Substituent Effects on Vinyl Radical Cyclizations onto Aryl Rings

Journal of Organic Chemistry

Volumn 63, Issue 3, 1998, Pages 537-542

  Montevecchi, Pier Carlo ^a   Navacchia, Maria Luisa ^a  

^a UNIVERSITY OF BOLOGNA   (Italy)

Author keywords

[No Author keywords available]

Indexed keywords

EID: 0002743531     PISSN: 00223263     EISSN: None     Source Type: Journal    
DOI: 10.1021/jo971399l     Document Type: Article

References (65)

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3. For leading references, see: (a) Curran, D. P. Radical Cyclizations. In Comprehensive Organic Synthesis; Pergamon Press: Oxford, U. K., 1991; Vol. 4, Chapter 4.2. (b) Curran, D. P.; Porter, N. A.; Giese, B. In Stereochemistry of Radical Reactions; VCH Publishers: New York, 1996; (c) Malacria, M. Chem. Rev. 1996, 96, 289.
4. For leading references, see: (a) Curran, D. P. Radical Cyclizations. In Comprehensive Organic Synthesis; Pergamon Press: Oxford, U. K., 1991; Vol. 4, Chapter 4.2. (b) Curran, D. P.; Porter, N. A.; Giese, B. In Stereochemistry of Radical Reactions; VCH Publishers: New York, 1996; (c) Malacria, M. Chem. Rev. 1996, 96, 289.
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7. Stereoelectronic effects mainly govern cyclization reactions. See: (a) Baldwin, J. E. J. Chem. Soc., Chem. Commun. 1976, 734. (b) Beckwith, A. L. J.; Easton, C. J.; Serelis, A. K. J. Chem. Soc., Chem. Commun. 1980, 482.
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20. Egger, K. W.; Cocks, A. T. Helv. Chim. Acta 1973, 56, 1537. Zabel, F.; Benson, S. W.; Golden, D. M. Int. J. Chem. Kinet. 1978, 10, 295. Ruchardt, C.; Beckhaus, H. D. Angew. Chem., Int. Ed. engl. 1980, 19, 429. Caronna, T.; Citterio, A.; Ghirardini, M.; Minisci, F. Tetrahedron 1977, 33, 793. Baban, J. A.; Roberts, B. P. J. Chem. Soc., Perkin Trns. 2 1991, 161. Shiraishi, H.; Ramby, B. Chem. Sci. 1977, 118. Korus, R.; O'Driscol, K. F. In Polymer Handbook; Brandrup J., Immergut, E. H., Eds.; Wiley: New York 1975, 11. Giese, B.; Kretzschmar, G.; Meixner, J. Chem. Ber. 1980, 113, 2787. Cekovic, Z.; Saicic, R. Tetrahedron Lett. 1986, 27, 5893. Clive, D. L. J.; Anogh, A. G. J. Chem. Soc., Chem. Commun., 1985, 980. Curran, D. P.; Chen, M. H. J. Am. Chem. Soc., 1987, 109, 6558.
21. Egger, K. W.; Cocks, A. T. Helv. Chim. Acta 1973, 56, 1537. Zabel, F.; Benson, S. W.; Golden, D. M. Int. J. Chem. Kinet. 1978, 10, 295. Ruchardt, C.; Beckhaus, H. D. Angew. Chem., Int. Ed. engl. 1980, 19, 429. Caronna, T.; Citterio, A.; Ghirardini, M.; Minisci, F. Tetrahedron 1977, 33, 793. Baban, J. A.; Roberts, B. P. J. Chem. Soc., Perkin Trns. 2 1991, 161. Shiraishi, H.; Ramby, B. Chem. Sci. 1977, 118. Korus, R.; O'Driscol, K. F. In Polymer Handbook; Brandrup J., Immergut, E. H., Eds.; Wiley: New York 1975, 11. Giese, B.; Kretzschmar, G.; Meixner, J. Chem. Ber. 1980, 113, 2787. Cekovic, Z.; Saicic, R. Tetrahedron Lett. 1986, 27, 5893. Clive, D. L. J.; Anogh, A. G. J. Chem. Soc., Chem. Commun., 1985, 980. Curran, D. P.; Chen, M. H. J. Am. Chem. Soc., 1987, 109, 6558.
