I am aware of one of these modifications, specifically on that magical 5-methoxy-position. This is the sterically similar, but metabolically totally dissimilar fluorine analogue, 5-fluoro-alpha-methyltryptamine, or 5-F-a-MT. The fluorine atom is the darling of the manipulators of molecular structure in that it is a form of fake hydrogen. True, as an atomic lump on an aromatic ring it is a lot larger and a lot heavier, but it is a lump that doesn't like to associate with anything else. It's bonding with a ring carbon is the same sort of two-electron bond that the hydrogen atom makes, but it cannot be oxidized off in the same way. So, when a drug has a position of sensitivity to oxidation, and that oxidation is thought to be responsible for some particular pharmacological property, put a fluorine there and you will deny the drug that property. This was discussed in the section on DET where the metabolism attacks a 6-position hydrogen.
And it may play such a role here. When one balances the sort-of stimulant nature of a-MT with the potent psychedelic properties of a,O-DMS, one can ask if the oxidation of the tryptamine system at this 5-position can be of some significance. Tryptamine becomes serotonin by this action. DMT becomes bufotenine by this action. If there is some extension of this to the a-MT world, then the placement of a fluoro group at that position of attack would be interesting. 5-F-a-MT has been made, and it appears to be of reduced activity in man. But it has proven to be an extremely potent monoamineoxidase inhibitor, and strongly influences the brain serotonin levels. The 6-fluoro isomer, 6-F-a-MT, is also effective.