Cyclobutane derivatives ring closure

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). 

Photodimerization reactions of some other simple alkenes and dienes follow/39-30 36-182 Although not a dimerization reaction, photochemical ring closures to yield cyclobutane derivatives are analogous and are included in this section 31-35 ... 

That the situation is different for photochemical reactions is indicated by a particularly interesting recent study of some dialkylketones (239). In solution, 5-nonanone, 152, reacts photochemically to yield the cyclobutanol 153 and its isomer 154 in comparable amounts. Within the urea clathrate, however, 153 is the dominant product, with only traces of 154 being formed. The cyclobutanols analogous to 153, that is, having methyl and hydroxyl cis, also predominate in the urea-clathrate-mediated photocyclization of 2-hexanone and 2-undecanone. It might be expected that the bulky cyclobutane derivatives, which almost certainly cannot be crystallized in a urea clathrate, would also not be formed in such a clathrate. There are decomposition pathways (cleavage reaction 0 of the diradical intermediate that occur both in the clathrate and in solution. Nevertheless, the ring closure is a major pathway of reaction even in the clathrate. 

The decay of the tetramethylene diradical derived from 2,2,5,5-t/4-cyclopenta-none is much slower than seen for the C4Hg diradical. Both principal decay modes, fragmentation to two ethylenes and ring-closure to cyclobutane, may be dependent dynamically on torsional motions of the terminal methylene groups. 

Ring closure to cyclobutanes and cyclopentane-s The synthesis of cyclo-propanones by elimination of lithium thiophenoxicMe with this base has been extended to similar syntheses of functionalized cyclob tanes and cyclopentanes, as shown in equations (I) and (II), When extended to synthesis of a cyclohexane derivative, this method resulted in a very low yield. One possible mechanism is formation of a dianion followed by loss of thiophenoxid e ion to give a carbene-anion, which cyclizes with loss of the second thiophenoxide ion. 

Direct irradiation of myrcene (100) gives the cyclobutane derivative 101 by an electrocyclic ring-closure reaction, together with )3-pinene (102) formed in a [2 + 2] cycloaddition process. The sensitized reaction, however, yields the bicyclic compound 103. 

Several examples of isomerism of heavily substituted dienes into cyclobutenes have been reported. The reactions are brought about by the use of quartz filtered light. The reactions are quite efficient and, for example, the bicyclo[4.4.1]undeca-dienes (130) can be converted efficiently into the isomeric products (131) and (132) in the yields shown below the appropriate structure. Ring strain does not adversely affect the reaction since it is also possible to bring about ring closure of the bicyclo[4.2.1]nonadienes (133) to afford the tricyclic products (134) and (135). The (2+2)-photocycloaddition of the lambertianate derivative (136) results in the formation of the cyclobutane adduct (137). ... 

Reaction of ethylene dibromide or trimethylene dibromide with an insufficiency of sodiomalonic ester can lead to ring closure to cyclopropane384 or cyclobutane derivatives,385 respectively (reaction b). 

In the absence of a 3-bromo substituent, such 1,4-zwitterions undergo ring closure to cyclobutane derivatives. ... 

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