Initiators, acetyl

A suspension of 40 g 3-acetylaminomethyl-5-amino-2,4,6-triodobenzoic acid in 180 ml acetic anhydride were mixed with 0.4 ml concentrated sulfuric acid. An exothermic reaction was thereby initiated. Acetylation was completed by heating to 80°C for three hours. The reaction mixture was then evaporated to dryness in a vacuum at a temperature not exceeding 50°C. The residue was treated with a mixture of 30 ml concentrated aqueous ammonium hydroxide and 40 ml water, whereby the solid material dissolved with spontaneous heating. Within a few minutes, the ammonium salt of the acetylated product started precipitating. The precipitate and residual liquid were cooled externally with ice after about 15 minutes. The salt was separated from the liquid by filtration with suction, and was washed with ice cold saturated ammonium chloride solution. 

The Pummerer reaction346 of conformationally rigid 4-aryl-substituted thiane oxides with acetic anhydride was either stereoselective or stereospecific, and the rearrangement is mainly intermolecular, while the rate-determining step appears to be the E2 1,2-elimination of acetic acid from the acetoxysulfonium intermediates formed in the initial acetylation of the sulfoxide. The thermodynamically controlled product is the axial acetoxy isomer, while the kinetically controlled product is the equatorial isomer that is preferentially formed due to the facile access of the acetate to the equatorial position347. The overall mechanism is illustrated in equation 129. 

Figure 8-2. Pathway for synthesis of palmitate by the fatty acid synthase (FAS) complex. Schematic representation of a single cycle adding two carbons to the growing acyl chain. Formation of the initial acetyl thioester with a cysteine residue of the enzyme preceded the first step shown. Acyl carrier protein (ACP) is a component of the FAS complex that carries the malonate covalently attached to a sulfhydryl group on its phosphopantatheine coenzyme (-SH in the scheme).

FIGURE 21-3 The overall process of palmitate synthesis. The fatty acyl chain grows by two-carbon units donated by activated malonate, with loss of C02 at each step. The initial acetyl group is shaded yellow, C-l and C-2 of malonate are shaded pink, and the carbon released as C02 is shaded green. After each two-carbon addition, reductions convert the growing chain to a saturated fatty acid of four, then six, then eight carbons, and so on. The final product is palmitate (16 0). 

The indirect conversion of an aromatic aldehyde into the corresponding nitrile by dehydration of an oxime is illustrated by the synthesis of veratronitrile (Expt 6.170). The dehydrating agent is acetic anhydride which probably effects an initial acetylation of the oximino group followed by the elimination of acetic acid. 

Certain quinoxaline (V-oxides (191), too, are converted into 1-acetyl-3-aroylbenz-imidazolinones (193) when they are heated in acetic anhydride. The reaction proceeds by an initial acetylation at N-3, and this is succeeded by formation and rearrangement of an intermediate oxaziridine (192 Scheme 111). Apart from their involvement in other reactions discussed earlier, such intermediates have also been invoked to explain the formation of 1,3-dibenzoylbenzimidazolinone (70%) when quinoxaline 1,3-dioxide is exposed to sunlight (74CRV279). 

The importance of both the cationic and anionic portions of the initiator was revealed by a study of various initiators for THF polymerization having different cations and different counter-ions. In this study, Yamashita et al. [84,85] concluded that the kp value with CH2CI2 solvent at 0°C is almost independent of counter-ion when triethyloxonium ions are used as initiators. They found much slower apparent rates when the different cation initiators, acetyl hexafluoroantimonate and 2-methyl-1,3-dioxolenium perchlorate, were used. They explained the slower rates by decreased rates of initiation. The apparent fep s can be increased to the... 

As can be seen from the equations above, the necessary amount of malonyl CoA is synthesized. Palmitate is subsequently synthesized from malonyl CoA and one initial acetyl CoA. Thus, acetyl CoA, NADPH, ATP, and HCOs are all necessary in this process. In contrast, FADH, is not utilized in fatty acid synthesis, but is one of the products of fatty acid oxidation. Vitamin is required for conversion of propionic acid to methylmalonic acid, a step in the p oxidation of odd-numbered fatty acid chains. 

