Total synthesis and evaluation of vinblastine analogues containing systematic deep-seated modifications in the vindoline subunit ring system: core redesign.

The total synthesis of a systematic series of vinblastine analogues that contain deep-seated structural modifications to the core ring system of the lower vindoline subunit is described. Complementary to the vindoline 6,5 DE ring system, compounds with 5,5, 6,6, and the reversed 5,6 membered DE ring systems were prepared. Both the natural cis and unnatural trans 6,6-membered ring systems proved accessible, with the latter representing a surprisingly effective class for analogue design. Following Fe(III)-promoted coupling with catharanthine and in situ oxidation to provide the corresponding vinblastine analogues, their evaluation provided unanticipated insights into how the structure of the vindoline subunit contributes to activity. Two potent analogues (81 and 44) possessing two different unprecedented modifications to the vindoline subunit core architecture were discovered that matched the potency of the comparison natural products and both lack the 6,7-double bond whose removal in vinblastine leads to a 100-fold drop in activity.