Isolation and structure determination of lyngbyastatin 3, a lyngbyastatin 1 homologue from the marine cyanobacterium Lyngbya majuscula. Determination of the configuration of the 4-amino-2,2-dimethyl-3-oxopentanoic acid unit in majusculamide C, dolastatin 12, lyngbyastatin 1, and lyngbyastatin 3 from