19 g of carbonyldiimidazole are added in portions to 33 g of the product obtained in Step B in 350 ml of dichloromethane. When the evolution of gas has stopped, the reaction mixture is stirred for 1 hour at room temperature and then a stream of methylamine is circulated through it. After 12 hours' stirring, the mixture is diluted with water, decanted, washed with 1N sodium hydroxide solution and then with 1N hydrochloric acid, decanted, dried and evaporated, enabling the expected product to be isolated.
