Synthesis and Biological Evaluation of Kibdelone C and Its Simplified Derivatives

Abstract

Poylcyclic tetrahydroxanthones comprise a large class of cytototoxic natural products. No mechanism of action has been described for any member of the family. We report the synthesis of kibdelone C and several simplified analogs. Both enantiomers of kibdeleone C show low nanomolar cytotoxicity toward multiple human cancer cell lines. Moreover, several simplified derivatives with improved chemical stability display higher activity than the natural product itself. In vitro studies rule out interaction with DNA or inhibition of topoisomerase, both of which are common modes of action for polycyclic aromatic compounds. However, celluar studies reveal that kibdelone C and its simplified derivatives disrupt the actin cytoseketon without directly binding actin or affecting its polymerization in vitro.

Figure 1

Dose-response curves of enantiomeric forms of kibdelone C (3) and methyl kibdelone C (57) against HCT116 colon cancer cells. Cell viability was determined with CellTiter-Glo®.

Figures 2

a. Interaction of (−)-methyl kibdelone C (57) with topoisomerase I (Topo I). Supercoiled plasmid DNA (Sc DNA, lane 1) is converted to relaxed plasmid DNA (lane 2) by Topo I. Linear DNA is shown for comparison (lane 3). Topo I generates nicked open circular DNA, which runs as a single band (lane 4). Methyl kibdelone does not inhibit Topo I up to 10 µM (lanes 6–11). b. Interaction of 57 with topoisomerase II. Catenated kinetoplast DNA (Cate. kDNA, lane 1) is converted to decatenated kDNA (lane 2) by topoisomerase II. A positive control, etoposide, induced the formation of linear DNA (lanes 3, 4). Methyl kibdelone does not inhibit Topo II up to 10 µM (lanes 6–11).

Figure 3

HeLa cells treated with A) DMSO, B) and C) kibdelone C [(−)−3], D) simaomicin α (4), E) bortezomib or F) paclitaxel for 12h. Representative stress fibers indicated with yellow arrow. Staining of nuclei (blue, DAPI) and actin (green, FITC-phalloidin) reveals cell contraction and stress fiber formation in the kibdelone and simaomicin α-treated cells.

Figure 4

(−)-Kibdelone C (3) has no effect on the polymerization rate of actin. The main chart shows the changes in fluorescence as recombinant actin was polymerized over time (2 µM actin including 10%-pyrene labeled protein). Single experiments are shown in the absence or presence of the indicated concentrations of 3. Inset shows the average rate of polymerization in the absence or presence of 3 relative to the rate in the absence of DMSO (n = 3).

Scheme 1

Hexacyclic tetrahydroxanthone and xanthone natural products.

Scheme 2

Synthetic strategy

Scheme 3

Synthesis of the AB-ring isoquinolinone. (a) (COCl)₂, DMF (cat), CH₂Cl₂; CH₂Cl₂/5M NaOH 1.5:1. (b) PhI(OAc)₂, TEMPO (10 mol%), CH₂Cl₂, 82% yield over 2 steps. (c) BCl₃ (1.0 equiv, then 1.5 equiv), 0 °C to rt. (d) TsOH•H₂O (1.5 equiv), toluene, reflux, 66% yield over 2 steps. (e) Pd(PhCN)₂Cl₂ (5 mol%), ^tBu₃P•HBF₄ (10 mol%), CuI (3 mol%), Et₂NH, 40 °C. (f) Bu₄NF (1.5 equiv), 85% yield over 2 steps.

Scheme 4

Enantioselective synthesis of the F-ring a) Shi catalyst (27, 0.6 equiv), oxone, H₂O (pH 10.5), CH₃CN, 0 °C, then b) BH₃•THF, Et₂O, 0 °C, 68% yield over 2 steps, >95:5 e.r and d.r. c) TsCl, pyridine/CH₂Cl₂ (1/1). d) TFAA, DMSO, CH₂Cl₂, then Et₃N, −78 °C to rt. e) I₂, pyridine (1.2 equiv), CH₂Cl₂. f) NaBH₄, CeCl₃•7H₂O, MeOH, −78 °C, 75% over 3 steps.

Scheme 5

Union of the D and F rings.

Scheme 6

Synthesis of ent-kibdelone C and derivatives a) Dess-Martin periodinane, CH₂Cl₂. b) HClO_4(aq), ^tBuOH, acetone, 66% yield over 2 steps. c) MOMCl, ⁱPr₂NEt, CH₂Cl₂. d) 17, [Pd(C₃H₅)Cl]₂ (5 mol%), ^tBu₃P, DABCO, CH₃CN/DMF 1:1, 77% yield, 2 steps. e. 10% Pd/C, NaHCO₃ (1.5 equiv),CH₂Cl₂/ⁱPrOH (1:1), H₂. f. CuCl(OH)•TMEDA (25 mol%), I₂, CH₂Cl₂. g) Boc₂O, DMAP, CH₂Cl₂. 66% yield over 3 steps. h. Pd(OAc)₂ (1.5 equiv), ^tBu₃P•HBF₄ (3.0 equiv), pivalic acid (6.0 equiv), NaHCO₃ (20 equiv), DMA, 90 °C, 63% yield, 1:1 mixture of atropisomers. i. 51, DMF/CH₂Cl₂ 5:1. j. THF/^tBuOH 2.5:1, HClO₄ (10 equiv). k. PhI(OAc)₂ (2 equiv), CH₃CN/H₂O 1:1. l. BCl₃ (6 equiv), CH₂Cl₂, −78 °C to 0 °C, then Na₂S₂O₄. m. BCl₃ (6 equiv), CH₂Cl₂, −78 °C.

Scheme 7

Synthesis of simplified kibdelone C derivatives a) 58 or 59, MeLi (1.05 equiv), Et₂O, then ^tBuLi (2.0 equiv), then 25, THF, −78 °C. c) HClO₄(aq), ^tBuOH, acetone. d) HCl, THF/H₂O, rt. e) SeO₂ (2.0 equiv), ^tBuOH, reflux. f) Benzyloxymethyl chloride (1.5 equiv), proton sponge (5.0 equiv), NaI (4 equiv), THF, rt. g) [Pd(allyl)Cl]₂ (5 mol%), ^tBu₃P•HBF₄ (20 mol %), DABCO (2.2 equiv), slow addition of 17, 50 °C. h) Pd/C, H₂ (1 atm), CH₂Cl₂/ⁱPrOH 1:1, rt. i) Cu(Cl)OH•TMEDA (0.25 equiv), I₂ (1 equiv), O₂ (1 atm), CH₂Cl₂, rt. j) (Boc)₂O, DMAP, CH₂Cl₂, rt. k) Pd(OAc)₂ (1.5 equiv), ^tBu₃P•HBF₄ (3.0 equiv), pivalic acid (6.0 equiv), NaHCO₃ (20 equiv), DMA, 90 °C. l) THF/^tBuOH 2.5:1, HClO₄ (10 equiv), m) BCl₃ (6 equiv), CH₂Cl₂, −78 °C to 0 °C.

Scheme 8

Structure-activity relationship