Abstract Our preliminary findings in the use of 1,3,2-oxazaphospholidin-2-ones (III) as useful synthons in the synthesis of naturally occurring helminthic (worm) phosphodiesters [i.e. Opheline(II, R=Me) and serine ethanolamine phosphate (I, R=NH2(CO2H)CHCH2,R′=H)] and of related compounds are reported. The 1,3,2-oxazaphospholidin-2-ones can be synthesised by routes A and B; once formed these can be ring-opened by acid treatment which causes exclusive N-P bond fission yielding the phosphodiesters (I). Guanidinated phosphodiesters (II) have been made by reacting the corresponding phosphodiesters (I,R′=H) with O-methylisourea.