There have been several successfull attempts to exchange hexamethylphosphoramide against the two urea derivatives  1,3-Dimethyl-2-imidazolidinone (DMEU) and  1,3-Dimethyltetrahydro-2(1H)-pyrimidinon (DMPU). A systematic survey is given by D.Seebach in

• D.Seebach,R.Henning,T.Mukhopadhyay, Chem.Ber. 115,1705-1720(1982)

Users of the substitutes are found in

• C.J. Gilmore, D.D. MacNicol, A. Murphy, M.A. Russel, Tetrahedron Lett. 24,3269-3272(1983)
• U. Schöllkopf, R. Lonsky, Synthesis, 675-676(1983)
• A.R. Bassindale, T. Stout, Tetrahedron Lett.,26,3403-3406(1985)
• U. Schöllkopf, R. Lonsky, Synthesis, 675-676(1983)
• T. Mukhopadhyay, D. Seebach, Helv.Chim.Acta,65,385-391(1982)

Some aspects make the changeover easier. It is quite difficult to purchase hexamethylphosphoramide and if there is a supplier, the price is higher than of the substitutes. Nevertheless is a question of the concrete synthetic problem if a substitute does work.