A metal-free cyclization of N-propargylamides for the synthesis of various oxazolines and oxazoles via a 5-exo-dig process is presented. Using (diacetoxyiodo)benzene (PIDA) as a reaction promoter and lithium iodide (LiI) as an iodine source, intramolecular iodooxygenation of N-propargylamides proceeded readily, leading to the corresponding (E)-5-iodomethylene-2-oxazolines in good to excellent isolated yields. In addition, using the PhI(OAc)2/LiI system, N-propargylamides can be converted to the corresponding oxazole-5-carbaldehydes in the presence of oxygen under visible light irradiation. The resulting products can be further converted into various oxazoline and oxazole derivatives after simple derivatizations, and this method ultimately offers an efficient route to a variety of biologically active structures.
A metal‐free method for the construction of oxazolidine‐2,4‐diones and oxindoles was discussed. Using iodine monochloride (ICl) as both the reaction promoter and iodide source, the iodolactonization of N‐Boc acrylamides proceeded readily and provided the corresponding iodo oxazolidine‐2,4‐diones and oxazolidin‐2‐ones in good isolated yields. The obtained oxazolidine‐2,4‐diones can be used as key intermediates in the synthesis of toloxatone. When N‐alkyl‐N‐arylacrylamide derivatives were subjected to the same reaction, iodocarbocyclization products 3,3‐disubstituted oxindoles were obtained. The obtained oxindoles can be used as key intermediates in the synthesis of the alkaloids (±)‐esermethole and (±)‐physostigmine.
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