Condensation reactions, base catalysed

Laks P E, Hemingway R W 1987 Condensed tannins Base-catalysed reactions of polymeric procyanidins with phenyl-methanethiol. Lability of the interflavanoid bond and pyran ring. J Chem Soc Perkin Trans I 465-470... 

The main example of a category I indole synthesis is the Hemetsberger procedure for preparation of indole-2-carboxylate esters from ot-azidocinna-mates[l]. The procedure involves condensation of an aromatic aldehyde with an azidoacetate ester, followed by thermolysis of the resulting a-azidocinna-mate. The conditions used for the base-catalysed condensation are critical since the azidoacetate enolate can decompose by elimination of nitrogen. Conditions developed by Moody usually give good yields[2]. This involves slow addition of the aldehyde and 3-5 equiv. of the azide to a cold solution of sodium ethoxide. While the thermolysis might be viewed as a nitrene insertion reaction, it has been demonstrated that azirine intermediates can be isolated at intermediate temperatures[3]. 

The long known catalyses of some ketone condensation reactions by secondary amines, can be postulated to have their basis in the reactions of enamine intermediates with ketones. The unsuitability of methyl ketones for azeotropic enamine formation is based on this phenomenon. Recent studies in cyclization reactions have added further support to this concept (354). 

Baekeland had to make important discoveries before he could bridge the gap between the initial concept and final products. In particular, he found that the base-catalysed condensation of phenol and formaldehyde can be carried out in two parts. If the process is carefully controlled, an intermediate product can be isolated, either as a liquid or a solid, depending on the extent of reaction. At this stage, the material consists of essentially linear molecules and is both fusible and soluble in appropriate solvents. When heated under pressure to 150 °C, this intermediate is converted to the hard, infusible solid known as bakelite . This second stage is the one at which the three-dimensional cross-linked network develops. 

The solid base catalysed aldol condensation of acetone was performed over a CsOH/Si02 catalyst using a H2 carrier gas. The products observed were diacetone alcohol, mesityl oxide, phorone, iso-phorone and the hydrogenated product, methyl isobutyl ketone. Deuterium tracer experiments were performed to gain an insight into the reaction mechanism. A mechanism is proposed. 

In liquid phase reactions the base catalysed reaction may only proceed as far as its self-condensation to DAA with minimal dehydration to MO, however when operating in the gas phase at elevated temperatures, secondary reactions can take place (4). 

The cyclizations to obtain cyclic thioureas have been performed using thiocarbonyldiimidazole.232 Reaction of methyl acetoacetate, thiourea and an aliphatic aldehyde in the presence of the zinc iodide (Znl2) was studied. Under the normal pressure, reaction has not been occurred whereas at high pressure (300 MPa) conditions 3,4-dihydropyrimidine-2-thione was obtained only in 10% yield.233 The same one-pot three-component cyclocondensation reaction in the presence of iodide (I2) provides a variety of 3,4-dihydropyrimi-dine-2-thione in high yields.234 Condensation reaction of thioureas with a,p-unsaturated ketones in the presence of the sodium methoxide in methanol affords 3,4-dihydropyrimidine-2-thione derivatives.235,236 Acylation of N,N -disubstituted thioureas with methyl malonyl chloride followed by base-catalysed cyclization leads in the formation of l,3-disubstituted-2-thiobarbituric acids (Scheme 78).237... 

The rate of the base-catalysed condensation of carbonyl compounds with alkyl groups activated by Jt-deficient aromatic systems is enhanced by the addition of quaternary ammonium salts. For example, 2-methylbenzoxazole, 2-methylbenzo-thiazole and 4-nitrotoluene react with a range of substituted benzaldehydes to produce the corresponding 2-styryl derivatives (62-80%) at room temperature over 1 -2 hours [61, 62]. The intermediate alcohol can also be isolated after a short reaction time. 

In temperature-sensitive reactions, aryl aldehydes and ketones produce the trimethylcarbinols at ca. 0°C and the glycolic acids at ca. 56°C in competition with the base-catalysed Cannizzaro or condensation reactions [3, 5, 8]. 2- and 3-Formylpyridines produce the trichloromethylcarbinols (20-30%), whereas 4-formylpyridine undergoes the Cannizzaro reaction [9]. Formylferrocene is converted into the a-ferrocenylglycolic acid (35%) [10],... 

A degree of stereoselective control of the course of a reaction, which is absent or different from that prevalent when the reaction is conducted in the absence of quaternary ammonium salts, may be achieved under standard phase-transfer catalysed reaction conditions. The reactions, which are influenced most by the phase-transfer catalyst, are those involving anionic intermediates whose preferred conformations or configurations can be controlled by the cationic species across the interface of the two-phase system. For example, in the base-catalysed Darzens condensation of aromatic aldehydes with a-chloroacetonitriles to produce oxiranes (Section 6.3), the intermediate anion may adopt either of the two conformations, (la) or (lb) which are stabilized by interaction across the interface by the cations (Scheme 12.1) [1-4]. 

The observations suggest that the two condensations are direct and distinct. Had the base-catalysed reaction first given the aryl-dihydro-triazine (CXXXIV) which, under the prevailing basic conditions rearranged to the anilino-dihydro-triazine (CXXXV), an equilibrium mixture of the two would have been expected. Since, however, the pure anilino-dihydrotriazine (CXXXV) was isolated, its formation is considered to be direct and independent of that of its isomer (ffS). The formation of (CXXXIII) from 1-aryl-l-alkylbiguanides (see preceding Section), which cannot proceed by way of an aryl-dihydrotriazine, bears out this conclusion. 

