Microwave-accelerated O-alkylation of carboxylic acids with O-alkylisoureas

Organic Letters

Volumn 4, Issue 17, 2002, Pages 2961-2963

  Crosignani, Stefano ^a   White, Peter D ^b   Linclau, Bruno ^a  

^a UNIVERSITY OF SOUTHAMPTON   (United Kingdom)

^b Novabiochem   (United Kingdom)

Author keywords

[No Author keywords available]

Indexed keywords

EID: 0012730030     PISSN: 15237060     EISSN: None     Source Type: Journal    
DOI: 10.1021/ol0263705     Document Type: Article

References (25)

1. (a) Lidstrom, P.; Tierney, J.; Wathey, B.; Westman, J. Tetrahedron 2001, 57, 9225-9283.
2. (b) Kaiser, N. F. K.; Bremberg, U.; Larhed, M.; Moberg, C.; Hallberg, A. Angew. Chem., Int. Ed. 2000, 39, 3596-3598.
3. (c) Larhed, M.; Hallberg, A. Drug Discov. Today 2001, 6, 406-416.
4. (d) Westman, J. Org. Lett. 2001, 3, 3745-3747.
5. (e) Brain, C. T.; Brunton, S. A. Synlett 2001, 382-384.
6. (f) Stadler, A.; Kappe, C. O. Eur. J. Org. Chem. 2001, 919-925.
7. (g) Watkins, K. J. Chem. Eng. News 2002, 80, 17-18.
8. (h) Leadbeater, N. E.; Torenius, H. M. J. Org. Chem. 2002, 67, 3145-3148.
9. (a) Kuhnert, N. Angew. Chem., Int. Ed. 2002, 41, 1863-1866.
10. (b) Sudrik, S. G.; Chavan, S. P.; Chandrakumar, K. R. S.; Pal, S.; Date, S. K.; Chavan, S. P.; Sonawane, H. R. J. Org. Chem. 2002, 67, 1574-1579.
11. (a) Mathias, L. J. Synthesis 1979, 561-76.
12. (b) Biftu, T.; Acton, J. J.; Berger, G. D.; Bergstrom, J. D.; Dufresne, C.; Kurtz, M. M.; Marquis, R. W.; Parsons, W. H.; Rew, D. R.; Wilson, K. E. J. Med. Chem. 1994, 37, 421-424.
13. (c) Nicolaou, K. C. Yue, E. W.; Naniwa, Y.; Dericcardis, F.; Nadin, A.; Leresche, J. E.; Lagreca, S.; Yang, Z. Angew. Chem., Int. Ed. 1994, 33, 2184-2187.
14. (d) Hugger, U.; Sharpless, K. B. Tetrahedron Lett. 1995, 36, 6603-6606.
15. Traditional ester formation between carboxylic acids and alcohols with silica gel as the acid source has been successfully accelerated under microwave conditions. See: Lami, L.; Casal, B.; Cuandra, L.; Merino, J.; Alvarez, A.; Ruiz-Hitzky, E. Green Chem. 1999, 1, 199-204.
16. A typical procedure is as follows: a microwave vial is charged with the carboxylic acid 4 (1.0 mmol) and O-alkylisourea 1 followed by addition of THF (2 mL). The vial is heated at the reported temperature in a Smith Synthesizer for 5 min. The white solid is filtered off and the solvent evaporated under vacuum. The residue is then further purified by column chromatography.
17. We recommend avoiding the use of lower boiling alcohols such as methanol as solvents for this reaction. In one case, the use of such solvents led to a large increase of the pressure inside the vial, which can be potentially hazardous.
18. Since the microwave reaction conditions involve a higher reaction temperature and a sealed tube, no direct comparison with the conventional thermal process can be made regarding the reaction rate. See ref 2a for a thorough discussion of thermal vs microwave effect.
19. Vowinkel, E. Chem Ber. 1967, 100, 16-22.
20. Klemm, H. P.; Hintze, U.; Gercken, G. J. Chromatogr. 1973, 75, 19-27.
21. Bergmeier, S. C.; Cobas, A. A.; Rapoport, H. J. Org. Chem. 1993, 58, 2369-2376.
22. Sheehan, J. C.; Cruickshank, P. A.; Boshart G. I. J. Org. Chem. 1961, 26, 2525-2528.
23. Gibson, F. S.; Park, M. S.; Rapoport, H. J. Org. Chem. 1994, 59, 7503-7507.
24. Crosignani, S.; White, P. D.; Lindau, B. Org. Lett. 2002, 4, 1035-1037.
25. A typical procedure is as follows: the carboxylic acid 4 (0.175 mmol) is dissolved in 2 mL of THF. The solution is added to resin 2 (200 mg. 0.35 mmol) in a microwave vial. The vial is heated at 120 °C for 15 min in a Smith Synthesizer, followed by filtration of the resin with subsequent resin wash (MeOH, 3 × 2 mL and DCM, 3 × 3 mL). The solvent is then evaporated under reduced pressure to give the desired product.