Synthesis of oxazolidinones from N -aryl-carbamate and epichlorohydrin under mild conditions

The reaction conditions for an enantiospecific synthesis of various N -aryl-oxazolidinones from N -aryl- carbamates and (R) or (S) epichlorohydrin were optimized. The N -aryl-oxazolidinones were applied to the synthesis of compounds of biological interest such as DuP 721, toloxatone and a linezolid analogue


Introduction
Oxazolidinones are a class of five-membered heterocycles containing nitrogen and oxygen with a broad range of applications.They are used as chiral auxiliaries in asymmetric synthesis 1,2 and are present in several biologically active compounds (Figure 1). 3,4articularly, N-aryl oxazolidinones have been extensively studied as antibacterial agents. 5,6In 1987, the pharmaceutical company DuPont identified the first oxazolidin-2-one, named DuP 721, that showed in vitro activity against Gram-positive pathogens 7 and M. tuberculosis. 8It shows liver toxicity in mice and, for this reason, there are no published results about clinical studies.Subsequent SAR studies, starting from DuP 721, led to the production of linezolid (ZYVOX ®), the first example of oxazolidinone approved for clinical use in 2000 by the FDA. 6This drug, binding the 50S ribosomal subunit, inhibits protein synthesis. 9It exerts antibacterial activity against Gram-positive multidrug-resistant pathogens such as methicillin-resistant Staphylococcus aureus (MRSA) and vancomycin-resistant enterococci (VRE). 102][13] For example, the replacement of the morpholine ring with a 1,2,4-oxadiazole ring, and the replacement of the carbonyl group with a thionyl in the side chain, gave new linezolid-like compounds with antibacterial activity against resistant S. aureus (linezolid analogue in Figure 1). 14he N-aryl-oxazolidinone ring is also the central core of several MAO inhibitors that are potential candidates for the treatment of several neurological diseases. 15,16To date, the oxazolidinone toloxatone (HUMORYL®) is in clinical use as a potent antidepressant drug. 17,18n terms of another example, the oxazolidinone rivaroxaban (XARELTO ®) is a direct inhibitor of the procoagulant factor Xa. It was approved in many countries for the prevention of several thromboembolic disorders. 19

