Design, synthesis, and biological evaluation of 14-substituted aromathecins as topoisomerase I inhibitors

Abstract

The aromathecin or "rosettacin" class of topoisomerase I (top1) inhibitors is effectively a "composite" of the natural products camptothecin and luotonin A and the synthetic indenoisoquinolines. The aromathecins have aroused considerable interest following the isolation and total synthesis of 22-hydroxyacuminatine, a rare cytotoxic natural product containing the 12 H-5,11a-diazadibenzo[ b, h]fluoren-11-one system. We have developed two novel syntheses of this system and prepared a series of 14-substituted aromathecins as novel antiproliferative topoisomerase I poisons. These inhibitors are proposed to act via an intercalation and "poisoning" mechanism identical to camptothecin and the indenoisoquinolines. Many of these compounds possess greater antiproliferative activity and anti-top1 activity than the parent unsubstituted compound (rosettacin) and previously synthesized aromathecins, as well as greater top1 inhibitory activity than 22-hydroxyacuminatine. In addition to potentially aiding solubility and localization to the DNA-enzyme complex, nitrogenous substituents located at the 14-position of the aromathecin system have been proposed to project into the major groove of the top1-DNA complex and hydrogen-bond to major-groove amino acids, thereby stabilizing the ternary complex.

Figure 1

Representative top1 inhibitors.

Figure 2

Structures of irinotecan and topotecan

Figure 3

Ligand overlay of aromathecin 27d (green) and indenoisoquinoline 4 (magenta). The positions of the lactam nitrogen and 14-position are indicated in their respective colors. The carboxylate group of 4 is perpendicular to the plane of the ring system.

Figure 4

Top1-mediated DNA cleavage induced by aromathecins 27a,d,g,i, 28d, and 28f: (lane 1) AG; (lane 2) DNA alone; (lane 3) top1 alone; (lane 4) camptothecin (1), 1 μM; (lane 5) 5, 1 μM; (lane 6) rosettacin (9), 100 μM; (lanes 7-30) (for compounds 27a,d,g,i, 28d, and 28f) Top1 + indicated compound at 0.1, 1, 10, and 100 μM, respectively.

Figure 5

Hypothetical model for the binding of aromathecin 27d in the ternary complex of DNA, top1, and the inhibitor. The diagram is programmed for wall-eyed (relaxed) viewing.

Scheme 1^a. ^aReagents and Conditions

(a) EtOCOCl, CHCl₃, reflux; (b) i. NaH, benzene, reflux, ii. methyl acrylate, reflux, iii. 6 M HCl, reflux; (c) ethylene glycol, cat. p-TsOH, benzene, reflux; (d) KOH, H₂O, reflux; (e) cat. FeCl₃, SOCl₂, reflux; (f) THF, Et₃N, r.t.; (g) polyphosphoric acid, 85% phosphoric acid, CH₂Cl₂, 100 °C.

Scheme 2^a. ^aReagents and Conditions

(a) i. cat. 2-cyclohexen-1-one, cyclohexanol, reflux, ii. maleic acid, EtOAc, r.t.; (b) MeOH, Et₃N, r.t.; (c) PDC, CH₂Cl₂, reflux; (d) polyphosphoric acid, CHCl₃, reflux.

Scheme 3^a. ^aReagents and Conditions

(a) i. BCl₃·Me₂S, 1,2,-dichloroethane, 0 °C, ii. chloroacetonitrile (for synthesis of 25), 4-chlorobutyronitrile (for synthesis of 26), AlCl₃, reflux. iii. 2 M HCl, reflux; (b) p-TsOH, benzene, reflux.

Scheme 4^a. ^aReagents and Conditions

(a) amine or amine salt + Et₃N, DMSO, r.t., 62-100 °C (27d); (b) NaN₃, DMSO; (c) i. (EtO₃)P, benzene, reflux, ii. 3 M HCl, MeOH, reflux.

Scheme 5^a. ^aReagents and Conditions

(a) amine, NaI, DMSO, 100 °C, then CF₃COOH, CHCl₃; (b) NaN₃, DMSO, 100 °C (c) i. (EtO)₃P, benzene, reflux, ii. 3 M HCl, MeOH, reflux.