aldehyde, AcOH, DCE, 40 °C, 20 min; (ii) NaBH(OAc)3, 40 °C, 16 h, 87%. (c) (i) 4-chlorobenzaldehyde, AcOH, DCE, 40 °C, 20 min. (ii) NaBH(OAc)3, 40 °C, 16 h, 72%. (d) TFA/H2O 8/2, 25 °C, 3 h, 48% for 14; 6 h, 25% for 15; 6 h, 37% for 16.

2.2  Chemistry
The retrosynthetic analysis of the dinucleoside 1 structure suggested the coupling reaction between the tosyl derivative 17 previously described by us (Scheme 2) [14], and the readily accessible 5′-amino-2′,3′-isopropylidene adenosine [21]. The direct N-alkylation at room temperatu