Synchronous and Zwitterionic Channels of the Reaction of (R)-5-Methyl-2-(propan-2-ylidene)cyclohexanone with 4-Phenyl-3H-1,2,4-triazole-3,5(4H)-dione

The reaction of pulegone with 4-phenyl-3H-1,2,4-triazole-3,5(4H)-dione as enophile gives two known epimers, 1-[(1R,4R)-4-methyl-2-oxo-1-(prop-1-en-2-yl)cyclohexyl]-4-phenyl-1,2,4-triazolidine-3,5-dione (RR isomer) and 1-[(1S,4R)-4-methyl-2-oxo-1-(prop-1-en-2-yl)cyclohexyl]-4-phenyl-1,2,4-triazolidine-3,5-dione (SR isomer). In addition, we isolated previously unknown 1-2-[(4R)-4-methyl-6-oxocyclohex-1-en-1-yl]propan-2-yl-4-phenyl-1,2,4-triazolidine-3,5-dione. According to the results of quantum chemical simulation, the isomeric products of hydrogen substitution in pulegone by the enophile are formed along three pathways. The SR isomer is formed via a concerted ene reaction. The other two channels involve approach of the enophile from the R side of pulegone and include two consecutive steps. The first step generates intermediate unsymmetrical cyclic aziridine cation, and elimination of proton from one of the geminal methyl groups of that intermediate yields the RR isomer with a terminal double bond. Proton elimination from methylene group of the pulegone fragment gives a cyclic compound with internal double bond. The ratio of the isomer with internal double bond and the RR isomer depends on the polarity of aprotic solvent.