Heterocyclic ketenimines

In a similar manner, the heterocyclic ketenimines 177 (X = SR, OR or NR2) undergo the 6 r electrocyclization to give the 6H-thieno[3,2b]pyrydine-7-ones 178 in 31-73 % yields... 

Ketenimines (144), generated from a-substituted benzophenone 1-acetamidoethyl-idenehydrazones with a mixture of triphenylphosphine, carbon tetrachloride and triethylamine in dichloromethane (Appel s conditions), have been used to synthesize a variety of 1,2,4-triazole-fiised heterocycles (see Scheme 56). Mechanistically, the... 

Annelatlon reactions. The ketenimine 1 acts chemically as a vinyl phosphonate activated by the cumulated imino group and is useful as an annelation reagent for the synthesis of heterocyclic compounds. As such it condenses with the sodium salt of salicylaldehyde (2) to afford the benzopyrane in 3 in 52% yield. Similarly, the pyrrolizine 5 is available from the sodium salt of 2-formylpyrrole in 51% yield. ... 

Ketenimine complexes from carbene complexes and isocyanides as building blocks for N-heterocycles 88AG(E)1456. 

Ketenimine complexes can serve as three-atom components in [3 + 2] cycloadditions. The reaction of the tungsten complex (378) with cyclohexyl isonitrile and phenyl isocyanate produces the heterocyclic carbene complex (379), which is the result of the in situ trapping of a ketenimine complex with the isocyanate. Complexes of the type (379) can be oxidatively cleaved to give the previously inaccessible 5-ethoxyhydantoins. [3 + 2] Cycloadditions of this type have also been carried out with aldehydes. 

Few reactions of alkynes with C=X bonds are valuable for the preparation of four-membered heterocycles. Ynamines - react with aldehydes and ketones in the presence of Lewis acids to give unstable oxetene derivatives which undergo electrocyclic opening (Scheme 43). Open-chain products are also obtained with thiocarbonyl compounds,Schiff bases and iminium salts. Reactions of ynamines with carbon dioxide, ketenes. - ketenimines - and isocyanates often give mixtures of products and are of little preparative value. 

Ketenimines, formed as intermediates in the reactions of aromatic amines with a dimer of hexafluoropropene (34), cyclize in an interesting process that yields perfluoroalkyl derivatives of various heterocyclic systems (Eq. 33).119... 

Fluoroalkylquinolines can also be obtained from iV-arylfluoroalkylketenimines generated by treating hexafluoropropene dimer with primary arylamines indeed, the intermediate ketenimines can be isolated only if the aryl group possesses no available o-hydrogens (see Scheme 20). The extension of this reaction to heterocyclic synthesis is discussed in detail elsewhere (see p. 464). 

A number of reports concern the attack of nucleophiles on perfluoro-olefins to give cumulenes or heterocumulenes, which in turn imdergo nucleophilic attack to give heterocyclic compounds with perfluoroalkyi substituents. o-Phenylenedi-amine reacts with hexafluoropropene to give 2-(o j8j8>tetrafluoroethyl)benzimidazole (301) ketenimine intermediates have also been utilized in the formation of quinolines (302) (see p. lll) 89.59o j,y intramolecular cyclization, and (303) (see p. 112) by intermolecular cyclization, naphthyridines (304), pyridopyrimidines (305), and isoquinolines (306). The pyran derivative (307) is formed via isomerization of an allene intermediate (see p. 100). 3-Fluoro-2-isopropyl-l,2,4-thiadiazolin-... 

Green chemistry oriented organic synthesis in water 12CSR1415. Highlights in ketenimine chemistry, in particular, synthesis of heterocycles 12EJ05637. 

The chemistry of ketenimines, particularly, synthesis of biologically interesting heterocycles 12CSR5687. 

Scheme 21 shows the preparative route to a novel phosphonylated ketenimine and also representative reactions of this species which lead to heterocyclic products. ... 

Ketenimines were first synthesized by Staudinger and Meyer in 1919. The various methods of synthesis of ketenimines are summarized in a review article by Krow in 1971 A more recent review on ketenimines in the synthesis of heterocyclic systems appeared in 2000. ... 

The reaction of ketenimines with sulfur dioxide at —78 °C affords the [2-1-2] cycloadducts, 68, which rearrange to give the heterocycles 69 in about 90 % yield... 

The imine-tethered ketenimine 166 undergoes intramolecular [4+2] cycloaddition on heating in toluene to form the heterocycle 167 °. ... 

In 1961, Smolinsky reported on vapor phase pyrolysis of a-azidostyrene (52, Scheme 5.8), which furnished 3-phenyl-2//-azirine as the main product and provided the first example of the synthesis of such strained heterocycles from vinyl azides. By analyzing the IR data of the pyrolysates produced from 52, it was shown one year later that N-phenylketenimine is formed as a side product. By utilizing IR and NMR spectroscopy at low temperature, Wentrup and coworkers have smdied recently the detailed structures of another azirine-ketenimine pair, generated by thermal or photochanical decomposition of an enazide. The transformation of vinyl azides into 2//-azirines is currently the most frequently used access to these heterocycles. The manifold chemistry of azirines was reviewed several times," " and most of the aspects of their synthesis from alkenyl azides are summarized in Chapter 6 (Gilchrist Alwes). Therefore, only some additional certain points are included here. 

A series of recent papers documents solid advance within the chemistry of thioacyl isocyanates. Thus a variety of heterocyclic compounds have been obtained in (4+2) cycloaddition reactions of thiobenzoyl isocyanate (276) with aldehydes, diarylketens, ketenimines, benzylideneamines, ... 

Heteroaromatic azides often show photochemistry that is very similar to the photochemistry of simple homoaromatic azides. Ring expansion to cycHc ketenimines is observed, if the photolysis is performed at ambient temperature, and triplet nitrenes are formed upon low-temperature irradiation. If a nitrogen atom is placed adjacent to the nitrene center, such as in 2-pyridylnitrene 100, addition generally takes place away from the nitrogen atom. In the case of the photolysis (or pyrolysis) of the 2-azidopyridine 99, the reaction leads to the cycHc carbodiimide 101. A comphcation arises from the fact that derivatives of 2-azidopyridine and related heterocycles frequently undergo a thermal isomerization to tetrazolopyridines, which also undergo photochemical dediazotation, albeit with a low quantum yield. ... 

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