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| - Heteroatom Influence on the π-Facial Selectivity of Diels−AlderCycloadditions to 1-Oxa-4-thia-6-vinylspiro[4.5]dec-6-ene,3-Methoxy-3-methyl-2-vinylcyclohexene, and3-Methoxy-2-vinylcyclohexene,
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| - The facial selectivities of the Diels−Alder cycloadditions of several dienophiles to the title substrateswere studied. The observed selectivities are interpreted as a consequence of the relative stericinteractions offered by the substituents. The addition of dimethylacetylene dicarboxylate (DMAD)is influenced by the electrostatic repulsion arising from the interaction of an electron pair orbitalon the acetal oxygen and the orthogonal π-orbital of the acetylene unit in DMAD in the syn-to-oxygen addition of the latter. This repulsion is offset on coordination of Li+ to the said oxygenelectron pair orbital, and the addition thus proceeds syn to oxygen. The enhanced and acceleratedsyn-to-oxygen addition in lithium perchlorate in nitromethane is interpreted as a consequence ofthe coordination of Li+ to both the acetal oxygen and a heteroatom in the dienophile that bringsthem in close proximity to facilitate a reaction. The Li+−oxygen combination, however, also exertssome steric effect that results in reduced syn-to-oxygen addition of dienophiles having largesubstituents such as N-phenylmaleimide.
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