Dienyl ketene intermediate

Meyers and Novachek described the Stille coupling of a chiral 2-bromooxazoline with a furylstannane to produce furyloxazoline [58]. Liebeskind and Wang conducted a benzannulation of a furylstannane using a Stille coupling with 4-chloro-2-cyclobutenone 62 to elaborate benzofuranol 63 via a dienyl ketene intermediate [59]. 

Ru(Tp)(PPh3)(CH3CN)2]PF6-catalyzed cydization of enynyl epoxides is shown to give various carbocyclic compounds depending on the types of epoxides a summary of the reaction protocol is provided in Scheme 6.22. Notably, these cyclized products are generated from dienyl ketene intermediate 59, which was trapped efficiently by alcohols to form the ester product. 

In a new benzannulation procedure, the 4 + 2-cycloaddition of lithium ynolates (131) with (trialkylsilyl)vinylketenes (132) yields, via an intermediate 3-(oxido)dienyl-ketene (133), the highly substituted phenols (134), which can be converted to ben-zofurans and benzopyrans (Scheme 37).135 The reaction of buta-2,3-dienoate with vinylketenimine yields the expected Diels-Alder adduct together with an unexpected aniline formed by a competing 2 + 2-cycloaddition.136 The first example of a... 

Intramolecular insertion of carbon monoxide into the metal-carbene bond of the (Ej-isomer of D leads to the t/4-vinyl ketene complex intermediate E. Experimental support for this type of intermediate has been provided by the isolation of Cr( CO) 3-coordinated dienyl ketenes related to 5 (Scheme 4) [15a], and by trapping the vinyl ketene intermediates as vinyl lactone derivatives in the course of the reaction of chromium carbene complexes with 1-alkynols [15b]. 

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