| Summary: | 博士 === 國立清華大學 === 化學系 === 103 === In the first part of this thesis, we reported the synthesis of glycosyl azides from peracetylated sugars (or glycosyl trichloroacetimidates) using VO(OTf)2 as the catalyst which can improve 1,2-trans selectivity to understand the role of this oxometallic species, we monitor the reaction progress by 1H NMR spectroscopic analysis of the reaction mixtures to confirm the different catalytic exerted by VO(OTf)2. An intermediate resulting from glycosyl rearrangement to α-form starting materialwas identified in the catalytic reaction mediated by TMSOTf. This novel vanadyl triflate also provided access to catalyze azidation of various, disaccharides and trisaccharides in high 1,2-trans stereoselectivity and good yields (92-99 %) under mild conditions in CH2Cl2. Not only does VO(OTf)2 enhance β-stereoselective glycosyl azidation but also avoids the rearrangement of C2 acetate group and the formation of orthoester byproducts.
The second part is dealt with azide-alkyne cycloaddition (CuAAC) in mild and green conditions by a combination of VOSO4 and Cu(0) for in-situ generation of Cu(I) in aqueous media. Vanadium compounds can exist in oxidation states ranging from -3 to +5 and their interconversion between different oxidation states is achieved by one-electron redox process. Its intrinsic redox nature permits the catalysis of a wide range of organic reactions by judicious combination with suitable metal reductants. Through an extensive survey of various conditions, we have established an effective recipe to generate Cu(I) speceies for the CuAAC reaction in both homogeneous (CH3CN) and heterogeneous (Methanol, t-BuOH/H2O, H2O) solvent systems. In this new click reaction, mild acidic condition plays an important role to stabilize the incipient Cu(I) species and a pronounced ligand effect on the vanadyl species is observed. This new catalytic methodology is attractive because it can be applied to aqueous solution which is important in biological systems at ambient temperature and physiologic pH conditions.
In the third part, we have disclosed an elegant and pragmatic catalytic system by using VO(OTf)2 catalyst for 1.2-trans β-selective and 1,2-cis α-selective glycosylationof thioglycosides. The catalytic strategy described herein can achieve stereospecific glycosylation in excellent yields. When low-concentration (10 mM) mixed solvent systems (CH2Cl2-CH3CN-EtCN) with N-Iodosuccimide (NIS, 1.25 equiv) as the activator was employed, preferential 1.2-trans glycosides(-selectivity) was attained. When VO(OTf)2-xCH3CN was used as a solution in DMF for catalytic glycosylation, highly 1,2-cis (α-selectivity) glycosylation was achieved. Finally, a reduced loading of VO(OTf)2-xCH3CN (3M in DMF) to 0.05 equiv still led to efficient catalysis and 1,2-cis -selective glycosylation can still be maintained.
In the second chapter of this thesis, a series of oxometallic species and metal acetylacetonates (acac) was examined as catalysts for oxidative beta-carbonylation of styrenes with benzaldehyde by using t-butylhydroperoxide as co-oxidant and trapping agent in warm acetonitrile. Among them, VO(acac)2 and vanadyl(IV) chloride were found to be the only two catalyst classes to achieve the cross-coupling processes by judicious turning the ligand electronic attributes, leading to β-hydroxylation and β-peroxidation of styrenes, respectively, in a complementary manner. Mechanistic studies indicated that vanadyl associated, acyl radicals generated by t-butoxy radical-assisted, homolytic cleavage of aldehyde C-H bond were involved in the tandem processes with exclusive syn diastereoselectivity in the case of -methylstyrene.
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