A simple route to enantiopure bis-lactones: synthesis of both enantiomers of epi-nor-canadensolide, nor-canadensolide, and canadensolide |
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Authors: | Sujit Mondal |
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Affiliation: | Department of Organic Chemistry, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032, West Bengal, India |
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Abstract: | A simple strategy has been developed for the synthesis of both enantiomers of nor-canadensolide, epi-nor-canadensolide, and an intermediate to canadensolide. An orthoester Claisen rearrangement of an appropriately constructed allyl alcohol derivative prepared from R-(+)-2,3-di-O-cyclohexylidine glyceraldehyde followed by epoxidation of the resulting unsaturated esters produced hydroxy-lactones, which on oxidation gave keto-lactones. Stereoselective reduction of the keto-carbonyl using either a chelation controlled or a non-chelation controlled process led to the natural or the epi-series, respectively. The interplay of the electronic effect between the polar groups and the steric effect of the β-substituent during reduction of the keto-lactones turned out to be the key factors in deciding the stereochemical outcome. Regeneration of the aldehyde functionality latent in the ketal moiety of the hydroxy-lactones provided the lactols, which on oxidation gave the bis-lactones. |
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Keywords: | Asymmetric synthesis Bis-lactones Chelation Stereocontrol |
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