Troponoid Atropisomerism: Studies on the Configurational Stability of Tropone-Amide Chiral Axes

Abstract

Configurationally stable, atropisomeric motifs are an important structural element in a number of molecules, including chiral ligands, catalysts, and molecular devices. Thus, understanding features that stabilize chiral axes is of fundamental interest throughout the chemical sciences. The following details the high rotational barriers about the Ar–C(O) bond of tropone amides, which significantly exceed those of analogous benzamides. These studies are supported by both experimental and computational rotational barrier measurements. We also report the resolution of an axially chiral α-hydroxytropolone amide into its individual atropisomers, and demonstrate its configurational stability at physiological pH and temperatures over 24 hours.

Graphical Absract:

Atropisomerism, a form of chirality arising from restricted rotation about an asymmetric axis, plays an important role in a number of functional molecules¹ including chiral biaryl ligands and catalysts (e.g., 1,² Figure 1A),³ as well as unidirectional molecular devices and switches.⁴ Single atropisomer therapeutics also exist, although they often have their roots in natural products (e.g., 2,⁵ Figure 1A).⁶ However, given the critical importance of chirality in drug development, they are also becoming increasingly prevalent in de novo drug design (e.g., 3,⁷ Figure 1A).⁸ Compared to 5- and 6-membered aromatic rings commonly found in drugs, troponoids should have an increased likelihood of being atropisomeric due to their decreased external bond angles (Figure 1B). However, other features such as tropylium characteristics,⁹ ring puckering,¹⁰ and decreased aromaticity¹¹ could influence the rotational barriers, as well. Only a few troponoids are known to exhibit atropisomerism, including colchicine (2, Figure 1A, ΔG^‡_{298 K} = 22 kcal/mol)⁵ and bistropone homodimer 4 (Figure 1C, ΔG^‡_{298 K} = 20.7 kcal/mol),¹² both of which have relatively low rotational barriers. Given the growing interest in troponoid drug development,¹³ as well as the importance of atropisomerism throughout the chemical sciences, understanding how this motif influences atropisomerism compared to benzenoids would be helpful in designing new, functional atropisomeric molecules.

Figure 1. Atropisomerism and troponoids.

(A) Examples of valuable molecules exhibiting atropisomerism. (B) External bond angle differences between 5, 6, and 7-membered aromatic rings, and their influence on proximity of ortho-substituents. (C) Bistropone homodimer 4 with an experimental rotational barrier measurement.

Our investigation began with DFT computations on a series of axially chiral tropolones and analogous benzenoids (Table 1).¹⁴ Troponoid substrates consistently exhibited higher rotational barriers than those of the corresponding benzenoids, with rotational energy barrier increases of up to 4.3 kcal/mol (Entry 4), and increases in half-lives to enantiomerization at room temperature of up to four orders of magnitude.

Table 1. Computed rotational energy barriers of related troponoids and benzenoids.

Rotational barriers were computed at the M06–2X/6–311++G(2d,3p)//B3LYP/6–31+G(d,p) level of theory at 298.15 K and 1 atm using the Gaussian 09 suite.

			Benzenoids			Troponoids
Entry	R1	R2	No.	t½ (s)	ΔG‡ (kcal/mol)	No.	t½ (s)	ΔG‡ (kcal/mol)
1	Ph	Me	5a	6 × 10–11	3.7	6a	1 × 10–8	6.9
2	N(CH2)5	H	5b	6 × 10–6	10.5	6b	1 × 10–4	12.3
3	N(CH2)5	Me	5c	4 × 10–4	13.0	6c	0.3	16.9
4	N(i-Pr)2	Me	5d	5 × 10–4	13.2	6d	0.7	17.5

To confirm these energy barriers experimentally, we turned to α-hydroxytropolone 7 and benzamide 8 (Figure 2A, B), which were readily accessible from the corresponding carboxylic acids.^{15,16 1}H NMR spectra of these molecules have diagnostic diastereotopic signals and observable E and Z amide isomers useful for probing the configurational stability of the Ar–C(O) and N–C(O) axes, respectively, through variable temperature NMR experiments (Figure 2A, B). Prior studies on hindered benzamides have established that, in addition to isolated Ar–C(O) and N–C(O) rotations, isomerization can also proceed through an often energetically intermediate, concerted Ar–C(O)/N–C(O) process (Figure 2C).¹⁷ Consistent with this precedent, computational modeling on both 7 and 8 revealed that Ar–C(O) rotational barriers were lower in energy than N–C(O) rotation, and the concerted Ar–C(O)/N–C(O) rotation was the lowest energy pathway for effective N–C(O) amide isomerization (Figure 2C). These results were also validated by variable temperature NMR experiments. Heating a solution of 7 in DMSO-d6 led to coalescence of only the diastereotopic signals, which allowed us to measure the individual rotational energy barriers of the Ar–C(O) bonds of both E and Z isomers (ΔG^‡_E = 16.7 kcal/mol, ΔG^‡_Z =16.5 kcal/mol). Conversely, cooling a sample of 8 revealed proton diastereotopicity, and allowed an experimental Ar–C(O) rotational barrier measurement (ΔG^‡_E = 12.2 kcal/mol, ΔG^‡_Z = 12.1 kcal/mol, Figure 2B). Finally, while no coalescence of the amide rotamers of 7 was observed at higher temperatures, heating a solution of 8 in DMSO-d₆ to 90 °C led to complete coalescence of its amide rotamers (Figure 2B). The increased stability of the N–C(O) axis of 7 relative to 8 demonstrates the ancillary rigidity provided by the tropone.

