Preparation of chiral isobenzofurans from 3-carene in the presence of modified clays

Isomerization of 3-carene to the reaction mixture, containing 2-carene and its further utilization in the synthesis of chiral isobenzofurans, was investigated in the presence of acid-modified clays at 140 degrees C. The catalysts were characterized by XRF, FTIR with pyridine and nitrogen adsorption. Quantification of the effect of the type and concentration of acid sites in different clays comprising commercial montmorillonite and natural illite on the yield of 2-carene as well as by-products was obtained for the first time. It was established that selectivity to 2-carene decreased with an increase of the Lewis to Bronsted acid sites ratio. The largest amount of 2-carene in the mixture (15.0 wt%) with the highest ratio of 2-carene to menthadienes (0.63) was observed at 50% 3-carene conversion over a commercial montmorillonite K-30 clay, which has a moderate (100 gmol/g) acidity. It was demonstrated for the first time that the mixture containing 2-carene can be effectively used as a starting material in the reaction with vanillin resulting in chiral isobenzofurans. The products obtained in this reaction exhibit neuroprotective activity in an animal Parkinson's disease model. It was established that isobenzofurans yield decreased with an increase in the Lewis to Bronsted acid sites ratio. The largest yield of these products was obtained in the presence of a commercial K-10 clay (59.8%), which is higher than using 2-carene per se. Catalytic synthesis of isobenzofurans based on a readily available monoterpene 3-carene instead of expensive 2-carene was thus developed.