Figure 19. Reaction scheme for solid-base catalysed Knoevenagel condensation reaction [36],...

The Claisen reaction (sometimes Claisen condensation) is formally the base-catalysed reaction between two molecules of ester to give a P-ketoester. Thus, from two molecules of ethyl acetate the product is ethyl acetoacetate. 

Only non-enolizable eneones give satisfactory yields of hydrodiroer on reduction in aptotic solvents [83]. A suitable aqueous buffer is needed with enolizable eneones to control base catalysed side reactions of condensation and oligomerisation. The polarographic behaviour of eneones in buffers is illustrated using cyclo-... 

The reaction is usually carried out in acid solution, but may also be base catalysed. This is the condensation between aldehydes, ammonia or a primary or secondary amine and a compound containing at least one active hydrogen atom e.g., ketones, b-ketoesters, b-cyanoesters, nitroalkanes, alkynes with C°H). For example. 

Retinol Derivatives. Aryl sulphones have been used in two new syntheses in the vitamin A series. Reaction of /8-cyclocitryl phenyl sulphone (102) with the bromo-compound (103) gives the intermediate sulphone (104), which on base-catalysed elimination affords methyl retinoate (98). Alternatively retinol (99) has been prepared in high yield by condensation of the C15 bromide (105) with the C5 hydroxy-sulphone (106), followed by elimination of sulphinic acid. The syntheses... 

Erom natural sources the (R)-enantiomer of ( )-a-ionone is detected with high enantiomeric purity (much more than 99%) hence, the authenticity of ( )-a-ionone is mostly proved via enantio-GC applications [27,65-67]. In the majority of cases synthetic ionones are produced via pseudoionone, prepared by base-catalysed condensation of citral with acetone. After acidic catalysis (using 85% phosphoric acid or concentrated sulphuric acid), this reaction yields racemic ( )-a-ionone and ( )-/l-ionone [68]. 

Aldol condensation reaction may be either acid or base catalysed. However, base catalysis is more common. The product of this reaction is called an aldol, i.e. aid from aldehyde and ol from alcohol. The product is either a P-hydroxyaldehyde or P-hydroxyketone, depending on the starting material. For example, two acetaldehyde (ethanal) molecules condense together in the presence of an aqueous base (NaOH), to produce 3-hydroxybutanal (a P-hydroxyaldehyde). 

In the work concerning the mechanism of solid-catalysed aldol reactions, the analogy between the homogeneous and heterogeneous mechanisms is usually assumed [370,372—375]. The mechanism of base-catalysed condensations, which has received much attention (cf. ref. 371), may be pictured in general as... 

The forward synthetic reaction is a base-catalysed condensation reaction between two carbonyl compounds, the aldol condensation leading to -hydroxy-aldehydes or / -hydroxyketones followed by dehydration. This sequence is one of the most important carbon-carbon bond forming reactions, and aldol-type condensation reactions are considered in a number of other sections of the text, for example, the Doebner reaction (Section 5.18.3, p. 805), the Knoevenagel reaction (Section 5.11.6, p. 681), the Perkin reaction (Section 6.12.3, p. 1036) and the Robinson annelation reaction (Section 7.2). 

Cross-aldol condensations have been performed with alkaline earth metal oxide, as base catalysts. A limitation of the cross-aldol condensation reactions is the formation of by-products throught the self-condensation of the carbonyl compounds, resulting in low selectivities for the cross-aldol condensation product. Thus, the cross-condensation of heptanal with benzaldehyde, which leads to jasminaldehyde (a-M-amylcinnamaldehyde), with a violet scent, has been performed with various solid base catalysts/13,541 particularly MgO, which gave excellent conversions of heptanal (97 %) at 398 K in the absence of a solvent (but the selectivity to jasminaldehyde was only 43 %). A low selectivity was also reported (40 %) for the cross-aldol condensation of acetaldehyde and heptanal catalysed by MgO.[55]... 

Blanc, A. C., Macquarrie, D. J., Valle, S., Renard, G., Quinn, C. R. and Brunei, D. The preparation and use of novel immobilised guanidine catalysts in base-catalysed epox-idation and condensation reactions, Green Chem., 2000, 2, 283-288. 

The mechanisms of the condensation reactions of polysilanols have been the subject of numerous reports (for reviews, see References 181 and 182) and are catalysed by acids and by bases. A simple outline of the mechanisms is given in Scheme 10. The hydrolysis of RSiCh species where R is small and no precautions are taken to remove HC1 as it is formed leads to the formation of ladder and branched polymers via RSi(OH)3 or chlorosilanols. The presence of larger groups or the avoidance of acidic or basic conditions allows the intermediate silanols to be formed as outlined in Section n. 

Diethyl oxalate also gives well-controlled condensations with ketones and we shall take the syn-j thesis of a new drug as an example. One way to try and prevent heart disease is to reduce the amount of bad lipoproteins in the blood, The drug Acifran does this, and a key step in its synthesis is the base-catalysed reaction between diethyl oxalate and a methyl ketone. 

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