本論文研究掩飾鄰苯醌與相關2,4-環己二烯酮的Diels-Alder反應，內容共分為三部份：第一部份係探討掩飾鄰苯醌與噻吩的環化加成反應；第二部份係探討矽烷取代之掩飾鄰苯醌的Diels-Alder反應及應用；第三部份為天然物obtunone的全合成及相關2,4-環己二烯酮的Diels-Alder反應。
第一部份：掩飾鄰苯醌與噻吩的分子間Diels-Alder反應為逆電子需求的環加成反應，具有很好的位置與立體選擇性。根據理論計算結果，此反應傾向非同步式的反應機構。本實驗所使用2-甲氧基酚及噻吩兩類簡單的平面芳香環類化合物，經氧化、環化加成反應可得高立體選擇性、位置選擇性的多官能基複雜化合物。

第二部份：第四位置含溴基或矽烷基的掩飾鄰苯醌可以降低雙聚合反應的反應速率，其Diels-Alder反應可在使用較少當量的親雙烯劑情況下，還可得到較高的產率。含矽烷基的環化加成產物可經由烯化、Cope重排反應得到含矽烷基的順式-十氫萘，再經由去矽反應可得順式-和反式十氫萘。

第三部份：香旱芹酚經由氧化反應、Diels-Alder反應和去保護三步可得(±)-obtunone，總產率為22%。與合成相關的6-甲基-6-甲氧基-2,4-環己二烯酮的Diels-Alder反應，具有很好的位置、立體與面向選擇性，根據理論計算結果，甲氧基的電子效應對面向選擇性影響很大。

Studies on Diels-Alder reactions of various 2,4-cyclohexadienones including masked o-benzoquinones (MOBs) are described. The thesis consists of three chapters: first chapter is concerned with Diels-Alder reactions of masked o-benzoquinones with thiophene and its derivatives. The second chapter deals with the synthesis of silyl substituted masked o-benzoquinones, their Diels-Alder reactions and applications in organic synthesis. The third chapter is concerned with the regio-, stereo- and facial selective Diels-Alder reactions of 6-methoxy-6-methyl-2,4- cyclohexadienones and their applications in the total synthesis of racemic obtunone.
The intermolecular Diels-Alder reactions of masked o-benzoquin- ones with thiophenes are discussed systematically in chapter 1. Thiophenes showed high regio- and stereoselectivities in the Diels-Alder reactions with MOBs, albeit with low reactivity in comparision to furans and pyrroles due the high resonance energy of the former. Using the present protocol multifunctional compounds with a high degree of structural complexity were synthesized from simple and commercially available aromatics. For the first time we have demonstrated the Diels-Alder reactions of several thiophenes with considerable generality. From the theoretical calculations the reaction appears to proceed via an asynchronous reaction mechanism.

In the second chapter, the Diels-Alder reactions of the 4-bromo-, 4-silyl- substituted masked o-benzoquinones with several dienophiles are described. The motive behind these substitutions is to produce stable MOBs and to use less equivalents of dienophile, which can offer better yields of the requisite cycloadducts by minimizing/ preventing the dimerization of MOBs. Several silyl- substituted 2-methoxyphenols have been prepared for this study. The Diels-Alder cycloadducts bearing silyl moiety were sequentially subjected to olefination and Cope rearrangement to provide substituted cis-decalins with silyl group at the bridgehead position, which can undergo desilylation to provide several substituted cis- and trans-decalins in good yields.

The investigations on selective Diels-Alder reactions of 6-methoxy-6-methyl-2,4-cyclohexadienones are detailed in chapter 3. The commercially available carvacrol was used to synthesize racemic obtunone, a natural product in three synthetic operations and 22% overall yield. Excellent regio-, stereo- and face-selectivities were observed in the cycloaddition step. Theoretical calculations have supported the experimentally observed facial selectivity i.e., syn-approach of the dienophile to methoxy group of the diene, which is apparently due to the electronic effect of the methoxy group.

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