, J. H.; McKinney, M. J. Am. Chem. Soc. 1991, 113, 4695. (-)-Cephalotaxine: (b) Isono, N.; Mori, M. J. Org. Chem. 1995, 60, 115. Immunosuppressant FR901483; (c) Snider, B. B.; Lin, H.; Foxman, B. M. J. Org. Chem. 1998, 63, 6442. (+)-Harichlorine: (d) Trauner, D.; Schwarz, J. B. Danishefsky, S. J. Angew. Chem., Int. Ed. 1999, 38, 3542. (e) Wright, D. L.; Schulte J. P., II; Page, M. A. Org. Lett. 2000, 13, 1847. Lactacystin: (i) Donohoe, T. J.; Sintim, H. O.; Sisangia, L.; Harling, J. D. Angew. Chem., Int. Ed. 2004, 43, 2293. Myriocin: (j) Lee, K.-Y.; Oh, C.-Y.; Kim, Y.-H.; Joo, J.-E.; Ham, W.-H. Tetrahedron Lett. 2002, 43, 9361.
28. Other targets, besides tetrodotoxin (ref 7), having nitrogenated quaternary centers include the following. (-)-Huperzine A: (a) Yamada, F.; Kozikowski, A. P.; Reddy, E. R.; Pang, Y.; Miller, J. H.; McKinney, M. J. Am. Chem. Soc. 1991, 113, 4695. (-)-Cephalotaxine: (b) Isono, N.; Mori, M. J. Org. Chem. 1995, 60, 115. Immunosuppressant FR901483; (c) Snider, B. B.; Lin, H.; Foxman, B. M. J. Org. Chem. 1998, 63, 6442. (+)-Harichlorine: (d) Trauner, D.; Schwarz, J. B. Danishefsky, S. J. Angew. Chem., Int. Ed. 1999, 38, 3542. (e) Wright, D. L.; Schulte J. P., II; Page, M. A. Org. Lett. 2000, 13, 1847. Lactacystin: (i) Donohoe, T. J.; Sintim, H. O.; Sisangia, L.; Harling, J. D. Angew. Chem., Int. Ed. 2004, 43, 2293. Myriocin: (j) Lee, K.-Y.; Oh, C.-Y.; Kim, Y.-H.; Joo, J.-E.; Ham, W.-H. Tetrahedron Lett. 2002, 43, 9361.
29. Other targets, besides tetrodotoxin (ref 7), having nitrogenated quaternary centers include the following. (-)-Huperzine A: (a) Yamada, F.; Kozikowski, A. P.; Reddy, E. R.; Pang, Y.; Miller, J. H.; McKinney, M. J. Am. Chem. Soc. 1991, 113, 4695. (-)-Cephalotaxine: (b) Isono, N.; Mori, M. J. Org. Chem. 1995, 60, 115. Immunosuppressant FR901483; (c) Snider, B. B.; Lin, H.; Foxman, B. M. J. Org. Chem. 1998, 63, 6442. (+)-Harichlorine: (d) Trauner, D.; Schwarz, J. B. Danishefsky, S. J. Angew. Chem., Int. Ed. 1999, 38, 3542. (e) Wright, D. L.; Schulte J. P., II; Page, M. A. Org. Lett. 2000, 13, 1847. Lactacystin: (i) Donohoe, T. J.; Sintim, H. O.; Sisangia, L.; Harling, J. D. Angew. Chem., Int. Ed. 2004, 43, 2293. Myriocin: (j) Lee, K.-Y.; Oh, C.-Y.; Kim, Y.-H.; Joo, J.-E.; Ham, W.-H. Tetrahedron Lett. 2002, 43, 9361.
30. Other targets, besides tetrodotoxin (ref 7), having nitrogenated quaternary centers include the following. (-)-Huperzine A: (a) Yamada, F.; Kozikowski, A. P.; Reddy, E. R.; Pang, Y.; Miller, J. H.; McKinney, M. J. Am. Chem. Soc. 1991, 113, 4695. (-)-Cephalotaxine: (b) Isono, N.; Mori, M. J. Org. Chem. 1995, 60, 115. Immunosuppressant FR901483; (c) Snider, B. B.; Lin, H.; Foxman, B. M. J. Org. Chem. 1998, 63, 6442. (+)-Harichlorine: (d) Trauner, D.; Schwarz, J. B. Danishefsky, S. J. Angew. Chem., Int. Ed. 1999, 38, 3542. (e) Wright, D. L.; Schulte J. P., II; Page, M. A. Org. Lett. 2000, 13, 1847. Lactacystin: (i) Donohoe, T. J.; Sintim, H. O.; Sisangia, L.; Harling, J. D. Angew. Chem., Int. Ed. 2004, 43, 2293. Myriocin: (j) Lee, K.-Y.; Oh, C.-Y.; Kim, Y.-H.; Joo, J.-E.; Ham, W.-H. Tetrahedron Lett. 2002, 43, 9361.
31. The identity of the orange solid formed when Ib is irradiated with a 450-W medium-pressure Hg Hanovia UV lamp in the presence of PhSH remains undetermined.
32. Although the final yield of IIb is greater than that of IIIb in entries 5 and 6 of Table 1, we believe that IIIb is still the major cyclization product but, unlike IIb, is converted into the above-mentioned orange solid under the reaction conditions of entries 5 and 6 (450-W UV lamp). That lib is stable under these conditions is suggested by its corrected yield (24% or 21%) being very similar to those obtained under the conditions of entries 3 and 4.
33. Compound IIc could be crystallized and its structure confirmed by X-ray crystallography. See Supporting Information for details.
34. Protic and Lewis acids were previously shown to favor the radical addition to ketoxime ethers; see for example ref 4c.
35. 1H NMR and MS; 20% and 12% for entries 5 and 6, respectively. See Supporting Information.