Study On The Synthesis Of Balasubramide Derivatives And Their Photoaffinity Probes

(+)-（5S,6R）-Balasubramide is an octameric lactam containing an indole ring isolated from the leaves of Clausenaindica（Datz）.Oliv.from the tropical rainforests of Sri Lanka.A series of natural products（+）-（5S,6R）-balasubramide derivatives have been synthesized by our group,mainly including the removal of methyl group on the 3-N of the eight-membered ring,esterification and etherification of the hydroxyl group at the 5-position,and introduction of different substituents on the indole benzene ring.A preliminary bioactivity evaluation was conducted as well.The results showed that except for a small number of compounds with cytotoxic properties,demethylation on N,ester and ether groups at position 5,and-CF₃ at position 6 on benzene ring still retained different degrees of neurocytoprotective,antioxidant and anti-neuroinflammatory activities;the indole ring substituted balasubramide derivatives,such as indole ring5-methoxy substitution and 7-methyl substitution also have certain biological activity.In this paper,we continued with the structural modification of the indole benzene ring using one of the more active balasubramide derivative（+）-3C-20 as a lead,and designed five new（+）-balasubramide derivatives to find a better active lead structure.At present,the targets of this class of natural products for neuroprotective effects are still unclear.Our group synthesized（+）-3C-20ester-based photoaffinity molecular probes in the previous stage.Considering that ester-based linkage bonds are unstable in biological environment,this paper continued to design and synthesize（+）-3C-20ether-based photoaffinity molecular probes.Synthesis of（+）-balasubramide derivatives:Firstly,（+）-（2S,3R）-3-（4-trifluoromethyl）phenyl-2-epoxycarboxylic acid methyl ester（（+）-L1）was synthesized with reference to the existing asymmetric synthetic route in this experiment.The key intermediate（+）-L1 was obtained from 4-trifluoromethylbenzaldehyde as raw material,followed by wittig reaction,asymmetric epoxidation and oxidative esterification.Subsequently,according to the improved synthesis method of 5-benzyloxytryptamine（D2）in the previous laboratory,the key intermediate 5-benzyloxytryptamine（D2）was obtained by Friedel-Craft acylation,ammonolysis,and reduction using 5-benzyloxyindole as raw material.Then the 5-hydroxy-substituted ester balasubramide（（+）-D6）was obtained by the acylation of（+）-L1 with5-benzyloxytryptamine on N atom,intramolecular cyclization,acylation on O atom and reductive debenzylation.The total yield of this route was about 29.7%in 7 steps（calculated as 5-benzyloxyindole）.The new target compounds（+）-D7a and（+）-D7b were obtained from the reaction between（+）-D6 with iodoethane and 2-iodopropane forming phenol ethers under alkaline conditions.（+）-D6 can also react with 4-nitrobenzoyl chloride to obtain a new target compound（+）-D7c（prepared from 4-nitrobenzoic acid）with an ee value of more than 98%.Subsequently,4-cyanotryptamine and 5-nitrotryptamine（E4a and E4b）were obtained by hydrocarbylation,selective reduction,Mitsunobu reaction and hydrazinolysis using 4-cyanindole and 5-nitroindole as raw materials.The target compounds（+）-E7a and（+）-E7b were then obtained by acylation on N atom,intramolecular cyclization,and acylation on O atom of E4a and E4b with the intermediate（+）-L1,respectively.Their ee values were more than 98%.The route involves 7 reaction steps and the total yield was 3.4～5.7%（calculated as substituted indole derivative）.Synthesis of（+）-balasubramide derivatives of ether-based photoaffinity probes:Pentinoic acid was use as raw material to react with meldrum acid,followed by refluxing with ethanol solution to obtain Ethyl3-carbonyl-6-ethinyl heptanoate（Q1）.Then the carbonyl group of Q1 was protected by ethylene glycol,reduced by lithium aluminum hydride,and deprotected to obtain intermediate 1-hydroxyhept-6-yn-3-one（Q4）.Aliphatic bisacridine photoaffinity probe（Q5）was obtained from Q4 in methanolic solution of ammonia,in the presence of tert-butyl hypochlorite.Aliphatic bisacridine iodoaffinity probe（Q6）was obtained from Q5 by reaction with monomeric iodine under the conditions of Apple reaction.Subsequent reactions with the iodinated probe Q6 using the 5-phenolic hydroxyl group of the balasubramide derivative（+）-D6 as the site of introduction of the photoaffinity group yielded a new ether-like photoaffinity probe molecule of the balasubramide derivative.The total yield of this route was about 6.1%（calculated as 4-pentynic acid）in 7 steps.A total of 39 compounds,including intermediates and target compounds,were synthesized in this paper,of which a total of 11 compounds were not reported.All of the new compounds were confirmed by 1H NMR,13 C NMR and HRMS.The ee% of the target compounds were all determined using a chiral column（polysaccharide-based chiral stationary phase）Chiralpak AD-H and Chiralcel OJ-H with ee% ≥ 98%.Bioactivity studies of the target products prepared herein and angling of the target proteins are in progress.