Ring contraction cyclobutanes

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). 

The photolysis of cyclic diazo ketones in hydroxylic solvents leads to ring contracted carboxylic acid derivatives via this ketocarbene -> ketene rearrangement. Examples of such reactions are given in (2.24)239) and (2.25) 240). In this last example a photoequilibrium between the diazo ketone and its valence isomer, a diazirine, has been observed, both products then eliminating nitrogen to afford the cyclobutane carboxylic acid. 

Carbenes are known intermediates in the thermolytic or photolytic decomposition of the lithium or sodium salts of tosylhydrazones, which, for endocyclic carbenes, results in ring contraction when the elimination of / - or y-hydrogens is impeded. Simple cyclobutanes generally cannot be prepared by this route from monocyclic cyclopentanone tosylhydrazones. However, the lithium salt of bicyclo[2.2.1]heptan-7-one tosylhydrazone gave bicyclo[3.2.0]hept-l-ene (4) as the major product (74%) by vacuum pyrolysis at 185 JC/20 Torr, together with bicyclo[2.2.1]heptane (14%) and tricyclo[2.2.1.02,7]heptane (12%) in 80% overall yield.67... 

Ring contraction of cyclopentenone and cyclohexenone derivatives to cyclobutanes was observed during direct irradiation of various j5,y-unsaturated cyclohexenones. By way of a Nor-rish type I cleavage in n,7i excited singlet (5t) and n,7t excited triplet (T2) states, an acyl/allyl diradical is formed which recombines in the allyl position to form the product of a 1,3-acyl migration,102 while sensitized irradiation leads mainly to oxa-di-rr-methane rearrangement (see Section 4.2.3.). 

Several photochemically induced vinylcyclopropane to cyclopentene rearrangements of nor-carene derivatives to form bicyclo[3.2.0]heptenes can be understood as ring contractions of cyclohexenes to cyclobutanes. Upon direct irradiation of norcar-2-ene (bicyclo[4.1.0]hept-2-ene) at 214 nm (pentane solution), however, complex product mixtures were obtained containing only small amounts of bicyclo[3.2.0]hept-2-ene, while toluene sensitized photolysis in 50 millimolar solution in degassed pentane at 254 nm gave mainly the cyclobutane derivative 13 in addition to EjZ-isomeric hepla-l,3,6-trienes.72... 

Two total syntheses of welwitindolinone A isonitrile have been completed so far. The first of them, due to Baran, uses a ring-contraction strategy to generate the cyclobutane ring, and is remarkable for the absence of protect-... 

Compound VIII thus formed would then undergo ring contraction as postulated before, followed by a now logical cyclobutane ring opening that would occur under the auspices of acetate and the C-C bond weakening effect of the sulfonium unit. 

Since the pioneering observation of Demjanov that both cyclobutylamine and cyclopropylcarbinylamine react with nitrous acid to give a mixture of cyclobutanol and cyclopropylcarbinoP , the facile interconversions which occur among related cyclobutane, cyclopropane and open-chain structures via carbenium ion intermediates have been extensively studied. This question has been discussed in Section III.A for a review concerning the formation, structure and ring contraction of cyclobutyl cations, see Ref. 264. 

Cava, M. P., LItle, R. L., Napier, D. R. Condensed cyclobutane aromatic systems. V. The synthesis of some a-dlazoindanones ring contraction In the Indan series. J. Am. Chem. Soc. 1958, 80, 2257-2263. 

J. Amer. Chem. Soc., 101, 5451 (1979) metallo-cyclopen-tane may lead to metallo-cyclobutane+carbene by ring contraction. The opposite path which involves insertion of a carbene in a metallo-cyclobutane is not unreasonable. 

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.). 

The ring contraction of monofunctionalized four-membered rings (i.e. halo- or tosyloxycyclobutanes), involving carbenium ion intermediates, usually affords mixtures of cyclobutyl, cyclopropylmethyl and but-3-enyl compounds, but are of limited synthetic applicability (Section 4.1.2.1.). On the other hand, the ring eontraction of vicinally difunctionalized cyclobutanes appears easier and does not necessarily require carbenium ion formation. 

Ring contractions of cyclobutane-1,2-diols have been widely reported. Both cis- and fran -cyclo-butane-l,2-diols rearrange under acid catalysis to give the corresponding cyclopropanecarb-aldehydes or ketones, generally in high yield. ... 

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). ... 

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