Figure 1. Bioactive N-aryl-oxazolidinones.
Organic chemists have reported several pathways for the building of the oxazolidinone frame.1][22] In the presence of these compounds, the oxazolidinone ring closure is mediated by reactants such as phosgene, disphogene, urea and carbodiimidazole, acting as the source of the carbonyl group. 20Particularly, the Pdcatalyzed oxidative carbonylation of aminoalcohols to give oxazolidinone compounds is an efficient synthetic approach known in the literature. 23Synthesis of enantiopure 5-substituted oxazolidin-2-ones from βaminoalcohols could also be performed through an initial carboxylation with CO2, followed by intramolecular Mitsunobu reaction. 24he opening of the epoxy ring in presence of isocyanate is an alternative way used to obtain the oxazolidinone core.This [3+2] coupling reaction is arguably the most useful approach for the synthesis of Naryl-oxazolidinones.6][27][28][29][30][31][32][33] Some of these methods involve the use of high temperatures or heavy metals and the use of isocyanate could be limited by polymerization. 30Researchers have tried to overcome these limits by introducing new, safer and more economical organo-catalyst systems.][36] Chiral oxazolidinone compounds in high enantiomeric excess can be obtained by reaction of commercially available (R)-glycidyl butyrate and readily available aryl carbamates.This strategy produces good yields, but requires low temperature (-78 °C) and the use of n-butyllithium as deprotonation agent. 37Also, solid phase synthesis on solid support have been reported, this time by using Solketal as building block. 38It should be noted that the synthesis of oxazolidinones was very recently reviewed. 39onsidering the important role of the N-aryl-oxazolidinones moiety in pharmaceutical chemistry and some limitations of literature synthetic methods, in particular, the use of high-cost chiral building blocks or organic bases and the use of low/high temperatures, we decided to develop a new efficient and stereoselective synthetic strategy involving mild conditions as well as available and cheap reagents.In this context, DuP 721, toloxatone, a linezolid analogue and other N-aryl-oxazolidinones have been synthetized from readly available N-aryl-carbamates and enantiopure epichlorohydrin, a very cheap chiral building block, using lithium hydroxide as base at room temperature.
The next key step involved the reaction of ethyl 4-bromo-3-fluorophenylcarbamate 3a and (R)epichlorohydrin 4 under a variety of conditions to give oxazolidinone 5a.As shown in Table 1, when acetonitrile or THF were used as solvent the product 5a was obtained in low yields when potassium tertbutoxide (t-BuOK) was used as base (Entries 1-3).Trying different bases in acetonitrile (Entries 3-10), we observed moderate yields just with 4-dimethylaminopyridine (DMAP) under reflux (Entry 8).When we moved to dimethyl sulfoxide (DMSO) at ambient temperature and using different bases (Entries 11-18), the formation of the corresponding oxazolidinone 5a in reasonable yields (40-62%) was observed, except when t-BuOK (Entry 11) or 1,8-diazabicyclo(5.4.0)undec-7-ene (DBU) (Entry 18) were used.In the end, lithium hydroxide proved to be the best base (Entry 15).Moreover, in the presence of LiOH the solvent substitution of DMSO with dimethylformamide (DMF) resulted in a further increase in the isolated product (Entry 19).Considering LiOH as a good compromise between good yield and low cost, we decided to further explore the stoichiometry of the reaction (Entries 20 -22).By using a stoichiometric amount of base we observed a lower yield, as well as conversion (Entry 20).With a stoichiometric amount of epoxide 4 (Entry 21), 5a was obtained in low yield, but a considerable conversion was observed, while an excess of 3 equiv.for LiOH and 4 (Entry 22), gave a moderate yield (51%), but with a similar conversion compared to Entry 19.Of particular interest, was that chiral HPLC revealed that the obtained oxazolidinone 5a was substantially obtained as single enantiomer (ee> 96%).
The optimized method, involving LiOH as base and DMF as solvent, was applied to the reaction of variously substituted N-aryl-carbamates 3a-i with enantiopure epichlorohydrin 4. Interestingly, in some cases, in addition to the desired 6a-i, epoxides 6a-i were also isolated in low yields.Table 2 explains how the N-aryloxazolidin-2-ones 5a-i and the corresponding epoxy derivatives 6a-i were obtained.
a The reactions were performed using 1 equiv. of ethyl 4-bromo-3-fluorophenylcarbamate 3a, 3 equiv.of (R)-epichlorohydrin 4 and 1.5 equiv. of the respective base; b Yields of purified products; c Quantitative formation of 2-aminobenzoic acid All compounds were synthetized using enantiopure (R)-epichlorohydrin 4, while for the derivatives 3a,b,i the same reaction was also conducted with (S)-epichlorohydrin with similar results.In some cases, the epoxy derivatives 6 were observed by TLC, but their reactivity made their isolation and characterization impossible.In the case of ester group of 3j, the limitation was due to concurrent saponification of the starting compound.Moreover, the presence of different electron-withdrawing substituents and their different positions on the aryl moiety gave different reaction time and product yields.Generally, the N-aryloxazolidin-2-ones 5(a-i) were obtained in greater quantities than the epoxy derivatives 6(a-i), except for 5f and 5h, which contained electron-donating groups (EDG).
The development of enantioselective synthesis is often a challenge for medicinal chemists.Particularly, in the synthesis of linezolid and its derivatives it is important to obtain the (S)-enantiomer that has higher antibacterial activity than the racemic mixture. 40In order to assess the enantiospecificity of the proposed method, we employed oxazolidinone 5i, obtained from the N-aryl-carbamate 3i and (R)-epichlorohydrin or (S)epichlorohydrin, as key intermediate in the synthesis of the already known active derivative 9. 14 As shown in Scheme 2, 5i was treated with sodium azide in dimethylformamide to synthesize 7. The reduction of 7 to its amino-derivative 8, with triphenylphosphine, and the acetylation performed with acetic anhydride gave the derivative 9. 14 Scheme 2. Synthesis of the linezolid analogue derivative 9.
The determination of the which enantiomer had been obtained from epichlorohydrin was achieved by means of chiral HPLC with a Daicel Chiralpak IA column and hexane/iPrOH (80:20) as mobile phase.Literature data concerning the enantioselective HPLC analysis of racemic linezolid and oxadiazole-containing analogues showed that under these conditions the (S)-enantiomer elutes before the (R)-enantiomer. 40The HPLC traces of compound 9, obtained using (R)-or (S)-epichlorohydrin, were compared with that of the racemic mixture (Figure 2).From the results obtained, all reactions are thus assumed to be perfectly enantiospecific and in general the proposed method allows us to obtain oxazolidinones as single enantiomer.In addition, the desired active (S)-enantiomer was obtained using (R)-epichlorohydrin.Secondly, the bioactive enantiopure oxazolidinone DuP 721 was synthetized from 5b, following the same synthetic pathway of the linezolid-like compound.The reaction of the aryl-carbamate 3b with (R)epichlorohydrin yielded R-5b, which could readily be converted into the active (S)-enantiomer of DuP 721 as shown (Scheme 3).Moreover, nucleophilic ring-opening of epoxide 6b with sodium azide, give the azidoderivative 10, thus demonstrating that epoxides 6 are not just reaction by-product, but could be used for obtaining biologically relevant oxazolidinones (Scheme 3).Similarly, oxazolidinone (R)-5h could be envisaged as intermediate in the synthesis of (S)-linezolid.
Considering the enantiospecific acquirement of compounds 5 and 6, we speculate that they originate from two concurrent cyclization pathway (a or b) involving intermediate II, in turn obtained from the nucleophilic ring opening of epichlorohydrin by anion I (Scheme 4).This pathway is also in agreement with the lower yields for derivatives 3f,h containing EDGs, thus reducing the electrophilic character of the carbamate and favoring path b.Finally, the synthesis of the antidepressant toloxatone was successful achieved by using two different strategies: the reaction of the oxazolidinone 5f with potassium acetate (KOAc), followed by selective saponification; 35 or in a single reaction step from the epoxy derivative 6f after treatment with KOAc (Scheme 5).

