α-Trialkylsilyl-substituted α-amino acids

Angewandte Chemie - International Edition

Volumn 39, Issue 13, 2000, Pages 2288-2290

  Bolm, Carsten ^a   Kasyan, Andrey ^a   Drauz, Karlheinz ^b   Günther, Kurt ^c   Raabe, Gerhard ^a  

^a RWTH AACHEN UNIVERSITY   (Germany)

^b EVONIK INDUSTRIES AG   (Germany)

^c Analytisch Technische Services   (Germany)

Author keywords

Amino acids; Insertions; Peptides; Rhodium; Silanes

Indexed keywords

ALKYL GROUP; ALPHA AMINO ACID;

AMINO ACID SYNTHESIS; ARTICLE; CATALYSIS; CHEMICAL REACTION; CHEMICAL STRUCTURE; ENANTIOMER; REACTION ANALYSIS; STEREOCHEMISTRY;

EID: 0034600893     PISSN: 14337851     EISSN: None     Source Type: Journal    
DOI: 10.1002/1521-3773(20000703)39:13<2288::AID-ANIE2288>3.0.CO;2-Y     Document Type: Article

References (55)

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12. For examples, see: a) R. J. Andersen, J. E. Coleman, E. Piers, D. J. Wallace, Tetrahedron Lett. 1997, 38, 317; b) P. Ettmayer, M. Huhner, A. Billich, B. Rosenwirth, H. Gstach. Bioorg. Med. Chem. Lett. 1994. 4, 2851; c) K. Paulvannan, T. Chen. Synlett 1999, 1371; d) G. Bold, A. Fässler, H.-G. Capraro, R. Cozens, T. Klimkait, J. Lazdins, J. Mestan, B. Poncioni, J. Rösel, D. Strover, M. Titelnot-Blomley, F. Acemoglu, W. Beck, H. Boss, M. Eschbach, T. Hürlimann, E. Masso, S. Roussel, K. Ucci-Stoll, D. Wyss, M. Lang, J. Med. Chem. 1998, 41, 3387; e) K. Paulvannan, T. Chen. Synlett 1999, 1371.
13. For examples, see: a) R. J. Andersen, J. E. Coleman, E. Piers, D. J. Wallace, Tetrahedron Lett. 1997, 38, 317; b) P. Ettmayer, M. Huhner, A. Billich, B. Rosenwirth, H. Gstach. Bioorg. Med. Chem. Lett. 1994. 4, 2851; c) K. Paulvannan, T. Chen. Synlett 1999, 1371; d) G. Bold, A. Fässler, H.-G. Capraro, R. Cozens, T. Klimkait, J. Lazdins, J. Mestan, B. Poncioni, J. Rösel, D. Strover, M. Titelnot-Blomley, F. Acemoglu, W. Beck, H. Boss, M. Eschbach, T. Hürlimann, E. Masso, S. Roussel, K. Ucci-Stoll, D. Wyss, M. Lang, J. Med. Chem. 1998, 41, 3387; e) K. Paulvannan, T. Chen. Synlett 1999, 1371.
14. For examples, see: a) R. J. Andersen, J. E. Coleman, E. Piers, D. J. Wallace, Tetrahedron Lett. 1997, 38, 317; b) P. Ettmayer, M. Huhner, A. Billich, B. Rosenwirth, H. Gstach. Bioorg. Med. Chem. Lett. 1994. 4, 2851; c) K. Paulvannan, T. Chen. Synlett 1999, 1371; d) G. Bold, A. Fässler, H.-G. Capraro, R. Cozens, T. Klimkait, J. Lazdins, J. Mestan, B. Poncioni, J. Rösel, D. Strover, M. Titelnot-Blomley, F. Acemoglu, W. Beck, H. Boss, M. Eschbach, T. Hürlimann, E. Masso, S. Roussel, K. Ucci-Stoll, D. Wyss, M. Lang, J. Med. Chem. 1998, 41, 3387; e) K. Paulvannan, T. Chen. Synlett 1999, 1371.
