Acetaldehyde, directed condensation

Fig. 2.103. Structures of all the compounds found in fraction B (a) anthocyanins and acylated anthocyanins direct condensation products between flavanols and anthocyanins (c) dimers resulting from the condensation mediated by acetaldehyde between anthocyanins and flavanols. Reprinted with permission from C. Alcalde-Eon el al. [236].

In carbohydrate chemistry, condensatiMi with carbonyl compounds such as acetone, acetaldehyde, formaldehyde and benzaldehyde is of great importance for the protection of pairs of hydroxyl groups. Trityla-tion may also be used in these instances for the protection of individual primary hydroxyl groups before carrying out the condensation with carbonyl compounds. After the condensation, the individual hydroxyl groups may be liberated by suitable detritylation methods. As yet, these possibilities have been explored very little. Wolfrom and coworkers described the condensation of 1,6-ditrityldulcitol with benzaldehyde to l,6-ditrityl-2,3,4,5-dibenzylidenedulcitol. Acetals other than those obtained by direct condensation are thus available. 

Aryl-substituted ketones react directly with elemental sulfur in hexa-methylphosphorus triamide with formation of l,2-dithiole-3-thiones.46,47 Aryl-substituted acetaldehydes can condense likewise with carbon disulfide to give the analogous 4-aryl-substituted l,2-dithiole-3-thiones.48 The same reaction was observed with aryl-substituted acetic acid esters, in which a methylthio group is incorporated into the reaction product (26).49 Thioketones and enthioles react analogously. 50,51 The parent l,2-dithiole-3-thione (27) has been prepared from the tetra-methyl acetal of malondialdehyde and phosphorus pentasulfide.52 In the presence of ammonia the reaction of cyclohexanone or cycloheptanone with carbon disulfide and sulfur gives as by-products the condensed 1,2-dithiole-3-thiones 28a and 28b, respectively.53... 

Several authors (Bakker et al, 1993 Garcla-Viguera et al., 1994 Escribano-Baildn et al., 1996 Es-Safi et al., 1999) have noted that the reaction via acetaldehyde is quicker than the direct condensation observed by Somers (1971). The initial products become new pigments with a high degree of polymerization, and they eventually precipitate. 

The highly purified wheat germ carboxylase of Singer carries out the simple decarboxylation of pyruvate to acetaldehyde and CO2. It can also synthesize AMC from pyruvate and acetaldehyde or form 2 moles of acetaldehyde, at a slower rate. The fact that this enzyme can form AMC from acetaldehyde alone indicates that this TPP-enzyme complex can activate acetaldehyde directly. This strongly suggests the formation of an acetaldehyde-TPP intermediate directly from acetaldehyde, which has the same capacity as that derived from the decarboxylation of pyruvate, to condense with a free acetaldehyde molecule to form AMC. 

Some TIQ, benzyltetrahydroisoquinoline, and tetrahydroxyberberine alkaloids known for their inhibitory activity on MAO can be formed within the human body. An example of such a TIQ is salsolinol (34), a weak MAO inhibitor formed by the direct condensation of acetaldehyde and dopamine. Following the metabolization of alcohol (oxidized to acetaldehyde) and dopamine, 34 can be found in rat brain [52], where it acts as a MAO inhibitor competitive to 5-HT, suggesting selectivity to MAO-A. The estimated Ki values for MAO inhibition by 34 were between 30 and 285 pM (see Table 3) [52-56]. In addition, the potency of 34 for MAO-A inhibition... 

Then detach and reverse the condenser, and reconnect it to the flask through a knee-tube for direct distillation, as shown in Fig. 60, p. 101, or Fig. 23(0), p. 45. Distil the mixture, by direct heating over a gauze, until about 8 ml. of distillate have been collected. Acetic acid is volatile in steam and an aqueous solution of the acid, containing, however, some acetaldehyde, is thus obtained. With a very small portion of this solution, perform the tests for acetic acid given on p. 347. 

The direct conversion of ethyl alcohol to ethyl acetate is beheved to take place via acetaldehyde and its condensation to ethyl acetate (Tishchenko reaction) (28-34). 

Methylated spirit contains, in addition to ethyl and methyl alcohols, water, fusel-oil, acetaldehyde, and acetone. It may be freed from aldehyde by boiling with a—3 per cent, solid caustic potash on the water-bath with an upright condenser for one hour, or if larger quantities are employed, a tin bottle is preferable, which is heated directly over a small flame (see Fig. 38). It is then distilled with the apparatus shown in Fig. 39. The bottle is here surmounted with a T-piece holding a thermometer. The distillation is stopped when most of the spirit has distilled and the thermometer indicates 80°. A further purification may be effected by adding a little powdered permanganate of potash and by a second distillation, but this is rarely necessary. The same method of purification may be applied to over-proof spirit, which will henceforth be called spirit as distinguished from the purified product or absolute alcohol. 

Alternatively, acetaldehyde and acetic anhydride are fed directly to the cracking reactor where the same sulfonic acid can catalyze the condensation of the aldehvde-anhydride mixture to EDA and the subsequent thermal elimination forming vinyl acetate. The best results are obtained when acetic anhydride is present as solvent to inhibit the competitive elimination to acetaldehyde and anhydride (see reverse of equation 2). Aldehyde degradation reactions are minimal under these conditions. 

There are a very large number of patented processes for the synthesis of pyridines, often in very small yields, from ketones and ammonia. Most of the syntheses must involve the condensation of two molecules of the carbonyl compound to give an a,/3 -unsaturated aldehyde or ketone, and such unsaturated compounds can be used directly. The early work on the vapour-phase catalytic processes dates from the 1920s and a series of papers by Chichibabin. In one of these (24JPR(107)154) he records the use of acrolein, acetaldehyde, and ammonia over an alumina catalyst at 370-380 °C to give a poor yield of pyridine and a very poor yield of 3-methylpyridine. The major problems are to separate the mixtures of products from the considerable amount of tarry material. Many catalysts and many mixtures of ketones have since been used a few of the better yields are reported here. 

The formation of deoxyribose, die pentose moiety of deoxyribonucleic acid, can occur directly from ribose while the latter is in the form of a nucleotide diphosphate. Deoxyribose-5-phosphate can also be formed by condensation of acetaldehyde and glyceraldehyde-3-phosphate. 

Stereochemically controlled synthesis of this subunit, which contains five stereogenic centers, is important to an efficient bleomycin synthesis. (2S,3S,4i )-4-(/er/-Butoxycarbonyl-amino)-3-hydroxy-2-methylpentanoic acid (15) was obtained via a stereoselective syn aldol addition of a boron Z-enolate with (27 )-2-(tert-butoxycarbonylamino)propanal (Scheme 4). Similarly, the L-threonine subunit 18 was prepared by diastereoselective syn aldol addition of an N- acy I ox azo I i di n one stannous Z-enolate with acetaldehyde. The bithiazole unit 19 was prepared using a direct DCC-promoted condensation of 3-(methylsulfanyl)propylamine. Convergent access to tetrapeptide S was obtained by coupling of acid 15 and deprotected 18 to give dipeptide 20, followed by further coupling with the bithiazole 19 to ultimately give tetrapeptide S (21). 

---