22. Egger, K. W.; Cocks, A. T. Helv. Chim. Acta 1973, 56, 1537. Zabel, F.; Benson, S. W.; Golden, D. M. Int. J. Chem. Kinet. 1978, 10, 295. Ruchardt, C.; Beckhaus, H. D. Angew. Chem., Int. Ed. engl. 1980, 19, 429. Caronna, T.; Citterio, A.; Ghirardini, M.; Minisci, F. Tetrahedron 1977, 33, 793. Baban, J. A.; Roberts, B. P. J. Chem. Soc., Perkin Trns. 2 1991, 161. Shiraishi, H.; Ramby, B. Chem. Sci. 1977, 118. Korus, R.; O'Driscol, K. F. In Polymer Handbook; Brandrup J., Immergut, E. H., Eds.; Wiley: New York 1975, 11. Giese, B.; Kretzschmar, G.; Meixner, J. Chem. Ber. 1980, 113, 2787. Cekovic, Z.; Saicic, R. Tetrahedron Lett. 1986, 27, 5893. Clive, D. L. J.; Anogh, A. G. J. Chem. Soc., Chem. Commun., 1985, 980. Curran, D. P.; Chen, M. H. J. Am. Chem. Soc., 1987, 109, 6558.
23. Egger, K. W.; Cocks, A. T. Helv. Chim. Acta 1973, 56, 1537. Zabel, F.; Benson, S. W.; Golden, D. M. Int. J. Chem. Kinet. 1978, 10, 295. Ruchardt, C.; Beckhaus, H. D. Angew. Chem., Int. Ed. engl. 1980, 19, 429. Caronna, T.; Citterio, A.; Ghirardini, M.; Minisci, F. Tetrahedron 1977, 33, 793. Baban, J. A.; Roberts, B. P. J. Chem. Soc., Perkin Trns. 2 1991, 161. Shiraishi, H.; Ramby, B. Chem. Sci. 1977, 118. Korus, R.; O'Driscol, K. F. In Polymer Handbook; Brandrup J., Immergut, E. H., Eds.; Wiley: New York 1975, 11. Giese, B.; Kretzschmar, G.; Meixner, J. Chem. Ber. 1980, 113, 2787. Cekovic, Z.; Saicic, R. Tetrahedron Lett. 1986, 27, 5893. Clive, D. L. J.; Anogh, A. G. J. Chem. Soc., Chem. Commun., 1985, 980. Curran, D. P.; Chen, M. H. J. Am. Chem. Soc., 1987, 109, 6558.
24. Egger, K. W.; Cocks, A. T. Helv. Chim. Acta 1973, 56, 1537. Zabel, F.; Benson, S. W.; Golden, D. M. Int. J. Chem. Kinet. 1978, 10, 295. Ruchardt, C.; Beckhaus, H. D. Angew. Chem., Int. Ed. engl. 1980, 19, 429. Caronna, T.; Citterio, A.; Ghirardini, M.; Minisci, F. Tetrahedron 1977, 33, 793. Baban, J. A.; Roberts, B. P. J. Chem. Soc., Perkin Trns. 2 1991, 161. Shiraishi, H.; Ramby, B. Chem. Sci. 1977, 118. Korus, R.; O'Driscol, K. F. In Polymer Handbook; Brandrup J., Immergut, E. H., Eds.; Wiley: New York 1975, 11. Giese, B.; Kretzschmar, G.; Meixner, J. Chem. Ber. 1980, 113, 2787. Cekovic, Z.; Saicic, R. Tetrahedron Lett. 1986, 27, 5893. Clive, D. L. J.; Anogh, A. G. J. Chem. Soc., Chem. Commun., 1985, 980. Curran, D. P.; Chen, M. H. J. Am. Chem. Soc., 1987, 109, 6558.
