Stereospecific single step cyclization to give belt-functionalized pillar[6]arenes

Two macrocycles were synthesized through cyclization reactions of secondary benzylic alcohols giving pillar[6]arenes with a methyl substituent at each belt position. These macrocycles form stereospecifically with the only observed isomer being the rtctct isomer with alternating up and down orientations of the belt methyl groups. Isolated yields were modest (7 and 9%) but the macrocycles are prepared in a single step from either a commercially-available alcohol or very readily-prepared precursor. X-ray crystal structures of the macrocycles indicate they have a capsule-like structure, which is far from the conventional pillar shape. Density functional theory calculations reveal that the energy barrier required to obtain the pillar conformation is significantly higher for these belt-functionalized macrocycles than for the conventional belt-unfunctionalized pillar[6]arenes.