Synthesis of Functionalized Cycloprop[<i>f</i>]indenes <i>via</i> the Carbene Addition Route

The synthesis of 4,5-dihydro-1H,3H-cycloprop[f]inden-4-ol (1) and diethyl 4,5-dihydro-1H,3H-cycloprop[f]-indene-4,4-dicarboxylate (26) starting from diene 4 is described. The cyclopentene ring is constructed by condensation of diethyl malonate to the dibromide 21. The key-step in the synthesis of 1 consists in a twofold Curtius degradation of 22, with subsequent reduction of the carbonyl group and aromatization. The skeleton of the isomer 31 is synthesized via cycloaddition of butadiene to cyclopent-4-ene-1,3-dione (7) and addition of dichlorocarbene to the adduct 27 after ketalisation. The attempted synthesis of dihydrocycloprop[f]indene (2) by base-induced elimination of several appropriately substituted precursors failed