Résumé : 1,3‐dimethyl‐4,5‐methylenephenanthrene has been synthesized in two different ways, starting from 1‐acenaphtylsuccinic anhydride. A stereochemical interpretation of anomalous reactions is proposed. Copyright © 1961 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim