Diastereoselective synthesis of a conformationally restricted dinucleotide with predefined alpha and beta torsional angles for the construction of alpha,beta-constrained nucleic acids (alpha,beta-CNA)

[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.