Bridged and Annulated 1,2-Cydohexadienes

the resulting 119 was optically active. This suggests that the progenitor of 119 has the allene structure 117 rather than other conceivable constitutions [80, 81]. Even early quantum-chemical calculations on 117 showed a strongly bent, chiral structure, although the enantiomerization barrier was not correctly estimated [19]. 

The DMS method has not been employed yet for the generation of 117 and 123, since the dibromocarbene adducts of norbomadiene and norbornene rearrange under the usual conditions for the preparation [89]. However, they could be synthesized at -60 °C by taking advantage of tetrabromomethane and methyllithium as a source of the carbene [90] and could prove stable enough to serve as precursors of 117 and 123. On the other hand, the adducts of bromofluorocarbene to norborna-diene and norbornene having the fluorine atom in a cis-orientation should be isol-able at room temperature and hence be usable as stable precursors of 117 and 123. These variations ofthe DMS method were published on the occasion ofthe preparation of cycloadducts of l-oxa-2,3-cyclohexadiene (351) (Section 6.3.6) [35, 91], 1,2,4-cyclohexatriene (162) and 3d2-lJ-f-naphthalene (221) (Section 6.3.4) [35, 92], 

In previous experiments, Bald et al. [2] had tried to liberate 131 from 3-bromo-6,7-benzobicyclo[3.2.1]octa-2,6-diene (134) by /3-elimination of hydrogen bromide with KOtBu. Both the trapping products 140 of the intermediate with DPIBF and the enol ether 138 appeared to provide evidence in favor of 131, but did not exclude the cycloalkyne 136 [94]. The generation of 136 in the presence of DPIBF by two routes that cannot lead to 131 gave rise to cycloadducts different from 140, which were converted into 140 on treatment with KOtBu, however [95, 96], On the basis of these results and further investigations [97, 98], the formation of 131 from 134 is considered unlikely. Instead of 131, 136 seems to arise and to be the source of the products observed. In contrast, the phenyl derivative 137 of 131 is the obvious intermediate in the reaction of 135 with KOtBu, which furnished the enol ether 139 [2]. 

The reaction of bis(cydohex-l-en-l-yl)acetylene (155) with dimethyl fumarate, proceeding at 180 °C via 156 and yielding 157 [107d], was repeated by Miller et al. [106], who established the configuration of 157 and that of the analogous product from 

155 and maleic anhydride. In these processes, the intermediates such as 153 and 

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