Stereo- and regiochemistries of the oxidations of 2-methoxy-5-tert-butyl-1,3,2-dioxaphosphorinanes and the cyclic methyl 3',5'-phosphite of thymidine by water/iodine and oxygen/AIBN to P-chiral phosphates. Oxygen-17 NMR assignment of phosphorus configuration to the diastereomeric thymidine cyclic me