15. For examples, see: a) R. J. Andersen, J. E. Coleman, E. Piers, D. J. Wallace, Tetrahedron Lett. 1997, 38, 317; b) P. Ettmayer, M. Huhner, A. Billich, B. Rosenwirth, H. Gstach. Bioorg. Med. Chem. Lett. 1994. 4, 2851; c) K. Paulvannan, T. Chen. Synlett 1999, 1371; d) G. Bold, A. Fässler, H.-G. Capraro, R. Cozens, T. Klimkait, J. Lazdins, J. Mestan, B. Poncioni, J. Rösel, D. Strover, M. Titelnot-Blomley, F. Acemoglu, W. Beck, H. Boss, M. Eschbach, T. Hürlimann, E. Masso, S. Roussel, K. Ucci-Stoll, D. Wyss, M. Lang, J. Med. Chem. 1998, 41, 3387; e) K. Paulvannan, T. Chen. Synlett 1999, 1371.
16. For examples, see: a) R. J. Andersen, J. E. Coleman, E. Piers, D. J. Wallace, Tetrahedron Lett. 1997, 38, 317; b) P. Ettmayer, M. Huhner, A. Billich, B. Rosenwirth, H. Gstach. Bioorg. Med. Chem. Lett. 1994. 4, 2851; c) K. Paulvannan, T. Chen. Synlett 1999, 1371; d) G. Bold, A. Fässler, H.-G. Capraro, R. Cozens, T. Klimkait, J. Lazdins, J. Mestan, B. Poncioni, J. Rösel, D. Strover, M. Titelnot-Blomley, F. Acemoglu, W. Beck, H. Boss, M. Eschbach, T. Hürlimann, E. Masso, S. Roussel, K. Ucci-Stoll, D. Wyss, M. Lang, J. Med. Chem. 1998, 41, 3387; e) K. Paulvannan, T. Chen. Synlett 1999, 1371.
17. Reviews: a) A. Studer, Synthesis 1996, 793; b) H. Waldmann, Synlett 1995, 133: c) D. J. Ager, I. Prakash, D. R. Schaad, Chem. Rev. 1996, 96, 835.
18. Reviews: a) A. Studer, Synthesis 1996, 793; b) H. Waldmann, Synlett 1995, 133: c) D. J. Ager, I. Prakash, D. R. Schaad, Chem. Rev. 1996, 96, 835.
19. Reviews: a) A. Studer, Synthesis 1996, 793; b) H. Waldmann, Synlett 1995, 133: c) D. J. Ager, I. Prakash, D. R. Schaad, Chem. Rev. 1996, 96, 835.
20. Further recent examples of successful asymmetric metal catalyses involving ligands based on tert-leucine: a) C. Bolm, K. Muñiz, J. P. Hildebrand, Org. Lett. 1999, 1, 491; b) C. Bolm, K. Muñiz-Fernandez, A. Seger, G. Raabe, K. Günther, J. Org. Chem. 1998, 63, 7860; c) C. Bolm, F. Bienewald, Synlett 1998, 1327; d) C. Bolm, T. K. K. Luong, G. Schlingloff, Synlett 1997, 1151; e) A. Lightfoot, P. Schnider, A. Plaltz, Angew. Chem. 1998, 110, 3047; Angew. Chem. Int. Ed. 1998, 37, 2897; f) M. Peer, J. C. de Jong, M. Kiefer, T. Langer, H. Rieck, II. Schell, P. Sennhenn, J. Sprinz, H. Steinhagen, B. Wiese, G. Helmchen, Tetrahedron 1996, 52, 7547; g) S. Kudis, G. Helmchen, Angew. Chem. 1998, 110, 3047; Angew. Chem. Int. Ed. 1998, 37, 3047; h) D. A. Evans, T. Rovis, M. C. Kozlowski, J. S. Tedrow, J. Am. Chem. Soc. 1999, 121, 1994.
21. Further recent examples of successful asymmetric metal catalyses involving ligands based on tert-leucine: a) C. Bolm, K. Muñiz, J. P. Hildebrand, Org. Lett. 1999, 1, 491; b) C. Bolm, K. Muñiz-Fernandez, A. Seger, G. Raabe, K. Günther, J. Org. Chem. 1998, 63, 7860; c) C. Bolm, F. Bienewald, Synlett 1998, 1327; d) C. Bolm, T. K. K. Luong, G. Schlingloff, Synlett 1997, 1151; e) A. Lightfoot, P. Schnider, A. Plaltz, Angew. Chem. 1998, 110, 3047; Angew. Chem. Int. Ed. 1998, 37, 2897; f) M. Peer, J. C. de Jong, M. Kiefer, T. Langer, H. Rieck, II. Schell, P. Sennhenn, J. Sprinz, H. Steinhagen, B. Wiese, G. Helmchen, Tetrahedron 1996, 52, 7547; g) S. Kudis, G. Helmchen, Angew. Chem. 1998, 110, 3047; Angew. Chem. Int. Ed. 1998, 37, 3047; h) D. A. Evans, T. Rovis, M. C. Kozlowski, J. S. Tedrow, J. Am. Chem. Soc. 1999, 121, 1994.
