Synthesis of apo-13- and apo-15-lycopenoids, cleavage products of lycopene that are retinoic acid antagonists

Abstract

Consumption of the tomato carotenoid, lycopene, has been associated with favorable health benefits. Some of lycopene's biological activity may be due to metabolites resulting from cleavage of the lycopene molecule. Because of their structural similarity to the retinoic acid receptor (RAR) antagonist, β-apo-13-carotenone, the "first half" putative oxidative cleavage products of the symmetrical lycopene have been synthesized. All transformations proceed in moderate to good yield and some with high stereochemical integrity allowing ready access to these otherwise difficult to obtain terpenoids. In particular, the methods described allow ready access to the trans isomers of citral (geranial) and pseudoionone, important flavor and fragrance compounds that are not readily available isomerically pure and are building blocks for many of the longer apolycopenoids. In addition, all of the apo-11, apo-13, and apo-15 lycopenals/lycopenones/lycopenoic acids have been prepared. These compounds have been evaluated for their effect on RAR-induced genes in cultured hepatoma cells and, much like β-apo-13-carotenone, the comparable apo-13-lycopenone and the apo-15-lycopenal behave as RAR antagonists. Furthermore, molecular modeling studies demonstrate that the apo-13-lycopenone efficiently docked into the ligand binding site of RARα. Finally, isothermal titration calorimetry studies reveal that apo-13-lycopenone acts as an antagonist of RAR by inhibiting coactivator recruitment to the receptor.

Keywords: apolipoprotein-13-lycopenone; apolycopenoids; chemical synthesis; diet and dietary lipids; gene expression; nuclear receptors; retinoic acid receptor antagonist; retinoids/vitamin A.

Fig. 1.

Structures of carotenoids and apocarotenoids (1, β-carotene; 2, RA; 3, apo-13-carotenone; 4, lycopene; 5, apo-15-lycopenoic acid; 6, apo-15-lycopenal; 9, geranial; 10, pseudoionone; 11, geraniol; 12, geranic acid).

Fig. 2.

Lycopene (4) and key cleavage products showing similarity to apo-13-carotenone (3).

Fig. 3.

Oxidation of geraniol (11) to geranial (9).

Fig. 4.

Claisen-Schmidt condensation of geranial (9) with acetone to give pseudoionone (10).

Fig. 5.

Synthesis of apolycopenoids from pseudoionone (10). Reagents and conditions: triethyl phosphonoacetate, NaH, THF, rt (a); DIBAL-H, THF, rt (b); MnO₂, ethyl acetate, rt (c); acetone, 3% aq. NaOH, 40°C (d); NaOH (aq), ethanol, benzene, rt (e); compound 18, n-butyllithium, DMPU, THF, 0 to −70°C (f); i) DIBAL-H, petroleum ether, rt, ii) MnO₂, petroleum ether, rt (g); KOH (aq), isopropanol, reflux (h) (17, apo-11-lycopenoic acid; 14, apo-11-lycopenol; 15, apo-11-lycopenal; 16, apo-13-lycpopenone; 5, apo-15-lycopenoic acid; 6, apo-15-lycopenal).

Fig. 6.

Synthesis of phosphonate 18. Reagents and conditions: NaBH₄, ethanol (a); PPh₃, CBr₄, CH₃CN (b); triethyl phosphite, 120°C (c).

Fig. 7.

Docking mode of apo-13-lycopenone to RAR LBD. A: Binding mode of RA (cyan) to the LBD, highlighting helix 12 (purple) and coactivator binding domain (green). B: Docking mode of apo-13-lycopenone (yellow) superimposed over RA (cyan) binding mode, Arg 267 is highlighted in blue.

Fig. 8.

Representative thermograms of the coactivator peptide (SRC-1) titrated into RAR LBD presaturated with RA (A), no ligand (apo) (B), apo-13-lycopenone (C), and BMS-195614 (D). The top panels show the raw data while the bottom panels show the binding isotherm created by plotting the integrated heat peaks against the molar ratio of the SRC-1 peptide to the RAR-LBD; note the data in (D) are indicative of essentially no coactivator binding in the case of the potent neutral antagonist.