25. Egger, K. W.; Cocks, A. T. Helv. Chim. Acta 1973, 56, 1537. Zabel, F.; Benson, S. W.; Golden, D. M. Int. J. Chem. Kinet. 1978, 10, 295. Ruchardt, C.; Beckhaus, H. D. Angew. Chem., Int. Ed. engl. 1980, 19, 429. Caronna, T.; Citterio, A.; Ghirardini, M.; Minisci, F. Tetrahedron 1977, 33, 793. Baban, J. A.; Roberts, B. P. J. Chem. Soc., Perkin Trns. 2 1991, 161. Shiraishi, H.; Ramby, B. Chem. Sci. 1977, 118. Korus, R.; O'Driscol, K. F. In Polymer Handbook; Brandrup J., Immergut, E. H., Eds.; Wiley: New York 1975, 11. Giese, B.; Kretzschmar, G.; Meixner, J. Chem. Ber. 1980, 113, 2787. Cekovic, Z.; Saicic, R. Tetrahedron Lett. 1986, 27, 5893. Clive, D. L. J.; Anogh, A. G. J. Chem. Soc., Chem. Commun., 1985, 980. Curran, D. P.; Chen, M. H. J. Am. Chem. Soc., 1987, 109, 6558.
26. Egger, K. W.; Cocks, A. T. Helv. Chim. Acta 1973, 56, 1537. Zabel, F.; Benson, S. W.; Golden, D. M. Int. J. Chem. Kinet. 1978, 10, 295. Ruchardt, C.; Beckhaus, H. D. Angew. Chem., Int. Ed. engl. 1980, 19, 429. Caronna, T.; Citterio, A.; Ghirardini, M.; Minisci, F. Tetrahedron 1977, 33, 793. Baban, J. A.; Roberts, B. P. J. Chem. Soc., Perkin Trns. 2 1991, 161. Shiraishi, H.; Ramby, B. Chem. Sci. 1977, 118. Korus, R.; O'Driscol, K. F. In Polymer Handbook; Brandrup J., Immergut, E. H., Eds.; Wiley: New York 1975, 11. Giese, B.; Kretzschmar, G.; Meixner, J. Chem. Ber. 1980, 113, 2787. Cekovic, Z.; Saicic, R. Tetrahedron Lett. 1986, 27, 5893. Clive, D. L. J.; Anogh, A. G. J. Chem. Soc., Chem. Commun., 1985, 980. Curran, D. P.; Chen, M. H. J. Am. Chem. Soc., 1987, 109, 6558.
27. Egger, K. W.; Cocks, A. T. Helv. Chim. Acta 1973, 56, 1537. Zabel, F.; Benson, S. W.; Golden, D. M. Int. J. Chem. Kinet. 1978, 10, 295. Ruchardt, C.; Beckhaus, H. D. Angew. Chem., Int. Ed. engl. 1980, 19, 429. Caronna, T.; Citterio, A.; Ghirardini, M.; Minisci, F. Tetrahedron 1977, 33, 793. Baban, J. A.; Roberts, B. P. J. Chem. Soc., Perkin Trns. 2 1991, 161. Shiraishi, H.; Ramby, B. Chem. Sci. 1977, 118. Korus, R.; O'Driscol, K. F. In Polymer Handbook; Brandrup J., Immergut, E. H., Eds.; Wiley: New York 1975, 11. Giese, B.; Kretzschmar, G.; Meixner, J. Chem. Ber. 1980, 113, 2787. Cekovic, Z.; Saicic, R. Tetrahedron Lett. 1986, 27, 5893. Clive, D. L. J.; Anogh, A. G. J. Chem. Soc., Chem. Commun., 1985, 980. Curran, D. P.; Chen, M. H. J. Am. Chem. Soc., 1987, 109, 6558.
28. Egger, K. W.; Cocks, A. T. Helv. Chim. Acta 1973, 56, 1537. Zabel, F.; Benson, S. W.; Golden, D. M. Int. J. Chem. Kinet. 1978, 10, 295. Ruchardt, C.; Beckhaus, H. D. Angew. Chem., Int. Ed. engl. 1980, 19, 429. Caronna, T.; Citterio, A.; Ghirardini, M.; Minisci, F. Tetrahedron 1977, 33, 793. Baban, J. A.; Roberts, B. P. J. Chem. Soc., Perkin Trns. 2 1991, 161. Shiraishi, H.; Ramby, B. Chem. Sci. 1977, 118. Korus, R.; O'Driscol, K. F. In Polymer Handbook; Brandrup J., Immergut, E. H., Eds.; Wiley: New York 1975, 11. Giese, B.; Kretzschmar, G.; Meixner, J. Chem. Ber. 1980, 113, 2787. Cekovic, Z.; Saicic, R. Tetrahedron Lett. 1986, 27, 5893. Clive, D. L. J.; Anogh, A. G. J. Chem. Soc., Chem. Commun., 1985, 980. Curran, D. P.; Chen, M. H. J. Am. Chem. Soc., 1987, 109, 6558.
