Methyl 1-hydroxyindole-3-acetate (1a) produced novel hexacyclic 8,17-bis(methoxycarbonylmethyl)-1,10-diaza-9,20-dioxakabutane (3) as a major product by the reaction with 85% formic acid, while its reaction with trifluoroacetic acid generated exclusively another 2,2'-bisindole dimer, 1-hydroxy-3,3'-di(methoxycarbonylmethyl)-2,2'-bisindole (4). Reaction of 1a with mineral acid such as HCl afforded nucleophile substituted 5-chloroindole derivative (6). Products and their distribution changed depending on the structure of 1-hydroxyindole. Species of acid, reaction conditions, side chain structure of 1-hydroxyindole at the 3-position are additional factors for governing the product formation. In the cases that the side chain has a C-C-N structure, nucleophilic substitution reaction occurred effectively, and was applied for the preparation of N-feruloylserotonin (46d), an alkaloid isolated from safflower seed. 1-Hydroxymelatonin (69), having a methoxy group on the benzenoid part, afforded (dl)-3a,3a'-bis(pyrrolo[2,3-b]indole) compound (72) by the treatment with 85% formic acid. Products' structures are unequivocally determined by X-ray single crystallographic analyses. © 2020 The Japan Institute of Heterocyclic Chemistry.