Transition Metal/Base-Catalyzed Aldol Reactions of Isocyanoacetic Acid Derivatives with Prochiral Ketones, a Straightforward Approach to Stereochemically Defined β,β-Disubstituted-β-hydroxy-α-amino Acids. 1 Scope and Limitations

Journal of Organic Chemistry

Volumn 62, Issue 11, 1997, Pages 3470-3479

  Soloshonok, Vadim A ^a   Kacharov, Alexey D ^b   Avilov, Dimitry V ^b   Ishikawa, Kohki ^c   Nagashima, Nobuya ^c   Hayashi, Tamio ^d  

^a NATIONAL INSTITUTE OF ADVANCED INDUSTRIAL SCIENCE AND TECHNOLOGY AIST   (Japan)

^b INSTITUTE OF BIOORGANIC CHEMISTRY AND PETROCHEMISTRY   (Ukraine)

^c AJINOMOTO CO INC   (Japan)

^d KYOTO UNIVERSITY   (Japan)

Author keywords

[No Author keywords available]

Indexed keywords

EID: 0000857641     PISSN: 00223263     EISSN: None     Source Type: Journal    
DOI: 10.1021/jo9623402     Document Type: Article

References (81)

1. For preliminary communications, see: (a) Soloshonok, V. A.; Hayashi, T.; Ishikawa, K.; Nagashima, N. Tetrahedron Lett. 1994, 35, 1055. (b) Soloshonok, V. A.; Kacharov, A. D.; Avilov, D. V.; Hayashi, T. Tetrahedron Lett. 1996, 37, 7845.
2. For preliminary communications, see: (a) Soloshonok, V. A.; Hayashi, T.; Ishikawa, K.; Nagashima, N. Tetrahedron Lett. 1994, 35, 1055. (b) Soloshonok, V. A.; Kacharov, A. D.; Avilov, D. V.; Hayashi, T. Tetrahedron Lett. 1996, 37, 7845.
3. For reviews of stereoselective aldol addition reactions, see: (a) Seebach, D. Angew. Chem. Int. Ed. Engl. 1990, 29, 1320. (b) Evans, D. A. Aldrichimica Acta 1982, 15, 23. Evans, D. A.; Nelson, J. V.; Taber, T. R. In Topics in Stereochemistry; Eliel, E. L., Allinger, N. L., Wilen, S. H., Eds.; Wiley Interscience: New York, 1982; Vol. 13, p 1. (d) Heathcock, C. H. In Asymmetric Synthesis; Morrison, J. D., Ed.; Academic Press: New York, 1984; Vol. 3, Chapter 2. (e) Kim, B. M.; Williams, S. F.; Masamune, S. In Comprehensive Organic Synthesis. Additions to C-X π Bonds; Heathcock, C. H., Ed.; Pergamon Press: Oxford; 1991; Vol. 2, Part 2, pp 239-275. (f) Gennari, C. In Comprehensive Organic Synthesis. Additions to C-X π Bonds; Heathcock, C. H., Ed.; Pergamon Press: Oxford, 1991; Vol. 2, Part 2, pp 629-660. (g) Bach, T. Angew. Chem., Int. Ed. Engl. 1994, 33, 417. (h) Franklin, A. S.; Paterson, I. Contemp. Org. Synth. 1994, 1, 317-416.
4. For reviews of stereoselective aldol addition reactions, see: (a) Seebach, D. Angew. Chem. Int. Ed. Engl. 1990, 29, 1320. (b) Evans, D. A. Aldrichimica Acta 1982, 15, 23. Evans, D. A.; Nelson, J. V.; Taber, T. R. In Topics in Stereochemistry; Eliel, E. L., Allinger, N. L., Wilen, S. H., Eds.; Wiley Interscience: New York, 1982; Vol. 13, p 1. (d) Heathcock, C. H. In Asymmetric Synthesis; Morrison, J. D., Ed.; Academic Press: New York, 1984; Vol. 3, Chapter 2. (e) Kim, B. M.; Williams, S. F.; Masamune, S. In Comprehensive Organic Synthesis. Additions to C-X π Bonds; Heathcock, C. H., Ed.; Pergamon Press: Oxford; 1991; Vol. 2, Part 2, pp 239-275. (f) Gennari, C. In Comprehensive Organic Synthesis. Additions to C-X π Bonds; Heathcock, C. H., Ed.; Pergamon Press: Oxford, 1991; Vol. 2, Part 2, pp 629-660. (g) Bach, T. Angew. Chem., Int. Ed. Engl. 1994, 33, 417. (h) Franklin, A. S.; Paterson, I. Contemp. Org. Synth. 1994, 1, 317-416.
