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Homo-aldol reaction

Scheme 4.12 Homo-aldol reaction catalyzed by L-proUne... Scheme 4.12 Homo-aldol reaction catalyzed by L-proUne...
Scheme 4.13 Two-step synthesis of allose by selective homo-aldol reaction... Scheme 4.13 Two-step synthesis of allose by selective homo-aldol reaction...
Although prolinamides have shown their versatility in the aldol reaction between ketones and aldehydes, their application towards the reaction between the cross or homo-aldol reaction between aldehydes have been scarcely studied. The homo-aldol dimerization reaction between neat propionaldehyde (5, R =Me and 2, R =Et in Scheme 4.23) catalyzed by (S)-prolinamide (40, 20 mol%) in the presence of... [Pg.273]

Scheme 4.24 Homo-aldol reaction of ethyl pyruvate catalyzed by prolinamine 75a... Scheme 4.24 Homo-aldol reaction of ethyl pyruvate catalyzed by prolinamine 75a...
In this cross-aldol reaction, formation of the enamine intermediate of an a-chloroaldehyde would be inhibited or significantly slowed down due to steric repulsion, and the formed enamine intermediate of a donor aldehyde reacts predominantly with the electronically activated a-chloroaldehyde (Scheme 17.7). The homo-aldol reaction of the donor aldehyde is suppressed probably due to the moderate nucleophilicity of (S)-4. [Pg.139]

Allyltitanium reagents have been used in the homo-aldol reaction [76]. Enan-tiodivergent tuning by virtue of the titanium reagents of chiral l-oxyallyUithium substrates leads to enantiomeric homo-aldol products with excellent enantiomeric excess. Hoppe and coworkers found the first example of a chiral nonracemic allyllithium with configurative stabiHty [77]. In their work they described the lithiation of optically pure aUyl carbamates. These lithiated allylic carbamates are substrates for homo-aldol reactions. The so-derived chiral allyllithium compounds can be transmetaUated to titanium in order to facilitate a subsequent aldol reaction (Scheme 3.48) [76a]. [Pg.175]

Scheme 3.48 Reagent-controlled enantioselective homo-aldol reaction with chiral 1-oxyallyllithium derivatives. Scheme 3.48 Reagent-controlled enantioselective homo-aldol reaction with chiral 1-oxyallyllithium derivatives.
The Cannizzaro reaction of formaldehyde is far from a simple kinetic process that could be characterized by first- or second-order kinetics, and that would be a single reaction in which methanol and formate are produced. Rather, it proceeds in alkaline medium in conjunction with the formose reaction, the autocatalytic self-addition of formaldehyde to produce glycolaldehyde, which is then followed by aldol reaction to afford higher aldoses and ketoses. Mono-, di-, tri-, and tetra-valent bases, as well as nitrogenous bases, have been reported to catalyze both reactions homo-... [Pg.203]

The intennolecular aldol reaction between aldehydes can be classified in two types the homo-aldol dinierization process of a single aldehyde and the CTOss-aldol process between two different aldehydes. The later is a more d anding transformation, since the two possible CTOss-aldol processes have to differentiated and the two possible homo-aldol dimerization processes must be prevented. [Pg.257]

Proline (1) has showed to be a effective catalyst for both the homo-aldol and the cross-aldol reactions (Schane 4.12). Thus, the homo-aldol process using propi-onaldehyde (R =Me in 5a and R =Et in 2) afforded the expected a-hydroxyaldehyde in an excellent enantioselectivity for the major anti-29. Under similar reaction conditions, the cross-aldol reaction between propionaldehyde as source of nucleophile (5, R =Me) and other different aliphatic and aromatic aldehydes has been performed, giving the anti-29 isomer as the main diastereoisomer [71], This reaction course has been explained due to the steric hindrance as well as the kinetic inaccessibility of the hydrogen for some a,a-disubstimted aldehydes which leads, in both cases, to a very thermodynamic unstable corresponding nucleophilic enamine intermediate. [Pg.257]

Fig. 4. 35 Organocatalysts used for the homo-cross-aldol reaction between aldehydes... Fig. 4. 35 Organocatalysts used for the homo-cross-aldol reaction between aldehydes...
The imididazolidinone 180 (10-20 mol%, Fig. 4.35) catalyzed the homo-aldol dimerization process of an aldehyde and also the cross-aldol reaction between eno-lizable aldehydes (5, source of nucleophile, 10 equiv.) and aromatic aldehydes (2, electrophile). For both cases, the yields were high (58-90%), the anti-diastereo-selectivity was moderated (60-86% de) and the enantioselectivity was excellent (91-97% ee). To prevent a hemiacetal reaction of the initial aldol product 29 with another equivalent of aldehyde, the reaction was quenched by a methanolysis process to form the corresponding dimethyl acetal [259]. [Pg.309]

