Inverted cucurbiturils or iCB[x] are CB analogues with one glycoluril repeating unit inverted. In this unit the methine protons actually point into the cavity and this makes the cavity less spacious. Inverted cucurbiturils form as a side-product in CB-forming reactions, with yields between 2 and 0.4%. Isolation of this type of CB compound is possible because it is more difficult to form inclusion compounds that ordinarily form with regular CBs. Inverted cucurbiturils are believed to be the kinetically controlled reaction products because the heating of iCB[6] in acidic medium results in a mixture of CB[5], CB[6] and CB[7] in a 24:13:1 ratio.
A cucurbituril cut in half along the equator is called a hemicucurbituril.