Revised structures and biosynthetic studies of tantazoles A and B

Revised structures are presented for tantazoles A ( 3 ) and B ( 4 ) and preliminary studies on their biosyntheses in the terrestrial blue-green alga Scytonema mirabile BY-8-1 are reported. Marfey analysis of the acid hydrolyzates shows that the absolute configuration of C4 in thiazoline ring A of the two alkaloids is R . Feeding experiments with [1,2- 13 C]acetate, [1,2- 13 C]glycine, [2- 13 C, 15 N]glycine and [ methyl - 13 C]methionine indicate that 3 and 4 are modified pentapeptides assembled from acetate, four cysteines and a threonine and that the methyl groups on the four thiazoline rings, the amide nitrogen, and the methine attached to ring A originate from the tetrahydrofolate C 1 pool via methionine.