[reaction: see text] The synthesis of a diastereopure 1,3,2-dioxaphosphorinane linkage in which two, alpha and beta, out of six torsion angles of the natural phosphodiester backbone are constrained with predefined values of ca. +60 degrees (g(+)) and 180 degrees (t), respectively, is described. The stable and unstrained six-membered cyclic phosphotriester structure represents the smallest possible ring allowing the conformational locking of the alpha torsion angle at a significant positive value that is typical of many bulged regions of nucleic acids.

The synthesis of the four possible diastereoisomers of α,β‐D‐CNA 5′‐XT (X = A, T, G and C) building units of nucleic acids, in which the α and β torsional angles are stereocontrolled by a dioxaphosphorinane ring structure (D‐CNA family) is described. The crystal structure analysis of the (RC5′,SP)‐configured α,β‐D‐CNA TT dimer demonstrates that the α and β torsional angles are restricted to the canonical {gauche(–),trans} conformation typically observed in DNA duplexes. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)