It can be prepared in several different ways: 1) By condensation of (I) with ethanolamine (II) by means of triethylamine in dioxane that gives 2-(2-hydroxyethylamino)-3-(2-chloroethyl)tetrahydro-2H-1,3,2-oxazaphosphorine-2-oxide (III), which is then esterified with methanesulfonyl chloride and triethylamine in dioxane. (1,2) 2) By condensation of 2-chloro-3-(2-chloroethyl)tetrahydro-2H-1,3,2-oxazaphosphorine-2-oxide (I) with 2-(methylsulfonyloxy)ethylamine by means of triethylamine (IV) in CH2Cl2. (1,2) 3) By condensation of (I) with 2-chloroethylamine (V) by means of triethylamine in CH2Cl2 that gives 2-(2-chloroethylamino)-3-(2-chloroethyl)tetrahydro-2H-1,3,2-oxazaphosphorine-2-oxide (VI), which is then transformed into the ester by means of silver methanesulfonate in refluxing acetonitrile. (1,2) 4) By cyclization at N-(3-hydroxypropyl)-2-chloroethylamine (VII) with N-(2-methylsulfonyl-oxyethyl)dichlorophosphoramide (VIII) by means of triethylamine in CH2Cl2. (1,2) 5) By mesylation of 2-(ethyleneimino)-3-(2-chloroethyl)tetrahydro-2H-1,3,2-oxaza-phosphorine-2-oxide (IX) with methanesulfonic acid in ethyl ether. (1,2)