February 20, 2012
Improve the synthesis and flow properties of an insomnia drug. Ramelteon (1), marketed as Rozerem by Takeda Pharmaceuticals, is used to treat insomnia. V. K. Kansal and co-inventors describe several processes that are used to prepare it, all of which require many steps. The inventors offer no comments about the relative merits of the processes, but they state that a new industrial-scale process is needed. Their main claims are to intermediate acid 2 as a racemic mixture and individual enantiomers; one enantiomer is converted to 1 by the route shown in Figure 1.
The inventors use diastereomeric crystallization to resolve the racemic mixture by forming its (S)-1-phenylethylamine salt. The salt of the (R)-isomer of 2 is recovered first; then the salt of (S)-2 is isolated from the solution and acidified to give the free acid, which is purified by using (R)-1-phenylethylamine. Both enantiomers are isolated with >99.0% purity and >99.0% ee.
The (S)-acid is converted to acid chloride 3 and then to amide 4 by reactions with SOCl2 and NH3 gas, respectively. The chloride is not isolated; the amide is recovered in 85–90% yield with 95–98% purity. When aq NH4OH is used instead of NH3 gas, the purity of 4 is slightly lower (93–96%). An alternative method for preparing 4 is to treat 2 with Et3N and ClCO2Et, followed by NH3. This method produces 4 in yields of 80–95% and 97–99% purity.
Amide 4 is reduced to amine 5 with NaBH4 and BF3·Et2O. The amine is purified by forming its chloride or oxalate salt in yields as high as 85% and 96–98% purity. The salts are used to prepare 1 by treating them with EtCOCl in the presence of base: NaOH for the chloride salt and Na2CO3 for the oxalate. In both cases, the yield of 1 is >92%, and the purity is as high as 99.9% after recrystallization from EtOH.
The inventors also recrystallized 1 from toluene to produce what they describe as a “nonelectrostatic” crystalline form, designated as form A. They describe the measurement of the electrostatic charge of the crystals in one of the patent’s examples. The measurements show that the average charge density of form A is ≈15 times lower than crystals obtained from EtOAc. Low electrostatic charge improves the flow characteristics of the solid, which is important in preparing drug formulations.
The inventors report the details of preparing rac-2 by a multistep procedure shown in Figure 2. In most of the reaction steps, the product is isolated in crude form; the inventors do not indicate whether the product is purified before it is used in the next stage. The synthesis of rac-2 begins with the conversion of benzofuran (6) to aldehyde 7 by treatment with POCl3 followed by hydrolysis. The crude product is isolated as a liquid in 85–90% yield and 90–92% purity.
In the next step, 7 is condensed with malonic acid (8) in the presence of piperidine and HOAc; acid 9 is isolated in 92–95% yield and 95% purity. Catalytic hydrogenation of 9 produces 10 in 95% yield and 94–96% purity. The hydrogenation also can be carried out in the presence of NaOH and HCO2NH4; the yield and purity of 10 are the same, but the reaction takes 6 h instead of 2 h. [The patent does not state why NaOH and HCO2NH4 would be used.—Ed.] Acid 10 is brominated to produce acid 11, isolated in 50–60% yield and 92–95% purity.
The next stage begins with treating 11 with SOCl2 to activate the carboxyl group by forming acid chloride 12. The chloride is not isolated but cyclized under Friedel–Crafts conditions to give tricyclic compound 13, isolated in yields of 85–92% and 90-95% purity. This reaction also produces two impurities, 14 and 15, but the amounts are not reported. Removing the impurities gives 13 in good yield, but the inventors do not describe how this is done. They do report that the impurities can be isolated, and 1H and 13C NMR data are provided for both.
In the next step, the bromine atoms in 13 are replaced by hydrogen to give 16 in 85–90% yield and 96–97%purity. This reaction produces two impurities, 17 and 18; again, the amounts are not reported, but 1H and 13C NMR data are. After MeOH reflux in the presence active carbon, 16 is isolated in 80–85% yield with 99.3–99.8% purity. It is then converted to ester 20 by treating it with a solution of phosphonate 19 that contains suspended NaH. Crude product 20 is isolated in 80–85% yield and 92–95% purity as a mixture of (E)- and (Z)-isomers. The isomer mixture is hydrogenated, and base hydrolysis gives rac-2 in 90–95% isolated yield and 95–98% purity.
The inventors claim that the overall process is suitable for producing ramelteon on an industrial scale in a crystalline form that has improved flow characteristics. (Teva Pharmaceutical Industries [Petah Tiqva, Israel]. US Patent 8,084,630, Dec. 27, 2011; Keith Turner)