22. Further recent examples of successful asymmetric metal catalyses involving ligands based on tert-leucine: a) C. Bolm, K. Muñiz, J. P. Hildebrand, Org. Lett. 1999, 1, 491; b) C. Bolm, K. Muñiz-Fernandez, A. Seger, G. Raabe, K. Günther, J. Org. Chem. 1998, 63, 7860; c) C. Bolm, F. Bienewald, Synlett 1998, 1327; d) C. Bolm, T. K. K. Luong, G. Schlingloff, Synlett 1997, 1151; e) A. Lightfoot, P. Schnider, A. Plaltz, Angew. Chem. 1998, 110, 3047; Angew. Chem. Int. Ed. 1998, 37, 2897; f) M. Peer, J. C. de Jong, M. Kiefer, T. Langer, H. Rieck, II. Schell, P. Sennhenn, J. Sprinz, H. Steinhagen, B. Wiese, G. Helmchen, Tetrahedron 1996, 52, 7547; g) S. Kudis, G. Helmchen, Angew. Chem. 1998, 110, 3047; Angew. Chem. Int. Ed. 1998, 37, 3047; h) D. A. Evans, T. Rovis, M. C. Kozlowski, J. S. Tedrow, J. Am. Chem. Soc. 1999, 121, 1994.
23. Further recent examples of successful asymmetric metal catalyses involving ligands based on tert-leucine: a) C. Bolm, K. Muñiz, J. P. Hildebrand, Org. Lett. 1999, 1, 491; b) C. Bolm, K. Muñiz-Fernandez, A. Seger, G. Raabe, K. Günther, J. Org. Chem. 1998, 63, 7860; c) C. Bolm, F. Bienewald, Synlett 1998, 1327; d) C. Bolm, T. K. K. Luong, G. Schlingloff, Synlett 1997, 1151; e) A. Lightfoot, P. Schnider, A. Plaltz, Angew. Chem. 1998, 110, 3047; Angew. Chem. Int. Ed. 1998, 37, 2897; f) M. Peer, J. C. de Jong, M. Kiefer, T. Langer, H. Rieck, II. Schell, P. Sennhenn, J. Sprinz, H. Steinhagen, B. Wiese, G. Helmchen, Tetrahedron 1996, 52, 7547; g) S. Kudis, G. Helmchen, Angew. Chem. 1998, 110, 3047; Angew. Chem. Int. Ed. 1998, 37, 3047; h) D. A. Evans, T. Rovis, M. C. Kozlowski, J. S. Tedrow, J. Am. Chem. Soc. 1999, 121, 1994.
24. Further recent examples of successful asymmetric metal catalyses involving ligands based on tert-leucine: a) C. Bolm, K. Muñiz, J. P. Hildebrand, Org. Lett. 1999, 1, 491; b) C. Bolm, K. Muñiz-Fernandez, A. Seger, G. Raabe, K. Günther, J. Org. Chem. 1998, 63, 7860; c) C. Bolm, F. Bienewald, Synlett 1998, 1327; d) C. Bolm, T. K. K. Luong, G. Schlingloff, Synlett 1997, 1151; e) A. Lightfoot, P. Schnider, A. Plaltz, Angew. Chem. 1998, 110, 3047; Angew. Chem. Int. Ed. 1998, 37, 2897; f) M. Peer, J. C. de Jong, M. Kiefer, T. Langer, H. Rieck, II. Schell, P. Sennhenn, J. Sprinz, H. Steinhagen, B. Wiese, G. Helmchen, Tetrahedron 1996, 52, 7547; g) S. Kudis, G. Helmchen, Angew. Chem. 1998, 110, 3047; Angew. Chem. Int. Ed. 1998, 37, 3047; h) D. A. Evans, T. Rovis, M. C. Kozlowski, J. S. Tedrow, J. Am. Chem. Soc. 1999, 121, 1994.
