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|Title:||Recent developments in michael reaction - a convenient tool for annelation and annulation|
|Publisher:||COUNCIL SCIENTIFIC INDUSTRIAL RESEARCH|
|Citation:||JOURNAL OF SCIENTIFIC & INDUSTRIAL RESEARCH,52(1)13-28|
|Abstract:||Improved methodologies evolved recently in Michael reaction are described. These involve usage of enol silanes or silyl ketene acetals, enamines or imines or enolates; phase transfer catalysts; transition metals; and heterogeneous catalysts. Lewis acid catalysed Michael reaction of enol silanes with alpha, beta-unsaturated carbonyl compounds have been used effectively for the synthesis of intermediates for various natural products. The reaction of chiral enones show a good to excellent 'UL' diastereofacial preference in TiCl4-mediated reactions with chiral and achiral enol-silanes. The diastereoselectivity reverses with the use of trityl salts, if the cyclic enones possessing alpha-substituents are used as acceptors. It has been pointed out that nu-acyl substituted-delta-hydroxy ketones have been stereoselectively synthesized through tandem Michael-aldol condensations. The lithium enolates undergo diastereoselective Michael additions under kinetically controlled conditions and the geometry of enolates determines the syn/anti nature of Michael adducts. Combination of crown ethers/KO(t)Bu has been found to be satisfactory phase transfer conditions for preparation of Michael adducts with good optical purity. Fluoride ion has been shown to be effective catalysts for Michael additions involving nitroalkanes as donors. It is pointed that complexes of Cu and Ni are mild catalysts for conjugate additions of beta-diketones, esters, etc. Certain heterogeneous catalysts such as clays, xonolites offer yet another set of mild catalysts for improving yields of the adducts.|
|Appears in Collections:||Review|
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