The first catalytic, asymmetric α-additions of isocyanides. Lewis-base-catalyzed, enantioselective Passerini-type reactions

Journal of the American Chemical Society

Volumn 125, Issue 26, 2003, Pages 7825-7827

  Denmark, Scott E ^a   Fan, Yu ^a  

^a UNIVERSITY OF ILLINOIS AT URBANA CHAMPAIGN   (United States)

Author keywords

[No Author keywords available]

Indexed keywords

ISOCYANIC ACID DERIVATIVE;

ARTICLE; CATALYSIS; CHEMICAL BINDING; CHIRALITY; CYCLOADDITION; ENANTIOSELECTIVITY; REACTION ANALYSIS;

EID: 0038203081     PISSN: 00027863     EISSN: None     Source Type: Journal    
DOI: 10.1021/ja035410c     Document Type: Article

References (49)

1. (a) Passerini, M. Gazz. Chim. Ital. 1921, 51, 126.
2. (b) Passerini, M.; Ragni, G. Gazz. Chim. Ital. 1931, 61, 964.
3. (a) Ugi, I.; Meyr, R.; Fetzer, U.; Steinbrückner, C. Angew. Chem. 1959, 71, 386.
4. (b) Ugi, I.; Steinbrückner, C. Angew. Chem. 1960, 72, 267.
5. (a) Dömling, A. Curr. Opin. Chem. Biol. 2002, 6, 306.
6. (b) Dömling, A.; Ugi, I. Angew. Chem., Int. Ed. 2000, 39, 3169.
7. (c) Armstrong, R. W.; Combs, A. P.; Tempest, P. A.; Brown, S. D.; Keating, T. A. Acc. Chem. Res. 1996, 29, 123.
8. Addition to chiral imines using carbohydrate auxiliaries: (a) Kunz, H.; Pfrengle, W. J. Am. Chem. Soc. 1988, 110, 651. (b) Kunz, H.; Rück, K. Angew. Chem. 1993, 105, 355. (c) Oertel, K.; Zech, G.; Kunz, H. Angew. Chem. 2000, 112, 1489; Angew. Chem., Int. Ed. 2000, 39, 1431. (d) Goebel, M.; Ugi, I. Synthesis 1991, 1095. (e) Lehnhoff, S.; Goebel, M.; Karl, R. M.; Klosel, R.; Ugi, I. Angew. Chem. 1995, 107, 1208; Angew. Chem., Int. Ed. Engl. 1995, 34, 1104. (f) Ross, G. F.; Herdtweck, E.; Ugi, I. Tetrahedron 2002, 58, 6127. (g) Ziegler, T.; Kaisers, H.-J.; Schlömer, R.; Koch, C. Tetrahedron 1999, 55, 8397. Linderman, R. J.; Binet, S.; Petrich, S. R. J. Org. Chem. 1999, 64, 336.
9. Addition to chiral imines using carbohydrate auxiliaries: (a) Kunz, H.; Pfrengle, W. J. Am. Chem. Soc. 1988, 110, 651. (b) Kunz, H.; Rück, K. Angew. Chem. 1993, 105, 355. (c) Oertel, K.; Zech, G.; Kunz, H. Angew. Chem. 2000, 112, 1489; Angew. Chem., Int. Ed. 2000, 39, 1431. (d) Goebel, M.; Ugi, I. Synthesis 1991, 1095. (e) Lehnhoff, S.; Goebel, M.; Karl, R. M.; Klosel, R.; Ugi, I. Angew. Chem. 1995, 107, 1208; Angew. Chem., Int. Ed. Engl. 1995, 34, 1104. (f) Ross, G. F.; Herdtweck, E.; Ugi, I. Tetrahedron 2002, 58, 6127. (g) Ziegler, T.; Kaisers, H.-J.; Schlömer, R.; Koch, C. Tetrahedron 1999, 55, 8397. Linderman, R. J.; Binet, S.; Petrich, S. R. J. Org. Chem. 1999, 64, 336.
