Abstract
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.