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The structure of the synthetic 9-ethyl analogue C21H300 2 of vitamin A acid has been determined in order to establish its conformation. The crystals are monoclinic, space group P2 Jc, with a = 13.335, b = 18.347, c = 8.205 A, fl = 95.20 °, Z = 4. The structure was refined to an R of 5.7% for 1393 counter reflections. The chain is slightly more curved than that of vitamin A acid and the cyclohexene ring is rotated 64 ° out of the scis conformation. The ring shows some conformational disorder. 505