An efficient one-pot synthesis of 3-substituted 1-phenyl-l,2dihydropentalenes from u,P-unsaturated ketones is described. These compounds can be rearranged to their 1,5-dihydro iso-mers using different methods such as acid catalysis and vacuum-flash thermolysis or by chromatography on alumina.
Dihydropentalenes" are versatile starting materials for the synthesis of complex molecules with a polyquinane structure2! Serveral methods have been applied to prepare these compounds either in their 1,2-dihydro or 1,Sdihydro form3). The well-known Hafner route'), which uses vinamidinium salts and cyclopentadienylsodium, leads to 6-(2-dialkylaminovinyl)pentafulvenes, which easily rearrange to 1-dialkylamino-I &dihydropentalenes in analogy to the electrocyclization of 6-vinylpentafulvene developed by Gajewski ' ). Only two reports describing the direct conversion of a,p-unsaturated carbonyl compounds into 1,2-or 1,5-dihydropentalenes are known. In one case the yields are very poor (< 5%)6', the other one describes special conditions (KF/DMSO) used in the synthesis of hexaphenyl-I ,2-dihydropentalene ' ).
A) CF3COOWCC1, B)550-600ยฐC/0.05 Tom C) A1203/CH,Cl,