MESITYLENE-DERIVED 1,3-ALTERNATE [1.1.1.1]METACYCLOPHANES

Synthetic procedures to conformationally immobile 1,3-alternate [1.1.1.1]metacyclophanes based on mesitylene units are described. Coupling of mesitol (19) with bis(chloromethyl)mesitol (20) and bis(chloromethyl)mesitylene (21) in nitroethane in the presence of SnCl4 affords metacyclophanes 6 and 7-10, respectively, possessing extra-annular hydroxyl groups. Metacyclophanes 12 and 14, which hold one or two pairs of carboxyl groups in the distal positions, have been obtained from the appropriate distal diol 8 and tetrol 6 by treatment with tert-butyl bromoacetate followed by basic hydrolysis. Functionalized molecular frameworks, such as 15 and 18, have been also prepared in high yield by direct methods. The H-1 NMR spectral characteristics of 1,3-alternate metacyclophanes synthesized are briefly discussed. The structures of mono-, tri-, and tetrahydroxy metacyclophanes 10, 7, and 6, respectively, have been determined by X-ray crystallography. All three macrocycles have very similar 1,3-alternate-biconic conformations with approximate 42mBAR symmetry. Molecules 10 and 7 are isomorphous, and the lone hydroxyl group in 10 and the three hydroxyl groups in 7 are disordered over four possible sites. A toluene of solvation is docked against the molecular cavity in both 10 and 7. Compound 6 has crystallographic 2-fold symmetry and what would have been voids in the crystal lattice are occupied by disordered solvent molecules.