Dibenzobarrelene-11,12-diesters

In addition to bimolecular [2-1-2] photocycloaddition reactions similar to the one discussed above [11], absolute asymmetric syntheses employing spontaneously grown chiral crystals have been studied for a number of unimolecular photorearrangement reactions as well. The first report of such an investigation dealt with the photoisomerization of dibenzobarrelene diester 6 to the corresponding dibenzosemibullvalene compound 7 [12], an example of the well-known di-7i-methane (Zimmerman) photorearrangement. Achiral diester 6 crystallizes spontaneously from cyclohexane in the now familiar chiral space... 

Approximately three years ago we initiated a study of the photochemistry of dibenzobarrelene-ll,12-diester derivatives (22, Scheme 9) as part of a general program aimed at investigating the di-jT-methane reaction in the solid state (review, 37). ciganek had shown some 20 years earlier that the dimethyl diester 22a undergoes smooth photorearrangement in solution to afford the dibenzosemibullvalene derivative 23a (38). This reaction is... 

Direct irradiation of dibenzobarrelene (106a) in solution yields the di-T-methane product (107) and the cyclooctatetraene (108). Acetone-sensitized irradiation affords compound (107) only. In the crystalline phase, however, a new product (109) is obtained as well as (107) and (108). A reinvestigation of this has suggested that the biradical (110) is involved in the formation of both the ester (109) and the cyclooctatetraene (108). The formation of the biradical could involve a tri-m-methane intermediate such as (111). Other examples (106b, c) have been studied and while the solid state irradiation does not yield the appropriate diester corresponding to (109) the cyclooctatetraene obtained does have the correct substitution pattern in confirmation of the involvement of a biradical analogous to (110) rather than the (2+2)-cycloaddition reaction path which has been favoured in the past. A further study of the asymmetric induction in the di-n-methane rearrangement has examined the... 

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