Design,Synthesis And SAR Studies On NGF Mimic Activity Of Pentacyclic Triterpenes

Nerve growth factor(NGF)is very difficult to be used as an effective medicine to treat Alzheimer’s disease(AD),because of its large molecular size,large polarity and NGF can not pass through the blood-brain barrier(BBB).Therefore,researching and developing small molecule compounds with NGF mimic activity for the treatment of AD become a hotspot.In this thesis,PC 12 cells were used as biological activity identification system to evaluate the NGF mimic activity of target compounds.3β,23,28-Trihydroxy-12-oleanene 3β-caffeate(la),which has significant neuritogenic activity,was isolated from the traditional Chinese medicine(TCM)Desmodium sambuense.However,the content of 1a in D.sambuense is very low(dry,0.000225%),it is difficult to get enough quantities from TCM for further experiments.la should be synthesized for the structure-activity relationship(SAR),and also for possible future animal experiments.Reverse synthesis analysis revealed that 1a can be obtained by esterification of caffeic acid and 3β,23,28-triol olean-12-ene(2).Oleanolic acid was used as the starting material,NaPdCl4 was used to activate the C-23 position,and finally 2 was synthesized through 9 steps with an overall yield of 50.9%.The synthetic route of 2 was optimized later:oleanolic acid also was used as the starting material,and transition metal ruthenium was used as a catalyst to synthesize 2 in 5 steps and the total yield was 74.1%.After that the two primary hydroxy groups of triol 2 were protected by tert-butyldimethylsilyl(TBS)to obtain 13,then 13 was esterified with O-TBS-protected caffeic acid,and the product was deprotected to yield la.Finally,the semi-synthesis of la was completed by 9 steps with oleanolic acid as the starting material,and the total yield was 33.6%.The spectra of synthesized 1a were consistent with those of natural one,and both of them had comparable NGFmimic activity.In order to find lead compounds for drug development,based on the chemical structure of la.Firstly,27 3β,23,28-triol olean-12-ene derivatives were designed and synthesized in this thesis.All of them are new compounds.The results of the SAR studies showed that:a.At least two ortho-hydroxy groups on cinnamic acid are important for neuritogenic activity;b.The double bond of caffeic acid is also important for neuritogenic activity;c.The presence of carboxyl group close to the aromatic ring renders an obvious decline in neuritogenic activity;d.C-3 substituted or C-23 substituted compounds have similar activity;e.The activity decreases after hydroxyacetylation on compound 1a.Compound 1e is more active than natural product 1a at the same concentration.Secondly,based on caffeic acid,caffeic acid esters of oleanolic acid,dihydroxy oleanane,glycyrrhetinic acid and asiatic acid were designed and synthesized.But their neuritogenic activity was significantly lower than that of the positive control.Based on the above SAR and related literatures,a series of pentacyclic triterpene derivatives and 3,4-dihydroxyphenolic acid derivatives were designed and synthesized.None of these synthesized compounds showed good activity.In summary,we completed the semi-synthesis of natural product la and studied its SAR,and identified 1e as the leading compound.Synthesis of other pentacyclic triterpene derivatives and 3,4-dihydroxyphenolic acid derivatives has also been performed.A total of 79 target compounds were synthesized.All synthetic derivatives were characterized by analysis of the spectral data,and their neuritogenic activities were evaluated in PC 12 cells.