Ketenes imidoyl

Flash vacuum thermolysis (FVT) of 2-substituted 4//-pyrido[l,2-n]pyrimidin-4-ones 126 above 800 °C afforded (2-pyridyl)iminopropadie-none (130) (99JCS(P2)1087). These reactions were interpreted in terms of reversible ring opening of 4//-pyrido[l,2-n]pyrimidin-4-ones to imidoyl-ketenes 127. A 1,5-H shift in 127 generated the N(l)H-tautomeric methylene ketene 128, in which facile elimination of HX took place via a six-membered cyclic transition state 129 to yield 130. In the case of 2-methoxy derivative 126 (X = OMe) another competing pathway was also identified at lower temperature, which resulted in the formation C3O2 and 2-methylaminopyr-idine via mesoionic isomer 131 (Scheme 9). The products were identified by IR spectroscopy. 

Nitro(imidoyl)ketene PhN=C(NEt2)C(N02)=C0 eliminates C02 on heating and rearranges to 2-diethylamino-3-hydroximino-3// -indole 14, presumably via nitrile oxide PhN=C(NEt2)C-N+O-(90). 

Mono-and disubstituted ketenes react with N-imidoyl phosphoranimines (19) to give pyrimidinone (20) and quinazoline (21) derivatives ... 

Cycloaddition of a,3-unsaturated ketones, acyl ketenes, acyl, thioacyl, imidoyl and vinyl isocyanates, as well as the corresponding isothiocyanates, to the C=C bond of vinylidenephosphoranes leads to the formation of six-membered heterocycles carrying an exocyclic ylide function e.g. equation 117). In some instances the mentioned thioisocyanates, however, may undergo [2 + 2] cycloaddition at the C=S bond. Interestingly N-aryliminovinylidenetriphenylphosphoranes dimerize in a [4 2] cycloaddition on heating alone. 

Acid amide-triphenylphosphine dihalide adducts (4) have found wide application in organic synthesis. - Synthetic equivalents are adducts (5) from acid amides and triphenylphosphine/CCU, which are prepared in situ from the educts. - With these reagents the following transformations have been performed dehydration of amides or aldoximes to nitriles, preparation of isonitriles from secondary form-amides, preparation of imidoyl halides from amides or acylhydrazines and preparation of ketene imines from amides. Using polymer-supported triphenylphosphine the work-up procedure is much easier to achieve. Triphenylphosphine can be replaced by tris(dialkylamino)phosphines. - Instead of CCI4 hexa-chloroethane, hexabromoethane or l,l,2,2-tetrabromo-l,2-dichloroethane can be used " the adducts thus formed are assumed to be more effective than those from the triphenylphosphine/CCU system. 

An interesting route to a-carboxy-8-lactones (81) and a-methylenelactones (80), based on hydrolysis of Knoevenagel products (79) of Meldrum s acid with cyclic aliphatic ketones (78), has been developed (Scheme 14). Reduction of 5-methylene derivatives of Meldrum s acid has been performed catalyti-caiiyi30 or by use of LAH. Imidoylation reaction of Meldrum s acid and subsequent solvolysis of the resulting (82) yields -enamino esters (83) in good yields.Flash vacuum pyrolysis of alkylidene derivatives of Meldrum s acid can be used to prepare methylene ketenes (84), a class of compounds difficult to prepare by conventional methods. By this procedure, methylene ketenes are obtained from aromatic aldehydes and ketones and from aliphatic ketones in only two steps. Intramolecular trapping of the methylene ketene obtained from the ketone (85) has been used successfully in the synthesis of the naphthol (86). ... 

The third method involves formation of both rings simultaneously. The ketene JV-imidoylimines (147) were shown to be transitory intermediates formed by the reaction of isocyanides (145) with sulfides (146) (Scheme 7). When the nitrogen atom of the imidoyl group bears a primary or secondary substituent, the imines (147) were converted to the diazatrienes (148) by a very fast 1,5 migration of the hydrogen atom of the imidoyl group. This then underwent a [4 + 2] cycloaddition to form the dihydrotriazines (149) <85JOC77l>. 

Acyl(imidoyl)ketenes bearing the acyl and imidoyl groups conjugated with the (2=C ketene bond is an interesting precursor for [4 + 2] cycloaddition reactions. Such compoimds can be subjected to a wide variety of [4 4- 2] cycloaddition reactions, interestingly as either a dienophile with... 

Intramolecular [4-1-2] cycloaddition reactions of the imidoyl ketene 241, giving rise to the formation of the heterocyclic compound 242, is also observed. ... 

Oxoketenimines (IR, 2076 cm ) are formed in a thermal rarrangement from imidoyl ketenes. The oxoketenimines are stable up to 0 °C. A-2-carbonylphenyl ketenimines undergo a [1,5] migration to produce vinylimino ketenes (see Section 4.1) . 

Some ketenimines, such as 169, undergo a thermal rearrangement to produce the isomeric imidoyl ketenes 170, which undergo intramolecular cyclization to give the quinolones 171. 

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