5. For reviews of stereoselective aldol addition reactions, see: (a) Seebach, D. Angew. Chem. Int. Ed. Engl. 1990, 29, 1320. (b) Evans, D. A. Aldrichimica Acta 1982, 15, 23. Evans, D. A.; Nelson, J. V.; Taber, T. R. In Topics in Stereochemistry; Eliel, E. L., Allinger, N. L., Wilen, S. H., Eds.; Wiley Interscience: New York, 1982; Vol. 13, p 1. (d) Heathcock, C. H. In Asymmetric Synthesis; Morrison, J. D., Ed.; Academic Press: New York, 1984; Vol. 3, Chapter 2. (e) Kim, B. M.; Williams, S. F.; Masamune, S. In Comprehensive Organic Synthesis. Additions to C-X π Bonds; Heathcock, C. H., Ed.; Pergamon Press: Oxford; 1991; Vol. 2, Part 2, pp 239-275. (f) Gennari, C. In Comprehensive Organic Synthesis. Additions to C-X π Bonds; Heathcock, C. H., Ed.; Pergamon Press: Oxford, 1991; Vol. 2, Part 2, pp 629-660. (g) Bach, T. Angew. Chem., Int. Ed. Engl. 1994, 33, 417. (h) Franklin, A. S.; Paterson, I. Contemp. Org. Synth. 1994, 1, 317-416.
6. For reviews of stereoselective aldol addition reactions, see: (a) Seebach, D. Angew. Chem. Int. Ed. Engl. 1990, 29, 1320. (b) Evans, D. A. Aldrichimica Acta 1982, 15, 23. Evans, D. A.; Nelson, J. V.; Taber, T. R. In Topics in Stereochemistry; Eliel, E. L., Allinger, N. L., Wilen, S. H., Eds.; Wiley Interscience: New York, 1982; Vol. 13, p 1. (d) Heathcock, C. H. In Asymmetric Synthesis; Morrison, J. D., Ed.; Academic Press: New York, 1984; Vol. 3, Chapter 2. (e) Kim, B. M.; Williams, S. F.; Masamune, S. In Comprehensive Organic Synthesis. Additions to C-X π Bonds; Heathcock, C. H., Ed.; Pergamon Press: Oxford; 1991; Vol. 2, Part 2, pp 239-275. (f) Gennari, C. In Comprehensive Organic Synthesis. Additions to C-X π Bonds; Heathcock, C. H., Ed.; Pergamon Press: Oxford, 1991; Vol. 2, Part 2, pp 629-660. (g) Bach, T. Angew. Chem., Int. Ed. Engl. 1994, 33, 417. (h) Franklin, A. S.; Paterson, I. Contemp. Org. Synth. 1994, 1, 317-416.
7. For reviews of stereoselective aldol addition reactions, see: (a) Seebach, D. Angew. Chem. Int. Ed. Engl. 1990, 29, 1320. (b) Evans, D. A. Aldrichimica Acta 1982, 15, 23. Evans, D. A.; Nelson, J. V.; Taber, T. R. In Topics in Stereochemistry; Eliel, E. L., Allinger, N. L., Wilen, S. H., Eds.; Wiley Interscience: New York, 1982; Vol. 13, p 1. (d) Heathcock, C. H. In Asymmetric Synthesis; Morrison, J. D., Ed.; Academic Press: New York, 1984; Vol. 3, Chapter 2. (e) Kim, B. M.; Williams, S. F.; Masamune, S. In Comprehensive Organic Synthesis. Additions to C-X π Bonds; Heathcock, C. H., Ed.; Pergamon Press: Oxford; 1991; Vol. 2, Part 2, pp 239-275. (f) Gennari, C. In Comprehensive Organic Synthesis. Additions to C-X π Bonds; Heathcock, C. H., Ed.; Pergamon Press: Oxford, 1991; Vol. 2, Part 2, pp 629-660. (g) Bach, T. Angew. Chem., Int. Ed. Engl. 1994, 33, 417. (h) Franklin, A. S.; Paterson, I. Contemp. Org. Synth. 1994, 1, 317-416.
