Aza-Diels-Alder reactions: An approach for construction of pro-mesogenic quinoline cores

Full text: Aza-Diels-Alder reaction is a powerful methodology for the preparation of useful 6-membered N-Heterocyclic. Schiff bases (SBs) are known to generated liquid crystals with very large mesophase range, but the main drawback is its inherent instability upon heating[1]. In this communication we are prompted to disclose our synthetics efforts for the construction of pro-mesogenic quinolone core by use of cycloaddition between SBs and dienophiles - alkynes and alkenes. A multicomponent methodology was used to synthesize tetrahidroquinolines and quinolones as exemplified by the Scheme 1. SBs were generated in situ by reaction of anilines 2 and aldehyde 1 and then, alkene or alkyne was added to yield the corresponding cycloadducts. Quinoline 3 was obtained by DDQ oxidation of tetrahidroquinole, whereas quinolines 5a-b and 7a-b were prepared by aromatization of the intermediate cycloadduct. All products were characterized by ¹H and ¹³C NMR and HRMS. MOLP showed some different textures for quinolines 5a-b and 7a-c, which are being investigated by X-ray, TGA and DSC analyses. [1] L. Fritsch, A. A. Merlo, ChemistrySelect vol 1. 23 – 29 (2016); [2] L. Fritsch, V. Lavayen, A. A. Merlo, Liq. Cryst. vol 45, 1-11 (2018). (author)