https://www.laserlab-europe.eu/publications/publications-access-2019-2024/reactivity-of-indolylmethylacetates-with-n-o-and-s-soft-nucleophiles-evidence-of-2-alkylideneindolenines-and-3-alkylideneindoleninium-generation-by-esi-ms-and-irmpd-spectroscopy