| Summary: | Thiacalix[4]arene monosulfoxide <b>4</b> possesses a very unusual chemistry, as demonstrated by several unprecedented derivatives in calixarene chemistry. Interestingly, compound <b>4</b> cannot be prepared by the dealkylation of its corresponding tetramethoxy derivative using BBr<sub>3</sub>. Instead, the borate complex is formed with a boron bound by the two neighboring phenolic oxygens and a sulfoxide group. A similar type of borate complex with a spirodienone fragment was then isolated as a by-product. The oxidation of monosulfoxide with Chloramine-T did not provide the expected spirodienone moiety, but rather a complex oxathiane-based spiroheterocyclic part containing a chlorine atom. X-ray analyses confirmed the structures of the unusual products and feasible formation mechanisms were proposed. These results provide further evidence of the distinction between thiacalixarene chemistry and the chemistry of classical CH<sub>2</sub> analogues.
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