Rearrangements cage compounds

Some unusual reactions have been described for 2-(4-chlorophenyl)-2-(3,3-dimethylallyl)-4-phenyl-5(277)-oxazolone 84. This compound undergoes a Lewis acid-catalyzed rearrangement to give a tetrahydrofuropyrrole 85. On the other hand, depending on the reaction conditions, thermolysis of 84 produces the azabicyclohexene 86 or a substituted 2,3-dihydropyridine 87 together with the caged compound 88 formed by dimerization of the 2,3-dihydropyridine and the azabicyclohexene (Scheme 7.21). " ... 

Studies on the photochemical reactions of dihydropyridines have proven to be interesting. There are a number of 1,4-dihydropyridines that are known to disproportionate when irradiated (equation 19) (B-76PH240). Analogous intramolecular reductions have also been observed by other workers (55JA447). In contrast to these results, the 1,4-dihydropyridine (59) rearranged to its 1,2-dihydro isomer (60). Further irradiation resulted in dimerization. Interestingly, the photodimer (61) cyclized to the cage compound (62). 

A new rearrangement of fused tetracyclic heterocycles, obtained by criss-cross intramolecular cycloadditions of homoallenylazines (40), mediated by HC1 in MeOH, leads to high yields of cage compounds (42) after reduction with NaBF CN (Scheme ll).33... 

Beside the practical importance of aromatic compounds, there has always been an interest in more or less theoretical problems like the scope, limitation and effects of electron delocalization in aromatic compounds (the aromaticity problem ). These investigations were strongly encouraged by the discovery of fullerene formation in a carbon plasma [18], in fuel-rich flames [24] or by the pyrolytic transformation of PAHs [25] together with a variety of the as yet potential application of these aromatic carbon cage compounds [18]. New selective C-C bond formation reactions as well as mechanisms of the rearrangement in carbon skeletons have been studied. 

Scheme 22. Rearrangement of the tetracyclic compound 81 to the new cage compounds 85 and 87, respectively...

When the cage compound 81e is heated in toluene for 5 days an interesting skeletal rearrangement occurs [72]. Product of the process is the novel cage compound 85a containing two phosphirane increments and an alkyne unit bound to a phosphorus atom. 

Hogeveen s group continues to explore the complex rearrangements following electrophilic additions to hexamethylDewarbenzene. Trapping of the intermediate carbonium ions with nucleophiles allows the synthesis of a variety of polycyclic and caged compounds. 

Sonochemical rearrangements are rare. The first case reported consists of the rearrangement of polycyclic hydrocarbons to ada-, dia-, and triamantane cage compounds (Eq. 75).223 The catalysts are usually superacids, such as trifluoro-methylsulfonic acid-antimony pentafluoride. 

The Favorskii rearrangement has frequently been employed in ring contractions. For example, a Favorskii ring contraction was employed in the synthesis of the cage compound 12 en route to a hexacyclotetradecane, and of cage compound 14 en route to pentaprismane. ... 

IV. THE FA VORSKII REARRANGEMENT IN BRIDGED POLYCYCLIC AND CAGE COMPOUNDS... 

Several useful reviews have appeared this year. Synthesis-related reviews include the synthetic applications of selenium-stabilized carbenium ions/ the use of C-acylnitrilium ions to initiate cyclization in the synthesis of heterocycles/ and the involvement of carbocation intermediates in the cyclization of olefins with mercuric triflate. Solvolysis and rearrangement in unsaturated carbon cage compounds is reviewed/ backside r-bond assistance is suggested as being always involved." The Marcus equation is shown to be very useful in the determination of intrinsic barriers to the formation and reactions of carbocations. ... 

Behr etal. studied the photoreaction of tetracyclic trienedione 139, which was prepared by the thermal addition of o-benzoquinone to barrelene (15%). Irradiation of 139 in deoxygenated methylcy-clohexane (MCH) or chloroform with benzophenone at 366 nm afforded the cage compound 140 (25%). The first step of this cycloaddition may be the formation of a biradical by a pathway similar to that of the oxa-di-Jt-methane rearrangement from the lowest (n 71 ) state (Scheme 25). Since the excitation energy of the a-dicarbonyl unit in 139 [EA (biacetyl) = 0.72 eV] is similar to that of 2-cyanonaphthalene, intramolecular excitation energy transfer from dione to diene similar to that from 2-cyanonaphthalene... 

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