10. Addition to chiral imines using carbohydrate auxiliaries: (a) Kunz, H.; Pfrengle, W. J. Am. Chem. Soc. 1988, 110, 651. (b) Kunz, H.; Rück, K. Angew. Chem. 1993, 105, 355. (c) Oertel, K.; Zech, G.; Kunz, H. Angew. Chem. 2000, 112, 1489; Angew. Chem., Int. Ed. 2000, 39, 1431. (d) Goebel, M.; Ugi, I. Synthesis 1991, 1095. (e) Lehnhoff, S.; Goebel, M.; Karl, R. M.; Klosel, R.; Ugi, I. Angew. Chem. 1995, 107, 1208; Angew. Chem., Int. Ed. Engl. 1995, 34, 1104. (f) Ross, G. F.; Herdtweck, E.; Ugi, I. Tetrahedron 2002, 58, 6127. (g) Ziegler, T.; Kaisers, H.-J.; Schlömer, R.; Koch, C. Tetrahedron 1999, 55, 8397. Linderman, R. J.; Binet, S.; Petrich, S. R. J. Org. Chem. 1999, 64, 336.
11. Addition to chiral imines using carbohydrate auxiliaries: (a) Kunz, H.; Pfrengle, W. J. Am. Chem. Soc. 1988, 110, 651. (b) Kunz, H.; Rück, K. Angew. Chem. 1993, 105, 355. (c) Oertel, K.; Zech, G.; Kunz, H. Angew. Chem. 2000, 112, 1489; Angew. Chem., Int. Ed. 2000, 39, 1431. (d) Goebel, M.; Ugi, I. Synthesis 1991, 1095. (e) Lehnhoff, S.; Goebel, M.; Karl, R. M.; Klosel, R.; Ugi, I. Angew. Chem. 1995, 107, 1208; Angew. Chem., Int. Ed. Engl. 1995, 34, 1104. (f) Ross, G. F.; Herdtweck, E.; Ugi, I. Tetrahedron 2002, 58, 6127. (g) Ziegler, T.; Kaisers, H.-J.; Schlömer, R.; Koch, C. Tetrahedron 1999, 55, 8397. Linderman, R. J.; Binet, S.; Petrich, S. R. J. Org. Chem. 1999, 64, 336.
12. Addition to chiral imines using carbohydrate auxiliaries: (a) Kunz, H.; Pfrengle, W. J. Am. Chem. Soc. 1988, 110, 651. (b) Kunz, H.; Rück, K. Angew. Chem. 1993, 105, 355. (c) Oertel, K.; Zech, G.; Kunz, H. Angew. Chem. 2000, 112, 1489; Angew. Chem., Int. Ed. 2000, 39, 1431. (d) Goebel, M.; Ugi, I. Synthesis 1991, 1095. (e) Lehnhoff, S.; Goebel, M.; Karl, R. M.; Klosel, R.; Ugi, I. Angew. Chem. 1995, 107, 1208; Angew. Chem., Int. Ed. Engl. 1995, 34, 1104. (f) Ross, G. F.; Herdtweck, E.; Ugi, I. Tetrahedron 2002, 58, 6127. (g) Ziegler, T.; Kaisers, H.-J.; Schlömer, R.; Koch, C. Tetrahedron 1999, 55, 8397. Linderman, R. J.; Binet, S.; Petrich, S. R. J. Org. Chem. 1999, 64, 336.
13. Addition to chiral imines using carbohydrate auxiliaries: (a) Kunz, H.; Pfrengle, W. J. Am. Chem. Soc. 1988, 110, 651. (b) Kunz, H.; Rück, K. Angew. Chem. 1993, 105, 355. (c) Oertel, K.; Zech, G.; Kunz, H. Angew. Chem. 2000, 112, 1489; Angew. Chem., Int. Ed. 2000, 39, 1431. (d) Goebel, M.; Ugi, I. Synthesis 1991, 1095. (e) Lehnhoff, S.; Goebel, M.; Karl, R. M.; Klosel, R.; Ugi, I. Angew. Chem. 1995, 107, 1208; Angew. Chem., Int. Ed. Engl. 1995, 34, 1104. (f) Ross, G. F.; Herdtweck, E.; Ugi, I. Tetrahedron 2002, 58, 6127. (g) Ziegler, T.; Kaisers, H.-J.; Schlömer, R.; Koch, C. Tetrahedron 1999, 55, 8397. Linderman, R. J.; Binet, S.; Petrich, S. R. J. Org. Chem. 1999, 64, 336.
