1.利用鈀金屬催化反應製備唾液酸衍生物 2.利用(TCT/DMF)試劑製備氯醣供體 3.二甲基甲醯胺促進(alpha)選擇性醣質化反應

Abstract

ABSTRACTS This dissertation is divided into four parts: Part I. A highly effective regio- and stereoselective palladium-catalyzed allylic substitution of 2,3-unsaturated derivatives of N-acetyl neuraminic acid (Neu5Ac2en) has been developed. We show that the efficiency of the allylation reaction depends on suitable protecting groups on the starting material and that, with sodium malonate anion as a nucleophile, regioselectivity can be fine tuned by the nature of the ligands associated with the palladium complex. The reaction was also applied to other nucleophiles for constructing C-C, C-N and C-O bonds and led to the major formation of the C-4 regioisomers. The selective transformation of some of the substitution products provided easy entry to a variety of modified sialic acid derivatives. Part II. Preparation of α-glycosyl chloride derived from glycosyl hemiacetals (1-hydroxy sugars) can be performed by treatment of Vilsmeier-Haack complex generated from stoichimetric 2,4,6-trichloro-1,3,5-triazine (TCT) and N,N-dimethylformamide (DMF) to afford the corresponding chloride in excellent yields. This mild condition has been approved being compatible with acid-labile functionalities. In addition, this protocol can be applied to the synthesis of N-acetylneuraminic acid glycal in one-pot manner. Part III. DMF-mediated α−selective glycosylation has been systematically investigated in aspects of mechanistic understanding, the applicable scope and experimental manipulation. We found the several distinguished features:1. Only stoichiometric DMF is required; 2. Better α−selectivity is usually obtained at the low temperature; 3. Pre-activation of glycosyl chlorides with DMF is for the first time proved feasible. 4. Plausible IAD mechanism may be contributed to stereoselectivity. This Applications by using DMF as an additive could be practical in glycol-assembly. . Part IV. A tetrasaccharide containing the core structure of a glycoglycerolipid isolated from Meiothermus taiwanensis ATCC BAA-400 has been efficiently synthesized by using the convergent [2+2] glycoyslation strategy. DMF-mediated α−selective and low-concentration 1,2-trans β−selective glycosylation strategy are for the first time applied to construct the specific α− or β−linkage in oligosaccharide synthesis.