These reactions can also be activated with SulTur and sulfur containing compounds. Ethylene thiourea was com- monly used as an activator, however suspected carcino- genicity is limiting its use(figure 13). Thiocarbanalide is potentially suitable for a replacement chemical, and other reagents have been proposed(ref. 15). Activation by ethyl- ene thiourea has been discussed previously(ref. 16). The activation is provided by the enhanced nucleophilicity of the sulfur atom. The metal oxide is still involved in the(continued on page 101)