Cyclobutenones synthesis

Strained ring compounds undergo insertion of a low-valence metal complex to give metallacycles and the cycloaddition of metallacycles has a potential in synthesis, as described above. This method is useful in ring transformations of cyclobutenediones and cyclobutenones. 

The cyclobutenone 70 is transformed to the r/4-vinylketene complex 72 with (t/5-indenyl)Co(PPh3)2 71. The vinylketene complex 72 undergoes cyclization with alkynes to produce the corresponding phenols 73. FeCl3 oxidation of the (2-phenylvinyl)ketene complex, however, leads to the naphthol 74. A catalytic synthesis of phenols via the vinylketene intermediates 72 is achieved by the use of Ni(COD)2 as a catalyst [36]. (Scheme 26)... 

The ring opening of cyclobutenone derivatives leads to the formation of vinylketenes (equation 3). The potential of these compounds in the synthesis of complex molecules has been investigated in the 1980s by a number of research groups, most notably those of Moore, Liebeskind and Danheiser. An excellent review describing work in this area prior to 1986 is available. ... 

Cycloadditions of ketenes with alkenes and alkynes constitute the most popular method for the synthesis of cyclobutanones and cyclobutenones. Unfoitunately, however, this process is truly general only for highly nucleophilic ketenophiles such as conjugated dienes and enol ethers. In general, unactivated alkenes and alkynes fail to react in good yield with either alkyl- or aryl-substituted ketenes, or with ketene itself To circumvent this limitation, dichloroketene is usually employed as a ketene equivalent, since this electrophilic ketene reacts well with many types of unactivated multiple bonds, and the resultant cycloadducts undergo facile dechlorination under mild conditions. ... 

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