July 8, 2013
Improve a drug synthesis by reducing byproducts. Atipamezole [5-(2-ethyl-2,3-dihydro-1H-inden-2-yl)-1H-imidazole, 1] is a veterinary drug that has been investigated for treating Parkinson's disease in humans. V. Lusis and co-inventors summarize several ways to synthesize 1. Some routes give a low yield of 1 and produce large quantities of an oxazole byproduct. Other processes involve a sluggish bromination reaction that leads to many byproducts.
The inventors’ process is intended to overcome these problems. In particular, it does not use the bromination reaction and thus avoids forming brominated byproducts. The process, outlined in the figure, begins with the reaction of imidazole 2 with i-PrMgCl to form iodo Grignard reagent 3, which is treated with DMF to give 4. This intermediate is not isolated but is treated with aq NH4Cl to give aldehyde 5, isolated in 73.2% yield. The aldehyde is condensed with phthalide (6) in the presence of NaOMe to produce imidazolylindane 7, recovered in crude form in 67.2% yield.
In the next stage, compound 7 is alkylated with EtI in the presence of K2CO3. Product 8 is isolated in 50.9% yield after being recrystallized from EtOH. Product 1 can be produced directly from 8 by making its HCl salt and hydrogenating the salt over Pd/C. Crude atipamezole is isolated as its HCl salt in 26.6% yield.
Alternatively, acid hydrolysis of 8 removes the trityl group to form dione 9, recovered as a white crystalline solid in 76.2% yield. The HCl salt of 9 is then hydrogenated to 1·HCl.
The patent’s claims cover the process to make 1 and new compounds 7 and 8. The overall yield of compound 1 is poor, partly because of the low yield from the hydrogenation step. The inventors claim, however, that the yield is higher than from earlier methods. They point out that the process is amenable to large-scale production without the use of specialized equipment. (JSC Grindeks [Riga, Latvia]. US Patent 8,431,717, April 30, 2013; Keith Turner)
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