Synthesis of 3,16,30-trioxolup-20(29)-ene, a selective butyrylcholinesterase inhibitor, from a natural triterpene

According to the cholinergic hypothesis, the inhibition of cholinesterase increases the levels of acetylcholine in the brain, thus improving cholinergic functions in Alzheimer´s Disease (AD) patients. Butyrylcholinesterase (BChE), is one of the enzymes involved in the metabolic degradation of acetylcholine, together with acetylcholinesterase (AChE). BChE activity increases as AD progresses, which suggests that BChE may play an important role at the latter stages of AD. Therefore, selective BChE inhibitors attract interest nowadays.Lup-20(29)-ene-3β,16β-diol (calenduladiol, 1), is a pentacyclic lupane-type triterpene with the interesting feature of being hydroxylated at C-16. Continuing our previous work on semisynthesis of derivatives obtained from the natural triterpene calenduladiol and their evaluation as potential cholinesterase inhibitors, we report here the synthesis of six new lupanes (3-8). These compounds have been obtained by sequential oxidations and reaction with NH₂OH of the starting compounds, the natural calenduladiol (1) and its analog 30-oxocalenduladiol (2), previously obtained by allylic oxidation with Se₂O. Their structures were confirmed by analysis of their 1H and 13C NMR and ESI-MS spectra. The complete assignation of the signals was achieved with the aid of 2D NMR experiments (COSY, HSQC, HMBC, NOESY). All of the new derivatives were evaluated as potential in vitro BChE inhibitors by the Ellman´s colorimetric method. In order to determine the BChE/AChE selectivity of compounds 3-8, their antiAChE activity was evaluated. All of them failed to inhibit AChE, but we found that the best BChE inhibition was observed for 3,16,30-trioxolup-20(29)-ene (4) with an IC₅₀ value of 21.5 μM, which elicited a selective inhibitor profile