Anomalous Direction of the Ene Reaction of Pulegone with 4-Phenyl-3 H -1,2,4-triazole-3,5(4 H )-dione

The ene reaction of pulegone with 4-phenyl-3 H -1,2,4-triazole-3,5(4 H )-dione, apart from the two previously described products, epimeric 1-[(1 S ,4 R )- and 1-[(1 R ,4 R )-4-methyl-2-oxo-1-(prop-1-en-2-yl)cyclohexyl]-4-phenyl-1,2,4-triazolidine-3,5-diones, gave one more isomer, 1-{1-methyl-1-[(4 R )-4-methyl-6-oxocyclohex-1-en-1-yl]ethyl}-4-phenyl-1,2,4-triazolidine-3,5-dione. The latter is formed as a result of addition of 4-phenyl-3 H -1,2,4-triazole-3,5(4 H )-dione to the Me 2 C= carbon atom of pulegone (anti-Markovnikov addition) and double bond migration to the six-membered carbocycle.