Mediation of metal chelation in cysteine-derived tetramate systems

Abstract

A study of bicyclic tetramates modified with a bulky ester, which leads to steric hindrance of distal chelating atoms as a route for the alteration of metal binding ability is reported. This approach required the development of a direct method for the synthesis of different esters of cysteine from cystine, which then provided access to bicyclic tetramates by Dieckmann cyclisation. Further derivation to ketones and carboxamides by Grignard addition and transamination reactions respectively provided rapid access to a chemical library of tetramates with diverse substitution. Of interest is that bicyclic tetramate ketones and carboxamides showed different tautomeric and metal binding behaviour in solution. Significantly, in both systems, the incorporation of bulky C-5 esters at the bridging position not only reduced metal binding, but also enhanced antibacterial potencies against Gram-positive MRSA bacteria. Those tetramates with antibacterial activity which was not metal dependent showed physiochemical properties of MSA of 559-737 Ų, MW of 427-577 Da, clogP of 1.8-6.1, clogD_7.4 of -1.7 to 3.7, PSA of 83-109 Ų and relative PSA of 12-15% and were generally Lipinski rule compliant. A subset of tetramates exhibited good selectivity towards prokaryotic bacterial cells. Given that the work reported herein is synthesis-led, without the underpinning detailed mechanistic understanding of biological/biochemical mechanism, that the most active compounds occupy a small region of chemical space as defined by MW, clogP, PSA and %PSA is of interest. Overall, the bicyclic tetramate template is a promising structural motif for the development of novel antibacterial drugs, with good anti-MRSA potencies and appropriate drug-like physiochemical properties, coupled with a potential for multi-targeting mechanisms and low eukaryotic cytotoxicity.

Fig. 1. Cysteine-derived tetramates with C-5 ester functionalisation.

Fig. 2. Target library of cysteine-derived tetramates.

Scheme 1. Reagents and conditions: (a) KO^tBu, THF, reflux, 4 h; (b) R¹OH, catalytic p-TsOH monohydrate or DMAP, toluene, reflux.

Scheme 2. (A) Direct transesterification of N-acylthiazolidine; (B) transesterification of the unprotected thiazolidine. Reagents and conditions: (a) R¹OH, R¹ = p-methoxybenzyl, catalytic DMAP, toluene, reflux, o.n.; (b) additional equiv. of R¹OH, toluene, reflux, o.n.; (c) R¹OH, R¹ = benzyl or p-methoxybenzyl, catalytic p-TsOH monohydrate or DMAP, toluene, reflux, o.n.

Scheme 3. Reagents and conditions: (a) N,N,N,N-tetramethylguanidine, anhydrous DMF, 0 °C for 30 min followed by addition of ethyl acetoacetate, 0 °C to r.t., 18 h; (b) RBr, (R = benzyl), r.t., 24 h.

Scheme 4. Reagents and conditions: (a) p-TsOH monohydrate, R¹OH, cyclohexane, reflux, 5–16 h; (b) DTT, Et₃N, DCM, N₂, r.t., 24 h; (c) R²CHO, petrol 40/60, reflux, 18 h; (d) mono-ethyl malonate, DCC, DMAP, DCM, 0 °C to r.t., 18 h; (e) N-methoxy-N-methylcarbomoyl chloride 22, dry pyridine, MgBr₂, DCM, 0 °C to r.t., 18–24 h; (f) DBU dry THF, r.t., 24 h; (g) R³MgBr (prepared fresh with R³Br, Mg, dibromoethane, dry THF, reflux, 1 h), dry THF, −15 °C, 1 h or o.n.; (h) R³NH₂, THF/toluene, reflux, 18 h.

Scheme 5

Fig. 3. Tautomeric behaviour of tetramate ketones.

Scheme 6

Scheme 7

Scheme 8

Fig. 4. Plot of post-column widths at half height/Hz for H-2 signals against van der Waals volume occupied by the respective C-5 ester. Subseries of compounds: orange – R³ = ethyl or phenethyl, R² = phenyl; red – R³ = cyclohexyl, R² = phenyl; green – R³ = N-cyclohexyl or N-(1-adamantyl), R² = phenyl; purple – R³ = N-cyclohexyl or N-(1-adamantyl), R² = pyridin-2-yl or bromo-pyridinyl.

Fig. 5. Plots of (A) clogP, (B) clogD_7.4, (C) PSA/Ų and (D) %PSA against MW/Da. Data representing the active tetramate are highlighted in red (MIC against MRSA, <8 μg ml⁻¹); tetramates exhibiting mild activity (8 μg ml⁻¹ < MIC against MRSA ≤ 32 μg ml⁻¹) are represented in yellow; and tetramates with no activity are shown in blue.

Fig. 6. Plot of MIC values against MRSA (μg ml⁻¹) against post-column widths at half height of the H-2 signal (Hz) for C-5 modified tetramates. Data representing the active tetramate are highlighted in red (MIC against MRSA, <8 μg ml⁻¹); tetramates exhibiting mild activity (8 μg ml⁻¹ < MIC against MRSA ≤ 32 μg ml⁻¹) are represented in yellow; and tetramates with no activity are shown in blue.