Total Synthesis Of Albomycins And Design,Synthesis And Biological Evaluation Of Siderophore-Methotrexate Conjugates

This thesis,aiming at the discovery of new antibiotics,consists of two parts:（1）Total synthesis and antimicrobial evaluation of natural albomycinsδ₁,δ₂ andε.（2）Design,synthesis and biological evaluation of siderophore-methotrexate conjugates.Part 1:Total synthesis and antimicrobial evaluation of natural albomycinsδ₁,δ₂andεAlbomycins are potent natural antibiotics,which strongly inhibit the growth of Gram-positive bacteria,including Streptococcus pneumoniae,Bacillus subtilis and staphylococcus aureus.They are also effective against a number of Gram-negative enterobacteria,for instance,albomycins exhibit an MIC value of 5 ng/mL against Escherichia coli.However,albomycins are mainly obtained as a mixture of albomycinsδ₁,δ₂ andεby fermentation,and their separatation is in trouble due to their similar polarity.The total synthesis of albomycins would not only expand the source of albomycins,but also could allow preparing albomycin derived analogs by structural modification and simplification.This part of the thesis mainly focuses on the total synthesis of albomycinsδ₁,δ₂ andε.Albomycins were constructed by a convergent synthetic strategy.The siderophore fragment and thionucleoside fragments were synthesized respectively,and then assembled to form albomycins.The synthesis of siderophore fragment was accomplished by first constructing the amino acid unit containing hydroxamic acid through direct oxidation of ornithine and then amino acids coupling.The synthesis of thionucleosides of albomycinsδ₁,δ₂ andεwere performed by a Pummerer reaction for nucleobase introduction and an aldol reaction to expand the side chain.Finally,the total synthesis of albomycinsδ₁,δ₂ andεwas achieved by combining the siderophore fragment with thionucleosides and removal of all the protecting groups.As the three naturally occurring albomycins became synthetically accessible,their MIC values against three Gram-positive and three Gram-negative bacteria species were determined.Albomycinδ₂ was further screened against a random collection of 27 clinical S.pneumoniae and S.aureus isolates（three of them are MRSA strains）,and compared it with ciprofloxacin,vancomycin,and penicillin G in inhibiting the growth of these clinical pathogens.Albomycinδ₂ exhibited excellent anti-S.pneumoniae and S.aureus activities,far better than the other three commonly used antibiotics in most cases.These results suggest that albomycinδ₂ is a promising antibiotic candidate for further antimicrobial drug evaluations.Part 2:Design,synthesis and biological evaluation of siderophore-methotrexate conjugatesAs a natural sideromycin,albomycins are actively transported into bacterial cells via efficient siderophore uptake pathways commonly found in bacterial pathogens,by the so-called“Trojan horse”strategy.Synthetic sideromycins,namely siderophore-antibiotic conjugates,which mimick the sideromycin approach for drug delivery,is an important strategy to develop new antibiotics.The study of siderophore-antibiotic conjugates has been widely reported,and even several compounds are in clinical trials.However,there are few studies on the development of siderophore-anti-tumor drug conjugates.In order to broaden antibiotic research pathway and provide more candidate antibiotics for clinic,the design,synthesis and biological studies of siderophore-methotrexate conjugates were studied in this part.Nine designed siderophore-methotrexate conjugates with different linkers were synthesized.Their MIC values against S.pneumoniae were determined.3-4（a–g）exhibited excellent anti-S.pneumoniae activities.The mammalian cell toxicity of 3-4（a–h）decreased significantly.Moreover,3-4b and 3-4g,which feature good antibacterial activity and low mammalian cell toxicity,are promising antibiotic candidates for further development.