Cyclobutenone, ring expansion

Lithiated O-silyl cyanohydrins add to cyclobutenediones to form acyl cyclobutenone intermediates 634, which can undergo ring expansion to provide 2/7-pyran-2-ones (Scheme 144) <1999JOC2145>. 

When the cyclopropenyl rhenium complex (CO)5Re( 7 -C3Ph3) was refluxed in hexane for 3-4 h, the isomeric / -oxocyclobutenyl and tricarbonyl( 7 -hexaphenylcyclohexadi-enyl)rhenium complexes are obtained, in addition to the tetracarbonyl rhenacyclobutadi-ene complex. In the presence of excess Me3NO, a mixture of two interconvertible triphenyl rhenafuran complexes is obtained. Furthermore, if the thermal reaction is conducted in the presence of phosphorus ligands, addition and ring expansion occur, affording octahedral -cyclobutenone complexes (equation 206) . ... 

In an interesting reaction involving ring expansions of cyclobutenones based on hexatriene electrocyclizations, the lithiated species 95 underwent conversion to the fused thiophene derivative 96 upon treatment with the cyclobutenone 97 <04JA1624>. 

Thermal ring expansion of polysubstituted 4-hydroxy-2-cyclobutenones, which can be prepared from squaric acid ester (see the previous section), has been extensively studied and its synthetic value has now been confirmed. The early works have been reviewed several times for the cases of cyclobutenones that have unsaturated bonds at the 4-position, such as (cyclo)alkenyl, alkynyl, and aromatic groups [19-23]. 

Scheme 31 Ring expansion of diazo-functionalized cyclobutenones 1,7-dipolar cycloaddition to diazepinedione [129]...

Scheme 32 Ring expansion of azido-functionalized cyclobutenones formation of stable 2-aza-2,4-cyclopentadienone and its reaction with some nucleophiles (Ohno et al. unpublished data)...

Moore et al. investigated the thermal rearrangements of differently substituted cyclobutenones. Reactions of 4-alkenyl-4-hydroxycyclobute-nones such as 18, in which the triple bond is replaced by a double bound, are complementary to the ring expansions of 4-alkynyl-4-hydroxycyclobute-nones and provide a route to the differently substituted benzoquinones, such as aurrantiogliocladin 19. The reaction proceeds via enyne-ketene 20. Since cyclization produces a derivative of hydroquinone 21, an additional oxidation step is required that is accomplished with the use of cerium ammonium nitrate on silica. This ring expansion process is independent of the... 

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