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By Jose L Vicario;Dolores Badia;Luisa Carrillo
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Additional resources for Organocatalytic enantioselective conjugate addition reactions : a powerful tool for the stereocontrolled synthesis of complex molecules
Example text
Yields of the final products were good to moderate and good diastereoselectivities and excellent enantioselectivities were obtained for a variety of enals and nitroalkene substrates, including the use of both b-aryl and b-alkyl substituted nitroalkenes. 3 Reactions under Non-conventional Methodologies Another issue that has led to intensive research in recent years is the development of more environmentally benign methodologies, especially regarding the solvent employed (with particular interest in the possibility of using water)66 and the chance of recycling the catalyst.
62 Experimental conditions change from one report to the other and while in one case MeCN (when nitrostyrenes were employed as Michael acceptors) or a DMF/iPrOH mixture (for b-alkyl substituted nitroolefins) was employed as the reaction solvent and the main efforts were focused on the manipulation of acetaldehyde to avoid oligomerization (a five-fold excess of a solution of freshly distilled acetaldehyde had to be employed and it was very slowly added to the reaction mixture via siring pump), in the other case the simple use of a ten-fold excess of acetaldehyde and 1,4-dioxane as reaction solvent was reported to be enough to reach the final Michael adducts in acceptable yields, although the use of a sealed tube as reaction vessel is also mentioned.
This results in a higher atom-efficiency of the process and also allows carrying out the reaction under neutral and smooth conditions. An additional outstanding advantage of the enamine activation strategy applied to the Michael reaction is the capacity to employ unmodified aldehydes as donor reagents without the need for prior derivatization to the corresponding enolate or silyl enol ether, avoiding the extremely problematic formation of these activated reactive derivatives. On the other hand, it has to be mentioned that this methodology is limited to the use of Michael donors capable of forming enamines (aldehydes or ketones) and the lower reactivity of such nucleophiles very often limits the application of this reaction to the use of rather activated Michael acceptors.