Hamish S. Sutherland

Synthesis and structure-activity studies of biphenyl analogues of the tuberculosis drug (6S)-2-nitro-6-{[4-(trifluoromethoxy)benzyl]oxy}-6,7-dihydro-5H-imidazo[2,1-b][1,3]oxazine (PA-824).

A series of biphenyl analogues of the new tuberculosis drug PA-824 was prepared, primarily by coupling the known (6S)-2-nitro-6,7-dihydro-5H-imidazo[2,1-b][1,3]oxazin-6-ol with iodobenzyl halides, followed by Suzuki coupling of these iodides with appropriate arylboronic acids or by assembly of the complete biaryl side chain prior to coupling with the above alcohol. Antitubercular activity was determined under both replicating (MABA) and nonreplicating (LORA) conditions. para-Linked biaryls were the most active, followed by meta-linked and then ortho-linked analogues. A more detailed study of a larger group of para-linked analogues showed a significant correlation between potency (MABA) and both lipophilicity (CLOGP) and the electron-withdrawing properties of terminal ring substituents ( summation operatorsigma). Selected compounds were evaluated for their efficacy in a mouse model of acute Mycobacterium tuberculosis infection. In vivo activity correlated well with the stability of compounds to microsomal metabolism. Three compounds bearing combinations of lipophilic, electron-withdrawing groups achieved >200-fold higher efficacies than the parent drug.

Synthesis and Structure−activity Relationships of Antitubercular 2-Nitroimidazooxazines Bearing Heterocyclic Side Chains

Recently described biphenyl analogues of the antituberculosis drug PA-824 displayed improved potencies against M. tuberculosis but were poorly soluble. Heterobiaryl analogues of these, in which the first phenyl ring was replaced with various 5-membered ring heterocycles, were prepared with the aim of identifying potent new candidates with improved aqueous solubility. The compounds were constructed by coupling the chiral 2-nitroimidazooxazine alcohol with various halomethyl-substituted arylheterocycles, by cycloadditions to a propargyl ether derivative of this alcohol, or by Suzuki couplings on haloheterocyclic methyl ether derivatives. The arylheterocyclic compounds were all more hydrophilic than their corresponding biphenyl analogues, and several showed solubility improvements. 1-Methylpyrazole, 1,3-linked-pyrazole, 2,4-linked-triazole, and tetrazole analogues had 3- to 7-fold higher MIC potencies against replicating M. tb than predicted by their lipophilicities. Two pyrazole analogues were >10-fold more efficacious than the parent drug in a mouse model of acute M. tb infection, and one displayed a 2-fold higher solubility.

Synthesis and Structure−Activity Relationships of Aza- and Diazabiphenyl Analogues of the Antitubercular Drug (6S)-2-Nitro-6-{[4-(trifluoromethoxy)benzyl]oxy}-6,7-dihydro-5H-imidazo[2,1-b][1,3]oxazine (PA-824)

New analogues of antitubercular drug PA-824 were synthesized, featuring alternative side chain ether linkers of varying size and flexibility, seeking drug candidates with enhanced metabolic stability and high efficacy. Both α-methyl substitution and removal of the benzylic methylene were broadly tolerated in vitro, with a biaryl example of the latter class exhibiting an 8-fold better efficacy than the parent drug in a mouse model of acute Mycobacterium tuberculosis infection and negligible fragmentation to an alcohol metabolite in liver microsomes. Extended linkers (notably propenyloxy, propynyloxy, and pentynyloxy) provided greater potencies against replicating M. tb (monoaryl analogues), with propynyl ethers being most effective under anaerobic (nonreplicating) conditions (mono/biaryl analogues). For benzyloxybenzyl and biaryl derivatives, aerobic activity was maximal with the original (OCH(2)) linker. One propynyloxy-linked compound displayed an 89-fold higher efficacy than the parent drug in the acute model, and it was slightly superior to antitubercular drug OPC-67683 in a chronic infection model.

Structure-activity relationships for amide-, carbamate-, and urea-linked analogues of the tuberculosis drug (6S)-2-nitro-6-{[4-(trifluoromethoxy)benzyl]oxy}-6,7-dihydro-5H-imidazo[2,1-b][1,3]oxazine (PA-824).

