Compounds of formula (22) can be prepared from 6-bromobenzothiazolone as shown in Scheme 3. 6-Bromobenzothiazolone is heated in the presence of NaOH base to provide a mixture of 2-amino-5-bromothiophenol (15) and its disulfide (16). The mixture is treated with chlorocarbonyl-acetic acid ethyl ester to provide 6-bromo-benzothiazol-2-yl-acetic acid ethyl ester (17) and 7-bromo-3-hydroxy-4H-benzo[1,4]thiazine-2-carboxylic acid ethyl ester (18), which can undergo rearrangement by treatment with zinc and ethyl acetate to provide compound (17). Compound (17) is reduced with NaBH4 to provide 2-(6-bromo-benzothiazol-2-yl)-ethanol (19). Compound (19) is treated with mesyl chloride in the presence of triethyl amine to afford the corresponding methanesulfonic acid 2-(6-bromo-benzothiazol-2-yl)-ethyl ester (20), which is treated with an amine of formula HNRaRb wherein Ra and Rb each is as defined for R4 and R5, to provide compounds of formula (21). Compounds of formula (21) can be treated with a boronic acid, wherein Rc is aryl or heteroaryl, to provide compounds of formula (22).
