Total synthesis of the cyclic peptide hepatotoxin motuporin is described, including an efficient synthesis of the constituent amino acid Adda. Three strategies to motuporin are outlined with their relative strengths and weaknesses. Cyclization of the linear peptide precursor was found to proceed moderately well for peptides containing the N-methyldehydrobutyrine residue masked as a threonine, but significant C-terminal epimerization occurred in the presence of the dehydroamino acid. Replacement of the N-methyldehydrobutyrine residue by L-alanine was explored to assess the contribution of this dehydroamino acid to the biochemical activity of motuporin. Some epimerization also was observed during cyclization of the alanine-containing peptide. Synthetic motuporin and both isomers of 5-[L-Ala]-motuporin inhibit the activity of protein phosphatase-1 (PP1) in rat adipocyte lysates with comparable IC(50) values. These results indicate that the N-methyldehydrobutyrine residue is not essential for PP1 inhibition.