- 學位論文
Stereochemical Process for Carbon-Carbon Bond Formation on Dihydropyran Ring System - Ferrier Rearrangement and Wittig Rearrangement
摘要
本論文包含了兩個題目: (一) 鈷媒介-費里爾類型重組反應合成掌性炔基環己酮化合物 利用糖苷化反應，在酸性條件下，以乙醯化葡萄烯糖和(三甲基甲硅烷基)乙炔為起始物來合成1-炔基醣類化合物。八羰基二鈷與炔基形成的已炔基鈷錯化合物可以用來穩定炔丙基碳正離子 (尼古拉斯碳正離子)，將此一特性運用於我們所合成的1-炔基-5-烯基醣類化合物，來進行費里爾類型重組反應得到炔基環己酮。從實驗結果發現2號碳位置保護基的大小對於立體選擇性有很大的影響，而此效應為鄰位交叉效應，實驗結果引起我們對於反應過程中立體化學的興趣。而近期所發表利用理論計算來推測炔基鈷錯化合物反應機構的文獻，更是令我們想對於此反應機構有更深入的了解。 (二) 研究丙炔基醚二氫吡進行喃威悌重排反應的立體化學 二號位置和四號位置具有丙炔基醚的二氫吡喃，利用[2,3]-威悌重排反應得到四號位置和二號位置具有高立體選擇性的碳碳鍵。反應機構：1) 對前手性為R或是S的氫進行去質子化、2) 具有立體異構的碳負離子進行平衡、3) 反應過渡態時分子構造進行翻轉、4) 進行[2,3]或[1,2]- 威悌重排反應、然後5) 終止反應。反應步驟第二步，對具有立體異構的碳負離子進行平衡，再某些為協同反應的[2,3]-威悌重排例子中是可以被省略的。而在某些例子中，[2,3]或[1,2]- 威悌重排反應是經由烯丙基的碳氧鍵斷裂而不是協同反應機構。而這一特殊反應機構，進一步利用密度泛函理論計算來加以證實。
並列摘要
This thesis consists of two topics: I. Cobalt-Assisted Ferrier-Type Rearrangement to Construct Chiral Alkynyl Cyclohexanones Functionalized 1-C-alkynyl sugars are prepared through so called C-glycosidation from a D-glucose derivative, 2-acetoxy-glucal triacetate with TMS-acetylene under Lewis acid condition. Dicobalt complexes of these C-alkynyl products, sugar acetylenes possess a special chemical character to stabilize the propargylic carbenium cations (known as Nicholas cation) at the anomeric position. This cation could promote the Ferrier-type rearrangement through the above propargyl vinyl cyclic cationic intermediates to afford alkynyl cyclohexanones after decomplexation. High stereoselectivity resulted from the suitably oriented steric interaction between the C-2 O-protecting group and the alkynyl substituents due to gauche effect. Such an effect prompted us to focus on stereochemical course of this rearrangement particularly found in the dihydropyran ring system. II. Stereochemical Course of Wittig Rearrangement of Dihydropyran Allyl Propargyl Ethers [2,3]-Wittig rearrangements of sugar-derived dihydropyran allyl propargyl ethers located at the 2- or 4-position have been studied as a means for extending the carbon chains to the 4- or 2-position with chirality transfer. The stereochemical courses of these reactions depend on the following factors: (1) deprotonation of propargylic pro-R or pro-S-H, (2) equilibration of the lithiated stereogenic carbanion, (3) conformational inversion at the transition state, (4) [2,3]- or [1,2]-Wittig-rearrangement, and (5) workup. The equilibration step-(2) may be skipped depending on the orientational change for the step-(3) leading to a concerted [2,3]-Wittig process. In some cases, a new mechanism occurs leading to allyl-C-O bond-cleavage before either the [2,3]- or [1,2]-Wittig rearrangement. The stereochemical courses of the rearrangements are compared among the lithiated reactants for determining the reaction pathways. This unique mechanism in the poly-oxygenated dihydropyran ring system was further supported by DFT calculations. Indeed, some examples are reported of the [2,3]-process going through a non-concerted process in 2-steps due to strong chelation effects.