Uncommon Regioselectivity in Thiacalix[4]arene Formylation

To reveal the alternative ways for upper-rim thiacalixarene derivatization, the formylation reactions (Gross and/or Duff conditions) of the corresponding tetrapropoxythiacalix[4]arene immobilized in the 1,3-alternate conformation were systematically studied. Surprisingly, albeit using an excess of the formylation agent, only two formyl groups were introduced exclusively into the meta positions of thiacalixarene skeleton. Unexpected regioselectivity of these reactions opens the door for a unique substitution pattern in thiacalixarene chemistry. The formation of meta-substituted aldehydes is another illustration showing remarkably different reactivity of the thiacalix[4]arene system compared with that of a classical calyx[4]arene analogue.