Since S-(−)-α-isopropyl-λ-acetobutyric acid (XVII) was obtained by degradation of 10- hydroxy-2, 5-dimethyl-8-isopropyl-decalone-1 (IX) derived from guaiol (I), the C-7 hydroxyisopropyl group in guaiol possesses the (α-configuration and guaiol should be represented by XVIII.