And the Perkin reaction

Furaldehyde is a colorless, poisonous, water-soluble Hquid, bp 162 °C, which slowly turns brown in air. Like benzaldehyde, it undergoes the Cannizzaro and the Perkin reaction as well as the Knoevenagel and acyloin condensations. The catalytic hydrogenation of 2-furaldehyde yields 2-(hydroxymethyl)oxolane (113, tetrahydro-2-furfuryl alcohol). This compound undergoes a nucleophilic 1,2-rearrangement to give 3,4-dihydro-2H-pyran (114) by the action of acid catalysts ... 

Olefinations (preparations of alkenes) by Knoevenagel and aldol condensations [43], and the Perkin reaction [44] have been complemented by Wittig [45] or Homer olefina-... 

It is interesting to note that altliough cis-lrans isomerism about the double bond is theoretically possible in cinnamic acid, the Perkin reaction gives rise only to the trans form, m.p. 133°, the cis form, m.p. 68° (termed aHo-cinnamic acid) being unstable and easily converted into the trails acid. 

The Perkin reaction, uti1i2ing an aromatic aldehyde, an acid anhydride, and a base such as an acid salt or amine, produces the corresponding a,P-unsaturated acid. 

Manufacture. The most widely employed method for the commercial synthesis of (H)-cinnamic acid uti1i2es ben2aldehyde, acetic anhydride, and anhydrous sodium or potassium acetate in a condensation reaction commonly referred to as the Perkin reaction (11). 

The Perkin reaction is of importance for the iadustrial production of coumarin and a number of modifications have been studied to improve it, such as addition of a trace of iodine (46) addition of oxides or salts of metals such as iron, nickel, manganese, or cobalt (47) addition of catalytic amounts of pyridine (48) or piperidine (49) replacement of sodium acetate by potassium carbonate (50,51) or by cesium acetate (52) and use of alkaU metal biacetate... 

The cinnamic acid is readily prepared by heating benzaldehyde with acetic anhydride and sodium acetate (the Perkin Reaction) (Figure 16.2). 

Because of the mild reaction conditions, and its broad applicability, the Knoevenagel reaction is an important method for the synthesis of a ,/3-unsaturated carboxylic acids. Comparable methods are the Reformatsky reaction, the Perkin reaction, as well as the Claisen ester condensation. The Knoevenagel reaction is of greater versatility however the Reformatsky reaction permits the preparation of a ,/3-unsaturated carboxylic acids that are branched in a-position. 

A variant of the Perkin reaction is the Erlenmeyer-Plochl-azlactone synthesis By condensation of an aromatic aldehyde 1 with an N-acyl glycine 5 in the presence of sodium acetate and acetic anhydride, an azlactone 6 is obtained via the following mechanism ... 

Azlactones like 6 are mainly used as intermediates in the synthesis of a-amino acids and a-keto acids. The Erlenmeyer-Pldchl reaction takes place under milder conditions than the Perkin reaction. 

The condensation of aromatic aldehydes with anhydrides is called the Perkin reaction When the anhydride has two a hydrogens (as shown), dehydration always occurs the P-hydroxy acid salt is never isolated. In some cases, anhydrides of the form (R2CHC0)20 have been used, and then the hydroxy compound is the product since dehydration cannot take place. The base in the Perkin reaction is nearly always the salt of the acid corresponding to the anhydride. Although the Na and K salts have been most frequently used, higher yields and shorter reaction times have been reported for the Cs salt. Besides aromatic aldehydes, their vinylogs ArCH=CHCHO also give the reaction. Otherwise, the reaction is not suitable for aliphatic aldehydes. ... 

As the chemical industry expanded, Perkin continued his own scientific research in the peace of his private laboratory. He had not lost his touch. Among the synthetic methods he discovered is one now called the Perkin reaction. He used it to make a synthetic substitute for a vegetable substance called coumarin, which has a pleasant, vanillalike odor. Coumarin spawned the synthetic perfume business and made luxurious scents available to all. Once again, a Perkin chemical started a new industry, albeit a modest one in comparison with dyes and pharmaceuticals. Despite the worldwide impact of Perkins discoveries, he was not knighted by the British monarchy until 1906, the fiftieth anniversary of his discovery of mauve. The world chemistry community feted him lavishly that year, and he traveled to the United States collecting further honors. A year later, at the age of 69, he died peacefully, at home. 

