Mitragyna speciosa Korth. (Rubiaceae) is a tropical plant indigenous to Thailand and the Malay Peninsula, whose leaves produce narcotic-like actions when smoked or chewed and have been traditionally used as a stimulant or opium substitute. Previous chemical and pharmacological studies demonstrated potent analgesic activity (due to opioid agonistic property) of its major indole alkaloid mitragynine and pseudoindoxyl derivatives, prompting investigation of the constituents in Malaysian M. speciosa. From the ethyl acetate extract of young leaves of M. speciosa collected in Malaysia, a new alkaloid (1) was isolated together with six known Corynanthe-type indole alkaloids. The structure of the novel monoterpenoid indole alkaloid 1, named mitralactonine, was elucidated by spectroscopic analysis as having a highly conjugated pentacyclic Corynanthe skeleton, representing the first Corynanthe-type indole alkaloid carrying an oxygen function at the C20 position. To establish the structure including the absolute configuration at the chiral C20 center, asymmetric total synthesis was performed: chiral epoxy ketone 5 was prepared via (S)-oxazaborolidine-catalyzed reduction and Sharpless asymmetric epoxidation, and the target compound was synthesized, whose chromatographic behavior, UV, 1H and 13C NMR, and mass spectra matched those of the natural product. Chiral HPLC analysis revealed that natural mitralactonine is a mixture slightly enriched with the (-)-enantiomer (ratio 54:46). Further chemical investigation on the components in Malaysian M. speciosa and pharmacological evaluation of these compounds are in progress.