Application of Flow Chemistry to Macrocyclization of Crown Ethers

This paper reports a new continuous-flow synthesis of chiral and achiral pyridino-18-crown-6 ethers. Macrocyclizations have been performed in a packed-bed flow reactor where deprotonation of a bifunctional primary or a secondary alcohol takes place with potassium hydroxide as a heterogeneous base avoiding the use of stronger and more dangerous one, sodium hydride. Ditosylate derivatives of pyridine as precursors for the macrocyclization used in batch condition were replaced by the appropriate diiodides and optimization of the parameters provided higher yields in shorter reaction times. The setup presented here is suitable for the preparation of different ethers by Williamsontype syntheses in continuous-flow reactions.