Ethylene derivatives cyclobutane ring

Thermal cycloaddition, to form a cyclobutane ring, is not confined to fluorinated ethylenes. Cyclobutadiene derivatives are formed as intermediates and dimerize to yield a cyclobutane ring 48... 

Irradiation (s. a. under CuBr, R B, (CH )fiO, (CHfiOOH, and HCl) Hi Cyclobutane ring from ethylene derivs. ... 

Ethylene deriv. molecules from cyclobutane ring... 

Acetic acid/irradiation Cyclobutane ring by dimerization of ethylene derivs. 

A survey of the literature reveals some characteristic photochemical features of homoquinones (1) [2+2]-photocycloaddition with alkenes and alkynes, (2) hydrogen abstraction and/or ring opening of incorporated cyclopropane, and (3) photoinduced electron transfer reactions, as described below. The photocycloaddition with alkenes and alkynes took place exclusively at the ethylene linkage of homobenzoquinones to afford cyclobutanes (or cyclobutene derivatives) regio- and exo/endo stereoselectively, depending on the identity of both the homobenzoquinones as well as the alkenes. 

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. 

The six-membered ring 85 is obtained from the allylamine 84 [31]. The sulfur-containing ring 87 was obtained from 86 using the Mo catalyst. The Ru catalyst is not active for this reaction [32]. The (S, f )-chromene derivative 89 was obtained in 97% yield by the Mo-catalysed intramolecular metathesis of (S,f )-cycloheptenyl styrenyl ether 88 under an atmosphere of ethylene. In the absence of ethylene, 89 and its dimer were obtained. The enantioselective total synthesis of (.S, / ,/ , / )-ncbivoIoI (90) has been carried out from 89 [33]. No cyclization of the cyclopentene 91 was observed, because the highly strained cyclobutane intermediate 92 is difficult to form. 

Photocycloaddition of ethoxyethene to the enone (81) at 254 nm in methanol affords the adduct (82). Addition of the same alkene to the enone (83) also proceeds via a cyclobutane intermediate but this is unstable and ring opens to afford the cyclo-octane derivative (84). A study of the addition of the optically active alkene (85) to the enone (86) affords four cyclobutane adducts two from head-to-head addition and two from head-to-tail addition. These cycloadditions load to a double induction giving either increased or decreased diastereoselectivity.The intermolecular cycloaddition of ethylene to the enone (87) yields the two adducts (88) and (89). The photoadducts are apparently susceptible to secondary irradiation and the maximum yields of the adducts was obtained at 50% consumption of the starting material (87). Under these conditions (88) and (89) were obtained in 71 and 23% respectively. The isomer (88) was taken on through several steps to afford ultimately racemic starpuric acid a... 

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). 

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