A convergent total synthesis of the kedarcidin chromophore: 20-years in the making.

Abstract

The kedarcidin chromophore is a formidible target for total synthesis. Herein, we describe a viable synthesis of this highly unstable natural product. This entailed the early introduction and gram-scale synthesis of 2-deoxysugar conjugates of both L-mycarose and L-kedarosamine. Key advances include: (1) stereoselective allenylzinc keto-addition to form an epoxyalkyne; (2) α-selective glycosylations with 2-deoxy thioglycosides (AgPF6/DTBMP) and Schmidt donors (TiCl4); (3) Mitsunobu aryl etherification to install a hindered 1,2-cis-configuration; (4) atropselective and convergent Sonogashira-Shiina cyclization sequence; (5) Ohfune-based amidation protocol for naphthoic acid; (6) Ce(III)-mediated nine-membered enediyne cyclization and ester/mesylate derivatisation; (7) SmI2-based reductive olefination and global HF-deprotection end-game. The longest linear sequence from gram-scale intermediates is 17-steps, and HRMS data of the synthetic natural product was obtained for the first time.