Synthesis of 1,N6-ethano-2′-deoxyadenosine, a metabolic product of 1,3-bis(2-chloroethyl)nitrosourea, and its incorporation into oligomeric DNA

1,N-6-Ethano-2'-deoxyadenosine (1) is one of the adducts formed during DNA reaction with the antitumor agent, 1,3-bis(2-chloroethyl)nitrosourea (BCNU), and was synthesized and incorporated into a site-specific deoxyoligonucleotide for the first time. The product 6-chloropurine-2'-deoxyriboside (11) was prepared in high yield by the reaction of 2'-deoxyinosine (6) with SOCl2, which then was derivatized to give compound 12 using tert-butyldimethylsilyl chloride, which was then reacted with 2-hydroxyethylamine to produce compound 13 in 86% yield. Reaction of 13 with (PhO)(8)P+MeI- in DMF gave the cyclized 1,N-6-ethano derivative 10 in 67% yield. Desilylation of 10 with triethylamine trihydrofluoride in THF gave 1,N-6-ethano-dA (1) in 91% yield. Tritylation of compound 1 with DMT+BF4- gave the 5'-O-DMT product 14 in 62% yield, which then was phosphitylated with 2-cyanoethyl N,N-diisopropylchlorophosphoramidite, which yielded a 1:1 mixture of the diastereoisomers 15 in 71% yield. This fully protected compound 15 was incorporated site-specifically into a 25-mer oligonucleotide. The coupling efficiency of ethano-dA phosphoramidite was 93%. Enzymatic hydrolysis and analysis by HPLC confirmed the incorporation of ethano-dA and base composition of the DNA oligomer. The latter is now under investigation for its biochemical and physical properties.