583. The formation of crystalline complexes between urea and esters, and their application to the separation of mixtures of esters

Polymer Inclusion Compounds

“…The earliest studies of the thermodynamic properties, physical characteristics, uses, and other aspects of the urea and TU channel ICs were reported by Schlenk and Bengen [1,[28][29][30], Redlich" et al [31], Zimmerschied and coworkers [32], Schlenk and Holman [33], Fetterely [34], Linstead and Whalley [35], and other investigators [36][37][38]. In 1946, Mikus, Hixon, and Rundle [39] first characterized the amylase-fatty acid and alcohol ICs, while Cramer [9] discovered a number of unique ICs using CDs.…”

Bis(trimethylsilyl)ketene Acetals as C,O‐Dinucleophiles: One‐Pot Formation of Polycyclic γ‐ and δ‐Lactones from Pyridines and Pyrazines

“…The search for a direct, one-pot transformation of the pyridines into lactones was therefore essential, but attempts to transform the acids into lactones by the classical protolactonization reaction failed. [51,52] The following observations gave us a first solution to this problem: the interaction of the ketene acetal 7b with pyridine in the presence of methyl chloroformate afforded the expected dihydropyridine 12b (vide supra) after fast silica gel chromatography, but during longer lasting purifications by the same technique, a second, slightly more polar, product (m.p. 108°C, 5-10 % yield) was isolated as a white solid.…”

Urea and Thiourea Complexes in Separating Organic Compounds

“…There are three main types of separation, namely, separations based on differences in chain length, degree of unsaturation, and branching, I n separations based on differences in chain length (34,41,61,70) advantage is taken of the fact that the longer chain compounds form urea complexes preferentially. Therefore, if insufficient urea is employed to combine with all the components of a mixture the longer chain components will combine with the urea and precipitate as complexes.…”