具葡萄醣為掌性輔助基掩飾鄰苯醌之合成及其不對稱Diels-Alder反應研究

The Synthesis and Asymmetric Diels-Alder Reactions of Masked o-Benzoquinones with D-Glucopyranosyl Auxiliaries

曹佩婷(Pei-Ting Tsao)

指導教授 ： 廖俊臣

國立清華大學/理學院/化學系/碩士(2007年)

https://doi.org/10.6843/NTHU.2007.00370

摘要

本論文係探討具葡萄醣為掌性輔助基之掩飾鄰苯醌99-104的合成及其不對稱Diels-Alder反應研究。
掌性酚90-95分別於三種溫度下氧化成掌性掩飾鄰苯醌99-104再與甲基乙烯基酮進行分子間Diels-Alder反應，得到的環加成產物為105-110 (a, b, c)，研究發現降低氧化溫度時有助於立體選擇性的提升。
將所得環加成粗產物於酸性條件下進行甲基化反應，切除葡萄醣掌性輔助基，即可得到鏡像超越值。其中以葡萄醣上取代基為第三丁基芐基，在-30 oC下氧化時的非鏡像選擇性最好；-78 oC下氧化，由於掌性酚立體障礙之因素，導致氧化不完全，反而使立體選擇性降低。
由X光單晶繞射ORTEP圖、比旋值及NOE實驗可以決定化合物105-110 (a, b, c)的絕對立體組態。我們因而得知掌性掩飾鄰苯醌在進行分子間Diels-Alder反應時，醣基具有誘導作用，而使得甲基乙烯基酮偏好從醣基方向加成。

關鍵字

掩飾鄰苯醌

並列摘要

The studies on the synthesis of masked o-benzoquinones (MOBs) 99-104 embedded with glucopyranoside as chiral auxiliary to induce chirality in MOB and their subsequent diastereoselective Diels-Alder reaction are described.
The in situ generated MOBs 99-104 by the oxidation of chiral phenols 90-95 in the presence of diacetoxyiodobenzene (DAIB) as oxidizing agent underwent diastereoselective Diels-Alder reaction with methyl vinyl ketone to provide cycloadducts 105-110 (a, b, c). Interestingly, the reaction temperature was found to affect the diastereoselectivities obtained. The best diastereoselectivities were obtained when the tandem oxidation/Diels-Alder reaction was carried out at -30 oC with R-group in 90-95 was t-butylbenzyl. However, when the temperature was further lowered to -78 oC, the bulky protecting group made the oxidation difficult and provided inferior diastereoselectivities.
A mixture of crude Diels-Alder adducts when subjected to methanolysis under acidic conditions replaced the glucopyranoside chiral auxiliary with methoxy- group and thus provided enantiomerically pure bicyclo[2.2.2]octenone systems 114.
The assigned absolute configuration of 105-110 (a, b, c) is based on single crystal X-ray analysis, optical rotation values and NOE data. This reaction was found to be highly facial selective and surprisingly, the dienophile preferred to approach from sterically more hindered face i.e., towards the glucopyranoside auxiliary side.

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