Oligonucleotide Analogues with Integrated Bases and Backbone. Part 16

The self‐complementary, ethylene‐linked U*[c a ]A ( * ) dinucleotide analogues 8, 10, 12, 14, 16 , and 18 , and the sequence‐isomeric A*[c a ]U ( * ) analogues 20, 22, 24, 26, 28 , and 30 were obtained by Pd/C‐catalyzed hydrogenation of the corresponding, known ethynylene‐linked dimers. The association of the ethylene‐linked dimers was investigated by NMR and CD spectroscopy. The U*[c a ]A ( * ) dimers form linear duplexes and higher associates ( K between 29 and 114 M −1 ). The A*[c a ]U ( * ) dimers, while associating more strongly ( K between 88 and 345 M −1 ), lead mostly to linear duplexes and higher associates; they form only minor amounts of cyclic duplexes. The enthalpy–entropy compensation characterizing the association of the U*[c x ]A ( * ) and A*[c x ]U ( * ) dimers ( x =y, e, and a) is discussed.