8. For reviews of stereoselective aldol addition reactions, see: (a) Seebach, D. Angew. Chem. Int. Ed. Engl. 1990, 29, 1320. (b) Evans, D. A. Aldrichimica Acta 1982, 15, 23. Evans, D. A.; Nelson, J. V.; Taber, T. R. In Topics in Stereochemistry; Eliel, E. L., Allinger, N. L., Wilen, S. H., Eds.; Wiley Interscience: New York, 1982; Vol. 13, p 1. (d) Heathcock, C. H. In Asymmetric Synthesis; Morrison, J. D., Ed.; Academic Press: New York, 1984; Vol. 3, Chapter 2. (e) Kim, B. M.; Williams, S. F.; Masamune, S. In Comprehensive Organic Synthesis. Additions to C-X π Bonds; Heathcock, C. H., Ed.; Pergamon Press: Oxford; 1991; Vol. 2, Part 2, pp 239-275. (f) Gennari, C. In Comprehensive Organic Synthesis. Additions to C-X π Bonds; Heathcock, C. H., Ed.; Pergamon Press: Oxford, 1991; Vol. 2, Part 2, pp 629-660. (g) Bach, T. Angew. Chem., Int. Ed. Engl. 1994, 33, 417. (h) Franklin, A. S.; Paterson, I. Contemp. Org. Synth. 1994, 1, 317-416.
9. For reviews of stereoselective aldol addition reactions, see: (a) Seebach, D. Angew. Chem. Int. Ed. Engl. 1990, 29, 1320. (b) Evans, D. A. Aldrichimica Acta 1982, 15, 23. Evans, D. A.; Nelson, J. V.; Taber, T. R. In Topics in Stereochemistry; Eliel, E. L., Allinger, N. L., Wilen, S. H., Eds.; Wiley Interscience: New York, 1982; Vol. 13, p 1. (d) Heathcock, C. H. In Asymmetric Synthesis; Morrison, J. D., Ed.; Academic Press: New York, 1984; Vol. 3, Chapter 2. (e) Kim, B. M.; Williams, S. F.; Masamune, S. In Comprehensive Organic Synthesis. Additions to C-X π Bonds; Heathcock, C. H., Ed.; Pergamon Press: Oxford; 1991; Vol. 2, Part 2, pp 239-275. (f) Gennari, C. In Comprehensive Organic Synthesis. Additions to C-X π Bonds; Heathcock, C. H., Ed.; Pergamon Press: Oxford, 1991; Vol. 2, Part 2, pp 629-660. (g) Bach, T. Angew. Chem., Int. Ed. Engl. 1994, 33, 417. (h) Franklin, A. S.; Paterson, I. Contemp. Org. Synth. 1994, 1, 317-416.
10. For reviews of stereoselective aldol addition reactions, see: (a) Seebach, D. Angew. Chem. Int. Ed. Engl. 1990, 29, 1320. (b) Evans, D. A. Aldrichimica Acta 1982, 15, 23. Evans, D. A.; Nelson, J. V.; Taber, T. R. In Topics in Stereochemistry; Eliel, E. L., Allinger, N. L., Wilen, S. H., Eds.; Wiley Interscience: New York, 1982; Vol. 13, p 1. (d) Heathcock, C. H. In Asymmetric Synthesis; Morrison, J. D., Ed.; Academic Press: New York, 1984; Vol. 3, Chapter 2. (e) Kim, B. M.; Williams, S. F.; Masamune, S. In Comprehensive Organic Synthesis. Additions to C-X π Bonds; Heathcock, C. H., Ed.; Pergamon Press: Oxford; 1991; Vol. 2, Part 2, pp 239-275. (f) Gennari, C. In Comprehensive Organic Synthesis. Additions to C-X π Bonds; Heathcock, C. H., Ed.; Pergamon Press: Oxford, 1991; Vol. 2, Part 2, pp 629-660. (g) Bach, T. Angew. Chem., Int. Ed. Engl. 1994, 33, 417. (h) Franklin, A. S.; Paterson, I. Contemp. Org. Synth. 1994, 1, 317-416.
11. Braun, M. Angew. Chem., Int. Ed. Engl. 1987, 26, 24.
12. For recent comprehensive reviews on asymmetric synthesis of α-amino acids, see: (a) Williams, R. M. Synthesis of Optically Active α-Amino Acids. In Organic Chemistry Series; Baldwin, J. E., Magnus, P. D., Eds.; Pergamon Press: Oxford, 1989; Vol. 7. (b) Duthaler, R. P. Tetrahedron 1994, 50, 1539.
