A large amount of stable ecteinascidin 770 (1b) was isolated from the Thai tunicate, Ecteinascidia thurstoni, which was pretreated with potassium cyanide in buffer solution (pH 7), along with a minor metabolite, ecteinascidin 786 (1c). A number of 6'-O-acyl derivatives 3-19 and three diacetyl derivatives 2a-c of the stable 1b were prepared and evaluated for activity against human tumor cell lines HCT116, QG56, and DU145. Nitrogen-containing heterocyclic ester derivatives such as 12, 13, and 16-19 showed similar in vitro cytotoxicity to 1b, whereas the other derivatives were less cytotoxic than 1b. Furthermore, we discovered that the N-indole-3-carbonyl derivative of ecteinascidin 770 (22) has higher cytotoxicity than 1b.