14. Addition to chiral imines using carbohydrate auxiliaries: (a) Kunz, H.; Pfrengle, W. J. Am. Chem. Soc. 1988, 110, 651. (b) Kunz, H.; Rück, K. Angew. Chem. 1993, 105, 355. (c) Oertel, K.; Zech, G.; Kunz, H. Angew. Chem. 2000, 112, 1489; Angew. Chem., Int. Ed. 2000, 39, 1431. (d) Goebel, M.; Ugi, I. Synthesis 1991, 1095. (e) Lehnhoff, S.; Goebel, M.; Karl, R. M.; Klosel, R.; Ugi, I. Angew. Chem. 1995, 107, 1208; Angew. Chem., Int. Ed. Engl. 1995, 34, 1104. (f) Ross, G. F.; Herdtweck, E.; Ugi, I. Tetrahedron 2002, 58, 6127. (g) Ziegler, T.; Kaisers, H.-J.; Schlömer, R.; Koch, C. Tetrahedron 1999, 55, 8397. Linderman, R. J.; Binet, S.; Petrich, S. R. J. Org. Chem. 1999, 64, 336.
15. Addition to chiral imines using carbohydrate auxiliaries: (a) Kunz, H.; Pfrengle, W. J. Am. Chem. Soc. 1988, 110, 651. (b) Kunz, H.; Rück, K. Angew. Chem. 1993, 105, 355. (c) Oertel, K.; Zech, G.; Kunz, H. Angew. Chem. 2000, 112, 1489; Angew. Chem., Int. Ed. 2000, 39, 1431. (d) Goebel, M.; Ugi, I. Synthesis 1991, 1095. (e) Lehnhoff, S.; Goebel, M.; Karl, R. M.; Klosel, R.; Ugi, I. Angew. Chem. 1995, 107, 1208; Angew. Chem., Int. Ed. Engl. 1995, 34, 1104. (f) Ross, G. F.; Herdtweck, E.; Ugi, I. Tetrahedron 2002, 58, 6127. (g) Ziegler, T.; Kaisers, H.-J.; Schlömer, R.; Koch, C. Tetrahedron 1999, 55, 8397. Linderman, R. J.; Binet, S.; Petrich, S. R. J. Org. Chem. 1999, 64, 336.
16. Addition to chiral imines using carbohydrate auxiliaries: (a) Kunz, H.; Pfrengle, W. J. Am. Chem. Soc. 1988, 110, 651. (b) Kunz, H.; Rück, K. Angew. Chem. 1993, 105, 355. (c) Oertel, K.; Zech, G.; Kunz, H. Angew. Chem. 2000, 112, 1489; Angew. Chem., Int. Ed. 2000, 39, 1431. (d) Goebel, M.; Ugi, I. Synthesis 1991, 1095. (e) Lehnhoff, S.; Goebel, M.; Karl, R. M.; Klosel, R.; Ugi, I. Angew. Chem. 1995, 107, 1208; Angew. Chem., Int. Ed. Engl. 1995, 34, 1104. (f) Ross, G. F.; Herdtweck, E.; Ugi, I. Tetrahedron 2002, 58, 6127. (g) Ziegler, T.; Kaisers, H.-J.; Schlömer, R.; Koch, C. Tetrahedron 1999, 55, 8397. Linderman, R. J.; Binet, S.; Petrich, S. R. J. Org. Chem. 1999, 64, 336.
17. Addition to chiral imines using carbohydrate auxiliaries: (a) Kunz, H.; Pfrengle, W. J. Am. Chem. Soc. 1988, 110, 651. (b) Kunz, H.; Rück, K. Angew. Chem. 1993, 105, 355. (c) Oertel, K.; Zech, G.; Kunz, H. Angew. Chem. 2000, 112, 1489; Angew. Chem., Int. Ed. 2000, 39, 1431. (d) Goebel, M.; Ugi, I. Synthesis 1991, 1095. (e) Lehnhoff, S.; Goebel, M.; Karl, R. M.; Klosel, R.; Ugi, I. Angew. Chem. 1995, 107, 1208; Angew. Chem., Int. Ed. Engl. 1995, 34, 1104. (f) Ross, G. F.; Herdtweck, E.; Ugi, I. Tetrahedron 2002, 58, 6127. (g) Ziegler, T.; Kaisers, H.-J.; Schlömer, R.; Koch, C. Tetrahedron 1999, 55, 8397. Linderman, R. J.; Binet, S.; Petrich, S. R. J. Org. Chem. 1999, 64, 336.
