Synthesis of spiro ɛ,ζ-D-CNA in xylo configuration featuring noncanonical δ/ɛ/ζ torsion angle combination

The synthesis of diastereoisomers of spiro xylo-ɛ,ζ-D-CNA dinucleotide building units of nucleic acids, in which the ɛ and ζ torsional angles are stereocontrolled by a dioxaphosphorinane ring structure (D-CNA family), is described from uridine with a Mukaïyamaʹs aldol condensation as key reaction. The NMR structure analysis showed that δ, ɛ and ζ torsional angles of the (SC3′,RP)- and (SC3′,SP)-configured xylo-ɛ,ζ-D-CNA TT dimer are restricted to the {δ, ɛ, ζ}={cis, trans, gauche(+)} or {cis, trans, gauche(−)} conformation, respectively, that are significantly different from those typically observed in A- or B-type duplexes.