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Substituent Effect in Color of Ehrlich's Test of Tetrahydrobenzofuran

Reaction of 4,5,6,7-tetrahydrobenzofuran derivatives, model compounds of furanoeremophilanes, with p-dimethylamino-benzaldehyde (Ehrlich's reagent) under acidic conditions was studied both on TLC and in a flask. 4-Hydroxy, 4-methoxy, and 4-acetoxy derivatives showed a pink/purple color, while 7-hydroxy, 7-methoxy, and 7-acetoxy derivatives showed a dark green color. The reaction of a 4-oxo derivative was slow showing an orange color on TLC. 7-Oxo derivatives and benzofuran were negative to Ehrlich's test. It was shown that an intermediate cation corresponding to the pink color was generated from both 4- and 7-substituted derivatives except for the oxo-derivatives.