Novel route to chaetomellic acid A and analogues: serendipitous discovery of a more competent FTase inhibitor

Abstract

A new practical route to chaetomellic acid A (ACA), based on the copper catalysed radical cyclization (RC) of (Z)-3-(2,2-dichloropropanoyl)-2-pentadecylidene-1,3-thiazinane, is described. Remarkably, the process entailed: (i) a one-pot preparation of the intermediate N-α-perchloroacyl-2-(Z)-alkyliden-1,3-thiazinanes starting from N-(3-hydroxypropyl)palmitamide, (ii) a two step smooth transformation of the RC products into ACA and (iii) only one intermediate chromatographic purification step. The method offers a versatile approach to the preparation of ACA analogues, through the synthesis of an intermediate maleic anhydride with a vinylic group at the end of the aliphatic tail, a function that can be transformed through a thiol-ene coupling. Serendipitously, the disodium salt of 2-(9-(butylthio)nonyl)-3-methylmaleic acid, that we prepared as a representative sulfurated ACA analogue, was a more competent FTase inhibitor than ACA. This behaviour was analysed by a molecular docking study.

Figure 1

Chaetomellic acid A (1).

Figure 2

Structural similarity between the dianionic form of chaetomellic acid A 2 and farnesyl pyrophosphate (FPP).

Figure 3

Sodium maleates used in the FTase assays.

Figure 4

Structure of the ACA analogues 29 and 30, and their IC₅₀ values against yeast FTase.

Figure 5

a) Alignment of FPP, in the conformation assumed in the 1FT2 pdb structure (blue), FPT-II FPP, in the conformation assumed in the 1TN8 pdb structure (yellow), and of the thia-analogue 27 in the quasi-extended conformation chosen (atom colors: carbon atoms are in green, oxygen atoms in red, and sulfur atom in orange). b) Superposition of the molecular volume of 27 (green) and the volume of the supermolecule (white) formed by FPP and FPT-II FPP. In the figure the hydrogen atoms are omitted for clarity.

Figure 6

The interaction of inhibitor 27 and the FTase binding site. The enzyme α-subunit is represented in grey, the β-subunit is represented in yellow. Aminoacid residues involved in the interations are colored by element type (grey: carbon, blue: nitrogen, red: oxygen). Compound 27 is represented according to the color code of Figure 5. The hydrogen atoms are not displayed for clarity. Top: focus on the interactions of the anionic head of 27 with the FTase binding site; bottom: focus on the interactions established by the hydrophobic tail of compound 27; inset: an overall view of the ternary complex FTase-27-peptide (CVLS).

Scheme 1

pH dependent equilibrium between the active dianionic open form 2 and the anhydride chaetomellic A (3).

Scheme 2

(a) 1) (COCl)₂, CH₂Cl₂, DMF, 23 °C, 2 h; 2) N-benzyl-3-Cl-2-propenylamine (PG₁ = Bn) or 2-(3-chloro-2-propenylamino)pyridine (PG₂ = 2-Py), Py, 23 °C, 1 h. (b) CuCl-TMEDA, CH₃CN, argon, 60 °C, 20 h. (c) 1) Na⁰, CH₃OH/diethyl ether, 25 °C, 20 h; 2) H⁺/H₂O. (d) 1) KOH, CH₃OH/THF, reflux, 2 h; 2) H⁺/H₂O. (e) 1) Na⁰, CH₃OH/diethyl ether, 25 °C, 20 h; 2) H₂SO₄ 4 N, 110 °C, 2 h.

Scheme 3

(a) CuCl-TMEDA, toluene, argon, 60 °C, 24 h. (b) Na⁰, toluene/MeOH, 25 °C, 24 h. (c) H₂SO₄/H₂O, 110 °C, 3 h. (d) One-pot procedure: 1) Na⁰, toluene/MeOH, 25 °C, 24 h; 2) H₂SO₄/H₂O, 110 °C, 3 h. (e) MnO₂, CH₂Cl₂, 25 °C, 20 h. (f) H₂SO₄/H₂O, 110 °C, 4 h.

Scheme 4

Possible mechanism for the RC of N-α-perchloroacyl six membered cyclic ketene-N, S-acetals A.

Scheme 5

Preparation of the 3,4-disubstituted-2,5-furandiones: (a) CuCl (5 mol%), TMEDA (10 mol%), CH₃CN, Na₂CO₃, argon, 17 h, 30 °C; (b) silica-sulfuric acid, NaNO₃, SiO₂/H₂O 3:2, CH₂Cl₂, 45 °C, 40 h; (c) KOH, CH₃OH-THF, reflux 2 h; (d) HCl (10 %), r.t..

Scheme 6

Preparation of the N-2,2-dichloropropanoyl ketene-N,S-acetal 10.

Scheme 7

Reactions of the N-2,2-dichloropropanoyl ketene-N,S-acetal 10: (a) CuCl (10 mol%), TMEDA (20 mol%), CH₃CN/toluene (3/2), Na₂CO₃, argon, 30 °C, 19–24 h; (b) KI, H₂O, r.t., 24 h; (c) NaOH, THF/H₂O, r. t., 12h, acidic work-up.

Scheme 8

Possible mechanism for deprotection of the thioacetal moiety in 12.

Scheme 9

Preparation of anhydride 18: (a) P₄S₁₀/HMDO, CH₂Cl₂, reflux, 5h; (b) 150 °C, 2 h, under vacuum; (c) CH₃CCl₂COCl, Py, CH₂Cl₂, 0–25°C, 20 h; (d) CuCl (10 mol%), TMEDA (20 mol%), CH₃CN/toluene (3/2), Na₂CO₃, argon, 30 °C, 19–24 h; (e) i) KI, H₂O, 24 h, ii) NaOH, THF/H₂O, r. t., 12h, acidic work-up.

Scheme 10

Radical addition of butanethiol to the terminal C=C group of 18.

Scheme 11

Preparation of anhydride 19: (a) P₄S₁₀/HMDO, CH₂Cl₂, reflux, 5h; (b) 150 °C, 2 h, under vacuum; (c) CH₃CCl₂COCl, Py, CH₂Cl₂, 0–25°C, 20 h; (d) i) CuCl (10 mol%), TMEDA (20 mol%), CH₃CN/toluene (3/2), Na₂CO₃, argon, 30 °C, 19–24 h, ii) KI, H₂O, r.t., 24 h, iii) NaOH, THF/H₂O, r. t., 12h, acidic work-up.