Eight photochemical cycloadditions

No concerted thermal 4 + 4 cycloadditions are known photochemical 4 + 4 additions are observed, but in most cases probably occur through biradicals.88 It should be noted that in the thermal butadiene dimerization (Equation 12.34), the eight-membered ring arises through the allowed [3,3]-sigmatropic isomerization of m-divinylcyclobutane.69... 

Cyclic enones with ring sizes of six-to-eight carbons can be photochemically induced to undergo [4+2] cycloadditions via isomerization to a strained trans isomer (Schs. 22-24). Irradiation of 2-cycloheptenone 99 leads to [2+2] dimerization of an intermediate r -2-cycloheptenone 100, but if this irradiation is conducted with an excess of cyclopentadiene 32 at —50 °C, a single [4+2] adduct 101 is isolated in very high yield [65,66]. The somewhat less strained nms-2-cyclo-octenone can be generated and trapped by subsequent addition of a cyclopentadiene [67,68]. Extension of this reaction to intramolecular examples has recently been reported [69]. 

Because dienes have relatively high-energy HOMOs and low-energy LUMOs they should be able to take part in cycloadditions with themselves. And they do. What they cannot do is form an eight-membered ring in one step (though this is possible photochemically or with transition metal catalysis as we shall see later). 

Eight-membered rings can be obtained by [4+4]-cycloadditions of 1,3-dienes [1] via diradicals or other intermediates. Synthesis of such compounds has been achieved by thermal, [2] photochemical, [3] and by metal-catalyzed [4] processes these reactions have been the subject of extensive mechanistic [5] and theoretical [5c] studies. Their strategic applications in natural product synthesis have been reviewed. [5d] The thermal version has generated little interest, except in orthoquino-dimethane dimerizations and in cycloreversions the Cope rearrangement of 1,2-divinyl-cyclobutanes [3] is more commonly used. [4+4]-Cycloadditions are also used with 1,3-dipoles or mesoionic heterocycles for the synthesis of six- and seven-membered rings. Sometimes also [6+4]-cycloadditions are... 

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