Synthesis, conformational analysis and transannular reactions of 5,9-propanobenzo[7]annulene derivatives

(5 alpha,9 alpha,11 beta)-11-Hydroxy-6,7,8,9-tetrahydro-5H-5,9-propanobenzo[7]annulen-7-one ethylene ketal (6a) and its 11 alpha-methyl derivative (6b) were prepared from monoketal 3. These compounds underwent acid-catalyzed transannular reactions leading to 6,7,8,9-tetrahydro-5H-5,9-propanobenzo[7]annulene derivatives 5a, 8a and 5b, 8b, respectively, depending on the reaction conditions. The compounds 6a and 6b were dehydrated to 6,7,8,9-tetrahydro-5H-5,9-prop[1]enobenzo[7]annulen-7-one (9a) and its 11-methyl derivative (9b), respectively. The conformational analysis of the 5,9-propanobenzo[7]annulene derivatives by molecular mechanics calculations (MM3 program) and the H-1 NMR data show that hydroxyketal 6a and the related compound (5 alpha,7 beta,9 alpha)-6,7,8,9-tetrahydro-5H-5,9-propanobenzo[7]annulen-7-ol (4) exist mainly in the boat-chair conformation with the boat cycloheptenol ring, while for hydroxyketal 6b the chair-bent conformation (chair cycloheptenol ring) seems to be the preferred one.