Neighboring group effect in Pd-catalyzed carbonylation terminated by lactonization: a need for a protective group and/or DMF |
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Authors: | Schiller Radan Pour Milan Fáková Helena Kunes Jirí Císarová Ivana |
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Institution: | Department of Inorganic and Organic Chemistry, Faculty of Pharmacy, Charles University, Heyrovského 1203, CZ-500 05 Hradec Králové, Czech Republic. |
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Abstract: | Synthesis of analogues of antifungal podolactones via Pd-catalyzed processes revealed that tandem 6-exo-alkyne carbopalladation/carbonylative lactonization sequence is strongly solvent-dependent. Contrary to earlier reports, premature esterification was the predominant pathway when the starting enynes derived from (Z)-2-iodohex-2-en-1,4-diol were subjected to Pd-catalyzed carbonylation in MeOH. Apparently, irreversible complexation of Pd by the OH group prevented decarbonylation and hence 6-exo-alkyne carbopalladation. Similarly, the influence of the chelation was also evident when the reaction was applied to the analogous preparation of 3-hydroxymethylbutenolides. The neighboring group effect can be efficiently overcome through using DMF as the solvent in combination with protection of the OH function. |
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