4-Benzyloxy-5-methoxy-2-nitrobenzoic acid (I) is converted to the corresponding acid chloride with SOCl2 and subsequently coupled to trans-4-hydroxy-L-proline methyl ester (II) to yield the benzamide (III). After protection of the hydroxyl group of (III) as the t-butyldimethylsilyl ether (IV), reduction of the ester function by means of DIBAL provides aldehyde (V). Simultaneous protection of the aldehyde function in (V) and O-desilylation in the presence of ethanethiol and chlorotrimethylsilane gives rise to thioacetal (VI). The secondary alcohol (VI) is oxidized to ketone (VII) by treatment with TPAP/NMMO. Then, Horner-Emmons olefination of (VII) with methyl diethylphosphonoacetate (VIII) affords the unsaturated ester (IX), which is further hydrolyzed to acid (X) under alkaline conditions.
Alkylation of 1,8-naphthalimide (XI) with N-Boc-1-amino-3-bromopropane (XII) provides (XIII), which is further deprotected with trifluoroacetic acid to yield the aminopropyl naphthalimide (XIV). Coupling between amine (XIV) and acid (X) affords amide (XV). After nitro group reduction in (XV) with SnCl2 in MeOH, the resultant amino thioacetal (XVI) is deprotected and cyclized by means of HgCl2/CaCO3 to furnish the title compound.