General method for cyclization of an enaminone

When 4//-l,3-benzothiazinc-4-thiones react with propargylamine, imidazoles arc formed in 53-72% yields via amidinc intermediates. This approach is, however, a little exotic to compete with the more common methods [25]. 

A mixture of DAMN (2.0 g, 18.5 mmol) and triethyl orthoformate (2.74 g, 18.5 mmol) in dioxane (31.5 ml) is healed in a flask fitted with a short Vigreux column, distillation head, condenser and receiver. Ethanol mixed with dioxane is removed continuously until the temperature at the distillation head reaches 90°C ( 17ml, 10 min). The clear brown solution remaining is allowed to cool overnight before the addition of hexane (16 ml), which precipitates dark brown crystals. Complete precipitation is ensured by the addition of further hexane. The filtered solid is dissolved in the minimum volume of hot diethyl ether, filtered to remove a dark brown solid impurity, then cooled to give (27) as white needles (2.55 g, 84%), m.p. 132.5°C (dec.). 

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