Studies on the Base‐Pairing Properties of N 7 ‐(2‐Deoxy‐β‐ D ‐ erythro ‐pentofuranosyl)guanine ( N 7 G d )

The base‐pairing properties of N 7 ‐(2‐deoxy‐β‐ D ‐erythro‐pentofuranosyl)guanine ( N 7 G d ; 1 ) are investigated. The nucleoside 1 was obtained by nucleobase‐anion glycosylation. The glycosylation reaction of various 6‐alkoxy‐purin‐2‐amines 3a ‐ i with 2‐deoxy‐3,5‐di‐ O ‐(4‐toluoyl)‐α‐ D ‐ erythro ‐pentofuranosyl chloride ( 8 ) was studied. The N 9 / N 7 ‐glycosylation ratio was found to be 1:1 when 6‐isopropoxypurin‐2‐amine ( 3d ) was used, whereas 6‐(2‐methoxyethoxy)purin‐2‐arnine ( 3i ) gave mainly the N 9 ‐nucleoside (2:1). Oligonucleotides containing compound 1 were prepared by solid‐phase synthesis and hybridized with complementary strands having the four conventional nucleosides located opposite to N 7 G d . According to T m values and enthalpy data of duplex formation, a base pair between N 7 G d and dG is suggested. From the possible N 7 G d dG base pair motives, Hoogsteen pairing can be excluded as 7‐deaza‐2′‐deoxyguanosine forms the same stable base pair with N 7 G d as dG.