Chemoselective esterification and amidation of carboxylic acids with imidazole carbamates and ureas

Organic Letters

Volumn 12, Issue 20, 2010, Pages 4572-4575

  Heller, Stephen T ^a   Sarpong, Richmond ^a  

^a UNIVERSITY OF CALIFORNIA   (United States)

Author keywords

[No Author keywords available]

Indexed keywords

EID: 77957829171     PISSN: 15237060     EISSN: 15237052     Source Type: Journal    
DOI: 10.1021/ol1018882     Document Type: Article

References (43)

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2. Methods include
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6. BOPCl: Diago-Meseguer, J.; Palomo-Coll, A. L.; Fernández-Lizarbe, J. R.; Zugaza-Bilbao, A. Synthesis 1980, 54, 7-551
7. DMFDMA: Brechbühler, H.; Büchi, H.; Hatz, E.; Schreiber, J.; Eschenmoser, A. Helv. Chim. Acta 1965, 48, 1746-1771
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13. Methods include
14. 3: Pfeffer, P. E.; Silbert, L. S. J. Org. Chem. 1976, 41, 1373-1379
15. 3: Jiang, X.; Tiwari, A.; Thompson, M.; Chen, Z.; Cleary, T. P.; Lee, T. B. K. Org. Process Res. Dev. 2001, 5, 604-608
16. 4/LiOH: Chakraborti, A. K.; Basak (née Nandi), A.; Grover, V. J. Org. Chem. 1999, 64, 8014-8017
17. 4/DIPEA: Raber, D. J.; Gariano, P.; Brod, A. O.; Gariano, A.; Guida, W. C.; Guida, A. R.; Herbst, M. D. J. Org. Chem. 1979, 44, 1149-1154
18. Alkylisoureas have been used as in situ alkylating reagents
19. Mathias, L. J. Synthesis 1979, 8, 561-576 and references therein
20. Crosignani, S.; White, P. D.; Steinauer, R.; Linclau, B. Org. Lett. 2003, 5, 853-856
21. A full safety evaluation has yet to be performed. MImC was stable for weeks at - 20 °C and could be stored for prolonged periods at 4 °C. All other prepared imidazole carbamates could be stored at room temperature without observable decomposition.
22. Staab, H. A. Angew. Chem., Int. Ed. 1962, 1, 351-367
23. Crumbie, R. L.; Nimitz, J. S.; Mosher, H. S. J. Org. Chem. 1982, 47, 4040-4045
24. Trost, B. M.; Zhang, Y.; Zhang, T. J. Org. Chem. 2009, 74, 5115-5117
25. Subjecting MImC to 2 equiv of imidazole in MeCN at 60 °C for 16 h led to nearly quantitative conversion to 1-methylimidazole.
26. Unless a minor product is reported, all yields reported herein are for reactions purified only by aqueous workup to give analytically pure products.
27. Imidazole carbamates were prepared either by treating imidazole (2 equiv) with the appropriate alkyl chloroformate (1 equiv) or by treating 1,1′-carbonyldiimidazole (1.05 equiv) with the appropriate alcohol (1 equiv). See the Supporting Information for details.
28. All esterification reactions were run for 24 h. No attempt was made to optimize reaction time for individual substrates.
29. In some cases, small amounts of N, N -dimethylbenzamide byproducts were observed. Using freshly distilled DMF usually suppressed this side reaction except in the case of very electron-poor benzoic acids such as 4-nitrobenzoic acid.
30. Efforts to mitigate amino acid racemization are underway.
31. For a discussion see, ref 8.
32. N2 displacement mechanism cannot be excluded.
33. Cowdrey, W. A.; Hughes, E. D.; Ingold, C. K.; Masterman, S.; Scott, A. D. J. Chem. Soc. 1937, 1271-1277
34. The product ester was assigned as the R enantiomer by comparison of its optical rotation with literature values: Chênevert, R.; Pelchat, N.; Morin, P. Tetrahedron: Asymmetry 2009, 20, 1191-1196
35. Retentive and regiospecific esterification can also be explained by a mechanism involving an acylimidazole intermediate. Further experimentation to delineate these mechanisms is underway.
36. 11% conversion to 4-methoxytoluene anisole was observed when MImC was mixed with p -cresol under the standard esterification conditions.
37. Mixing ethyl and allyl imidazole carbamates (EImC and AllImC) with p -cresol led to <1% and 6% conversion to p -ethoxytoluene and p -allyloxytoluene, respectively.
38. 2/MeOH for 4 h at room temperature, only the methyl ester was observed.
39. 4-Hydroxy-3-nitrobenzoic acid was obtained in 31% isolated yield when using MImC (eq 2, top entry).
40. Grzyb, J. A.; Shen, M.; Yoshina-Ishii, C.; Chi, W.; Brown, R. S.; Batey, R. A. Tetrahedron 2005, 61, 7153-7175
41. Other methods for the direct conversion of carboxylic acids to Weinreb amides include: Niu, T.; Zhang, W.; Huang, D.; Xu, C.; Wang, H.; Hu, Y. Org. Lett. 2009, 11, 4474-4477
42. Tunoori, A. R.; White, J. M.; Georg, G. I. Org. Lett. 2000, 2, 4091-4093
43. The transformation of hindered acids can be problematic when standard conditions are used. See: Woo, J. C. S.; Fenster, E.; Dake, G. R. J. Org. Chem. 2004, 69, 8984-8986