Dibenzobarrelene substituted

The influence of radical stabilization on the outcome of the rearrangement reactions of a variety of dibenzobarrelenes has been evaluated178. A detailed analysis of the acetophenone-sensitized conversion of the cyano-substituted barrelenes into the corresponding semibullvalenes has been presented179. The outcome of the irradiation of the dibenzobarrelene 331 is dependent upon the excited state involved. Thus direct irradiation affords a cyclooctatetraene and sensitized irradiation converts it into the two... 

It has been known for over two decades that optically active products can be formed from achiral precursors without the intervention of preexisting optical activity. This was illustrated by Penzien and Schmidt when they showed that 4,4/-dimethylchalcone, although itself achiral, crystallizes spontaneously in a chiral space group [140]. When these crystals are treated with bromine vapor in a gas-solid reaction, a chiral dibromide is produced in 6% enantiomeric excess. Since then several research groups have carried out similar reactions wherein the achiral substrate crystallizes in a chiral space group [141-184]. Photolyses of these chiral crystals lead to optically active photoproducts. An example of these types of reactions is the di-ir-methane rearrangement of substituted dibenzobarra-lenes shown in Scheme 7 [151,178]. The dibenzobarrelene derivatives 16a and... 

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... 

Benzonorbornadienes and Related Benzopolycyclics.—Cycloaddition routes to benzopolycyclics have been extended by investigations, which show the synthetic importance of [4 + 4] photocycloaddition, and by further examples of [4 + 2] thermal cycloaddition. Addition of dimethyl acetylenedicarboxylate to 2-substituted anthracenes provides a satisfactory route to substituted dibenzobarrelenes. Increasing methyl substitution in a series of polymethyl-naphthalenes facilitates Diels-Alder addition of maleic anhydride, "" and as expected the kinetically favoured product is the endo-addact but the thermodynamically favoured product is the exo-adduct. Details of the preparation of (347) by a benzyne addition and of further additions of dimethyl acetylenedicarboxylate to substituted naphthalenes are given.In order to investigate their conformational equilibria, (348) and (349) were synthesized by [4 + 2] cycloaddition of benzocyclobutadiene to 2,7-dimethylanthracene and by [4 + 4] cycloaddition of o-xylylene to 2,7-dimethylanthracene, respectively. The estimated barrier to inversion in (349) is 8 kcal mol . ... 

Interesting photochemistry involving the tri-Ti-methane rearrangement of bridgehead-substituted dibenzobarrelenes 50a,b was reported earher (Scheme 9)P When 50a was irradiated in benzene, compounds 51a and 52a were obtained, whereas 51a was the only product from acetone-sensitization, and... 

---