Electrophilic iodocyclization reaction scheme

The [2,3- ]> [2,3- ]. and [i,Z-b] isomers of 2,3-disubstituted furopyridines can be obtained from alkynes and o-iodoacetoxypyridines by a palladium-catalyzed cross-coupling reaction followed by an electrophilic iodocyclization reaction or a palladium-catalyzed cyclization in the presence of CO, as illustrated in Scheme 32 for [2,3-3] and [3,2-3] derivatives. When the pyridine ring contains strongly electron-withdrawing substituents such as carboethoxy, the reaction proceeded smoothly with benzyloxyalkynylpyridines <20020L2409>. 

Verma et al. [62] observed a chemoselective behavior of iodine in diSeient solvents in the electrophilic iodocyclization of o-alkynyl aldehydes. o-Alkynyl aldehydes on reaction with in CH C with appropriate nucleophiles provided pyrano[4,3-6]quinolines 40 via the formation of cyclic iodonium intermediate 39. In case of using alcohols as a solvent as well as nucleophile, o-alkynyl esters 42 were obtained selectively in good to excellent yields via the formation of hypoio-dide intermediate 41. Subsequently, o-alkynyl esters 42 were converted into py-ranoquinolinones and isocoumarins by electrophilic iodocyclization. The developed oxidative esterification provides a novel access for the chemoselective synthesis of esters 43 from aldehydes without oxidizing primary alcohol present in the substrate (Scheme 10.28). 

An expedient method for synthesis of 2,3-disubstituted 1-benzoselenophenes (91) by the electrophilic cyclization of various 1 -(1 -alkynyl)-2-(methylseleno)arenes (90) has been reported recently (Scheme 24). This method tolerates a wide of variety of functional groups, and proceeds under exceptionally mild reaction conditions [141], Moreover, an efficient solid phase synthesis of 91 based on a combination of palladium-mediated coupling and iodocyclization protocols has been developed [142],... 

Formation of a highly electrophilic iodonium species, transiently formed by treatment of an alkene with iodine, followed by intramolecular quenching with a nucleophile leads to iodocyclization. The use of iodine to form lactones has been elegantly developed. Bartlett and co-workers216 reported on what they described as thermodynamic versus kinetic control in the formation of lactones. Treatment of the alkenoic acid 158 (Scheme 46) with iodine in the presence of base afforded a preponderance of the kinetic product 159, whereas the same reaction in the absence of base afforded the thermodynamic product 160. This approach was used in the synthesis of serricorin. The idea of kinetic versus thermodynamic control of the reaction was first discussed in a paper by Bartlett and Myerson217 from 1978. It was reasoned that in the absence of base, thermodynamic control could be achieved in that a proton was available to allow equilibration to the most stable ester. In the absence of such a proton, for example by addition of base, this equilibration is not possible, and the kinetic product is favored. 

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