2-Halomethyl-, 2-alkoxymethyl- and 2-acyloxymethyl-3-methyl-3-formyl-bicyclo[2.2.1]heptane and -hept-5-ene and a process for their preparation. The new compounds are obtained by reacting cyclopentadiene or dicyclopentadiene with the corresponding 4-halo-, 4-alkoxy- or 4-acyloxy-2-methyl-crotonaldehydes under the conventional conditions for Diels-Alder reactions, with or without subsequent catalytic hydrogenation of the Diels-Alder adduct obtained. The new compounds provide an industrially easily realized and advantageous method of synthesis of the sought-after essential ingredients of natural sandalwood oils.