In organocatalytic cross-aldol reactions of two different aldehydes through the enamine intermediate first reported by MacMillan and Northrop, the a d-cross-aldol adduct could be obtained in a highly stereoselective fashion. However, most such reactions required the use of sterically hindered aliphatic aldehydes, from which the enamine intermediates are rather difficult to form, or aromatic aldehydes as electrophile. In the direct aldol reaction between simple aliphatic aldehydes (enolisable aldehydes), both aldehydes can perform the double role of nucleophile and electrophile, and consequently, two cross-aldol adducts and two homo-aldol adducts would be possible products with each having four stereoisomers. To differentiate two... [Pg.138]

Other reports deal with a pyrrolidine-catalysed homo-aldol condensation of aliphatic aldehydes (further accelerated by benzoic acid), a diastereoselective aldol-type addition of chiral boron azaenolates to ketones,the use of TMS chloride as a catalyst for TiCU-mediated aldol and Claisen condensations, a boron-mediated double aldol reaction of carboxylic esters, gas-phase condensation of acetone and formaldehyde to give methyl vinyl ketone, and ab initio calculations on the borane-catalysed reaction between formaldehyde and silyl ketene acetal [H2C=C(OH)OSiH3]. ... [Pg.24]

We decided to evaluate our hypothesis with readily available substrates having a tethered enone to the starting aldehyde in order to maximize the chances of bond formation (Scheme 18) [141]. After screening for reactimi conditicms, we found that the use of 10 mol% of azolium salt 7 in the presence of DBU was the most efficient combination to promote the reaction and afford 3,4-dihydrocoumarins structures in high yield. The reaction was performed in acetonitrile at low cmicentration to prevent homo-aldol coupling. In this process, acylation of the alkoxide followed by intramolecular Michael addition was a plausible pathway, but it was ruled out after subjecting the acylated substrate to the reaction conditimis, which was found to be mneactive. [Pg.249]

A cyclic transition state model, that differs from the Zimmerman-Traxler and the related cyclic models inasmuch as it does not incorporate the metal in a chelate, has been proposed by Mulzer and coworkers [78] It has been developed as a rationale for the observation that, in the aldol addition of the dianion of phenylacetic acid 152, the high ti-selectivity is reached with naked enolate anions (e.g., with the additive 18-crown-6). Thus, it was postulated that the approach of the enolate to the aldehyde is dominated by an interaction of the enolate HOMO and the n orbital of the aldehyde that functions as the LUMO (Scheme 4.31), the phenyl substituents of the enolate (phenyl) and the residue R of the aldehyde being oriented in anti position at the forming carbon bond, so that the steric repulsion in the transition state 153 is minimized. Mulzer s frontier molecular orbital-inspired approach reminds of a 1,3-dipolar cycloaddition. However, the corresponding cycloadduct 154 does not form, because of the weakness of the oxygen-oxygen bond. Instead, the doubly metallated aldol adduct 155 results. Anh and coworkers also emphasized the frontier orbital interactions as being essential for the stereochemical outcome of the aldol reaction [79]. [Pg.151]

Soler, A., Garrabou, X., Hernandez, K., Gutierrez, M. L., Busto, E., Bujons, J., PareUa, T., Joglar, J., and Clapes, R, Sequential biocatalytic aldol reactions in multi-step asymmetric synthesis Pipecolic acid, piperidine and pyrrolidine (homo)iminocycUtol derivatives from achiral buUding blocks. Adv. Synth. Gatal. 2014,356 (14—15), 3007-3024. [Pg.302]


See other pages where Homo-aldol reaction is mentioned: [Pg.15]    [Pg.75]    [Pg.15]    [Pg.258]    [Pg.259]    [Pg.260]    [Pg.290]    [Pg.317]    [Pg.386]    [Pg.249]    [Pg.286]    [Pg.126]    [Pg.15]    [Pg.75]    [Pg.15]    [Pg.258]    [Pg.259]    [Pg.260]    [Pg.290]    [Pg.317]    [Pg.386]    [Pg.249]    [Pg.286]    [Pg.126]    [Pg.291]    [Pg.152]    [Pg.549]    [Pg.17]    [Pg.456]    [Pg.187]    [Pg.92]    [Pg.96]    [Pg.101]    [Pg.121]    [Pg.299]    [Pg.139]    [Pg.139]    [Pg.109]    [Pg.647]    [Pg.158]    [Pg.92]    [Pg.95]    [Pg.795]    [Pg.286]   
See also in sourсe #XX -- [ Pg.257 , Pg.258 , Pg.259 , Pg.273 , Pg.276 , Pg.290 , Pg.317 ]




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