Resurrecting the cornforth model for carbonyl addition: Studies on the origin of 1,2-asymmetric induction in enolate additions to heteroatom-substituted aldehydes

Angewandte Chemie - International Edition

Volumn 42, Issue 15, 2003, Pages 1761-1765

  Evans, David A ^a   Siska, Sarah J ^a   Cee, Victor J ^a  

^a HARVARD UNIVERSITY   (United States)

Author keywords

Aldol reaction; Reaction mechanisms; Stereoselectivity; Transition states

Indexed keywords

ALDEHYDES; REACTION KINETICS;

ALDOL REACTIONS; CORNFORTH MODEL;

CARBONYLATION;

ALDEHYDE DERIVATIVE; CARBONYL DERIVATIVE;

ADDITION REACTION; ALDOL REACTION; ARTICLE; ASYMMETRIC CATALYSIS; CHEMICAL MODEL; CONJUGATION; CYCLIZATION; EMPIRICISM; PREDICTION; RELIABILITY; STEREOCHEMISTRY; SUBSTITUTION REACTION;

ALDEHYDES; BORON COMPOUNDS; KETONES; LITHIUM COMPOUNDS; MODELS, CHEMICAL; MOLECULAR CONFORMATION; STEREOISOMERISM;

EID: 0037541165     PISSN: 14337851     EISSN: None     Source Type: Journal    
DOI: 10.1002/anie.200350979     Document Type: Article

References (44)

