ChemInform Abstract: Nucleic-Acid Analogues with Constraint Conformational Flexibility in the Sugar-Phosphate Backbone ′Tricyclo-DNA′. Part 1. Preparation of [(5′R,6′R)-2′-Deoxy-3′,5′-ethano-5′,6′-methano-β-D-ribofuranosyl] thymine and -adenine, and the C