The Lithiation/Oxygenation Approach to Calix[6]arenes Selectively Functionalized at a Pair of Opposite Methylene Bridges

Unusual Calixarenes Incorporating Chromene and Benzofuran Moieties Obtained via Propargyl Claisen Rearrangement

Monopropargyloxy-tripropoxy-calix[4]arene 1 was subjected to a propargyl Claisen rearrangement to give unusual calix[3]arene[1]chromene and homocalix[3]arene[1]benzofuran macrocycles. Quantum mechanical density functional theory calculations indicated that an initial [3,3] sigmatropic reaction affords a highly reactive allene intermediate, stabilized by two main diradical pathways leading to six- and five-membered oxygenated rings. In the presence of a n-butylammonium guest, calix[3]arene[1]chromane 6 forms two stereoisomeric complexes stabilized by ⁺N–H···O and cation···π interactions.

Threading of Conformationally Stable Calix[6]arene Wheels Substituted at the Methylene Bridges

Calix[6]arenes disubstituted at the methylene bridges, which are stable in the cone or 1,2,3-alternate conformation, form pseudorotaxanes with dialkylammonium axles. The cone wheel-based pseudorotaxanes are 10-100 times more stable than those obtained with the native conformationally mobile calix[6]arene wheel, as a consequence of their higher degree of preorganization. The threading of conformationally stable 1,2,3-alternate calix[6]arenes is unprecedented in the literature. Therefore, very peculiar NMR features are here evidenced for this threading process involving the less symmetrical 1,2,3-alternate calix[6]arene conformation, which implies a peculiar rototranslation motion of the axle.

Calixrotanes: Calixarenes Incorporating Spiro-Linked Cyclopropyl Groups

Macrocyclic compounds (calix[4]- and calix[6]arene derivatives) with aryl rings interconnected by spirocyclopropyl groups have been synthesized and structurally characterized. The compounds were prepared by the reaction of dichlorocarbene with calixarenes possessing exocyclic double bonds at the bridges, followed by reductive perdechlorination of the spirocyclopropyl groups. In all systems, pairs of geminal rings connected to the quaternary spiro carbon atoms are oriented anti, and the methylene groups of the cyclopropyl rings are located in isoclinal positions. Calix[6]rotane adopts in the crystal and in solution a 1,3,5-alternate conformation. The presence of the spirocyclopropyl groups increases the rigidity of the macrocyclic ring.