Saponification of ester (IV) yields the alkoxycarboxylic acid (VII), which is coupled with 5-aminotetrazole in the presence of 1,1'-carbonylbis(1H-imidazole) (CDI) to provide the carboxamidotetrazole (VIII). Reaction of (VIII) with sodium hydroxide furnishes CI-959 (IX) as the tetrazole sodium salt.
Alternate routes to the key benzo[b]thiophene ester intermediates (IV) are described: 1) Alkylation of the thiol ester (I) yields the diester (II), and a Dieckmann condensation of (II) with base provides the benzo[b]thiophene enol ester (III). Alkylation of (III) with 2-bromopropane then yields the ester intermediate (IV, R = CH3). 2) A shorter route to (IV) begins with cyclization of the cinnamic acid derivative (V) with thionyl choride (2) to give the chloro acid chloride (VI). Reaction of (VI) with the potassium salt of 2-propanol yields the ester (IV, R = CH(CH3)2).