Discovery of highly potent analgesic activity of isopulegol-derived (2R,4aR,7R,8aR)-4,7-dimethyl-2-(thiophen-2-yl)octahydro-2H-chromen-4-ol

A large set of chiral heterocyclic compounds with the octahydro-2H-chromene scaffold was first obtained by a reaction of (−)-isopulegol and (+)-neoisopulegol with furan-2-carbaldehyde, thiophene-2-carbaldehyde and their derivatives and isomers in the presence of montmorillonite K10 clay. Most of the (−)-isopulegol-derived compounds exhibited a significant analgesic activity in the acetic acid-induced writhing test. Compound 3b obtained by a reaction of (−)-isopulegol with thiophene-2-carbaldehyde demonstrated a significant analgesic effect in this test within 15 min after oral administration at the dose of 1 mg/kg and retains the effect for at least 24 h. Compound 3b exhibited analgesic activity in the hot plate test also. A change in the sulfur atom position in the aromatic ring was found to lead to the effect reversal in the hot plate test.