13. For recent comprehensive reviews on asymmetric synthesis of α-amino acids, see: (a) Williams, R. M. Synthesis of Optically Active α-Amino Acids. In Organic Chemistry Series; Baldwin, J. E., Magnus, P. D., Eds.; Pergamon Press: Oxford, 1989; Vol. 7. (b) Duthaler, R. P. Tetrahedron 1994, 50, 1539.
14. (a) Peptide Chemistry: Design and Synthesis of Peptides, Conformational Analysis and Biological Functions; Hruby V. J.; Schwyzer, R., Eds.; Tetrahedron-Symposia-in-Print, 31; Tetrahedron 1988, 44, 661.
15. (b) Giannis, A.; Kolter, T. Angew. Chem., Int. Ed. Engl. 1993, 32, 1244.
16. (c) Robl, J. A.; Cimarusti, M. P.; Simpkins, L. M.; Weller, H. N.; Pan, Y. Y.; Malley, M.; DiMarco, J. D. J. Am. Chem. Soc. 1994, 116, 2348,
17. For general review on fluorine-containing amino acids, see: Fluorine-Containing Amino Acids. Synthesis and Properties; Kukhar, V. P.; Soloshonok, V. A., Eds.; Wiley: Chichester, 1994. α-Amino-β-hydroxy carboxylic acids are naturally occurring compounds. Some are proteinogenic and/or essential amino acids are involved in various physiological processes in living organisms; therefore, analogs of these amino acids are very interesting as biomedical tools or potential inhibitors. Other α-amino-β-hydroxy carboxylic acids are constituents of complex natural products such as cyclosporin, bouvardin, peptides, and glycopeptides, which usually possess high antibiotic activity. In addition, these amino acids are valuable precursors to β-lactam antibiotics.
18. (a) Soloshonok, V. A.; Kukhar, V. P.; Galushko, S. V.; Svistunova, N. Yu.; Avilov, D. V.; Kuz'mina, N. A.; Raevski, N. I.; Struchkov, Yu. T.; Pysarevsky A. P.; Belokon Yu. N. J. Chem. Soc., Perkin Trans. 1 1993, 3143.
19. (b) Soloshonok, V. A.; Avilov, D. V.; Kukhar', V. P. Tetrahedron: Asymmetry 1996, 7, 1547.
20. (c) Soloshonok, V. A.; Avilov, D. V.; Kukhar', V. P. Tetrahedron 1996, 52, 12433.
21. For reviews: (a) Schollkopf, U. Angew. Chem., Int. Ed. Engl. 1977, 16, 339. (b) Matsumoto, K.; Moriya, T.; Suzuki, M. J. Synth. Org. Chem., Jpn. 1985, 43, 764.
22. For reviews: (a) Schollkopf, U. Angew. Chem., Int. Ed. Engl. 1977, 16, 339. (b) Matsumoto, K.; Moriya, T.; Suzuki, M. J. Synth. Org. Chem., Jpn. 1985, 43, 764.
23. (a) Ito, Y.; Matsumura, T.; Saegusa, T. Tetrahedron Lett. 1985, 26, 5781.
24. (b) Saegusa, T.; Ito, Y.; Kinoshita, H.; Tomita, S. J. Org. Chem. 1971, 36, 3316, and references cited therein.
25. (a) Soloshonok, V. A.; Kacharov, A. D.; Hayashi, T. Tetrahedron 1996, 52, 245.
26. (b) Soloshonok, V. A.; Hayashi, T. Tetrahedron Lett. 1994, 35, 2713.
27. (c) Soloshonok, V. A.; Hayashi, T. Tetrahedron: Asymmetry 1994, 5, 1091.
28. (d) Hayashi, T.; Sawamura, M.; Ito, Y. Tetrahedron 1992, 48, 1999.
29. (e) Ito, Y.; Sawamura, M.; Hamashima, H.; Emura, T.; Hayashi, T. Tetrahedron Lett. 1989, 30, 4681.
30. (f) Sawamura, M.; Ito, Y.; Hayashi, T. Tetrahedron Lett. 1989, 30, 2247.
31. (g) Ito, Y.; Sawamura, M.; Hayashi, T. Tetrahedron Lett. 1988, 29, 239.
32. (h) Ito, Y; Sawamura, M.; Kobayashi, M.; Hayashi, T. Tetrahedron Lett. 1988, 29, 6321.
33. (i) Ito, Y.; Sawamura, M.; Shirakawa, E.; Hayashizaki, K.; Hayashi, T. Tetrahedron Lett. 1988, 29, 235.