Figure 2.

Variable temperature 1H-NMR spectra of (A) troponoid, 7 (DMSO-d6), and (B) benzamide, 8 (CD₂Cl₂ and DMSO-d6) on an Agilent 500 MHz NMR spectrometer. The peaks assigned to the protons of the Z and E amide conformers are denoted by (*) and (**), respectively. Peak intensities normalized for clarity. (C) Proposed pathways to enantiomerization (Ar–C(O) rotation) and amide isomerization (N–C(O) rotation) for differentially substituted thiazolidine systems with energy values (kcal/mol) indicated for ground states, as well as computed rotational barriers denoted in the subsequent table (calculated at the M06–2X-D3/6–311++G(2d,3p) level of theory with using the Gaussian 09 suite). a Obtained from considering both isolated Ar-C(O) and concerted pathways.

In order to observe these rotational barrier differences in more configurationally stable systems, we modeled brominated variants of the compounds shown in Table 1 (Figure 3A).^7,14,18 Rotational barriers increased by upwards of 11 kcal/mol for the benzenoids and by 14 kcal/mol for the troponoids. A corollary of this effect is observed in the half-life to enantiomerization of 10c, which was computed to take place over centuries at room temperature (t_½, _{298 K} = ~300 years), relative to hours for benzenoid 9c (t_½, _{298 K} = ~10 hours). Based on the classification system put forward by LaPlante and co-workers, these two molecules are considered to be class 3 (ΔG^‡ > 28 kcal/mol) and class 2 (ΔG^‡ ≈ 20–28 kcal/mol) atropisomers, respectively (Figure 3B).¹⁹ Class 2 molecules fall into a category where the molecules are atropisomeric, but the rotational barriers might not be high enough to develop them as single atropisomers. Class 3 molecules are those with rotational barriers sufficiently high to confidently develop as single-atropisomer drugs.

Figure 3.

(A) Computed rotational energy barriers of brominated troponoids and benzenoids, with (B) comparison to non-brominated molecules. Class 2 = intermediate barrier and Class 3 = stable atropisomerism. Rotational barriers were computed at the M06–2X/6–311++G(2d,3p)//B3LYP/6–31+G(d,p) level of theory at 298.15 K and 1 atm using the Gaussian 09 suite.

To obtain the quantities of enantioenriched 10c necessary to confirm these high energy barriers experimentally, we turned our attention to a peptide-catalyzed dynamic kinetic atroposelective halogenation^14,18,20 that had been established on structurally analogous benzamides (i.e. 11 vs. 6c, Figure 4A/B).^20a Employing slightly modified conditions to those used previously on 11, we were able to obtain 10c in 75:25 er.²¹ We next monitored enantioerosion of a solution of enantioenriched 10c (93:7 er) in triglyme over 60 minutes at 145 °C, and obtained an experimental energy barrier of 30.1 kcal/mol.¹⁶ A discrepancy between this value and that computed using DFT at the same temperature (ΔG^‡_{418 K} = 32.7 kcal/mol) may be due to the triglyme solvent.

Figure 4.

(A) Atroposelective halogenation of close troponoid structural homolog 11 using peptide 13. (B) Peptide 14 was found to deliver enantioenriched 10c. (C) a-Hydroxytropolone atropisomers of 15. (D) Enantiomerically enriched (–)-15 before (blue) and after (red) incubation at 37 °C in phosphate buffer (pH = 7.4) as monitored by analytical HPLC (CHIRALPAK® IC, 250 mm, i.d. 4.6 mm, 30% acetonitrile in water with 0.1% TFA, 1.5 mL/min). Peak intensities normalized for clarity.

Methoxytropolones, such as 10c, are known precursors to α-hydroxytropolones,²² which are promising dinuclear metalloenzyme-inhibiting fragments we have been leveraging in drug-development pursuits for a number of different diseases.²³ The high energy barriers we identified for 10c suggested that structurally analogous α-hydroxytropolones could be studied as single atropisomers. α-Hydroxytropolone 15 was thus synthesized through demethylation of 10c and resolved into individual enantiomers with a preparatory scale chiral stationary phase column.¹⁶ To test stability under physiological conditions, an enantiomerically enriched sample consisting of 93% (−)-15 and 7% (+)-15 was dissolved in phosphate buffer (pH = 7.4) and heated to 37 °C for 24 hours; no change was observed in the enantiomeric ratio over this time period (Figure 4E). This result is important for tropolone development as single atropisomers, since ionization states can change rotational barriers²⁴ and tropolones are likely to exist in an anionic state at physiological pH.²⁵

In conclusion, computational and experimental rotational barrier measurements demonstrate that tropone-amide chiral axes are substantially higher than those of analogous benzamides. We also synthesized and resolved a configurationally stable, axially chiral α-hydroxytropolone amide, and we found the enantiomers to be stable at physiological temperature and pH for over 24 hrs. Given the importance of configurationally stable atropisomeric molecules throughout the chemical sciences, these studies suggest that tropones could find a valuable role in atropisomeric molecule design.