From ketenes

In a sophisticated variation of the Knoevenagel condensation ("Panizzi ) methyl 3,3-dimethoxypropanoate (from ketene and dimethoxymethenium tetrafluoroborate D.J. Crosby, 1962) is used as a d -reagent. Because only one carbonyl group activates the methylene group, a strong base with no nucleophilic properties (p. 10) has to be used. A sodium-sand mixture, which presumably reacts to form silicate anions in the heat, was chosen... 

P-Hydroxy acids lose water, especially in the presence of an acid catalyst, to give a,P-unsaturated acids, and frequendy P,y-unsaturated acids. P-Hydroxy acids do not form lactones readily because of the difficulty of four-membered ring formation. The simplest P-lactone, P-propiolactone, can be made from ketene and formaldehyde in the presence of methyl borate but not from P-hydroxypropionic acid. P-Propiolactone [57-57-8] is a usehil intermediate for organic synthesis but caution should be exercised when handling this lactone because it is a known carcinogen. 

Economic Aspects. Due to the physical nature and instabHity of these materials, aH ketene production is used captively and production figures are not readily available. The economic aspects of the products made from ketenes wHl be addressed later. 

Significant producers include Daiichi, Hoffmann-La Roche, Takeda, and several factories in mainland China. Takeda and Daiichi practice processes based on alanine by ethoxyoxazole (12), at least one Chinese producer from alanine by oxazole acid (13), and Roche from ketene via cyanooxazole (15). 

Oxirene is probably a true intermediate, but is separated from ketene by only a very low barrier. Since its instability results from unimolecular isomerization rather than from attack of other molecules, the only viable current technique for its direct observation seems to be generation and spectroscopic examination in an inert matrix at temperatures near absolute zero. 

Ketene (4, 39) It has been found that the formation of acetanilide from ketene and aniline takes place more satisfactorily if the ketene is passed directly into excess aniline without any dry ether present The excess aniline may then be removed by vacuum distillation until the temperature of the vapors is io-is° higher than the boiling point of aniline An alternative plan is to remove excess aniline by dilute hydrochloric acid, to filter the acetanilide and to wash with water. 

Wiskur SL, Fu GC (2005) Catalytic asymmetric synthesis of esters from ketenes. J Am Chem Soc 127 6176-6177... 

Quantitative yields of allenes were obtained from keten and the stable phosphoranes (48). Addition of similar phosphoranes occurred at the j3-position of the allenic ketones (49) to give the phosphoranes (50). The compound (50 = COPh) eliminated phosphine oxide under the... 

Eastman Chemicals produces cyclopropylaldehyde from epoxy butene via 2,5-dihydrofuran and 2,3-dihydrofuran in a continuous plant for safety reasons. Another example cited by Eastman chemicals is that of the production of diketene from ketene, for which process technology dictates a continuous plant (Higgins, 1998). 

Oxadiazines (120) are formed from ketene acetals and DMAZD in moderate yields.106... 

New Chain Transfer Agents from Ketene Acetals... 

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. 

General procedure for the synthesis of a-bromovinyl tellurides from ketene stannyl (telluro) acetals To a solution of the appropriate ketene stannyl(telluro) acetal (1.0 mmol) in CH2CI2 (10 mL) cooled at -78°C (at 0°C for 5 h) was added dropwise a solution of NBS (0.445 g, 2.5 mmol) in CH2CI2 (25 mL). Th reaction mixture was stirred for 6 and 12 h (R=H). It was transferred to an Erlenmeyer flask and treated with a solution... 

An interesting bifunctional system with a combination of In(OTf)3 and benzoyl-quinine 65 was developed in p-lactam formation reaction from ketenes and an imino ester by Lectka [Eq. (13.40)]. High diastrereo- and enantioselectivity as well as high chemical yield were produced with the bifunctional catalysis. In the absence of the Lewis acid, polymerization of the acid chloride and imino ester occurred, and product yield was moderate. It was proposed that quinine activates ketenes (generated from acyl chloride in the presence of proton sponge) as a nucleophile to generate an enolate, while indium activates the imino ester, which favors the desired addition reaction (66) ... 

Figure 2. Synthesis of orthoesters from ketene acetals and diols.

Lastly, Corey has developed B LA species 65, derived from zwitterionic oxazaboro-lidine 64 and tri-n-butyltin trifalte, as a novel catalybc system for the enanboselec-bve synthesis of P-lactones from ketene and aldehydes (Scheme 5.81) [152]. The reaction of B LA 65 with ketene generates intermediate A. The subsequent addition of the ketene acetal unit to the coordinated aldehyde (intermediate B) followed by extrusion of the P-lactone completes the catalybc cycle. 

Ynamines from Ketene S,N-Acetals as Described in Equation (30) [19]... 

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