July 30, 2012
This synthetic sequence provides a useful drug—at a price. Several dihydropteridione derivatives effectively prevent cell proliferation. G. Linz and co-inventors report a comprehensive method for preparing pharmacologically active crystalline and anhydrous forms of compound 1 (Figure 1) that are suitable for drug formulations.
The inventors list several criteria for the properties of 1 and its manufacturing procedure:
- favorable bulk characteristics such as drying times, filterability, solubility in biologically acceptable solvents, and thermal stability;
- purity of the pharmaceutical composition;
- low hygroscopicity;
- no or low tendency toward polymorphism; and
- scalability to a convenient commercial process.
They describe their finding that the tri-HCl salt of 1 satisfies these criteria as “surprising”.
Free base 1 is prepared by condensing cyclopropylmethylpiperazine derivative 2 with pteridinone 3 in the presence of p-toluenesulfonic acid (TsOH), as shown in Figure 1. After the reaction is complete, the crude free base 1 is recovered as a viscous oil. It is then treated with HCl in an organic solvent to form 1·3HCl, isolated in 91% yield. Alternatively, the free base is not isolated; instead, concd HCl is added to the reaction mixture, followed by acetone. The crude salt is recovered in 92% yield.
The salt is purified by crystallization from refluxing EtOH, adding water, and cooling to precipitate the crystals. The inventors do not report the purity of this or any other reaction product.
The inventors obtained a hydrated form of the tri-HCl salt by dissolving the free base in EtOH at room temperature, followed by adding concd HCl and cooling to 2 °C. An anhydrous form can be recovered by drying the hydrate at 130 °C. The solubility of the hydrated salt in aqueous and organic media is reported, as are X-ray diffraction data for the hydrated form. The hydrated salt has good solid-state stability.
The patent also contains the syntheses of reactants 2 and 3 (Figures 2 and 3). The preparation of 2 begins with the formation of amide 7. Acid 4 is treated with SOCl2–DMF to form acid chloride 5; the crude product is added to a suspension of chiral difunctionalized cyclohexane 6 in THF and aq K2CO3 to produce 7. The crude product is recovered in 98% yield and oxidized to 8 with RuCl3 and N-methylmorpholine N-oxide (NMMO) in 91% yield.
Amide 8 reacts with cyclopropylmethylpiperazine 9 in the presence of methanesulfonic acid (MsOH). The solvent is evaporated, and the reaction mixture is treated with NaBH4. After further workup, product 10 is isolated in 46% yield. The nitro group is then hydrogenated over Raney Ni to give 2 in 90% yield. An alternative method for preparing 10 is also described.
To prepare 3, readily available amino acid 11 is esterified and alkylated to form 12. In a multistep, one-pot procedure, 11 is first treated with HC(OMe)3 and SOCl2. Further reaction with NaBH(OAc)3, acetone, and NH4OH produces 12 as its HCl salt in 90% yield. The salt is treated with aq NaOH to form the free base, which reacts with pyrimidine 13 in the presence of NaHCO3 to form 14 in 79% isolated yield.
The pteridinone system is formed by hydrogenating 14 over a Pt/C catalyst in the presence of V(acac)3. Precursor 15 is recovered in 90% yield and methylated with (MeO)2CO and K2CO3 to give 3 in 82% isolated yield.
The inventors succeeded in developing a route for making a crystalline salt that is suitable for preparing pharmaceutical formulations. The many synthetic steps, however, use a large number of solvents that are frequently evaporated to dryness. [This observation implies that the processes have a significant environmental burden. —Ed.] (Boehringer Ingelheim International [Ingelheim am Rhein, Germany]. US Patent U.S. 8,188,086, May 29, 2012; Keith Turner)