TOTAL SYNTHESIS OF GALANTIN-I - ACID-CATALYZED CYCLIZATION OF GALANTINIC ACID

The proposed structure of galantin 1, a peptide antibiotic isolated from Bacillus pulvifaciens as a mixture of congeners (1a with the D-ornithine residue and lb with D-lysine; 1a/1b = 9/1), was shown to be incorrect by total synthesis. The substructure 3a, named galantinic acid, was an artifact, and its correct structure was assumed to be the hydroxylated form, 20a or 20b, by spectroscopic comparisons of synthetic la with natural galantin I. The synthesis of both diastereomers, 4a and 4b, again suggested that the sequence of the spermidine moiety of galantin I should be N5,N8. Finally, the correct structure of galantin I as 5a was confirmed by the synthesis of diastereomers 5a and 5b. The synthesis of 5a was accomplished in a convergent manner by the coupling of the protected forms of the constituent amino acids: D-ornithine, 6b, D-alanine, 23b, 11b, 8c, and 19a. Galantinic acid residue 20a, present in natural galantin 1, was found to undergo cyclization with retention of its C3 configuration under the chemical degradation conditions to give the artifact 3a. In order to elucidate the mode of cyclization of 20a to 3a, the synthesis of 20a and its analogues was accomplished in a stereoselective manner from D-serine. The synthesis was characterized by the stereoselective epoxidation of hydroxymethyl (Z)-allylamine 34 and alpha,beta-unsaturated delta-lactone 39. Acidolysis of 20a, 20b, and their analogues suggested that the stereoselective cyclization of galantinic acid was initiated by the formation of delta-lactone 54, which through the sequence of reactions should afford the artifact 3a.