Michael additions Lewis acid catalysed

Ketoacetals are available from the Lewis acid catalysed Michael addition of hemiacetal vinylogues to 3,4-dihydropyrans. The products are a source of hydroxy- and amino- acetals and hence give access to annulated tetrahydropyrans (95JCS(P 1)2103). 

Figure 3.8 Lewis acid catalysed Michael addition.

The Evans mechanism for the Lewis acid-catalysed Michael addition to chiral oxa-zoUdinones and related systems has been challenged rather than chelation, open-chain mono- and bis-coordinated complexes with antiparaUel carbonyls (120) have been proposed, based on the NMR and DFT computational studies. This new mechanism is consistent with the experimental results reported for the conjugate addition of amines to a,j8-unsaturated acylimidazolidinones (119) with aluminium, boron, or zinc Lewis acids. ... 

There are only few examples of organic reactions catalysed effectively by Lewis acids which can be carried out in pure water without any organic co-solvent. While water can be used successfully for the uncatalysed Michael addition of 1,3-diketones (Table 4, entry D)22, the corresponding reaction of /i-kctocsters does not give satisfactory results. On the other hand, the Yb(OTf)3 catalysed Michael reaction of various /i-ketoesters (Table 21, entry A)257 and a-nitroesters (Table 21, entry B)258 takes place. 

There are only few examples of organic reactions catalysed effectively by Lewis acids which can be carried out in pure water without any organic co-solvent. While water can be used successfully for the uncatalysed Michael addition of 1,3-diketones (Table 4, entry... 

Itoh, K. and Kanemasa, S. (2002) Enantioselective Michael additions of nitromethane by a catal3ftic double activation method using chiral Lewis acid and chiral atitine catalysts. Journal of the American Chemical Society, 124, 13394—13395 Itoh, K., Oderaotoshi, Y. and Kanemasa, S. (2003) Enantioselective Michael addition reactions of malononitrile catalysed by chiral Lewis acid and achiral antine catalysts. Tetrahedron Asymmetry, 14, 635 39. 

Due to the unique bivalent carbene and diversity of the N-heterocyclic motif, NHCs have been demonstrated to be efficient organocatalysts for various enantioselective reactions. In addition to the traditional thioazolium and imidazolium NHCs, triazolium NHCs have become the most successful organocatalysts. Recently, NHC/Lewis acid cocatalysis and bifunctional NHCs have shown a very promising future. Beyond the classic NHC-catalysed umpolung of aldehydes, the extended umpolung of functionalised aldehydes are extremely successful. A series of NHC-catalysed reactions of ketenes have been developed for the synthesis of various enantioriched heterocycles. Esters, anhydrides, carboxylic acids and even Michael acceptors are useful alternative substrates for NHC-catalysed reactions. With increasing interest and rapid development of NHC catalysis, new structures of the catalysts, new reaction modes, and synthetic applications can be expected in the near future. 

The Mukaiyama-Michael addition of diazo-silylenol ethers CH2=C(OTBS)-C(N2)C02R to enones R CH=CHCOR, catalysed by a chiral Cu(II) complex (10mol%), acting as a Lewis acid, affords the corresponding y-functionalized diazoacetoacetates with <96% ee ... 

These bulky aluminium aryloxides will also promote a variety of carbon-carbon bond forming reactions with a high degree of regio and stereoselectivity. Examples include Michael addition to a,/S-unsaturated ketones, Diels-Alder additions, and Claisen rearrangements. In the case of Diels-Alder reaction, ATPH promoted the exo-selective condensation of a,/6-unsaturated ketones with dienophiles. The Claisen rearrangement can be catalysed by ATPH and its more Lewis acidic 4-bromo derivative. A series of chiral aluminium aryloxides were also synthesized and have been applied to asymmetric Claisen rearrangements, aldol reactions, and aldehyde alkylations. ... 

Diones are important intermediates in a number of syntheses of cyclopent-2-enones by aldol cyclization, and several routes to them have been published (see Review section). An attractive new approach involves the Michael addition of silyl enol ethers to nitro-olefins catalysed by Lewis acids, which leads directly to the 1,4-dione without isolation of nitroketone or related intermediates (Scheme 15). A feature of the method is that it is highly regiospecific in the selective formation of 1,4-diones with no double-bond migration in the silyl enol ether. 

A condensation of vinyl sulphides of synthetic value involves the reaction of a-ethylthio-styrenes with an aldehyde in the presence of a Lewis acid catalyst to give 3-ethylthio-indenes. Regiospecific and stereospecific pentannelation i.e, fusion of a cyclopentane ring) of an alicyclic ketone involves arylthiomethylen-ation followed by base-catalysed addition to an aj8-unsaturated ester. a-Phenyl-thiobutenolides are less reactive Michael acceptors than their sulphoxide counterparts, and /SjS-dichlorovinyl sulphides are less reactive in electrophilic reactions than their oxygen counterparts. Dimerization of aryl a-cyanovinyl sulphides occurs more readily than that of alkyl analogues, since the intermediate 1,4-diradical ArSC(CN)CH2CH2d(CN)SAr, which cyclizes to the cyclobutane, is more stable. Products obtained by photolysis of methyl vinyl sulphide are accounted for by homolysis of both vinyl—S and methyl—S bonds. The kinetics of catalysed cis-trans isomerization of aryl vinyl sulphides have been studied. ... 

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