2′-Deoxy-β-D-ribofuranosides of N6-Methylated 7-Deazaadenine and 8-Aza-7-deazaadenine: Solid-phase synthesis of oligodeoxyribonucleotides and properties of self-complementary duplexes

The syntheses of 7‐deaza‐N6‐methyladenine N9‐(2′‐deoxy‐β‐D‐ribofuranoside) (2) as well as of 8‐aza‐7‐deaza‐N6‐methyladenine N8− and N9−(2′‐deoxyribofuranosides) (3 and 4, resp.) are described. A 4,4′‐dimeth‐oxylritylation followed by phosphitylation yielded the methyl phosphoramidites 12–14. They were employed together with the phosphoramidite of 2′‐deoxy‐N6v‐methyladenosine (15) in automated solid‐phase oligonucleotide synthesis. Alternating or palindromic oligonucleotides derived from d(A‐T)6 or d(A‐T‐G‐C‐A‐G‐A*‐T‐C‐T‐G‐C‐A) but containing one methylated pyrrolo[2,3‐d]pyrimidine or pyrazolo[3,4‐d]pyrimidine moiety in place of a N6‐methylaminopurine (A*) were synthesized. Melting experiments showed that duplex destabilization induced by a N6‐Me group of 2′‐deoxy‐N6‐methyladenosine is reversed by incorporation of 8‐aza‐7‐deaza‐2′‐deoxy‐N6‐meihyladenosine, whereas 7‐deaza‐2′‐deoxy‐N6‐methyladenostne decreased the Tm value further. Regiospecific phosphodiester hydrolysis of d(A‐T‐G‐C‐A‐G‐m6A‐T‐C‐T‐G1‐C‐A) by the endodeoxyribonuclease Dpn I, yielding d(A‐T‐G‐C‐A‐G‐m6A) and d(pT‐C‐T‐G‐C‐A), was prevented when the residue c7m6Ad (2), c7m6z8Ad (3), or c7m6z8Ad′ (4) replaced m6Ad (1) indicating that N(7) of N6‐methyladenine is a proton‐acceptor site for the endodeoxyribonuclease.