The Syntheses of 16a‘-homo-Leurosidine and 16a‘-homo-Vinblastine. Generation of Atropisomers
journal contributionposted on 2001-07-07, 00:00 authored by Martin E. Kuehne, Yong Qin, Anne E. Huot, Susan L. Bane
The synthesis of 16a‘-homo-leurosidine was achieved through enantioselective generation of a ring D‘-seco-precursor 33 (without requirement of a chiral auxiliary). Its cyclization provided the Nb‘-quaternary salt 35 with a configuration corresponding to the atropisomeric form 8a rather than 8b of the target product. On debenzylation, the amine 8a was obtained and found not to isomerize thermally to the anticipated atropisomer 8b (in contrast to its lower homologue, with its formation of natural leurosidine). However, on protonation, a 1:1 mixture of atropisomers of 16a‘-homo-leurosidine was obtained. A synthesis of 16a‘-homo-vinblastine provided two atropisomers 5a and 5b for the free base at equilibrium (1:2.3 at room temperature in CDCl3), with a shift to the major conformer 5b with increasing solvent acidity or decreasing temperature. The synthesis was achieved through a stereoselective inversion of the tertiary hydroxyl function in the enantioselectively generated C-20‘ progenitor 39.