Scheme 4

Synthesis of the phosphoramidite reagents. a) BnOH, Et3N, Et2O; b) 4, dry DCM, 1H-Tetrazole at 0°C, 15 min at rt; c) 9, dry DCM, 1H-Tetrazole at 0°C, 15 min at rt.
Synthesis of the phosphoramidite reagents. a) BnOH, Et3N, Et2O; b) 4, dry DCM, 1H-Tetrazole at 0°C, 15 min at rt; c) 9, dry DCM, 1H-Tetrazole at 0°C, 15 min at rt.