Ketenes, preparation from acids

This Asian species is a major agricultural pest. The pheromone has been proposed to consist of three male-specific compounds, only one of which, (Z)-exo-a-bergamotenal 150, has been reported in the literature [114]. The racemic compound was synthesized starting from farnesoic acid chloride 146 (Scheme 25) [114]. Thus, the vinyl ketene prepared from acid chloride 146 underwent 2+2 cycloaddition to give bicyclic ketone 147. The ketone function was removed by reaction with hydrazine followed by treatment of the resulting hy-... 

Uses. The lowest member of this class, ketene itself, is a powerful acetylating agent, reacting with compounds containing a labile hydrogen atom to give acetyl derivatives. This reaction is used only when the standard acetylation methods with acetic anhydride or acetyl chloride [75-36-5] do not work weU. Most of the ketene produced worldwide is used in the production of acetic anhydride. Acetic anhydride is prepared from the reaction of ketene and acetic acid. 

Such copolymers of oxygen have been prepared from styrene, a-methylstyrene, indene, ketenes, butadiene, isoprene, l,l-diphen5iethylene, methyl methacrjiate, methyl acrylate, acrylonitrile, and vinyl chloride (44,66,109). 1,3-Dienes, such as butadiene, yield randomly distributed 1,2- and 1,4-copolymers. Oxygen pressure and olefin stmcture are important factors in these reactions for example, other products, eg, carbonyl compounds, epoxides, etc, can form at low oxygen pressures. Polymers possessing dialkyl peroxide moieties in the polymer backbone have also been prepared by base-catalyzed condensations of di(hydroxy-/ f2 -alkyl) peroxides with dibasic acid chlorides or bis(chloroformates) (110). 

Ketene acetals prepared from fluorinated esters by trimethylsilylation undergo Lewis acid-promoted aldol condensations giving satisfactory yields but low diastereoselectivity [27] (equation 22). 

Amides prepared from carboxylic acids and primary amines using azolides obtained from acid chloride/imidazolea) or ketene/imidazole systems.b)... 

Several approaches to the 1,2,3-triazole core have been published in 2000. Iodobenzene diacetate-mediated oxidation of hydrazones 152 led to fused 1,2,3-triazoloheterocycles 153 <00SC417>. Treatment of oxazolone 154 with iso-pentyl nitrite in the presence of acetic acid gave 1,2,3-triazole 155, a precursor to 3-(W-l,2,3-triazolyl)-substituted a,P-unsaturated a amino acid derivatives <00SC2863>. Aroyl-substituted ketene aminals 156 reacted with aryl azides to provide polysubstituted 1,23-triazoles 157 <00HC387>. 2-Aryl-2T/,4/f-imidazo[43-d][l,2,3]triazoles 159 were prepared from the reaction of triethyl AM-ethyl-2-methyl-4-nitro-l//-imidazol-5-yl phosphoramidate (158) with aryl isocyanates <00TL9889>. 

Cyelobutanone has been prepared by (1) reaction of diazomethane with ketene,4 (2) treatment of methylenecyclobutane with performic acid, followed by cleavage of the resulting glycol with lead tetraacetate,s (3) ozonolysis of methylenecyclobutane, (4) epoxidation of methylene-cyclopropane followed by acid-catalyzed ring expansion,7 and (5) oxidative cleavage of cyclobutane trimethylene thioketal, which in turn is prepared from 2-(co-chloropropyl)-l,3-dithiane.8... 

Acetic anhydride is prepared from distilled acetic acid by the ketene method with ethyl phosphate as a catalyst. The water from the first cyclonite wash on the vacuum filter contains on an average 20% of acetic acid which is recovered by extraction with ethyl acetate. 

The synthesis of natural products by chirality transfer from carbohydrates has been used for a total synthesis of (-)-(7S)-nonactic acid (199). The furanoid glycal (197) was prepared from D-mannose, which is the appropriate chiral precursor (Scheme 46) (80JOC4259). A [3,3]-sigmatropic rearrangement of the silylated ketene-acetal (198) led to the control of the C-2 configuration. The intermediate furanoid glycal was prepared in ten steps from the carbohydrate precursor. 

Alcohols can also be prepared from support-bound carbon nucleophiles and carbonyl compounds (Table 7.4). Few examples have been reported of the a-alkylation of resin-bound esters with aldehydes or ketones. This reaction is complicated by the thermal instability of some ester enolates, which can undergo elimination of alkoxide to yield ketenes. Traces of water or alcohols can, furthermore, lead to saponification or transesterification and release of the substrate into solution. Less prone to base-induced cleavage are support-bound imides (Entry 2, Table 7.4 see also Entry 3, Table 13.8 [42]). Alternatively, support-bound thiol esters can be converted into stable silyl ketene acetals, which react with aldehydes under Lewis-acid catalysis (Entries 3 and 4, Table 7.4). 

