Uncommon Regioselectivity in the Thiacalix[4]arene Series: Gross Formylation of the Cone Conformer

Thiacalix[4]arene immobilized in the cone conformation undergoes a direct Gross formylation reaction (Cl2CH-O-CH3/SnCl4/CH2Cl2) to give the upper-rim formylated thiacalixarene. Albeit using excess of the formylation agent and various reaction temperatures, only one formyl group is introduced into the meta position of the thiacalixarene skeleton. The surprising regioselectivity indicates dramatically different reactivity of the thiacalix[4]arene system when compared with a classical calix[4]arene analogue, which yields exclusively para isomers. The introduction of functional groups into the meta position represents an exceptional substitution pattern in thiacalixarene chemistry, which imparts in interesting conformational behavior to these compounds. The systematic NMR study revealed that the pinched cone-pinched cone equilibrium is remarkably shifted toward one pinched cone structure depending on the substitution.