34. (j) Ito, Y.; Sawamura, M.; Hayashi, T. Tetrahedron Lett. 1987, 28, 6215.
35. (k) Ito, Y.; Sawamura, M.; Hayashi, T. J. Am. Chem. Soc. 1986, 108, 6405.
36. (a) Pastor, S. D.; Kesselring, R.; Togni, A. J. Organomet. Chem. 1992, 429, 415.
37. (b) Pastor, S. D.; Togni, A. Helv. Chim. Acta 1991, 74, 905.
38. (c) Togni, A.; Blumer, R. E.; Pregosin, P. S. Helv. Chim. Acta 1991, 74, 1533.
39. (d) Togni, A.; Pastor, S. D. J. Org. Chem. 1990, 55, 1649.
40. (e) Pastor, S. D.; Togni, A. Tetrahedron Lett. 1990, 31, 839.
41. (f) Togni, A.; Pastor, S. D. Tetrahedron Lett. 1989, 30, 1071.
42. (g) Togni, A.; Pastor, S. D.; Rihs, G. Helv. Chim. Acta 1989, 72, 1471.
43. (h) Pastor, S. D.; Togni, A. J. Am. Chem. Soc. 1989, 111, 2333.
44. (a) Hayashi, T.; Uozumi, Y.; Yamazaki, A.; Sawamura, M.; Hamashima, H.; Ito, Y. Tetrahedron Lett. 1991, 32, 2799.
45. (b) Sawamura, M.; Hamashima, H.; Ito, Y. J. Org. Chem. 1990, 55, 5935.
46. Heinzer, F.; Bellus, D. Helv. Chim. Acta 1981, 64, 2279.
47. The common name, 2-oxazoline, is used throughout. Following nomenclature rules, the systematic name for 3 and 4 is 4,5-dihydro-5-(substituent)-5-(substituent)-4-oxazolecarboxylic acid, methyl ester.
48. Cahn, R. S.; Ingold, C.; Prelog, V. Angew. Chem. Int. Ed. Engl. 1966, 5, 385.
49. Silverstein, R. M.; Bassler, G. C.; Morrill, T. C. Spectrometric Identification of Organic Compounds, 3rd ed.; Wiley: New York, 1974.
50. The (2R*,3R*)-configuration, a consequence of the Cahn-Ingold-Prelog priority (ref 14), is stereochemically equivalent to the (2R*,3S*)-configuration in the hydrocarbon analogs.
51. Hansch, C.; Leo, A. Substituent Constants for Correlation Analysis in Chemistry and Biology; Wiley: New York, 1979.
52. Smart, B. E. In Organofluorine Chemistry: Principles and Commercial Applications; Banks, R. E.; Smart, B. E.; Tatlow, J. C., Eds.; Plenum Press: New York, 1994; pp. 57-88.
53. Taft, R. W. In Steric Effects in Organic Chemistry; Newman, M. S., Ed.; Wiley: New York, 1956; p 556.
54. MacPhee, A.; Panaye, A.; Dubois, J.-E. Tetrahedron 1978, 34, 3553.
55. Bott, G.; Field, L. D.; Sternhell, S. J. Am. Chem. Soc. 1980, 102, 5618.
56. 2"; see ref 18. For asymmetric results, which do not follow this statement, and the corresponding rationale, see ref 23k.
57. (a) Aaron, C.; Dull, D.; Schmiegel, J. L.; Jaeger, D.; Ohashi, Y. O.; Mosher, H. S. J. Org. Chem. 1967, 32, 2797.
58. (b) Nasipuri, D.; Bhattacharya, P. K. J. Chem. Soc. Perkin Trans 1 1977, 576.
59. (c) Midland, M. M.; Kazubski, A. J. Org. Chem. 1982, 47, 2495.
60. (d) Bucciarelli, M.; Forni, A.; Moretti, I.; Torre, G. Synthesis 1983, 897.
61. (e) Brown, H. C.; Pai, G. G. J. Org. Chem. 1985, 50, 1384.
62. (f) Brown, H. C.; Cho, B. T.; Park, W. S. J. Org. Chem. 1988, 53, 1231.
63. (g) Corey, E. J.; Bakshi, R. K. Tetrahedron Lett. 1990, 31, 611.
64. (h) Chong, J. M.; Mar, E. K. J. Org. Chem. 1991, 56, 893.
65. (i) Corey, E. J.; Link, J. O.; Sarshar, S.; Shao, Y. Tetrahedron Lett. 1992, 33, 7103.
66. (j) Corey, E. J.; Link, J. O.; Bakshi, R. K. Tetrahedron Lett. 1992, 33, 7107.
67. (k) Ramachandran, P. V.; Teodorovic, A. V.; Brown, H. C. Tetrahedron 1993, 49, 1725.
