2.4- Hexadiene, photocycloaddition with

An intramolecular photocycloaddition of unspecified quantum efficiency has been reported for l,5-hexadien-3-one.429 Again the product may be postulated to result from closure to a biradical with a five-membered ring. 

Transient absorption spectra of singlet diradical in addition to exciplex have been observed in the course of the photocycloaddition reaction of 9-cyanoan-thracene (CNA) with 2,5-dimethyl-2,4-hexadiene (DMHD), while no intermediate was detected in CNA-l,3-cyclohexadiene (CHD). The cycloadduct with CHD shows an efficient adiabatic photodissociation into CNA and CHD immediately after excitation. 

An alternative approach to the same mitomycin intermediate was based on the unusual photocycloaddition of an azidoquinone with a suitably functionalized (Z,Z)-2,4-hexadiene, involving the diastereoselective formation of C —C and C —N bonds154 155. A single adduct 18 from four possible diastereomers was isolated from either the (Z,Z)- or (.E,.E)-diene, the double-bond geometry was preserved in either case, as determined by H-NMR analysis. 

A few specific examples of cyclopropanation using the above methods are shown in Scheme 2.134. The naturally occurring insecticide /ra -chrisanthemic acid 396 served as an obvious target to check the viability of carbene addition as a preparative method. This compound was first synthesized (in the mixture with the cis isomer) by the monocyclopropanation of 2,5-dimethyl-2,4-hexadiene. Since then, numerous analogs of 396 were prepared by similar reactions. Some of the analogs are now widely used as efficient and ecologically safe pesticides. The formation of the tricyclic hydrocarbon 397 from 1,5-hexadiene proceeds as a sequence of inter- and intramolecular carbene transfer reactions. An initial carbene precursor, CHBr3, is actually employed here as an equivalent of a unique tetradentate Ci synthon The preparation of 398 via intramolecular [2 + 1] photocycloaddition is a typical example of the efficiency of this route for the construction of the polycyclic framework frequently encountered in the structures of natural compounds. 

Two intramolecular [2 + 2] photocycloadditions of note have been reported. The quantitative conversion of 7-oxanorbornadiene to the corresponding oxaquadri-cyclane has now been achieved despite many earlier efforts. Few syntheses of prismane derivatives can match the efficiency displayed in the cyclization to afford (45) which results from cycloaddition across the 1,4-diene system of the bicyclo-hexadiene, being best effected with light of wavelength 350 nm, which the product... 

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