Quantitative acetylation

Although the acetylation of alcohols and amines by acetic anhydride is almost invariably carried out under anhydrous conditions owing to the ready hydrolysis of the anhydride, it has been shown by Chattaway (1931) that phenols, when dissolved in aqueous sodium hydroxide solution and shaken with acetic anhydride, undergo rapid and almost quantitative acetylation if ice is present to keep the temperature low throughout the reaction. The success of this method is due primarily to the acidic nature of the phenols, which enables them to form soluble sodium derivatives, capable of reacting with the acetic... 

Acetylating Mixture. Equal vols. of acetic anhydride and glacial acetic add (p. 107). (A special mixture is used for quantitative acetylation see p. 451.)... 

Crystalline derivatives, suitable for identification and characterisation are dealt with in Section IV, 114, but the preparation of the following, largely liquid, derivatives will be described in the following Sections. When phenols are dissolved in aqueous sodium hydroxide solution and shaken with acetic anhydride, they undergo rapid and almost quantitative acetylation if the temperature is kept low throughout the reaction. This is because phenols form readily soluble sodium derivatives, which react with acetic anhydride before the latter undergoes appreciable hydrolysis, for example ... 

The solvents and coupling reagents were reagent grade materials. The tricaprylmethylammonium chloride (Adogen 464) was not purified further. Intrinsic viscosities were measured in chloroform at 25. Infrared spectra for hydroxyl analysis were measured on 2.5% solutions in carbon disulfide (vs. carbon disulfide2.in a cell with a 1.00 cm path length. The absorbance at 3610 cm was subtracted from a similar spectrum of 2 which had been quantitatively acetylated. 

Quantitative Acetylation of Lignins. To avoid fractionation of the lignins and loss of low molecular weight components, which occur in the usual acetylation procedures, the quantitative acetylation described by H. Chum et al. (20,21) was employed. 

The preferred method for reduction and acetylation is that of Blakeney et al. (1983). Their innovation was to use DMSO as the solvent for sodium borohydride and to use 1-methylimidazole as the catalyst for the quantitative acetylation of alditols in the presence of borate. Methods commonly used to prepare alditol acetates prior to this had incorporated steps in which the borate was removed by repeated evaporations with methanol, which was slow and tedious. 

Aminolysis is a laborious procedure, e.g., for wood samples, a 6-7-h reaction period may be required for the aminolysis step alone. The accuracy of this method is critically dependent upon both a quantitative acetylation of phenolic hydroxyl groups and a selective deacetylation of phenolic acetyl groups. Although these requirements may not represent a serious concern in the analysis of soluble or reagent-accessible lignin preparations, they could present a problem in the case of lignocellulosic materials. 

The usefulness of hydrolases like the immobilized lipase LIP to provide chiral substances for elaborated synthetic strategies is demonstrated in the example represented by Scheme 4. meso-UvA 8 was quantitatively acetylated to the monoacetate 9, an important intermediate in the preparation of synthon 10 used for prostaglandin syntheses [27]. 

We carried out the synthesis of drim-7P,8p,9a-triol (149) starting with drim-8-en-7-one (27) [95]. The latter was epoxidised to the epoxy ketone (150), which was reduced to the epoxy alcohol (151), whose acetate (152) was transformed, on interaction with HC104, to a mixture of the triol (149) (41% yield) and its monoacetate (153) (54%). Compound (153) gives the triol (149) on saponification, and vice versa, the triol (149) is quantitatively acetylated to the monoacetate (153). The triol (149) forms the acetonide (154). That conversion proves its absolute configuration at C-8 and C-9, confirmed by spectral data. 

The easiest way of generating it is to pass acetone vapour over a red-hot wire filament [38, 39]. If the gas is bubbled into an aqueous solution of an amine or amino acids, all amino groups are smoothly and quantitatively acetylated without acidic by-products, but none of the hydroxyls [40, 41]. Alcohols can be acetylated by bubbling ketene directly through the alcohol or mixture of alcohols, either neat or dissolved in a non-polar solvent [39]. 

Hyocholic acid forms an acetonide (IV). Although it could not be crystallized [Ziegler (7) Haslewood (24)], its formation was substantiated by chromatographic mobility and data of quantitative acetylation. Formation of the acetonide gave evidence for the m-glycol structure in hyocholic acid. The a-orientation of this 6,7-glycol was deduced from data of molecular rotations. Based on values from Barton and Klyne (34), the calculated molecular rotation of 3a,6a,7a-trihydroxy-5/3-cholanic acid would be —13 and that of the 3a,6a,7 -isomer, +249. The observed molecular rotation of hyocholic acid was +19. It was therefore concluded that hyocholic acid is the 3a,6a,7a rather than the 3a,6/3,7/3-isomer [Ziegler (7)]. 

Amyrol cannot be esterified with phthalic acid anhydride quantitative acetylation is equally impossible. Amyrol is probably a sesquiterpene alcohol, CijHggO. In addition to this body. West Indian aandal-wood oil contours a sesquiterpene, whose nature has been investigated by E. Deussen. He introduced hydrochloric acid gas up to saturation into the oil dissolved in dry ether, and obtained crystals of cadinene dihydrochloride. The corresponding compounds of hydrogen bromide and iodide, of which the constants are given below, were produced in a similar manner —... 

Structure determination of NONA unfolded along similar lines (Blix et al., 1956). Glycolic acid was cleaved with base and identified as the calcium salt by X-ray diffraction. Quantitative acetylation revealed six free hydroxyl groups, one more than for NANA. The absence of C-methyl groups, determined by the Kuhn-Roth method, implied that the extra hydroxyl is located on the iV-acetyl group. Blix found a close similarity in the chemical reactions of NONA as compared to NANA, and included the observation that heating with methanolic-HCl gave... 

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