Barrelene photoisomerization

The di-TT-methane rearrangement was discovered and conceptually developed by H. E. Zimmerman (University of Wisconsin), one of the earliest examples being the photoisomerization of barrelene (1) into semibullvalene (2) (equation 2). Through intensive subsequent work, this photoreaction was shown to be quite general for 1,4-dienes. In view of the major contributions by the original author, the rearrangement is also referred to as the Zimmerman rearrangement. 

The benzoquinoxalinobarrelene 86 with methoxy functionalities at the 1,4-positions of the benzene moiety photorearranges to 87 and 88 in a 1 1 ratio, indicating that benzo and quinoxalino groups are in equal competition (Equation 32.15). A hvefold enhancement of the DPM mode is observed in the reactions of 86 in comparison with those of 78 (see Equation 32.14). This enhancement arises from the introduction of methoxy groups in the benzene ring of 78. As in the case of 43, the hydroquinone-fused barrelene 89 did not undergo photoisomerization under direct or sensitized excitation (Equation 32.16). 

As a further extension of the photochemistry of heteroarene-fused barrelenes, dibenzo(/,/i]-quinoxali-nobarrelenes have been subjected to photoisomerization in this laboratory. Upon direct irradiation of 101 in cyclohexane, the DPM product 102 and the ADPM products 103 and 104 are obtained. The vinyl-vinyl bridged product 102 is minor (Scheme 12). Similarly, the reaction of heterobarrelene 105 under direct irradiation proceeds through both the DPM and the ADPM modes and produces SBs 106-108. Again, the DPM product 106 is formed as a minor photoproduct (Scheme 13). ... 

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