1,3-Dimethyl-2-oxo-hexahydropyrimidine (DMPU, or N,N′-dimethylpropyleneurea) is found to be a good substitute for the carcinogenic hexamethylphosphoric triamide (HMPT) as cosolvent in the alkylation of lithium 1-alkynides, key intermediates in moth sex pheromone synthesis.