Synthesis And Spectrogram Interpretation Of IEPOX,the Photo Oxidation Intermediates Of Isoprene

Isoprene epoxydiols?IEPOX?are important intermediates of isoprenephotooxidation.Synthetic preparation of IEPOX can lay a foundation for further study of formation mechanisms ofsecondary organic aerosols from photooxidation of atmospheric isoprene.In this study,four isomers of IEPOX were synthesized,and the hydrogen nuclear magnetic resonance?H-NMR?spectra of the four isomers were obtained.Moreover,after derivatization of different IEPOX isomers with N,O-bis?trimethylsilyl?trifluoroacetamide?BSTFA?,the standard electron impact ion source?EI?mass spectra and chemical ionization?CI?mass spectra of the trimethylsilyl ester?TMS?derivatives were also obtained.In this paper,the synthetic routes,the principle of the synthetical reactions,the structure of fragmental ions in EI and CI spectra and the absorption peaks in the H-NMR spectra were analyzed detailedly.Finally,on the basis of the standard spectra,four IEPOX isomers were identified in ambient aerosol samples.Under the catalysis of osmium tetroxide?OsO₄?,2-methyl-2-vinyloxirane was oxidized by N-methylmorpholine-N-oxide?MNO?,the addition reactions took place on the unsaturated double bold of 2-methyl-2-vinyloxirane,and 1-?2?-methyloxiran-2?-yl?ethane-1,2-diol?EPOX-1?,a product with vicinal diols,were generated.The product was purified by flash silica gel column chromatography.The yield of IEPOX-1 was approximately 68%.Hydrolyzing 2-methyl-2-vinyloxirane in a warm water bath at 50?to produce the ring-open products,2-methyl-3-butene-1,2-diol.Then,the cyclization of the diol with m-chloroperoxybenzoic acid?mCPBA?gave rise to the products 2-?oxiran-2?-yl?-propane-1,2-diol?IEPOX-2?.After purification by flash silica gel column chromatography,the yield of IEPOX-2 was approximately 24%.3-Methyl-2-?5H?-furanone was reduced by lithium aluminum hydride?LAH?to produce the intermediate 2-methyl-2-butene-1,4-diol.Then,the ring-forming reaction of the diol with mCPBA produce the final product,cis-2-methyl-2,3-epoxybutane-1,4-diol?IEPOX-3?.The ultimate yield of IEPOX-3 was approximately 50%.There are a lot of steps in the systhesis of IEPOX-4.Firstly,tert-butyl dimethyl chlorosilane was reacted with 3-methyl-2-butene-1-ol to generate t-butyldimethylsilyl?TBDMS?derivative of 3-methyl-2-butene-1-ol.The hydroxyl group in the molecule of3-methyl-2-buten-1-ol was protected.Subsequently,selenium dioxide?SeO₂?and tert-butyl hydroperoxide?t-BuOOH?were used to oxidize the TBDMS derivative of3-methyl-2-butene-1-ol to add a hydroxyl group to the last atom of the carbon chain of the substrate.Next,tetrabutylammonium fluoride?TBAF?was added to the reaction system to reduce the pre-protected hydroxyl group in the substrate,and generate trans-2-methyl-2,3-butene-1,4-butanediol.Finally,the cyclization of trans-2-methyl-2,3-butene-1,4-butanediol with mCPBA gave rise to trans-2-methyl-2,3-epoxy-1,4-butanediol?IEPOX-4?.After the product was purified by silica gel column,the yield of IEPOX-4 was about 15%.The synthesized four isomers were derivatized with BSTFA,and TMS derivatives were injected into a gas chromatograph/mass spectrometer?GC/MS?to obtain standard electron impact ion source?EI?mass spectra and chemical ionization?CI?mass spectra.There are subtle differences in the EI and CI mass spectra of TMS derivatives of four IEPOX isomers.The peak times of the four isomeric eluting from the column were:22.34min,22.70 min,24.70 min,and 25.40 min,respectively.Two diastereoisomers of IEPOX-2?erythro-and threo-configurations?were fould in the chromatograms.The four synthesized isomers were also dissolved in deuterochloroform?CDCl₃?and determined by a 600 MHz nuclear magnetic resonance hydrogen spectrometer to obtain a standard H-NMR spectra of IEPOX.The structure of the IEPOX isomers was further confirmed.In the end,the study identified four isomers of IEPOX in environmental aerosol samples.The concentrations of IEPOX in ambient aerosol sample are much lower than those of 2-methylterols and C₅-alkene triols.Among the four IEPOX isomers,IEPOX-2?erythro-and threo-?had the highest concentrations in ambient samples.