18. Addition to chiral imines with ferrocenyl auxiliaries: (a) Ugi, I.; Offermann, K. Angew. Chem. 1963, 75, 917; Angew. Chem., Int. Ed. Engl. 1963, 2, 624. (b) Ugi, I.; Offermann, K.; Herlinger, H. Angew. Chem. 1964, 76, 613; Angew. Chem., Int. Ed. Engl. 1964, 3, 656. (c) Ugi, I.; Offermann, K.; Herlginer, H.; Marquarding, D. Justus Liebigs Ann. Chem. 1967, 709, 1. (d) Demharter, A.; Ugi, I. J. Prakt. Chem. 1993, 335, 244.
19. Addition to chiral imines with ferrocenyl auxiliaries: (a) Ugi, I.; Offermann, K. Angew. Chem. 1963, 75, 917; Angew. Chem., Int. Ed. Engl. 1963, 2, 624. (b) Ugi, I.; Offermann, K.; Herlinger, H. Angew. Chem. 1964, 76, 613; Angew. Chem., Int. Ed. Engl. 1964, 3, 656. (c) Ugi, I.; Offermann, K.; Herlginer, H.; Marquarding, D. Justus Liebigs Ann. Chem. 1967, 709, 1. (d) Demharter, A.; Ugi, I. J. Prakt. Chem. 1993, 335, 244.
20. Addition to chiral imines with ferrocenyl auxiliaries: (a) Ugi, I.; Offermann, K. Angew. Chem. 1963, 75, 917; Angew. Chem., Int. Ed. Engl. 1963, 2, 624. (b) Ugi, I.; Offermann, K.; Herlinger, H. Angew. Chem. 1964, 76, 613; Angew. Chem., Int. Ed. Engl. 1964, 3, 656. (c) Ugi, I.; Offermann, K.; Herlginer, H.; Marquarding, D. Justus Liebigs Ann. Chem. 1967, 709, 1. (d) Demharter, A.; Ugi, I. J. Prakt. Chem. 1993, 335, 244.
21. Addition to chiral imines with ferrocenyl auxiliaries: (a) Ugi, I.; Offermann, K. Angew. Chem. 1963, 75, 917; Angew. Chem., Int. Ed. Engl. 1963, 2, 624. (b) Ugi, I.; Offermann, K.; Herlinger, H. Angew. Chem. 1964, 76, 613; Angew. Chem., Int. Ed. Engl. 1964, 3, 656. (c) Ugi, I.; Offermann, K.; Herlginer, H.; Marquarding, D. Justus Liebigs Ann. Chem. 1967, 709, 1. (d) Demharter, A.; Ugi, I. J. Prakt. Chem. 1993, 335, 244.
22. Addition to chiral imines with ferrocenyl auxiliaries: (a) Ugi, I.; Offermann, K. Angew. Chem. 1963, 75, 917; Angew. Chem., Int. Ed. Engl. 1963, 2, 624. (b) Ugi, I.; Offermann, K.; Herlinger, H. Angew. Chem. 1964, 76, 613; Angew. Chem., Int. Ed. Engl. 1964, 3, 656. (c) Ugi, I.; Offermann, K.; Herlginer, H.; Marquarding, D. Justus Liebigs Ann. Chem. 1967, 709, 1. (d) Demharter, A.; Ugi, I. J. Prakt. Chem. 1993, 335, 244.
23. Addition to chiral imines with ferrocenyl auxiliaries: (a) Ugi, I.; Offermann, K. Angew. Chem. 1963, 75, 917; Angew. Chem., Int. Ed. Engl. 1963, 2, 624. (b) Ugi, I.; Offermann, K.; Herlinger, H. Angew. Chem. 1964, 76, 613; Angew. Chem., Int. Ed. Engl. 1964, 3, 656. (c) Ugi, I.; Offermann, K.; Herlginer, H.; Marquarding, D. Justus Liebigs Ann. Chem. 1967, 709, 1. (d) Demharter, A.; Ugi, I. J. Prakt. Chem. 1993, 335, 244.
24. Addition to chiral imines with α-methylbenzylamine auxiliary: (a) Divanfard, H. R.; Lysenko, Z.; Wang, P. C.; Joullie, M. M. Synth. Commun. 1978, 8, 269. (b) Semple, J. E.; Wang, P. C.; Lysenko, Z.; Joullié, M. M. J. Am. Chem. Soc. 1980, 102, 7505.
