Anne Loesche

Ranunculus muricatus L. is a spiny fruit buttercup that is used in various traditional medicinal systems. In the current investigation of R. muricatus, the new chalcone 4-benzyloxylonchocarpin (1), the new anthraquinone muracatanes A (2), the new-to-nature anthraquinone muracatanes B (3), and the new naphthalene analog muracatanes C (4) were isolated, in addition to the three previously reported compounds, 4-methoxylonchocarpin (5), β-sitosterol (6), and β-sitosterol β-D-glucopyranoside (7). Their structures were elucidated using 1D (¹H and ¹³C) and 2D (COSY, HSQC, and HMBC) NMR spectroscopy and HR-ESI-MS. Chalcone 1 showed potent acetylcholinesterase inhibitory effects with K_i of 5.39 µM and K_i′ of 3.54 µM, but none of the isolated compounds showed inhibitory activity towards butyrylcholinesterase. Anthraquinone 3 illustrated α-glucosidase inhibitory effects with IC₅₀-values of 164.46 ± 83.04 µM. Compound 5 displayed moderate cytotoxic activity towards ovarian carcinoma (A2780, IC₅₀ = 25.4 µM), colorectal adenocarcinoma (HT29, IC₅₀ = 20.2 µM), breast cancer (MCF7, IC₅₀ = 23.7 µM), and thyroid carcinoma (SW1736, IC₅₀ = 26.2 µM) while it was inactive towards pharynx carcinoma (FaDu: IC₅₀ > 30 µM).

The reactions of phenylglyoxylic acids during the synthesis and biological evaluation of fungal metabolites led to the discovery of hitherto unknown compounds with a p-quinone methide (p-QM) structure. The formation of these p-QMs using ¹³C-labelled starting materials revealed a key-step of this reaction being a retro-Friedel–Crafts alkylation.

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.

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 K_i of 0.07 ± 0.01 µM (K_i′ = 2.38 ± 0.48 µM) for the inhibition of BChE.

The synthesis of novel types of furanosyl nucleoside analogues, namely N‐(benzyltriazolyl)methyl glucuronamide derivatives, N‐dodecyl glucuronamide‐based phenyltriazole nucleosides, and theobromine xylosyl 5′‐isonucleosides, as potential cholinesterase inhibitors is described herein. O‐Substituted and partially O‐substituted N‐propargyl glucuronamides, accessed from glucofuranurono‐6,3‐lactone, were engaged in Cu^I‐catalyzed cycloaddition with benzyl azide, whereas their N‐dodecyl uronamide counterparts were converted in three steps into glycosyl azides, which were subjected to cycloaddition with phenylacetylene. A xylofuranose derivative having a free 5‐OH group was coupled with theobromine by Mitsunobu reaction and the obtained isonucleoside was functionalized at C‐1′ with a sulfonamide moiety, leading to a prospective nucleotide mimetic. Five compounds displayed selective inhibition of acetylcholinesterase in the micromolar concentration range, with an α‐glycosyl triazole (K_i = 3.53 µm) and its 1‐azido‐uronamide precursor (K_i = 1.73 µm) being the most active. Docking studies were performed to give insights into the different inhibitory behavior within glycosyl azide anomers. Two of the best inhibitors showed low toxicity in both a neural cell line and human fibroblasts, rendering them promising lead compounds and supporting further investigations.

The synthesis of new isonucleosides comprising purine and pyrimidine-derived systems linked to methyl glucopyranosidyl units at C-6 and evaluation of their cholinesterase inhibitory profiles is reported. Their access was based on the Mitsunobu coupling of partially acetylated and benzylated methyl glucopyranosides with purine and pyrimidine derivatives. While the reactions with purines and theobromine proceeded with complete regioselectivity, affording exclusively N⁹- or N¹-linked 6′-isonucleosides, respectively, the use of pyrimidine nucleobases led to N¹ and/or N³-glucopyranosid-6′-yl pyrimidines and/or to N¹,N³/2-O,4-O-pyrimidine-linked pseudodisaccharides through bis-coupling, depending on the substitution pattern of the sugar precursor and on the nature of the nucleobase. From this series of compounds, four were shown to be effective and selective inhibitors of acetylcholinesterase with inhibition constants in the micromolar concentration range. A tri-O-acetylated N¹-glucopyranosid-6′-yl theobromine and a benzylated N¹,N³-bis-glucopyranosid-6-yl thymine were the most active molecules with K _i values of 4 μM. A tri-O-benzylated glucopyranosid-6′-yl uracil displayed good and selective inhibition of butyrylcholinesterase (K _i=8.4±1.0 μM), similar to that exhibited by the standard galantamine. Molecular docking simulations, performed with the two most effective acetylcholinesterase inhibitors, showed interactions with key amino acid residues located at the enzyme’s active site gorge, which explain the competitive component of their inhibitory activities.