A novel entry to cyclopenta[b]quinolines via thermal ring-expansion of (2-aminophenyl)-ethynyl-substituted squaric acid derivatives

Substituted cyclopenta[b]quinolin-1-ones were prepared by thermal ring-expansion of substituted N-Boc protected 4-(2-aminophenylethynyl)-4-hydroxy-2-cyclobuten-1-ones forming the corresponding 2-aminophenylmethylidene substituted 4-cyclopentene-1,3-diones. Deprotection of the amine resulted in spontaneous condensation giving cyclopenta[b]quinolin-1-ones. Sodium borohydride reduction of these products produced cyclopenta[b]quinolin-1-ols.