Synthese, Struktur und Reaktionen Phenylethinyl‐substituierter Bicyclo[3.3.0]octane

Syntheses, Structures, and Reactions of Phenylethynyl‐Substituted Bicyclo[3.3.0]octanes [1] The bicyclo[3.3.0]octane‐3,7‐dione 5 adds phenylethynyl‐cerium(III) dichloride to afford an almost quantitative yield of the 3,7‐diols exo,exo‐, exo,endo ‐, and endo,endo ‐ 6 (1:2:2) which are separated by flash chromatography. While both exo,endo ‐ and endo,endo ‐ 6 are dehydrated by sulfuric acid in acetic acid to furnish the isomeric dienes C 2 ‐ and C s ‐ 7 , exo,exo ‐ 6 rearranges to a bicyclic ketone ( 8 ), two tricyclic ketones ( 10a, b ), and a tetracyclic ketone ( 12 ) as a consequence of the proximity of the endo ‐phenylethynyl groups. The rearrangements of exo,exo ‐ 6 are avoided when chlorotrimethylsilane is used as the dehydrating agent. Thus, a 90% yield of C 2 ‐ and C s ‐ 7 is realized from the mixture of the 3,7‐diols 6 . – The 2,6‐dione 13 reacts with phenylethynylcerium(III) dichloride to yield the hydroxyketone endo ‐ 14 and the 2,6‐diols exo,endo ‐ and endo,endo ‐ 16 (1:1:3) which are separated by cyclic medium‐pressure liquid chromatography. Dehydration of endo ‐ 14 furnishes the enone 15 which adds phenylethynylcerium(III) dichloride to afford the endo alcohol 17 . Eventually, this is dehydrated to the diene 18 . Formation of 18 from exo,endo ‐ 16 , and endo,endo ‐ 16 as well, is accompanied by dehydrating cyclization to the oxanortwistane 19 (20%) readily separated from 18 by chromatography. – The encumbered diphenyldione 20 is converted into the highly substituted bicyclo[3.3.0]octa‐2,6‐diene 24 in 68% overall yield in a four‐step sequence involving additions of phenylethinyl‐cerium(III) dichloride ( 20→21 and 22→23 ) followed by dehydration of the endo alcohols 21 (→22 ) and 23 (→24 ). – In contrast to 20 , the dicyanodiketone 25 adds phenylethynylcerium(III) dichloride at both carbonyl groups and, surprisingly, from the endo face, thus affording the exo,exo ‐diol 26 . At 150°C, 26 undergoes quantitatively an intramolecular dehydro Diels‐Alder reaction to yield the 1‐phenylnaphthalene derivative. 30 . The lack of incorporation of deuterium from deuterated solvents attests to the intramolecular nature of the 1,5‐hydrogen shift converting the intermediate six‐membered cyclic cumulene 29 to the final product 30 . – Dehydration of 26 produces the highly substituted bicyclo[3.3.0]octadiene‐2,6‐dicarbonitrile 27 which rearranges into an equilibrium mixture of the diastereomeric bisallenes exo,exo‐, exo,endo ‐, and endo,endo ‐ 28 on treatment with a strong base. In the same way, the identical mixture of diastereomers is obtained from each of the bisallenes 28 which had been separated by preparative medium‐pressure liquid chromatography. – The gross structures of all new compounds are based on spectroscopic evidence including IR, NMR, and mass spectra. The structures of endo,endo ‐ 6, 12 , exo ‐ 14 , endo,endo ‐ 16 (α‐ and β‐form), 19, 24, 26 · 2 Me 2 SO, 27 , exo,exo ‐ 28 , and endo,endo ‐ 28 are established by X‐ray diffraction analyses. The preferred conformations of exo ‐ 14 , endo,endo ‐ 16 (α‐ and β‐form), and 26 · 2 Me 2 SO in the crystalline state, as well as the stereochemistry of the nucleophilic attack to bicyclo[3.3.0]octanediones, and the mechanisms of the intramolecular reactions between the phenylethynyl groups are discussed.