Nucleotides. Part XLVII. Synthesis of 3′‐Deoxyadenylyl‐(2′–5′)‐3′‐deoxyadenylyl‐(2′–5′)‐3′‐ O ‐(2‐hydroxyethyl)adenosine and 3′‐Deoxyadenylyl‐(2′–5′)‐3′‐deoxyadenylyl‐(2′–5′)‐3′‐ O ‐{2‐[(cholest‐5‐en‐3β‐yloxy)carbonyloxy]ethyl}adenosine: A New Type of (2′–5′)Adenylate Trimer Conjugate

A suitably protected adenosine derivative bearing an acetyl‐protected 2‐hydroxyethyl moiety in 3′‐ O ‐position was attached to the 2′‐terminus of a cordycepin (3′‐deoxyadenosine) dimer. Coupling was performed by phosphoramidite chemistry using two alternative approaches – condensation of 5′‐phosphoramidite 16 with 2′‐OH cordycepin dimer 18 and condensation of dimeric cordycepin phosphoramidite 19 with 5′‐OH adenosine derivative 15 – of which the latter synthesis worked best (→ 20 ). After cleavage of the acetyl protecting group (→ 21 ), cholesteryl carbonate was introduced into the OH function of the spacer (→ 24 ). Final deblocking of trimer 24 with, or trimer 21 without the cholesterol moiety afforded the modified cordycepin conjugates 25 and 23 , respectively.