posted on 2007-09-14, 00:00authored byHana Dvořáková, Jan Lang, Jiří Vlach, Jan Sýkora, Michal Čajan, Michal Himl, Michaela Pojarová, Ivan Stibor, Pavel Lhoták
NMR spectroscopy, X-ray diffraction analysis, and quantum chemical calculations were used for
conformational behavior study of partially alkylated thiacalix[4]arenes bearing methyl (1), ethyl (2), or
propyl (3) groups at the lower rim. The conformational properties are governed by two basic effects: (i)
stabilization by intramolecular hydrogen bonds, and (ii) sterical requirements of the alkoxy groups at the
lower rim. While the monosubstituted derivatives 1a and 3a adopt the cone conformation in solution, distally
disubstituted compounds 1b, 1‘b, 2b, 2‘b, 3b, and 3‘b exhibit several interesting conformational features.
They prefer pinched cone conformation in solution, and, except for 3‘b, they form also 1,2-alternate
conformation, which is flexible and undergoes rather fast transition between two identical structures. The
crystal structures of the compounds 1b, 2b, 2‘b, and 3b revealed yet quite rare 1,2-alternate conformation
forming molecular channels held together by π−π interactions. Different channelswith hexagonal symmetry,
0.26 nm wideare formed in the crystal structure of the pinched cone conformation of 3b. An uncommon
hydrogen bonding pattern was found in dimethoxy and dipropoxy derivatives 1‘b and 3‘b that adopt distorted
cone conformations in crystal. Trialkoxy-substituted compounds 1c and 3c adopt the partial cone conformation
in solution. A higher mobility of methyl derivative 1c enables also existence of the cone conformer.