A modification of Leimgruber-Batcho synthesis of indoles was employed to convert 3-methyl-4-nitrobenzoic acid (I) to the 3,5-disubstituted indole (V)). In this process, the intermediate enamine (II) was alkylated with the bromobenzoate (III) prior to hydrolysis and reductive cyclization to the indole nucleus. Alkylation of (V) with iodomethane under basic conditions, followed by amide formation using (R)-4,4,4-trifluoro-2-methylbutylamine (VII) afforded the key intermediate amidoester (VIII). Hydrolysis of the methyl ester moiety of (VIII) with aqueous lithium hydroxide, followed by sulfonimide formation gave ZD 3523).
The synthesis of the key chiral amine component (VII) is outlined: Conversion of 4,4,4-trifluorobutyric acid (XI) to the corresponding acid chloride and subsequent reaction with the anion derived from the Evans oxazolidinone (XII) yielded the acyloxazolidinone (XIII). Treatment of (XIII) with a strong base such as sodium hexamethyldisilazide and alkylation of the resultant enolate with iodomethane at -35 C afforded a mixture of the diastereomeric acyloxazolidinones (XIV). The desired diastereomer was obtained by crystallization of the crude product. Reductive cleavage of the chiral auxiliary gave the enantiomerically pure alcohol (XV), which was converted in two steps to the desired enantiomerically pure amine hydrochloride (VII). Alternative routes to the amine, such as resolution of racemic 4,4,4-trifluoro-2-methylbutyric acid followed by amide formation and reduction to (VII) could also be utilized.