Total Syntheses of the Thiopeptides Amythiamicin C and D

The thiopeptides amythiamicin C and D were synthesized by employing amide bond formation, a Stille cross-coupling reaction, and two Negishi cross-coupling reactions as key transformations. The central 2,3,6-trisubstituted pyridine ring of the target compounds was introduced as a 2,6-dibromo-3-iodopyridine, which was selectively metalated at the 3-position and connected to the complete Southern fragment of the amythiamicins by a Negishi cross-coupling. For the synthesis of amythiamicin C, this step was followed by a Negishi cross-coupling at C-6 of the pyridine core. Subsequent attachment of the Eastern fragment was achieved by amide bond formation and macrolactam ring closure by a Stille cross-coupling at C-2. The Eastern bithiazole fragment of the amythiamins was constructed also by regioselective metalation and cross-coupling reactions. The pivotal step involved the diastereoselective addition of 4-bromo-thiazole-2-magnesium bromide to a chiral sulfinyl imine. For the synthesis of amythiamicin D, the order of cross-coupling at C-6, amide bond formation, and cross-coupling at C-2 was changed. The amide bond formation to the Eastern fragment was performed first and it was subsequently attempted to close the macrolactam by an intramolecular regioselective Stille cross-coupling at C-2. Despite the low regioselectivity of this reaction it paved the way to the immediate completion of the amythiamicin D synthesis when followed by a Negishi cross-coupling at C-6 with 2-zincated methyl thiazole-5-carboxylate.