The Construction And Mechanism Of ?-mannosidic Bond Based On Remote Acyl Participation

P-Mannosyl glycosides and its derivatives are widely distributed in nature,especially in N-linked glycoproteins.Biological studies have shown that oligosaccharides containing P-mannosyl motifs play an important role in many biological and cellular processes.High stereoselective construction of beta-mannosyl bond is one of the long-standing challenges in glycochemical synthesis.The main reasons are as follows:1)The formation of beta-glycosides is not favored due to the anomeric effect;2)Neighboring group participation(NGP)effect or steric hindrance at C-2 position on mannosyl ring leads to the easy formation of alpha-mannosides.In this thesis,two major topics have been investigated:(1)Highly stereoselective ?-mannosylation has been studied using a variety of mannosyl lactone donors with different leaving groups at anomeric position in the presence of catalytic amount of promotors.Four kinds of donors were prepared with different leaving groups at anomeric position,such as trichloroacetimide ester group,p-methylphenylthio group,p-methoxyphenylthio group and 2,2-dimethyl-S-but-3-ynyl.Among thoses donors,2,2-dimethyl-S-but-3-ynyl mannosylthioside has turn out to be the best donor for the construction of beta-mannosides in the presence of 20%Hg(NTf2)2.(2)Remote acyl group participation has been proved by trapping the key intermediates via the treatment of mannosyl lactone donors in the presence of different promotors.Lactone donors were synthesized with different protecting groups at C-4 position such as Ac,Boc,Bz and trichloroacetimido groups and with different leaving groups at the anomeric position such as ethylthio,p-methylphenylthio,trichloroacetimido and 2,2-dimethyl-S-butyl-3-ynyl.The trapping experiments were conducted under mild conditions and two types of intermediates were obtained to support the remote acyl group participation effect during beta-mannosylation:1)Treatment of the lactone imidate donor with trichloroacetamido protecting group at O-4 position resulted in the formation of oxazole 35;2)Treatment of 2,2-dimethyl-S-but-3-ynyl mannosylthio lactone donor with Bz group at O-4 position led to the isolation of orthoacid chloride 36 in the presence of AuCl3.