cycloPentane—spiro-cyclopentanone has been synthesised from cyclopentane-I-carboxy­I-butyric acid, which has been synthesised in a way which leaves no doubt regarding its configuration. The lactone obtained by the reduction of the anhydride of cyciopentane-I-carboxy-I-acetic acid yields a bromo-ester which by the action of sodiomalonic ester and subsequent hydrolysis gives cyclopentane-I-carboxyI-butyric acid. This acid has been converted into the Spiro-ketone both by Dieckmann condensation of its ester, followed by hydrolysis, as also by its pyrogenetic decomposition in presence of baryta.