Biocatalytic aromatization of indolines into indoles by monoamine oxidase (MAO-N) whole cells

Indoles represent one of the most important heterocyclic rings which provide privileged scaffolds for the discovery of new drug candidates with different pharmacological activities. Also, the indole moiety can be found in many therapeutically active natural alkaloids. In view of the pharmacological significance of these heterocyclic compounds, the synthesis of indoles has been a major object of research for over hundred years in organic chemistry. Among the typical approaches for the synthesis of indoles, the Fischer’s reaction remains an extremely useful and important method; other methods to access indoles include the use of metal catalysts like Pd, Au, Co or oxidising agents like DDQ. All these methods often require the use of harsh conditions such as high temperatures and acid catalysts that are in many cases environmentally unfriendly and hazardous. With the aim to overcome these limitations, a novel, greener and more sustainable approach for the preparation of indole compounds has been developed. In our previous work, we described the aromatizing properties of monoamine oxidase (MAO-N) biocatalysts. Herein, the aromatization of differently-substituted indolines into the respective indole products exploiting freeze-dried MAO-N whole cell biocatalysts is presented. In the optimization of this methodology the use of different co-solvents as well as the choice of MAO-N variant was explored. Indoline substrates were obtained via photocatalysis from anilines or through arylative dearomatization of indoles. The new methodology proves to be a sustainable alternative to standard catalytic methods for the synthesis of indoles under milder conditions.