Cyclobutene-1,2-diones

Dehmlow49) found that the photochemical extrusion of carbon monoxide from the cyclobutene dione system is possible as exemplified by the conversion of diethyl squarate (43) to diethoxy cyclopropenone (44) ... 

Finally, a reaction should be mentioned in which a nucleophile gives support to another reacting species without appearing in the final product. Diphenyl cyclopropenone interacts with 2,6-dimethyl phenyl isocyanide only in the presence of tri-phenylphosphine with expansion of the three-ring to the imine 344 of cyclobutene-dione-1,2229,230 Addition of the isocyanide is preceded by formation of the ketene phosphorane 343, which can be isolated in pure formss 231 it is decomposed by methanol to triphenyl phosphine and the ester 52. 

Carboxylic acid derivatives, as well as aldehydes and ketones, together with the corresponding sulfur compounds, have been included in this section. The reactivity of the 1- and 3-positions is such that in syntheses using acetic anhydride it is often impossible to prevent acetylation. Acyl derivatives may be formed from acid chlorides,44,159-161 cyclobutene-dione derivatives,162 or esters.163 Thus, reaction of a substituted in-... 

It was found that nanosize Ti02 particles experience an adjustment in the coordination geometry of the Ti atoms near the particle surface from octahedral to square-pyramidal in order to accommodate the large surface curvature [57]. X-ray absorption near edge structure reveals that surface modification with enediol ligands (ascorbate, ortho-hydroxy cyclobutene dione, catechol, etc.) restores the pre-edge features of octahedrally coordinated Ti in the anatase crystal environment. Specific binding of the enediol modifiers to surface comer defects ... 

Liebeskind and co-workers have published a series of papers dealing with the chemistry of cyclobutenones and cyclobutene-diones. Thus an unprotected stannyl derivative bearing an amino function was reacted with an appropriate aryl iodide, leading in excellent yield to a pyrido-annelation product [64] (Scheme 4-23). The reaction of a monoprotected derivative with chlorocyclobutenones led to benzannelated cyclobutenediones [65], and stannyl derivatives of p-benzoquinone also underwent benzannelation when treated with these substrates [66]. Acylation of cyclobutenediones via stannyl derivatives has also been carried out [67], as has functionalization of cyclobutenones [68]. 

Diamino-cyclobutene-diones as a-amino carboxylic acid surrogates Diamino-cyclobutene-dione was proposed by Kinney et al e m original surrogate of the a-amino carboxylic acid function (Figure 15.25). 

Alkoxy-2-cyclopetenones. In a cycloaddition process with alkenes, cyclobutene-diones participate after losing [CO]. This method is complementary to the Pauson-Khand reaction... 

Diamino-cyclobutene-dione, was proposed by Kirmey et al as an original surrogate of the a-amino carboxyUc acid function (Fig. 13.12). 

Scheme 3.55 Rh(l)-catalyzed intramolecular decarbonylative annulation of cyclobutene-diones.

Figure 5.7 General structure of donor-acceptor-donor quadrupolar system of cyclobuten-dione (C4O2) in the bridge with deficient electronic system.

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