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3. (a) J. W. Cornforth, R. H. Cornforth, K. K. Mathew, J. Chem. Soc. 1959, 112-127; (b) D. J. Cram, D. R. Wilson, J. Am. Chem. Soc. 1963, 85, 1245-1249; (c) G. J. Karabatsos, J. Am. Chem. Soc. 1967, 89, 1367-1371; (d) M. Chérest, H. Felkin, N. Prudent, Tetrahedron Lett. 1968, 2199-2204. For additional discussion of some of these models, see: (e) E. L. Eliel in Asymmetric Synthesis, Vol. 2 (Ed.: J. D. Morrison), Academic Press, New York, 1983, pp. 125-155. For a discussion of electrostatic versus frontier molecular orbital (FMO) effects in carbonyl additions, see: f) B. W. Gung, Tetrahedron 1996, 52, 5263-5301.
4. (a) J. W. Cornforth, R. H. Cornforth, K. K. Mathew, J. Chem. Soc. 1959, 112-127; (b) D. J. Cram, D. R. Wilson, J. Am. Chem. Soc. 1963, 85, 1245-1249; (c) G. J. Karabatsos, J. Am. Chem. Soc. 1967, 89, 1367-1371; (d) M. Chérest, H. Felkin, N. Prudent, Tetrahedron Lett. 1968, 2199-2204. For additional discussion of some of these models, see: (e) E. L. Eliel in Asymmetric Synthesis, Vol. 2 (Ed.: J. D. Morrison), Academic Press, New York, 1983, pp. 125-155. For a discussion of electrostatic versus frontier molecular orbital (FMO) effects in carbonyl additions, see: f) B. W. Gung, Tetrahedron 1996, 52, 5263-5301.
5. (a) J. W. Cornforth, R. H. Cornforth, K. K. Mathew, J. Chem. Soc. 1959, 112-127; (b) D. J. Cram, D. R. Wilson, J. Am. Chem. Soc. 1963, 85, 1245-1249; (c) G. J. Karabatsos, J. Am. Chem. Soc. 1967, 89, 1367-1371; (d) M. Chérest, H. Felkin, N. Prudent, Tetrahedron Lett. 1968, 2199-2204. For additional discussion of some of these models, see: (e) E. L. Eliel in Asymmetric Synthesis, Vol. 2 (Ed.: J. D. Morrison), Academic Press, New York, 1983, pp. 125-155. For a discussion of electrostatic versus frontier molecular orbital (FMO) effects in carbonyl additions, see: f) B. W. Gung, Tetrahedron 1996, 52, 5263-5301.
6. (a) J. W. Cornforth, R. H. Cornforth, K. K. Mathew, J. Chem. Soc. 1959, 112-127; (b) D. J. Cram, D. R. Wilson, J. Am. Chem. Soc. 1963, 85, 1245-1249; (c) G. J. Karabatsos, J. Am. Chem. Soc. 1967, 89, 1367-1371; (d) M. Chérest, H. Felkin, N. Prudent, Tetrahedron Lett. 1968, 2199-2204. For additional discussion of some of these models, see: (e) E. L. Eliel in Asymmetric Synthesis, Vol. 2 (Ed.: J. D. Morrison), Academic Press, New York, 1983, pp. 125-155. For a discussion of electrostatic versus frontier molecular orbital (FMO) effects in carbonyl additions, see: f) B. W. Gung, Tetrahedron 1996, 52, 5263-5301.
7. (a) J. W. Cornforth, R. H. Cornforth, K. K. Mathew, J. Chem. Soc. 1959, 112-127; (b) D. J. Cram, D. R. Wilson, J. Am. Chem. Soc. 1963, 85, 1245-1249; (c) G. J. Karabatsos, J. Am. Chem. Soc. 1967, 89, 1367-1371; (d) M. Chérest, H. Felkin, N. Prudent, Tetrahedron Lett. 1968, 2199-2204. For additional discussion of some of these models, see: (e) E. L. Eliel in Asymmetric Synthesis, Vol. 2 (Ed.: J. D. Morrison), Academic Press, New York, 1983, pp. 125-155. For a discussion of electrostatic versus frontier molecular orbital (FMO) effects in carbonyl additions, see: f) B. W. Gung, Tetrahedron 1996, 52, 5263-5301.
