Pyridinium triiodides

Furo[2,3- ]pyridines can be synthesized from alkynylpyridones and iodonium sources (Scheme 31) <20060L1113>. Iodine proved to be much more effective at promoting the iodocyclization reaction than other iodonium sources (ICl, A -iodosuccinimide (NIS)). The pyridinium triiodide salt, 104, can be converted into the corresponding pyridinone by treatment with an external source of iodide. In a variation of the reaction, a one-pot synthesis of the furopyridine derivatives 105 can be achieved, with overall yields of 79-92%, by treatment with iodine followed by sodium iodide without isolation of the triiodide salt. Another similar one-pot synthesis involves 3-iodo-2-pyridones, terminal alkynes, and organic halides in a series of two palladium cross-coupling reactions (Equation 45) <20030L2441>. This reaction could also be carried out in a two-step sequence, but the overall reaction yields were typically improved for the one-pot method. 

The crude 2-(2-thiophenyl)pyridine 219 (R2 = H) in ethanol was treated with a saturated solution of iodine in ethanol until the oxidation and precipitation of the crystalline l,2-benzisothiazolo[2,3-a]pyridinium triiodide 222a was complete. This salt 222a was filtered and recrystallized from nitromethane as brown needles (67%) (85CJC882, Scheme 78). 

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