Lewis acid mediated diastereoselective synthesis of fused fluorinated spiroketal as potential biologically active compounds

A series of substituted dibenzalacetones prepared using standard procedures were condensed with 5-methyl, 5-phenyl, and 5-trifluoromethyl-1,3-cyclohexanediones respectively, in toluene containing BF3·OEt as the Lewis acid catalyst. The reaction was found to be highly diastereoselective (dr, 9:1). The resulting spiroketals 1a-r were formed in moderate to good yields. In addition, a synthetically and biologically useful by-product identified as chromenone 7 was observed.