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| - Selective Reductions. 59. Effective Intramolecular AsymmetricReductions of α-, β-, and γ-Keto Acids withDiisopinocampheylborane and Intermolecular AsymmetricReductions of the Corresponding Esters withB-Chlorodiisopinocampheylborane
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| - A comparison of the stereochemistry of the products obtained from the intramolecular asymmetricreduction of a series of keto acids with (−)-diisopinocampheylborane and intermolecular asymmetricreduction of the corresponding series of keto esters with (−)-B-chlorodiisopinocampheylborane ((−)-DIP-Chloride) has been made. The stereochemistry of the hydroxy acids from the reduction ofketo acids is dependent only on the enantiomer of the reagent used. The stereochemistry of theproducts from the reduction of keto esters is also consistent, except those of aliphatic α-keto esters.α-, β-, and γ-keto acids provide the corresponding hydroxy acids in 77−98% ee, and the α- and γ-keto esters afford the hydroxy esters in 82−≥99% ee. β-Keto esters do not undergo reduction.Although the reduction of δ-keto acids does not proceed under the same reaction conditions, thereduction of δ-keto esters is facile. All of the products from the reduction of γ-keto acids and estersand δ-keto esters were converted to the corresponding lactones. This study revealed that DIP-Chloride is an efficient reagent for the reduction of α-keto esters at low temperatures.
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