8. (a) J. W. Cornforth, R. H. Cornforth, K. K. Mathew, J. Chem. Soc. 1959, 112-127; (b) D. J. Cram, D. R. Wilson, J. Am. Chem. Soc. 1963, 85, 1245-1249; (c) G. J. Karabatsos, J. Am. Chem. Soc. 1967, 89, 1367-1371; (d) M. Chérest, H. Felkin, N. Prudent, Tetrahedron Lett. 1968, 2199-2204. For additional discussion of some of these models, see: (e) E. L. Eliel in Asymmetric Synthesis, Vol. 2 (Ed.: J. D. Morrison), Academic Press, New York, 1983, pp. 125-155. For a discussion of electrostatic versus frontier molecular orbital (FMO) effects in carbonyl additions, see: f) B. W. Gung, Tetrahedron 1996, 52, 5263-5301.
9. (a) N. T. Anh, O. Eisenstein, Nouv. J. Chim. 1977, 1, 61-70;
10. (b) N. T. Anh, Top. Curr. Chem. 1980, 88, 145-162.
11. For theoretical studies which support the polar Felkin-Anh model, see: a) Y.-D. Wu, K. N. Houk, J. Am. Chem. Soc. 1987, 109, 908-910; (b) S. S. Wong, M. N. Paddon-Row, J. Chem. Soc. Chem. Commun. 1990, 456-458; (c) Y.-D. Wu, J. A. Tucker, K. N. Houk, J. Am. Chem. Soc. 1991, 113, 5018-5027; (d) S. S. Wong, M. N. Paddon-Row, J. Chem. Soc. Chem. Commun. 1991, 327-330; (e) S. S. Wong, M. N. Paddon-Row, Aust. J. Chem. 1991, 44, 765-770; (f) G. Frenking, K. F. Köhler, M. T. Reetz, Tetrahedron 1991, 47, 9005-9018; (h) N. T. Anh, F. Maurel, J.-M. Lefour, New J. Chem. 1995, 19, 353-364.
12. For theoretical studies which support the polar Felkin-Anh model, see: a) Y.-D. Wu, K. N. Houk, J. Am. Chem. Soc. 1987, 109, 908-910; (b) S. S. Wong, M. N. Paddon-Row, J. Chem. Soc. Chem. Commun. 1990, 456-458; (c) Y.-D. Wu, J. A. Tucker, K. N. Houk, J. Am. Chem. Soc. 1991, 113, 5018-5027; (d) S. S. Wong, M. N. Paddon-Row, J. Chem. Soc. Chem. Commun. 1991, 327-330; (e) S. S. Wong, M. N. Paddon-Row, Aust. J. Chem. 1991, 44, 765-770; (f) G. Frenking, K. F. Köhler, M. T. Reetz, Tetrahedron 1991, 47, 9005-9018; (h) N. T. Anh, F. Maurel, J.-M. Lefour, New J. Chem. 1995, 19, 353-364.
13. For theoretical studies which support the polar Felkin-Anh model, see: a) Y.-D. Wu, K. N. Houk, J. Am. Chem. Soc. 1987, 109, 908-910; (b) S. S. Wong, M. N. Paddon-Row, J. Chem. Soc. Chem. Commun. 1990, 456-458; (c) Y.-D. Wu, J. A. Tucker, K. N. Houk, J. Am. Chem. Soc. 1991, 113, 5018-5027; (d) S. S. Wong, M. N. Paddon-Row, J. Chem. Soc. Chem. Commun. 1991, 327-330; (e) S. S. Wong, M. N. Paddon-Row, Aust. J. Chem. 1991, 44, 765-770; (f) G. Frenking, K. F. Köhler, M. T. Reetz, Tetrahedron 1991, 47, 9005-9018; (h) N. T. Anh, F. Maurel, J.-M. Lefour, New J. Chem. 1995, 19, 353-364.
14. For theoretical studies which support the polar Felkin-Anh model, see: a) Y.-D. Wu, K. N. Houk, J. Am. Chem. Soc. 1987, 109, 908-910; (b) S. S. Wong, M. N. Paddon-Row, J. Chem. Soc. Chem. Commun. 1990, 456-458; (c) Y.-D. Wu, J. A. Tucker, K. N. Houk, J. Am. Chem. Soc. 1991, 113, 5018-5027; (d) S. S. Wong, M. N. Paddon-Row, J. Chem. Soc. Chem. Commun. 1991, 327-330; (e) S. S. Wong, M. N. Paddon-Row, Aust. J. Chem. 1991, 44, 765-770; (f) G. Frenking, K. F. Köhler, M. T. Reetz, Tetrahedron 1991, 47, 9005-9018; (h) N. T. Anh, F. Maurel, J.-M. Lefour, New J. Chem. 1995, 19, 353-364.
15. For theoretical studies which support the polar Felkin-Anh model, see: a) Y.-D. Wu, K. N. Houk, J. Am. Chem. Soc. 1987, 109, 908-910; (b) S. S. Wong, M. N. Paddon-Row, J. Chem. Soc. Chem. Commun. 1990, 456-458; (c) Y.-D. Wu, J. A. Tucker, K. N. Houk, J. Am. Chem. Soc. 1991, 113, 5018-5027; (d) S. S. Wong, M. N. Paddon-Row, J. Chem. Soc. Chem. Commun. 1991, 327-330; (e) S. S. Wong, M. N. Paddon-Row, Aust. J. Chem. 1991, 44, 765-770; (f) G. Frenking, K. F. Köhler, M. T. Reetz, Tetrahedron 1991, 47, 9005-9018; (h) N. T. Anh, F. Maurel, J.-M. Lefour, New J. Chem. 1995, 19, 353-364.
