Selective synthesis and isolation of all possible conformational isomers of proximally para-disubstituted calix[4]arene.

All six possible conformational isomers of the proximally p-dibrominated calix[4]arene tetraalkyl ether, 1a-f*, were selectively synthesized by appropriate control of stereochemistry during di-O-alkylation reactions of 5,11-dibromocalix[4]arene syn-dialkyl ethers, namely, 5,11-dibromo-27,28-dihydroxy-25,26-dipropoxy-, 5,11-dibromo-25,26-dihydroxy-27,28-dipropoxy-, 5,11-dibromo-25,28-dihydroxy-26,27-propoxy-, and 5,11-dibromo-26,28-dihydroxy-25,27-dipropoxycalix[4]arenes. Their conformations were confirmed by (1)H and (13)C NMR spectroscopy and are cone for 1a (u(Br)(Pr), u(Br)(Pr), u(H)(Pr),u(H)(Pr)), partial cone for 1b (u(Br)(Pr), d(Br)(Pr), u(H)(Pr),u(H)(Pr)) and 1d (u(Br)(Pr), u(Br)(Pr), u(H)(Pr),d(H)(Pr)), 1,2-alternate for 1c (u(Br)(Pr), u(Br)(Pr), d(H)(Pr),d(H)(Pr)) and 1e (u(Br)(Pr), d(Br)(Pr), d(H)(Pr),u(H)(Pr)), and 1,3-alternate for 1f (u(Br)(Pr), d(Br)(Pr), u(H)(Pr),d(H)(Pr)). Although both 1c and 1e are in the 1,2-alternate conformation, the conformation of 1e was found to be strongly distorted and distinct from that of 1c.