FIGURE 21-5 Sequence of events during synthesis of a fatty acid. The fatty acid synthase complex is shown schematically. Each segment of the disk represents one of the six enzymatic activities of the complex. At the center Is acyl carrier protein (ACP), with its phosphopantetheine arm ending in an —SH. The enzyme shown in blue is the one that will act in the next step. As in Figure 21-3, the initial acetyl group is shaded yellow, C-1 and C-2 of malonate are shaded pink, and the carbon released as CO2 is shaded green. Steps (T) to are described in the text. 

In a supplementary experiment it was learned that under the conditions of the experiment the acetate content within the wood decreased by 85% (over the theoretical value based on the initial acetyl content). Thus, within the five-minute period in the treating solution, saponification of the acetate occurred, and 85% of the resulting sodium acetate had diffused out of the wood. This rapid diffusion is consistent with earlier work (37), which showed that the diffusion rate of water-soluble material through the cell walls is increased several times by NaOH treatment. The amount of sodium held as sodium acetate should thus be only 15% of that computed on the basis of the initial acetyl content. With this correction, the computed total equivalent NaOH content in the treated... 

Fig. 33.15. Synthesis of palmitate on the fatty acid synthase complex. Initially, acetyl CoA adds to the synthase. It provides the to-methyl group of palmitate. Malonyl CoA provides the 2-carbon units that are added to the growing fatty acyl chain. The addition and reduction steps are repeated until palmitate is produced. 1. Transfer of the malonyl group to the phosphopantetheinyl residue. 2. Condensation of the malonyl and fatty acyl groups. 3. Reduction of the P-ketoacyl group. 4. Dehydration. 5. Reduction of the double bond. P = a phosphopantetheinyl group attached to the fatty acid synthase complex Cys-SH = a cysteinyl residue.

The 1,4-cycloaddition of ketenes to A, A -disubstituted enaminones allows a general synthesis of pyranones. Berchtold and co-workers (61JOC4776 65JOC2642) showed that treatment of ethyl 3-pyrrolidinocrotonate 341 (R = EtO) or 4-pyrrolidino-3-penten-2-one 341 (R = Me) with an excess of ketene resulted in the formation of pyranones 342, Scheme 96. The mode of formation of 342 apparently involves initial acetylation with one mole of ketene at the enaminone fi position followed by cycloaddition with a second mole of ketene. 

The first step is acetylation of the sulfoxide oxygen and the release of an acetate ion (7 8 Scheme 20.31. Recent computational studies have shown that the initial acetylation step is rate determining and that the release of the acetate ion proceeds by stabilization of the acetate by coordination with a positively charged sulfur, which yields an achiral sulfurane 11. It is generally accepted that the formation of this sulfurane is responsible for the poor enantioselectivity shown in the reactions of chiral sulfoxides. However, sulftirane formation can be avoided by some additives, for exanple, iV,iV-dimethylacetamide (DMAC), iV-methyl-2-pyrrolidone (NMP), or the acetate trap DCC. ... 

Suggest a mechanism. Hint A pericyclic reaction is involved after the initial acetylation. 

The Pummerer reaction between a chiral sulfoxide and acetic anhydride has been investigated using computational methods and involves initial acetylation of the sulfoxide (Scheme 171)2" ... 

The kinetics of VCM polymerization with two initiators (acetyl cyclo-hexanesulfonyl peroxide and diethyl peroxydicarbonate), which decompose at distinctly different rates at a given temperature, has been studied in an isothermal pressure calorimeter. In this work, the ratio of water to monomer was 1.7 1, and the level of the unspecified suspending agent was 0.3% of the monomer [107]. The use of two such initiators may reduce the problems caused by the heat kick. ... 

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