16. For theoretical studies which support the polar Felkin-Anh model, see: a) Y.-D. Wu, K. N. Houk, J. Am. Chem. Soc. 1987, 109, 908-910; (b) S. S. Wong, M. N. Paddon-Row, J. Chem. Soc. Chem. Commun. 1990, 456-458; (c) Y.-D. Wu, J. A. Tucker, K. N. Houk, J. Am. Chem. Soc. 1991, 113, 5018-5027; (d) S. S. Wong, M. N. Paddon-Row, J. Chem. Soc. Chem. Commun. 1991, 327-330; (e) S. S. Wong, M. N. Paddon-Row, Aust. J. Chem. 1991, 44, 765-770; (f) G. Frenking, K. F. Köhler, M. T. Reetz, Tetrahedron 1991, 47, 9005-9018; (h) N. T. Anh, F. Maurel, J.-M. Lefour, New J. Chem. 1995, 19, 353-364.
17. (g) G. Frenking, K. F. Köhler, M. T. Reetz, Tetrahedron 1993, 49, 3983-3994;
18. For theoretical studies which support the polar Felkin-Anh model, see: a) Y.-D. Wu, K. N. Houk, J. Am. Chem. Soc. 1987, 109, 908-910; (b) S. S. Wong, M. N. Paddon-Row, J. Chem. Soc. Chem. Commun. 1990, 456-458; (c) Y.-D. Wu, J. A. Tucker, K. N. Houk, J. Am. Chem. Soc. 1991, 113, 5018-5027; (d) S. S. Wong, M. N. Paddon-Row, J. Chem. Soc. Chem. Commun. 1991, 327-330; (e) S. S. Wong, M. N. Paddon-Row, Aust. J. Chem. 1991, 44, 765-770; (f) G. Frenking, K. F. Köhler, M. T. Reetz, Tetrahedron 1991, 47, 9005-9018; (h) N. T. Anh, F. Maurel, J.-M. Lefour, New J. Chem. 1995, 19, 353-364.
19. The Cornforth model has been modified from its original form to incorporate torsional effects and the Bürgi-Dunitz trajectory. For a discussion of torsional effects in transition states, see: M. N. Paddon-Row, N. G. Rondan, K. N. Houk, J. Am. Chem. Soc. 1982, 104, 7162-7166.
20. Addition of cyanide anion to fluoroacetaldehyde in solution: a) A. S. Cieplak, K. B. Wiberg, J. Am. Chem. Soc. 1992, 114, 9226-9227. Addition of cyanide anion to chlorofluoroacetaldehyde in the gas phase: (b) G. Frenking, K. F. Köhler, M. T. Reetz, Tetrahedron 1994, 50, 11197-11204. Allylboration of α-methoxypropanal in the gas phase: (c) B. W. Gung, X. Xue, Tetrahedron: Asymmetry 2001, 12, 2955-2959.
21. Addition of cyanide anion to fluoroacetaldehyde in solution: a) A. S. Cieplak, K. B. Wiberg, J. Am. Chem. Soc. 1992, 114, 9226-9227. Addition of cyanide anion to chlorofluoroacetaldehyde in the gas phase: (b) G. Frenking, K. F. Köhler, M. T. Reetz, Tetrahedron 1994, 50, 11197-11204. Allylboration of α-methoxypropanal in the gas phase: (c) B. W. Gung, X. Xue, Tetrahedron: Asymmetry 2001, 12, 2955-2959.
22. Addition of cyanide anion to fluoroacetaldehyde in solution: a) A. S. Cieplak, K. B. Wiberg, J. Am. Chem. Soc. 1992, 114, 9226-9227. Addition of cyanide anion to chlorofluoroacetaldehyde in the gas phase: (b) G. Frenking, K. F. Köhler, M. T. Reetz, Tetrahedron 1994, 50, 11197-11204. Allylboration of α-methoxypropanal in the gas phase: (c) B. W. Gung, X. Xue, Tetrahedron: Asymmetry 2001, 12, 2955-2959.
23. (a) R. W. Hoffmann, Chem. Scr. 1985, 25, 53-60 (Sp. Iss.);
24. (b) W. R. Roush, M. A. Adam, A. E. Walts, D. J. Harris, J. Am. Chem. Soc. 1986, 108, 3422-3434;
25. (c) R.W. Hoffmann, R. Metternich, J. W. Lanz, Liebigs Ann. Chem. 1987, 881-887;
26. (d) H. Brinkmann, R. W. Hoffmann, Chem. Ber. 1990, 123, 2395-2401.
27. (a) D. A. Evans, J. V. Nelson, E. Vogel, T. R. Taber, J. Am. Chem. Soc. 1981, 103, 3099-3111. For an extensive review of boron enolate based aldol additions see: (b) C. J. Cowden, I. Paterson, Org. React. 1997, 51, 1-200.