25. Further recent examples of successful asymmetric metal catalyses involving ligands based on tert-leucine: a) C. Bolm, K. Muñiz, J. P. Hildebrand, Org. Lett. 1999, 1, 491; b) C. Bolm, K. Muñiz-Fernandez, A. Seger, G. Raabe, K. Günther, J. Org. Chem. 1998, 63, 7860; c) C. Bolm, F. Bienewald, Synlett 1998, 1327; d) C. Bolm, T. K. K. Luong, G. Schlingloff, Synlett 1997, 1151; e) A. Lightfoot, P. Schnider, A. Plaltz, Angew. Chem. 1998, 110, 3047; Angew. Chem. Int. Ed. 1998, 37, 2897; f) M. Peer, J. C. de Jong, M. Kiefer, T. Langer, H. Rieck, II. Schell, P. Sennhenn, J. Sprinz, H. Steinhagen, B. Wiese, G. Helmchen, Tetrahedron 1996, 52, 7547; g) S. Kudis, G. Helmchen, Angew. Chem. 1998, 110, 3047; Angew. Chem. Int. Ed. 1998, 37, 3047; h) D. A. Evans, T. Rovis, M. C. Kozlowski, J. S. Tedrow, J. Am. Chem. Soc. 1999, 121, 1994.
26. Further recent examples of successful asymmetric metal catalyses involving ligands based on tert-leucine: a) C. Bolm, K. Muñiz, J. P. Hildebrand, Org. Lett. 1999, 1, 491; b) C. Bolm, K. Muñiz-Fernandez, A. Seger, G. Raabe, K. Günther, J. Org. Chem. 1998, 63, 7860; c) C. Bolm, F. Bienewald, Synlett 1998, 1327; d) C. Bolm, T. K. K. Luong, G. Schlingloff, Synlett 1997, 1151; e) A. Lightfoot, P. Schnider, A. Plaltz, Angew. Chem. 1998, 110, 3047; Angew. Chem. Int. Ed. 1998, 37, 2897; f) M. Peer, J. C. de Jong, M. Kiefer, T. Langer, H. Rieck, II. Schell, P. Sennhenn, J. Sprinz, H. Steinhagen, B. Wiese, G. Helmchen, Tetrahedron 1996, 52, 7547; g) S. Kudis, G. Helmchen, Angew. Chem. 1998, 110, 3047; Angew. Chem. Int. Ed. 1998, 37, 3047; h) D. A. Evans, T. Rovis, M. C. Kozlowski, J. S. Tedrow, J. Am. Chem. Soc. 1999, 121, 1994.
27. Further recent examples of successful asymmetric metal catalyses involving ligands based on tert-leucine: a) C. Bolm, K. Muñiz, J. P. Hildebrand, Org. Lett. 1999, 1, 491; b) C. Bolm, K. Muñiz-Fernandez, A. Seger, G. Raabe, K. Günther, J. Org. Chem. 1998, 63, 7860; c) C. Bolm, F. Bienewald, Synlett 1998, 1327; d) C. Bolm, T. K. K. Luong, G. Schlingloff, Synlett 1997, 1151; e) A. Lightfoot, P. Schnider, A. Plaltz, Angew. Chem. 1998, 110, 3047; Angew. Chem. Int. Ed. 1998, 37, 2897; f) M. Peer, J. C. de Jong, M. Kiefer, T. Langer, H. Rieck, II. Schell, P. Sennhenn, J. Sprinz, H. Steinhagen, B. Wiese, G. Helmchen, Tetrahedron 1996, 52, 7547; g) S. Kudis, G. Helmchen, Angew. Chem. 1998, 110, 3047; Angew. Chem. Int. Ed. 1998, 37, 3047; h) D. A. Evans, T. Rovis, M. C. Kozlowski, J. S. Tedrow, J. Am. Chem. Soc. 1999, 121, 1994.
28. Further recent examples of successful asymmetric metal catalyses involving ligands based on tert-leucine: a) C. Bolm, K. Muñiz, J. P. Hildebrand, Org. Lett. 1999, 1, 491; b) C. Bolm, K. Muñiz-Fernandez, A. Seger, G. Raabe, K. Günther, J. Org. Chem. 1998, 63, 7860; c) C. Bolm, F. Bienewald, Synlett 1998, 1327; d) C. Bolm, T. K. K. Luong, G. Schlingloff, Synlett 1997, 1151; e) A. Lightfoot, P. Schnider, A. Plaltz, Angew. Chem. 1998, 110, 3047; Angew. Chem. Int. Ed. 1998, 37, 2897; f) M. Peer, J. C. de Jong, M. Kiefer, T. Langer, H. Rieck, II. Schell, P. Sennhenn, J. Sprinz, H. Steinhagen, B. Wiese, G. Helmchen, Tetrahedron 1996, 52, 7547; g) S. Kudis, G. Helmchen, Angew. Chem. 1998, 110, 3047; Angew. Chem. Int. Ed. 1998, 37, 3047; h) D. A. Evans, T. Rovis, M. C. Kozlowski, J. S. Tedrow, J. Am. Chem. Soc. 1999, 121, 1994.
