Vanillic acid (I) was alkylated with ethyl bromobutyrate (II) to afford ether (III). The ester function of (III) was then subjected to saponification, yielding diacid (IV). Nitration of (IV) gave the nitro compound (V). Selective esterification of the aliphatic carboxyl group of (V) was achieved employing p-toluenesulfonic acid in methanol, and the resultant mono-acid was further converted to the corresponding acid chloride (VI) upon treatment with oxalyl chloride. Coupling of acid chloride (VI) with (S)-pyrrolidine-2-carbaldehyde diethyl dithioketal (VII) provided amide (VIII). After the reduction of the nitro group of (VIII) by means of SnCl2, the resultant amino compound (IX) was protected as the N-Fmoc derivative (X). Thioketal deprotection of (X) with concomitant cyclization in the presence of HgCl2 and CaCO3 gave rise to the pyrrolobenzodiazepinone system (XI). The methyl ester of (XI) was then hydrolyzed under acidic conditions to furnish acid (XII).

Acid (XII) was coupled with the indolyl amine (XIII) by means of EDC to give amide (XIV). Finally, simultaneous removal of the Fmoc protecting group of (XIV) and cyclization of its seco-cyclopropapyrroloindole moiety were accomplished in one step by treatment with tetrabutylammonium fluoride in DMF, producing the target compound.