Cyclobutane-1,2-diones ring contraction

Within the cubane synthesis the initially produced cyclobutadiene moiety (see p. 329) is only stable as an iron(O) complex (M. Avram, 1964 G.F. Emerson, 1965 M.P. Cava, 1967). When this complex is destroyed by oxidation with cerium(lV) in the presence of a dienophilic quinone derivative, the cycloaddition takes place immediately. Irradiation leads to a further cyclobutane ring closure. The cubane synthesis also exemplifies another general approach to cyclobutane derivatives. This starts with cyclopentanone or cyclohexane-dione derivatives which are brominated and treated with strong base. A Favorskii rearrangement then leads to ring contraction (J.C. Barborak, 1966). 

Acyloin-type cyclization of dimethyl 2,3-di-tert-butylbutanedioate by sodium in refluxing toluene gave the 3,4-di-/erf-butyl-2-hydroxycyclobutanone (1) in 90% yield.Oxidation with dimethyl sulfoxide and acetic anhydride - gave 3,4-di-tm-butylcyclobutane-l,2-dione (2, 55%). Irradiation of this a,/1-diketone gave 2,3-di-terf-butylcyclopropanone (3) and a polymeric product. (For the base- or acid-induced ring contraction of cyclobutane-1,2-diones, see Section 4.1.2.2.7.). 

Ring contraction of a cyclobutane-1,2-dione was first postulated in order to account for the formation of an a-oxo acid 2 from 2-phenylcyclobutane-1,2-dione (1). ... 

The parent compound cyclobutane-1,2-dione itself was proposed as an intermediate in the acid- or base-catalyzed ring contraction of l,2-dichlorocyclobutyl-l,2-diylcarbonate (5), adduct of dichlorovinylene carbonate with ethene under photolysis, which on dissolution in cold dilute potassium hydroxide and subsequent acidification or on dissolution in hydrochloric acid, gave the 1-hydroxycyclopropanecarboxylic acid (6) as a single product. (For other examples of such a reaction, see ref 70.)... 

Cyclobutane-l,2-dione has been isolated from the bromination of 1,2-bis(trimethyl-siloxy)cyclobutene, prepared by sodium induced acyloin cyclization of butanedioate in the presence of chlorotrimethylsilane . The acyloin cyclization of 1,4-, 1,5- and 1,6-diesters to give four-, five- and six-membered rings and the cyclization of )9-chloro esters to three-membered rings in the presence of chlorotrimethylsilane, which usually requires highly dispersed sodium, has been recently simplified by the use of sonochemical activation. The ring contraction of cyclobutane-1,2-diones to 1-hydroxyeyclopropanecarboxylic acid (7) and derivatives may be catalyzed by aqueous acid or base, by methoxide anion, amines or hydrazines and even takes place, though more slowly, in pure water or ethanol. ... 

For the photochemical ring contraction of cyclobutane-1,2-diones with carbon monoxide elimination, see Section 4.1.1. 

Photochemical ring contraction of various substrates, e.g. cyclobutane-1,3-diones and 1 -pyrazolin-4-ones - gives either isolable cyclopropanes or cyclopropanones as intermediates which are usually intercepted as adducts or which fragment to alkenes and carbon monoxide. The photoextrusion process most often leads to fragmentation products. 

The base-catalysed ring contraction of substituted cyclobutanes continues to attract attention and has been used for the synthesis of bicyclo[n,l,0]alkyl derivatives (n = 3 or 4) of known configuration. However, when applied to chlorotrialkylcyclobutane-1,3-diones, cyclopropanone-2-carboxylates are not obtained instead, ring cleavage occurs to give y-chloro-P-keto-esters, perhaps because of the ring strain associated with cyclopropanone. The dione (31), a photoproduct from bicyclo[2,2,l]hept-5-ene-... 

---