3aH-Indenes. Part 2. Cycloaddition reactions of 3-methoxy- and 3-trimethylsiloxy-3a-methyl-3aH-indene

Abstract

The title 3a-methyl-3aH-indenes (1a) and (1b) have been prepared from indan-1-one via the dienone (5) and the trienone (6), and the preparation of the key intermediates (5) and (6) has been improved. The 3aH-indene (1b) behaves similarly to the methoxy derivative (1a), and rearranges on heating to the 1H-isomer. Cycloaddition reactions of (1a) to N-phenylmaleimide and maleic anhydride give isolable [4 + 2]-adducts (12), which rearrange on heating to give the exo- and endo-[8 + 2]-adducts (13) and (14). In contrast, both 2-chloroacrylonitrile and 2-chloroacryloyl chloride give [8 + 2]-adducts with (1a). The 2-chloroacryloyl chloride adducts are readily converted into the tricyclic ketone (23). The trimethylsiloxy 3aH-indene (1b) undergoes cycloaddition reactions with dimethyl acetylenedicarboxylate, 2-chloroacryloyl chloride, and dichloroketene. The trimethylsilyl derivative (1b) offers some advantages over the methoxyindene (1a) in synthetic work.