CONVERGENT TOTAL SYNTHESIS OF VINEOMYCINONE-B2 METHYL-ESTER AND ITS C(12)-EPIMER

Total syntheses of vineomycinone B2 methyl ester (7) and its C(12)-epimer (epi-7) have been completed. The key reaction for construction of the aryl C-glycoside linkage is the O --> C-glycoside rearrangement starting from D-olivosyl fluoride derivative 11 and anthrol derivative 21, which provides the regio- and stereocontrolled formation of the aryl C-glycoside sector of the target. The combination of Cp2HfCl2-AgClO4 serves as a particularly efficient promoter for this reaction. An extensive model study for attaching the side chain is presented. The Lochmann-Schlosser base cleanly effects ortho metalation of anthracene derivatives 19 and 20. The metalated species can be trapped as stannyl derivatives, from which the corresponding aryllithium species are generated by using n-BuLi or preferably MeLi in toluene. These specific reaction conditions are necessary to suppress the abnormal reaction of RLi reagents at the C(9)/C(10)-positions of the anthracenes. Coupling of the side chain moiety was efficiently carried out by such metalation of anthracene derivative 25 followed by reaction with chiral aldehyde (S)-29. The chiral aldehyde was derived from enantiomerically pure acid (S)-37 obtained by enzymatic kinetic resolution. Deoxygenation of the benzylic alcohol function followed by several steps allowed the total ynthesis of 7. Starting from (R)-aldehyde 29, the same sequence of reactions accomplished the total synthesis of epi-7. The epi series of intermediates provided firm evidence for the stereochemical homogeneity of synthetic 7.