Intermediate (III) is prepared by the following route. Acylation of 2-Boc-tetrahydropyridoindole (I) with dimethylcarbamoyl chloride provides the 9-carbamoyl derivative (II). Subsequent Boc group cleavage under acidic conditions furnishes amine (III).
Tetrahydrobenzo[cd]indol-2-one (IV) is selectively alkylated at position 2a with 1,4-dibromobutane (V) in the presence of NaH to provide the bromobutyl derivative (VI). Then, condensation of bromide (VI) with amine (III) employing K2CO3 in DMF leads to the title compound.