Structure Elucidation of a Pungent Compound in Black Cardamom: Amomum tsao-ko Crevost et Lemarié (Zingiberaceae)

Natural plant extracts containing taste modifier compounds will gain more commercial interest in the future. Black cardamom, Amomum tsao-ko Crevost et Lemarié, used as a spice in Asia, produces a nice refreshing effect in the mouth. Therefore, an ethyl acetate extract was prepared, and constituents were separated by liquid chromatography. Guided by the tasting of each fraction (LC tasting), a new pungent compound was discovered, (±)-trans-2,3,3a,7a-tetrahydro-1H-indene-4-carbaldehyde. To confirm this new structure, a synthesis was performed starting from cyclopentene-1-carbaldehyde. The Wittig conditions were determined to control the stereochemistry of the ring fusion to prepare (±)-trans-(2,3,3a,7a-tetrahydro-1H-inden-4-yl) methanol and (±)-cis-(2,3,3a,7a-tetrahydro-1H-inden-4-yl) methanol. After oxidation, (±)-trans-2,3,3a,7a-tetrahydro-1H-indene-4-carbaldehyde and (±)-cis-2,3,3a,7a-tetrahydro-1H-indene-4-carbaldehyde were tasted in water and only the trans-2,3,3a,7a-tetrahydro-1H-indene-4-carbaldehyde, present in black cardamom, produced a trigeminal effect in the mouth.