Analogues of clinical tuberculosis drug (6S)-2-nitro-6-{[4-(trifluoromethoxy)benzyl]oxy}-6,7-dihydro-5H-imidazo[2,1-b][1,3]oxazine (PA-824), in which the OCH(2) linkage was replaced with amide, carbamate, and urea functionality, were investigated as an alternative approach to address oxidative metabolism, reduce lipophilicity, and improve aqueous solubility. Several soluble monoaryl examples displayed moderately improved (∼2- to 4-fold) potencies against replicating Mycobacterium tuberculosis but were generally inferior inhibitors under anaerobic (nonreplicating) conditions. More lipophilic biaryl derivatives mostly displayed similar or reduced potencies to these in contrast to the parent biaryl series. The leading biaryl carbamate demonstrated exceptional metabolic stability and a 5-fold better efficacy than the parent drug in a mouse model of acute M. tuberculosis infection but was poorly soluble. Bioisosteric replacement of this biaryl moiety by arylpiperazine resulted in a soluble, orally bioavailable carbamate analogue providing identical activity in the acute model, comparable efficacy to OPC-67683 in a chronic infection model, favorable pharmacokinetic profiles across several species, and enhanced safety.

Insertion of alkynes into diterpenoid chromium aminocarbenes: synthesis of ring-C aromatic steroidal analogues

Abstract The insertion of alkyl- or aryl-substituted alkynes into chromium aminocarbenes derived from podocarpic acid gives good to excellent yields of cyclopentaannulated products. The presence of a heteroatom bonded directly to the alkyne lowers the yield of the indanones. Although no steroidal derivatives could be isolated from the use of acetylene or its synthons, the electron-deficient alkyne ethyl 4,4-dimethylpentyn-2-oate gave a good yield, as did ethynylferrocene. Novel diterpenoid ferrocenyl quinones were synthesised by reacting diterpenoid chromium alkoxycarbenes with ethynylferrocene.

Synthesis of chromium aminocarbene complexes of diterpenoids

Abstract Chromium aminocarbene complexes of podocarpane diterpenoids have been synthesised in good to excellent yields, either by aminolysis of the corresponding alkoxycarbene, or by treatment of the morpholino amide with disodium pentacarbonylchromium and subsequent elimination of silyloxide.

Synthesis of diterpenoid indole derivatives via tethered chromium alkynylaminocarbenes

Abstract Thermolysis of podocarpane-tethered chromiumalkynylaminocarbenes gives indole derivatives in moderate yields. (Arylalkynyl)aminocarbenes could not be isolated. Ester derivatives were formed due to trapping of the intermediate ketene in situ with butan-1-ol.

β-Lactams (including polycyclic) derived from chromium carbenes

Abstract The photolytic reaction of N-methylbenzylideneimine with 12-methoxypodocarpane chromium carbenes gave products derived either from carbon monoxide dissociation followed by 12-methoxy ligation or from oxidation of the carbene metal moiety, while the reaction of N-methylbenzylideneimine with 12-desmethoxy diterpenoid carbenes gave diterpenoid β-lactams. The relative stereochemistry of two monocyclic 3,4-diaryl β-lactams prepared from a methoxy- or ethylthio-phenylcarbene by photolysis with an imine has been determined by X-ray crystallography.

6-Cyano Analogues of Bedaquiline as Less Lipophilic and Potentially Safer Diarylquinolines for Tuberculosis

Bedaquiline (1) is a new drug for tuberculosis and the first of the diarylquinoline class. It demonstrates excellent efficacy against TB but induces phospholipidosis at high doses, has a long terminal elimination half-life (due to its high lipophilicity), and exhibits potent hERG channel inhibition, resulting in clinical QTc interval prolongation. A number of structural ring A analogues of bedaquiline have been prepared and evaluated for their anti-M.tb activity (MIC90), with a view to their possible application as less lipophilic second generation compounds. It was previously observed that a range of 6-substituted analogues of 1 demonstrated a positive correlation between potency (MIC90) toward M.tb and drug lipophilicity. Contrary to this trend, we discovered, by virtue of a clogP/M.tb score, that a 6-cyano (CN) substituent provides a substantial reduction in lipophilicity with only modest effects on MIC values, suggesting this substituent as a useful tool in the search for effective and safer analogues of 1.

Synthesis and evaluation of analogues of the tuberculosis drug bedaquiline containing heterocyclic B-ring units

Graphical abstract