Selective Preparation of Isomeric Tetrahydroxypillar[5]arenes and Pillar[3]arene[2]quinones

The isomeric tetrahydroxylated pillar[5]arenes were synthesized through partial oxidation of 1,4‐dimethoxypillar[5]arene or 1,4‐dimethoxypillar[4]arene[1]quinone by (NH4)2[Ce(NO3)6] and then reduction by Na2S2O4. The intermediate 1,4‐dimethoxypillar[3]arene[2]quinones have two constitutional isomers. The yield of the 1,3‐isomer 1 can reach 42 %, and was far higher than that of the 1,2‐isomer 2 (formed in trace amounts). The approach provides an efficient way to prepare constitutional‐isomeric selectively functionalized pillar[5]arenes 3 and 4. Furthermore, the host–guest complexation of 3 and 4 with a bis(imidazolium) salt was investigated. Tetrahydroxylated pillar[5]arenes were synthesized selectively through the partial oxidation of 1,4‐dimethoxypillar[5]arene or 1,4‐dimethoxypillar[4]arene[1]quinone by (NH4)2[Ce(NO3)6] and then reduction. The intermediates have two constitutional isomers: 1,3‐isomer 1 (42 % yield) and 1,2‐isomer 2 (trace). The complexation of two pillar[5]arene isomers with a bis(imidazolium) salt was investigated.