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The heterocyclic cycloprop[f]indene 19 was synthesized via cycloaddition of diene 10 to the cyclopropene 11a and subsequent base-induced aromatization. While 19 is isolable, although very short-lived, the oxa analogue 18 decomposed under the conditions required for its preparation. The difluoro derivatives 14 and 15, in which the heterocyclic moiety is saturated, are accessible by the cycloaddition approach, but the corresponding dichlorides are again not isolable. Cycloprop[f]indenes carrying substituents at C(4) have been obtained via cycloaddition of 22b to 1-bromo-2-chlorocyclopropene. The key step of the sequence is a double Curtius degradation of the cycloadduct 23b to the ketone 27a. While aromatization of the alcohol 27b provided the cycloprop[f]indenol 28b, the reaction failed with 27a. Attempts to convert 28b to 1,3-dihydrocycloprop[f]indene (25) via the methanesulfonate 28d failed.

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.