9‐(2′‐Deoxy‐β‐ D ‐xylofuranosyl)adenine Building Blocks for Solid‐Phase Synthesis and Properties of Oligo(2′‐deoxy‐xylonucleotides)

The 9‐(2′‐deoxy‐à‐ D ‐threo‐pentofuranosyl)adenine (=9‐(2′‐deoxy‐à‐ D ‐xylofuranosyl)adeninc, xA d ; 2) was protected at its 6‐NH 2 group with cither a benzoyl ( 5a ) or a (dimethyfamino)methylidcnc ( 6a ) residue and with a dimethoxytntyl group at 5′‐OH ( 5b, 6b ). Compounds 5b and 6b were then converted into the 3′‐phosphonates 5c and 6c ; moreover, the 2‐cyanoethyl phosphoramidite 6d was synthesized starting from fib. The DNA building blocks were used for solid‐phase synthesis of d[(xA) 12 2‐A] ( 8 ). The latter was hybridized with d[(xT)12‐T] ( T m = 35°); in contrast, with d(T 12 ), complex formation was not observed. Moreover, xAd and xTd were introduced into the self‐complementary dodccamcr d(G‐T‐A‐G‐A‐A‐T‐T‐C‐T‐A‐C) ( 12 ) at different positions lo give the oligomcrs 13 – 16 . All oligonucleotides were characterised by temperature‐dependent CD and UV spectroscopy, and in addition, 14 by T ‐jump experiments. From concentration‐dependent T m measurements, the thermodynamic paraneters of the melting as well as the tendency of hairpin formation of the oligonucleotides were deduced. Oligemer 14 was hydrolyzed by snake‐venom phosphodiesterase in a discontinuous way implying a fast hydrolysis of unmodified 3′‐ and 5′‐flanks followed by a slow hydrolysis of the remaining modified tetramer. In contrast to this, oligonucleotide 16 was hydrolyzed in a continuous reaction. In both cases, calf‐spleen phosphodiesterase hydrolyzed the oligomer only marginally.