Aldehydes aldol condensation with ketones

In contrast, fluorinated ketones have been used as both nucleophilic and electrophilic reaction constituents The (Z)-lithium enolate of 1 fluoro 3,3-di-methylbutanone can be selectively prepared and undergoes highly diastereoselec-tive aldol condensations with aldehydes [7] (equation 8) (Table 4)... 

Stereoselective aldol-type condensation.1 Enol silyl ethers do not undergo aldol condensation with aldehydes or ketones in the presence of this triflate,. but the reaction occurs at —78° (4-12 hours) with the corresponding acetals or ketals (and certain orthoesters). Moreover the erythro-aldol is formed with high stereoselectivity. 

Aldol condensation with aldehydes and ketones gives hydroxy compounds (265 — 267) which usually spontaneously lose water (by a reverse Michael addition) to give unsaturated compounds (268). 

Conjugate addition-aldol reactions. A novel synthesis of a-substituted a,(3-enones involves conjugate addition of 1 to an a,p-enone the resulting 0-phenyl-selenoboron enolate undergoes aldol condensation with aldehydes. The adduct on oxidative elimination furnishes unsaturated p-hydroxy ketones.1 Example ... 

Aldol condensation. With aldehydes, successful with either acid or base catalysis. m With ketones, conditions (strong acid or strong base catalysis) under which dehydration occurs are usually used to shift equilibrium toward the product.. ... 

Further transformations of the intermediate alkenyloxydialkylboranes (46), as shown in Scheme 10, include aqueous hydrolysis to the homologated aldehydes or ketones 30 electrophilic additions to afford the corresponding a-dialkylaminomethyl aldehydes and ketones (47),31 and a-halogenated (48)32 or a-arylselenylated aldehydes or ketones (49) 33 transmetallation-alkylation to afford a-substituted aldehydes or ketones (50) 34 and aldol condensations with aldehydes (51).35 These reagents offer the equivalent of the tandem 1,4-conjugate addition-electrophile-trapping protocol. A prototypical prosta-... 

Enanantioselective aldol reactions. Divalent tin enolates of aldehydes and aryl ketones generated with tin(II) triflate undergo aldol condensation with aldehydes to form aldols.2 The reaction is highly enantioselective if conducted in the presence of chiral diamines derived from (S)-proline, such as l.3... 

Aldol reactions of methyl ketones. The optically active 1,3-oxazolidine (1) formed from a methyl ketone and ( — (-norephedrine aftir conversion to the tin(ll) enolate undergoes enantioselective aldol condensation with aldehydes. The enantioselectivity is partic-... 

Anions formed from group 6 and manganese Fischer carbene complexes undergo aldol condensations with aldehydes and ketones. Allylic carbenes exclusively react in the y position with aldehydes affording dienyl-substituted carbenes. For alkoxy-substituted carbenes, the presence of an excess Lewis acid see Lewis Acids Bases), such as boron trifluoride etherate, titanium tetrachloride, or tin tetrachloride is required for the reaction to proceed in reasonable yield. The initial aldol product can be isolated without elimination (Scheme 12). ... 

Aldol condensations with aldehydes and ketones proceed smoothly at temperatures below — 60 °C and usually give high yields [3]. 

KNOEVENAGEL OOEBNER STOBBE Condensation Base catalyzed aldol condensation of aldehydes or ketones with an activated methylene group of a malonic ester (Knoevenagel Doebner) or a succinic ester (Stobbe)... 

O-Trimethylsilyl nitronates 1036 have been used in fluoride-catalyzed aldol-type condensations with aldehydes and ketones to give a-trimethylsilyloxy-nitro com-... 

The third hypothesis (C) is that reaction takes place through an aldol condensation between the ethyl acetoacetate and the carbonyl form of the sugar. There is some precedent indicating that it is possible for /3-ketonic esters to undergo aldol condensation. These compounds react mole to mole with the aliphatic aldehydes and afford unsaturated substances, which could be formed via an aldol condensation with subsequent dehydration. 

Methyl-3,6-diacetylcarbazole undergoes a double aldol condensation with aromatic aldehydes. The cyclic ketone 197 (R = Hj) condenses with... 

