Aldols silylated reagent reactions

Sitro-aldol reaction fl-amino alcohols. Primary nitro compounds form silyl nitronates (1) when treated in sequence with LDA (THF, -78°) and then a silylating reagent. These silyl nitronates undergo aldol condensation with aldehydes in the presence of tetra-n -butylammonium fluoride (there is no reaction in the absence of the catalyst). The products 2 are reduced to /3-amino alcohols (3) in good yield by lithium aluminum hydride (equation 1). Secondary nitroalkanes undergo the same reaction sequence, but the silyl nitronates are less stable and are obtained in only... 

Enolsilylation. Silyl enolates derived from this reagent (in the presence of i-PrNEt2) undergo aldol and Michael reactions without additional catalysts. 

Fluoral hydrate and hemiacetals are industrial products. They are stable liquids that are easy to handle, and they react as fluoral itself in many reactions. Thus, in the presence of Lewis acids, they react in Friedel-Crafts reactions. They also react very well with organometallics (indium and zinc derivatives) and with silyl enol ethers.Proline-catalyzed direct asymmetric aldol reaction of fluoral ethyl hemiac-etal with ketones produced jS-hydroxy-jS-trifluoromethylated ketones with good to excellent diastereo- (up to 96% de) and enantioselectivities. With imine reagents, the reaction proceeds without Lewis acid activation. The use of chiral imines affords the corresponding 8-hydroxy ketones with a 60-80% de (Figure 2.49). ° ... 

Organoaluminum reagents, 202 1,1,1-Trifluoroacetone, 323 Trityllithium, 338 Zinc chloride, 349 Stereoselective aldol reactions With boron enolates Boron trichloride, 43 Chlorodimethoxyborane, 73 9-(Phenylseleno)-9-borabicyclo-[3.3.1]nonane, 245 With silyl enol ethers... 

It has been reported that the chiral NMR shift reagent Eu(DPPM), represented by structure 19, catalyzes the Mukaiyama-type aldol condensation of a ketene silyl acetal with enantiose-lectivity of up to 48% ee (Scheme 8B1.13) [29-32]. The chiral alkoxyaluminum complex 20 [33] and the rhodium-phosphine complex 21 [34] under hydrogen atmosphere are also used in the asymmetric aldol reaction of ketene silyl acetals (Scheme 8BI. 14), although the catalyst TON is quite low for the former complex. 

Aldol condensation.1 These O-silyl enol derivatives of amides are available by hydrosilylation of a,P-unsaturated amides catalyzed by Wilkinson s catalyst. A typical reagent of this type, 1, reacts with aldehydes in the absence of a catalyst to form aldol adducts (2) with unusual anti-selectivity. This silyl aldol reaction can be ex-... 

Exposure of 270 to the TBSOTf/DIPEA reagent couple once more triggered a cascade of reactions, namely chemoselective thiolactone enolization, intramolecular aldolization, and hydroxyl silylation, which ultimately gave rise to 3,4-trans and 3,4-cis bicyclic thiolactones 271 and 272. Final unmasking of the pseudoanomeric thiol function and terminal hydroxymethyl moiety was performed on the individual precursors 271 and 272 to deliver, as expected, the corresponding thiocarbasugars 273 and 274 in high isolated yields. 

As above (eq 1), a major drawback of this reagent is the lack of a readily available enantiomer. There are many alternative methods for the enantioselective propionate aldol reaction. The most versatile chirally modified propionate enolates or equivalents are N-propionyl-2-oxazolidinones, a-siloxy ketones, boron enolates with chiral ligands, as well as tin enolates. Especially rewarding are new chiral Lewis acids for the asymmetric Mukaiyama reaction of 0-silyl ketene acetals. Most of these reactions afford s yw-aldols good methods for the anri-isomers have only become available recently. ... 

This reagent is the best promoter of the aldol reaction of 2-(trimethylsiloxy)acry-late esters, prepared by the silylation of pyruvate esters, to afford y-alkoxy-a-keto esters (Eq. 80) [135] These esters occur in a variety of important natural products. 

This reaction of silyl ketene acetals with aldehydes, using 29 as a stoichiometric chiral reagent (Eq. 46), was reported by Reetz et al. [42]. The aldol addition of l-(trimethyl-siloxy)-l-methoxy-2-methyl-l-propene and 3-methylbutanal provides the aldol in only 57 % yield, but with 90 % ee. 

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