Synthesis of 2-butenyl linked α-carboxy nucleoside phosphonates 6i–6m with different nucleobases [(i) 3g (1.0 equiv.), 4 (1.2 equiv.), PPh3 (2.1 equiv.), DIAD (2.0 equiv.), THF, −40 °C-RT, 24 h (ii) Pd/C (10%), H2 (1 atm), MeOH, rt, 12 h (iii) a. TMSBr, CH3CN, 0 °C-rt, 16 h, H2O, 1h; b. aq. NaOH, 50 °C, 12 h, charcoal column].