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-25,28dihydroxy-26,27-propoxy-, and 5,11-dibromo-26,28-dihydroxy-25,27-dipropoxycalix[4]arenes. Their conformations were confirmed by H-1 and C-13 NMR spectroscopy and are cone for 1a (u(Pr)(Br), U-Pr(Br), u(Pr)(H), u(Pr)(H)), partial cone for 1b* (u(Pr)(Br), d(Pr)(Br), u(Pr)(H)) and 1d* (u(Pr)(Br), u(Pr)(Br), d(Pr)(H)), 1,2-alternate for 1c (u(Pr)(Br), u(Pr)(Br), d(Pr)(H),d(Pr)(H)) and 1e* (u(Pr)(Br), d(Pr)(Br), d(Pr)(H),U-Pr(H)), and 1,3-alternate for 1f* (u(Pr)(Br)) d(Pr)(Br), U-Pr(H),d(Pr)(H)). 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.