The synthesis of dithymidine boranophosphate by the oxathiaphospholane approach

The application of the oxathiaphospholane approach for the synthesis of dit hymidine boranphospate was evaluated. It was shown, that although the nucle oside-3'-O-oxathiaphospholane-borane complexes 2 or 6 could not be chromato graphically separated into diastereomerically pure species due to their app arent instability to moisture, they can be successfully applied to the non- stereocontrolled formation of internucleotide boranophosphate bond by react ion with 5'-OH-nucleoside in the presence of DBU. Attempts to apply the rel ated dithiaphospholane approach for the preparation of dithymidine boranoph osphorothioate were unsuccessful.