A convenient synthetic method for preparing 2,5-disubstituted 1,6- methano[10]annulenes from 1,6-diacetylcyclohepta-1,3,5-triene

The reaction of 1,6-diacetylcyclohepta-1,3,5-triene with various Grignard reagents gave high yields of addition products 5, which were treated with a catalytic amount of p-toluenesulfonic acid or pyridinium p- toluenesulfonate to afford 2,5-disubstituted 2,3-dihydro-1,6- methano[10]annulenes 6 via 1,6-bis(1-substituted vinyl)cyclohepta-1,3,5- trienes 8 and 2,5-disubstituted 3,4-dihydro-1,6-methano[10]annulenes 9. On the other hand, reactions under more forcing conditions gave the rearrangement products: 4,7-disubstituted 1,2-benzocyclohepta-1,3-dienes 7. The oxidation of 6 with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone gave the 2,5-disubstituted 1,6-methano[10]annulens 4 in moderate-to-good yields. Conversion from 5 into 4 can be achieved by a one-flask procedure, providing a convenient synthetic method for preparing the title annulenes, particularly 2,5-diaryl ones. The reaction mechanism from 8 to 6, 7, and 9 is discussed on the basis of semiempirical molecular orbital calculations.