Dewar-benzenes 1,2-bridged, synthesis

The silver(i)-catalysed rearrangements of the bicyclopropenyls (234) and (235) have been the subject of mechanistic studies. This rearrangement affords a useful synthesis of bridged Dewar-benzenes (236) and (237). The trimethylene-bridged Dewar-benzene (236) is stable to thermolysis at 300°C in a flow system. The penta-methylene-bridged Dewar-benzene (237) rearranges at 282 °C to benzocycloheptene and the spirocyclic compound (238). The strained benzene (239) and the biradical (240) are suggested as intermediates in the formation of benzocycloheptene and (238) from (237). 

The 2-I-2-cycloaddition of l,l-difluoro-2,2-bis(dimethylamino)ethene with ethyl propiolate yields a cyclobutene which readily undergoes electrocyclic ring opening to form a diene. The 1,4-diacetal-bridged cycloadduct (7), produced in four steps from the 2 -I- 2-photocycloaddition of DMAD with l,4-dichlorobut-2-ene, reacts with DMAD to provide a synthesis of the Dewar-benzene structure (8) (Scheme 3). ... 

Gleiter and Treptow reported the synthesis of the first doubly bridged prismane derivatives 46a (n = 5) and 46b (n = 6). ° Irradiation (500 W high-pressure Hg lamp) of the Dewar benzene derivatives 45a (n = 5) at 250 nm in diethyl ether under Ar yields only polymeric material because of the instability of the initially formed [5]paracyclophane 47a, whereas irradiation at 280 nm gives the prismane 46a (15%, 72 h). When 45b (n = 6) was irradiated at 250 nm, rapid isomerization to the [6]paracyclophane 47b (100%, 1 h) took place increasing the wavelength to 280 nm led to the prismane 46b (30%, 24 h). 

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