Ion pairing effects on the regioselectivity of arylic versus benzylic C-O bond reductive cleavage: synthetic applications |
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Authors: | Ugo Azzena Giovanna Dettori Ilaria Mascia Luisa Pisano Mario Pittalis |
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Institution: | Dipartimento di Chimica, Università di Sassari, via Vienna 2, I-07100 Sassari, Italy |
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Abstract: | The regioselectivity of the reductive cleavage of 3,4,5-trimethoxybenzyl methyl ether strongly depends on the alkali metal employed as a reducing agent and solvent effects. Reactions run using Na as a reducing agent led to aromatic C(4)-O bond cleavage, whilst reductions run in the presence of Na/15-crown-5, or using Li as a reducing agent, led to highly regioselective benzylic C-O bond cleavage. This regioselectivity turnaround is discussed in terms of major solvent effects affecting the fragmentation paths of a common reaction intermediate. Synthetic applications of these findings led to the synthesis of biologically active compounds, like 2,5-dialkyl-substituted resorcinols, or 1-(3,4,5-trimethoxyphenyl)-2-arylethanes structurally related to combretastatin. |
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Keywords: | Alkali metals Metallation Cleavage reactions Regioselectivity Ionic pairs Natural products |
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