Aldehydes reaction with enolates

Besides aldehydes and ketones, TiCl4 activates acetals effectively for reaction with enolates (Scheme 18).70,71 The reactivity of acetals is higher than that of the corresponding aldehydes in... 

Monocyanohydrins of P-diketones.3 In the presence of TiCl4, acetyl cyanide reacts with enol silyl ethers of ketones at - 78° to afford monocyanohydrins of diketoncs in excellent yield. The corresponding reaction with enol silyl ethers aldehydes proceeds in only about 35% yield. A low temperature is essential for this reaction. A similar reaction is possible with allyltrimethylsilane. 

Aldol reactions with enol silyl ethers5 (Mukaiyama reaction). Although TiCl4 is commonly used to effect aldol reactions with enol silyl ethers, BF3 etherate is more effective for aldol reactions of tetrasubstituted enol ethers such as (Z)- and (E)-3-methyl-2 (trimethylsilyloxy)-2-pentene (1). These enolates react with aldehydes in the same sense as the corresponding metal enolates, but with generally higher levels... 

The different methods available for doing aldol reactions with enolates of aldehydes, ketones, and esters... 

Chiral a-methyl aldehydes (43) show exceptional diastereofacial preferences in their Lewis acid mediated reactions with enol silanes (equation i6) 2i 25c-26-64 selected data are reported in Table 8. The reason for this selectivity may be due to an approach trajectory of the nucleophile closer to the stereocenter when the carbonyl group is bound to the Lewis acid. Additions to chiral a-alkoxy aldehyde (48) were studied with both nonstereogenic (equation 17 Table 9) and stereogenic enol silanes (equation 18 Table 10). (Stereogenic and nonstereogenic are defined according to Mislow and Siegel.) ... 

Early extensive accounts of the 4v participation of a,/)-unsaturated carbonyl compounds in [4 + 2] cycloadditions detailed their reactions with electron-deficient dienophiles including a,/3-unsaturated nitriles, aldehydes, and ketones simple unactivated olefins including allylic alcohols and electron-rich dienophiles including enol ethers, enamines, vinyl carbamates, and vinyl ureas.23-25 31-33 Subsequent efforts have recognized the preferential participation of simple a,/3-unsaturated carbonyl compounds (a,/3-unsaturated aldehydes > ketones > esters) in inverse electron demand [4 + 2] cycloadditions and have further explored their [4 + 2]-cycloaddition reactions with enol ethers,34-48 acetylenic ethers,48 49 ke-tene acetals,36-50 enamines,4151-60-66 ynamines,61-63 ketene aminals,66 and selected simple olefins64-65 (Scheme 7-1). Additional examples may be found in Table 7-1. 

It is proposed that the benzo[c]pyrylium cation 39, produced from o-alkynylbenzaldehydes by AuBra catalysis, behaves as the 4n component in an inverse electron demand DA reaction with enols. Dehydration and bond rearrangement leads to naphthalene derivatives. Simple a, 3-unsaturated aldehydes can also be benzannulated in this way <04JA7458>. 

P-Hydroxy amides. Cerium(III) enolates of amides show better reactivities than the Li enolates toward ketones and aldehydes. Reaction with camphor gives >94% yield of the aldol. 

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