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, Ki = 3.1 µM) and butyrylcholinesterase (BChE, Ki = 5.4 µM), and a 2-O,4-O-bis-xylofuranos-5′-yl uracil derivative, which displayed moderate inhibition of AChE (Ki = 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.

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The synthesis of purine/triazole 6'-isonucleosides and of glucuronic acid/glucuronamide-derived N-glycosylsulfonohydrazides through efficient and stereo-or regioselective methodologies is described. Their structures were envisaged to mimic nucleosides, sugar phosphates, or nucleotides, and were expected to provide potential inhibitors of therapeutically relevant enzymes, the active sites of which could potentially bind their structural fragments or functional groups. Such enzymes include cholinesterases, carbonic anhydrase II (CA-II) and cyclin-dependent kinase 2 (CDK-2). A (triazolyl)methyl amide-linked disaccharide nucleoside, based on a new prospective structural framework for analogues of nucleoside diphosphate sugars, was synthesized. The synthetic strategies employed unprotected or partially protected carbohydrate derivatives as precursors, including ribose, glucuronic acid, glucuronolactone, and glycopyranosides and relied on stereoselective N-glycosylation, regioselective Mitsunobu coupling and 'click chemistry' approaches. Some 6'-isonucleosides and triazole-containing glycoderivatives displayed moderate selective acetylcholinesterase inhibitory activities. The best inhibitor was an aminomethyltriazole 6'-isonucleoside with a K i value of 11.9 μM. N-Glucuronylsulfonohydrazide showed good inhibition of CA-II (K i = 9.5 μM). Molecular docking of the most active compounds into the effected enzymes showed interactions with key amino acid residues for substrate recognition. In addition, the tested compounds did not show toxicity to normal cells.

A set of overall 40 carboxamides was prepared from five different natural occurring triterpenoids including oleanolic, ursolic, maslinic, betulinic, and platanic acid. All of which were derived from ethylene diamine holding an additional substituent connected to the ethylene diamine group. These derivatives were evaluated regarding their inhibitory activity of the enzymes acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) employing Ellman’s assay. We further determined the type of inhibition and inhibition constants. Carboxamides derived from platanic acid have been shown to be potent and selective BChE inhibitors. Especially the mixed-type inhibitor (3β)-N-(2-pyrrolidin-1-ylethyl)-3-acetyloxy-20-oxo-30-norlupan-28-amide (35) showed a remarkably low Ki of 0.07 ± 0.01 µM (Ki′ = 2.38 ± 0.48 µM) for the inhibition of BChE.