Covalent Analogues of DNA Base-Pairs and Triplets V. Synthesis of Purine-Purine and Purine-Pyrimidine Conjugates Connected by Diverse Types of Acyclic Carbon Linkages

The title 1,2-bis(purin-6-yl)acetylenes, -diacetylenes, -ethylenes and -ethanes were prepared as covalent base-pair analogues starting from 6-ethynylpurines and 6-iodopurines by the Sonogashira cross-coupling or oxidative alkyne-dimerization reactions followed by hydrogenations. 6-[(1,3-Dimethyluracil-5-yl)ethynyl]purine (11) was prepared analogously and hydrogenated to the corresponding purine-pyrimidine conjugates linked via vinylene and ethylene linkers. Unlike the cytostatic bis(purin-6-yl)acetylenes and -diacetylenes, the purine-pyrimidine conjugates were inactive. Crystal structures of bis(purin-6-yl)acetylene 6a, -diacetylene 8a and -ethane 5a were determined by single-crystal X-ray diffraction.