The reduction of a-amino mixed anhydrides with lithium tri-/ert-butoxyaluminum hydride in THF at —70 °C is a very efficient method for synthesis of amino aldehydes (Table 9). 551 Three approaches were taken for the reduction of a-amino mixed anhydrides. 55 The first approach reduced Boc-Ala-OC02Et with lithium tri-terf-butoxyaluminum hydride was unsuccessful due to intramolecular rearrangements that gave Boc-Ala-OEt in addition to the Boc-Ala-H. The second approach involved reduction of diphenylacetic anhydride derivatives, which were prepared from Boc amino acids and diphenyl ketene, gave a diphenylacetic acid byproduct that was very difficult to remove unless the aldehyde was converted into its semicarbazone and separated chromatographically yields were 51-69%. The last and... 

Except for the well-documented conjugate additions of diethylaluminum cyanide,92 triethylaluminum-hydrogen cyanide and Lewis acid-tertiary alkyl isonitriles,93 examples of Lewis acid catalyzed conjugate additions of acyl anion equivalents are scant Notable examples are additions of copper aldimines (233),94, 94b prepared from (232), and silyl ketene acetals (234)940 to a,(3-enones which afford 1,4-ketoal-dehydes (235) and 2,5-diketo esters (236), respectively (Scheme 37). The acetal (234) is considered a glyoxylate ester anion equivalent. 

Keteniminium salts are more electrophilic than ketenes and are thus able to react with less nucleophilic olefins. Ketoketeniminium salts can be conveniently prepared from the corresponding a-chloroenamines and Lewis acids.3 However, the method cannot be applied well to the preparation of the less stable aldoketeniminium salts. 

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. 

Presumably, 1-acetoxycyclopropanol (88) is the intermediate in this reaction as well as that between the hydrate and ketene. 80> Although 88 has not been isolated in the above cases, it may be prepared from acetic acid and cyclopropanone and reacts with ketene to give 87. 5>15>... 

S)-fl-Amino acids.1 The Mannich reaction of the Schiff base 1, prepared from 2,3,4,6-tetra-O-pivaloyl-fl-D-galactosylamine, reacts with the silyl ketene acetal 2 in... 

One method for the synthesis of hydroxyalkyl-substituted P-lactams is by the Staudinger reaction, the most frequently used method for the synthesis of P-lactams.86 This method for the preparation of 4-acetoxy- and 4-formyl-substituted P-lactams involves the use of diazoketones prepared from amino acids. These diazoketones are precursors for ketenes, in a diastereoselective, photochemically induced reaction to produce exclusively tram-substituted P-lactams. The use of cinnamaldimines 96, considered as vinylogous benzaldimines, resulted in the formation of styryl-substituted P-lactams. Ozonolysis, followed by reductive workup with dimethyl sulfide, led to the formation of the aldehyde 97, whereas addition of trimethyl orthoformate permitted the production of the dimethyl acetal 98 (Scheme 11.26). 

Treatment of 1-morpholino-l-cyclohexene with ketene gives l-morpholino-2-acetyl-l-cyclohexene251 as the main product, whereas enamines prepared from aliphatic aldehydes yield cyclo-butanones.252,253 Ketene may be generated directly in the reaction medium from acid chlorides and triethylamine. 

Since sUyl ynol ethers have an electron-rich triple bond, they are useful for Lewis acid catalyzed synthetic reactions. Lithium ynolates 175 are silylated by TIPSCl or TIPSOTf and TBSCl to afford the corresponding silyl ynol ethers 176 and 177, which are thermally stable and isolable, but sensitive toward acids (equation 71) . See also equations 9 and 10 in Section ll.C. An experimentally improved procedure for the purification of 176 derived from Kowalski s method is described. Lithium ynolate derived from Julia s method is also used for the preparation of 176. TMSCl and TESCl provide silyl ketenes 179, however, by C-silylation. These small silyl chlorides primarily gave the silyl ynol ethers 178, but, upon warming the reaction mixture, isomerization to the more stable silyl ketenes takes place. The soft electrophilic silyl chlorides like PhsSiCl afford silyl ketenes. Disi-lyl ynol ethers, prepared from ynolate dianions, are rearranged to disilylketenes mediated by salts . 

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