25. Addition to chiral imines with α-methylbenzylamine auxiliary: (a) Divanfard, H. R.; Lysenko, Z.; Wang, P. C.; Joullie, M. M. Synth. Commun. 1978, 8, 269. (b) Semple, J. E.; Wang, P. C.; Lysenko, Z.; Joullié, M. M. J. Am. Chem. Soc. 1980, 102, 7505.
26. Addition of a chiral isocyanide to aldehydes and imines: Bock, H.; Ugi, I. J. Prakt. Chem. 1997, 339, 385.
27. Lewis Acids in Organic Synthesis; Yamamoto, H., Ed.; Wiley-VCH: Weinheim, 2001.
28. 3 catalysis: Muller, E.; Zeeh, B. Liebigs Ann. Chem. 1966, 696, 72.
29. 2O catalysis: Muller, E.; Zeeh, B. Liebigs Ann. Chem. 1968, 715, 47.
30. (c) Saegusa, T.; Taka-Ishi, N.; Fujii, H. Tetrahedron 1968, 24, 3795.
31. 2/TMSCl promoted Passerini reaction: Xia, Q.; Ganem, B. Org. Lett. 2002, 4, 1631.
32. 4 promoted Passerini reaction: Schiess, M.; Seebach, D. Helv. Chim. Acta 1983, 66, 1618. (b) Seebach, D.; Adam, G.; Gees, T.; Schiess, M.; Weigand, W. Chem. Ber. 1988, 121, 507. (c) Carofiglio, T.; Cozzi, P. G.; Floriani, C.; Chiesi-Villa, A.; Rizzoli, C. Organometallics 1993, 12, 2726.
33. 4 promoted Passerini reaction: Schiess, M.; Seebach, D. Helv. Chim. Acta 1983, 66, 1618. (b) Seebach, D.; Adam, G.; Gees, T.; Schiess, M.; Weigand, W. Chem. Ber. 1988, 121, 507. (c) Carofiglio, T.; Cozzi, P. G.; Floriani, C.; Chiesi-Villa, A.; Rizzoli, C. Organometallics 1993, 12, 2726.
34. 4 promoted Passerini reaction: Schiess, M.; Seebach, D. Helv. Chim. Acta 1983, 66, 1618. (b) Seebach, D.; Adam, G.; Gees, T.; Schiess, M.; Weigand, W. Chem. Ber. 1988, 121, 507. (c) Carofiglio, T.; Cozzi, P. G.; Floriani, C.; Chiesi-Villa, A.; Rizzoli, C. Organometallics 1993, 12, 2726.
35. (a) Denmark, S. E.; Wynn, T. J. Am. Chem. Soc. 2001, 123, 6199.
36. (b) Denmark, S. E.; Wynn, T.; Beutner, G. L. J. Am. Chem. Soc. 2002, 124, 13405.
37. (c) Denmark, S. E.; Beutner, G. L. J. Am. Chem. Soc. 2003, 125, 7800.
38. Recent examples of Passerini reactions: (a) Owens, T. D.; Araldi, G.-L.; Nutt, R. F.; Semple, J. E. Tetrahedron Lett. 2001, 42, 6271-4. (b) Banfi, L.; Guanti, G.; Riva, R. Chem. Commun. 2000, 985. (c) Beck, B.; Magnin-Lachaux, M.; Herdtweck, E.; Domling, A. Org. Lett. 2001, 3, 2875. (d) Owens, T. D.; Semple, J. E. Org. Lett. 2001, 3, 3301. (e) Semple, J. E.; Owens, T. D.; Nguyen, K.; Levy, O. E. Org. Lett. 2000, 2, 2769. (f) Banfi, L.; Guanti, G.; Riva, R.; Basso, A.; Calcagno, E. Tetrahedron Lett. 2002, 43, 4067.
39. Recent examples of Passerini reactions: (a) Owens, T. D.; Araldi, G.-L.; Nutt, R. F.; Semple, J. E. Tetrahedron Lett. 2001, 42, 6271-4. (b) Banfi, L.; Guanti, G.; Riva, R. Chem. Commun. 2000, 985. (c) Beck, B.; Magnin-Lachaux, M.; Herdtweck, E.; Domling, A. Org. Lett. 2001, 3, 2875. (d) Owens, T. D.; Semple, J. E. Org. Lett. 2001, 3, 3301. (e) Semple, J. E.; Owens, T. D.; Nguyen, K.; Levy, O. E. Org. Lett. 2000, 2, 2769. (f) Banfi, L.; Guanti, G.; Riva, R.; Basso, A.; Calcagno, E. Tetrahedron Lett. 2002, 43, 4067.