29. Further recent examples of successful asymmetric metal catalyses involving ligands based on tert-leucine: a) C. Bolm, K. Muñiz, J. P. Hildebrand, Org. Lett. 1999, 1, 491; b) C. Bolm, K. Muñiz-Fernandez, A. Seger, G. Raabe, K. Günther, J. Org. Chem. 1998, 63, 7860; c) C. Bolm, F. Bienewald, Synlett 1998, 1327; d) C. Bolm, T. K. K. Luong, G. Schlingloff, Synlett 1997, 1151; e) A. Lightfoot, P. Schnider, A. Plaltz, Angew. Chem. 1998, 110, 3047; Angew. Chem. Int. Ed. 1998, 37, 2897; f) M. Peer, J. C. de Jong, M. Kiefer, T. Langer, H. Rieck, II. Schell, P. Sennhenn, J. Sprinz, H. Steinhagen, B. Wiese, G. Helmchen, Tetrahedron 1996, 52, 7547; g) S. Kudis, G. Helmchen, Angew. Chem. 1998, 110, 3047; Angew. Chem. Int. Ed. 1998, 37, 3047; h) D. A. Evans, T. Rovis, M. C. Kozlowski, J. S. Tedrow, J. Am. Chem. Soc. 1999, 121, 1994.
30. A fully protected ethyl α-trimethylsilyl-α-amino ester was identified as minor isomer in a product mixture that was obtained by reaction of a lithiated aminosilane with ethyl chloroformate. S. McSieburth, J. J. Somers, H. K. O'Hare Tetrahedron 1996, 52, 5669.
31. For the synthesis of benzyl α-diazoacetate, which - like the corresponding, commercially available ethyl ester - can be silylated, see: S. R. Angle, D. S. Bernier, N. A. El-Said, D. E. Jones, S. Z. Shaw, Tetrahedron Lett. 1998, 39, 3919.
32. a) M. Martin, Synth. Commun. 1983, 13, 809;
33. b) A. Fronda, F. Krebs. B. Daucher, T. Erle, G. Maas, J. Organomet. Chem. 1992, 424, 253;
34. c) G. Maas, S. Bender, Synthesis 1999, 1175;
35. d) general overview of diazocarbonyl chemistry: T. Ye, M. A. McKervey, Chem. Rev. 1994, 94, 1091.
36. Reviews on silyl-substituted diazocarbonyl compounds, α-silylcarbenes and -carbenoids: a) G. Maas in The Chemistry of Organic Silicon Compounds, Vol. 2, Part 1 (Eds.: Z. Rappoport, Y. Apeloig), Wiley, Chichester, 1998, chap. 13, p. 703; b) H. Tomioka, Methoden Org. Chem. (Houben Weyl), 4th ed., 1952-, Vol. E 19b, 1989, p. 1410.
37. Reviews on silyl-substituted diazocarbonyl compounds, α-silylcarbenes and -carbenoids: a) G. Maas in The Chemistry of Organic Silicon Compounds, Vol. 2, Part 1 (Eds.: Z. Rappoport, Y. Apeloig), Wiley, Chichester, 1998, chap. 13, p. 703; b) H. Tomioka, Methoden Org. Chem. (Houben Weyl), 4th ed., 1952-, Vol. E 19b, 1989, p. 1410.
38. Examples of other Rh-catalyzed reactions of α-silyl-α-diazoacetates: a) G. Maas, M. Gimmy, M. Alt, Organometallics 1992, 11, 3813; b) V. Gettwert, F. Krebs, G. Maas, Eur. J. Org. Chem. 1999, 1213; c) S. N. Kablean, S. P. Marsden, A. M. Craig, Tetrahedron Lett. 1998, 39, 5109; d) S. P. Marsden, W.-K. Pang, Tetrahedron Lett. 1998, 39, 6077; e) S. P. Marsden, E.-K. Pang, Chem. Commun. 1999, 1199.