29. Egger, K. W.; Cocks, A. T. Helv. Chim. Acta 1973, 56, 1537. Zabel, F.; Benson, S. W.; Golden, D. M. Int. J. Chem. Kinet. 1978, 10, 295. Ruchardt, C.; Beckhaus, H. D. Angew. Chem., Int. Ed. engl. 1980, 19, 429. Caronna, T.; Citterio, A.; Ghirardini, M.; Minisci, F. Tetrahedron 1977, 33, 793. Baban, J. A.; Roberts, B. P. J. Chem. Soc., Perkin Trns. 2 1991, 161. Shiraishi, H.; Ramby, B. Chem. Sci. 1977, 118. Korus, R.; O'Driscol, K. F. In Polymer Handbook; Brandrup J., Immergut, E. H., Eds.; Wiley: New York 1975, 11. Giese, B.; Kretzschmar, G.; Meixner, J. Chem. Ber. 1980, 113, 2787. Cekovic, Z.; Saicic, R. Tetrahedron Lett. 1986, 27, 5893. Clive, D. L. J.; Anogh, A. G. J. Chem. Soc., Chem. Commun., 1985, 980. Curran, D. P.; Chen, M. H. J. Am. Chem. Soc., 1987, 109, 6558.
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31. Curran, D. P.; Kim, D.; Ziegler, C. Tetrahedron 1991, 32, 6189. Ichinose, Y.; Matsunaga, S.; Fugami, K.; Oshima, K.; Utimoto, K. Tetrahedron Lett. 1989, 30, 31555.
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34. Minisci, F.; Vismara, E.; Fontana, F. Heterocycles 1989, 28, 489. Minisci, F.; Citterio, A.; Giordano, C. Acc. Chem. Res. 1983, 16, 27. Minisci, F. In Substituents Effects in Radical Chemistry; Viehe H. G., Ed., Reidel: Dordrecht, 1986; p 391. Minisci, F. Top. Curr. Chem. 1976, 62, 1.
35. Minisci, F.; Vismara, E.; Fontana, F. Heterocycles 1989, 28, 489. Minisci, F.; Citterio, A.; Giordano, C. Acc. Chem. Res. 1983, 16, 27. Minisci, F. In Substituents Effects in Radical Chemistry; Viehe H. G., Ed., Reidel: Dordrecht, 1986; p 391. Minisci, F. Top. Curr. Chem. 1976, 62, 1.
36. Minisci, F.; Vismara, E.; Fontana, F. Heterocycles 1989, 28, 489. Minisci, F.; Citterio, A.; Giordano, C. Acc. Chem. Res. 1983, 16, 27. Minisci, F. In Substituents Effects in Radical Chemistry; Viehe H. G., Ed., Reidel: Dordrecht, 1986; p 391. Minisci, F. Top. Curr. Chem. 1976, 62, 1.
37. For our reports on 2-sulfanyl vinyl radical cyclization and rearrangements, see: (a) Montevecchi, P. C.; Navacchia, M. L. J. Org. Chem. 1997, 62, 5600. (b) Montevecchi, P. C.; Navacchia, M. L. Tetrahedron Lett. 1996, 37, 6583. (c) Capella, L.; Montevecchi, P. C.; Navacchia, M. L. J. Org. Chem. 1996, 61, 6783. (d) Benati, L.; Capella, L.; Montevecchi, P. C.; Spagnolo, P. J. Org. Chem. 1995, 60, 7941. (e) Benati, L.; Capella, L.; Montevecchi, P. C.; Spagnolo, P. J. Org. Chem. 1994, 59, 2818.
38. For our reports on 2-sulfanyl vinyl radical cyclization and rearrangements, see: (a) Montevecchi, P. C.; Navacchia, M. L. J. Org. Chem. 1997, 62, 5600. (b) Montevecchi, P. C.; Navacchia, M. L. Tetrahedron Lett. 1996, 37, 6583. (c) Capella, L.; Montevecchi, P. C.; Navacchia, M. L. J. Org. Chem. 1996, 61, 6783. (d) Benati, L.; Capella, L.; Montevecchi, P. C.; Spagnolo, P. J. Org. Chem. 1995, 60, 7941. (e) Benati, L.; Capella, L.; Montevecchi, P. C.; Spagnolo, P. J. Org. Chem. 1994, 59, 2818.
