dl-1-(1-Oxo-3,4-threo-3,4,5-trihydroxy-1-pentyl)-β-carboline 16a was synthesized from 1-formyl-β-carboline in 13 steps. The prepared compound is one of the diastereomers of an alkaloid 3 produced by the inter-generic somatic hybrid cell culture of Rauwolfia serpentina Benth and Rhazya stricta Decaisne (family: Apocynaceae). The N9-benzyl and N9-methyl derivatives 16b,c were also prepared. The final compounds and some of the intermediates showed cytotoxic activity against human promyelocytic leukemia cells HL 60 and/or human diploid embryonic lung fibroblast cells.