4-Alkynyl-4- cyclobutenones

The Moore reaction is also effective for the synthesis of annulated derivatives of quinines. Moore et al. have established that 4-alkynyl cyclobutenones are precursors in the efficient syntheses of some members of the family of naturally occurring isochroman-1,4-naphthoquinones, which show biological activity as antibiotics and antymycotics. Benzocyclo-... 

Here we want to discuss the course of the reaction when alkynyl cyclobutenone 2 is refluxed in toluene to yield naphthofuran 3. The structure of this compound suggests that a cascade cyclization process has occurred. A new benzene ring has been formed and the cyclobutenone ring has disappeared, which could indicate that this is the key reacting group. 

Alkynyl-3-methoxy-4-hydroxycyclobutenones [31] have also been shown to cleave supercoiled DNA, and DNA damage was believed to be mediated by diradicals (Fig. 9) arising from the thermal decomposition of cyclobutenones at 49 °C. 

An intramolecular allenylidene-alkynyl coupling was also observed in the reaction of the mixed alkynyl-allenylidene rhodium(I) complex 73 with carbon monoxide (Scheme 25). In this case, the initially formed thermally unstable allenyl derivative 74 evolved into the metallated cyclobutenone 75 when an excess of CO was present [276]. 

In a study of annelation reactions of 4-alkynylcyclobutenones <91JOC6104>, thermolysis of a 4-alkynyl-4-(propargyloxy)cyclobutenone was observed to give a high yield of a quinone methide as evidenced by the formation of a hetero-Diels-Alder product in the presence of butoxyethene as a... 

Cyclobutenones are fairly common side-products in the reaction of chromium arylalkoxy-carbenes with internal alkynes. As indicated in Scheme 5-1, the branch point in the formation of cyclobutenone versus naphthol products is believed to be vinylketene intermediate 4, which may undergo electrocyclic ring closure to 9, followed by reductive elimination to the product [7 a]. Cyclobutenone formation occurs only in the presence of internal or external ligands that can coordinate to unsaturated chromium species sufficiently well to prevent complexation to an internal n-system and thus divert the system toward 9. Depending on the alkyne and aryl substitution patterns and the reaction conditions, cyclobutenone formation can be made to predominate. Thus, solvents of good coordinating ability such as acetonitrile, o-OMe aryl substitution (which allows internal coordination to chromium), and bulky alkynyl substituents all favor cyclobutenone formation [Eq. (23)] [13]. In fact, the effect of solvent alone can be even more dramatic for the reaction partners in Eq. (21), a 0.5 M concentration of the carbene complex in acetonitrile gives instead a 78 % yield of cyclobutenone and only a combined 17 yield of quinone and indene products [9]. 

This methodology has been extended to 4-alkynyl-4-(propargyloxy)cyclobutenones, formed by addition of lithium acetylides to dimethyl squarate (3) followed by treatment of the 1,2-adducts with propargyl iodides. These products (4) when heated at 135° rearrange to methylenebenzofurans (5), which isomerize in an acidic medium to benzofurans (6). 

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]. 

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... 

In the reaction of the rhodium alkynyl complexes 7 with excess carbon monoxide at room temperature, the cyclobutenone 8 is isolated in 90 % yield... 

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