28. (a) D. A. Evans, J. V. Nelson, E. Vogel, T. R. Taber, J. Am. Chem. Soc. 1981, 103, 3099-3111. For an extensive review of boron enolate based aldol additions see: (b) C. J. Cowden, I. Paterson, Org. React. 1997, 51, 1-200.
29. This effect is well known for α-methyl chiral aldehydes. See: a) D. A. Evans, J. V. Nelson, T. R. Taber, Top. Stereochem. 1982, 13, 1-115; (b) W. R. Roush, J. Org. Chem. 1991, 56, 4151-4157.
30. This effect is well known for α-methyl chiral aldehydes. See: a) D. A. Evans, J. V. Nelson, T. R. Taber, Top. Stereochem. 1982, 13, 1-115; (b) W. R. Roush, J. Org. Chem. 1991, 56, 4151-4157.
31. Boron enolates: a) H. C. Brown, K. Ganesan, R. K. Dhar, J. Org. Chem. 1993, 58, 147-153; Z lithium enolate: (b) C. H. Heathcock, C. T. Buse, W. A. Kleschick, M. C. Pirrung, J. E. Sohn, J. Lampe, J. Org. Chem. 1980, 45, 1066-1081; E lithium enolate: (c) L. Xie, K. M. Isenberger, G. Held, L. M. Dahl, J. Org. Chem. 1997, 62, 7516-7519.
32. Boron enolates: a) H. C. Brown, K. Ganesan, R. K. Dhar, J. Org. Chem. 1993, 58, 147-153; Z lithium enolate: (b) C. H. Heathcock, C. T. Buse, W. A. Kleschick, M. C. Pirrung, J. E. Sohn, J. Lampe, J. Org. Chem. 1980, 45, 1066-1081; E lithium enolate: (c) L. Xie, K. M. Isenberger, G. Held, L. M. Dahl, J. Org. Chem. 1997, 62, 7516-7519.
33. Boron enolates: a) H. C. Brown, K. Ganesan, R. K. Dhar, J. Org. Chem. 1993, 58, 147-153; Z lithium enolate: (b) C. H. Heathcock, C. T. Buse, W. A. Kleschick, M. C. Pirrung, J. E. Sohn, J. Lampe, J. Org. Chem. 1980, 45, 1066-1081; E lithium enolate: (c) L. Xie, K. M. Isenberger, G. Held, L. M. Dahl, J. Org. Chem. 1997, 62, 7516-7519.
34. 2B enolates of cyclohexanone give similar results, indicating that enolate configuration is primarily responsible for the selectivity difference.
35. Lithium enolates: a) C. H. Heathcock, S. D. Young, J. P. Hagen, M. C. Pirrung, C. T. White, D. VanDerveer, J. Org. Chem. 1980, 45, 3846-3856. Boron enolates: (b) C. Gennari, A. Bernardi, S. Cardani, C. Scolastico, Tetrahedron 1984, 40; 4059-4065; (c) D. R. Williams, J.L. Moore, M. Yamada, J. Org. Chem. 1986, 51, 3916-3918. Titanium enolates: (d) R. Annunziata, M. Cinquini, F. Cozzi, P. G. Cozzi, E. Consolandi, Tetrahedron 1991, 47, 7897-7910; (e) C. Esteve, M. Ferreró, P. Romea, F. Urpí, J. Vilarrasa, Tetrahedron Lett. 1999, 40, 5079-5082. Samarium enolates: (f) L. Lu, H.-Y. Chang, J.-M. Fang, J. Org. Chem. 1999, 64, 843-853.
36. Lithium enolates: a) C. H. Heathcock, S. D. Young, J. P. Hagen, M. C. Pirrung, C. T. White, D. VanDerveer, J. Org. Chem. 1980, 45, 3846-3856. Boron enolates: (b) C. Gennari, A. Bernardi, S. Cardani, C. Scolastico, Tetrahedron 1984, 40; 4059-4065; (c) D. R. Williams, J.L. Moore, M. Yamada, J. Org. Chem. 1986, 51, 3916-3918. Titanium enolates: (d) R. Annunziata, M. Cinquini, F. Cozzi, P. G. Cozzi, E. Consolandi, Tetrahedron 1991, 47, 7897-7910; (e) C. Esteve, M. Ferreró, P. Romea, F. Urpí, J. Vilarrasa, Tetrahedron Lett. 1999, 40, 5079-5082. Samarium enolates: (f) L. Lu, H.-Y. Chang, J.-M. Fang, J. Org. Chem. 1999, 64, 843-853.
37. Lithium enolates: a) C. H. Heathcock, S. D. Young, J. P. Hagen, M. C. Pirrung, C. T. White, D. VanDerveer, J. Org. Chem. 1980, 45, 3846-3856. Boron enolates: (b) C. Gennari, A. Bernardi, S. Cardani, C. Scolastico, Tetrahedron 1984, 40; 4059-4065; (c) D. R. Williams, J.L. Moore, M. Yamada, J. Org. Chem. 1986, 51, 3916-3918. Titanium enolates: (d) R. Annunziata, M. Cinquini, F. Cozzi, P. G. Cozzi, E. Consolandi, Tetrahedron 1991, 47, 7897-7910; (e) C. Esteve, M. Ferreró, P. Romea, F. Urpí, J. Vilarrasa, Tetrahedron Lett. 1999, 40, 5079-5082. Samarium enolates: (f) L. Lu, H.-Y. Chang, J.-M. Fang, J. Org. Chem. 1999, 64, 843-853.
