precursor of all cucurbiturils, its sulfur analog represents a yet unmet syntheticchallenge. The reaction between glyoxal and thiourea was found to stop at the level of dihydroxyimidazolidine-2-thione, which is quite unstable under various acidic conditions. In an attempt to answer these questions, several stable analogs of thioglycoluril, that is, monothioglycoluril, ditolylthioglycoluril, and its diether