Convergent synthesis of (-)-mirabazole C using a chloroimidazolidium coupling reagent, CIP

Convergent synthesis of (-)-mirabazole C (1), a tetra thiazoline/thiazole alkaloid isolated from blue-green alga, has been described. The successive thiazoline rings of (-)-mirabazole C were formed by a single-step cyclization mediated by TiCl4 treatment of tripeptide amide 4. Convergent synthesis of the key intermediate 33 derived from three a-methylcysteine residues was first achieved using a newly developed coupling reagent, 2-chloro-1,3-dimethylimidazolidium hexafluorophosphate (CIP). The effectiveness of CIP for the coupling of alpha,alpha-dialkyl amino acids and the reaction pathway of the activation were clarified by the syntheses of model peptides containing an alpha,alpha-dimethylamino acid. A practical method of asymmetric synthesis of 2-methylcysteine by alkylation of 2,4-cis-oxazolidinone 23 has also been described.