Substituted 1-(pyridin-2-yl)-2,4,6-triphenylpyridinium perchlorates 1b-1e were converted with potassium ferricyanide and potassium hydroxide to sterically crowded 2-phenyl-3-[(Z)-1,3-diphenyl-3-oxopropenyl]imidazo[1,2-a]pyridines 2b-2e accompanied by minor isomeric 2-benzoyl-3,5-diphenyl-1-(pyridin-2-yl)pyrroles 3c-3e. 4-Phenyl-2,6-di(4-substituted phenyl)-1-(pyridin-2-yl)pyridinium salts 4a, 4b gave exclusively corresponding imidazo[1,2-a]pyridines 5a, 5b while the ferricyanide oxidation of 1-(5-iodo- and 5-cyanopyridin-2-yl)-2,4,6-triphenylpyridinium perchlorates 6a, 6b led to mixtures of major imidazo[1,2-a]pyridines 7a-7c and minor pyrroles 8a-8c. Some mechanistic aspects of the oxidation procedure are discussed in connection with a resistance of 2,6-diphenyl-1,3,5-trimethylpyridinium perchlorate (9c) towards the oxidizing agents.
用
钾亚
铁氰化钾和
氢氧化钾将1-(
吡啶-2-基)-
2,4,6-三苯基吡啶盐酸盐1b-1e转化为空间位阻的2-苯基-3-[(Z)-1,3
-二苯基-3-氧代
丙烯基]
咪唑[1,2-a]
吡啶2b-2e,伴随着少量异构体2-苯甲酰基-3,5
-二苯基-1-(
吡啶-2-基)
吡咯3c-3e。4-苯基-2,6-二(4-取代苯基)-1-(
吡啶-2-基)
吡啶盐4a、4b仅产生相应的
咪唑[1,2-a]
吡啶5a、5b,而对5-
碘和5-
氰基吡啶-2-基-1-(
吡啶-2-基)-
2,4,6-三苯基吡啶盐6a、6b的亚
铁氰化氧化导致主要的
咪唑[1,2-a]
吡啶7a-7c和次要的
吡咯8a-8c混合物。讨论了氧化程序的一些机理方面,涉及2,6
-二苯基-1,3,5-三
甲基吡啶盐酸盐9c对氧化剂的抵抗性。