Constituents of the Japanese sea hare Dolabella auricularia collected in Mie Prefecture, Japan were examined by using bioassay, and new cytotoxic depsipeptides, dolastatins G (1) and H (14) and isodolastatin H (15) were isolated. Dolastatin G (1) showed cytotoxicity against HeLa-S_3 cells with an IC_<50> of 1.0μg/mL. On the basis of 2D NMR technique, dolastatin G (1) has proved to be a 35-membered cyclic depsipeptide which consists of a hexapeptide and two new hydroxy acids. The absolute stereochemistry of the hexapeptide moiety was determined by the chiral HPLC analysis of amino acids obtained by acidic hydrolysis of dolastatin G (1). The absolute stereochemistry of two hydroxy acid parts was determined by the enantioselective synthesis of two corresponding fragments obtained by degradation of dolastatin G (1). For the purpose of confirming the stereostructure of dolastatin G (1), synthetic studies on dolastatin G (1) have been carried out. Three subunits, 7, 10, and 11, were synthesized, coupling of which gave seco acid 13. The synthesis of dolastatin G (1) from seco acid 13 is in progress. A 1:1 mixture of dolastatin H (14) and isodolastatin H (15) showed potent cytotoxicity against HeLa-S_3 cells with an IC_<50> of 0.00381μg/mL. On the basis of spectroscopic data, dolastatin H (14) has proved to be a linear tetrapeptide which contains two unusual amino acids and is esterified at the C-terminus by the primary hydroxyl group of 3-phenyl-1,2-propanediol. Isodolastatin H (15) is the structural isomer of dolasatin H (15), in which the tetrapeptide is esterified at the C-terminus by the secondary hydroxyl group of 3-phenyl-1,2-propanediol. The absolute stereochemistry of dolastatin H (14) and isodolastatin H (15) was unambiguously determined by the enantioselective total synthesis.
