Synthetic Approach Towards Hexaprismane. A Novel Entry to Homosecohexaprismane Skeleton by Cage Enlargement

A series of caged 1,4-diols, 26, 29a/29b, 31 , were synthesized from the Diels-Alder cycloadduct 21 of 1,2,3,4-tetrachloro-5,5-dimethoxycyclopentadiene and 1,4-benzoquinone. Reduction of enedione 21 with aqueous TiCl₃, followed by base-catalyzed enolization in the presence of acetic anhydride, gave diene 23b , which reacted with maleic anhydride and 1,4-benzoquinone to produce the corresponding [4+2] cycloadducts 24b and 27 , respectively. The adduct 24b was converted to birdcaged 1,4-diol 26 by photocyclization followed by decarboxylative olefination of resulted caged anhydride 25b . The adduct 27 was photocyclized to cage compound 28 , which was aromatized to dihydroquinone 29a by acid-catalyzed enolization, or benzo-annulated compound 31 by reduction and dehydration. The birdcaged 1,4-diols 26 and 31 underwent fragmentation induced by (diacetoxyiodo)benzene to give enediones 38/39 and 41/42 , respectively. Photocyclization of 38 and 39 produced the corresponding caged compounds 43 and 44 , respectively, possessing homosecohexaprismane skeletons. The corresponding monohydrate 43a of 43 was structurally characterized by single-crystal X-ray diffraction.