In this case, the 2-aminobenzyl alcohol is oxidized to 2-aminobenz-aldehyde, which undergoes an aldol condensation with the ketone to give an 0, /3-unsaturated ketone. This is followed by cyclodehydratisation to form quinoline. An excess of ketone is necessary to act as a sacrificial hydrogen acceptor. 

Aldol reaction. The reagent (I) adds in a 1,4-fashion to an a,/J-unsaturated ketone lo give an aluminum enolate, which undergoes aldol condensation with an aldehyde. The adduct is converted into an a-substituted-a./l-unsaturated ketone on sulfoxide elimination.1... 

When an ionic organic reaction (the kind catalyzed by most enzymes) occurs a nucleophilic center joins with an electrophilic center. We use arrows to show the movement of pairs of electrons. Tire movement is always away from the nucleophile which can be thought of as "attacking" an electrophilic center. Notice the first step in the second example at right. The unsaturated ketone is polarized initially. However, this is not shown as a separate step. Rather, the flow of electrons from the double bond, between the a- and (1-carbons into the electron-accepting C=0 groups, is coordinated with the attack by the nucleophile. Dotted lines are often used to indicate bonds that will be formed in a reaction step, e.g., in an aldol condensation (right). Dashed or dotted lines are often used to indicate partially formed and partially broken bonds in a transition state, e.g., for the aldol condensation (with prior protonation of the aldehyde). However, do not put arrows on transition state structures. 

Diastereospecific aldol condensations,u The titanium enolate of the chiral ketone 1 reacts with aldehydes to give mainly the syn-aldol (—90 10). However, use of excess titanium reagent or addition of 12-crown-4 (which complexes Li+) results in >99 1 diastereoselectivity. 

Acid Catalyzed. Although ketonic carbonyl groups are less reactive than aldehydic carbonyls in the presence of basic catalysts, this is not the case with acid catalysts. Thus acetone undergoes aldol addition in the presence of sulfuric acid to give mesityl oxide, which then condenses with a third molecule of acetone to give a mixture ofphorone (2,6-dimethyl-2,6-heptadien-4-one) and mesitylene (1,3,5-trimethylbenzene). Ketones also condense with activated aromatic products in the presence of sulfuric acid to give coupled aromatic products. For example, acetone and phenol condense to bisphenol A (4,4 -isopropylidenediphenol), which is used in the manufacture of epoxy resins (qv) and polycarbonates (qv). 

If die enolate nucleophile is derived from an aldehyde or ketone different than die carbonyl electrophile, a crossed-aldol condensation results. Normally best success is achieved if the carbonyl electrophile employed for the crossed-aldol condensation is more reactive than the carbonyl electrophile from which the enolate is derived. For example, ketone etiolates react with aldehydes effectively, but aldehyde enolates do not give the crossed aldol with most ketones but self-condense instead. 

Asymmetric alkylation andaldol condensations.2 The enolate (2) of 1 reacts with primary iodides to give essentially a single product (3), in which the alkyl group is syn to the cyclopentadieny ring. Aldol condensation with acetone leads to only one observable product (4). Only two isomeric products are obtained on aldol condensation with prochiral aldehydes and ketones as expected for a rranx-enolate, the i/ww-aldol predominates or is the exclusive product (5) as in the case of pivaldehyde. 

Ketones are less reactive towards aldol condensations than aldehydes. With acid catalysts, however, small amounts of aldol product can be formed. But the Aldol product that forms will rapidly dehydrate to form a resonance-stabilized product. This dehydration step drives the reaction to completion. 

Carbonyl activation and deactivation.1 Aldehydes, but not ketones, undergo aldol condensation with silyl enol ethers at —78° in the presence of dibutyltin bistriflate. In contrast, the dimethyl acetals of ketones, but not of aldehydes, can undergo this condensation (Mukaiyama reaction) with silyl enol ethers at -78° with almost complete discrimination, which is not observed with the usual Lewis-acid catalysts. Thus dibutyltin bistriflate activates aldehydes, but deactivates acetals of... 

Some of the chemistry developed by the industry more recently, to produce new monohydric alcohols, is just as interesting as the linalool chemistry. Sandalore, a recent new Givaudan chemical with a persistent, sandalwood odor is made according to the scheme in Figure 15 (9). Alpha-pinene, the starting material, is converted to the epoxide which is catalytically rearranged to campholen-ic aldehyde. Aldol condensation with methyl ethyl ketone followed by hydrogenation yields Sandalore . 

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