38. Lithium enolates: a) C. H. Heathcock, S. D. Young, J. P. Hagen, M. C. Pirrung, C. T. White, D. VanDerveer, J. Org. Chem. 1980, 45, 3846-3856. Boron enolates: (b) C. Gennari, A. Bernardi, S. Cardani, C. Scolastico, Tetrahedron 1984, 40; 4059-4065; (c) D. R. Williams, J.L. Moore, M. Yamada, J. Org. Chem. 1986, 51, 3916-3918. Titanium enolates: (d) R. Annunziata, M. Cinquini, F. Cozzi, P. G. Cozzi, E. Consolandi, Tetrahedron 1991, 47, 7897-7910; (e) C. Esteve, M. Ferreró, P. Romea, F. Urpí, J. Vilarrasa, Tetrahedron Lett. 1999, 40, 5079-5082. Samarium enolates: (f) L. Lu, H.-Y. Chang, J.-M. Fang, J. Org. Chem. 1999, 64, 843-853.
39. Lithium enolates: a) C. H. Heathcock, S. D. Young, J. P. Hagen, M. C. Pirrung, C. T. White, D. VanDerveer, J. Org. Chem. 1980, 45, 3846-3856. Boron enolates: (b) C. Gennari, A. Bernardi, S. Cardani, C. Scolastico, Tetrahedron 1984, 40; 4059-4065; (c) D. R. Williams, J.L. Moore, M. Yamada, J. Org. Chem. 1986, 51, 3916-3918. Titanium enolates: (d) R. Annunziata, M. Cinquini, F. Cozzi, P. G. Cozzi, E. Consolandi, Tetrahedron 1991, 47, 7897-7910; (e) C. Esteve, M. Ferreró, P. Romea, F. Urpí, J. Vilarrasa, Tetrahedron Lett. 1999, 40, 5079-5082. Samarium enolates: (f) L. Lu, H.-Y. Chang, J.-M. Fang, J. Org. Chem. 1999, 64, 843-853.
40. Lithium enolates: a) C. H. Heathcock, S. D. Young, J. P. Hagen, M. C. Pirrung, C. T. White, D. VanDerveer, J. Org. Chem. 1980, 45, 3846-3856. Boron enolates: (b) C. Gennari, A. Bernardi, S. Cardani, C. Scolastico, Tetrahedron 1984, 40; 4059-4065; (c) D. R. Williams, J.L. Moore, M. Yamada, J. Org. Chem. 1986, 51, 3916-3918. Titanium enolates: (d) R. Annunziata, M. Cinquini, F. Cozzi, P. G. Cozzi, E. Consolandi, Tetrahedron 1991, 47, 7897-7910; (e) C. Esteve, M. Ferreró, P. Romea, F. Urpí, J. Vilarrasa, Tetrahedron Lett. 1999, 40, 5079-5082. Samarium enolates: (f) L. Lu, H.-Y. Chang, J.-M. Fang, J. Org. Chem. 1999, 64, 843-853.
41. 2 is reported to proceed with good to excellent selectivity for the 2,3-anti, 3,4-anti diastereomer. See: a) S. F. Martin, W. Li, J. Org. Chem. 1989, 54, 6129-6133; (b) J. Mulzer, L. Kattner, A. R. Strecker, C. Schröder, J. Buschmann, C. Lehmann, P. Luger, J. Am. Chem. Soc. 1991, 113, 4218-4229.
42. 2 is reported to proceed with good to excellent selectivity for the 2,3-anti, 3,4-anti diastereomer. See: a) S. F. Martin, W. Li, J. Org. Chem. 1989, 54, 6129-6133; (b) J. Mulzer, L. Kattner, A. R. Strecker, C. Schröder, J. Buschmann, C. Lehmann, P. Luger, J. Am. Chem. Soc. 1991, 113, 4218-4229.
43. The matched/mismatched relationship between Z and E chiral γchloroallylborane reagents and α-chiral oxygen-substituted aldehydes is also consistent with Cornforth transition states (Figure 1, III and IV). See: S. Hu, S. Jayaraman, A.C. Oehlschlager, J. Org. Chem. 1998, 63, 8843-8849.
44. Abbreviations: PFA = polar Felkin-Anh; DIPEA = diisopropylethylamine; 9-BBNOTf = 9-borabicyclo[3.3.1]nonyl trifluoromethanesulfonate; TEA = triethylamine; LiHMDS = lithium hexamethyldisilazide; TMS = trimethylsilyl; THF = tetrahydrofuran; Bn = benzyl; TBS = tert-butyldimethylsilyl; pyr=pyridine; LDA = lithium diisopropylamide; TBDPS = tert-butyldiphenylsilyl.