... The 2-alkynyl analogue 13 was synthesized from 2-iodoadenosine 28, prepared in 5 steps from guanosine (Scheme 4).(26) Sonogashira coupling was then performed to ... figure Scheme 4. Synthesis of 13 a. ... (d) biotin–NHS, Cs 2 CO 3 , DMF, 0 → 25 °C; (e) 50% aqueous TFA, 0 ...