January 7, 2013
Here’s an improved synthesis of an ADHD drug. The HCl salt of atomoxetine [5, with the (R)-configuration], which is marketed under the trade name Strattera, is used for treating attention-deficit hyperactivity disorder (ADHD). The (R)-enantiomer is 9 times more effective than the (S)-enantiomer. It has been prepared by several methods, some of which involve asymmetric reduction or chiral resolution and use expensive reagents such as diethyl azodicarboxylate or chiral boranes.
B.-F. Chen and co-inventors describe a synthesis of 5 that avoids costly reagents. It includes the preparation of the chiral amino alcohol 3 as a key intermediate. The route for preparing 5 starts with a Mannich reaction between benzophenone, N,O-dimethylhydroxylamine, and paraformaldehyde to give compound 1, isolated in 87.6% yield. The purity of 1 is not reported, but 1H-NMR data are provided.
Ketone 1 is asymmetrically reduced to form alcohol 2 by using the chiral ruthenium catalyst RuCl2-[(S)-DMSEGPHOS)][(S)-DAIPEN]. The hydrogen pressure is described as “predetermined”, but no value is given. Product 2 is recovered as an oily product in 98.7% yield with 98.8% purity and 99% ee. It appears that the catalyst is not removed before the next step in which the oil is hydrogenated over a Raney nickel catalyst to form amino alcohol 3.
Intermediate 3 is also isolated as an oil in 96.4% yield, 96.5% % purity, and 99% ee. After recrystallization from toluene–heptane, the solid product is recovered with 100% ee. In the last step, 3 is treated with fluorotoluene 4 in the presence of t-BuOK to form atomoxetine, isolated as an oil in 91% yield with 97% ee. The purification of 5 and its conversion to the HCl salt are not described.
The inventors provide basic 1H-NMR data for all compounds except 5. The example describing the preparation of 1 lists one of the reactants as 2-acetylthiophene, which is clearly incorrect; and another reagent is called “32% hydrochloride”. These errors should have been spotted by anyone with a fundamental knowledge of chemistry who was involved in writing the patent—perhaps none were. (Sci Pharmtech [Taiwan]. US Patent 8,299,305, Oct. 30, 2012; Keith Turner)