Ketene, acetylation with

The Hydrate and Enol Form. In aqueous solutions, acetaldehyde exists in equihbrium with the acetaldehyde hydrate [4433-56-17, (CH2CH(0H)2). The degree of hydration can be computed from an equation derived by BeU and Clunie (31). Hydration, the mean heat of which is —21.34 kJ/mol (—89.29 kcal/mol), has been attributed to hyperconjugation (32). The enol form, vinyl alcohol [557-75-5] (CH2=CHOH) exists in equihbrium with acetaldehyde to the extent of approximately 1 molecule per 30,000. Acetaldehyde enol has been acetylated with ketene [463-51-4] to form vinyl acetate [108-05-4] (33). 

In aqueous solutions, acetaldehyde exists in equilibrium with the acetaldehyde hydrate [CH3CH(OH)2], The enol form, vinyl alcohol (CH2=CHOH) exists in equilibrium with acetaldehyde to the extent of 0.003% (1 molecule in approximately 30,000) and can be acetylated with ketene (CH2=C=0) to form vinyl acetate (CH2=CHOCOCH3). 

Acetylation with ketene at room temperature a few days to a week minutes (19)... 

Potassium tert-butoxide Alkaline O-acetylation with ketene... 

Ketones with labile hydrogen atoms undergo enol acetylation on reaction with ketene. Strong acid catalysis is required. If acetone is used, isoptopenyl acetate [108-22-5] (10) is formed (82—85). Isopropenyl acetate is the starting material for the production of 2,4-pentanedione (acetylacetone) [123-54-6] (11). 

Carbonyl Compounds. Cychc ketals and acetals (dioxolanes) are produced from reaction of propylene oxide with ketones and aldehydes, respectively. Suitable catalysts iaclude stannic chloride, quaternary ammonium salts, glycol sulphites, and molybdenum acetyl acetonate or naphthenate (89—91). Lactones come from Ph4Sbl-cataly2ed reaction with ketenes (92). 

Reaction of 3-(4-methylphenylamino)-4-(4-methylphenylimino)-4//-pyr-ido[l,2-a]pyrazine (373, R = 4-MePh) with ketenes 393, prepared in situ from the appropriate acetyl chloride with NEt3, yielded tricyclic derivatives... 

After filtration of the reaction mixture, treatment of the toluene filtrate with ketene gas accomplishes a smooth mono-C-acetylation of the nucleophilic enamine function in 25, and provides keto enamine... 

Acetyl-l//-azepine (20%) is formed by A-acetylation of 1//-azcpine with ketene.9... 

Phosphoric Acid and its Derivatives.—Triacetyl phosphate18 and diammonium monoacetyl phosphate14 have been obtained by acetylation of phosphoric acid with keten at —10 to —15 °C in diethyl ether and ethyl acetate, respectively the reaction can be controlled to give 90% yields of either product. Long-chain monoalkyl... 

Figure 4.4 Acetylation of wood with ketene gas (a) and acetoacetylation of wood with diketene (b).

Four different methods (vapour-phase acetylation using acetic anhydride, acetylation using ketene gas, liquid phase acetylation using acetic anhydride/xylene, or neat acetic anhydride) were used to acetylate pine wood chips to a variety of WPGs for the production of MUF-bonded particleboards (Nilsson etal., 1988). Composite boards were exposed to unsterile soil in fungal cellar tests. Boards made from ketene acetylated chips were not found to be resistant to decay at the maximum WPG level achieved (17 %) with a liquid acetic anhydride modification, no decay was recorded at a WPG level of c. 18 % after 12 months exposure, whereas with a vapour-phase treatment at the same WPG, evidence for decay was found. 

Reactive organic chemicals can be bonded to cell wall hydroxyl groups on cellulose, hemicelluloses, and lignin. Much of our research has involved simple epoxides (1 3) and isocyanates (4), but most of our recent effort has focused on acetylation. Acetylation studies have been done using fiberboards (5f6), hardboards (7 11) particleboards (12-20), and flakeboards (21-23), using vapor phase acetylation (8,2 257, liquid phase acetylation (, ), or reaction with ketene (28). 

