Process Research and Development of a Dihydropyrimidine Dehydrogenase Inactivator: Large-Scale Preparation of Eniluracil Using a Sonogashira Coupling Jason W. B. Cooke,* Robert Bright, Mark J. Coleman, and Kevin P. Jenkins Chemical DeVelopment DiVision, GlaxoSmithKline Medicines Research Centre, Gunnels Wood Road, SteVenage, Hertfordshire. SG1 2NY, UK Abstract: Eniluracil (5-ethynyluracil) is a potent inactivator of the enzyme dihydropyrimidine dehydrogenase, which is the rate-limiting enzyme in the metabolism of 5-fluorouracil, a widely used anti- cancer drug. The process research and development of a three- stage route to eniluracil is described. A Sonogashira coupling between 5-iodouracil and trimethylsilylacetylene was used to synthesise 5-(2-trimethylsilylethynyl)uracil on a >60 kg scale. Sodium hydroxide deprotection and acidification with acetic acid completed the synthesis of eniluracil in high yield and quality. The optimisation of this process is described with particular attention paid to minimising the input of palladium and copper catalysts and ensuring that the copper catalyst is well suspended in the reaction mixture. Introduction 5-Fluorouracil (5-FU) is one of the most widely used anti- cancer drugs for solid tumours. 5-FU is administered by slow intravenous infusion owing to low and variable oral bio- availability due to metabolism by the enzyme dihydropyri- midine dehydrogenase 1 (DPD, also known as uracil reduc- tase), and it is this enzyme that is the target of eniluracil 1 (5-ethynyluracil). Eniluracil is a potent, suicide substrate for DPD and becomes covalently linked to the enzyme, 2 thereby inactivating it. Co-administration of eniluracil with 5-FU leads to higher and more prolonged serum levels 3 of 5-FU and allows oral administration of 5-FU. The synthesis of eniluracil is reported in the literature 4 from the 1970s. Patents were filed 5 by the Wellcome Foundation covering the use of eniluracil as a DPD inactivator to improve therapy with 5-FU. The Sonogashira reaction 6 is well-known in the literature and has achieved widespread use as a mild method for introducing an acetylene into a molecule via coupling with a vinyl or aryl halide. The use of trimethylsilylacetylene 7 (TMSA) was introduced in 1980, and this provides a mild, safe and high-yielding method for the introduction of an unsubstituted ethyne fragment. Later papers describe Sono- gashira-type coupling of acetylenes with 5-iodouracil 8 and close analogues. 9 It is the development and scale-up of a Sonogashira coupling as the foundation of a route to eniluracil that forms the subject of this paper. Issues with the Initial Process When the project was transferred to Stevenage, a three- stage pilot-plant process 10 had already been devised and operated (Scheme 1). Stage 1 consisted of a Sonogashira coupling of TMSA with 5-iodouracil 2 (5-IU) catalyzed by 1.5 mol % PdCl 2 and 10 mol % CuI. This was followed by two charcoal treatments (2 × 0.9 weights of charcoal) and recrystallisations, which were required to control the heavy metal contamination of 5-(2-trimethylsilylethynyl)uracil 3 (stage 2). Finally, stage 3, a deprotection with aqueous sodium hydroxide, was used to remove the trimethylsilyl group. The route was considered suitable for the routine long- term manufacture of eniluracil; however, a number of key * Author for correspondence. Telephone: +44 1438 764407. Fax: +44 1438 764414. E-mail: jwbc16317@gsk.com. (1) Heggie, D.; Sommadossi, J.-P.; Cross, D. S.; Huster, W. J.; Diasio, R. Cancer Res. 1987, 47, 2203. (2) Porter, D. J. T.; Chestnut, W. G.; Merrill, B. M.; Spector, T. J. Biol. Chem. 1992, 267 (8), 5236. (3) For a review of eniluracil see: Spector, T.; Porter, D. J. T.; Nelson, D. J.; Baccanari, D. P.; Davis, S. T.; Almond, M. R.; Khor, S. P.; Amyx, H.; Cao, S.; Rustum, Y. M. Drugs Future 1994, 19 (6), 565. (4) (a) Perman, J.; Sharma, R. A.; Bobek, M. Tetrahedron Lett. 1976, 51, 2427. (b) Barr, P. J.; Jones, A. S.; Walker, R. T. Nucleic Acids Res. 1976, 3, 2845. (5) Spector, T.; Porter, D. J. T.; Rahim, S. G. WO 9201452 and WO 9204901. (6) (a) Sonogashira, K.; Tohda, Y.; Hagihara, N. Tetrahedron Lett. 1975, 50, 4467. (b) For a review see: Campbell, I. B. The Sonogashira Cu-Pd- Catalyzed Alkyne Coupling Reaction. Organocopper Reagents; Taylor, R. J. K., Ed.; IRL Press: Oxford, UK, 1994; pp 217-35. (7) Takahashi, S.; Kuroyama, Y.; Sonogashira, K.; Hagihara, N. Synthesis 1980, 627. (8) Imamura, K.; Yamamoto, Y.; Bioorg and Med. Chem. Lett. 1996, 6 (15), 1855-1858. (9) (a) Robins, M. J.; Barr, P. J. Tetrahedron Lett. 1981, 22, 421-424. (b) De Clercq, E.; Descamps, J.; Balzarini, J.; Giziewicz, J.; Barr, P. J.; Robins, M. J. J. Med. Chem. 1983, 26, 661-666. (10) See the Supporting Information for details of this process. Scheme 1. Route to eniluracil Organic Process Research & Development 2001, 5, 383-386 10.1021/op0100100 CCC: $20.00 © 2001 American Chemical Society and The Royal Society of Chemistry Vol. 5, No. 4, 2001 / Organic Process Research & Development • 383 Published on Web 05/23/2001