Phosphorothioates diastereoselective synthesis

In continuation of work in Just s laboratory on the diastereoselective synthesis of dinucleoside phosphorothioates, the chlorophosphoramidite 219 (for the synthesis of the aminoalcohol see Chapter 14) was converted as indicated in Scheme 8 into the Sp-isomer 220, in a 6 1 ratio with the diastereomer, and this ratio could be improved by the use of a more hindered base in step ii. The auxiliary 221, derived from L-tryptophan, has also been used. When 221 was treated sequentially with 5 -0-Tbdms-thymidine, 3 -0-Tbdms-thymidine and Beaucage s reagent, a phosphorothioate triester was produced. The auxiliary could be removed as the aminomethyl compound by treatment with ammonia, to give the i p-isomer of 220, in 40 1 excess. ... 

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