Conversion of ketene

The same reaction can be used for conversion of ketene bis(phenylthio)acetals (3) to the vinyllithium reagents 4. This method may be more useful in some cases than deprotonation of vinyl sulfides. Thus 4 generated in this way reacts as expected with cyclohexanone and carbon dioxide. Both these reactions fail when 4 is generated by deprotonation of vinyl sulfides. 

Amide groups are also formed by the conversion of ketene derivatives and amino groups. This reaction is used in what is used in what is known as the negative process in reproduction technology Azoketones decompose in light to carbenes which rearrange to ketenes. The ketenes then cross-link in situ with poly(/7-amino styrene), for example ... 

Scheme 4.15 Energies of conversion of ketene dimer 40 to CO2 and allene (kcal mol ). ° ...

This work was extended to a one-pot multicomponent procedure involving two different ketenes derived from diazo ketones 145 and 149, respectively, and using 4-methoxyphenylazide (ArN3), and rhodium acetate, with in situ conversion of ketene 150 derived from diazo ketone 149 to the imine 147 by reaction with the arylazide, and then subsequent cycloaddition of 147 with ketene 146, also generated in situ, from diazo ketone 145, again forming the P-lactam 148 (Eqn (4.90)). ... 

While this discussion has shown how the PMO method can be extended to cover the main phenomena of photochemistry, it is by no means exhaustive. We have deliberately omitted reference to several types of photochemical processes because although it is not difficult to extend the PMO treatment to them, such an extension would take an undue amount of space. One example which we previously mentioned is the photochemical conversion of ketene or diazomethane to carbene. 

The manufacture of the highly pure ketene required for ketenization and acetylation reactions is based on the pyrolysis of diketene, a method which has been employed in industrial manufacture. Conversion of diketene to monomeric ketene is accompHshed on an industrial scale by passing diketene vapor through a tube heated to 350—600°C. Thus, a convenient and technically feasible process for producing ketene uncontaminated by methane, other hydrocarbons, and carbon oxides, is available. Based on the feasibiHty of this process, diketene can be considered a more stable form of the unstable ketene. 

Another principal use of ketene is in the production of sorbic acid [110-44-1] (80,81). In this process, which requires an acidic or manganese(II) catalyst, ketene adds to crotonaldehyde [123-73-9] (8) with subsequent conversion of the P-lactone and the polyester to sorbic acid (qv) (9). 

The isomerization of carbenaoxirane (81) to oxirene seems unlikely, as ab initio calculations predict a more facile conversion to ketene (Figure 3). However, oxirene involvement has been demonstrated for ketene photolysis (Scheme 102) (80PAC1623) and this may proceed through vibrationally excited (81) cold carbenaoxiranes would appear to be even... 

Of the many interesting and productive stages in Corey s synthesis of ginkgolide B, it is the intramolecular ketene-olefin [2+2] cycloaddition step that is perhaps the most impressive (see 14—>13). The conversion of 14 to 13 is attended by the formation... 

Acetylsultam 15 is also used for stereoselective syntheses of a-unsubstituted /1-hydroxy-carboxylic acids. Thus, conversion of 15 into the silyl-A/O-ketene acetal 16 and subsequent titanium(IV) chloride mediated addition to aldehydes lead to the predominant formation of the diastereomers 17. After separation of the minor diastereomer by flash chromatography, alkaline hydrolysis delivers /f-hydroxycarboxylic acids 18, with liberation of the chiral auxiliary reagent 1919. 

Conversion of allyl alcohol esters into their corresponding trimethylsilyl or t-butyldimethylsilyl ketene acetals, followed by mild thermolysis, results in... 

Conversion of Free or Silylated Carboxylic Acids into Esters, Thioesters, Lactones, or Ketenes. Transesterification of Esters with Alcohols... 

Zinc chloride in conversion of dimethyl-ketene /3-lactone dimer to 2,2,4-trimethyl-3-oxovaleryl chloride, 48,126... 

Clear formation of ketene—zirconocene complexes upon treatment of acylzirconocene chlorides with a hindered amide base indicates that the carbonyl group of the acylzirconocene chloride possesses usual carbonyl polarization (Scheme 5.10). However, these zirconocene—ketene complexes are exceptionally inert due to the formation of strongly bound dimers [13a], Conversion of the dimer to zirconocene—ketene—alkylaluminum complexes by treating with alkylaluminum and reaction with excess acetylene in toluene at 25 °C has been reported to give a cyclic enolate in quantitative yield. Although the ketene—zirconocene—alkylaluminum complex reacts cleanly with acetylene, it does not react with ethylene or substituted acetylenes [13b]. Thus, the complex has met with limited success as a reagent in organic synthesis. 

A remarkable reaction, discovered by McGuire and Hegedus in 1982 [292], is the photochemical conversion of heteroatom-substituted chromium and molybdenum carbene complexes into intermediates with ketene-like character (Figure 2.21). This reaction has been reviewed by Hegedus [203]. 

An important extension of the ketene acetal cycloaddition involved the diazoniapentaphenes. An example is the conversion of 4a,8a-bis(azonia)pentaphene (31) to a mixture of two diadducts of which 32 represents the stereoisomer formed by addition of both mole-... 

In contrast to the foregoing observations, there are other reactions that generate oxirane derivatives as intermediates that undergo further transformations to a variety of compounds. For example, oxidation of ketenes 15 gives polyesters 17 via the intermediacy of oxiranones 16. The conversion 15 16 probably involves a nucleophilic attack of oxygen end of the... 

Robert and co-workers (239,240) discovered novel conversions of 2-amino-1,3-dithiolium-4-olates (348) into other mesoionic heterocycles. For example, reaction of 348 with carbon disulfide, phenyl isocyanate, or phenyl isothiocyanate affords l,3-dithiolium-4-thiolates (349), l,3-thiazolium-4-olates (350), and 1,3-thiazolium-4-thiolates (351), respectively. Some of these reactions proceed via the ring-opened ketene tautomer of 348 (240). 

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