39. For our reports on 2-sulfanyl vinyl radical cyclization and rearrangements, see: (a) Montevecchi, P. C.; Navacchia, M. L. J. Org. Chem. 1997, 62, 5600. (b) Montevecchi, P. C.; Navacchia, M. L. Tetrahedron Lett. 1996, 37, 6583. (c) Capella, L.; Montevecchi, P. C.; Navacchia, M. L. J. Org. Chem. 1996, 61, 6783. (d) Benati, L.; Capella, L.; Montevecchi, P. C.; Spagnolo, P. J. Org. Chem. 1995, 60, 7941. (e) Benati, L.; Capella, L.; Montevecchi, P. C.; Spagnolo, P. J. Org. Chem. 1994, 59, 2818.
40. For our reports on 2-sulfanyl vinyl radical cyclization and rearrangements, see: (a) Montevecchi, P. C.; Navacchia, M. L. J. Org. Chem. 1997, 62, 5600. (b) Montevecchi, P. C.; Navacchia, M. L. Tetrahedron Lett. 1996, 37, 6583. (c) Capella, L.; Montevecchi, P. C.; Navacchia, M. L. J. Org. Chem. 1996, 61, 6783. (d) Benati, L.; Capella, L.; Montevecchi, P. C.; Spagnolo, P. J. Org. Chem. 1995, 60, 7941. (e) Benati, L.; Capella, L.; Montevecchi, P. C.; Spagnolo, P. J. Org. Chem. 1994, 59, 2818.
41. For our reports on 2-sulfanyl vinyl radical cyclization and rearrangements, see: (a) Montevecchi, P. C.; Navacchia, M. L. J. Org. Chem. 1997, 62, 5600. (b) Montevecchi, P. C.; Navacchia, M. L. Tetrahedron Lett. 1996, 37, 6583. (c) Capella, L.; Montevecchi, P. C.; Navacchia, M. L. J. Org. Chem. 1996, 61, 6783. (d) Benati, L.; Capella, L.; Montevecchi, P. C.; Spagnolo, P. J. Org. Chem. 1995, 60, 7941. (e) Benati, L.; Capella, L.; Montevecchi, P. C.; Spagnolo, P. J. Org. Chem. 1994, 59, 2818.
42. We have previously shown that toluenesufanyl radical addition to terminal alkynes having radical acceptor moieties in the radical chain (terminal and styrene double bonds or aryl rings) leads to vinyl radicals which undergo π-exo cyclization onto the radical acceptor in competition with the 5-(π-endo) cyclization onto the aryl ring. See ref 13a.
43. Crich, D.; Sun, S.; Brunckova, J. J. Org. Chem. 1996, 61, 605. Mawson, S. D.; Routledge, A.; Weavers, R. T. Tetrahedron 1995, 51, 4665.
44. Crich, D.; Sun, S.; Brunckova, J. J. Org. Chem. 1996, 61, 605. Mawson, S. D.; Routledge, A.; Weavers, R. T. Tetrahedron 1995, 51, 4665.
45. Nevertheless, it should be stated that the "philicity" is a relative concept. In principle, a radical can behave as an electrophile or nucleophile, depending on the nature of the radical-acceptor counterpart. For example, reactions of vinyl radicals with olefins are usually dominated by SOMO-LUMO interaction; in this cases the vinyl radical behaves as a nucleophile and the olefin as an electrophile (see ref 6).
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51. This claim was been supported by semiempirical calculations (performed with a AM1 Spartan program) which gave ΔH = ca. -22 kcal/mol.
52. Ring expansion of 5-exo to 6-endo radicals are well documented in cyclizations of β-multiply bonded alkyl or vinyl radicals. See: Beckwith A. L. J.; O'Shea, D. M. Tetrahedron Lett. 1986, 27, 4525. Stork, G.; Mook, R., Jr. Tetrahedron Lett. 1986, 27, 4529. Jasperse, C. P.; Curran, D. P.; Fevig, D. L. Chem. Rev. 1991, 91 1237. Beckwith A. L. J.; O'Shea, D. M.; Westwood, S. W. J. Am. Chem. Soc. 1988, 110, 2565. Nanni, D.; Pareschi, P.; Tundo, A. Tetrahedron Lett. 1996, 37, 9337.
