Säurekatalysierte Umlagerungen von 1,5‐Dimethyl‐6‐methyliden‐tricyclo[3.2.1.0 2,7 ]oct‐3‐en‐8 endo ‐olen

Acid Catalysed Rearrangement of 1,5‐Dimethyl‐6‐methyliden‐tricyclo[3.2.1.0 2,7 ]oct‐3‐en‐8 endo ‐ols The tricyclic alcohols 2,3,4 and 6 (Scheme 1) are synthesized by the reaction of the tricyclic ketone 1 with sodiumborohydrid or metalloorganic reagents. Their configuration at C(8) is determined by NMR. in the presence of Eu(fod) 3 . The exo ‐attack of 1 by the nucleophil forming the endo ‐alcohol is favored, the π‐electrons of C(3) = C(4) hindering the endo ‐attack. On treatment with sulfuric acid in dioxane/water at 25° the tertiary alcohols yield aryl‐substituted ketones. 3 gives in 78.5% yield a mixture of the 3‐(dimethylphenyl)‐2‐butanones 12 and 13 , in addition to 16.5% of (2,3,4‐trimethylphenyl)‐2‐propanon ( 14 ) (Scheme 2) . The alcohols 4 and 6 yield mixtures of the 2‐(dimethylphenyl)‐3‐pentanones 19 and 20 (72%), and 2‐(dimethylphenyl)‐propiophenones 21 and 22 (68%), respectively (Scheme 2) . In the case of the secondary alcohol 2 mainly products derived from hydration at the C(6), C(9) double bond are formed, namely the mixture of diols 23 and 24 (21%), and the mixture of the isomeric 2‐(dimethylphenyl)propanals 25, 26 and 27 (3%) (Scheme 3) . ‐ The structures of 12–14, 19/20, 21/22, 23/24 and 25/26/27 were established by spectroscopic data. In the case of 12 and 13 the degradation of their mixture to the known 1‐(dimethylphenyl)ethanols 17/18 confirmed the assignment. ‐ The most probable mechanism for the rearrangement of 3 is shown in Schemes 4 and 5 . The reaction proceeds from 3 through a, b and g to 12 and 13; 14 is formed via e, f and i . In the case of 4 and 6 only the reaction analogue to 3 → a → b → g ⇉12/13 takes place. The isomeric aldehyds 25–27 formed from 2 could have the structures s, t , and v . The former two could be generated in a similar way as 12/13 from 3 , the latter one as shown in Scheme 8 .