40. Recent examples of Passerini reactions: (a) Owens, T. D.; Araldi, G.-L.; Nutt, R. F.; Semple, J. E. Tetrahedron Lett. 2001, 42, 6271-4. (b) Banfi, L.; Guanti, G.; Riva, R. Chem. Commun. 2000, 985. (c) Beck, B.; Magnin-Lachaux, M.; Herdtweck, E.; Domling, A. Org. Lett. 2001, 3, 2875. (d) Owens, T. D.; Semple, J. E. Org. Lett. 2001, 3, 3301. (e) Semple, J. E.; Owens, T. D.; Nguyen, K.; Levy, O. E. Org. Lett. 2000, 2, 2769. (f) Banfi, L.; Guanti, G.; Riva, R.; Basso, A.; Calcagno, E. Tetrahedron Lett. 2002, 43, 4067.
41. Recent examples of Passerini reactions: (a) Owens, T. D.; Araldi, G.-L.; Nutt, R. F.; Semple, J. E. Tetrahedron Lett. 2001, 42, 6271-4. (b) Banfi, L.; Guanti, G.; Riva, R. Chem. Commun. 2000, 985. (c) Beck, B.; Magnin-Lachaux, M.; Herdtweck, E.; Domling, A. Org. Lett. 2001, 3, 2875. (d) Owens, T. D.; Semple, J. E. Org. Lett. 2001, 3, 3301. (e) Semple, J. E.; Owens, T. D.; Nguyen, K.; Levy, O. E. Org. Lett. 2000, 2, 2769. (f) Banfi, L.; Guanti, G.; Riva, R.; Basso, A.; Calcagno, E. Tetrahedron Lett. 2002, 43, 4067.
42. Recent examples of Passerini reactions: (a) Owens, T. D.; Araldi, G.-L.; Nutt, R. F.; Semple, J. E. Tetrahedron Lett. 2001, 42, 6271-4. (b) Banfi, L.; Guanti, G.; Riva, R. Chem. Commun. 2000, 985. (c) Beck, B.; Magnin-Lachaux, M.; Herdtweck, E.; Domling, A. Org. Lett. 2001, 3, 2875. (d) Owens, T. D.; Semple, J. E. Org. Lett. 2001, 3, 3301. (e) Semple, J. E.; Owens, T. D.; Nguyen, K.; Levy, O. E. Org. Lett. 2000, 2, 2769. (f) Banfi, L.; Guanti, G.; Riva, R.; Basso, A.; Calcagno, E. Tetrahedron Lett. 2002, 43, 4067.
43. Recent examples of Passerini reactions: (a) Owens, T. D.; Araldi, G.-L.; Nutt, R. F.; Semple, J. E. Tetrahedron Lett. 2001, 42, 6271-4. (b) Banfi, L.; Guanti, G.; Riva, R. Chem. Commun. 2000, 985. (c) Beck, B.; Magnin-Lachaux, M.; Herdtweck, E.; Domling, A. Org. Lett. 2001, 3, 2875. (d) Owens, T. D.; Semple, J. E. Org. Lett. 2001, 3, 3301. (e) Semple, J. E.; Owens, T. D.; Nguyen, K.; Levy, O. E. Org. Lett. 2000, 2, 2769. (f) Banfi, L.; Guanti, G.; Riva, R.; Basso, A.; Calcagno, E. Tetrahedron Lett. 2002, 43, 4067.
44. 3 showed no reaction after 4 h at room temperature.
45. We speculate that the catalyzed reaction is first order in isocyanide by analogy to the allylation and aldol processes (ref 16), whereas the "uncatalyzed reaction" is promoted by the isocyanide and is likely higher order.
46. (a) Denmark, S. E.; Fu, J. J. Am. Chem. Soc. 2000, 122, 12021.
47. (b) Denmark, S. E.; Pham, S. M. Helv. Chim. Acta 2000, 83, 1846.
48. (c) Denmark, S. E.; Fu, J. J. Am. Chem. Soc. 2001, 123, 9488.
49. Catalyst 5a is commercially available from Obiter Research, LLC. Contact waboulanger@obiterresearch.com.