Dehydrotoluenes have likewise been obtained by double photochemical elimi- nation from chlorobenzylsilanes and chlorophenylacetic acids [59,60].
This has been synthesized starting from the 2+2 cycloadduct between cyclopentenone and cyclobutene by introducing the diazo function and applying the Wolff rearrangement for building a tricyclic alkene. A further photochemical cycloaddition with a cyclopentenone derivative, followed by a new introduction of the diazo function, photo-Wolff rearrangement, and reduction of the acidic function for enabling insertion of the chain lead to the acid, after this succession of (five!) photochemical reactions (Scheme6.19).
In many cases such strained structures are smoothly opened to form further interesting compounds. An example is the intramolecular Paterno`–Bu¨chi reaction of Diels–Alder cycloadducts between enones and dienes. Upon acid treatment, oxetanes open up to chloroalcohols, but the intermediate cation generally rearranges before addition of chloride. The actual process occurring depends on the bridges length, as shown below for the case of the rearranged products formed from two oxetanes differing by one carbon in a bridge, and thus in the forced pyramidalization of the cationic center (Scheme6.20) [63].
Scheme 6.19 Synthesis of pentacycloanammoxic acid ester45
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The anti-aromaticity of cyclobutadiene (46) has made this molecule a topic of choice in research by matrix isolation, where it occupies an important chapter. It was initially obtained by matrix photodecomposition ofα-pyrone by a prolonged irradiation at 20.4 K in argon matrix via a series of intermediates (Scheme6.21).
The peaks marked in the unexpectedly simple spectrum obtained were attributed to such species (Fig.6.30). To finally establish the structure of this molecule, rectan- gular or square, many further experiments were required, in particular the genera- tion from a different precursor and testing the effect of specific deuteration in the spectrum [8].
Scheme 6.20 Synthesis of chloroalcohols by ionic opening polycyclic oxetanes prepared by Paterno`–Bu¨chi cycloaddition [63]
6.5 Anything You Can Do Photochemistry Can Do Better: Making (Strained) Molecules 177
The interest for poly(acenes) is growing because of the interesting properties of these materials as low-temperature photoconductive materials. However, their stability is limited, particularly from hexacene onward. Double elimination from the corresponding diketone has been proved to offer a valuable access. In the case of heptacene49, the reaction takes place from precursor50via both the singlet and the triplet manifold and is complete within 7 ns [64]. This approach has been extended to octacene and nonacene from the doubly bridged precursors (Scheme6.22). Long wavelength irradiation causes partial decarbonylation, but prolonged irradiation in the UV eliminates also the second bridge [64].
Scheme 6.21 Generation of cyclobutadiene46from bicyclo[2.2.0]cyclohexenone47and from tetracyclic ether48 [8]
Fig. 6.30 IR spectrum of richest portion of photolyzed compound after 75 h of irradiation of a sample ofα-pyrone in an argon matrix. Reprinted with permission from [8]
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