The replacement of the furanose moiety of DNA by a cyclohexene ring gives a new nucleic acid structure: cyclohexene nucleic acids or CeNA. CeNAs can be obtained by the classical phosphoramidite chemisty starting from protected cyclohexenyl nucleoside building blocks. Incorporation of cylcohexenyl nucleosides in a DNA chain increases the stability of a DNA/RNA hybrid. The complex formed between cyclohexenyl oligoadenylate and its DNA or RNA complement is of similar stability. Circular dichroism (CD) and NMR studies indicate easy conformational adaptation of a cyclohexenyl nucleoside when incorporated in a natural nucleic acid structure. CeNA is stable against degradation in serum and a CeNA/RNA hybrid is able to activate E. Coli RNase H, resulting in cleavage of the RNA strand.

In this review, I describe various natural manifestations of purine systems, i.e., methylated, higher-alkylated, and glycosylated forms. These comprise the purine alkaloids, cytokines, as well as the purine nucleoside antibiotics. In part, the compounds described herein were isolated from natural sources already long ago. However, some have been reported only during the last few years. The biological activities of most of the purine derivatives are briefly described, and, in some cases, syntheses are formulated. In particular, this article introduces the main synthetic principles for the generation of the purine ring system. The last chapter describes modern preparative routes for C-alkylation and C-arylation of purines.

Oligonucleotides constructed of 1',5'-anhydrohexitol nucleoside building blocks (hexitol nucleic acids, HNA) are completely stable towards 3'-exonuclease and form very stable self-complementary duplexes as well as sequence-selective stable duplexes with the natural DNA and RNA. Triple-helix formation has also been observed. These hybridisation characteristics are highly dependcnt on the base sequence and the experimental conKeywords antisense systems -DNA recognitionnucleic acids * oligonucleotides -RNA ditions. When using a phosphate buffer containing 0.1 M NaCI, a homopurine HNA dodecamer gives a ATm of + 1.3 ' C i base pair with DNA as complement and a ATm of +3.O0C/base pair with RNA as complement. These oligomers may therefore be of considerable interest as antisense constructs.

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