53. Ring expansion of 5-exo to 6-endo radicals are well documented in cyclizations of β-multiply bonded alkyl or vinyl radicals. See: Beckwith A. L. J.; O'Shea, D. M. Tetrahedron Lett. 1986, 27, 4525. Stork, G.; Mook, R., Jr. Tetrahedron Lett. 1986, 27, 4529. Jasperse, C. P.; Curran, D. P.; Fevig, D. L. Chem. Rev. 1991, 91 1237. Beckwith A. L. J.; O'Shea, D. M.; Westwood, S. W. J. Am. Chem. Soc. 1988, 110, 2565. Nanni, D.; Pareschi, P.; Tundo, A. Tetrahedron Lett. 1996, 37, 9337.
54. Ring expansion of 5-exo to 6-endo radicals are well documented in cyclizations of β-multiply bonded alkyl or vinyl radicals. See: Beckwith A. L. J.; O'Shea, D. M. Tetrahedron Lett. 1986, 27, 4525. Stork, G.; Mook, R., Jr. Tetrahedron Lett. 1986, 27, 4529. Jasperse, C. P.; Curran, D. P.; Fevig, D. L. Chem. Rev. 1991, 91 1237. Beckwith A. L. J.; O'Shea, D. M.; Westwood, S. W. J. Am. Chem. Soc. 1988, 110, 2565. Nanni, D.; Pareschi, P.; Tundo, A. Tetrahedron Lett. 1996, 37, 9337.
55. Ring expansion of 5-exo to 6-endo radicals are well documented in cyclizations of β-multiply bonded alkyl or vinyl radicals. See: Beckwith A. L. J.; O'Shea, D. M. Tetrahedron Lett. 1986, 27, 4525. Stork, G.; Mook, R., Jr. Tetrahedron Lett. 1986, 27, 4529. Jasperse, C. P.; Curran, D. P.; Fevig, D. L. Chem. Rev. 1991, 91 1237. Beckwith A. L. J.; O'Shea, D. M.; Westwood, S. W. J. Am. Chem. Soc. 1988, 110, 2565. Nanni, D.; Pareschi, P.; Tundo, A. Tetrahedron Lett. 1996, 37, 9337.
56. Ring expansion of 5-exo to 6-endo radicals are well documented in cyclizations of β-multiply bonded alkyl or vinyl radicals. See: Beckwith A. L. J.; O'Shea, D. M. Tetrahedron Lett. 1986, 27, 4525. Stork, G.; Mook, R., Jr. Tetrahedron Lett. 1986, 27, 4529. Jasperse, C. P.; Curran, D. P.; Fevig, D. L. Chem. Rev. 1991, 91 1237. Beckwith A. L. J.; O'Shea, D. M.; Westwood, S. W. J. Am. Chem. Soc. 1988, 110, 2565. Nanni, D.; Pareschi, P.; Tundo, A. Tetrahedron Lett. 1996, 37, 9337.
57. 6 ratio = 6-7:1 has been reported for related π-endo vinyl radical cyclizations carried out at room temperature. See: Citterio, A.; Sebastiano, R.; Maronati A.; Santi, R.; Bergamini, F. J. Chem. Soc., Chem. Commun. 1994, 1517.
58. The cyano group do not affect the rate of vinyl radical cyclization in 3-position (see Table, entry 9) and thus the 6-membered orthocyclization reactions leading to 5h and 5j, respectively, would occur at the same rate. In contrast, the formation of sulfide 9j should be 5.05 times slower than 9h (see Table, entry 8). The calculated [9h]/[5h] = 5.05[9j]/[5j] = 12.75 ratio value is actually strictly closed to the observed [9h]/[5h] = 12-13.
59. We have previously suggested that samarium diiodide-promoted vinyl radicals can undergo 5-exo cyclization onto C-C triple bonds in competition with the E/Z interconversion. (Capella, L.; Montevecchi, P. C.; Navacchia, M. L. J. Org. Chem. 1995, 60, 7424.)
60. Capella, L.; Montevecchi, P. C.; Nanni, D. J. Org. Chem. 1994, 59, 3368.
61. Benati, L.; Montevecchi, P. C.; Spagnolo, P. J. Chem Soc., Perkin Trans 1 1991, 2103 and references therein.
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63. Marshall, J. A.; Robinson, E. D.; Zapata, A. J. Org. Chem. 1989, 54, 5854.
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