Rigid polycyclic molecules having two isolated double bonds located in the laticyclic topology 1 and spatially in close proximity have provided suitable frameworks for the study of transannular interactions and reactions. 2 In the aspect of chemical reactions, face-proximate double bonds in these systems may undergo transannular reactions by three modes (Chart 1). Photochemical [2 + 2] cycloadditions to produce cyclobutane rings (closure, O-type) are very facile and well known. 3 Stepwise additions to double bonds usually lead to the transannular bridge formation in either cross (N-type) or parallel (U-type) manner or both. 4 We recently reported that transannular bromination of endo-5,12:endo-6,11-dietheno-5,5a,6,11,11a,12-hexahydronaphthacene (1) produced both N-type and U-type adducts. 5 Our continuous interest in the transannular reactions of rigid polycyclic hydrocarbons containing π-face-proximate double bonds 6 led us to prepare the tetrahydrofuran-grafted syn-sesterbicyclo-[2.2.2]octene 2 7 which contains three parallel, face-to-face double bonds. Herein, we report the results of the synthesis, transannular photocyclization and bromination of 2, and the X-ray crystal structure of the resulting bromination product.The synthesis of 2 from readily accessible chlorinated tetracyclic diol 3 is outlined in Scheme 1. The underlying synthetic approach is that a dechlorination-deacetalization-decarbonylation process 6 is employed to unmask the cyclohexadiene moiety in 3 or 7 (e.g. 7 f 8 f 6 f 5) for connecting another cyclohexadiene moiety by Diels-Alder reaction using p-benzoquinone as an equivalent (5/6 f 9 f f 12), setting up for subsequent construction of the third bicyclo[2.2.2]octene substructure by another Diels-Alder reaction with maleic anhydride. Thus, chlorinated tetracyclic diol 3 was converted by our previously reported procedure 6d to keto diol 4 which was then subjected to acid-catalyzed dehydration in toluene in the presence of triethyl orthoformate at 100°C to produce a mixture of decarbonylated product 5 and dienone 6 in 65% and 25% yields, respectively. Increasing the reaction temperature or elongation of the reaction time did not improve the yield of 5. Alternatively, dienone 6 was prepared in an overall yield of 70% from 3 by dehydration of tetracyclic diol 3 followed by dechlorination with sodium/tert-butyl alcohol in THF and deacetalization with 3 M HClO 4 in CH 2 Cl 2 . Reaction of dienone 6 with p-benzoquinone in refluxing benzene resulted in slow decarbonylation of 6 (formation of 5), followed by cycloaddition to furnish single cycloadduct 9 in nearly quantitative yield. The syn orientation of the cyclohexendione moiety with respect to the bridged CdC double bond is expected based on the Alder rule, and as observed in the Diels-Alder reactions of related 1,3cyclohexadienes with substituted dienophiles. 6 Reduction of 9 with sodium borohydride/cerium chloride in methanol at 0°C gave enediol 10 in 89% yield. Conversion of enediol 10 to the corresponding dimesylate 11 with methanesulfo...
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