Conclusions
A new synthetic procedure was developed for the preparation of chiral oxazolidinones 5a-i from N-arylcarbamates 3a-i with enantiopure epichlorohydrin 4. We have succeeded in finding appropriate conditions for this reaction by varying the temperature, solvent and base.The use of stirring at room temperature, LiOH as base and DMF as solvent gave the best yields.Epoxy derivative 6 were obtained as secondary product, except for substates bearing EDGs such as 6f and 6h.N-aryl-oxazolidinones 5b,f,i were used as precursors for the synthesis of the linezolid analogue 9, DuP 721 and toloxatone, respectively.Additionally, HPLC analysis of the linezolid-like derivative 9 allowed us to demonstrate that this synthetic strategy is enantiospecific.The correct relative configuration of biologically active oxazolidinone, such as (S)-Linezolid, was obtained using (R)epichlorohydrin.

Experimental Section
General.All solvent and reagents were obtained from commercial sources and were used without purification.Hygroscopic solvents were purchased as anhydrous in sealed bottles with septa and over molecular sieves.The reactions were monitored by thin layer chromatography (TLC) on Merck silica gel plates.The synthesized compounds were purified by silica flash chromatography, using Merck silica gel (particle size 0.040-0.063mm) and mixtures of ethyl acetate and petroleum ether (fraction boiling in the range of 40-60 °C) in various ratios.Melting points were determined on a Reichart-Thermovar hot-stage apparatus. 1H NMR, 13 C NMR and HPLC/MS are utilized to verify the structure and purity of synthesized compounds. 1H NMR and 13 C NMR were recorded on a Bruker Advance ( 1 H: 300 MHz, 13 C: 75.5 MHz); DMSO-d6 or CDCl3 were used as solvent and TMS as an internal standard.Chemical shifts (δ) are expressed in ppm and coupling constants as J values in Hertz.HRMS spectra were recorded in positive mode with HPLC/MS (6540 UHD Accurate Mass Q-TOF LC/MS -Agilent Technologies) and Dual AJS ESI source.Chiral HPLC (HPLC/UV 1260 infinity -Agilent Technologies, Inc., Santa Clara, CA, USA) with a Daicel Chiralpak IA column and hexane/iPrOH (80:20) as mobile phase, as previously reported. 40he compounds 3c and 3d were purchased from Sigma Aldrich (Sigma Aldrich, St. Louis, MO, USA).The derivatives 3i,7-9 14 and toloxatone 35 were synthetized as previously reported. 1H NMR spectroscopic data of the known compounds 3b, 41 3e, 42 3f, 43 3g, 44 3h, 45 5a, 46 5b, 37 5d, 46 5f, 35 5h, 47 6h, 44 9, 14 10, 37 and DuP 721 37 are consistent with literature.