68. (l) Ramachandran, P. V.; Teodorovic, A. V.; Gong, B.; Brown, H. C. Tetrahedron: Asymmetry 1994, 5, 1075.
69. 5-cyclopentadienyl); Ojima, I.; Kwon, H. B. J. Am. Chem. Soc. 1988, 110, 5617.
70. 3P)(CO)CpFeCOMe; see for examples: (a) Davies, S. G.; Walker, J. C. J. Chem. Soc., Chem. Commun. 1985, 209. (b) Liebeskind, L. S.; Welker, M. E.; Fengle, R. W. J. Am. Chem. Soc. 1986, 108, 6328.
71. 3P)(CO)CpFeCOMe; see for examples: (a) Davies, S. G.; Walker, J. C. J. Chem. Soc., Chem. Commun. 1985, 209. (b) Liebeskind, L. S.; Welker, M. E.; Fengle, R. W. J. Am. Chem. Soc. 1986, 108, 6328.
72. (a) (Enantiocontrolled Synthesis of Fluoro-Organic Compounds; Tetrahedron: Asymmetry, Special Issue; Hayashi, T.; Soloshonok, V. A., Eds.) (b) Tetrahedron 1996, 52, No 1 (Fluoroorganic Chemistry: Synthetic Challenges and Biomedical Rewards; Resnati, G.; Soloshonok, V. A. Eds.; Tetrahedron Symposium-in-Print No. 58). (c) EPC-Synthesis of Fluoro-Organic Compounds: Stereochemical Challenges and Biomedicinal Targets; Soloshonok, V. A. Ed.; Wiley: Chichester, scheduled to appear in 1997.
73. (a) (Enantiocontrolled Synthesis of Fluoro-Organic Compounds; Tetrahedron: Asymmetry, Special Issue; Hayashi, T.; Soloshonok, V. A., Eds.) (b) Tetrahedron 1996, 52, No 1 (Fluoroorganic Chemistry: Synthetic Challenges and Biomedical Rewards; Resnati, G.; Soloshonok, V. A. Eds.; Tetrahedron Symposium-in-Print No. 58). (c) EPC-Synthesis of Fluoro-Organic Compounds: Stereochemical Challenges and Biomedicinal Targets; Soloshonok, V. A. Ed.; Wiley: Chichester, scheduled to appear in 1997.
74. (a) (Enantiocontrolled Synthesis of Fluoro-Organic Compounds; Tetrahedron: Asymmetry, Special Issue; Hayashi, T.; Soloshonok, V. A., Eds.) (b) Tetrahedron 1996, 52, No 1 (Fluoroorganic Chemistry: Synthetic Challenges and Biomedical Rewards; Resnati, G.; Soloshonok, V. A. Eds.; Tetrahedron Symposium-in-Print No. 58). (c) EPC-Synthesis of Fluoro-Organic Compounds: Stereochemical Challenges and Biomedicinal Targets; Soloshonok, V. A. Ed.; Wiley: Chichester, scheduled to appear in 1997.
75. Soloshonok, V. A. In Biomedical Frontiers of Fluorine Chemistry; Ojima, I.; McCarthy, J. R.; Welch, J. T. Eds.; ACS Symposium Series 639; American Chemical Society: Washington, D. C., 1996, pp 26-41.
76. Schlosser, M.; Michel, D. Tetrahedron 1996, 52, 99.
77. The state of controversy in this area was best exemplified recently by Schlosser: "All in all, the more results one sees, the more one feels frustrated. Whenever the question about the effective size of fluorine is asked again, a different answer is obtained." See ref. 28.
78. Creary, X. J. Org. Chem. 1987, 52, 5026.
79. Ramachandran, P. V.; Gong, B.; Teodorovic, A. V.; Brown, H. C. Tetrahedron: Asymmetry 1994, 5, 1061.
80. Bonati, F.; Minghetti, G. G. Gazz. Chim. Ital. 1973, 103, 373.
81. Atomic coordinates for the structure (9)have been deposited with the Cambridge Crystallographic Data Centre. The coordinates can be obtained, on request from The Director, Crystallographic Data Centre, 12 Union Road, Cambridge, CB2 1EW, UK.