Highly enantioselective Mannich-type reactions of A-(2-hydroxyphenyl) aldi-mines with ketene trimethylsilyl acetals and of A-Boc-aldimines with acetyl acetone or furan are catalyzed by chiral phosphonic acids 9 derived from 3,3 -diaryl-(l )-BlNOL and POCI3 (Scheme 12.7). ... 

Methylbenzofuran-3-carbaldehydes undergo ready condensation with Meldrum s acid (isopropylidene malonate) to afford arylmethylene derivatives 83. These on flash vacuum pyrolysis at 500-600 C give 3-dibenzofuranols 84 (Scheme 21). The arylmethylene derivative, e.g., 85, presumably undergoes conversion to a methylene ketene (86, Scheme 22) on pyrolysis, which undergoes a [1,5-H] shift and subsequent cyclization and tautomerization, yielding the dibenzofuranol 87. The derived methyl ether 88 has been converted by mild acetylation with acetyl chloride and aluminum chloride and subsequent boron trichloride-induced demethylation to the natural product ruscodibenzofuran (8). A limitation is imposed on this method because 3-acetyl-2-methyldibenzofurans fail to condense with Meldrum s acid so that l-methyl-3-dibenzofuranols are not available by this method. ... 

Treatment of chromium (III) acetylacetonate with acetic anhydride and boron trifluoride etherate yielded a complex mixture of acetylated chelates but very little starting material. Fractional crystallization and chromatographic purification of this mixture afforded the triacetylated chromium chelate (XVI), which was also prepared from pure triacetylmethane by a nonaqueous chelation reaction (8, 11). The enolic triacetylmethane was prepared by treating acetylacetone with ketene. The sharp contrast between the chemical properties of the coordinated and uncoordinated ligand is illustrated by the fact that chromium acetylacetonate does not react with ketene. 

The slow acetylation of the hydroxyl group is difficult to explain. One is inclined to suggest that this group is coordinated to the metal ion and consequently rendered inactive. Such a possibility requires either a coordination number of 7 for chromium (III) or displacement of carboxylate from the coordination sphere by hydroxyl. In the latter instance an uncoordinated functional group would still be present to react with ketene (COO or COOH) and anhydride should be detected in the crude reaction product. This is not the case. 

The first oxidative studies on N-acylglycosylamines were conducted on a preparative scale by Niemann and Hays.65 The N-acetyl-D-glucopyranosylamine (69) obtained by acetylation of D-glucopyranos-ylamine with ketene (or by acetylation of this amine, followed by ammonolysis) was oxidized by way of the dialdehyde 70 to the acid 71, isolated as the strontium salt. Hydrolysis of 71 led to D(+)-glyceric acid (72), thus showing the pyranose structure of 69. 

TV-Acylation of (73) can be achieved with acyl chlorides and pyridine or triethylamine, or with ketene (75S547 p. 560). 7V-Trimethylsilylazetidin-2-ones have been reported to undergo acyldesilylation on treatment with acetyl chloride. Acylation of (73) with imidates (78) at 150-160 °C was followed by ring expansion to give the pyrimidin-4-ones (79). Pyrimid-4-ones were obtained from a similar reaction of imidates with 4-alkoxyazetidin-2-ones (74CB270). Acylation with subsequent ring expansion also occurs with dithioimidates (73CPB1305). 

The reaction of cyclopropanone with hydrogen chloride affords the chlorohydrin 90 (65%). 5 15) Acetylation of 90 with ketene gives the expected acetoxy compound 91 (34%) while the addition of methanol to a methylene chloride solution of 90 affords cyclopropanone methyl hemiketal (100%). The latter reaction illustrates the use of 91 as an in situ source of cyclopropanone. 

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