Described herein are thiosulfonate reagents of Structure IIa useful for the synthesis of phosphorothiotriesters from H-phosphonates in a stereospecific fashion.wherein,X is alkyl, cycloalkyl, or heteroaryl;R is R1-R2;R1 is selected from -S-alkenylene-, -S-alkylene-, -S-alkylene-aryl-alkylene-, -S-CO-aryl-alkylene-, and -S-CO-alkylene-aryl-alkylene-; andR2 is selected from heterocyclo-alkylene-S-, heterocyclo-alkenylene-S-,aminoalkyl-S-, and (alkyl)4N-alkylene-S-.