Furylacrylic acid has usually been prepared from furfural by the Perkin reaction.1 The use of potassium acetate is advantageous, since it allows the reaction to proceed rapidly at relatively lower temperatures. The acid has also been prepared from furfural and malonic acid in the presence of pyridine,2 and by oxidation of furfuralacetone with bleaching powder.3... 

Formylfuran behaves in a very similar manner to benzaldehyde and undergoes the usual reactions of an aromatic aldehyde, e.g. (i) the Cannizzaro reaction with cone, sodium hydroxide to give furan-2-ylmethanol and the sodium salt of furoic acid, (ii) the Perkin reaction with acetic anhydride and sodium acetate to yield an aldol product that dehydrates to 3-(furan-2-yl)propenoic acid, and (iii) a condensation with potassium cyanide in alcoholic solution to form furoin (under these conditions, benzaldehyde undergoes the benzoin condensation) (Scheme 6.32). 

Until the late 1890s, coumarin was obtained commercially only from natural sources by extraction from tonka beans. Synthetic methods of preparation and industrial manufacturing processes were developed starting principally from ortho-creso (Raschig process), phenol (Pechmann reaction) and salicylaldehyde (Perkin reaction). Various methods can be used to obtain coumarin from each of these starting materials. In order to be suitable for perfumery uses, synthetic coumarin must be highly pure (Bauer et al., 1988 Boisde Meuly, 1993). 

Hydroxy benzo[6]thiophene-4-carboxaldehyde, which is prepared from 5-hydroxybenzo[6]thiophene by the Duff reaction 338 or, better, by a modified Gattermann reaction,340 reacts with diethyl bromo-malonate to give the thienobenzofuran (282), and undergoes the Perkin reaction to give the thienobenzo-a-pyrone (283).338... 

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). 

A condensation occurs between 5-hydroxymethylfurfural and malonic ester20 and in a similar way, two molecules of malonic ester react with furan 2,5-dialdehyde.88 A condensation product, XXXV, has also been obtained with hydantoin.89 5-Hydroxymethylfurfural and its acetyl derivative undergo the Perkin reaction with sodium acetate and acetic anhydride giving 5-acetoxymethylfuran 2-acrylic acid (XXXVI).70 Similar products of the same reaction are obtained from 5-methyl-furfural71 and 5,5 -diformyl-l,l -furylmethyl ether (XXVII).61,72... 

The condensation product of dithiooxamide with benzaldehyde has been shown to be 2,5-diphenylthiazolo[5,4-d]thiazole (equation 75) (60JA2719). Its formation involves dehydration of the reactants and further dehydrogenation. The reaction is comparable in scope with condensations such as the Perkin reaction, and many 2,5-diarylthiazolo[5,4-c ]thiazoles have been prepared from a variety of aryl aldehydes. Condensation products were not obtained from simple aliphatic aldehydes (71JMC743). 

Enols or enolates from anhydrides are not used very often in aldol reactions other than in one important application, usually known as the Perkin reaction. An acid anhydride, such as acetic anhydride, is combined with a non-enolizable aldehyde and a weak base, usually the salt of the acid. This base is used so that nucleophilic attack on the anhydride does no hami, simply regenerating the anhydride. q... 

Selenophene-2-aldehyde takes part in the Hantzsch synthesis [Eq. (I)]108 and reacts readily with ammonia, aromatic amines and diamines,109 hippuric, barbituric, and malonic acids, malononitrile,70 and rhodanine.109 /3-(Selenien-2-yl)acrylic acid has been obtained from selenophene-2-aldehyde by the Perkin reaction and by Knoevenagel condensation with malonic acid.70 Esters of /9-(selenien-2-yl)acrylic acid are easily formed by condensation of the aldehyde... 

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