Cucurbit[n]uril analogues:: Synthetic and mechanistic studies

The synthesis of cucurbit[n]uril analogues (18, 19, (+/-)-20, 33, 34, 35, 36, and 37) is presented. These CB[5], CB[6], and CB[7] analogues all contain bis(phthalhydrazide) walls that are incorporated into the macrocycle. The tailor-made synthesis of these CB[n] analogues proceeds by the condensation of the appropriate bis(electrophile) (4, 7, or 9) with bis(phthalhydrazide) (17), which delivers the CB[6] and CB[7] analogues in good yield, whereas the CB[5] analogue is formed in low yield. To improve the solubility characteristics of the CB[n] analogues for recognition studies in water or organic solution, the CO2Et groups were transformed to CO2H and CO2(CH2)(9)CH3 groups. On the basis of the results of product resubmission experiments, we conclude that these macrocycles are kinetic products. To help rationalize the good yields obtained in the CB [6] and CB [7] analogue macrocyclization reactions, we performed mechanistic studies of model methylene bridged glycoluril dimers, which suggest an intramolecular isomerization during CB[n] analogue formation.