2. Other bases, including KOH, RbOH, and CsOH, also work with approximately the same results, but LiOH is considerably inferior.
3. The (solvated) product can be obtained in crystalline form but, on standing in air for a few minutes, the colorless, glistening needles change to a white powder as the result of loss of solvent of crystallization. Considerable variation in the melting point of this material is noted. The product generally melts above 400°C, but sometimes the melting point falls to ca. 395°C or even lower. Undoubtedly, this is due to impurities such as metal ions and/or other cyclic oligomers that are incompletely removed in the recrystallization (see (Note
4)).
4. Evaporation of the
xylene filtrate and trituration of the residue with
methylene chloride yields ca. 11 g of solid that consists mainly of cyclic hexamer and cyclic tetramer. The
methylene chloride contains, inter alia, cyclic pentamer, cyclic heptamer, and bishomo compound, and these can be obtained as pure samples in low yield by fractional crystallization procedures. The composition of the reaction mixtures can be qualitatively established by TLC by means of the following
Rf values in 9 : 1
petroleum ether/
acetone and 1 : 1
petroleum ether/
methylene chloride, respectively: (a) cyclic octamer—0.54, 0.85, (b) cyclic heptamer—0.40, 0.78, (c) cyclic hexamer—0.54, 0.76, (d) cyclic pentamer—0.74, 0.68, (e) cyclic tetramer—0.63, 0.66, and (f) bishomooxa compound—0.66, 0.56.