Studies On Total Synthesis Of Natural Products Tumescenamide A And Brevianamide C And D

Total synthesis of natural products is an important field in modern natural products research,which plays an important role in the research on natural products.Through total synthesis,it can not only further confirm the structure of natural products,but also solve the problem that sufficient compounds could not be obtained through separation and extraction to a certain extent,which provides a strong guarantee for further study on the properties of natural products.And through the study of total synthesis of natural products,it is theoretically expected to discover new reactions or new reagents to develop and enrich the content of organic chemistry.Tumescenamide A is a novel cyclic peptide ester compound isolated from the fermentation broth of the marine bacterium Streptomyces YM23-20.It is composed of cyclic peptide and side chain 2,4-dimethylheptanoic acid（Dmh）.The amino acids forming the cyclic peptide structure are L-Thr,α,β-unsaturated L-Thr,D-Tyr,L-Val and L-Leu.At present,tumescenamide A-C have been reported,which are composed of cyclic peptide and side chain.The amino acid configurations in the tumescenamide A were determined by comparing the retention time using Marfey method,while the configuration of the side chain was not determined.Brevianamide C and brevianamide D were isolated from Pennicillum brevi-compactum,belonging to the 2,5-diketopiperazine series.Brevianamides have been reported as more than 20species of these compounds,which are named brevianamide A-Y in turn.Most of them show antibacterial,pest-resistant and tuberculosis-resistant potential.Brevianamide C and brevianamide D both contain a bridge ring[2.2.2]diazooctane and 3-indoleone structure,and the two structures are connected by an extrapyclic double bond.They are stereoisomers,and the only difference between them is cis and trans forms of the double bond in their structure.The total synthesis of the two compounds has not been reported.In this paper,tumescenamide A and brevianamide C and D are selected as the target molecules.The absolute configuration of tumescenamide A was confirmed by total synthesis,and the wrong results in the literature were corrected.A novel asymmetric synthesis method of tumescenamide A side chain 2,4-dimethylheptanoic acid（Dmh）was developed.A new method forα,β-unsaturated amino acids and related small peptides was established.Total synthesis of brevianamide C and D has been explored.The biological activities of natural products and their derivatives were preliminarily evaluated,and three tumescenamide A analogues with high inhibitory activity against tumor cells were found.The research contents and results of this paper are summarized as follows:1.A novel asymmetric synthesis method of tumescenamide A side chain 2,4-dimethylheptanoic acid（Dmh）was developed.Two chiral centers of 2,4-dimethylheptanoic acid were constructed from Evans chiral auxiliary（S/R）-4-benzyl-2-azolidone by acylation,asymmetric alkylation,reduction,asymmetric alkylation and hydrolysis.The four isomers of 2,4-dimethylheptanoic acid were synthesized in a highly stereoselective manner.The optical purity of four isomers reached 100%by chiral HPLC analysis.The yield of the compounds with different configurations was almost the same,and the total yield of the five-steps reaction was 76.5%.2.A new method was developed for the preparation of unsaturated amino acids and small peptides fromβ-hydroxyl amino acids by elimination reaction using nano-K₂CO₃as base.In the preparation process ofα,β-unsaturated amino acids and related small peptides,aiming at the reported defects of using strong base or high temperature and narrow substrate spectrum.The“one-pot”selectively E2 reaction of small peptides containingβ-hydroxy amino acids was established by using nano-K₂CO₃as base.And the yield did not decrease significantly when the reaction scale was enlarged to 100 g.Nano-K₂CO₃can be recycled and reused at least six times.It was found that the elimination reaction of threonine or serine could be carried out well whether the threonine or serine was at the C-terminal or N-terminal of the peptide chain.The configuration of carbon-carbon double bond is Z configuration through NOESY spectrum analysis.3.Total synthesis of tumescenamide A was completed and its stereochemistry was established.Firstly,the Model molecule was designed and synthesized by heptanic acid instead of Dmh,and the synthesis route was explored.After that,six cyclic peptides were synthesized by changing the configuration of amino acids and side chain Dmh.By comparing the ¹H NMR,¹³C NMR and specific optical rotation data of the synthesized six cyclic peptides with natural tumescenamide A,the amino acid configurations of tumescenamide A were determined to be L-Thr,D-Tyr,D-Val and L-Leu.The Val configuration reported was revised to D configuration,and the configuration of Dmh was determined to be 2S and 4S.The structure of tumescenamide A confirmed in the original literature is wrong by the comparison of total synthesis results and literature data,and tumescenamide A and tumescenamide C are the same compound.4.The synthesis of Brevianamide C and Brevianamide D was studied.Using L-Pro as raw material,the construction of the main structure dicyclic[2.2.2]diazooctane fragment has been completed through the carbonylα-isopentenylation reaction,the reaction of constructing diketopiperazine ring and aza-Prins reaction.The total synthesis of brevianamide C and D will be completed through Z and E selective Wittig reaction according to the designed route,and the absolute configuration will be determined.5.Study on anti-tumor activity of XH series cyclic peptides at cellular level.More than 10 tumor cell lines were selected as biological models to study the antitumor activity of 16 XH series cyclic peptides.It was found that some compounds showed high inhibition rate at 25μM.Moreover,compounds XH-K and XH-O showed high activity against a variety of cells.The IC₅₀value of compound XH-J against HT-29 cells reached 4.3μM.It provides a new type of structure for further study.By analyzing the anticancer activity of 16 XH series cyclic peptides at the cellular level,it was found that the amino acid configuration and side chain structure of these cyclic peptides had significant effects on the anticancer activity,and the more detailed structure-activity relationship needed to be further studied.