Modular Total Synthesis and Cell-Based Anticancer Activity Evaluation of Ouabagenin and Other Cardiotonic Steroids with Varying Degrees of Oxygenation

Abstract

A Cu(II)-catalyzed diastereoselective Michael/aldol cascade approach is used to accomplish concise total syntheses of cardiotonic steroids with varying degrees of oxygenation including cardenolides ouabagenin, sarmentologenin, 19-hydroxysarmentogenin, and 5-epi-panogenin. These syntheses enabled the subsequent SAR studies on 37 synthetic and natural steroids to elucidate the effect of oxygenation, stereochemistry, C3-glycosylation and C17-heterocyclic ring. Based on this parallel evaluation of synthetic and natural steroids and their derivatives, glycosylated steroids cannogenol-L-α-rhamnoside (79a), strophanthidol-L-α-rhamnoside (92), and digitoxigenin-L-α-rhamnoside (97) were identified as the most potent steroids demonstrating broad anticancer activity at 10–100 nM concentrations and selectivity (nontoxic at 3 μM against NIH-3T3, MEF and developing fish embryos). Further analyses indicate that these molecules show a general mode of anticancer activity involving DNA damage upregulation that subsequently induces apoptosis.

Recent Progress in Steroid Synthesis Triggered by the Emergence of New Catalytic Methods

“…Despite these numerous studies, the synthesis of highly oxygenated cardenolides, such as ouabagenin, represents a significant challenge, and only recent advances in catalysis and synthetic methodology allowed to overcome some of these challenges. Such landmark efforts include the recently disclosed syntheses of ouabagenin and 19‐hydroxysarmentogenin by the Deslongchamps, Baran, and Inoue groups as well as the studies by the Nagorny group that will be the focus of this mini‐review…”

“…Based on these findings, in 2015 Nagorny group demonstrated that the formation of 130 is possible in a highly enantioselective and diastereoselective fashion from simple and readily available building blocks 127 and 1 28 (Scheme ). [49g] Thus, it was demonstrated that readily available chiral bis (oxazoline) copper(II) complex 129 could promote Michael reactions with high levels of stereocontrol and the stereochemistry required for their subsequent elaboration to natural cardiotonic steroids (Scheme A). These Michael adducts could be subjected to various acidic or basic conditions that promote the intramolecular aldolization and provide various steroid‐like cores 130A‐J .…”

“…These studies were subsequently translated into the synthesis of more complex cardenolide cores 146 and 147 (Scheme ) that were later elaborated to a variety of natural cardenolides. [49a, d, e], The modified approach required adjusting the oxidation state of both the β‐keto ester and enone fragments in order to install the C3 and, optionally, C11 oxygenation. To address the problems associated with the installation of the C3 stereochemistry, vinyl chloride‐containing β‐keto ester 137 was employed.…”

“…Enantioselective and diastereoselective Cu(II)‐catalyzed Michael/aldol cascade reactions for the synthesis of functionalized cardenolides cores 1 46 and 147 . [49a, d, e]…”

“…The aforementioned strategy for the assembly of ouabagenin could be generally applied to a variety of other cardiotonic steroids (Scheme ). [49c], [49d] Thus, key intermediate 147 was successfully elaborated to diastereomeric cardenolides trewianin aglycon (12 steps, 14 % yield),[49a], [9d] 19‐hydroxysarmentogenin (12 steps, 8 % yield),[49a], [9d] panogenin (9 steps, 5 % yield)[49a] and 5‐ epi ‐panogenin (9 steps, 23 % yield)[49a], [49d] that have different configurations at the C5 and C11 stereocenters. In addition, these efforts provided sarmentologenin (14 steps, 7 % yield) that lacks C1‐oxygenation in comparison to ouabagenin.…”