Reaction 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). 

The aromatic primary and secondary stibines are readily oxidized by air, but they are considerably more stable than their aHphatic counterparts. Diphenylstibine is a powerful reducing agent, reacting with many acids to Hberate hydrogen (79). It has also been used for the selective reduction of aldehydes and ketones to the corresponding alcohols (80). At low temperatures, diphenylstibine undergoes an addition reaction with ketene (81) ... 

The reaction doe.s not involve dehydrogenation and may also be applied (with low yield) to aliphatic acid chlorides. The reaction with aryl isocj anates proceeds analogously to the reaction with ketenes leading through a-ketoiminoketenes to atylimino-4-pyrones, identical to those obtained by Bardone-Gaudemar and described at the end of Section II,B,2,a (see Scheme 6). 

Scheeren et al. reported the first enantioselective metal-catalyzed 1,3-dipolar cycloaddition reaction of nitrones with alkenes in 1994 [26]. Their approach involved C,N-diphenylnitrone la and ketene acetals 2, in the presence of the amino acid-derived oxazaborolidinones 3 as the catalyst (Scheme 6.8). This type of boron catalyst has been used successfully for asymmetric Diels-Alder reactions [27, 28]. In this reaction the nitrone is activated, according to the inverse electron-demand, for a 1,3-dipolar cycloaddition with the electron-rich alkene. The reaction is thus controlled by the LUMO inone-HOMOaikene interaction. They found that coordination of the nitrone to the boron Lewis acid strongly accelerated the 1,3-dipolar cycloaddition reaction with ketene acetals. The reactions of la with 2a,b, catalyzed by 20 mol% of oxazaborolidinones such as 3a,b were carried out at -78 °C. In some reactions fair enantioselectivities were induced by the catalysts, thus, 4a was obtained with an optical purity of 74% ee, however, in a low yield. The reaction involving 2b gave the C-3, C-4-cis isomer 4b as the only diastereomer of the product with 62% ee. 

Dipolar cydoadditions are one of the most useful synthetic methods to make stereochemically defined five-membered heterocydes. Although a variety of dia-stereoselective 1,3-dipolar cydoadditions have been well developed, enantioselec-tive versions are still limited [29]. Nitrones are important 1,3-dipoles that have been the target of catalyzed enantioselective reactions [66]. Three different approaches to catalyzed enantioselective reactions have been taken (1) activation of electron-defident alkenes by a chiral Lewis acid [23-26, 32-34, 67], (2) activation of nitrones in the reaction with ketene acetals [30, 31], and (3) coordination of both nitrones and allylic alcohols on a chiral catalyst [20]. Among these approaches, the dipole/HOMO-controlled reactions of electron-deficient alkenes are especially promising because a variety of combinations between chiral Lewis acids and electron-deficient alkenes have been well investigated in the study of catalyzed enantioselective Diels-Alder reactions. Enantioselectivities in catalyzed nitrone cydoadditions sometimes exceed 90% ee, but the efficiency of catalytic loading remains insufficient. 

Reaction of 4a with TiCl4 was carried out in the presence of siloxyalkene 3 as nucleophile and the results are summarized in Table III. In the reaction with ketene silyl acetals 3a and 3e at -78 °C, y-ketoesters 15a and 15e were obtained instead of chloride product 8 which is a major product in the absence of 3. Formation of product 15 is likely to result from trapping of alkylideneallyl cation 5 with 3 at the sp2 carbon. In contrast, the reactions with silyl enol ethers 3f and 3g gave no acyclic product 15, but gave cyclopentanone derivatives 16-18. The product distribution depends on the mode of addition of TiCl4 (entries 4-7). 

The reaction with silyl enol ethers 3f and 3g gave only the [3 + 2] cycloadducts in comparison with effective formation of acyclic adduct 15 in the reaction with ketene silyl acetals 3a and 3e at lower reaction temperature. This can be explained by the reactivity of cationic intermediates 19 the intermediates from 3f and 3g are more reactive owing to lower stabilization by the oxy group than those from 3a and 3e, and react with the internal allene more efficiently to give the cycloadduct(s). Cyclic product 17a could be obtained at higher temperature via the reaction of 3a (entry 2). 

Intramolecular acylation reactions with ketene complexes, generated, for instance, by thermolysis or photolysis of carbene complexes, can also be used for the preparation of six-membered rings. Illustrative examples are shown in Table 2.23. 

Further methods for preparing seven-membered rings with the aid of heteroatom-substituted carbene complexes include the intramolecular acylation reactions with ketene complexes discussed in Section I.2.6.2. 

The three-component reaction with ketene as the heterocumulene component and 6-aminouracil (Scheme 136) leads to both zwitterionic het-eropolycyclic uracils and their monohydro products. For example, 377 is obtained by treatment of diphenylketene and pyridine in a 42% yield. However, diphenylketene and quinoline are transformed into the zwitterionic 378 in a satisfactory yield of 54% (94UP2). 

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). 

In the presence of acids, linalool isomerizes readily to geraniol, nerol, and a-terpineol. It is oxidized to citral by chromic acid. Oxidation with peracetic acid yields linalool oxides, which occur in small amounts in essential oils and are also used in perfumery. Hydrogenation of linalool gives tetrahydrolinalool, a stable fragrance material. Its odor is not as strong as, but fresher than, that of linalool. Linalool can be converted into linalyl acetate by reaction with ketene or an excess of boiling acetic anhydride [34]. 

Vanpee and Grard133 made a quantitative study of the formation of saturates (mainly ethane) in the photolysis of CH2CO with added methane (ratios CH4/CH2CO = 1 to 7) at 28 to 250 °C. and found that the results could be explained by a mechanism involving competition between CH4 and CH2CO for methylene by reactions of the first order in methylene. The rate of reaction of CH2 with CH4 was found to be 0.183 that of the reaction with ketene. Decomposition of excited ethane by the reaction... 

Considerable evidence shows that the reaction is fast. For example, Vanpee and Grard133 found the reaction of CH2 with CH4 to be only about five times slower than the reaction with ketene, which has a collision efficiency > 10 2. Further evidence is found in the results of the... 

Ketene itself reacts with enamines and if in excess produces pyran-2-ones which are substituted in the 4-, 5- and 6-positions (65JOC2642). It seems likely that the acylated enamine undergoes a 1,4-addition with ketene to give an intermediate 4-amino-3,4-dihydropyranone, which eliminates the secondary amine on further reaction with ketene. 

Jarrahpour et al. [135] have described the synthesis of novel mono- and bis-spiro-[S-lactams 231 and 233, respectively, from benzylisatin 229 (Scheme 52). The starting substrate, benzylisatin 229 was prepared by reaction of isatin 228 with benzyl bromide and calcium chloride in DMF. The benzylisatin substituted imines 230 and di-imines 232 were further subjected to Staudinger reaction with ketenes derived from methoxy, phenoxy, and phthaloglycyl chlorides to afford novel mono- and bis-spiro-p-lactams 231 and 233, respectively. The configuration of benzylisatin 229 and monocyclic spiro-p-lactams 231 was established by X-ray crystallographic studies. These spiro-p-lactams will be studied as precursors of modified p-amino acids, (3-peptides and monobactam analogues. 

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