Synthesis and Biological Evaluation of Structurally Varied 5'-/6'-Isonucleosides and Theobromine-Containing N-Isonucleosidyl Derivatives

Isonucleosides are rather stable regioisomeric analogs of nucleosides with broad therapeutic potential. We have previously demonstrated the ability of 5' and 6'-isonucleosides to inhibit the activity of acetylcholinesterase, a major target for Alzheimer's disease therapy. Continuing with our research on this topic, we report herein on the synthesis and biological evaluation of a variety of novel terminal isonucleosides and theobromine isonucleotide analogs. Xylofuranose-based purine or uracil 5'-isonucleosides and xylofuranos-5'-yl or glucos-6'-yl theobromine derivatives were accessed via Mitsunobu coupling between partially protected xylofuranose or glucofuranose derivatives with a nucleobase using conventional or microwave-assisted heating conditions. Theobromine-containing N-isonucleosidyl sulfonamide and phosphoramidate derivatives were synthesized from isonucleosidyl acetate precursors. The most active compounds in the cholinesterase inhibition assays were a glucopyranose-based theobromine isonucleosidyl acetate, acting as a dual inhibitor of acetylcholinesterase (AChE, K_i = 3.1 µM) and butyrylcholinesterase (BChE, K_i = 5.4 µM), and a 2-O,4-O-bis-xylofuranos-5'-yl uracil derivative, which displayed moderate inhibition of AChE (K_i = 17.5 µM). Docking studies revealed that the active molecules are positioned at the gorge entrance and at the active site of AChE. None of the compounds revealed cytoxic activity to cancer cells as well as to non-malignant mouse fibroblasts.