3- dienylketene intermediate

Z-Dienynes Phis Carbene Complexes Alternative benzannulation through reaction of dienynes (e.g., 173, Scheme 17.24) and carbene complexes provides a net [5 -I-1] approach for the construction of benzofurans (e.g., 176) [71-73]. In most of the examples reported, the carbene complex is relatively simple and the molecular complexity resides in the diene group. In this transformation, the critical dienylketene intermediate for benzannulation (174) is accessed through regioselec-tive alkyne insertion followed by CO insertion. The reaction was designed to produce the phenol-enol ether derivatives (e.g., 175) however, these complexes are unstable... 

Scheme 3)10. Indeed, independent photolysis of 2,4-cyclohexadien-l-ones 12 and 13 afforded the macrolides 15. These reactions likely proceed via a common intermediate, in this case dienylketene 14, which is trapped intramolecularly by the pendant hydroxyl group. Adjustment of the oxidation level and functional group interconversion then led efficiently to the desired macrolide 17. The sulfonyl group was used for two reasons first, to easily transform lactones 15 into dienyl lactones 16 needed for 17, and secondly, to control the regiochemistry of the Wessely oxidation of phenolic precursor needed to produce the photolysis substrates 12 and 13. 

The competition of nucleophilic attack by solvent at the ketene sp center has been shown to be useful for obtaining aza analogues of dienylketenes, namely dienylketenimines (297) fixHn ketene intermediates generated from (295). Upon dehydration, the addition product (296) afforded directly the Af-alkylated cy-clohexadienimines (298) in go( yield. The expected dienylketenimines (297) were not isolated. Regeneration of spectroscopically detectable (297) can be achieved by photochemical irradiation of (298), its stability above 0 C being solvent depen nt. ... 

Irradiation of chromium dienyl(amino) carbene complexes produces o-amino aromatic alcohols. An intermediate in this reaction is presumably a photogenerated dienylketene which undergoes electrocyclisation and aromatisation. Irradiation of optically active chromium aminocarbenes,... 

Cyclohexadienones and dienylketenes are readily interconverted, as reported for the carbonylation of a rhodium complex forming a dienyUcetene intermediate which undergoes 6tc electron electrocyclization forming the carbazole product after enolization with aromatization of an initially formed cyclohexadienone (Eqn (4.76)). ... 

An intermediate dienylketene 149 is also generated in the reaction of the trisubstituted vinyl ketene 148 with lithium enolates, and subsequent 6 jr-electrocyclization followed by tautomerization to give the highly substituted resorcinol monosilyl ethers 150 in good yields . ... 

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