November 26, 2012
Here’s an improved synthesis of benzazepines. Benzazepines such as compound 3 are intermediates for synthesizing drugs used to treat heart and kidney disorders. Y. Torisawa and co-inventors state that existing methods for preparing the intermediates are inefficient for industrial production because of low yields and purity.
The route they used for preparing 3 is the reaction of chlorotetrahydrobenzazepine 1 with bromoaniline derivative 2 in the presence of CO and a Pd–Ph3P catalyst (Figure 1); DBU is 1,8-diazabicyclo[5.4.0]undec-7-ene. The product is isolated, purified by column chromatography, and recovered in 85% yield and 99.1% purity. The patent contains 1H and 13C NMR, IR, and mass spectroscopy data.
The reaction gives small quantities (≈0.01–0.03 wt%) of four byproducts that are formed by the reaction of 3 with 1. These compounds are easily separated from 3, and their 1H NMR data are reported.
The inventors also describe the synthesis of 2 and several structurally related compounds by routes outlined in Figure 2. The preparation of 2 begins with the condensation of toluidine 4 and benzoyl chloride 5 in the presence of NaOH to form amide 6, isolated in 96.6% yield. In the second step, 6 is brominated in HOAc, and 2 is isolated in 97% yield. Although the purities of 2 and 6 are not reported, both have sharp melting points; 1H NMR data are provided.
Compound 6 can also be used to prepare acid 7 by treating it with (COCl)2 in the presence of AlCl3, and then hydrolyzing the product. The crude acid is isolated, treated with aq NaOH, and recovered in 65.8% yield and 99.4% purity after recrystallization from MeOH.
The reaction of 6 and AcCl in the presence of AlCl3 gives compound 8, isolated in 66.8% yield. The purity is not reported, but the compound has a sharp melting point, and 1H NMR data are given. Oxidizing 8 with NaOCl forms 7, which is isolated in 77.2% yield and 99.8% purity after recrystallization from MeOH.
A key feature of the processes described in this patent is the commercial availability of starting materials 1 and 2. The inventors claim that the processes give the desired compounds in higher yields and purities than existing methods. (Otsuka Pharmaceutical [Tokyo]. US Patent 8,273,735, Sept. 25, 2012; Keith Turner)