Stereochemie der Addition von Phenylmetall‐Reagenzien an Bicyclo[3.3.0]octandione und ‐octadiendione. — Synthese phenylsubstituierter Bicyclo[3.3.0]octadiene

Stereochemistry of the Addition of Phenylmetal Reagents to Bicyclo[3.3.0]octanediones and ‐octadienediones. — Synthesis of Phenyl‐Substituted Bicyclo[3.3.0]octadienes Conjugate addition of dilithium diphenylcyanocuprate is observed only on one side of the dienedione 4b , affording the enedione 5b , but on both sides in the presence of boron trifluoride yielding the diphenyldione 6b . Likewise, the latter reagent gives rise to the formation of 6a from 4a . Phenylcerium(III) dichloride is much superior to phenyllithium and phenylmagnesium bromide in the 1,2 addition reaction to encumbered, enolizeable bicyclo[3.3.0]octanediones. Thus, a mixture of the diols exo,exo ‐ 8 , exo,endo ‐ 8 , and endo,endo ‐ 8 is formed in almost quantitative yield from the 3,7‐dione 7 . Furthermore, the diphenyldione 6a is transformed into the tetraphenyldiol 14 in this way and in high yield. The 2,6‐dione 11b adds phenylcerium(III) dichloride affording a mixture of the hydroxyketone endo ‐ 16 , which is inert towards an excess of the reagent, and the diol exo,endo ‐ 12b . The inertness of endo ‐ 16 is attributed to the formation of the cyclic hemiacetal 20 which eventually is trapped quantitatively by O ‐silylation with chlorotrimethylsilane in the presence of pyridine. The hydroxyketone endo ‐ 16 is dehydrated to give the enone 17 which subsequently is treated with phenylcerium(III) dichloride to form exo ‐ and endo ‐ 19 . A solution of sulfuric acid in acetic acid dehydrates the tertiary benzyl alcohols 8 (→ 9 + 10 ), endo,endo ‐ 12a (→ 13a ), exo,endo ‐ 12b (→ 13b ), 14 (→ 15 ), endo ‐ 16 (→ 17 ), and 19 (→ 13b ) in high yields. The same results are achieved even more conveniently with a solution of chlorotrimethylsilane in dichloromethane in the absence of a base. The oxanortwistane 18 is obtained as a byproduct in the dehydration of exo,endo ‐ 12b with both reagents. The gross structures of all compounds are based on spectroscopic evidence, in particular NMR spectra. Concentration‐independent IR and low‐field NMR absorptions of two equivalent hydroxy groups are indicative of intramolecular hydrogen bonds and hence the endo , endo configuration of the diols endo,endo ‐ 8 , endo,endo ‐ 12a , and 14 . Eventually, the configurations of 5b , 6b , endo,endo ‐ 8 , endo,endo ‐ 12a , exo,endo ‐ 12b , 14 , 15 , endo ‐ 16 , and the structure of 18 as well, are established by X‐ray diffraction analyses. In the solid state, the bicyclo[3.3.0]‐octanols endo,endo ‐ 12a , exo , endo ‐ 12b , 14 , and endo ‐ 16 adopt conformations that allow the phenyl groups the occupation of quasi equatorial positions. The optimized procedures described open high‐yield routes to phenyl‐substituted bicyclo‐[3.3.0]octadienes starting from readily available bicyclooctanediones.