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23.13.2.1 Synthesis of Product Subclass 2

DOI: 10.1055/sos-SD-023-00504

Danheiser, R. L.; Dudley, G. B.; Austin, W. F.Science of Synthesis, (200623522.

Schemes 46 and 47 outline the principal methods for the synthesis of ketenes belonging to Product Subclass 2. Members of this subclass include 1,3-dienylketenes as well as (2-arylvinyl)ketenes (i.e., dienylketenes in which the terminal C=C bond is incorporated in a benzoaromatic or heteroaromatic ring). In some methods, ketenes in which the central double bond has the E-configuration may be formed in addition to or in preference to the Z-isomer depicted in Scheme 46. Method A, involving the elimination of carboxylic acid derivatives, can lead to either stereoisomeric ketene, depending on the configuration of the double bond in the β,γ-unsaturated acid starting material and the reaction conditions. The Wolff rearrangement of certain α-diazo-α,β-unsaturated ketones (Method B) provides a second route to 1,3-dienylketenes and (2-arylvinyl)ketenes, and the electrocyclic ring opening of suitably substituted cyclobutenones (Method C) constitutes a third general strategy. Finally, the photochemical six-electron electrocyclic ring opening of appropriately functionalized cyclohexa-2,4-dien-1-ones constitutes a fourth approach to 1,3-dienylketenes and (2-arylvinyl)ketenes as outlined in Scheme 47.

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