| Abstract
| - Inverted cucurbit[n]urils (iCB[n]) form as intermediates during the synthesis of cucurbit[n]urils fromglycoluril and formaldehyde in HCl (85 °C). Product resubmission experiments establish that thediastereomeric iCB[6] and iCB[7] are kinetic products that are less stable thermodynamically thanCB[6] or CB[7] (>2.8 kcal mol-1). When iCB[6] or iCB[7] is heated under aqueous acidic conditions,a preference for ring contraction is noted in the formation of CB[5] and CB[6], respectively. Interestingly,under anhydrous acidic conditions ring size is preserved with iCB[6] delivering CB[6] cleanly. To establishthe intramolecular nature of the iCB[6] to CB[6] conversion under anhydrous, but not aqueous, acidicconditions we performed crossover experiments involving mixtures of iCB[6] and its 13CO labeledisotopomer 13C12-iCB[6]. An unusual diastereomeric CB[6] with a Möbius geometry (13) is proposed asa mechanistic intermediate in the conversion of iCB[6] to CB[6] under anhydrous acidic conditions. Theimproved mechanistic understanding provided by this study suggests improved routes to CB[n]-typecompounds.
|