39. Examples of other Rh-catalyzed reactions of α-silyl-α-diazoacetates: a) G. Maas, M. Gimmy, M. Alt, Organometallics 1992, 11, 3813; b) V. Gettwert, F. Krebs, G. Maas, Eur. J. Org. Chem. 1999, 1213; c) S. N. Kablean, S. P. Marsden, A. M. Craig, Tetrahedron Lett. 1998, 39, 5109; d) S. P. Marsden, W.-K. Pang, Tetrahedron Lett. 1998, 39, 6077; e) S. P. Marsden, E.-K. Pang, Chem. Commun. 1999, 1199.
40. Examples of other Rh-catalyzed reactions of α-silyl-α-diazoacetates: a) G. Maas, M. Gimmy, M. Alt, Organometallics 1992, 11, 3813; b) V. Gettwert, F. Krebs, G. Maas, Eur. J. Org. Chem. 1999, 1213; c) S. N. Kablean, S. P. Marsden, A. M. Craig, Tetrahedron Lett. 1998, 39, 5109; d) S. P. Marsden, W.-K. Pang, Tetrahedron Lett. 1998, 39, 6077; e) S. P. Marsden, E.-K. Pang, Chem. Commun. 1999, 1199.
41. Examples of other Rh-catalyzed reactions of α-silyl-α-diazoacetates: a) G. Maas, M. Gimmy, M. Alt, Organometallics 1992, 11, 3813; b) V. Gettwert, F. Krebs, G. Maas, Eur. J. Org. Chem. 1999, 1213; c) S. N. Kablean, S. P. Marsden, A. M. Craig, Tetrahedron Lett. 1998, 39, 5109; d) S. P. Marsden, W.-K. Pang, Tetrahedron Lett. 1998, 39, 6077; e) S. P. Marsden, E.-K. Pang, Chem. Commun. 1999, 1199.
42. Examples of other Rh-catalyzed reactions of α-silyl-α-diazoacetates: a) G. Maas, M. Gimmy, M. Alt, Organometallics 1992, 11, 3813; b) V. Gettwert, F. Krebs, G. Maas, Eur. J. Org. Chem. 1999, 1213; c) S. N. Kablean, S. P. Marsden, A. M. Craig, Tetrahedron Lett. 1998, 39, 5109; d) S. P. Marsden, W.-K. Pang, Tetrahedron Lett. 1998, 39, 6077; e) S. P. Marsden, E.-K. Pang, Chem. Commun. 1999, 1199.
43. Simple trialkylsilylcarbenes do not react under N,H-insertion: a) R. L. Kreeger, H. Shechter, Tetrahedron Lett. 1975, 2061; b) in ref. [10b], p. 1417.
44. max: 45%): C. F. García, M. A. McKervey, T. Ye, Chem. Commun. 1996, 1465.
45. max: 45%): C. F. García, M. A. McKervey, T. Ye, Chem. Commun. 1996, 1465.
46. max: 45%): C. F. García, M. A. McKervey, T. Ye, Chem. Commun. 1996, 1465.
47. max: 45%): C. F. García, M. A. McKervey, T. Ye, Chem. Commun. 1996, 1465.
48. max: 45%): C. F. García, M. A. McKervey, T. Ye, Chem. Commun. 1996, 1465.
49. max: 45%): C. F. García, M. A. McKervey, T. Ye, Chem. Commun. 1996, 1465.
50. max: 45%): C. F. García, M. A. McKervey, T. Ye, Chem. Commun. 1996, 1465.
51. Crystallographic data (excluding structure factors) for the structures reported in this paper have heen deposited with the Cambridge Crystallographic Data Centre as supplementary publication no. CCDC-135676 (rac-4a) and -135677 ((S)-4a). Copies of the data can he obtained free of charge on application to CCDC, 12 Union Road. Cambridge CB21EZ, UK (fax: (+44) 1223-336-033: e-mail: deposit@ ccdc.cam.ac.uk).
52. 1.
53. F. H. Allen, O. Kennard, D. G. Watson, L. Brammer, A. G. Orpen, R. Taylor, J. Chem. Soc. Perkin Trans. 2 1987, S1.
54. Reactions of the like have already been utilized to synthesize simple dipeptides; cf. ref. [13].
55. This transformation was performed with racemic N-Boc-2. It is still to be investigated how enantiopure α-trialkylsilyl-α-amino acids will react.