S-alkylation of thiacalixarenes: how the regio- and stereoselectivities depend on the starting conformation.

S-alkylation of all four thiacalix[4]arene conformations was accomplished using alkyl triflates. The corresponding sulfonium salts are formed in a highly regio- and stereoselective manner depending on the conformation used. Interestingly, only mono- or disubstituted sulfonium salts can be prepared. Although many regio- and stereoisomers are theoretically possible, only one dialkylated cone and 1,2-alternate derivatives were formed, while only a single isomer of monoalkylated partial cone and 1,3-alternate were isolated. The combination of experimental results with the quantum-chemical approach using the B3LYP/6-311G(d,p) method resulted in the elucidation of the rules governing the regio- and stereochemical outcomes of the alkylation reactions. All S-alkylated compounds represent a novel type of substitution pattern in calixarene chemistry showing the wide-ranging possibility of thiacalixarene skeleton modifications.