TOTAL SYNTHESIS OF THE MACROLIDE ANTITUMOR ANTIBIOTIC LANKACIDIN-C

The first total synthesis of natural (-)-lankacidin C (I) has been achieved by a convergent, enantioselective sequence starting from D-arabinose and L-aspartic acid, proceeding through the tricyclic carbamate 15 as an advanced relay intermediate. Specifically, the beta-lactam diene intermediate 41 is acylated by the thiopyridyl ester 34c. The resulting beta-ketolactam 42 is stereospecifically reduced by KEt(3)BH to carbinol 43, which on desilylation undergoes acid-catalyzed N --> O acyl migration to yield the delta-lactone 44. The derived iodo aldehyde 46 undergoes Stille coupling to give tetraene 54a, which upon Stork-Takahashi cyclization to ketone 56 and CBS reduction gives the key relay 15. N-acylation of the latter, and then regioselective carbamate scission followed by Dess-Martin oxidation, produces the target antibiotic (-)-lankacidin C (1).